DK Earth: Volcanoes
A volcano is a vent or weak spot in Earth?s crust through which magma (hot, melted rock) escapes as LAVA. In some places, lava oozes slowly out of the ground. In others, there is a violent eruption.
A volcano erupts when magma wells up from deep inside Earth. In violent eruptions, the magma fills a hollow chamber below a vent blocked by cooled and hardened rock. Gas and water mingle with the magma, forming an explosive mixture. The pressure builds up in the chamber until the magma, gas, and steam are forced upward and blast through the vent.
There are around 25 major volcanic eruptions on land every year and thousands of minor ones, many of which take place under the sea. Active volcanoes are those that may erupt at any time. Dormant (sleeping) volcanoes have not erupted for centuries but may still do so. Extinct volcanoes are no longer likely to erupt.
Lava is the name given to magma once it has reached the Earth?s surface. Lava may be thick and sticky or thin and runny, depending on the minerals the lava contains and the temperature and pressure when it was formed.
Volcanoes are different shapes depending on the type of lava and shape of their vents. Shield volcanoes have broad, shallow cones and are made of runny, flowing lava. Fissure volcanoes are long cracks in the crust. Violent eruptions usually produce steep-sided conical mountains. Composite cones are built up from alternate layers of lava and ash.