Volcanism

World Organization of Volcano Observatories (WOVO)

The World Organization of Volcano Observatories was established as the result of a meeting of representatives from world-wide volcano observatories, held in Guadeloupe in 1981. WOVO became International Association of Volcanology and Chemistry of the Earth’s Interior Commission in the following year.

The principal aims of the World Organization of Volcano Observatories are:

1. To stimulate cooperation between scientists working in observatories and to create or improve ties between observatories and institutions directly involved in volcano mmonitoring.

2. To facilitate an exchange of views and experience in volcano monitoring by convening periodic meetings including field-based ones, by periodic newsletters, and by promoting a specific e-mail observatories network service.

3. To maintain an up-to-date inventory (Directory of Volcano Observatories) of networks and of instrumentation and manpower that could be made available to any of the member institutions if a situation arises that requires scientific support.

4. To supply technical support at observatories in developing countries and to create a WOVO ffund to be used by these observatories (activities of WOVO generally speaking are focused on dangerous volcanoes in developing countries).

5. To organise an international task force and to promote funding from international organisations that could help defray travel and related eexpenses of scientific support teams.

Volcano Monitoring Techniques

Background:

Geologists have developed several methods to monitor changes in active volcanoes. These methods allow geologists to forecast and, in some cases, predict, the onset of an eruption. A forecast indicates that the volcano is „ready“ to erupt. A prediction states that a volcano will erupt within a specified number of hours or days (Wright and Pierson, 1992). Several methods developed by and currently used at the Hawaiian Volcano Observatory are introduced in the following paragraphs. The Teaching Suggestions provide additional description and activities.

How do Volcanologists predict volcanic eruptions?

The prediction of volcanic eruptions is difficult because, to be of practical use, they must be made before eruptions! Its a lot easier to see ppatterns in monitoring data after an eruption has occurred. But great progress has been made because of the lessons learned over many years at Kilauea volcano in Hawaii, and applied and modified at Mt. St. Helens before and during in eruption sequence in the 1980s.

Meaningful prediction requires careful monitoring of a volcano’s vital signs. Seismometers can be used to pinpoint earthquakes which track the rise of magma and its movement along fissures. Measurements of the tilt of the entire mmountain provide additional information about the „breathing“ of the volcano as magma moves inside it. Instruments that sniff SO2, CO2 and other gases also can signal changes in the volcano. At some volcanoes the seismic information seems most reliable, at others the tilt tells the story. But the best predictions come from the combination of all of these methods into a volcano monitoring and prediction system.

You must remember that each volcano is unique. The pattern of events that signifies an eruption at one volcano may not occur before an eruption at a different volcano. And the same volcano may change its eruptive behavior at any time! The good news is that general trends in precursor behavior our being observed at a variety of volcanoes around the world so that volcanologists are getting better at predicting eruptions.

How does a thermocouple measure the temperature of lava without melting?

A thermocouple works on the principle that the electrical resistance at the point where two wires of different composition join, is very sensitive to the temperature. So.a thermocouple consists of two wires joined to an electrical source. Current passes through the wires and they only touch eachother out at the tip of tthe thermocouple probe. The temperature of that tip controls the resistance of the current passing through the place where they join, and this resistance can be measured and converted to temperature information. These little wires are usually sheathed in a ceramic insulation with an outer sheath of stainless steel. The melting points of stainless steel, ceramic, and the two wires are all higher than the temperature of lava, so none of them melt.

Lava

Background:

There are three types of lava and lava flows: pillow, pahoehoe, and aa. Pillow lavas are volumetrically the most abundant type because they are erupted at mid-ocean ridges and because they make up the submarine portion of seamounts and large intraplate volcanoes, like the Hawaii-Emperor seamount chain. Pahoehoe is the second most abundant type of lava flow.

Eruptions under water or ice produce pillow lava. James Moore of the U.S. Geological Survey made the first underwater observations of the formation of pillow lavas (Moore and others, 1973). Moore and his coworkers studied lava from the Mauna Ulu eruption They described pillow lavas as elongate, interconnected flow lobes that are elliptical or circular in cross-section (Moore, 1975). This photo shows pillow lava forming off the south coast of KKilauea volcano, Hawaii. Photograph by Gordon Tribble and courtesy of U.S. Geological Survey.

Lava flows near the coast tend to spread out laterally and enter the ocean over a large front. Several lava tubes may be active across the flow front. Larger lava tubes feed down slope and maintain pressure in the growing lobes that extend into the water. Flow lobes develop most readily on steep slopes and result in stacks of pillow „tongues“ that parallel the offshore slope. This offshore stack of lava produces beds of rubble parallel to the offshore slope. Geologists call these steeply dipping beds foreset beds.

On November 8, 1992, lava entered the ocean just east of the archeological sites at Kamoamoa on the south flank of Kilauea volcano. By the end of the month, nearly all of Kamoamoa was buried under lava. By May 1993, a lava delta had extended the coastline about 1,600 feet (500 m) to the south. This aerial view is to the west. Photograph by Christina Heliker, U.S. Geological Survey, May 12, 1993.

The foreset beds are part of a lava delta that grows towards the ocean and provides a platform on which subaerial lavas can extend, thus adding new land to the

island. However, some lobes are not continuous and break apart to form pillow sacks, thus adding rubble to the flank of the volcano. The active front of a lava delta is called a bench. Like the delta, the bench is built on rubble. However, the bench is relatively unstable and can collapse, falling into the ocean (Mattox, 1993).

A bench collapsed at the Lae Apuki entry in April 1993, removing a block 690 feet (210 m) long, 46 feet (14 m) wwide, and 26 feet (8 m) thick from the front of the delta. A visitor, standing on the bench at the time of the collapse, disappeared into the ocean. This aerial view shows an active bench adding new land out from the old sea cliff (bottom of photo). Dark areas are black sand. Photograph by Christina Heliker, U.S. Geological Survey, August 25, 1994.

Pillow lavas are also found near the summit of Mauna Kea These pillow lavas were produced by a ssubglacial eruption that occurred 10,000 years ago. The pillow is about 3 feet (1 m) in diameter and has a glassy rim. Figure 21.11 from Porter, 1987.

Pillow lavas can also form when flows enter a river or lake. The pillows iin the photo formed in the Wailuku River above Hilo, Hawaii about 3,500 years ago. The round cobbles were transported by the river. Photograph by Jack Lockwood, U.S. Geological Survey, June 14, 1982.

Pahoehoe lava is characterized by a smooth, billowy, or ropy surface. A ropy surface develops when a thin skin of cooler lava at the surface of the flow is pushed into folds by the faster moving, fluid lava just below the surface.

Pahoehoe flows can evolve into lava tubes. One way that tubes form is by the crusting over of channelized lava flows. As the crust on a flow becomes thicker, it insulates the lava in the interior of the flow. The lava drains down slope and feeds tthe advancing front or flows into the ocean. When the eruption stops or the vent is abandoned, the lava drains from the tube. Thurston lava tube is an excellent example of a lava tube. The rock surrounding a lava tube serves as a insulator to keep the lava hot and fluid. Because the lava remains hot, it can travel great distances from the vent. For example, tube-fed pahoehoe lava traveled 7.3 miles (11.7 km) from the Kupaianaha vent to the vvillage of Kalapana (Mattox and others, 1993). Photo of volcanologist looking through a skylight to see inside of a lava tube. Photograph by J.D. Griggs, U.S. Geological Survey, February 2, 1989.

Pahoehoe flows tend to be relatively thin, from a few inches to a few feet thick. Road cuts and craters expose stacks of lava flows that make the volcano.

Aa is characterized by a rough, jagged, spinose, and generally clinkery surface. Aa flows advance much like the tread of a bulldozer. This photo is looking across an aa channel. Note the character of the aa that makes the wall of the channel. Photo by Steve Mattox, October 12, 1990.

The interior of the flow is molten and several feet thick. The molten core grades upwards and downwards into rough clinkers. As the flow advances, clinker on the surface is carried forward relative to the molten interior. The clinkers continue to move forward until they roll down the steep front. The clinkers are then overridden by the molten core. Aa lava flows tend to be relatively thick compared to pahoehoe flows. Aa flows from the 1984 Mauna Loa eruption ranged from 6 to 23 feet (2-7 m) in thickness.

During the early episodes of tthe current eruption, aa flows up to 36 feet (11 m) thick surged through the Royal Gardens subdivision at rates as great as 108 ft/min (33 m/min)(Neal and Decker, 1983). The molten core of the flow is exposed. Note vehicles for scale. Photograph by R.W. Decker, U.S. Geological Survey, July 2, 1983.

Since the mid-1800s, geologists have tried to explain the causes of pahoehoe and aa lava. Several factors were offered to explain the transition: impediment of flow by obstacles, flowage during cooling, quantity of lava, conditions under the flow, and deep cooling. In the last few decades, with careful observations of numerous lava flows, geologists have reached a better understanding of the transition of pahoehoe to aa. One important influence is the viscosity of the lava. Viscosity is the resistance of a fluid to flow. For example, molasses has a higher viscosity that water. The following paragraphs outline some of the more important observations on the transition of pahoehoe to aa.

How is lava formed?

First, there is a definition we need to make. Just to keep things straight, geologists use the word „magma“ for molten rock that is still underground, and the word „lava“ when it has reached the ssurface.

Rocks in the mantle and the core are still hot from the formation of the Earth about 4.6 billion years ago. When the Earth formed, material collided at high speeds. These collisions generated heat (try clapping your hands together – they get hot) that heat became trapped in the Earth. There is also heat within the earth produced by radioactive decay of naturally-occurring radioactive elements. It is the same process that allows a nuclear reactor to generate heat, but in the earth, the radioactive material is much less concentrated. However, because the earth is so much bigger than a nuclear power plant it can produce a lot of heat. Rocks are good insulators so the heat has been slow to dissipate.

This heat is enough to partially melt some rocks in the upper mantle, about 50-100 km below the surface. We say partially melt because the rocks don’t completely melt. Most rocks are made up of more than one mineral, and these different minerals have different melting temperatures. This means that when the rock starts to melt, some of the minerals get melted to a much greater degree than others. The main reason this is important is that the

liquid (magma) that is generated is not just the molten equivalent of the starting rock, but something different.

You could think of making a „rock“ out of sugar, butter, and shaved ice. Pretend that they are mixed equally so that your rock is 1/3 sugar, 1/3 butter, and 1/3 shave ice. If you start melting this „rock“, however, the „magma“ that is generated will be highly concentrated in the things that melt more easily, namely the ice (now water) and bbutter. There will be a little bit of molten sugar in your magma, but not much, most of it will still be crystalline.

The most common type of magma produced is basalt (the stuff that is erupted at mid-ocean ridges to make up the ocean floors, as well as the stuff that is erupted in Hawai’i). Soon after they’re formed, little drops of basaltic magma start to work their way upward (their density is slightly less than that of the ssolid rock), and pretty soon they join with other drops and eventually there is a good flow of basaltic magma towards the surface. If it makes it to the surface it will erupt as basaltic lava.

What is lava made oof?

Lava is made up of crystals, volcanic glass, and bubbles(volcanic gases). As magma gets closer to the surface and cools, it begins to crystallize minerals like olivine and form bubbles of volcanic gases. When lava erupts it is made up of a slush of crystals, liquid, and bubbles. The liquid „freezes“ to form volcanic glass.

Chemically lava is made of the elements silicon, oxygen, aluminum, iron, magnesium, calcium, sodium, potassium, phosphorus, and titanium (plus other elements in very small concentrations. Have a look at the background information in Minerals, Magma, and Volcanic Rocks.

Chemistry:

Most rocks are made of the following elements:

Element (Symbol)

Weight percent

Oxygen (O) 46.6

Silicon (Si) 27.7

Aluminum (Al) 8.1

Iron (Fe) 5.0

Calcium (Ca) 3.6

Sodium (Na) 2.8

Potassium (K) 2.6

Magnesium (Mg) 2.1

Total: 98.5

The amount of these different elements in a rrock can be determined. Geologists report the values as oxides (the element combined with oxygen). For example, a rock might have 6 weight percent FeO (iron oxide).

Different types of lava have different chemical compositions.

How does lava change its composition?

Geochemists use several elements to track the history of a batch of magma. For our example we’ll start with magnesium (and think of it as magnesium oxide, MgO, like the geochemists do). Beneath Hawaii, when the mantle melts it produces aa magma with about 17 weight percent MgO. Volcanologists call magma with this composition a picrite. As the magma rises crystals begin to form. The first crystal to form is olivine, which contains the elements magnesium, silicon, and oxygen in the proportions 2 to 1 to 4. The crystal is more dense than the surrounding magma and it begins to settle. It is estimated that an than the surrounding magma and it begins to settle. It is estimated that an olivine crystal 1 mm in diameter falls through the magma at a rate of 20 meters per hour. By settling the crystals are being removed from the magma, causing the chemical composition of system to change. As more and more olivine crystals settle, the magma has less and less magnesium oxide and more and more silica. For most volcanoes, by the time the original magma from the mantle is erupted its composition has changed from a picrite to a basalt.

If the magma is not erupted, the same process continues but more and different minerals become involved. In general, the removal of these crystals drives the composition away from that of a basalt and towards that of a rhyolite. The aamount of magnesium oxide and iron oxide continues to decrease and the amount of silica, sodium oxide, and potassium oxide increases.

Other processes can become involved. The magma may melt and incorporate crustal rocks that tend to contain more silica. This drives the composition towards rhyolite. The rising magma may also intersect and mix with a magma that has evolved differently. This will also change its composition.

These processes (crystal settling, assimilation, and magma mixing) influence the composition of the magma. Geochemists study the minerals and the chemistry of the lava to determine which processes were involved and how big of a role each one played.

How hot is lava?

The temperatures of lavas vary depending on their chemical composition. Hawaiian lava (basalt) is usually around 1100 C. Volcanoes such as Mt. St. Helens erupt lava that are around 800 C.

Here is a list of temperatures for the common types of lava:

Rock type

Temperature (C) Temperature (F)

Rhyolite 700-900 1292-1652

Dacite 800-1100 1472-2012

Andesite 950-1200 1742-2192

Basalt 1000-1250 1832-2282

How long does it take lava to cool?

Lava cools very quickly at first and forms a thin crust that insulates the interior of the lava flow. As a result, basaltic lava flows can form crusts that are thick enough to walk on in 10-15 minutes bbut the flow itself can take several months to cool! Because of the insulating properties of lava, it cools slower and slower over time. Thick stacks of lava flows (30 m or 100 ft thick) can take years to cool completely. An extreme example is a lava flow that was erupted in 1959 and partly filled a pit crater (Kilauea Iki). The „ponded“ flow was about 85 meters thick (about 280 ft thick). It was drilled in 1988, and there was still some mushy, not-quite-solid stuff down near the bottom, 29 years after it erupted!

How far can lava flow?

The longest recent flow in Hawai’i was erupted from Mauna Loa in 1859. It is about 51 kilometers long from the vent to the ocean (we don’t know how much longer it went out under water, but probably not too much farther). There is a lava flow at Undara in Queensland, Australia is 100 miles (160 km) long.

How fast does lava flow?

In Hawaii the fastest flows we’ve recorded were those of the 1950 Mauna Loa eruption. These were going about 6 miles (10 kilometers) per hour through thick forest. That was the velocity of the flow front. Once the lava

flows became established and good channels developed, the lava in the channels was going at more like 60 km/hour!

On January 10,1977, a lava lake at Nyiragongo drained in less than one hour. The lava erupted from fissures on the flank of the volcano and moved at speeds up to 40 miles per hour (60 km/hr). About 70 people were killed.

How close can I get to lava and will it hurt or kill me?

