Tuesday, December 2, 2008

A volcano is a vent, or fissure, in a planet's crust, through which molten rock, ash, and gases are discharged with violence. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time. The term volcano is commonly applied both to the vent and to the conical mountain built up around the opening by the erupted rock materials. Volcanoes are described as active, dormant, or extinct. The soil resulting from decomposition of volcanic materials is very fertile.

Volcanoes can be caused by mantle plumes and are usually found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart. The Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. Volcanoes can also form where there is stretching and thinning of the Earth's crust, called non-hotspot intraplate volcanism, such as in the African Rift Valley, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America. By contrast, volcanoes are not created where two tectonic plates slide past one another.

According to its shape, there are six types of volcanoes; fissure volcano, shield volcano, dome volcano, ash-cinder volcano, composite volcano, and caldera volcano. Fissure volcanoes are hard to recognize as there is no central crater at all, but a giant crack in the ground. A shield volcano is a broad, shallow volcanic cone, built up by running lava; it is fluid and hot and cools slowly. A dome volcano has a steep, convex slope from thick, fast-cooling lava. An ash-cinder volcano spews out much more ash and lava into the air. A composite volcano is built up by alternate layers of lava and ash but, besides its main crater, it has many little craters on its slope. Finally, a caldera volcano is an older volcano with a large crater which can be 62 miles wide; in its crater there are little new craters.