What causes a volcano to erupt?


THERE are approximately 1500 potentially active volcanoes worldwide. Volcanoes occur as a result of the rise of magma (red-hot liquid rock inside the earth’s core) to the surface of the earth, causing bubbles of gas to appear which leads to pressure build up in the mountain that eventually erupts.

The three predominant factors of volcano eruption are the buoyancy of the magma, the pressure of the gases in the magma, and the injection of a new batch of magma into the existing magma chamber.

As rock inside the earth melts, its mass remains the same while its volume increases-producing a melt that is less dense than the surrounding rock. This lighter magma then rises toward the surface by virtue of its buoyancy. If the density of the magma between the zone of its generation and the surface is less than that of the surrounding and overlying rocks, the magma reaches the surface and erupts.

Furthermore, magma contains dissolved volatiles such as water, sulfur dioxide and carbon dioxide. As magma moves toward the surface, the solubility of the water in the magma decreases, and so the excess water separates from the magma in the form of bubbles. When the volume of bubbles reaches about 75 per cent, the magma disintegrates into partially molten and solid fragments and erupts explosively.

The third factor is the injection of magma into the existing magma chamber which forces some of the magma in the chamber to move up in the volcano conduit and erupt at the surface.