How close you can get depends oon what kind of lava flow it is, and whether you are upwind or downwind. For example, the most approachable lava is pahoehoe. This is because each toe forms an insulating skin seconds after emerging on the surface. This skin is at first flexible and then hardens, but even when flexible it is a good insulator. This serves to keep the interior of an active pahoehoe toe hot and fluid but also prevents you from getting burned by the radiant hheat. If the wind is at your back, you can easily approach long enough and close enough to get a sample with a hammer. It is still hot, and unless yo! are well-protected you can only be that close for aa minute or so. You also notice that as soon as you peel the skin off to get at the molten interior, th! heat goes way up. This is heat that you can’t stand, you have to get back otherwise blisters start to form. It is hot enough that you can’t accidentally step on active lava.

Skylights into lava tubes on pahoehoe flows are quite hot, and have to be approached from upwind. They are so hot that the air shimmers over them so they are hard to miss. They are dangerous not as much because of the radiant heat from the lava inside but because of the super-heated plume of air coming out. You have to be really careful tthat the wind doesn’t shift, and many a volcanologist has gotten singed skin and hair when the wind changed.

An ‘a’a flow is terrible to work near. Instead of a relatively continuous skin! ‘a’a flows have discontinuous layers of clinker, and a huge amount of radiant heat escapes from between the clinker. ‘A’a flows also move faster so you really have to be quick on your feet if you want a sample. Additionally, ‘a’a flows tend to form open channels rrather than lava tubes. The channels can sometimes have completely incandescent surfaces because they are flowing so fast that any skin that forms is immediately torn or sunk. We remember once flying over a large channel in a helicopter. We must have been at least 200-40! meters above the flow, but as soon as we were over the channel we could immediately feel the radiant heat through the windows! We would bet that nobod! has been downwind of an active ‘a’a channel.

Lava won’t kill you if it briefly touches you. You would get a nasty burn, but unless you fell in and couldn’t get out, you wouldn’t die. With prolonged contact, the amount of lava „coverage“ and the length of time it was in contac! with your skin would be important factors in how severe your injuries would be! The health of the individual, the amount of time before care can be given and the quality of that care would also be important. In fact there have been 2 cases at the Hawaiian Volcano Observatory where a geologist fell into lava. Fortunately in both instances the lava was not very deep and they were able to get out quickly. BBoth ended up in the hospital and it was a scary and painful experience. Both recovered fine. Blong (1984) points out that little research has been done on injuries caused by lava. People have been killed by very fast moving lava flows. A recent example was the 1977 eruption at Nyiragongo.

Why is lava different colors?

The color of lava depends on its temperature. It starts out bright orange (1000-1150 C). As it cools the color changes to bright red (800-1000 C), then do dark red (650-800 C), and to brownish red (500-650 C). Solid lava is black (but can still be very hot).

It is the same reason that electric stove elements turn colors. First red then orange when turned on high (don’t do that experiment by yourself). As things get hotter they start to glow, first red, then orange, then yellow, then green, then blue, then violet. The sun’s color, for example, is sort of yellow-green, but you can’t tell by looking at it (and remember, you should NEVER look at the sun).

Why is lava so hot?

Lava is hot for two reasons:

1. It’s hot deep in the Earth (about 100 km down) where rocks melt to make magma. <

2. The rock around the magma is a good insulator, so the magma doesn’t lose much heat on the way to the surface.

How do volcanoes affect people?

Painting of Mount Vesuvius

Volcanoes affect people in many ways, some are good, some are not. Some of the bad ways are that houses, buildings, roads, and fields can get covered with ash. As long as you can get the ash off (especially if it is wet), your house may not collapse, but often the people leave because of the ash and are not around to continually clean off their roofs. If the ashfall is really heavy it can make it impossible to breathe.

Lava flows are almost always too slow to run over people, but they can certainly run over houses, roads, and any other structures.

Pyroclastic flows are mixtures of hot gas and ash, and they travel very quickly down the slopes of volcanoes. They are so hot and choking that if you are caught in one it will kill you. They are also so fast (100-200 km/hour) that you cannot out-run them. If a volcano that is known for producing pyroclastic flows is looking like it may erupt soon,

the best thing is for you to leave before it does.

Some of the good ways that volcanoes affect people include producing spectacular scenery, and producing very rich soils for farming.

Gases

Water vapor, the most common gas released by volcanoes, causes few problems. Sulfur dioxide, carbon dioxide and hydrogen are released in smaller amounts. Carbon monoxide, hydrogen sulfide, and hydrogen fluoride are also released but typically less than 1 percent by volume.

Gases pose the greatest hazard close to the vvent where concentrations are greatest. Away from the vent the gases quickly become diluted by air. For most people even a brief visit to a vent is not a health hazard. However, it can be dangerous for people with respiratory problems.

The continuous eruption at Kilauea presents some new problems. Long term exposure to volcanic fumes may aggravate existing respiratory problems. It may also cause headaches and fatigue in regularly healthy people. The gases also limit visibility, especially on the lleeward side of the island where they become trapped by atmospheric conditions.

The biggest eruption

The 1815 explosive eruption of Tambora volcano in Indonesia and the subsequent caldera collapse produced 9.5 cubic miles (40 cubic kilometers) of ash. The eruption killed 110,000 people. An additional 80,000 people died from crop loss and famine.

Aircraft

To put it mildly, ash is bad for jet aircraft engines. Apparently the problem is much more severe for modern jet engines which burn hotter than the older ones. Parts of these engines operate at temperatures that are high enough to melt ash that is ingested. Essentially you end up with tiny blobs of lava inside the engine. This is then forced back into other parts where the temperatures are lower and the stuff solidifies. As you can imagine this is pretty bad. One problem that I heard about is that pilots start losing power and apply the throttle, causing the engine to be even hotter and melt mmore ash.

Added to this is the fact that ash is actually tiny particles of glass plus small mineral shards–pretty abrasive stuff. You can imagine that dumping a whole bunch of abrasive powder into a jet engine is not good for the engine. This has been a pretty non-scientific explanation of the problem. I just found an article that describes the problem a little more technically.

„The ash erodes sharp blades in the compressor, reducing its efficiency. The ash melts iin the combustion chamber to form molten glass. The ash then solidifies on turbine blades, blocking air flow and causing the engine to stall.

Safe distance

The distance you have to evacuate depends entirely on what kind of eruption is going on. For example, Pinatubo, one of the largest recent eruptions sent pyroclastic flows at least 18 km down its flanks, and pumice falls were hot and heavy even beyond that. For example, pumice 7 cm across fell at Clark Air base which is 25 km from the volcano! A 7 cm pumice won’t necessarily kill you but it does mean that there is a lot of pumice falling, and if you don’t get out and continuously sweep off your roof it may fall in and you’ll get squashed.

On the other hand, the current eruption at Ruapehu is relatively small. In fact, there were skiers up on the slopes when the eruptions commenced, and even though they were only 1-2 km from the vent they managed to escape. The volcanologists routinely go up on the higher slopes of Ruapehu during these ongoing eruptions to collect ash and take photographs.

So you see, you need to know something about what you tthink the volcano is going to do before you decide how far to run away. I guess if you have no idea of what the volcano is planning, and have no idea of what it has done in the past, you might want to be at least 25-30 km away, make sure you have a good escape route to get even farther away if necessary, and by all means stay out of low-lying areas!

Cities and Towns

Mount Mayon , in the Philippines, is a classic example of a stratovolcano. Photograph copyrighted and provided by Steve O’Meara of Nature.Stock.

The effect an eruption will have on a nearby city could vary from none at all to catastrophic. For example, atmospheric conditions might carry ash away from the city or topography might direct lahars and pyroclastic flows to unpopulated areas. In contrast, under certain atmospheric, eruption and/or topographic conditions, lahars, pyroclastic flows, and/or ash fall could enter the city causing death and destruction.

This scenario brings up several interesting problems. How do you evacuate a large population if there is little warning before the eruption? Where do these people go? If an eruption is highly likely yet hasn’t happened yet how long ccan people be kept away from their homes and businesses?

I should point out that in most volcanic crises geologists advise local civil defense authorities. The civil defense authorities decide what to do concerning evacuations, etc.

The IAVCEI has a program to promote research on „Decade“ Volcanoes. Decade volcanoes are likely to erupt in the near future and are near large population centers. Mount Rainier in Washington and Mauna Loa in Hawaii are two Decade volcanoes in the U.S. Other Decade volcanoes include Santa Maria, Stromboli, Pinatubo, and Unzen.

What happens to the towns around a volcano when it erupts depends on many things. It depends of the size and type of eruption and the size and location of the town. A few examples might help. The 1984 eruption of Mauna Loa in Hawaii sent lava towards Hilo but the eruption stopped before the flows reached the town. The 1973 eruption of Heimaey in Iceland buried much of the nearby town of Heimaey under lava and cinder. The 1960 eruption of Kilauea in Hawaii buried all of the nearby town of Kapoho under lava and cinder. In 1980, ash from Mount St. Helens fell on many towns in Washington

and Oregon. The 1902 eruption of Mount Pelee on the island of Martinique destroyed the town of Saint Pierre with pyroclastic flows. In 1985, the town of Armero was partially buried by lahars generated on Ruiz. For more examples see Decker and Decker (1989).

How do volcanoes affect the atmosphere and climate?

There are two things to think about. The first is how the weather near an erupting volcano is being affected. The second is how large eruptions will affect the wweather/climate around the world. I think more people are worried about the second issue than the first.

As far as I know, the main effect on weather right near a volcano is that there is often a lot of rain, lightning, and thunder during an eruption. This is because all the ash particles that are thrown up into the atmosphere are good at attracting/collecting water droplets. We don’t quite know how the lightning is caused but it probably involves the pparticles moving through the air and separating positively and negatively charged particles.

Another problem that we are having here in Hawai’i involves the formation of vog, or volcanic fog. The ongoing eruption is very quiet, with lava flowing through lava ttubes and then into the ocean. Up at the vent is an almost constant plume of volcanic fume that contains a lot of sulfur dioxide. This SO2 combines with water in the atmosphere to form sulfuric acid droplets that get carried in the trade winds around to the leeward side of the Big Island. The air quality there has been really poor since the eruption started in 1983 and they are getting pretty tired of it.

As for the world-wide affects of volcanic eruptions this only happens when there are large explosive eruptions that throw material into the stratosphere. If it only gets into the troposphere it gets flushed out by rain. The effects on the climate haven’t been completely ffigured out. It seems to depend on the size of the particles (again mostly droplets of sulfuric acid). If they are big then they let sunlight in but don’t let heat radiated from the Earth’s surface out, and the net result is a warmer Earth (the famous Greenhouse effect). If the particles are smaller than about 2 microns then they block some of the incoming energy from the Sun and the Earth cools off a little. That seems to have bbeen the effect of the Pinatubo eruption where about a 1/2 degree of cooling was noticed around the world. Of course that doesn’t just mean that things are cooler, but there are all kinds of effects on the wind circulation and where storms occur. Some folks think that large eruptions can cause the weather phenomena called „El Nino“ to start. This is a huge disruption of the Earth’s atmospheric circulation. The connection hasn’t been accepted by everybody though.

An even more controversial connection involves whether or not volcanic activity on the East Pacific Rise (a mid-ocean spreading center) can cause warmer water at the surface of the East Pacific, and in that way generate an El Nino. Dr. Dan Walker here at the University of Hawai’i has noticed a strong correlation between seismic activity on the East Pacific Rise (which he presumes indicates an eruption) and El Nino cycles over the past ~25 years.

How do volcanoes affect plants and animals?

Lava flows covering the Kamoamoa area of Hawai`i Volcanoes National Park. Photograph by Steve Mattox, November 14, 1992.

Plants are destroyed over a wide area, during an eruption. The good thing is that volcanic soil is very rich, so once eeverything cools off, plants can make a big comeback!

Livestock and other mammals have been killed by lava flows, pyroclastic flows, tephra falls, atmospheric effects, gases, and tsunami. They can also die from famine, forest fires, and earthquakes caused by or related to eruptions.

Mount St. Helens provides an example. The Washington Department of Game estimated that 11,000 hares, 6,000 deer, 5,200 elk, 1,400 coyotes, 300 bobcats, 200 black bears, and 15 mountain lions died from the pyroclastic flows of the 1980 eruption.

Aquatic life can be affected by an increase in acidity, increased turbidity, change in temperature, and/or change in food supply. These factors can damage or kill fish.

Eruptions can influence bird migration, roosting, flying ability, and feeding activity.

The impact of eruptions on insects depends on the size of the eruption and the stage of growth of the insect. For example, ash can be very abrasive to wings.

How quickly do plants begin to grow back?

The answer is that it depends on how much rain falls in the particular area. For example, on the rainy side of the island of Hawai’i, flows that are only 2 years old already have ferns and small trees growing on tthem. Probably in 10 years they’ll be covered by a low forest. On the dry side of Hawai’i there are flows a couple hundred years old with hardly a tuft of grass in sight. This means that when you are looking at old lava flows and trying to determine how old they are based on the amount of vegetation, you have to take the climate into effect as well.

What are some good things that volcanoes do?

Aso viewed from the visitors center. Small plume above Aso during a period of mild Strombolian eruptions, December 30, 1991. Photograph by Mike Lyvers.

That’s a good question. I guess the main good effect that volcanoes have on the environment is to provide nutrients to the surrounding soil. Volcanic ash often contains minerals that are beneficial to plants, and if it is very fine ash it is able to break down quickly and get mixed into the soil.

Perhaps the best place to look for more information about this would be to look up references about some of the countries where lots of people live in close proximity to volcanoes and make use of the rich soils on volcanic flanks. These would

include Indonesia, The Philippines, Japan, Italy, etc.

I suppose another benefit might be the fact that volcanic slopes are often rather inaccessible, especially if they are steep. Thus they can provide refuges for rare plants and animals from the ravages of humans and livestock.

Finally, on a very fundamental scale, volcanic gases are the source of all the water (and most of the atmosphere) that we have today. The process of adding to the water and atmosphere is pretty slow, bbut if it hadn’t been going on for the past 4.5 billion years or so we’d be pretty miserable.

Volcanoes have done wonderful things for the Earth. They helped cool off the earth removing heat from its interior. Volcanic emissions have produced the atmosphere and the water of the oceans. Volcanoes make islands and add to the continents.

Volcanic deposits are also used as building materials. In the 1960’s Robert Bates published Geology of the Industrial Rocks and Minerals. He nnoted that basalt and diabase are quarried in the northeastern and northwest states. Most of the basalt and diabase is used for crushed stone: concrete aggregate, road metal, railroad ballast, roofing granules, and riprap. High-denisity basalt and diabase aggregate is uused in the concrete shields of nuclear reactors. Some diabase is used for dimension stone („black granite“).

Pumice, volcanic ash, and perlite are mined in the west. Pumice and volcanic ash are used as abrasives, mostly in hand soaps and household cleaners. The finest grades are used to finish silverware, polish metal parts before electroplating, and for woodworking. Bates reports that in ancient Rome lime and volcanic ash were mixed to make cement. In modern times pumice and volcanic ash have been used to make cement for major construction projects (dams) in California and Oklahoma. Pumice and volcanic ash continue to be used as lightweight aggregate in concrete, especially precast concrete blocks. Crushed and ground pumice are also used for lloose-fill insulation, filter aids, poultry litter, soil conditioner, sweeping compound, insecticide carrier, and blacktop highway dressing. Perlite is volcanic glass (made of rhyolite) that has incorporated 2-5% water. Perlite expands rapidly when heated. Perlite is used mostly as aggregate in plaster. Some perlite is used as aggregate in concrete, especially in precast walls.

How were the volcanoes of Hawaii created?

About 95% of the world’s volcanoes are located near the boundaries of tectonic plates. The other 5% are thought to be associated wwith mantle plumes and hot spots. Mantle plumes are areas where heat and/or rocks in the mantle are rising towards the surface. A hot spot is the surface expression of the mantle plume. Geologists think the hot spots are stationary and the tectonic plates are mobile. The hot spot provides magma which supplies volcanoes. The movement of the plate carries the volcano off the hot spot and it gradually becomes extinct and usually subsides below sea level. A new volcano begins to form on the sea floor and grow towards the surface. Some of these volcanoes rise above sea level to make islands. The classic example is the Hawaii-Emperor volcanic chain, a line of volcanoes and seamounts that extends from Hawaii to Daikakuji, 2,200 miles (3,500 km) to the northwest. Suiko seamount is 65 million years old, the oldest seamount associated with the Hawaiian hot spot. The hot spot is probably older but any volcanoes it produced have been destroyed by subduction.

