How are volcanoes caused?

The main causes of volcanism can be summarised as follows:

1. The temperature inside the earth's crust increases with increase in depth at the rate of 1°C per 32 metres. The high temperature and pressure inside the earth crust, which increases with increase in depth is responsible for instability. Molten magma under great pressure has the capacity to liquefy great volume of gases.
2. The surface water that percolates through the cracks and crevices, turns into steam due to hot rocks. The steam being very powerful, forces itself through the weak points of the earth crust along with molten material.
3. The plate tectonics or the movement and splitting of the major and minor plates of the earth cause cracks, crevices and fissures in the earth's crust through which the magma is poured out.