The whole of the earth consisted of a single piece of land which later broke into pieces and that is why we have the seven continents at presents. The earth was divided into plates and these plates continue to collide with each other.

When these two plates hit each other, one goes under the other and due to the pressure becomes so hot that it melts. This molten stuff is called magma. This molten material is looking for an escape and it finds a way out through a hole in the surface of the earth.

When the magma explodes out, it  is called lava. The explosions in the volcanoes are caused because of the pressure of gas that builds up under a volcano.