Volcanoes erupt to release pressure built up in the magma chamber by expelling the magma as lava. Along with lava, a volcanic eruption releases ash and greenhouse gases such as sulfur dioxide and carbon dioxide.
The intensity of a volcanic eruption depends on how viscous the magma is. If the magma is very viscous, then the eruption tends to be more explosive, with rocks and ash shooting up several feet. But if the magma is not viscous, the magma may gently flow out of the crater to form lava streams. Some volcanic eruptions are accompanied by pyroclastic flows. A pyroclastic flow is a fast-moving mass of hot gas and rock that flows down the side of the volcano. If ash is released during the eruption, the heavier particles fall back to the ground. The ash is very dense, and if it accumulates on roofs, the weight of the ash can cause the roof to collapse. The lighter particles remain suspended in the air and can block out the sun. People living in areas surrounding a volcano can have difficulty breathing during an eruption due to the ash suspended in the air. Greenhouse gases such as sulfur dioxide and carbon dioxide are released into the atmosphere and can condense as acid rain.