At first glance the shape and topography of the Hawaii-Emperor volcanic chain might remind you of an island arc. However, volcanoes of the Hawaii-Emperor volcanic chain are progressively younger towards Hawaii and made of basalt. Island arcs are pproduced at convergent plate boundaries where an ocean plate is subducted beneath an adjacent ocean plate. The Aleutians of Alaska is an example. There is no age progression and andesite is the most common rock.

What is the history of Mount. St. Helens prior to the May 1980 eruption?

According to Volcanoes of the World, by Simkin and Siebert (1994, Geoscience Press, P.O. Box 42948, Tucson, AZ 85733-2948), Mt. St. Helens erupted 23 times prior to 1831, based on charcoal dates. After that (and before the 1980 eruption) it erupted in 1831, 1835, 1842, 1847, 1848, 1849, 1853, 1854, and 1857.

Eruptions began at Mount St. Helens about 40,000 years ago. The deposits are air-fall tephra and pyroclastic flows, the type of material produced by explosive eruptions. One pumiceous tephra deposit produced during this episode had a volume as great as any subsequent tephra eruption at Mount Saint Helens. I think it would qualify as a big eruption.

A recent paper suggests that eruptions at Mount Saint Helens began as long ago as 80,000 years. Berger and Busacca (1995) dated a loess (wind-blown slit-sized material) deposit just below a tephra layer in eastern Washington that is known to be from Mount SSt. Helens. The loess is about 80,000 years old. The tephra is thought to be slightly younger.

What happened when Mount St. Helens erupted on May 18, 1980?

Chronologies and summaries of the May 18, 1980 eruption For more information on what happen during the May 18, 1980 eruption of Mount St. Helens click here and here. The USGS’s Cascade Volcano Observatory has a very detailed account of the eruption, with a list of references.

Time

The climatic eruption began at 08:32 PDT on May 18, 1980.

Deaths

57 people were killed directly by the eruption. There was also a plane crash, a traffic accident, and shoveling ash which killed a total of 7 more.

Height of Mount St. Helens

The summit elevation was 9,760 feet (2,975 m) before the eruption. After the eruption the elevation of the new summit was about 8,525 feet (2,600 m).

Volume

About 0.25 cubic kilometers of new volcanic rock was erupted on May 18, 1980. This would make a cube about 600 meters (~2000 ft) on a side! But much more material was moved by the eruption: the entire northern side of the volcano collapsed and flowed down hill. The volume of this collapse was about 2.5 cubic kilometers

– ten times bigger than the new lava.

Ash

The eruption began at 8:45 a.m. At noon, the ash plume (in the upper troposphere and lower stratosphere) had reached Moscow, Idaho. By about 3 p.m. it was near Missoula, Montana and starting to spread south. By 6 pm it was eas! of Pocatello, Idaho. At the end of the day, about 16 hours after the eruption started, the ash plume was near central Colorado.

A huge volume of ash was created bby the various 1980 eruptions of Mount St. Helens. Every community affected had its own ways of dealing with the ash. Tons of ash was probably washed down storm drains and into sewer systems as people cleaned roofs and sidewalks. Local landfills received ash. Many tons of ash came down rivers and streams into the Columbia River. The river had to be dredged to allow shipping to pass, and the sand dredged from the bottom was deposited in large dikes aalong the Columbia. These dikes are now covered in grass and trees, but if a person was to dig down a few feet they would find the ash.

Ash can still be found on the floor of forested areas all aaround the mountain, and the blast zone is still heavily covered by ash. Souvenir „ash“ trays and mugs made from Mount St. Helens ash can be purchased at gift shops near the mountain.

Will Mt. St. Helens erupt again?

Mt. St. Helens still emits steam pretty much every day, but it hasn’t had any eruptions since 1985. We can not say if it will erupt again this century, but studies indicate that there is still hot magma under the mountain, so it will probably erupt sometime. Fortunately, Mt. St. Helens is still carefully monitored by volcanologists, so that if or when the mountain begins to become active again, we should have early warning.

It is likely that Mt. St. Helens will hhave one or more small eruptions during your lifetime, but probably unlikely that it will have another big one. At the time of the 1980 eruption there was the weight of the whole volcanic cone available to keep the magma pressure from erupting. This allowed a large amount of pressure to build up and consequently the eruption was large. Now the whole top of the mountain is gone so there is a good deal less weight available as a counter bbalance to the pressure. This means that eruptions occur after lesser amounts of pressure have built up and the eruptions are therefore smaller. Eventually, of course, Mt. St. Helens will recover its cone-shaped shape by filling in the amphitheater with lava domes and ash. Once this occurs (probably over a period of a few hundred years) it’ll be ready to suffer another large 1980-size eruption.

Since the famous 1980 eruption there has been a lava dome growing within the caldera, and every once in a while part of the dome either collapses or is blasted away by either gas or steam explosions. These eruptions seem to be getting less and less frequent but have probably not stopped.

Mount St. Helens erupted at least 10 times in the 200 years before the 1980 eruption. It is considered to be the most active volcano in the Cascades. Volcanologists that study Mount St. Helens believe it is likely to erupt again within a few decades or a century at most.

It can be difficult to know when a volcano is dead. Many cinder cones, are monogenetic, erupting only once before the volcano is dead. Stratovolcanoes can erupt many times in only a few ccenturies, like Llaima in Chile, which has erupted at least 40 times in the last 350 years. Stratovolcanoes can also go many centuries without an eruption, like Newberry volcano in Oregon, which has not erupted in 1,300 years but will probably erupt again. Large volcanic systems, like Yellowstone, can go hundreds of thousands of years between eruptions.

What is the current status of Mt. St. Helens?

The U.S. Geological Survey’s Cascade Volcano Observatory reports that the number of small magnitude earthquakes (less than magnitude 1) beneath Mount St. Helens has increased slowly and steadily from less than 10 events per month in January (1995) to about 100 events per month in September (1995). They point out that the volcano has been quiet during this period and that no explosion or emission of gas or ash has occurred from the lava dome. Similar seismic activity was observed prior to and during a series of small gas explosions from the dome in 1989-1991. Although these explosions were relatively small, they did throw dome rocks 1 foot (30 cm) in diameter 0.5 miles (0.8 km) from the dome and sent ash plumes as high as 20,000 feet (6,100 m) above the sea level. Because tthese events can happen without warning some trails in the area have been closed.

What were the effects on people when Mt St Helens erupted?

The eruption killed 57 people, in the lateral blast, ashfall, and lahars. The causes to death included asphyxiation, thermal injuries, and trauma. Four indirect death were caused by a cropduster hitting powerlines during the ashfall, a traffic accident during poor visibilty, and two heart attacks from shoveling ash.

The Toutle River was flooded by melting snow and ice from the mountain. About 12 million board feet of stockpiled lumber were sweep in the river. Eight bridges were destroyed. 200 homes were destroyed or damaged. Debris dams were added to help control sediment in the rivers.

Thirty logging trucks, 22 transport vehicles, and 39 railcars were damaged or destroyed along with 4.7 billion board feet of timber.

Shipping was stopped on the Columbia River and some vessels were stranded. In eastern Washington, falling ash stranded 5,000 motorist. Ash had to be cleared from runways and highways.

For a limited time, some people living near the eruption suffered from post traumatic stress syndrome: depression, troubled sleep, irritability, ans a sense of powerlessness.

From 1980-1990, 74 research projects were

funded by the National Science Foundation at a total cost of just less than $5 million. The Mount St. Helens Visitors Center at Castle Rock cost $5.5 million to construct. Trails, roads in the park, and interpretive centers cost another $42.3 million. New highway and bridges from the Toutle River to Johnston Ridge cost $145 million. facilities along this road will cost another $25 million.

What On June 23, 1994, a lava lake formed in Nyiragongo volcano in eastern Zaire. AActivity has continued at the lava lake. Nyiragongo is 3,459 m tall and at 1.5S, 29.3E.

Erta Ale has been in continuous eruption since 1967.

Was the most recent volcanic eruption in Africa?

On June 23, 1994, a lava lake formed in Nyiragongo volcano in eastern Zaire. Activity has continued at the lava lake. Nyiragongo is 3,459 m tall and at 1.5S, 29.3E.

Erta Ale has been in continuous eruption since 1967.

Nyiragongo.

Nyiragongo (1.5S, 29.3E) is associated with the East AAfrican Rift. The volcano is in Zaire, not far from the border with Rwanda, in Virunga National Park. The town of Goma is 11 miles (18 km) south of the summit of Nyiragongo and on the shore of Lake Kivu. GGoma served as an encampment for nearly a million refugees from the civil war in Rwanda.

Nyiragongo (elevation: 11,365 feet, 3,465 m) is a stratovolcano. A crater at the summit of Nyiragongo contained a lava lake from 1894 to 1977. On January 10, 1977, the lava lake drained in less than one hour. The lava erupted from fissures on the flank of the volcano and moved at speeds up to 40 miles per hour (60 km/hr). About 70 people were killed. The fluid lava reached within 2,000 feet (600 m) of the Goma airport. From June 1982 to early 1982 the volcano was active with a lava lake in the crater and phreatic explosions and lava fountaining.

The most recent aactivity at Nyiragongo began in June of 1994. A lava lake once again filled part of the crater. The lava lake was approximately 130 feet (40 m) in diameter and sent lava flows onto the floor of the 2,600 foot (800 m) diameter crater. The surface of the lava lake was about 500 feet (150 m) below the level of the lake when it drained in 1977.

The lava at Nyiragongo is a nephelinite. Nephelinite is a type of alkaline ((high concentration of alkali elements, Na and K) lava. It tends to be aphanitic (no crystals visible to the unaided eye). With a microscope crystals of the following minerals might be seen: nepheline, clinopyroxene, olivine, iron-titanium oxides, feldspar and possibly leucite.

Cameroon.

Volcanoes in Cameroon are part of the Cameroon line, a chain of volcanoes extending from Annobon Island in the Atlantic Ocean northeastward through Cameroon. The oldest rocks have been dated at 70 million years old. Nine volcanoes along the line are active. A fissure eruption occurred at Mt. Cameroon in 1982.

Volcanism along the Cameroon line is related to rifting – where a continent breaks into two pieces. About 110 million years ago a giant rift broke apart what became Africa and South America and the South Atlantic Ocean began to form. A smaller rift formed within the African continent. This older rift, called the Benue Trough, is north of and parallel to the Cameroon line. About 80 million years ago, during a reorganization of plate boundaries, the African plate rotated counter-clockwise. Then a new rift formed that failed to split Africa but apparently did form conduits that allowed magma to ultimately reach the surface and form the vvolcanoes of the Cameroon line.

1986 In 1986, a cloud of gas burst out of Lake Nyos because a landslide disturbed the stratification with the lake. The stratification (layering) is caused by different amounts of CO2 dissolved in the water. The layers are stable (they don’t mix or changes position). The alternative mechanism, that the burst was caused by a phreatic eruption (involving lava into the lake), is not generally accepted.

Dr. Niels Oskarsson proposed reducing the amount of CO2 in the lake by continuously draining the bottom waters from the lake. Water from rainfall would displace an equal volume of water from the bottom of the lake. Over time this would establish low CO2 levels in the lake.

The 1987 conference recommended continuous telemetered monitoring of the deep water temperature, pH, conductivity, and alkalinity of the lake. Unfortunately, no funding or plans for implementation were developed (in 1987).

I think there is a pretty good consensus as to what happened at Lake Nyos (in 1986, by the way). A few folks disagree on the details, but the main event was the sudden release of a large cloud of carbon dioxide gas from the lake. Lake Nyos is a water-filled throat oof an old volcano and it is deep and funnel-shaped. Although no longer erupting, there is still gas being released by the old plumbing system under the lake. Carbon dioxide gas was released directly into the deepest waters of the lake, where it could remain in solution (the way that carbon dioxide stays in solution in an un-opened soda or beer). In this situation the lake could build up a large amount of carbon dioxide dissolved in the deeper water. This was a stable situation. The carbon-dioxide charged water was slightly denser than the normal water in the upper levels of the lake, and the weight of the overlying water kept the carbon dioxide in solution in the deeper parts of the lake.

However, nature decided to unbalance the situation. This is where the disagreement among volcanologists comes in. It is agreed that somehow some of that carbon dioxide-rich water was displaced upward into shallower depths to the point where the overlying water pressure was lower and carbon dioxide bubbles could start to form (like when you lower the pressure on a soda by opening the bottle and suddenly bubbles start to form). At Lake Nyos, once these bubbles started

to form they wanted to rise to the top, this brought up more carbon dioxide-rich water which then also started to develop bubbles, and pretty soon there was a big rush of carbon dioxide bubbles to the surface. What people don’t agree on is what the trigger for this unbalancing event was. Most people, I think, feel that there was some sort of landslide into the lake that stirred up the water. There are a few volcanologists who think there wwas some type of eruption in the deeper part of the lake, but they are in the minority.

Once all this carbon dioxide reached the surface, it splashed some lake water out of the lake, like a big bubble bursting. Carbon dioxide is denser than air, so it hugged the ground and flowed down the stream valley that leads away from the lake. Unfortunately many homes and at least one town are also along this valley and the inhabitants were ccaught by this cloud of ground-hugging gas. Carbon dioxide usually kills people by displacing the air that they need to breathe, but in high-enough concentrations it is poisonous as well.

So you see that although volcanologists might disagree on the ttectonic details underlying Cameroon and might even disagree on the triggering mechanism for the 1986 disaster, but the danger is pretty well understood. Obviously one way to minimize the chances of this happening again is to prevent the deep lake waters from becoming gas-charged. A program was started to have a pump running that brought up the deep water (in a small controlled way), where it was pumped into the air like a fountain. This allowed smaller amounts of deep water to lose their carbon dioxide gradually rather than having the potential of a big bubble occurring again. Another worry is that the lake walls themselves are not very strong (they are constructed of tuff, partially solidified ash). The problem iis that there is this lake at an elevation higher than the main towns nearby, and if for any reason the walls of the lake were breached there would be a flood of water that could be just as dangerous as a flood of carbon dioxide. I’m not sure but I think there were plans to pump water out of the lake to try and keep the level and pressure down.

Mt. Erebus.

The most recent eruption of Mount Erebus bbegan in 1972 and stopped in 1992. It shares some similarities with both Kilauea and Mount St. Helens but also has some significant differences. Like Kilauea current eruption, the vent was a lava lake that produced some lava flows. Unlike Kilauea, Mount Erebus is a stratovolcano, the same type of volcano as Mount St. Helens. Eruptive activity at Mount Erebus tends to be strombolian , a little more explosive than Kilauea but far less than Mount St. Helens. The composition of the volcanic products at each volcano is different. The silica content of lava from Kilauea and Mount Erebus are about 50 weight percent. However, Mount Erebus rocks have greater amounts of alkali elements (sodium and potassium), about 9-10 weight percent compared to the 3 weight percent for rocks from Kilauea. Ash and lava from Mount St. Helens are called dacite because they contain about 64 weight percent silica, much more than the other two volcanoes.

The tectonics around Erebus are not too clearly understood, but as summarized by Tom Simkin and Lee Seibert in „Volcanoes of the World“, there is a large continental rift that is cutting through the W part of Antarctica (I guess it is this splitting-apart tthat has formed the Ross Sea). The most famous continental rift is the E. African Rift, and it too is associated with volcanism.

