Washington, July 18 : Scientists have determined that a single granite boulder found high atop a glacier in Antarctica may provide additional key evidence to support a theory that parts of the southernmost continent once were connected to North America hundreds of millions of years ago.
An international team of US and Australian investigators made the investigation.
The findings, which were made in the Transantarctic Mountains, are significant to the problem of piecing together what an ancient supercontinent, called Rodinia, looked like.
Previous lines of scientific evidence led researchers to theorise that about 600-800 million years ago, a portion of Rodinia broke away from what is now the southwestern United States and eventually drifted southward to become eastern Antarctica and Australia.
The team's find, they argue, provides physical evidence that confirms the so-called southwestern United States and East Antarctica (SWEAT) hypothesis.
The boulder find came by chance while the researchers were picking though rubble carried through the Transantarctic Mountains by ice streams-rivers of ice, that flow at literally a glacial pace from East Antarctica.
John Goodge, an NSF-funded researcher with the Department of Geological Sciences at the University of Minnesota-Duluth, and his team were searching for rocks that might provide keys to the composition of the underlying continent crust of Antarctica.
One rock in particular, small enough to heft in one hand, found atop the Nimrod Glacier, was later determined to be a very specific form of granite with, as Goodge describes it, "a particular type of coarse-grained texture."
Subsequent chemical and isotopic tests conducted in laboratories in the US revealed that the boulder had a chemistry "very similar to a unique belt of igneous rocks in North America" that stretches from what is now California eastward through New Mexico to Kansas, Illinois and eventually through New Brunswick and Newfoundland in Canada.
That belt of rocks is known to have been a part of what is called Laurentia, which was a component of the supercontinent of Rodinia.
"There is a long, linear belt of these igneous rocks that stretches across Laurentia. But 'bang' it stops, right there at the (western) margin where we knew that something rifted away from what is now the West Coast of the United States," said Goodge.
"It just ends right where that ancient rift margin is. And these rocks are basically not found in any other part of the world," he added.
That it should turn up on a glacier high up in the mountains of Antarctica is strong evidence in support of the SWEAT model that parts of North America continue into part of the frozen continent at the bottom of the Earth.
"There's no other explanation for how it got where we found it," said Goodge. "It was bull-dozed over from that interior region of Antarctica," he added.