What Makes a Volcano Erupt?
Even though the earth is 4.6 billion years old, it is still generating heat, and that's due to radioactive decay, says USGS volcano expert John Eichelberger. "And more heat is getting generated than can just calmly flow out -- conduct out of the planet. So excess heat builds up, and this causes melting. So volcanoes are earth sweat, in a way...."