When Will It Blow?

PREDICTING VOLCANIC ERUPTIONS IS A NOTORIOUSLY inexact science, and no one knows that better than people along the "ring of fire" that extends through Southeast Asia. Without warning last week the Philippines' Mayon volcano blew up, killing at least 68 and driving thousands from their homes.

There is hope, though, that forecasts will improve. In Nature, geophysicist Hazel Rymer and colleagues at England's Open University reported a possible sign of an impending eruption: shifts in gravity. They found that the gravitational field around Italy's Mount Etna increased sharply six months before it spewed forth in December 1991.

Rymer cautions that gravity data should be used only with more conventional forecasting methods. Moreover, the technique can be dangerous, since researchers often climb into a volcano to take gravity readings. Last month Rymer's colleague Geoff Brown died when Colombia's Galeras volcano blew just as he was setting up his equipment.