The surface of the earth is made up of large plates which we call the continents. These plates float on the sea of molten rock which is called Magma.  It is under great pressure and is of very high temperature. The surface of earth which feels stable is in fact constantly in motion and the continental plates are continuously rubbing against each other. Sometimes these plates even go under one another. This continuous movement causes earthquakes and volcanoes.

Volcanoes erupt where the surface of the earth has softened due to the activity between plates and the magma and hot gases erupt from that part of the surface.

Volcanoes exist in each part of the world however they may be either dormant or active. Dormant volcanoes may go through violent activity over a few hundred to several thousand years before becoming active suddenly.

The three reasons often cited for a volcanic eruption are density, gases and extra magnum.

As we know, the earth has a solid core, a liquid outer core, a more or less solid mantle and a solid crust. The magma is hot and hotter parts melt and need more space. These parts become less dense and rise to the surface. Once its density becomes lighter than the rocks in the crust around the volcano, it erupts.

Water vapors and other gases are dissolved in magma and cooler the magma the more gases can be dissolved in it. But as the magma gets hotter, its capacity to hold the gases decreases. As both the magma and gases occupy the same place inside the chamber of the volcano, it erupts when the volume of gases exceeds too much in the chamber.

Extra magma is the commonest cause of an eruption. When the magma under the volcano becomes too much and since it is less dense, it takes the path with least resistance and the volcano erupts.