There are chapters on Mt. Erebus in „Volcanoes of the Antarctic Plate and Southern Oceans“ by WE LeMasurier and JW Thomson, editors. In the summary chapter on the Mt. Erebus volcanic province, PR Kyle writes „Mt Erebus is an active volcano.and contains a persistent convecting lava lake of anorthoclase phonolite magma. Small Strombolian eruptions occur on a daily basis.often ejecting anorthoclase phonolite bombs onto the crater rim. From September to December 1984, larger Strombolian eruptions occurred more frequently, ejecting bombs up to 2 km from the crater and sending small eruption columns to over 2 km high.,,“

The lava lake in the summit crater counts as an ongoing eruption but it is not a particularly active eruption. The fact that the volcano is composed of layers of lava and ash erupted mainly from the summit means that it does erupt in a bigger way, it is just that no humans have seen it happen.

What happened at Ruapehu in 1995?

Since the activity in 1995, the volcano has quieted down and the only threat is to people in tthe immediate vicinity of the summit. The nearest large town is Taupo, with a population that probably varies between 20,000 and 60,000 depending on whether it is summer or winter. Taupo is ~100 km NE of Ruapehu. Other smaller but closer towns are Turangi, Waiouru, Ohakune, and National Park. Even tinier places are Whakapapa, Iwikau, Tukino, and Turoa Villages, all within~10 km of the summit.

As far as I know there hasn’t been any damage from the 1995 eruption other than the minor inconvenience of slight dustings of ash. There have also been the inconvenience to lots of skiers hoping to enjoy the last month of ski season and being kept off the slopes. The first day of the eruption 4 volcanologists were injured in a plane crash. Two received only minor scrapes and bruises but the other two were more seriously hurt and are still recovering. I have forwarded your question to a friend who is working at the volcano observatory there and hopefully she can provide some more answers.

The current research involves studying both the activity that led up to the 1995 eruption and the activity during the most recent eruption. The former is an attempt to

try and understand what the precursors were and can the be better identified prior to the next eruption. The latter is to try and understand the mechanisms of explosions, ashfall, and lahar generation and emplacement.

History

Ruapehu has had 50 historic eruptions, more than any other crater lake in the world. Most of these eruptions have been phreatic. The last major eruption was in March of 1992. In this century the longest period of repose between eruptions at Ruapehu has been 112 years (1906-1908). The average period of repose was about 4 years.

Eruptions at Mount Ruapehu are caused by the interaction of lava and water. Volcanologists call this a phreatic eruption. Because the lava erupts into the bottom of a crater lake, a lot of heat energy converts water to steam. The steam expands violently, throwing water and sometimes ash into the air. The recent increase in activity was caused by a greater volume of lava being erupted into the llake. Similar, but usually smaller, phreatic eruptions have been occurring at Ruapehu since 1889.

Has there ever been any volcanic activity in Australia?

There has not been any eruptions in Australia in this century. The most recent eruption in Australia was aat Mt. Gambier, a shield volcano in the Newer Volcanic Province, Victoria. The Newer Volcanics Province in Victoria Australia is made of four shield volcanoes and associated vents: Red Rock, Mt. Napier, Mt. Schank, and Mt. Gambier. They last erupted between 5850 and 2900 B.C. The eruptions were explosive and some generated lava flows. It is impossible to say if the volcanoes will erupt again. However, there have been rare earthquakes in the area, most recently in 1976. There are numerous volcanic islands north and east of Australia including North Island, New Zealand, the islands of Vanuatu, the Solomon Islands, New Britian, and Indonesia. There are numerous interesting volcanic provinces in Australia. There are flood basalts of Cambrian age (about 6650 million years old) northeast of Halls Crossing in northern Australia. Volcanism commenced about 70 million years ago at volcanic centers in southeast Queensland and northeast South Wales. Compositions range from basalt to rhyolite and includes shields, plugs, and domes. In north Queensland there are some very long basaltic lava flows. For example, at Undara a flow is 100 miles (160 km) long. You might have a look at Johnson and others (1989).

The 1883 eruption of Krakatau.

Krakatau erupted iin 1883, in one of the largest eruptions in recent time. Krakatau is an island volcano along the Indonesian arc, between the much larger islands of Sumatra and Java (each of which has many volcanoes also along the arc). There is a very fine book about the Krakatau eruption by Tom Simkin and Richard Fiske (Simkin, T., and Fiske, R.S., Krakatau 1883: The volcanic eruption and its effects: Smithsonian Institution Press: Washington, D.C., 464 p.), so if you really want to know about the eruption you should go to the nearest bookstore or library to find that.

Here are some highlights from their summary of effects:

1. The explosions were heard on Rodriguez Island, 4653 km distant across the Indian Ocean, and over 1/13th of the earth’s surface.

2. Ash fell on Singapore 840 km to the north, Cocos (Keeling) Island 1155 km to the SW, and ships as far as 6076 km west-northwest. Darkness covered the Sunda Straits from 11 a.m. on the 27th until dawn the next day.

3. Giant waves reached heights of 40 m above sea level, devastating everything in their path and hurling ashore coral blocks weighing as much as 600 tons.

4. At least 36,417 people were killed, mmost by the giant sea waves, and 165 coastal villages were destroyed.

5. When the eruption ended only 1/3 of Krakatau, formerly 5×9 km, remained above sea level, and new islands of steaming pumice and ash lay to the north where the sea had been 36 m deep.

6. Every recording barograph in the world documented the passage of the atmospheric pressure wave, some as many as 7 times as the wave bounced back and forth between the eruption site and its antipodes for 5 days after the explosion.

7. Tide gauges also recorded the sea wave’s passage far from Krakatau. The wave „reached Aden in 12 hours, a distance of 3800 nautical miles, usually traversed by a good steamer in 12 days“.

8. Blue and green suns were observed as fine ash and aerosol, erupted perhaps 50 km into the stratosphere, circled the equator in 13 days.

9. Three months after the eruption these products had spread to higher latitudes causing such vivid red sunset afterglow that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration. Unusual sunsets continued for 3 years.

10. Rafts of floating pumice-locally thick enough to support men, trees, and no doubt other bbiological passengers-crossed the Indian Ocean in 10 months. Others reached Melanesia, and were still afloat two years after the eruption.

11. The volcanic dust veil that created such spectacular atmospheric effects also acted as a solar radiation filter, lowering global temperatures as much as 1.2 degree C in the year after the eruption. Temperatures did not return to normal until 1888. The book is full of many more amazing bits of information. Hopefully these small excerpts will be useful to you.

Recent activity

Krakatau is still active. The presently-active vent has formed a small island in the middle of the ocean-filled caldera that developed during the famous big eruption of 1883. The island is called Anak Krakatau, which means child-of-Krakatau. It is pretty much erupting all the time at a low level, but once or twice a year it has slightly larger eruptions that people notice and sometimes report in the news. Of course none of these are anywhere near the size of the famous 1883 eruption.

Krakatau is following a pattern that is pretty common for volcanoes. This pattern involves hundreds to thousands of years of small eruptions to build up the volcano followed by 1 or more huge eruptions that

causes the volcano to collapse into a caldera, and then the cycle starts over again.

I think the chances of a huge 1883-style eruption are very small for the time being. However, it is certainly dangerous to go onto Anak Krakatau, especially if it is one of its more agitated moods. It is probably not even very smart to spend too much time on the small islands that form the remnants of what was once the main Krakatau island. This iis because even a small collapse of Anak Krakatau could generate a small tsunami that could sweep towards these islands. Since they are so close to Anak Krakatau there wouldn’t be very much time for a warning.

Pinatubo.

The 1991 eruption of Mt. Pinatubo was the third largest volcanic eruption this century (after Santa Maria, Guatemala in 1902 and Novarupta, Alaska in 1912). Because it was so large – and it strongly effected both greenhouse warming and the ozone hole &– there have been lots of scientific publications about it.

When is the last time Mt. Fuji erupted?

Mt. Fuji is a beautiful example of a stratovolcano, and is almost a perfect symmetric cone (at least when viewed from far away). IIt is mostly basalt, which is a little bit unusual for stratovolcanoes. Most stratovolcanoes are constructed of andesite or dacite compositions. The fact that it is a stratovolcano means that it is composed of layers of both lava and ash. The fact that it is such a beautiful cone probably indicates that it hasn’t recently suffered a big eruption.

„Volcanoes of the World“ by Tom Simkin and Lee Seibert lists 63 eruptions of Mt. Fuji since about 9000 years ago. Obviously most of these have been determined by using carbon-14 dating rather than accounts by witnesses. However, the most recent 22 eruptions are listed as having been recorded by people. The most recent eruption was in 1709 but probably the mmost famous was in 1707.

How were the Deccan Flood basalts formed?

The currently popular theory is that once in a while great teardrop-shaped blobs of magma work their way to the surface from near the core-mantle boundary. These huge blobs result in extensive volcanism (that we call flood basalts). Following behind the teardrop shaped blob is a long skinny tail that persists for millions of years, and results in what we call hotspot volcanism. Some of the evidence for tthis is that most flood basalt provinces can be tied to a hotspot trace – in the Deccan Traps example the hotspot is now under Reunion Island. On the Indian Ocean floor you can trace a line of now-extinct volcanoes that follow back to NW India.

Are there any volcanoes erupting in Costa Rica?

November 1996 activity at Rincon de la Vieja Rincon de la Vieja began erupting volcanic ash and water vapor on November 6, 1995. Rincon de la Vieja is an active composite volcano located 30 miles (50 km) south of Lake Nicaragua. Local authorities have evacuated 300 families. Scientists are on their way to the remote volcano. Great amounts of ash have been reported in local rivers. Heavy cloud formations have prevented satellite observations.

In 1982, J. Bruce Gemmel climbed the volcano and provided a description to McClelland and others (1989). Collapse craters trend east-northeast to west-southwest across the summit of the volcano. The main cone is heavily vegetated with the exception of three craters to the west. At that time, the most recently active crater (diameter 800 feet, 250 m) was 0.6 mile (1 km) northwest of the main cone. A lake covered the crater floor.

The mmost recent confirmed eruption of Rincon de la Vieja was in early 1992. Rincon de la Vieja has erupted at least 16 times since 1851. Most eruptions are phreatic and include the emission of gas and ash. Lahars are often generated by displacement of the crater lake.

What type of volcanic damage prevention is there in Central America?

Unfortunately most of the Central American countries are too poor to do much with damage prevention. I remember being told by a Guatemalan geologist that they can’t keep their seismometers going because the people come along and steal the batteries for their cars. A dedicated engineering geologist would go a long way towards lowering the problems, by determining the places where the danger from lahars is the greatest, by determining the most unstable slopes, by determining which roads are the best for evacuation routes

What’s the most recent eruption of Vesuvius and will it erupt again?

Vesuvius has erupted about three dozen times since 79 A.D., most recently from 1913-1944. The 1913-1944 eruption is thought to be the end of an eruptive cycle that began in 1631. It has not erupted since then. Vesuvius is an active volcano, it will erupt again.

Background

The ooldest dated rock at Mt Vesuvius is about 300,000 years old. It was collected from a well drilled near the volcano.

Vesuvius erupted catastrophically in 79 A.D., burying the towns of Herculaneum and Pompeii. The Somma Rim, a caldera-like structure formed by the collapse of a stratovolcano about 17,000 year ago, flanks Vesuvius to the east.

The 79 A.D. eruption of Vesuvius was the first volcanic eruption ever to be described in detail. From 18 miles (30 km) west of the volcano, Pliny the Younger, witnessed the eruption and later recorded his observations in two letters.

Volcanologists now refer to sustained explosive eruptions which generate high-altitude eruption columns and blanket large areas with ash as plinian eruptions. It is estimated that at times during the eruption the column of ash was 20 miles (32 km) tall. About 1 cubic mi (4 cubic km) of ash was erupted in about 19 hours. Vesuvius has erupted about three dozen times since 79 A.D., including a large, explosive eruption in 1631 that killed 4,000 people. The most recent eruption was from 1913-1944.

An excellent source of information on Vesuvius is an article titled „The Eruption of Vesuvius in A.D. 79″ by Sigurdsson and

others (Sigurdsson, H., Carey, S., Cornell, W., and Pescatore, T., 1985, The eruption of Vesuvius in A.D. 79:

What happened at Pompeii?

Probably one of the most detailed studies of a large explosive eruption anywhere was that done on the AD 79 Vesuvius eruption by Sigurdsson et al (1985) in the National Geographic Research vol. 1, no. 3, pp. 332-387. It should be required reading for any students of Vesuvius and Pompeii. It is a long article so I won’t re-type mmore than just a short part that deals with the people being killed.

The citizens of Pompeii became directly aware of Vesuvius’ eruption sometime in the early afternoon of 24 August as coarse pumice-fall began to plunge the city into darkness. No trace remains of the initial fine ashfall that affected areas to the north and east farther up the slopes of the volcano. The initial explosive phase of the eruption (A-1) may have been witnessed from Pompeii — an eexcellent view of the summit — but without direct consequences; it probably only generated curiosity.

Pumice and lithics rained continuously from early afternoon on 24 August to early the next morning, according to accounts of Pliny the Younger. During this ttime 130-140 cm of white pumice (A-2) accumulated, on top of which another 110 to 130 cm of gray pumice was laid down (A-3 to A-5). Although the pumice layer appears relatively homogeneous, the diameter of lithics and pumice varies throughout (Figure 23), reflecting variations in height of the eruption column during the Plinian phase. For example, relatively large, dense pieces of pumice are concentrated about 10 cm above the base of the gray pumice layer.

Pompeii happened to be located on the secondary thickness maximum of the fallout deposit, and thus received the thickest accumulation of pumice- and lithic-fall. Roofs probably collapsed about halfway through the deposition of white pumice (some 40 cm). With most structures unsafe for habitation, aan exodus from the city is likely to have begun. Escape of most of Pompeii’s residents can thus be attributed to the extended yet comparatively innocuous Plinian fallout phase.

The first surge to reach Pompeii swept against the north wall of the city in the early morning of 24 August, depositing dark gray ash near the Herculaneum Gate (S-3). Neither Vesuvius Gate, 200 m to the east, nor other sites in the Pompeii area evidence this surge. Therefore it is ppossible that the surge cloud did not extend inside the city walls but flowed just west of Pompeii over the villa dei Misteri and Villa Diomede. The surge must have caused alarm and almost unbearable conditions in the city. Evidence from Mount St. Helens in 1980 and El Chichon in 1982 indicates a peripheral zone of high heat associated with the distal ends of surge clouds (Moore & Sisson 1981; Sigurdsson, Carey et al. 1984). The first surges of the eruption (S-1 and S-2), which form a characteristic doublet in the middle of the pumice-fall at Boscoreale and Oplontis, did not reach as far southeast as Pompeii.

Within the city, 3 cm of pumice- and lithic-fall (A-6) accumulated before the next surge (S-4) overwhelmed the city. This surge also extended to Bottaro, 1 km south of Pompeii (Figure 8), and to Tricino, 3 km east of Pompeii (Figure 10).

The majority of human remains discovered in the excavations have been found on top of the pumice-fall layer, lying within surge deposits S-4 and S-5, but principally buried by the thick S-6 surge (Figure 24). Because of their fine-grained, silty nature, the surges have preserved accurate molds of the victims including ddetails of facial expressions and sometimes clothing. With time, the soft tissues have decayed, leaving only bones in the hollow cavities. In 1860, Giuseppe Fiorelli developed the ingenious technique of making plaster casts of these impressions before the surrounding surge is disturbed (figure 25). Many hundreds of casts of the dead in Pompeii have since been made in this manner. In 1966, for example, casts of 13 victims were made in the Garden of the Fugitives, where they fell in various groups of adults and children on top of the pumice-fall deposit. Thus evidence is compelling that the S-4 surge was the lethal event in Pompeii. Since detailed stratigraphic studies of the deposits have not yet been feasible inside Pompeii, the extent of building damage resulting from the S-4 surge cannot be judged. By this time the ground-floor levels of buildings had already been buried and only the upper stories protruded from the pumice blanket.

