Answer:
Volcanoes erupt when molten rock called magma rises to the surface. Magma is formed when the earth's mantle melts. Melting may happen where tectonic plates are pulling apart or where one plate is pushed down under another. Magma is lighter than rock so rises towards the Earth's surface. The explosivity of an eruption depends on the composition of the magma. If magma is thin and runny, gases can escape easily from it.
