What is a volcano?

Volcanoes often resemble mountains. They are cracks in the earth which go from the surface right down to the hot, melted rocks below. The rocks are known as magma. The volcano erupts when the boiling magma rises up through the crack to the top, i.e., the earth’s surface. Then it loses its gases and turns into lava. The eruption is caused by pressure in the gases that are trapped inside the magma.