As I said, this is a very comprehensive study, and you should get a copy or two to read. It seems as if the people survived the fall of over a meter of pumice but that this pumice did start to collapse walls. When pyroclastic ssurges (fast-moving, horizontally-directed, ash and rock-laden blasts) occurred, the remaining people were killed, either by the heat, by lack of oxygen in the thick dusty cloud, or by being struck by larger blocks. They were buried by a later surge. Most of what you see as the „bodies“ are apparently plaster casts rather than preserved bodies.

Why are there volcanoes in Iceland?

Iceland is an island made of numerous overlapping volcanoes. The volcanoes are associated with a divergent plate boundary, the Mid-Atlantic Ridge, where new oceanic plate is added to the North American and Eurasian plates and with a hot-spot that underly. No wonder there is such prolific volcanic activity in Iceland!

Surtsey

Surtsey erupted from November of 1963 to June of 1967. At the end of the eruption the new island was about 2,500 feet (800 m) in diameter. The island has not grown since the last eruption. The volcano is made of lava flows and tephra on a submarine (under water) base of pillow lava. The subaerial (under air) lava flows serve as a protective cap on part of the island. The part of the island that is made of only tephra is prone to wave erosion. As tephra

is removed by waves the island will shrink in size.

Surtsey is featured in a number of volcano books because it popped to the surface of the Atlantic Ocean and was able to be studied pretty closely almost from the first moments.

Prior to the eruption that location was known to be shallow so it wasn’t a huge surprise; the shallow water was actually the summit of an undersea volcano that wasn’t quite at the surface yet. The first eruptions wwere very explosive because of the mixture of hot lava and ocean water, and even after the island built above sea level, water was still able to seep through the ash to make more explosions. Eventually, however, the vent got sealed off from the ocean and the eruptions became non-explosive Hawaiian-style lava fountains instead of steam explosions. In fact, explosive water-lava eruptions are now called „Surtseyan“ after Surtsey. A number of times water would suddenly gain access to the vent aand instantly the activity would go from quiet Hawaiian-style fountaining to explosive Surtseyan explosions. This created quite a hazard to those studying the volcano.

Surtsey is still around. Volcanoes of the World (by Tom Simkin and Lee Seibert) lists its llast activity as 1967. It is now being attacked by waves, and if it does not erupt again it may eventually disappear. Many times during the early days of its eruption it was nearly washed away by the waves. That is because it was originally mostly ash (formed by the explosive interaction of erupting lava and ocean water). As soon as the cone had built to the point that water could no longer gain access to the vent, the eruption style changed to more Hawaiian style, with lava fountains producing lava flows. These hardened flows are much more resistant than the earlier-produced ash, and they guarantee that the island won’t be washed away very quickly.

What is Etna doing?

Background

Etna (location: 337.7N, 15.0E) is a 10,791 foot (3,290 m) tall stratovolcano on the northeastern edge of Sicily. It certainly deserves to be on a list of famous volcanoes because it has the longest documented record of volcanism in the world, erupts frequently, and is the largest volcano in Europe. Etna probably does not belong on a list of great (individual) volcanic eruptions. In mythology, Etna was identified as the location of the forge of Volcan, home of the Cyclopses, and where tthe giant Enceladus laid (eruptions being his breath and earthquakes his motion). Etna has erupted nearly 160 times since the first recorded eruption in 1500 B.C. Most eruptions consist of lava flows. Small to moderate explosive eruptions occur near the summit less often. Etna is not known for any eruptions that caused great loss of life. Known violent eruptions occurred in 1169, 1669, 1752-1754, 1893-1899, 1917, 1940, and 1945. Known fatal eruptions occurred in 141 B.C., 1329 A.D., 1536 A.D., 1832 A.D., 1843 A.D., 1928 A.D., 1979 A.D., and 1987 A.D. I could not find information on the number and cause of fatalities during most of these eruptions. Nine people were killed and 23 were injured (150 tourist were in the area) on September 12, 1979, by a 30-second explosion that threw large blocks near the crater rim. Blocks 10 inches (25 cm) in diameter fell 1,300 feet (400 m) away. Two people were killed and 7 others injured by falling volcanic material 1,600 feet (500 m) from the crater in the 1987 phreatic eruption.

Eruption mechanism

The general accept explanation for volcanism at Mt. Etna, Vulcano, and Stromboli is the subduction of part of the northward-moving African Plate beneath the EEurasian Plate. Subduction may be in a late stage of evolution or may have ceased. Mt. Etna is unusual because it is adjacent to but just outside of the subduction zone. It has been suggested that material associated with the subduction zone migrates into the lithosphere adjacent to the Aeolian arc. Lava from Etna shares a few chemical similarities to hot spot volcanoes. However, these may result from modification at the source or during ascent of the magma.

What’s the cause of volcanism in Yellowstone and is it likely to erupt again?

The U.S. Geological Survey experts on Yellowstone have an excellent homepage that describes the volcanic system. They state that Yellowstone will erupt again but at the present level of activity there is no need for immediate concern about eruptions.

Yellowstone is the result of three very large eruptions. The first was about 2.1 million years ago and erupted 600 cubic miles (2,500 cubic km) of ash. It created the Island Park caldera about 45 miles (75 km) long. The second eruption was about 1.3 million years ago. It produced the Henrys Fork caldera in the west end of the older (2.1 million year) caldera. About 600,000 years ago aan eruption produced about 250 cubic miles (1,000 cubic km) of ash and the present-day Yellowstone caldera. Since then there have been large volume (250 cubic miles; 1,000 cubic km) rhyolitic lava flows between 150,000 and 70,000 years ago. Yellowstone is still an active volcano. It sits above a hot spot and will erupt again. To learn more about hot spots.

North Dakota’s volcanic future

Yellowstone is not coming to us. We are going towards Yellowstone (no offense to North Dakotans but this sounds good to me). The location of the Yellowstone hot spot is fixed within the Earth. The North American plate is moving about 2.3 cm to the southwest each year. Assuming plate motions remain constant, the North Dakota-Montana state line will be over the hotspot in 22 million years. The town of Bismarck will be over the hotspot in 33 million years. Fargo will be over the hotspot in 44 million years. Duluth will have to wait 59 million years before it basks in the warmth of geothermal splendor.

What’s going on at Mammoth Lakes?

The surface of part of the Long Valley caldera has been doming upward gradually for some time. Volcanologists do not believe any material is

working its way to the surface. Have a look at Current Activity and Eruptions. Past eruptions at Long Valley caldera were very large. Future eruptions might also be large but the recent flurry of earthquakes is nothing to be concerned about, so far.

Activity at Long Valley caldera has returned to background levels after an increase in the number and size of earthquakes prompted a change in alert status over the last few days.

A swarm of earthquakes occurred beneath the ssouth moat of Long Valley caldera on Thursday and Friday. A series of earthquakes with magnitude greater than 3 occurred beneath the south moat on Friday. Earthquake activity beneath the south moat declined after a magnitude 3.6 earthquake on the late afternoon on Friday, February 16.

Two other earthquakes (magnitudes 3.7 and 2.3) occurred near Red Slate Mountain 6 miles (4 km) south of the caldera and 1 mile (0.6 km) south-southwest of Mammoth Lakes on Sunday evening and Monday mmorning, respectively.

This level of unrest occurs at Long Valley caldera every few weeks or months.

Current activity is consistent with a NORMAL STATUS.

Normal activity in Long Valley caldera includes as many as 10 to 20 earthquakes of magnitude 22 of less per day with occasional swarms of small earthquakes. The earthquakes are accompanied by steady uplift of the resurgent dome at a rate of about 1 inch (2 to 3 cm) per year.

Why is there volcanic activity there?

The Long Valley Caldera is situated along the east edge of the Sierra Nevada Mountains and the western edge of the Basin and Range Province. Earthquakes, extension, and faulting indicate the area is tectonically active. You are correct that the area is not associated with subduction. The volcanism is probably related to extension. As the crust thins, the asthenosphere rises closer to the surface and it may melt, producing magma. The magma probably rises into the crust where it heats aand melts rocks. The melted crustal rocks are the rhyolite magmas that are erupted during caldera collapse.

Is Mt. Rainier a dangerous volcano?

Mount Rainier is potentially the most dangerous volcano in the Cascades because it is very steep, covered in large amounts of ice and snow, and near a large population that lives in lowland drainages. Numerous debris avalanches start on the volcano. The largest debris avalanche traveled more than 60 miles (100 km) to Puget Sound. The most recent eeruption was about 2,200 years ago and covered the eastern half of the park with up to one foot (30 cm) of lapilli, blocks, and bombs.

Are there volcanoes in Canada?

There are more than 100 volcanic centers in British Columbia and the Yukon. Many are monogenetic cones but there are also large shield and dome complexes. Eruptions began a few million years ago. The most recent eruption was about 200-250 years ago Stikine Volcanic Belt.

The volcanic rocks are divided into five groups with diverse types of volcanoes and tectonic settings. In southern British Columbia, the Pemberton and Garibaldi volcanic belts and the Chilcotin Group plateau are related to the subduction of the Juan de Fuca and Explorer plates beneath the North American continent. The Anahim Volcanic Belt trends easterly across central British Columbia and is probably related to a mantle hot spot. The Stikine Volcanic Belt forms a broad zone of volcanoes in northwestern British Columbia and the southern Yukon. These volcanoes are probably related to shear along the Queen Charolette transform fault to the west. The Wrangell Volcanic Belt is an arc of continental volcanoes associated with the subduction of the Pacific plate beneath the North American plate. VVolcanism has also occurred at the Clearwater-Quesnel and McConnell Creek area and at Alert Bay.

Why are there no active volcanoes in the Great Plains and Eastern U.S.?

Most subaerial eruptions are associated with subduction zones, places where oceanic plates are forced under overriding, less dense plates. In this setting, heat and water in the mantle cause rocks to melt. If the melt (magma) makes it to the surface there is an eruption. There are no active subduction zones on the east coast (or the Great Plains).

Why are there so many volcanoes in the Pacific Northwest and Alaska?

The distribution of volcanoes in the northwest and Alaska is the result of plate tectonics. In the northwest, the oceanic Farallon Plate is being pushed beneath (subducted) the continental margin of the North American Plate. When the subducted plate comes in contact with the hot asthenosphere beneath the continental plate conditions are right for the rocks in the asthenosphere to melt. The melt, called magma, rises to the surface to build volcanoes. Since the subduction zone is a long curvi-linear feature it produces a similar line of volcanoes, called an arc, on the continent. Alaskan volcanoes are the result of the subduction oof the Pacific plate under the North American plate.

Subduction

If new oceanic lithosphere is created at mid-ocean ridges, where does it go? Geologists had the answer to this question before Vine and Matthews presented their hypothesis. In 1935, K. Wadati, a Japanese seismologist, showed that earthquakes occurred at greater depths towards the interior of the Asian continent. Earthquakes beneath the Pacific Ocean occurred at shallow depths. Earthquakes beneath Siberia and China occurred at greater depths. After World War II, H. Benioff observed the same distribution of earthquakes but could not offer a plausible explanation.

The movement of oceanic lithosphere away from mid-ocean ridges provides an explanation. Convection cells in the mantle help carry the lithosphere away from the ridge. The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere. Thus, oceanic lithosphere is created at mid-ocean ridges and consumed at subduction zones, areas where the lithosphere sinks into the asthenosphere. Earthquakes are generated in the rigid plate as it is subducted into the mantle. The dip of the plate under the continent accounts for the distribution of the earthquakes.

How many volcanoes are there in North America?

We get so many questions about

how many volcanoes are in Washington or Oregon or Canada, we thought we’d break it down and give you a listing of volcanoes by state (and country). If your state is not listed it means there are no volcanoes there (as described below). This list is compiled from Volcanoes of North America: United States and Canada, by Wood and Kienle. They list all volcanoes that are younger that 5 million years old and are morphologically distinct. There are about 262 vvolcanoes and volcanic fields in North America.

Location

Number of Volcanoes

and Volcanic Fields

Canada* 21

USA*

241

Alaska*

108

Arizona*

9

California*

24

Colorado*

1

Idaho*

6

Hawai’i*

19

Nevada*

7

New Mexico*

16

Oregon*

34

Utah*

6

Washington*

9

Wyoming*

2

Why were there many active volcanoes in southern California about 10,000 years ago, but not now?

I will give you two hypothesis.

The first one is that perhaps the volcanoes in that region (which part of California you’re talking about, I don’t know) aren’t really extinct. There are a number of volcanoes that have average repose periods between eruptions that are measured in thousands of years. To a human iit might seem as if the volcano is dead, but in fact it is just between eruptions.

The second idea is that magma is required to produce volcanoes and the tectonic setting of California has changed from one that produces mmagma to one that doesn’t. Tectonic changes such as these take millions of years to occur though, so I don’t think this second hypothesis is very likely.

Which volcano killed the most people?

The eruption of Tambora in Indonesia in 1815 killed the most people. It was a huge eruption that sent ash into the stratosphere that then spread around the world. World climate was noticeably cooler the following year, and in places it was called „the year without a summer“. Closer to the eruption itself thousands of people were killed, and due to the destruction of crops, disease, contamination of water, etc., tens of thousands more died in the years that followed. Overall about 92,000 people died as a result oof the eruption. Back in 1815 there wasn’t much news coming out of Indonesia to the western world but if it were to happen today it would definitely have been a big deal.

How many act ive volcanoes are there?

The absolute number of volcanoes that exists depends on your definition: active only, active, dormant plus extinct volcanoes? And even if we decide on a definition, nobody has really counted all of the volcanoes, especially the tens on thousands on the ssea floor. The best guess is 1511 volcanoes have erupted in the last 10,000 years and should be considered active. This number is from the new Smithsonian Institution book, „Volcanoes of the World: Second Edition“ compiled by Tom Simkin and Lee Siebert.

What’s the biggest volcano in the Solar System?

So far, the largest volcano in our Solar System is Olympus Mons on Mars. It is about 17 miles (27 km) tall. That’s a long hike for some future explorer. Mount Everest is about 6 miles (10 km) tall

What’s the tallest volcano in the world?

The highest volcano is Ojos del Salado in Chile. It is 22,589 feet (6,887 m) tall.

From base to summit, the tallest would be Mauna Kea, which, when measured from its base on the ocean floor, is more than 30,000 feet high.

What is the largest eruption ever?

The biggest eruption I know of was at Yellowstone about 2.2 million years ago. An explosive eruption produced 2,500 cubic kilometers of ash! That’s about 2,500 times more ash than Mount St. Helens erupted!

Why do volcanoes erupt?

Volcanoes erupt because of density and pressure. The lower density of the magma relative to the surrounding rocks causes it to rise (like aair bubbles in syrup). It will rise to the surface or to a depth that is determined by the density of the magma and the weight of the rocks above it. As the magma rises, bubbles start to form from the gas dissolved in the magma. The gas bubbles exert tremendous pressure. This pressure helps to bring the magma to the surface and forces it in the air, sometimes to great heights.

It’s sort of like the bubbles of gas in a bottle of soda. Before you open the soda you don’t see many bubbles because the pressure in the bottle keeps the gas dissolved in the soda. When you open the bottle the pressure is released and the gas bubbles leave the soda. If you shake up the bottle first, the soda gets pushed out by the bubbles of gas as they rush out. You might try this, but do it outside.

What are the s igns that a volcano is about to erupt?

Short answer

Several things happen when a volcano is about to erupt, some of the most obvious are listed here:

1. The number and size of earthquakes increase in and around the volcano.

2. The ground deforms or „bulges“ aat the eruption site.

3. A lot more gas comes out of the volcano.

Longer answer

There are lots of signs that are examined, depending on how closely monitored the particular volcano is. Probably the most common type of monitoring is by seismicity. Even one seismometer can tell if there is an increase of seismic activity on a usually seismically-quiet volcano. If you have at least 3 seismometers, and they are strategically placed, you can triangulate on earthquakes to see if they are occurring in a place that indicates perhaps magma movement. By examining the seismic data over a period of time you may be able to determine if the earthquakes are migrating towards the surface (suggesting that magma is also migrating towards the surface since the earthquakes are probably being generated as magma breaks rocks that are in its way).

