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Environment

The Year in Science: Earth 1997

Antarctic Puzzle

By Kathy A SvitilJanuary 1, 1998 6:00 AM

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Geologists had been wondering where to stick a sizable piece of Gondwanaland, but this past year they found where it belongs. Gondwanaland is the southern supercontinent that started breaking up some 180 million years ago. Reassembling it from its scattered pieces— Africa, Australia, South America, and Antarctica—is not too hard, except that to get a nice fit you have to break a chunk off Antarctica, move it up the coast and reattach it. This sounds like cheating, but last April two researchers reported that something like this really did happen between 80 and 60 million years ago.

The chunk in question includes the Bellingshausen Sea and probably even the neighboring Antarctic Peninsula, which curls up toward South America. The idea that a Bellingshausen plate had once separated from the main body of Antarctica was first suggested around a decade ago; tracks in the seafloor south of New Zealand were the evidence. David McAdoo of the National Oceanic and Atmospheric Administration in Silver Spring, Maryland, and Seymour Laxon of University College London found clearer signs of this scenario by using data from the European radar satellite ers-1. The radar soundings reveal the topography of the sea surface—and thus indirectly of the seafloor.

When McAdoo and Laxon made an ers-1 map of the seafloor off Antarctica, they discovered previously unknown fracture zones in what is now the ice-covered Amundsen Sea. Those fractures represent the tracks of the Bellingshausen plate as it moved 100 to 200 miles northeast, presumably carrying the Antarctic Peninsula to its present position. A trough paralleling the coast marks the former rift between the Bellingshausen and Antarctic plates, a rift that was healed again around 60 million years ago. We’ve found firm evidence for an extinct plate boundary between the Bellingshausen and the rest of Antarctica, McAdoo says. What we need now is for people to survey with ice-breakers to confirm it.

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