Space Radar Reveals Hidden History of Antarctica
Modern space technology is helping to reveal Earth's geological past. In a paper published in the 25 April issue of Science, Dr. Seymour Laxon of the Mullard Space Science Laboratory in Surrey, England and Dr. David McAdoo of the Geosciences Laboratory, National Oceanic and Atmospheric Administration in Maryland, USA have used data from the European Space Agency's ERS-1 satellite to confirm scientists' ideas about how Antarctica evolved more than 60 million years ago.
The last remaining mysteries in the tectonic evolution of the Earth's crust lie in the polar regions. This is because the oceans are opaque to all types of electromagnetic wave which might otherwise be used to map structures on the ocean floor from space. Mapping by surface survey ships is extremely limited owing to the difficulties of operating a ship in these remote and inhospitable polar areas which suffer dense and permanent cover by floating pack ice. As a result, some areas of the sea floor surrounding Antarctica are more poorly mapped than the surfaces of Mars or Venus. In many places regions of the sea floor larger than the United Kingdom have never been subjected to any kind of geophysical survey.
BREAK UP OF A SUPERCONTINENT. 180 million years ago, most of the individual continents that we know today were joined to form one supercontinent known as Gondwanaland. At this time, Antarctica was part of Gondwanaland and New Zealand was attached to Antarctica. What happened next is 'written' in the rocks. The sea floor adjacent to Antarctica contains the clues needed to solve the mystery surrounding the earliest stages in the break-up of this giant land mass.
The first steps in unravelling the mystery were made ten years ago. Workers in the United States, who were working to fit the jigsaw of current continents back together, discovered a mismatch in the area where the Campbell Plateau, on which New Zealand is now located, broke away from Antarctica. To explain this mismatch they proposed that a previously unknown tectonic plate, the "Bellingshausen plate", must have existed during the time of the break-up. Since this plate no longer exists, it must have fused with the current Antarctic plate at a later date and lost its separate identity. The theory remained unproved due to a lack of data concerning the structure of the seafloor in the region.
RADAR REVEALS THE SEA FLOOR. Satellite data now seems to have confirmed this hypothesis. Although satellites cannot directly map the ocean floor, the topography of the sea surface can be mapped with a high degree of accuracy using spaceborne radar altimeters. The technology is such that instruments can measure the surface elevation to within a few cm from an altitude of 800km. On this scale bumps and dips in the sea surface reflect minute changes in the strength of the Earth's gravity. These, in turn, can be related to changes in the topography and density of rocks on the ocean floor. This technique has been used to generate highly detailed maps of the tectonic structure of the ocean floor. These show spreading ridges, trenches and fracture zones which reveal important clues regarding the past movement of the continents.
The data used by McAdoo and Laxon came from the ERS-1 satellite, which, amongst other sensors, carried a radar altimeter. From a polar orbit, the satellite provided coverage of the high polar latitudes for the first time. A large part of the new areas covered by the satellite was, however, subject to cover by seasonal or permanent pack ice cover. Changes in the return echo shape, used by the satellite to determine the elevation of the sea surface, rendered conventional techniques useless where pack ice was present.
Dr. Laxon, in collaboration with David McAdoo, developed techniques to re-analyse the individual radar echoes to retrieve accurate height measurements even when sea ice is present. Three years ago, they applied their technique to data over the Arctic Ocean and discovered a long suspected, but previously unmapped spreading ridge in the ocean basin north of Canada, thereby solving a long standing problem in tectonic development of the Arctic.
The generation of the Antarctic map required analysis of a much larger, dense grid of satellite data collected by the satellite over a number of years. "In the Antarctic the signals we were looking for were much more faint than was the case for the Arctic and we therefore had to work a lot harder", said Dr. Laxon. "We had to analyse more than 500 Gbytes of data and employ new processing techniques before the fracture zones off the coast of Antarctica became discernible. By mapping the fracture zones adjacent to Antarctica we were able to precisely trace the paths taken by Antarctica and the Campbell Plateau during the earliest stages".
They discovered that the fracture zones formed in a splayed pattern just offshore from Antarctica. This unusual pattern arises from asymmetrical seafloor spreading which led to the separation of the Campbell Plateau portion of the New Zealand microcontinent from Antarctica 60 to 85 million years ago. The fracture zones provide the first firm proof that Antarctica behaved as two plates at this time - an Antarctic plate and a separate Bellingshausen plate.
"This result is likely to have a major impact on our understanding of the break-up of Gondwanaland and the tectonic evolution of Antarctica", said Dr. McAdoo. The new results will be of great interest to geologists and to those interested in the tectonic evolution of our polar regions.
Higher resolution versions of the Arctic Ocean gravity field have already attracted considerable interest from oil exploration companies for whom the data provides a valuable overview of regions allowing the identification of target areas for more detailed surface survey.