volcano

There are two main types of volcano, but three distinctive cone shapes. Composite volcanoes emit a stiff, rapidly solidifying lava which forms high, steep-sided cones. Volcanoes that regularly throw out ash build up flatter domes known as cinder cones. The lava from a shield volcano is not ejected violently, flowing over the crater rim forming a broad low profile.

Mount Etna photographed in eruption. The volcano can be extremely violent, and destroyed several towns in the 1950s, yet the area at its foot is densely populated, with vineyards, orchards, and orange groves. The first recorded eruption was in c. 700 BC.

A satellite photograph showing the Augustine volcano in Cook Inlet, south of Anchorage, Alaska. Alaska is at the northern end of the Pacific rim and experiences high levels of volcanic activity.

A knife-edge ridge of volcanic rock on the Napali coast of Kauai, one of the westernmost of the Hawaiian islands. Such a ridge may be formed when lava flows along a surface and drops into a crack or crevice, filling it up; the supporting sides eventually erode away, leaving only the shape of the original fissure.

From Hawaiian shield volcanoes, lava may flow smoothly and consistently for long distances. The smoothness of flow is partly due to the chemical composition of the lava which has a comparatively low proportion of silica (silicon dioxide) and a comparatively high proportion of calcium (as feldspar) and magnesium (as pyroxene).

Alaska has a number of active volcanoes, such as the Augustine Volcano in the southwest of the state. This is because Alaska is situated at the edge of a tectonic plate, as indeed are virtually all the countries of the Pacific rim.

The slopes of Helgafell, a volcano on the island of Heimaey, Iceland. Helgafell last erupted in 1973. The eruption created the new Eldfell cone and a 3 km/1.9 mi lava flow. Although the lava slopes have stabilized, the soil remains very hot, and at a depth of 1 m/3.3 ft, the soil and rock hold temperatures up to 300°C/572°F. In the far distance another volcanic island can be seen.

Mount Rainier in Washington State is the highest volcano of the Cascade Range. Although not especially active recently it has a significant cover of ice and snow, which if melted rapidly would produce catastrophic flooding of nearby populated areas.

A large snow-capped volcano in Iceland. Located on a plate margin where the North American and European plates diverge, Iceland has about 20 active volcanoes. An extensive field of jagged and irregularly shaped blocks of volcanic rock – an old lava flow – stretches into the distance.

Volcanic plug in Assekrem, Algeria. This outcrop is the remains of a mass of volcanic material which once filled the vent of a volcano. The softer rock surrounding it has eroded away, leaving behind the more resistant ‘plug’.

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.

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