Another type of data that is used is the study of ground deformation. When magma moves up into the shallow plumbing of a volcano, it takes up space and pushes the surrounding rock outward. This also causes the surface of the volcano to deform. Some points move upward and any two points will move farther apart. By using very accurate leveling

and distance-measuring techniques, these surface changes can be measured. Usually the changes are a few mm over a distance of a few hundred meters, but sometimes they are dramatic. For example prior to many eruptions at Kilauea, the summit bulges 1-2 meters upward. In the last few days prior to the big Mt. St. Helens eruption the northern flank was bulging outward at a few meters per day!

Some people like to monitor volcanoes by constantly monitoring gases that come oout of fumaroles. Most active volcanoes have fumaroles where volcanic gases escape to the surface. It is relatively easy to monitor the temperatures of these gases, and an anomalous increase in temperature might be a sign that magma has moved closer to the surface. Monitoring the composition of the gases is more difficult to do, and changes in the composition are way more difficult to interpret. Many times just visual changes to fumarole areas are indications of impending activity. If tthe area of active degassing gets larger, if the plants nearby die suddenly, if the color of any lakes or ponds nearby changes.Many volcanoes have summit lakes through which heat and gases rise to the surface and escape. Many of tthese lakes have strange colors due to all the dissolved minerals in them, and many of the colored ones change color, pH, temperature, etc. These too, are signs of change below but are often difficult to interpret.

A number of people are studying ways in which to use satellite data to monitor volcanoes. It is possible to obtain thermal images of volcanic areas, and by comparing images on a monthly or bi-weekly basis, increases or decreases in temperatures can be detected. Additionally, some new technologies have allowed for the determination of very accurate topography from satellite data. This technology may someday allow for the remote monitoring of surface deformation associated with sub-surface magma movement. This process is still being developed. IIt usually takes too long to get satellite data processed for this technique to be useful in a rapidly-escalating crisis so it would be used over the long term, in the years to months prior to an eruption rather than the hours prior.

Why are some eruptions gentle and others violent?

Volcanic eruptions could be thought as a continuum between two end members. At one extreme is the gentle effusion of lava. Most Hawaiian eruptions would be a examples of this ttype of eruption. At the other extreme is the explosive ejection of ash from a vent. The May 18, 1980 Mount St. Helens eruption would be an example of this type of eruption.

The two main factors that influence how a volcano will erupt are viscosity and gas content. Both are related to the composition of the magma. Hawaiian volcanoes tend to erupt basalt, which is low in viscosity and low in gas content (about 0.5 weight percent). The gas that is present can readily escape and little pressure builds up in the magma. At the other extreme, rhyolite magmas are very viscous and can contain a lot of gas (up to 7-8 weight present). As the magma moves into the vent and the pressure drops, the gas wants to escape. The magma is very sticky and resists the expansion of the gas bubbles. Ultimately, enough bubbles grow and expand to blow the magma into ash size fragments and eject them violently into the atmosphere.

How high can explosive eruptions go and how far can the ash be spread?

Well, that depends on how big the eruption is and how big the debris is that you are concerned about. As yyou might imagine a big eruption will send material farther. Additionally, the big material from any eruption doesn’t get thrown as far as the finer stuff.

Volcanologists go out into the field to figure out the distribution of erupted pyroclastic material. They will go to numerous sites around the volcano and measure (in general) 3 things: 1) the total thickness of the pyroclastic deposit at each location; 2) the average size of the 10 largest pumice at each location; and 3) the average of the 10 largest lithic clasts at each location (a lithic is a pre-existing rock that is blown apart in the explosive eruption). They then draw contours around the data that they have collected. In some cases, if the geologists are studying a very old eruption, they may not even know where the vent was. The contours of the thickness and size measurements should close around the vent so that its location can be determined.

Data from a very extensive study of the AD 79 Vesuvius eruption by Haroldur Sigurdsson shows that for each of two particularly strong blasts during the eruption, the pumice layer was 100 cm thick up to ~20 km downwind, 50 cm thick oout to about 50 km, 25 cm thick out at ~60 km, and so on. Pumice 15 cm in diameter made it out ~6 km downwind, 10-cm pumice made it ~7 km, and 5-cm pumice even further.

The finest dust often gets carried hundreds or even thousands of km downwind.

Explosive eruption plumes such as those generated at volcanoes like Mt. St. Helens or Pinatubo can reach high into the atmosphere. The highest Mt. St. Helens plume on May 18, 1980 reached about 31 km (101,700 feet), and the highest Pinatubo plume got as far as 45 km (147,600 feet).

Here are some highlights very fine book about the 1883 Krakatau eruption by Tom Simkin and Richard Fiske (Simkin, T., and Fiske, R.S., Krakatau 1883: The volcanic eruption and its effects: Smithsonian Institution Press: Washington, D.C., 464 p.). They should give you and idea about how far ash can travel during a large eruption.

-Ash fell on Singapore 840 km to the north, Cocos (Keeling) Island 1155 km to the SW, and ships as far as 6076 km west-northwest. Darkness covered the Sunda Straits from 11 a.m. on the 27th until dawn the next day.

-Blue and green suns

were observed as fine ash and aerosol, erupted perhaps 50 km into the stratosphere, circled the equator in 13 days.

-Three months after the eruption these products had spread to higher latitudes causing such vivid red sunset afterglow that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration. Unusual sunsets continued for 3 years.

-The volcanic dust veil that created such spectacular atmospheric effects also acted as a solar radiation filter, lowering gglobal temperatures as much as 1.2 degree C in the year after the eruption. Temperatures did not return to normal until 1888.

How long do eruptions last?

Historic eruptions have lasted less than a day to thousands of years. In 1977, the lava lake at Nyiragongo drained in less than one hour. In contrast, Stromboli has had a low-level of activity since 450 BC (about 2,400 years).

Percent of Volcanoes Duration

9% < 1 day

16% < 2 days <

24% < 1 week

30% < 2 weeks

43% < 1 month

53% < 2 months

17% > 1 year

7% > 3 years

0.5% > 30 years

Durations of eruption based oon 3,211 historic eruptions. Data from Simkin and Siebert (1994).

The median duration of historic eruptions is 7 weeks.

Simkin and Siebert (1994) make several important observations:

1. the paroxysmal phase of an eruption can occur during any interval of a volcanoes eruption, for example the 1980 eruption of Mount St. Helens and the 1883 eruption of Krakatau were preceded by months of low-level activity;

2. some eruptions can reach their paroxysmal phase within an hour after the eruption starts, for example the 1886 eruption at Tarawera and the 1977 eruption of Usu;

3. to uninstrumented observers, unrest prior to some eruptions can be too short to provide a warning of an impeding eruption – highlighting the need for careful instrumental monitoring of aactive volcanoes.

What is the approxiamate temperature of an eruption cloud?

The cloud that rises above a volcano is surprisingly cold, probably close to freezing! This is because the gasses are expanding so rapidly.

The temperature of the ash flows that can be erupted, on the other hand, are on the order of hundreds of degrees Celsius.

Could a series of eruptions possibly have caused the extinction of the dinosaurs due to the ash depoited in the atmosphere?

Lots of people have definitely pproposed that idea, but it is still not decided. For one thing there would have to be eruptions such as we’ve never seen before. About the only possibility would be a flood basalt eruption (and one did indeed occur around the 65 million years-ago time). The only problem is that flood basalt eruptions aren’t explosive. They are persistent though so they might have filled the atmosphere with enough sulfuric acid aerosols (vog) to make life difficult. There’s no doubt that a big meteorite hit around 65 million years also. One idea I’ve heard lately is that the flood basalt eruption (the Deccan Traps in NW India) had stressed life on the planet, and the big meteorite impact was the last straw.

There have been other global extinctions, and if you go by the percentage of Earth’s life that has disappeared in them, the one 65 million years ago is not the most drastic. What I wrote above would require that a meteorite hits during a time of flood basalt volcanism more than once. Is that too much of a coincidence to occur multiple times? I don’t know.

Why are there volcanoes?

Volcanoes are a natural way that the Earth and other planets hhave of cooling off. Planets are warm in their mantles. Heat inside planets escapes towards their surfaces. Heat sometimes melts rocks, which then rise buoyantly toward the planet’s surface. When the hot rocks – called magma – and included gases break through the crust, an eruption occurs. The buildup of ash and lava flows around the eruption hole (or vent) makes a volcano. Some volcanoes erupt for only a short time – a few days to weeks and never erupt again. Large volcanoes such as stratovolcanoes and shields erupt many thousands of times throughout their lifetimes of hundreds of thousands to a few million years.

The Earth has volcanoes because it is hot inside. In some places it is hot enough to turn solid rock into liquid rock. Geologists call the liquid rock magma. The magma rises towards the surface because it is less dense than the surrounding rock (like a hot air balloon rising through the cooler air). If the magma reaches the surface it is called lava and lava accumulates to make a volcano.

The Earth’s interior is hot, and, as all hot things will do, the interior is trying to lose its heat and cool. It cools mmostly by releasing its heat through volcanic eruptions. That is why there are volcanoes.

What are the different types of volcanoes?

There are pretty much 3 types of volcanoes, but as with all kinds of classifications you can find exceptions. The three types that I am thinking of are: 1) shield volcanoes (such as we have here in Hawai’i); 2) stratovolcanoes (such as Mt. St. Helens and Pinatubo); and 3) large rhyolite complexes (such as Yellowstone and Taupo). You might also want to add the mid-ocean ridges, flood basalts, and monogenetic fields to make that 6 types in all. „Volcanoes of the World“ by Tom Simkin and Lee Seibert, lists 26 different types, but that’s probably kind of extreme.

Here are some brief descriptions:

Shield volcanoes–the largest of all volcanoes on Earth (not counting flood basalt flows). The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted. For this reason these volcanoes are not steep (you can’t pile up a fluid that easily runs downhill). These volcanoes are only explosive if water somehow gets into the vent, otherwise they are characterized by low-explosivity fountaining

that forms cinder cones and spatter cones at the vent, however, 95% of the volcano is lava rather than pyroclastic material. Shield volcanoes are the common product of hotspot volcanism but they can also be found along subduction-related volcanic arcs and out by themselves as well.

Stratovolcanoes–making up the largest percentage (~60%) of the Earth’s volcanoes, these are characterized by eruptions of cooler and more viscous lavas than basalt. The usual lavas that erupt from stratovolcanoes are andesite, dacite, and ooccasionally rhyolite. These more viscous lavas allow gas pressures to build up to high levels (they are effective „plugs“ in the plumbing), therefore these volcanoes often suffer explosive eruptions. They are usually about 50/50 lava and pyroclastic material, and the layering of these products gives them their other common name of composite volcanoes. Stratovolcanoes are commonly found along subduction-related volcanic arcs.

Large rhyolite caldera complexes–the most explosive of Earth’s volcanoes. These are volcanoes that often don’t even look like volcanoes. TThey are usually so explosive when they erupt that they end up collapsing in on themselves rather than building any tall structure. The collapsed depressions are called calderas, and they indicate that the magma chambers associated with the eruptions are hhuge. Fortunately we haven’t had to live through one of these since 83 AD when Taupo erupted. Yellowstone is the most famous U.S. example of one of these. Their origin is still not well-understood. Many folks think that Yellowstone is associated with a hotspot, however, a hotspot association with most other rhyolite calderas doesn’t work.

Monogenetic fields. These also don’t look like a „volcano“, rather they are a collection of sometimes hundreds to thousands of separate vents and flows. These are the product of very low supply rates of magma. The supply rate is so slow and spread out that between the times of eruptions the plumbing doesn’t stay hot so the next batch of magma doesn’t have any preferred ppathway to the surface and it makes its own path. A monogenetic field is kind of like taking a single volcano and spreading all its separate eruptions over a large area. There are a number of monogenetic fields in the American southwest, and there is a famous one in Mexico called the Michoacan-Guanajuato field.

Flood basalt provinces–another strange type of „volcano“. Some parts of the world are covered by thousands of square kilometers of thick basalt lava flows–some flows are mmore than 50 meters thick, and individual flows extend for hundreds of kilometers. The old idea was that these flows went whooshing over the countryside at incredible velocities. The new idea is that these flows are emplaced more like pahoehoe flows–slow moving, with most of the great thickness being accomplished by injecting lava into the interior of an initially thin flow. The most famous U.S. example of a flood basalt province is the Columbia River Basalts, covering most of SE Washington State, and extending all the way to the Pacific and into Oregon. The Deccan Traps of northwest India are a much larger flood basalt province.

Mid-ocean ridge volcanism occurs at plate margins where oceanic plates are created. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins–some folks consider this the largest volcano on Earth. Here, the plates are pulled apart by convection in the upper mantle, and basalt lava intrudes to the surface to fill in the space. Or, the basalt intrudes to the surface and pushes the plates apart. Or, better yet, it is a combination of these two processes. Either way, this is how the oceanic plates aare created. A recent mid-ocean ridge eruption took place along the Gorda Rise–the mid-ocean ridge that separates the Juan de Fuca plate from the northern part of the Pacific plate.

What comes out of volcanoes?

Lava, pyroclasts (broken pieces of lava that fly through the air), and gas come out of volcanoes. We have described lava before. Pyroclasts are classified by size. Ash is the smallest pyroclast. Blocks and bombs are the largest. Cinder and pumice are also types of pyroclasts. Water vapor, carbon dioxide, and sulfur dioxide are the most common volcanic gases.

Specific examples of different kinds of eruptions:

Lots of different things come out of a volcano when it erupts depending on what kind of eruption it is. If it is a shield volcano like we have here in Hawai’i, then there is usually a fountain of molten lava that reaches anywhere from 10 to 500 meters into the air. This fountain builds a spatter cone or cinder cone around the vent. Meanwhile, if enough lava is falling from the fountain, a lava flow can develop. If the amount of lava feeding the flow is high, then the flow will move rapidly downhill away from the vent. Rapid-moving flows ccontinually disrupt their surfaces and are constantly exposing more red-hot lava to the atmosphere. This means that the flow is losing a lot of heat and consequently its viscosity increases. As the lava continues to flow rapidly, but now with a high viscosity it starts to get torn into jagged pieces rather than flow nicely. This is how an ‘a’a flow develops.

In some eruptions there is almost no fountaining and the lava just flows slowly away from the vent. In these cases the surface of the lava is not disrupted and can solidify even while the inside is still molten. This is how pahoehoe flow move. If these pahoehoe flows go on long enough then lava tubes can develop within the flow. These lava tubes allow lava to reach the flow front from the vent without losing much heat so it is still pretty fluid even 10’s of kilometers from the vent.

At more explosive volcanoes eruptions are very different. The main difference is that the viscosity of the magma (how fluid or how pasty it is) is much higher. This really viscous magma acts as an effective plug on the vent and allows gas pressures to build to

very high. Eventually the gas pressure is higher than even the viscous lava can stand, and an explosive eruption occurs. These explosions remove the cap of viscous lava that was plugging the vent so that the pressure is now lower. With the new low pressure, more gas bubbles can expand and push more lava out of the vent, and on and on and on. Once one of these explosive eruptions starts it pretty much continues until the available magma is uused up. These big explosions reach 10’s of km into the atmosphere sometimes, and spread fine ash over huge areas.

Sometimes instead of going up, the hot mixture of gas and ash flows out of the vent and hugs the ground. These fast-moving hot mixtures are called pyroclastic flows and they are very dangerous. Because they are mostly gas, they can move quickly, up to 200 km/hour. They are sometimes up to 600 degrees centigrade. With this combination of speed aand heat they are the most dangerous phenomenon that a volcano can produce. They may leave only a thin layer of ash after they pass through, but for those few moments while the pyroclastic flow is passing through nothing can llive. Pyroclastic flows killed about 25,000 people in the town of St. Pierre in 1902. This disaster prompted Thomas A. Jaggar to dedicate his life to studying volcanoes, and he went on to found the Hawaiian Volcano Observatory.

