Volcanoes are among the most dramatic features of the earth, and a sure sign that the planet is alive. But what we see on land is only a small part of the story. In the middle of the oceans about 5 cubic miles of liquid rock is added to the sea floor every year to make new plates. This activity poses a major paradox. We know that the outer part of the earth is solid. How then is it able to produce such quantities of molten rock? With submersible ships we can travel to where plates are created at mid-ocean ridges, where we find a sea floor belching clouds of superheated black water from chimneys. The runny magma that pours from the earth where plates separate can flow huge distances on land and make spectacular displays of fountains and bubbling cauldrons of liquid rock. By contrast, the magma produced where plates converge is often much stickier and hardly flows at all; instead it erupts in catastrophic explosions. Why should these magmas be so different? (60 minutes)