East Antarctica is sliding sideways
by Staff Writers
San Francisco CA (SPX) Dec 12, 2013
It’s official: East Antarctica is pushing West Antarctica around. Now that West Antarctica is losing weight–that is, billions of tons of ice per year–its softer mantle rock is being nudged westward by the harder mantle beneath East Antarctica.
The discovery comes from researchers led by The Ohio State University, who have recorded GPS measurements that show West Antarctic bedrock is being pushed sideways at rates up to about twelve millimeters–about half an inch–per year. This movement is important for understanding current ice loss on the continent, and predicting future ice loss.
They reported the results on Thursday, Dec. 12 at the American Geophysical Union meeting in San Francisco.
Half an inch doesn’t sound like a lot, but it’s actually quite dramatic compared to other areas of the planet, explained Terry Wilson, professor of earth sciences at Ohio State. Wilson leads POLENET, an international collaboration that has planted GPS and seismic sensors all over the West Antarctic Ice Sheet.
She and her team weren’t surprised to detect the horizontal motion. After all, they’ve been using GPS to observe vertical motion on the continent since the 1990’s.
They were surprised, she said, to find the bedrock moving towards regions of greatest ice loss.
“From computer models, we knew that the bedrock should rebound as the weight of ice on top of it goes away,” Wilson said. “But the rock should spread out from the site where the ice used to be. Instead, we see movement toward places where there was the most ice loss.”
The seismic sensors explained why. By timing how fast seismic waves pass through the earth under Antarctica, the researchers were able to determine that the mantle regions beneath east and west are very different. West Antarctica contains warmer, softer rock, and East Antarctica has colder, harder rock.
Stephanie Konfal, a research associate with POLENET, pointed out that where the transition is most pronounced, the sideways movement runs perpendicular to the boundary between the two types of mantle.
She likened the mantle interface to a pot of honey.
“If you imagine that you have warm spots and cold spots in the honey, so that some of it is soft and some is hard,” Konfal said, “and if you press down on the surface of the honey with a spoon, the honey will move away from the spoon, but the movement won’t be uniform. The hard spots will push into the soft spots. And when you take the spoon away, the soft honey won’t uniformly flow back up to fill the void, because the hard honey is still pushing on it.”
Or, put another way, ice compressed West Antarctica’s soft mantle. Some ice has melted away, but the soft mantle isn’t filling back in uniformly, because East Antarctica’s harder mantle is pushing it sideways. The crust is just along for the ride.
This finding is significant, Konfal said, because we use these crustal motions to understand ice loss.
“We’re witnessing expected movements being reversed, so we know we really need computer models that can take lateral changes in mantle properties into account.”
Wilson said that such extreme differences in mantle properties are not seen elsewhere on the planet where glacial rebound is occurring.
“We figured Antarctica would be different,” she said. “We just didn’t know how different.”
Ohio State’s POLENET academic partners in the United States are Pennsylvania State University, Washington University, New Mexico Tech, Central Washington University, the University of Texas Institute for Geophysics and the University of Memphis. A host of international partners are part of the effort as well. The project is supported by the UNAVCO and IRIS-PASSCAL geodetic and seismic facilities.
|Mount Sidley is the youngest volcano rising above the ice in West Antarctica’s Executive Committee Range. A group of seismologists has detected new volcanic activity under the ice about 30 miles ahead of Mount Sidley.
Credit: Doug Wiens
SAN FRANCISCO — A big, hot blob hiding beneath the bottom of the world could be evidence of a long-sought mantle plume under West Antarctica, researchers said Monday (Dec. 9) here at the annual meeting of the American Geophysical Union.
The possible hotspot — a plume of superheated rock rising from Earth’s mantle — sits under Marie Byrd Land, a broad dome at West Antarctica’s edge where manyactive volcanoes above and below the ice spit lava and ash. The hot zone was discovered with seismic imaging techniques that rely onearthquake waves to build pictures of Earth’s inner layers, similar to how a CT scan works. Beneath Marie Byrd Land, earthquake waves slow down, suggesting the mantle here is warmer than surrounding rocks. The strongest low-velocity zone sits below Marie Byrd Land’s Executive Committee Range, directly under the Mount Sidley volcano, said Andrew Lloyd, a graduate student at Washington University in St. Louis.
“The slow velocities suggest that it’s a mantle hotspot,” Lloyd said. The hot zone also matches up with Marie Byrd Land’s high topography and active volcanoes, Lloyd said.
Many researchers have long suspected that Marie Byrd Land sits atop a hotspot, because the region swells above the surrounding topography like the top of a warm soufflé (and it has lots of volcanoes). But with few seismometers sitting on the ice, scientists were left speculating about what lies beneath Antarctica’s ice.
The evidence for the new hot zone, called a thermal anomaly, comes from a massive, temporary earthquake-monitoring network called Polenet that was installed between 2010 and 2012, giving scientists an unprecedented look at Antarctica’s crust and mantle. (A gravity survey conducted at the same time also suggests there is a big warm spot beneath this part of West Antarctica.)
