From Envirowiki

A Volcano is a vent of molten rock and/or gasses from the Earth's mantle through the crust and on to the surface. Volcanoes are sometimes violent and devastating to their immediate surrounds, covering the land with ash and mud.

Volcanoes can have an impact on the global climate: some release greenhouse gasses, while others sometimes release sulphur dioxide, which, when pushed high enough, can become sulfuric acid in the upper atmosphere, and increase reflection of the sun's heat, causing global cooling. However, Only fairly large eruptions cause this on a global scale - ie. over a million tonnes of S02.