Explain in brief, how a volcanic eruption is caused.

The temperature inside the Earth increases at the rate of 1° per 32 metres of descent. It results in very high temperature as well as extreme pressure deep inside the Earth. The molten lava inside the Earth, turns surface water (which percolates through the cracks and crevices to the interior) into steam. This steam and other compressed gases put pressure on the Earth's crust. The movement of Tectonic plates cause cracks, crevices and fissures in the Earth's crust through which the magma is poured out.