Volcanoes

Volcanoes normally originate in the mantle,the layer of earth about 80km below the crust.The heat inside this layer causes the rocks to melt and produce gas.The molten rocks,called magma,mix with the gas and become lighter than the surrounding rocks.Magma gradually rises upward making its way through the solid rocks above.Here it undergoes two types of action -intrusive and extrusive.In intrusive action,the magma solidifies within the faults and cracks and in its extrusive action,it moves upward.In its upward course,the magma forms a large magma chamber when it is just a few kilometers from the earth.There are solid rocks around magma chamber.The pressure of the solid rocks causes the magma to blast through the weaker parts crevices of the rock.Magma that finds its way through weaker rocks and spurts out is known as lava.

The volcanoes which have a constant emission of lava are known as active volcanoes.Those that spurt after a very long time span are dormant ones.The volcanoes which have erupted once and are not likely to emit lava ever again are known as extinct volcanoes.The crates of such volcanoes may fill with rainwater and eventually become a lake.