The centre of the earth is hot and molten. The temperature rises as we go deeper form the surface. When rocks melt, they expand, creating new mountains. Under them, there would be a space of low pressure. Molten lava fills up this place, building pressure. When the pressure exceeds the strength of the overlying crust of the Earth, it breaks out as a volcanic eruption.
The eruption lasts until the gas is gone and pressures decreases. What comes out is mainly gaseous, but large quantities of molten rock, called lava and solid particles that look like cinders and ash are also thrown out.
Central America, bordering the Pacific Ocean is an area of one of the most active volcanic activity in the world. Nearly two-third of the active ones and a large number of the dormant volcanoes are found in this area.
Volcanoes can cause great loss of life and property if it happens in a place of human dwelling.
Below Volcanoes 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/Volcanoes
