ANTARCTICA CAME CLOSE TO SPLITTING.

Byline: Usha Sutliff Staff Writer

PASADENA - A team of scientists that included a Caltech researcher has found a missing piece in the geologic puzzle of how tectonic plates moved and formed in Antarctica.

Joann Stock of California Institute of Technology; Steve Cande of the Scripps Institute of Oceanography in San Diego; and their colleagues in Australia and Japan found that a rift between East and West Antarctica started about 43 million years ago and abruptly ended 17 million years later, after the division between them had grown to 112 miles.

``The two pieces of Antarctica pulled apart and then stopped,'' said Stock, a professor of geology and geophysics. ``If it had kept on going, there would eventually have been two Antarcticas.''

Tectonic plates are the dozen or so plates that make up the surface of the earth. For more than 25 years, scientists have been piecing together the story of how the earth's plates have moved and developed over tens of millions of years.

As one plate moves, the edge of an adjoining plate is affected, much like the pieces of a giant jigsaw puzzle.

The team's findings, being published in today's issue of the journal Nature, are significant because they provide some answers about the plate motion around Antarctica.

Geophysicists have had a hard time understanding how the motion in Antarctica fits into the grand scheme of global plate tectonics, Stock said.

``You have to know how one piece moved relative to the other pieces to understand how it all fits together,'' she said. ``A lot of the tectonic plate history for western North America, for example, depends on what happened in Antarctica. You wouldn't think so, but that's the way plate tectonic movements work.''

Cande, a professor of geophysics at Scripps and the study's lead author, said that knowing about the correct plate motion around Antarctica is the key to understanding the motions between the Pacific plate and the North American plate.

The Antarctic plate consists of Antarctica and some of the sea floor around it, he said.

``So, by determining the plate motions in Antarctica, we are defining the plate motions up here in California, for example,'' Cande said.

Stock and the others made their discovery after taking several cruises in the mid-1990s in the northern Ross Sea off the Antarctic coast.

An underwater trough off Cape Adare is the key to the discovery, according to the researchers.

The trough is about 142 miles long and runs roughly northwest-southeast near the 170th meridian. Stock said the sharp break in the direction of the magnetic lines on each side of the trough allowed the researchers to infer the ancient relative motions of the plates.