A volcanic eruption is caused by the injection of magma into a full magma chamber, the buoyancy of magma and the pressure exerted by the gases in the magma. The volcano simply acts as an opening through which magma and dissolved gases are discharged.
Magma is formed when part of the Earth's lower crust or upper mantle melts. Since magma has a lower density compared to the surrounding rocks, it rises. As the magma rises, bubbles are formed from the dissolved gas within the magma. These gas bubbles exert immense pressure. This pressure brings magma to the surface and forces it into the air at great heights. During volcanic eruptions, cinders, red-hot ash and chunks of hot rock shoot out forming fiery clouds over the volcano.
Volcanic eruptions can occur without any prior warning, making them hard to predict. Understanding how a volcano works and how its eruptions can be predicted is important to the preservation and well-being of the inhabitants of volcanically vulnerable areas. There are three main classifications of volcanoes. Active volcanoes have recently erupted, dormant volcanoes have not erupted in a long while but are likely to erupt in the future and extinct volcanoes have not erupted in a long while and are not expected to erupt in the future.