volcano

Crack in the Earth's crust through which hot magma (molten rock) and gases well up. The magma is termed lava when it reaches the surface. A volcanic mountain, usually cone-shaped with a crater on top, is formed around the opening, or vent, by the build-up of solidified lava and ash (rock fragments). Most volcanoes occur on plate margins (see plate tectonics), where the movements of plates generate magma or allow it to rise from the mantle beneath. However, a number are found far from plate-margin activity, on hot spots where the Earth's crust is thin, for example in Hawaii. There are two main types of volcano: composite volcanoes and shield volcanoes.

The type of volcanic activity also depends on the age of the volcano. The first stages of an eruption are usually vigorous as the magma forces its way to the surface. As the pressure drops and the vents become established, the main phase of activity begins. Composite volcanoes emit pyroclastic debris, while shield volcanoes produce lava flows. When the pressure from below ceases, due to exhaustion of the magma chamber, activity wanes and is confined to the emission of gases, and in time this also ceases. The volcano then enters a period of quiescence, after which activity may resume after a period of days, years, or even thousands of years. Only when the root zones of a volcano have been exposed by erosion can a volcano be said to be truly extinct.

Many volcanoes are submarine and occur along mid-ocean ridges. The main volcanic regions are around the Pacific rim (Cape Horn to Alaska); the central Andes of Chile (with the world's highest active volcano, Guallatiri, 6,063 m/19,892 ft); North Island, New Zealand; Hawaii; Japan; and Antarctica. There are more than 1,300 potentially active volcanoes on Earth. Volcanism has also helped shape other members of the Solar System, including the Moon, Mars, Venus, and Jupiter's moon Io.