Crack in the Earth 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 ashes (rock fragments). Most volcanoes arise on plate, 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 crust is thin. There are two main types of volcano:

Composite volcanoes such as Stromboli and Vesuvius in Italy, are found at destructive plate margins (areas where plates are being pushed together), usually in association with island arcs and coastal mountain chains. The magma often clogs the volcanic vent, causing violent eruptions as the blockage is blasted free, as in the eruption of Mount St Helens, USA, in 1980.

Shield volcanoes, such as in Hawaii, are found along the rift valleys and ocean ridges of constructive plate margins (areas where plates are moving apart), and also over hot spots. The magma is derived from the Earth mantle and is quite free-flowing. The lava formed from this magma - usually basalt - flows for some distance over the surface before it sets and so forms broad low volcanoes.

The type of volcanic activity is also governed by 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 giving debris and shield volcanoes giving 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 times 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.

There are more than 1,300 potentially active volcanoes on Earth. Volcanism has helped shape other members of the Solar System, including the Moon, Mars, Venus, and Jupiter moon too.

There are several methods of monitoring volcanic activity. They include seismographic instruments on the ground, aircraft monitoring, and space monitoring using remote sensing satellites.