What is a hot spot?

Mantle plumes are areas of hot, upwelling mantle. A hot spot develops above the plume. Magma generated by the hot spot rises through the rigid plates of the lithosphere and produces active volcanoes at the Earth’s surface. As oceanic volcanoes move away from the hot spot, they cool and subside, producing older islands, atolls, and seamounts. As continental volcanoes move away from the hot spot, they cool, subside, and become extinct.

Hot spots are places within the mantle wwhere rocks melt to generate magma. The presence of a hot spot is inferred by anomalous volcanism (i.e. not at a plate boundary), such as the Hawaiian volcanoes within the Pacific Plate. The Hawaiian hot spot has been active at least 70 million years, producing a volcanic chain that extends 3,750 miles (6,000 km) across the northwest Pacific Ocean. Hot spots also develop beneath continents. The Yellowstone hot spot has been active at least 15 million years, producing a chain oof calderas and volcanic features along the Snake River Plain that extends 400 miles (650 km) westward from northwest Wyoming to the Idaho-Oregon border.

Can volcanoes form just anywhere?

I guess you can think of 3 main places where volcanoes originate, hot spots, divergent plate boundaries(such as rifts and mid-ocean ridges), and convergent plate boundaries(subduction zones).

The origin of the magma for hot spots is not well known. We do know that the magma comes from partial melting within the upper mantle, probably from depths not too much greater than 100 km. The actual source of the heat that causes the partial melting (the actual hotspot itself) is almost certainly much deeper than that, but we really don’t know how deep or even exactly what a hotspot is!

At a divergent margin, two tectonic plates are moving apart, and magma that is generated in the upper mantle flows upward to fill in the space. This magma is probably generated at depths that are shallower than those for hotspot magmas. People argue about whether the magma forcing its way to the surface causes the plates to move apart or whether the plates move apart and the magma just reacts to that and ffills in the space. Perhaps it is a combination of these two. The most extensive example of this type of volcanism is the system of mid-ocean ridges. Continental examples include the East African Rift, the West Antarctic Rift, and the Basin and Range Province in the southwestern US.

The final major place where volcanism originates is at convergent margins (subduction zones)–where an oceanic plate dives under either another oceanic plate or perhaps a continental plate. As the plate gets pushed further and further it starts to give off its volatiles (mostly water), and these migrate upwards into the mantle just under the overriding plate. The addition of these volatiles to this overriding mantle probably lowers the melting point of that mantle so that magma is generated. Part of the magma may also be generated by the downgoing plate actually starting to melt as it gets into the hotter and hotter interior.

How are volcanoes and earthquakes related?

Some, but not all, earthquakes are related to volcanoes. For example, most earthquakes are along the edges of tectonic plates. This is where most volcanoes are too. However, most earthquakes are caused by the interaction of the plates not the movement of magma.

Most eearthquakes directly beneath a volcano are caused by the movement of magma. The magma exerts pressure on the rocks until it cracks the rock. Then the magma squirts into the crack and starts building pressure again. Every time the rock cracks it makes a small earthquake. These earthquakes are usually too weak to be felt but can be detected and recorded by sensitive instruments. Once the plumbing system of the volcano is open and magma is flowing through it, constant earthquake waves, called harmonic tremor, are recorded (but not felt).

There is a general relationship, in that volcanic eruptions and earthquakes occur along the same major areas of the world. . .because both occur most often at plate boundaries. . .such as around the edge of the Pacific Ocean.

What is a volcano?

A volcano is a vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the form or structure, usually conical, that is produced by the ejected material.

What are the different parts of a volcano?

The basic parts are the magma supply system (which may or may not include a magma chamber), the plumbing system that gets magma from the magma chamber to

the surface, the layers of lava and/or ash, and perhaps a summit crater or caldera.

How many active volcanoes are there in the world?

The Smithsonian Institution’s list of historical volcanic eruptions lists about 800 active or dormant volcanoes

What is the difference between active, dormant, and extinct volcanoes?

An active volcano is one that has erupted sometime during the last few hundred years.

A dormant volcano is one that has not erupted during the last few hundred years, but it has erupted during tthe last several thousand years.

An extinct volcano is one that has not erupted during the last several thousand years.

How do you make a model of a volcano?

Volcano world has an excellent section that for those interested in building volcano models . It has everything from simple volcanoes to ones that actually erupt! They are all pretty easy and cheap to make.

How did hot spots get formed?

Nobody really knows the answer. One thought is that they represent bbumps on the surface that represents the boundary between the Earth’s outer core and its lower mantle. these bumps cause upward streaming of material that we call hotspots. The honest answer is that lots of folks are working on it bbut haven’t come up with the answer yet.

What part of the world do scientists call the „ring of fire“?

The „ring of fire“ refers to the rim of the Pacific ocean, from southern Chile all the way around to New Zealand.

DISTRIBUTION OF ACTIVE VOLCANOES

The earth is a dynamic planet. Its rigid outer surface layer is broken into several tectonic plates which are in constant motion relative to one another. As demonstrated in the world map below, most of the ~550 active volcanoes on earth are located along the margins of adjacent plates.

PLATE MOTION, MANTLE CONVECTION, AND MAGMA GENERATION

Tectonic plates are composed of lithosphere, the rigid outer portion of the earth. With a thickness of about 100 km, the llithosphere is composed of an upper layer of crust (~7 km thick under the oceans, and ~35 km thick under the continents) and a lower, denser layer of the earth’s upper mantle. The lithosphere is underlain by the asthenosphere, a hot, mobile layer of partially molten rock lying within the earth’s upper mantle. (For detailed information, click the Earth’s Interior.)

Volcanic eruptions above these lithospheric plates are driven by the ascent of magma (molten rock) from deep beneath the surface. The vvarious magma types are described in Physicochemical Controls on Eruption Style. They vary from mafic, intermediate, to felsic as their silica (SiO2) content increases. Mafic (basaltic) magmas are generated directly from the mantle, either within the asthenosphere or within the overlying mantle lithosphere. Many mafic-to-intermediate (basaltic-to-andesitic) magmas appear to be derived from the melting of hydrated lithospheric mantle. More differentiated, intermediate-to-felsic magmas, on the other hand, are partly derived from the melting of continental crust by hot, mafic magmas that either pond at the crust-mantle boundary, or intrude into the overlying continents where they reside in magma chambers located at various crustal levels.

Volcanism is typically widespread along plate boundaries. Although volcanism in the interior of plates is less common, these intraplate regions can also generate voluminous eruptive products. The regional volcano-tectonic processes associated with plate-boundary environments and intraplate environments are described in more detail below.

VOLCANISM AT PLATE TECTONIC BOUNDARIES

Plate boundaries mark the sites where two plates are either moving away from one another, moving toward one another, or sliding past one another. Adjacent plates are delineated by three types of boundaries defined by this relative motion:

• Divergent plate boundaries — Plates diverge from one another at the site of thermally buoyant mmid-oceanic ridges. Oceanic crust is created at divergent plate boundaries.

• Convergent plate boundaries — Plates converge on one another at the site of deep oceanic trenches. Oceanic crust is destroyed at convergent plate boundaries.

• Transform plate boundaries — Plates slide past one another.

Although volcanism is abundant at divergent and convergent plate boundaries, there is a distinct lack of significant volcanism associated with transform plate boundaries. Spreading center volcanism occurs at divergent plate margins, and subduction zone volcanism occurs at convergent plate margins. Intraplate volcanism describes volcanic eruptions within tectonic plates.

The most volcanically active belt on Earth is known as the Ring of Fire, a region of subduction zone volcanism surrounding the Pacific Ocean. Subduction zone volcanism occurs where two plates are converging on one another. One plate containing oceanic lithosphere descends beneath the adjacent plate, thus consuming the oceanic lithosphere into the earth’s mantle. This on-going process is called subduction. As the descending plate bends downward at the surface, it creates a large linear depression called an oceanic trench. These trenches are the deepest topographic features on the earth’s surface. The deepest, 11 kilometers below sealevel, is the Mariana trench, which lies along the western margin of the RRing of Fire. Another example, forming the northern rim of the Ring of Fire, is the Aleutian trench.

The crustal portion of the subducting slab contains a significant amount of surface water, as well as water contained in hydrated minerals within the seafloor basalt. As the subducting slab descends to greater and greater depths, it progressively encounters greater temperatures and greater pressures which cause the slab to release water into the mantle wedge overlying the descending plate. Water has the effect of lowering the melting temperature of the mantle, thus causing it to melt. The magma produced by this mechanism varies from basalt to andesite in composition. It rises upward to produce a linear belt of volcanoes parallel to the oceanic trench, as exemplified in the above image of the Aleutian Island chain. The chain of volcanoes is called an island arc. If the oceanic lithosphere subducts beneath an adjacent plate of continental lithosphere, then a similar belt of volcanoes will be generated on continental crust. This belt is then called a volcanic arc, examples of which include the Cascade volcanic arc of the U.S. Pacific northwest, and the Andes volcanic arc of South America.

HOT SPOTS AND MANTLE PLUMES

Although most volcanic

rocks are generated at plate boundaries, there are a few exceptionally active sites of volcanism within the plate interiors. These intraplate regions of voluminous volcanism are called hotspots. Twenty-four selected hotspots are shown on the adjacent map. Most hotspots are thought to be underlain by a large plume of anomalously hot mantle. These mantle plumes appear to be generated in the lower mantle and rise slowly through the mantle by convection. Experimental data suggests that they rise as a plastically ddeforming mass that has a bulbous plume head fed by a long, narrow plume tail. As the head impinges on the base of the lithosphere, it spreads outward into a mushroom shape. Such plume heads are thought to have diameters between ~500 to ~1000 km.

Many scientists believe that mantle plumes may be derived from near the core-mantle boundary, as demonstrated in this computer simulation from the Minnesota supercomputing lab. Note the bulbous plume heads, the narrow plume tails, and tthe flattened plume heads as they impinge on the outer sphere representing the base of the lithosphere.

Decompressional melting of this hot mantle source can generate huge volumes of basalt magma. It is thought that the massive flood basalt provinces oon earth are produced above mantle hotspots. Although most geologists accept the hotspot concept, the number of hotspots worldwide is still a matter of controversy.

HOTSPOT TRACKS

The Pacific plate contains several linear belts of extinct submarine volcanoes, called seamounts, an example of which is the Foundation seamount chain

The formation of at least some of these intraplate seamount chains can be attributed to volcanism above a mantle hotspot to form a linear, age-progressive hotspot track. Mantle plumes appear to be largely unaffected by plate motions. As lithospheric plates move across stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the mantle plume in the direction oof plate movement. Thus, a linear belt of inactive volcanic islands and seamounts will be produced. A classic example of this mechanism is demonstrated by the Hawaiian and Emperor seamount chains.

The „Big Island“ of Hawaii lies above the mantle plume. It is the only island that is currently volcanically active. The seven Hawaiian Islands become progressively older to the northwest. The main phase of volcanism on Oahu ceased about 3 million years ago, and on Kauai about 5 million years aago. This trend continues beyond the Hawaiian Islands, as demonstrated by a string of seamounts (the Hawaiian chain) that becomes progressively older toward Midway Island. Midway is composed of lavas that are ~27 million years old. Northwest of Midway, the volcanic belt bends to the north-northwest to form the Emperor seamount chain. Here, the seamounts become progressively older until they terminate against the Aleutian trench. The oldest of these seamounts near the trench is ~70 million years old. This implies that the mantle plume currently generating basaltic lavas on the Big Island has been in existence for at least 70 million years!

The Hawaiians were very good at recognizing the difference in the older, eroded volcanic islands and newer islands to the southeast, where volcanic features are more pristine. Legend has it that Pele, the Hawaiian goddess of fire, was forced from island to island as she was chased by vairous gods. Her journey is marked by volcanic eruptions, as she progressed from the island of Kaua’i to her current home on the Big Island. The legend corresponds well with the modern scientific notion of the age progression of these volcanic islands.

THE EARTH’S HEAT FURNACE

The Earth’s internal heat source provides the eenergy for our dynamic planet, supplying it with the driving force for plate-tectonic motion, and for on-going catastrophic events such as earthquakes and volcanic eruptions. This internal heat energy was much greater in the early stages of the Earth than it is today, having accumulated rapidly by heat conversion associated with three separate processes, all of which were most intense during the first few hundred thousand years of the Earth’s history: (1) extraterrestrial impacts, (2) gravitational contraction of the Earth’s interior, and (3) the radioactive decay of unstable isotopes.

EXTRATERRESTRIAL IMPACTS

Most scientists believe that our solar system evolved from the accretion of solid particles derived from a large nebular cloud – the so-called Nebular Hypothesis. Under this scenario, proto-planet Earth would have grown over time from a barrage of extraterrestrial impacts, increasing its mass with each bombardment. As the proto-planet grew in size its increased gravitational field would have attracted even more objects its surface. The composition of these colliding bodies would have included metal-rich fragments (i.e.., iron meteorites), rocky fragments (i.e., stony meteorites), and icy fragments (i.e., comets). Although accretion was much more prevalent in the early stages of the Earth’s history, these extraterrestrial collisions are still occurring today, exemplified bby shooting stars and fireballs in the night sky, and by the occasional impact of larger bodies on the Earth’s surface.

Such particles travel at great velocities, typically ~30,000–50,000 km/hr, similar to that of the Earth as it rotates around the Sun. The very large amount of kinetic energy inherent in these moving bodies is instantly converted to heat energy upon impact, thus providing a component to the Earth’s internal heat source.

GRAVITATIONAL CONTRACTION

In the early stages of planetary accretion, the earth was much less compact than it is today. The accretionary process led to an increasingly greater gravitational attraction, forcing the Earth to contract into a smaller volume. Increased compaction resulted in the conversion of gravitational energy into heat energy, much like a bicycle pump heats up due to the compression of air inside it. Heat conducts very slowly through rock, so that the rapid build up of this heat source within the Earth was not accommodated by an equally rapid loss of heat through the surface.

DECAY OF RADIOACTIVE ELEMENTS

Radioactive elements are inherently unstable, breaking down over time to more stable forms. The unstable isotope Uranium-238, for example, will slowly decay to Lead-206. All such radioactive decay processes release heat as

a by product of the on-going reaction. In its early stages of formation, the young earth had a greater complement of radioactive elements, but many of these (e.g., aluminum-26) are short-lived and have decayed to near extinction. Others with a more lengthy rate of decay and are still undergoing this radioactive process, thus still releasing heat energy. The greater complement of unstable elements in the early Earth thus generated a greater amount of heat energy in its initial stages of fformation.

MELTING AND COMPOSITIONAL DIFFERENTIATION OF THE EARLY EARTH

The heat buildup inside earth reached a maxim early in the Earth’s history and has declined significantly since. The greater heat content of the early Earth was the product of (1) a greater abundance of radioactive elements, (2) a greater number of impacts, and (3) the early gravitational crowding. The initial accretion of particles resulted in a rather homogeneous sphere composed of a loose amalgam of metallic fragments (iron meteorites), rocky fragments (stony mmeteorites), and icy fragments (comets). However, the increased heat content of the early Earth resulted in melting of the Earth’s interior, so that the young planet became density stratified with the heavier (metallic) materials sinking to the center of the eearth, and the lighter (rocky) materials floating upward toward the surface of the earth. The very lightest volatile materials (derived from comets) were easily melted or vaporized, rising beyond the earth’s rocky surface to form the early oceans and the atmosphere. We now have a differentiated earth due to melting and mobilization of materials driven by the earth’s internal heat engine. This has resulted in the development of a series of concentric layers that are both density and compositionally stratified. This demonstrated in the diagram below, courtesy of the USGS.