But confirming that Marie Byrd Land is truly above a hotspot may require a return trip to Antarctica for another seismic experiment, said Doug Wiens, principal investigator on Polenet.
“What’s absolutely sure is there’s a big thermal anomaly, a big blob,” said Wiens, a seismologist at Washington University. “What’s less sure is whether that anomaly goes deeper.”
The thermal anomaly extends 125 miles (200 kilometers) below Marie Byrd Land, Lloyd said. Below about 255 miles (410 km), where a mantle plume’s trailing tail would also leave a hotter-than-average mark in mantle rocks, there’s little evidence for a rising hotspot, said Erica Emry, a postdoctoral researcher at Pennsylvania State University.
“There’s no smoking gun,” Emry said. However, more work remains to be done on the Polenet data, which could reveal new clues and further refine what the mantle looks like under West Antarctica, Emry told LiveScience’s OurAmazingPlanet.
Antarctica’s thinnest crust
The discovery is one of many new insights reported Monday into the geologic mysteries concealed by Antarctica’s thick ice. Other findings include extremely thin crust, just 10 miles (17 km) thick, in West Antarctica’s Ross embayment near the Transantarctic Mountain Range, said Xinlei Sun, a postdoctoral researcher at Washington University. The Ross embayment is one of Antarctica’s two big coastal divots; the gap is filled by the Ross Ice Shelf. Here, the crust is as thin as in the Gulf of California, where continental rifting (also called extension) is tearing Baja California from mainland Mexico and building a new ocean basin.
“This is the thinnest crust [in Antarctica] and is probably related to an extensional environment,” Sun said.
On the other side of the Transantarctic Mountains lies the thick, old crust of East Antarctica, similar to the relatively stable interiors of continents such as North America and Africa. Antarctica’s thickest crust is found here, beneath the Gamburtsev Mountain Range. The Gamburtsevs are spectacular Alpine peaks completely buried in ice; the crust here is about 31 miles (50 km) thick. The crust beneath Marie Byrd Land is about 15 miles (25 km) thick, Sun said.
Scientists Find New Volcano Rumbling Under Antarctica ice: 1,370 tremors: “It may blow or it may not. We don’t know.”
November 18, 2013 – ANTARCTICA – A volcano may be stirring more than a half-mile beneath a major ice sheet in Antarctica, raising the possibility of faster base melting that could ultimately affect climate. Seismologists working in a mountainous area of Marie Byrd Land in western Antarctica detected a swarm of low-magnitude earthquakes in 2010 and 2011 similar to those that can precede volcanic eruptions, according to a study published online Sunday in Nature Geoscience. The area of activity lies close to the youngest in a chain of volcanoes that formed over several million years, and the characteristics and depth of the seismic events are consistent with those found in volcanic areas of Alaska’s Aleutian Islands, the Pacific Northwest, Hawaii and Mt. Pinatubo in the Philippines, the study concludes. The tight focus of the 1,370 tremors and their deep, long-period waves helped researchers rule out ice quakes, glacial motion or tectonic activity as causes. So, too, did their apparent depth: At 15-25 miles beneath the sub-glacial surface, they are close to the local boundary between Earth’s crust and mantle. “At first I had no idea it was something volcanic, and then, as I started putting together all the pieces, it started looking more and more like I’d found a volcano,” said study co-author Amanda Lough, a seismology graduate student at Washington University in St. Louis.
Chances of a massive fire-and-ice catastropheare slim, however. Clusters with these wave characteristics have sometimes preceded eruptions, but not always, Lough said. And it would take a staggering release of energy to punch through more a half-mile of ice, she noted. “Does this mean that something is going to be happening in the next 20 years or so? I have no idea,” she said. “It’s not something that’s going to cause major issues. You’d have to have a huge, huge eruption.” Still, even a small eruption could increase base melting and lubricate the ice sheet. “If you have a future eruption it’s going to increase the heat flow, so you’re going to have more melting in the surrounding area, which will then lead to more water at the base of the ice sheet and cause the overlying ice flow to increase in velocity because it’s been lubricated.” How such melting could affect the vast ice sheet remains unknown. The data came from seismic equipment used to study the interaction of ice with the crust in Antarctica, part of a broader program studying polar ice caps. That data were collected in 2010-2011, but most of the events analyzed occurred in two swarms during the first two months of 2010 and in March 2011.
Radar imaging also revealed a buried ash layer believed to be from an eruption of Mt. Waesche about 8,000 years ago. There also is evidence of small flows of magma on the sub-ice topography, and the surface closest to the swarm appears to be a mound of volcanic material, according to the study. Lough, who expects to complete her PhD work next year, has been bowled over by reaction to the discovery. “This is my first paper and people keep calling me wanting to know about it,” she said. “This is going to be one of the chapters in my thesis — probably the most exciting one.” The study was led by geophysicist and seismologist Douglas Wiens of Washington University, who is supervising Lough’s thesis. Other research team members hailed from UC Santa Cruz, Penn State University, New Mexico Tech, Colorado State University, the University of Texas at Austin, Central Washington University and Ohio State. –LA Times
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