In the interior of the Earth, the temperature is very high. At the centre of the Earth, the temperature could be as high as 10,000 degrees Fahrenheit.
When the rocks melt inside the earth, it expands and needs more space. When the pressure is very great, it bursts out causing an eruption.
When a volcano erupts, it throws out hot gaseous liquid called lava. The material piles up around the opening and a cone-shaped mound is formed.
Below Volcano info from Wikipedia (Read More):-
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface. The word volcano is derived from the name of Vulcano island off Sicily which in turn, was named after Vulcan, the Roman god of fire.[1]
Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes.
http://en.wikipedia.org/wiki/Volcano
