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Magma Boils Beneath Antarctic Ice
17 November 2013 1:00 pm
Marie Byrd Land is a desolate region of Antarctica buried deep beneath the West Antarctic Ice Sheet. But while the surface of Earth may be frozen, underneath is a different story. Historic eruptions have punctured the ice sheet, creating a chain of volcanoes amid the ice. Now, researchers have shown that molten rock still stirs deep underground. Only the largest eruptions could melt all the ice above them and poke through to the surface, but even smaller eruptions could potentially cause global sea level to rise, although no one knows how big the rise might be.
The curiously named Executive Committee Range is a series of volcanoes in Marie Byrd Land lying in a roughly straight line, with the volcanoes becoming progressively younger as one heads south from Whitney Peak, which is between 13.2 million and 13.7 million years old, to Mount Waesche, which formed in the last 1 million years. The crust is thinned by the West Antarctic Rift System, a series of giant rift valleys beneath the ice sheet, and erupted lava from underground magma chambers has burst through the ice repeatedly over geological history as the plates moved over the top. No one knew whether magma was still stirring underneath the Executive Committee Range, however, until seismic monitoring stations were installed on the ice between 2007 and 2010.
Researchers built the stations to study the shifting crustal blocks of the West Antarctic Rift System. But seismologist Amanda Lough of Washington University in St. Louis in Missouri and colleagues in California, Pennsylvania, Colorado, Texas, Washington, and Ohio found another use. They noticed a series of small earthquakes, mainly occurring during two “seismic swarms” in January and February 2010 and March 2011. These earthquakes were unusual: The ground was shaking much more slowly during the quakes than one would expect from the plates grinding against each other.
Lough worked out the origin of these tremors. “I looked at two different types of waves that come in—the P wave, which is the primary wave, and the S wave, which is the secondary wave,” she says. Her calculations revealed that the waves had come from 25 to 40 kilometers below Earth's surface and were centered around a point 55 kilometers south of Mount Waesche—approximately the point the volcanic activity should have reached if it had continued the linear trend of volcanoes to the south. The exact cause of these deep quakes is not understood, but they are thought to result from the movement of magma deep below active or soon-to-be active volcanoes. Scientists have recorded them around other active volcanoes in Hawaii, for example, sometimes around the time of eruptions.
Geomagnetic data and radar mapping by Lough's colleagues confirmed the presence of magmatic activity within the rocky crust. They found that the area showed a slightly higher magnetic field than the surrounding area and that there was a bump in the crust—common signals of magmatic activity. Radar mapping also indicated a layer of volcanic ash embedded in the ice. The team thinks this probably came from an eruption of Mount Waesche about 8000 years ago—very recent geological history. There is no evidence of an actual eruption since then, but, because magma is still moving deep under the Earth, an eruption could occur at any time, the team reports online today in Nature Geoscience.
The current center of volcanic activity is covered by at least 1 kilometer of ice, and it would take an exceptionally large eruption to melt all this. But an eruption could make its presence felt in subtler ways. As fresh snow adds to their own mass, ice sheets flow downward into the sea. By melting the base of the ice sheet, an eruption could speed up this flow, potentially raising the level of the ocean. No one knows how significant such a rise might be.
Any effect on the ice sheet above, and thus any effect on the oceans, would probably be quite small, says glaciologist Robert Bindschadler of the NASA Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the work. Still, he says, a proper study is needed to find out how significant volcanic activity could be to future sea levels. “It's a wild card.”