Active volcanoes shake, rattle and roll; scientists can’t always be sure whether such activity indicates that molten rock is rising. Now, volcanologists have linked earthquake swarms at Mount St. Helens in Washington state with fresh magma pulses. A team led by Kate Saunders of the University of Bristol in England studied more than 300 rock crystals formed during the 1980–1986 eruption of Mount St. Helens. These crystals (one shown), of a mineral called orthopyroxene, grow in zoned “rims,” each formed as fresh magma arrives from below and cools. Saunders’ team calculated when the crystals grew and compared that with the timing of small earthquakes shaking the mountain. In many cases the crystal rim growth and seismic swarms matched, the team reports in the May 25 Science. Studying crystals erupted from other volcanoes could help scientists better understand how often new magma arrives, Saunders says.
