What happens when a volcano erupts?

Answer

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.

Reference:
Q&A Related to "What happens when a volcano erupts?"
To make a volcano erupt is simple if you have the right tools. If you are speaking of a model volcano there are steps to take that will make a volcano erupt. You can find more information
http://www.ask.com/web-answers/Science/Nature/how_...
A volcano erupts because the gas in it is trapped which will make it erupt
http://wiki.answers.com/Q/Where+are+volcanos+that+...
Volcanoes erupt as the earth's crust becomes heated and melts. This melted material is less dense than the rock surrounding it, and begins to rise. Eventually, this material breaks
http://www.ehow.com/facts_5344340_volcano-erupt.ht...
Inside the earth is pressurised red hot magma - trying to get out! When it finds a weak spot in the earths crust (usually where the tectonic plates are meeting) it "pops"
http://answers.yahoo.com/question/index?qid=201301...
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