Not-so-early warning

In the minutes or hours before an explosive volcano erupts, it trembles. The tiny vibration, easily measured with seismology equipment but usually too subtle for humans to feel, is the best indication that the volcano is about to blow.

Scientists have known about this signature rumble for decades, but its cause remained a mystery. Regardless of the shape or size of the volcano, every one vibrates at a similar frequency before erupting. In Nature, a pair of geophysicists has finally proposed a simple equation to explain the phenomenon.

David Bercovici of Yale, one of the authors, studies the earth’s mantle—but not, typically, volcanoes. He got interested in the tremor phenomenon after chatting with a colleague at a seminar in British Columbia. On the flight home, he worked on the problem. By the time he touched down in Hartford, he’d worked out the “simple, elegant model” at the heart of the paper. “I was so excited, I got off the airplane and I called him to tell him the result,” Bercovici says. They talked so long that when he hung up, “I looked around and I was the only one in the airport lobby.”

The formula posits that the tremor is caused when rising magma in the volcano cone “rattles” inside a sheath of hot gases. “All the stuff around it is like bubble wrap,” Bercovici says. “It basically bobs and shakes or moves back and forth within the bubble wrap.” According to the team’s calculations, the shifting magma inside that gassy sheath will always generate a similar low tremor, regardless of the geometry of the volcano. Other scientists had proposed theories, but they all involved more variables and predicted the tremors less accurately.

The equation alone can’t make eruptions any easier to forecast, but Bercovici thinks the work may give experts a better understanding of the dynamics inside the volcano, which in turn may enable them to find out further in advance when an eruption is coming. A good physical model “gives you a better handle on how to use [the tremor] for prediction if you don’t even know where to start,” he says.