GPS measurements of ground inflation help forecast ash plumes

by Jesse Davenport
Thursday, February 20, 2014

When Grimsvötn Volcano in Iceland erupted in May 2011, northern European airspace was closed for seven days and about 900 passenger flights were canceled. Scientists were charged with trying to read the volcano — to tell how high the ash plume would go and to figure out how long the violent eruption would last. Such features are hard to predict, but in a novel study, one research team has found a correlation between the height and composition of the Grimsvötn ash plume and ground deformation before and during the eruption. The findings, the team says, could improve volcanic plume dispersion models, which in turn could help air traffic managers determine when and where it’s safe to fly when volcanoes like Grimsvötn and Eyjafjallajökull suddenly erupt.

Perched atop the mid-Atlantic ridge, which separates the Eurasian Plate from the North American Plate, Iceland is frequented by volcanic eruptions. Grimsvötn is Iceland’s most active volcano. Before the 2011 eruption, Grimsvötn spewed lava and ash most recently in 2004. The 2011 eruption is the largest recorded from the volcano since an eruption in 1873. Grimsvötn is also one of the most well-instrumented volcanoes in Iceland, with GPS sensors monitoring uplift and unrest since 1992. Various international collaborations have continued to build up the GPS monitoring network throughout Iceland to close to 80 stations in the past 20 years. Iceland is also home to an extensive network of 60 seismic monitoring stations, five strain-monitoring stations and two radar stations. The combination of observations from all of these techniques help scientists monitor volcanic and seismic activity in real-time and study patterns associated with individual events.

Before and during an eruption, magma can significantly deform land above the chamber; by monitoring the deformation, geologists can learn about the volume, shape and depth of the chamber. Sigrún Hreinsdóttir, a geophysicist at the University of Iceland, says that she and her colleagues wondered if they could use GPS measurements of this ground deformation to say something about what is happening inside the volcano and how that activity might affect the eruption aboveground.

During the 2011 Grimsvötn eruption, Hreinsdóttir and her team — using a combination of GPS, seismic and radar monitoring before and during the event — measured changes within the Grimsvötn magma chamber as well as corresponding changes around the volcano. A massive swarm of earthquakes told the team that Grimsvötn had begun to erupt. Hreinsdóttir and her team then translated the GPS data into a model that could explain what happened inside the magma chamber.

The conclusion based on this model was simple and intuitive, Hreinsdóttir says, but until now had not been shown. Basically, the height of a volcanic plume directly correlates to the amount of pressure in a magma chamber. The more pressure in the magma chamber, the faster it expels its magma, and therefore the higher the volcanic plume. Using the GPS data and their model, the team estimated the height of the ash plume to be between 15 and 20 kilometers. Hreinsdóttir and her colleagues then checked the model results with data from nearby weather stations, which indicated the volcanic ash plume indeed reached a height of 20 kilometers.

Such calculations were possible, Hreinsdóttir says, because “we were getting data points almost every second in real time.” The combination of real-time GPS monitoring and magma chamber models can potentially be used to forecast future volcanic eruptions, or at least to provide a clearer view into magma chambers, the team reported in Nature Geoscience.

Many factors make forecasting a volcanic eruption difficult, such as new batches of magma entering a magma chamber, but Hreinsdóttir and her colleagues demonstrated the correlation between ground deformation before and during the Grimsvötn eruption and the height of the ash plume, wrote Paul Segall, a geophysicist at Stanford University, in a commentary in the same issue of Nature Geoscience. He noted that the team’s correlation between deformation and ash from the relatively simple eruption at Grimsvötn in 2011 is encouraging to scientists everywhere monitoring and forecasting eruptions. By using the data and capabilities of satellite and GPS systems, this team has shown that scientists may be able to provide timely warnings of possible eruptions, Segall wrote.

“There are many things that make it hard to predict volcanoes,” such as the small time window for data collection immediately before or during an eruption and volcanoes' rather “infrequent occurrence,” Hreinsdóttir says. “But we knew about Grimsvötn months before it happened — thanks to intensifying seismic events — and we are learning more and more with each new eruption.”

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