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All About Volcanoes
A volcano is simply defined as an opening in the earth's surface crust through which molten lava, ash, and gases are ejected. Volcanoes occur most often on or near crustal plates or boundaries, although certain volcanoes known as hotspot volcanoes can occur virtually anywhere. Today’s existing terrestrial volcanoes are believed to have formed when molten rock or magma welled up in the Earth’s interior pushed its way to the surface causing the volcanoes’ initial eruptions. Volcanoes all generally fit into one of six different categories: Shield volcanoes, Cinder volcanoes, Stratovolcanoes, Submarine volcanoes, Subglacial volcanoes, and the most powerful volcano of all, the [|Supervolcano]. A Supervolcanic eruption can be catastrophic to entire regions both climatically and geologically. Individual volcanoes are typically described as being either active (currently erupting), dormant (not currently active) or extinct (not expected to ever erupt again). Some of the most famous volcanic eruptions around the world include the Yellowstone Caldera in Yellowstone National Park, Lake Toba in Sumatra, Indonesia, and the devastating eruption of Mt. St. Helens in southwest Washington State in 1980. For more [|volcano information], click on any of the [|volcano pictures], volcano articles or other [|interactive volcano] features below.

There are few geological events that are as dramatic as a volcanic eruption except maybe a hurricane, tornado or a tsunami. Each is dramatic in its own way. A volcano is more localized, however. Its impact will affect a small local area unless, of course, it erupts like Mount St. Helens which was monumental.

Mt. Vesuvius erupted in Italy and buried Pompei. Paricutin, in Mexico, grew in a farmer's field into a 1400- hundred foot volcano over the course of nine years.


 * What is a Volcano?**

Okay, so what is a volcano? It is a geologic formation which has fissures that lead to a deep part of the earth. These fissures, like pipes, have magma or molten rock. When the magma comes out of the volcano, it is called lava. The magma is formed when the heat and pressure in the earth melts the rock.


 * What makes a volcano erupt?**

The pressure under the earth is usually caused by tectonic plates which can come together or come apart, causing these fissures to appear. The magma pressure is from a heat source. It comes from natural radioactive decay within the Earth.

This concentration of radioactive elements is not very high, but this is offset by the volume of the Earth. There is enough earth to add a significant amount of heat produced from the decay to cause the melting. The material that melts, the magma, has a lower density and it migrates to the surface.

The increasing pressure pushes the magma topside. The magma pressure can get high if the supply of gases and earth volume is high. The magma pressure can also get high if magma is just sitting around in the magma chamber within the volcano. This is because as the magma in the chamber starts to cool off, it releases gases. These gases can increase the pressure until the volcano fractures and you have an eruption.


 * Active and Dormant Volcanoes**

Volcanoes can also be classified as dormant or active. A dormant volcano is one which has no activity present. A volcano which may have been active at one time but where no activity is seen for years or centuries is dormant.

Active volcanoes are the ones where there is still some activity present. Usually you can see plumes of steam or ash rise from the surface. Very active ones have lava spill out from the fissures. Many active eruptions are spectacular sights with lava jets shooting hundreds of feet into the air or cascading veins of red lava coming down mountain sides.


 * Famous Eruptions**

Volcanic eruptions are a thing of awe but can be deadly despite their appearance. Take Mount St. Helens in Washington, 1980, for instance. The volcano blew its top. Literally. This was one of the most dramatic eruptions ever captured on film.

Mount Etna in Sicily had a series of eruptions in 2002 and 2003 that consumed the town of Piano Provenzana and whose activity was felt across the Mediterranean Sea in Libya. The eruption was seen from space.

In 1883, Krakatau in Indonesia created a tsunami and caused over 36,000 deaths. This eruption and the next were dramatic and deadly. Mount Pelee in Martinique during 1802 lead to ash flows and caused 29,000 deaths.

In answer to the question of what makes a volcano erupt, the easiest answer is that it erupts when the pressure of the magma inside becomes so great that the volcano splits and the magma gets out.

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