"If you think of a freshman geology textbook, there is always a picture of a volcano with a magma chamber, large compared to the volcano. I think that most people walk away from that with the sense that these magma chambers exist for millennia and that -- for some reason never quite spelled out -- once in a while they burp out a bit of magma, which becomes lava when it comes to the surface," says Nye. "But in fact things are happening much more quickly than that, on time scales of days or weeks for magma to move from a magma chamber and then up to the surface to erupt."

The world's most explosive -- and devastating -- volcanic eruptions usually occur in subduction zones, because subducting oceanic plates are soaked with water, and that water helps the overlying rock melt. Ultimately, the result is a particularly gassy magma. This andesitic magma, as it is called, is very viscous -- that is, resistant to flow, like maple syrup compared to water. Such is the case in the Cascades Range of the Pacific northwest, the home of Mount St. Helens and 14 other large volcanoes.

Andesitic magma impedes the escape of gases out of the magma chamber. The trapped gases form bubbles and pockets in the magma. Eventually, the pressure of the collected gases rises so high that they blow through the magma like a cork out of a champagne bottle. The result is an explosion of gas, ash, and fiery fragments of volcanic rock.

Explosive volcanoes typically have a characteristic shape -- tall, with a steep summit, created out of alternating layers of lava and volcanic rock fragments -- known as a composite cone or stratovolcano. Many of history's most famous volcanoes -- Etna, Vesuvius, St. Helens, Fujiyama -- are stratovolcanoes. Page 3