What causes volcanoes to erupt?

By studying bits of material trapped in volcanic crystals, Fangquiong Lu, a University of Chicago graduate student, has poked some holes in a common theory of why volcanoes erupt. Scientists know that molten rock from deep in the Earth gathers in chambers near the planet's surface. The magma cools in these chambers for many thousands of years, then bursts out in sudden eruption. Many believe these discharges are triggered by new magma entering the chamber from below. Lu maintains that this can not be the entire story. Her research indicates that some of the old magma remains in the ground up to 1,000 years after new magma first enters the chamber.

Lu examined hundreds of volcanic rock samples from the area around Bishop, Calif., where there was a massive eruption 700,000 years ago. Volcanoes in this class can eject up to several thousand cubic kilometers of lava and ash, enough to spread a layer one foot deep over the entire U.S. None of these volcanoes have erupted in recent times, but magma chamber systems of this type are still active in Yellowstone National Park; Naples Bay, Italy; and the island of New Guinea.

Three types of equipment--an electron microprobe, X-ray synchrotron, and ion microprobe--were used to study the rock from the Bishop area. Lu looked at "inclusions"--tiny drops of liquid magma and crystal that were trapped in larger crystals as the molten material cooled in the...