These layers include (1) the dense inner core composed largely of solid Fe and subordinate Ni, with radius of about 1200 km, (2) the molten outer core composed largely of liquid FFe, with subordinate sulfur, with a radius of about 2250 km, (3) the mantle, composed of relatively dense rocky materials, with radius of about 2800 km thick, and (4) the crust which comprises the thin relatively light outer skin of the earth, is divisible into two types: the oceanic crust (~7 km thick) and the continental crust (about 35 km thick). Whereas oceanic crust is composed of basaltic rock, the less dense continental crust is composed of a great variety oof rock types having an overall average composition akin to granite.

Within the mantle exists the asthenosphere (Grk. asthenos = weak), between about 100 km and 350 km, which is a special zone composed of hot, weak material that is capable of gradual flow. The layer above the asthenosphere is the lithosphere (Grk. lithos = rock), the rigid and relatively cool outer layer of the earth, composed of both crust and a portion of the upper mantle.

Lying above the lithosphere is (1) the liquid hydrosphere, comprising 71% of the Earth’s surface, and (2) that the still lighter gaseous atmosphere, both of which were ultimately derived from the accretion of comets. The occurrence of these volatile components along the outermost portion of the Earth is a product of volcanic outgassing during the differentiation event.

PHYSICOCHEMICAL CONTROLS ON ERUPTION STYLE

There is a great range in the explosivity of volcanic eruptions. Many eruptions are relatively quiescent and are characterized by the calm, nonviolent extrusion of lava flows on the earth’s surface. Other eruptions, however, are highly explosive and are characterized by the violent ejection of fragmented volcanic debris, called tephra, which can extend tens of kilometers into the atmosphere above the volcano.

Whether or not aan eruption falls into one of these end-member types depends on a variety of factors, which are ultimately linked to the composition of the magma (molten rock) underlying the volcano. Magma composition is discussed below, followed by a description of the controlling factors on explosivity — viscosity, temperature, and the amount of dissolved gases in the magma.

MAGMA COMPOSITION AND ROCK TYPES

Only ten elements make up the bulk of most magmas: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), titanium (Ti) calcium (Ca), sodium (Na), potassium (K), and phosphorous (P). Because oxygen and silicon are by far the two most abundant elements in magma, it is convenient to describe the different magma types in terms of their silica content (SiO2). The magma types vary from mafic magmas, which have relatively low silica and high Fe and Mg contents, to felsic magmas, which have relatively high silica and low Fe and Mg contents. Mafic magma will cool and crystallize to produce the volcanic rock basalt, whereas felsic magma will crystallize to produce dacite and rhyolite. Intermediate-composition magmas will crystallize to produce the rock andesite. Because the mafic rocks are enriched in Fe and Mg, they tend to be darker colored tthan the felsic rock types.

SiO2 CONTENT

MAGMA TYPE VOLCANIC ROCK

~50% Mafic Basalt

~60% Intermediate Andesite

~65% Felsic (low Si) Dacite

~70% Felsic (high Si) Rhyolite

There also exists more unusual magmas that erupt less commonly on the Earth’s surface as ultramafic, carbonatite, and strongly alkaline lavas.

STRONGLY ALKALINE LAVAS

One way to classify lavas is by their alkali content, reflected in their weight percent of Na2O + K2O. In contrast to the common lava types (basalt, andesite, dacite, and rhyolite), there exists less common lavas that define mildly alkaline trends (e.g., with increasing silica content: alkali basalt, trachybasalt, trachyandesite, trachyte, and comendite), and strongly alkaline trends (e.g., with increasing silica content: tephrite, phonotephrite, tephriphonolite, and phonolite). Although these lavas can occur in a variety of tectonic settings, they are typically found in either (1) continental or oceanic intraplate settings, where there is often a lack of significant tectonic control, (2) continental rift zones, and (3) the back-arc setting of subduction zones.

Alkali-rich lavas are often charateristic of the waning stages of volcanism, as demonstrated, for example, in the late-stage parasitic cones and flows found on Hawaiian shield volcanoes. Differentiated trachytic and phonolitic lavas typically erupt as low-volume flows with high aspect ratios, as demonstrated above in the phonolite coulées of western Saudi Arabia. However, extensive sheetflows of

similar lavas have been recognized in continental rift zones, as exemplified in the voluminous phonolitic lavas of the Ethiopian rift system.

CARBONATITES

Carbonatites are perhaps the most unusual of all lavas. They are defined, when crystalline, by having more than 50% carbonate (CO3-bearing) minerals, and typically they are composed of less than 10% SiO2. There are only 330 known carbonatite localities on Earth, most of which are shallow intrusive bodies of calcite-rich igneous rock in the form of volcanic necks, dikes, and ccone-sheets. These generally occur in association with larger intrusions of alkali-rich silicate igneous rocks. Extrusive carbonatites are particularly rare, and appear to be restricted to a few continental rift zones, such as the Rhine valley and the East African rift system.

Most carbonatite lavas have low eruption temperatures, between 500 and 600 degrees Centigrate (compared with >1100 degrees Centigrate for basaltic lavas). They typically have low viscosities due to their lack of silica polymerization. Thus, carbonatite flows are generally only a ffew centimeters thick, with surface textures that vary from a’a to pahoehoe. Although they often resemble flowing lobes of black mud, they are hot enough to display glowing, deep-red colors when seen at night. Some active carbonatite flows are enriched iin alkalies (Na and K) – these are called natrocarbonatites. Soon after their eruption, dark natrocarbonatite flows will cool and rapidly turn white due to reaction with atmospheric water.

OLDOINYO LENGAI, TANZANIA

The East African Rift System contains a vast array of igneous rocks, dominated by voluminous floods of basaltic lava. Its highest peaks, Mt. Kilamanjoro and Mt. Kenya, are active volcanoes located along the eastern rift of Lake Victoria. Lying between these impressive peaks is the only know active carbonatite volcano in the world – Oldoinyo Lengai. Rising over 2200 meters from the valley floor, the bulk of this volcanic cone is composed phonolitic tephra. However, its upper portion is dominated by natrocarbonatite lava flows. Historic eruptions of natrocarbonatite have filled mmuch of the summit crater, shown here courtesy of Marco Fulle – stromboli.net. Note the tall conical hornitos protruding from the crater floor.

ORIGIN OF CARBONATITE

A history of controversy has surrouned the origin of carbonatites, and whether or not calcite carbonatites are primary or secondary lavas. Some scientists believe that the Ca-carbonatites are generated by fractional melting of crustal carbonate rocks. However, others believe that the Ca-carbonatites are not primary magmas at all, but rather derived from the alteration of Na-rich nnatrocarbonatite lavas. Virtually, all natrocarbonatites on Earth are found in historic eruptions, and none in ancient rocks. Many have attributed this to the very high solubility of sodium carbonate. The argument being that all carbonatites were originally natrocarbonatites that have had sodium removed by hydrothermal solutions and rainwater.

The natrocarbonatites themselves appear to have clear isotopic signatures indicating their origin from a mantle source. The very high Na2O and very low MgO compositions of the Oldoinyo Lengai natrocarbonatites suggest that they are highly differentiated magmas. The common association of carbonatites with alkali-rich silicate rocks (e.g., nephelinite) suggests further that liquid immiscibility between silicate and carbonate magmas may have played a role in their origin.

KOMATIITES

Named from their type locality along the Komati River in South Africa, komatiites are ultramafic volcanic rocks, having very low silica contents (~40-45%) and very high MgO contents (~18%). These lavas are exceptional not only for their compositions, but also for their very old, restricted ages. These lavas have no modern analogs. The youngest komatiites (from Gorgona Island, Columbia) have been dated at about 90 million years; however, all other komatiites are about three billion years old or older. These ancient lava flows erupted at a time when tthe Earth’s internal heat was much greater than today, thus generating exceptionally hot, fluid lavas with calculated eruption temperatures in excess of 1,600 degrees C (2,900 degrees F). In comparison, typical basaltic lavas erupting today have eruption temperatures of about 1,100 degrees C.

A spectauclar identifying trait of komatiites is their spinifex texture, which resembles a lacey mesh of acicular (needle-like) olivine crystals, typically surrounded by lighter-colored, interstitial minerals such as plagioclase, tremolite, and/or chlorite. The elongated nature of the komatiite olivine crystals is quite distinct when compared with the equant to tabular olivine crystals seen in most basaltic rocks.

Since we have never observed a komatiite eruption, we have had to deduce the fluid flow character and eruption style of these lavas from the properties and textures of the ancient rocks. Their high eruption temperatures, for example, are calculated from their olivine-rich compositions. The charateristic spinifex textures on the otherhand, indicate that these lavas cooled very rapidly. Combined, the high eruption temperatures and the low silica contents indicate that komatiites erupted as very fluid lavas, having exceptionally low viscosities and low aspect ratios. It is believed that these hot, fluid lavas would have been turbulent, and therefore capable of a significant aamount of both mechanical and thermal erosion. Indeed, many scientists believe that the sinuous rilles exposed on the moon’s surface are erosive valleys produced by komatiite lava flows, contemporaneous with the ancient eruption of similar lavas on Earth.

LAVA FLOW TYPES

INTRODUCTION

Volumetrically, most lava is of basaltic composition. Basaltic melts have overall lower gas contents and are more fluid than their andesitic-to-rhyolitic counterparts. Their higher fluidity (lower viscosity) is a product of their lower SiO2 (silica) contents. When gases exsolve from basaltic melts they are allowed to rise unimpeded through the fluid magma without a significant build up of gas pressure. This results in relatively calm, nonexplosive eruptions, and a preponderance of lava. In contrast, when gases exsolve from felsic magmas, their upward mobility is impeded by the high viscosity of the melt. This results in the buildup of gas pressure, which generates explosive eruptions associated with a preponderance of pyroclastic ejecta. The low viscosity of basaltic lavas allows them to be extruded over great distances, often producing high-volume lava flows with low aspect ratios (ratio of thickness to area). Under the right conditions, de-gassed felsic magmas can also erupt lava in a nonviolent manner. However, felsic lavas tend to be much

thicker than basaltic lavas and have much higher aspect ratios.

EFFUSION RATE

The volume of magma generated over a given amount of time is known as the effusion rate. The effusion rates for historical eruptions of basaltic lava are highly variable, from 0.5 to 5000 m3/sec. Historic flows from Mt. Etna average about 0.5 m3/sec, whereas the fissure-generated flows associated with the Icelandic Laki eruption in 1783 were released at a rate of ~5000 m3/sec. The effusion rates for andesite and dacite aare much lower (~10 to 0.05 m3/sec) due to their higher viscosities.

COMPOSITIONAL TYPES:

Lava is highly variable in composition and it therefore varies greatly in flow character. Once crystallized, this compositional diversity is reflected in variations in rock texture, mineralogy, and volcanic landforms. Click on the links below for a more detailed description of the compositional lava types:

• BASALTIC LAVA

• ANDESITE, DACITE, AND RHYOLITE LAVA

• UNUSUAL LAVA TYPES

BASALTIC LAVA

SURFACE STRUCTURE

Basaltic lava flows erupt primarily from shield volcanoes, fissure systems, scoria ccones, and spatter cones. These fluid lava flows can be subdivided into two end-member structural types, based primarily on the nature of lava flow surfaces:

• Pahoehoe Lava — Surfaces are smooth, billowy, or ropy.

• A’a lava — Surfaces are fragmented, rough, aand spiny, with a „cindery“ appearance.

PAHOEHOE LAVA

Pahoehoe can take several different forms. As the smooth lava surface cools to turns to a dark gray color and becomes less fluid and more viscous, behaving more like a plastic substance than a truly liquid substance. As lava continues to flow underneath this plastic skin, the surface can bunch up or wrinkle into a form that resembles coiled rope. Such a surface is called ropy pahoehoe. In addition to these ropy surfaces, solidified basalt flows can also display shelly to slabby surfaces. Shelly pahoehoe contains a billowy flow top with a frothy vesicular surface skin, only a few centimeters thick, overlying large cavities, generally 5-30 centimeters thick. These shelly surfaces often collapse when wwalking on the top of the flow. Slabby pahoehoe contains a series of closely spaced slabs, a few meters across and a few centimeters thick, broken and tilted by mass movement, or drainage, of the underlying lava. Slabby pahoehoe is often gradational to a’a lava.

Pahoehoe lavas are typically the first to erupt from a vent. They are relatively thin (1-2 m) and very fluid with low viscosities. They advance downslope in a sort of smooth „rolling motion.“ The front of tthe flow usually advances as a thin (< 20 cm) glowing lobe that will chill and crust over after 1-2 meters of flow. It will slow and be overrun by a new lobe that propagates downslope until it also chills, and in turn is overrun by another flow. Overriding lavas and breakouts on the flow top and sides thus produce compound flows composed of several lobes cooling against one another. Slower moving pahoehoe flows will advance through the protrusion of small bulbous appendages at the flow front, called pahoehoe toes. The image above shows a breakout and the advancement of pahoehoe toes along the sides of a ropy pahoehoe lava flow. As the lava surface cools and thin skin becomes more viscous, progressive breakouts will occur, thus advancing the flow forward, as demonstrated below. Where pahoehoe toes advance rapidly, usually down steeper slopes, an elongated protrusions may emerge, called entrail pahoehoe.

INFLATED PAHOEHOE SHEETFLOWS

High effusion rates may result in the development of inflated pahoehoe sheetflows. These large-volume pahoehoe flows are emplaced initially as thin sheets, 20-30 cm thick. Cooling of these flow sheets will produce a smooth pahoehoe crust which initially behaves in a plastic fashion. However, after attaining a thickness oof 2-5 cm, the crust behaves more rigidly and develops strength. As the underlying liquid core of the flow increases in size due to sustained lava injection, the hydrostatic head of the flow is distributed evenly throughout. This can result in flow inflation and uniform uplift of the entire flow sheet. Such flows will inflate and thicken by as much as 4 meters. Many of these inflated flows will crystallize in place; however, some will deflate after emplacement as the fluid core drains beneath the solidified crust. Deflation is evident in many Hawaiian flows which contain tree molds that currently stand 1-2 meters above the current surface of the pahoehoe sheet flows. An example of deflation on Kilauea Volcano, Hawaii, is demonstrated to the left by solidified lava around high-standing tree trunks to produce hollow tree molds standing above the deflated pahoehoe surface.

A’A LAVA

The a’a flows shown below are advancing over older pahoehoe surfaces. Although these lava flows are often more viscous, and typically thicker, than pahoehoe lavas they tend to advance at greater rates. Their flow fronts can vary from two meters to as much as twenty meters thick. Their forward motion is similar to the movement of a ttractor tread. A jumbled mass of debris steepens at the flow front until a section breaks off and tumbles forward. Inward from the flow front, the flow usually contains an upper rubbly flow top, and a lower massive part of viscous lava insulated from the overlying rubble. The jagged cinder blocks that break off the front are then overridden by the massive lava core of the flow which pushes forward.

CONVERSION OF PAHOEHOE TO A’A

Pahoehoe is often converted to a’a as lava advances downslope, away from the volcano. Conversion of a’a to pahoehoe, on the other hand, never takes place. The pahoehoe-to-a’a conversion can be caused by either an increase in flow viscosity, or an increase in the rate of shear. Cooling, gas loss, and crystallization of the lava will cause it to become increasingly more polymerized and viscous as it advances farther downslope. Although the downslope increase in viscosity can convert pahoehoe to a’a, a higher rate of shear can also cause the conversion. Shear increases in flows that have higher effusion rates. A’a begins to form when effusion rates are >5-10 cubic meters per second. The rate of shear can also increase as lava advances down increasingly steeper slopes.

This is demonstrated, for example, by the conversion of pahoehoe to a’a in recent lava flows that have advanced down the Hilina Pali escarpment during the ongoing eruption of the Pu’u O’o Volcano, Hawaii.

ANDESITIC LAVA

Whereas basalt forms a’a and pahoehoe surface forms, andesite generally produces blocky lava. Here, the surface contains smooth-sided, angular fragments (blocks) that are not as splintery or vesicular as a’a lava fragments. The blocky nature of these flows is attributed to the higher viscosity of andesite. TThese viscous lavas have relatively high aspect ratios (thickness/area), generally > 1/100, and some are thick enough ...