Thirty-two years ago this month an explosive eruption reshaped Mount Saint Helens in a matter of seconds. An earthquake under the volcano in Washington State on May 18, 1980, triggered the largest landslide in recorded history as billions of cubic meters of mountainside tumbled away, initiating a massive release of gas, lava and ash. The cataclysm killed 57 people and sent a plume some 20 kilometers into the sky.
The 1980 eruption was not totally unexpected. For the two months prior, earthquakes and steam explosions had rattled the mountain, and the north slope of the volcano had noticeably swelled outward. But interpreting just what was happening beneath the mountain to cause those shudders and belches was not possible.
Now a team in England and Germany has linked the seismic record from that era to magmatic processes beneath Mount Saint Helens by closely examining crystallized minerals formed in the volcano's innards just before eruption.
In a study appearing in the May 25 issue of Science, the researchers report that crystals of the silicate mineral orthopyroxene from 1980 and from subsequent eruptions trace various injections of magma, as well as other chemical changes, within the bowels of the volcano.
The crystals contain concentric rings of differing chemical composition. Some orthopyroxene crystals, for instance, have a magnesium-rich core surrounded by an iron-rich rim; others have an iron-rich core and a magnesium-rich rim. Each type of crystal zonation can record the conditions of the magma reservoir from which it emerged.
"We chemically fingerprint each of those zones to determine how they formed," says lead study author Kate Saunders, a volcanologist of the University of Bristol in England. The outer rim of an orthopyroxene crystal, she says, represents the most recent stage of crystal formation and typically grew just months before the crystal’s emergence in volcanic ejecta. That allowed the researchers to make precise estimates of when, and how, the crystals acquired their chemical forms. "Mount Saint Helens is really good—because the samples, we know exactly when they erupted," Saunders says.
In several cases, a flurry of crystallization matched up well with internal activity at the volcano, as inferred from seismic records. For instance, crystals with iron-rich rims increased in number in the weeks and months leading up to the May 1980 eruption, as earthquakes shook the mountain with low-frequency seismic waves. Such long-period quakes, Saunders says, are characteristic of magma giving off gas. "Previously it's been reported that iron-rich rims on orthopyroxene are due to changes in the water content of the magma, so it makes sense that if we've got degassing or fluxion of water or CO2 through the system, we may get more iron-rich rims," she says. Other crystals with magnesium-rich rims seem to mark an influx of hot magma from deep reservoirs just before eruption.
Those events are also captured in the seismic record, but the root cause of the tremors is not always clear from seismology alone. That is where the crystals can provide important corroboration—or critical insight into ancient eruptions, for which seismic records do not exist. In the case of Mount Saint Helens, the study's authors "are providing fairly convincing arguments that the periods of enhanced seismicity are related to inputs of magma," says Timothy Druitt, a volcanologist at Blaise Pascal University in Clermont–Ferrand, France. "Just because you've got seismicity doesn't mean you've got magma intruding at depth," he adds. "They're providing evidence that that's probably what happened. If you can establish that, then that helps you better interpret your seismicity.”