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What is a Volcano?

Volcanoes form when molten rock, called magma, created inside the crust of the Earth, moves upward and breaks through a vent in the surface of the Earth. Over time a mountain like structure forms as the erupted matter hardens. Located at the top of the volcano is a depression called a crater.

There are two main types of volcano - a non-explosive, or effusive erupting volcano, and an explosive volcano. In the simplest of terms a non-explosive volcano is a lava type volcano - lava is the name given to magma once it reaches the surface.

Mt St.Helen’s was an excellent example of an explosive volcano. Here extreme pressure is formed below the surface by gases within the magma and water vapour, formed by the magma heating groundwater.

Pyroclastic flows, more commonly associated with an explosion eruption, are one of the most dangerous and destructive products of a volcano. Made of hot clouds of gas, ash, and volcanic material the flows speed down the slopes of a volcano by as much as 150km/h destroying everything in their path. They are produced either when a lava dome collapses, or when a dense ash column collapses under it's own weight. A Pyroclastic flow can reach temperatures of 850 degrees C, or higher.nbsp;
A volcano is "active" when it is erupting, or has erupted recently. Volcanoes that have not erupted recently but are considered likely to erupt in the future are said to be "dormant." A volcano that has not erupted for a long time and is not expected to erupt in the future is referred to as "extinct".

Information obtained from the Wikipedia web site. Wikipedia the free encyclopedia.

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