August 7, 2008
Minnesota Duluth Geologist’s Find Could Prove Existance of Supercontinent
By Steve Kuchera, Duluth News-Tribune, Minn.
Aug. 7--IT DOESN'T LOOK EXTRAORDINARY -- but a rock found in Antarctica by a University of Minnesota Duluth professor is helping researchers reconstruct what a supercontinent that existed a billion years ago looked like.Analysis of the chunk of granite collected by UMD geology professor John Goodge in 2005 indicate that part of Antarctica and North America were joined 1.4 billion years ago.
"We got really lucky -- I had no idea what we were going to find when we started looking at these glacier samples," Goodge said. "It was a one-in-a-million chance to find this one piece of rock that has such special character."
The discovery, detailed in the July 11 issue of Science Magazine, is important, said Eldridge M. Moores, vice president of the International Union of Geological Sciences.
"For the first time, we have a fairly firm connection between North America and Antarctica," he said. "The proposed connection makes a lot of sense. ... Goodge's interpretation is compelling."
The Earth's continents sit on large plates that drift slowly over the planet's surface. Currently, they are relatively spread out. But in the past, they have joined in various combinations to form massive supercontinents. At one time, all the continents were together in the supercontinent Pangaea, which broke apart about 200 million years ago.
The structure of Pangaea is fairly well agreed-on. But researchers trying to fit the pieces of more ancient supercontinents together face formidable challenges.
"We are dealing with rocks that formed prior to the development of animals, so we essentially have few if any fossils to help us," Moores said. "The key is in radiometric age dating, and these techniques have only been available for the past decade or so."
A lack of knowledge about the geology of some areas also hinders researchers.
"Ninety-eight percent of Antarctica is covered with ice, and we can't really see the geology," Goodge said. "However, the ice dredges material up that we can work with."
Goodge has visited Antarctica 10 times, trying to piece together the geologic history of the frozen continent by examining the rocks that glaciers have dug up. In October 2005, Goodge and his National Science Foundation-funded team were based at America's McMurdo Station, flying in a ski plane to various field sites along the Transantarctic Mountains.
"My goal was to see what the modern glacier systems are eroding from beneath the ice caps and depositing along the mountains," Goodge said.
One day, the team landed at an altitude of about 8,000 feet to examine a moraine where the Nimrod Glacier cuts through the mountains, draining ice from the South Polar Plateau to the Ross Sea. Where the glacier entered the mountains, acres of dark, rusty brown-red rocks scraped from the mountains littered the area.
"It was pretty easy to spot anything that didn't belong," Goodge said.
One thing that didn't belong was a rock the size of a small watermelon. It was coarse-grained granite, pinkish red-gray in color and containing large feldspar crystals.
"It stood out, but I had no idea it would give us the results it did," Goodge said.
Subsequent tests revealed that the rock was the same age -- 1.4 billion years -- and had the same chemical, textural, mineral and isotopic characteristics as a belt of rocks in North America stretching between California and Newfoundland. The findings, the team argues, confirms that the Southwestern United States and East Antarctica were once joined -- an idea called the SWEAT hypothesis. Under the hypothesis, parts of Antarctica and America were connected more than 1.4 billion years ago. About 1.1 billion years ago they joined other land masses to form Rodinia -- the earliest known supercontinent. Some researchers believe that between 600 million to 800 million years ago a portion of Rodinia broke away from what is now the southwestern United States, drifting south to become eastern Antarctica and Australia.
Ian Dalziel of the University of Texas-Austin helped develop the SWEAT hypothesis.
"I think the paper by Goodge and others is really definitive -- it's a major steppingstone in understanding what the pre-Pangaea world was like," Dalziel said. "This is the most definitive piece of evidence that has been brought to bear on the (SWEAT) hypothesis."
STEVE KUCHERA can be reached at (218) 279-5503 or by e-mail at [email protected]
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