Frequently Asked Questions about Volcanoes

How are volcanoes created?

Volcanoes are a natural way that the Earth and other planets have of cooling off. Planets are warm in their mantles. Heat inside planets escapes towards their surfaces. For reasons that are not well understood, 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.

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 air 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.

At any given time, what is the average number of volcanoes that are erupting in the world?

That number is probably around 20, with 12-15 of those being volcanoes that pretty much are erupting all the time.

Why are most volcanoes located around the Pacific area?

Most of the Earth's volcanoes are located around the Pacific Ocean because that is where most of the Earth's subduction zones. A subduction zone is a place where one plate of oceanic lithosphere (= the crust + uppermost mantle) is shoved under another plate. The down going plate eventually starts to melt, and the material rises up to erupt through the overlying plate. If the overlying plate is a continent, you get a chain of volcanoes such as the Andes or Cascades. If the overlying plate is ocean you get a chain of volcanic islands such as the Marianas or Aleutians. This is also where the Earth's deep ocean trenches are and where the Earth's deep earthquakes are. The trenches form because the down going plate is bent downward as it subducts. The earthquakes form as the two plates scrape against each other (earthquakes down to about 150 km) and then as the down going plate bends (earthquakes down to about 700 km). The earthquakes do a very good job of tracing the position of the down going plate. These zones of earthquakes are called Wadati-Benioff zones, after the two seismologists who first recognized them.

What happens when a volcano erupts?

There are lots of things that happen when a volcano erupts, and they depend on what kind of eruption it is. If it is a shield volcano like for instance in Hawaii, 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 continually 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 used 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 and 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 live. 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.


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What are the different types of volcanoes?

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 occasionally 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. They 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 huge. 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 pathway 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 more 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 are 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.

Is there evidence for a cause and effect relationship between eruptions that occur at about the same time from volcanoes located hundreds to thousands of km apart?

No. Since there are on average between 50 and 60 volcanoes that erupt each year somewhere on Earth (about 1 every week), some of Earth's volcanoes may actually erupt within a few days or hours of each other. Upon closer inspection, however, the eruptions are almost always preceded by very different build-up periods in terms of time (days to weeks to months to years) and type of activity (earthquakes, ground deformation, gas emissions, and small eruptions). The "trigger" of this precursory activity is the key to understanding what causes an eventual eruption at any one volcano, not the timing of significant eruptions hundreds to thousands of km apart.

According to the theory of plate tectonics, the location and frequency of volcanism on Earth is due primarily to the way in which our planet's surface is divided into large sections or plates and how they move relative to each other, and the formation of deep "thermal plumes" that rise from the core-mantle boundary about 3,200 km below the surface. These mechanisms and the fact that even nearby volcanoes erupt magma with different and often

Can an eruption at one volcano trigger an eruption at another nearby volcano (for example, within about 10 km)?

There are a few historic examples of simultaneous eruptions from volcanoes or vents located within about 10 km of each other, but it's very difficult to determine whether one might have caused the other. To the extent that these erupting volcanoes or vents have common or overlapping magma reservoirs and hydrothermal systems, magma rising to erupt from one volcano may effect the other volcano's "plumbing" system and cause some form of unrest, including eruptions. For example, the huge explosive eruption of Novarupta vent in Alaska triggered the summit of nearby Mt. Katmai volcano to collapse, thereby forming a new caldera (but no eruption!).

Is there a relationship between large earthquakes (>M 6) that occur along major fault zones and nearby volcanic eruptions?

Sometimes, yes. A few historic large regional earthquakes (>M 6) are considered by scientists to be related to a subsequent eruption or to some type of unrest at a nearby volcano. The exact triggering mechanism for these historic examples is not well understood, but the volcanic activity probably occurs in response to a change in the local pressure surrounding the magma reservoir system as a consequence of (1) severe ground shaking caused by the earthquake; or (2) a change in the "strain" or pressure in the Earth's crust in the region surrounding where the earthquake occurred.

How many active 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 sea 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.

Someone recently told me that volcanoes put out more pollution than cars and factories, is that true?

No, that is a holdover from a quote by Ronald Regan who claimed that Mount St. Helens was adding more atmospheric pollutates than human activities add. The scientific truth is that the activities of people produce more than a hundred more times the pollutants than volcanoes.

What are the 30 most famous volcanoes?

Caribbean Pelee
Colombia Ruiz
Congo Nyiragongo
Costa Rica Arenal
Greece Santorini
Guatemala Santa Maria
Hawaii Kilauea
Hawaii Mauna Loa
Iceland Hekla
Indonesia Tambora
Indonesia Merapi
Indonesia Krakatau
Indonesia Agung
Italy Vesuvius
Italy Stromboli
Italy Etna
Japan Fuji
Japan Sakurajima
Mexico El Chichon
Mexico Popocatepetl
Mexico Colima
Mexico Paricutin
New Zealand Ruapehu
New Zealand White Island
Papua New Guinea Rabaul
Papua New Guinea Lamington
Philippines Mayon
Philippines Taal
Alaska Katmai
California Lassen Peak
California Long Valley
Washington Mount St. Helens
Wyoming Yellowstone