Volcanoes can be unpredictable beasts, threatening not only people living nearby but aircraft flying over and downwind of their ash plumes.
An eight-day-long eruption on Iceland in May 2011 sent a blast of ash--what scientists call tiny flecks of fractured rock--20 kilometers into the sky, closing airspace downwind and cancelling hundreds of flights for four days during the eruption.
A new study of data gathered from the eruption of that Icelandic peak suggests that scientists might be able to better predict the onset of an impending eruption—and, for the first time, estimate the height of the resulting ash plume—if the right sensors are installed on a rumbling volcano.
Weeks before the eruption of Iceland’s Grímsvötn volcano, the island nation’s most active, a nearby network of seismographs had picked up signs of increasing unrest beneath the peak, said Freysteinn Sigmundsson, a volcanologist at the University of Iceland in Reykjavik.
A pre-eruption boost in seismic activity isn’t unusual, and has been long noted at restive volcanoes elsewhere, Sigmundsson noted. But in this case, GPS equipment and tiltmeters—an instrument that measures the inclination of the terrain— installed on the southern rim of one of the craters atop the largely ice-covered Grímsvötn in 1992 provided even more data.
Before now, eruption forecasts have often been informed only by data from seismometers, which provide only rough insights into magma movement inside a grumbling peak.
Ever since Grímsvötn’s previous eruption in 2004, the sensors on its rim had discerned movement at the site upward and toward the southeast, away from the center of the crater complex—a trend suggesting that molten rock was recharging the magma chamber beneath Grímsvötn’s caldera, said Sigmundsson.
But on the afternoon of May 21, 2011, the sensors began moving in the opposite direction. One hour later, lava and ash burst from a fissure on the southwestern edge of the crater, about 6 km away.
During the following week, Grímsvötn spewed an estimated 270 million cubic meters of material—and all the while, the sensors sank toward the northwest, monitoring the volcano’s deflation.
Analyzing the data, Sigmundsson and his colleagues found that the sensor’s movement—and in particular, how quickly the sensors moved—provided insights into how pressure was changing in Grímsvötn’s magma chamber. Those pressure changes, in turn, were linked to fluctuations in the height of the ash plume spewing from the peak, the researchers noted.
"This is a very exciting development," said Sigrún Hreinsdóttir, a geophysicist now at GNS Science in Lower Hutt, New Zealand, and a leading member of the team. The findings suggest that with the proper instruments and with near-real-time analysis, researchers could track an eruption as it unfolds and possibly gain insights into how long it might last, she said.
The new findings are the first to measure the height of an ash plume and correlate it with data gathered from the volcano, said Paul Segall, a geophysicist at Stanford University in California. "These observations are very encouraging, and offer hope that volcanoes can be modeled and eruptions can be forecast," he noted.
Nevertheless, Segall admitted, such models might have their limits. For example, between one eruption and another, the characteristics of magma beneath a volcano can change.
Even subtle variations in the chemical composition of that molten material—especially in the concentrations of volatile substances, such as water, dissolved in the magma—can give an eruption a dramatically different personality, from explosive in one instance to gentle oozing in another.
"It’s a long journey ahead, but we’re starting down the path to more physics-based models of volcanoes," said Segall.
The study of Grímsvötn volcano was published January 12 in the journal Nature Geoscience.