http://www.helium.com/knowledge/6230-volcanoes-what-they-do-and-how-theyre-formed.xml Helium.com > Geology & Geophysics > Volcanoes, what they do and how they're formed en-us Copyright Helium.com Mon, 05 May 2008 19:39:55 GMT Mon, 05 May 2008 19:39:55 GMT