Scientists harness natural forces to image earth beneath Antarctica's ice
Posted May 20, 2011
Scientist Philip Wannamaker lets nature do half the work when it comes to studying the deep geology of the Earth.
A geophysicist at the University of Utah’s Energy and Geoscience Institute , Wannamaker led an international team of researchers on a field project to the central Transantarctic Mountains (CTAM) to investigate the nature of the deep margin between east and west Antarctica and the range that divides the two sides of the continent.
This past austral summer the group was based out of a large field camp located near the Beardmore Glacier, one of Antarctica’s greatest rivers of ice that flows from East Antarctica through the mountains into the Ross Ice Shelf . From there the scientists made day trips on helicopters or Twin Otter aircraft to deploy recording instruments that would allow them to map the earth below the ice sheet using a technique called magnetotelluric sounding.
It’s a method similar to employing seismic waves used by other polar researchers to image the bedrock of the continent. But instead of using waves of energy from an earthquake or explosion to learn about the properties of the Earth’s subsurface, magnetotelluric sounding relies on natural electromagnetic (EM) waves generated by the sun and by lightning bolts.
Such phenomena cause natural variations in the Earth’s magnetic field, creating electric currents under the Earth’s surface. The rocks and geologic structures through which the telluric currents pass have different conductivities. By measuring the electrical resistivity — how strongly the material resists the current — the researchers can identify the stuff that makes up the lithosphere, which consists of the Earth’s crust and upper mantle, up to about 200 kilometers in depth.
“We’re applying this method in this area in an attempt to understand something about how West Antarctica and East Antarctica differ from each other. In particular, how the transition where they meet really behaves,” explained Wannamaker, who has employed the technique in central West Antarctica and at the South Pole during previous field seasons.
The Transantarctics that divide the continent are one of largest mountain ranges in the world at 3,500 kilometers long and upwards of 4,500 meters high. Most mountains form when tectonic plates collide, but the Transantarctics emerged as the Earth’s crust rifted apart.
That’s the part of the story most researchers agree upon today. However, such rifting typically doesn’t result in a mountain range as high as the Transantarctics. In recent years, some scientists, including Michael Studinger at NASA and Audrey Huerta at Central Washington University , have argued that the mountains are the remnant of an ancient plateau. [See previous article: Challenging orthodoxy.]
Wannamaker’s data may shed some additional light on types of forces that created the Transantarctic Mountains, which happens to be one of the world’s biggest rift zones. He hopes to learn about the geologic structures that “hold up” the mountains. Is the rock below East Antarctica simply older and stronger, or are there volcanic forces related to West Antarctic rifting that “creep” under the margin and push the mountains up?
Understanding how the mountains formed isn’t just an important geologic question. The mountains also play a role in how the Antarctic ice sheet formed — a mystery that still occupies glaciologists.
Getting the measurements is ordinarily a time-consuming task, requiring the scientists to install temporary stations to record the electromagnetic waves along a line that can stretch hundreds of kilometers. Doing the job in Antarctica adds a little extra excitement to the project.
“Antarctica is a challenging place to do geophysics work in because of the access to things,” said Wannamaker, who after weeks of doing installations on the highly reflective glaciers sported a face burnished brown by the sun but for two pale circles around the eyes.
Most good weather days found the team leap-frogging by air along a line perpendicular to the mountains, from the Ross Ice Shelf to the polar plateau. A couple of mountaineers worked with the scientists to ensure each site, located at 10-kilometer intervals required for the measurements, was safe.
“Walking out from the center of the site to the ends of the line is probably one of the more worrisome or hazardous aspects because there’s a lot of crevassing out there,” Wannamaker said.
“Many places were too rough for the Twin Otter, so the helicopter was the only option,” wrote team mountaineer Danny Uhlmann on his blog, Antarctica and Beyond.
At the center of each installation were a data logger, battery and solar panel. Four 150-meter electrical lines emanated from this hub, each ending with a perforated sheet of titanium that served as the electrode. A voltage meter took a reading, much like testing a car battery.
To measure the magnetic part of the EM wave requires a different sensor, a sensitive solenoid with copper wire wrapped in a cylindrical pattern, which magnifies the magnetic field. In effect, the device converts the magnetic field to voltage, which is also sent through a cable to the recording box. Each set-up was left in the field for several days.
“We’re out there for a while, waiting for all of these waves to come in,” Wannamaker noted.
The team managed to complete 33 installations over the course of the field season, with plans for a second visit later this year during the upcoming austral summer to extend the line to about 500 kilometers.
Patterns of backscattered EM radiation from all the stations will eventually be assembled by a computer program to create a cross-sectional view of the crust and upper mantle between East and West Antarctica.
Preliminary results suggest the region looks “old and cold,” Wannamaker said, meaning high resistivity because the telluric currents don’t move through cold spots as well as they do warm spots where there may be thermal activity. Nevertheless, Wannamaker cautions that their transect must be extended onto the Ross Sea and account for its influence on their current results before firm conclusions can be made.
“That’s just an eyeball [estimate] and a limited amount of data, and there could be some big surprises,” he added. “I think the information content is going to be in there that we can say something about it.”
NSF-funded research in this story: Philip Wannamaker, University of Utah, Award No. 0838914 .
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