Throughout Earth's turbulent geological history, volcanoes have relentlessly erupted vast quantities of molten rock onto the Earth's surface. Of the 1500 or so'active' volcanoes around the world today, about 60 erupt each year and there are around 20 currently in eruption at any one time. But with its upper 2900km essentially solid, how does the Earth produce so much molten rock, where does it all come from and why are volcanoes confined to certain well-defined zones?