Page last updated at 16:42 GMT, Monday, 8 September 2008 17:42 UK

Animated guide: Volcanoes

Graphic showing the upward movement of molten rock underground leading to a volcanic eruption
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The Earth is a 4.5-billion-year-old ball of molten rock with an iron core and a cool crust.

In-between lies a solid mantle.

The crust is made up of plates, moving at the rate of about 10cm a year. They are propelled by convection currents in the mantle.

Areas along edges of plates are prone to geological upheaval. Molten rock magma finds its way into the upper crust. If it reaches the surface, it becomes a volcano.

An eruption is caused by the pressure of dissolved gas building up in the magma. When it becomes greater than the surface can take, the volcano erupts.

Large rocks thrown out during the eruption bombs - crush and set fire to anything they hit.

A super-hot avalanche of rock and ash pyroclastic flow - engulfs everything in its path in dense ash.

Ash and toxic gases, including sulphur dioxide, create an eruption cloud, which pollutes the atmosphere over a large area and can contribute to acid rain.

Lava - molten rock - flows slowly, but is very destructive, burning anything in its path and cutting off escape routes.

Mud flows can be swift, silent and lethal, engulfing homes many kilometres away from the volcano.

A stratovolcano is formed by layers of lava from successive eruptions.

But other types of volcano also exist.

Caldera volcanoes are formed when the dome of a stratovolcano collapses, leaving a broad circular vent.

Shield volcanoes are formed by eruptions of runny basalt lava which flows swiftly and does not build up in a cone shape.

Most of the 550 active volcanoes are found along the edges of the Earth's plates.

More than half of these encircle the Pacific Ocean in the so-called "Ring of Fire".

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