VOLCANO

A volcano is a vent in the earth from which molten rock and gas erupt. The molten rock that erupts from the volcano forms a hill or mountain around the vent. The lava may flow out as a viscous liquid, or it may be exploded from the vent as solid or liquid particles.

The most fluid magmas are erupted quietly. They flow from the vent to form gently sloping shield volcanoes. The lava flows from the shield volcanoes are usually only 1 to 10 meter thick. They may extend for great distances away from the vent.

Magmas with high gas contents and high viscousities are usually more explosive. These gas-rich magmas are blown high into the air during an eruption. The magma falls as volcanic bombs, which accumulate around the vent. They form steep-sided cinder cones.

Most of the tallest volcanoes are stratovolcanoes. These form from a cycle of quiet eruptions of fluid lava followed by explosive eruptions of viscous lava.

A volcano may exhibit different styles of eruptions at different times. Eruptions may change from one type to another as the eruption progresses. The least violent type of eruption is termed Hawaiian. It is characterized by extensive fluid lava flows from central vents. Occasionally it is accompanied by lava fountains. Strombolian eruptions are characterized by moderately fluid lava flows. Vulcanian eruptions are marked by viscous magmas.

Volcanic activity has yielded a variety of economic resources. Volcanoes supply both lightweight and heavy aggregate, as well as pumice for abrasives and elements such as sulfur, copper and many other. Recently volcanic areas have become important sources of geothermal energy.