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| Vol.24, No. 04 | Sept. 25, 1998 |
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| Vol.24, No. 04 | Sept. 25, 1998 |
By Hannah Fairfield
Two Columbia researchers believe that a recently discovered valley in the South Pacific may be a fresh tear- rather than a 200-million-year-old scar-in the seafloor, a hypothesis that could change present scientific thought about the behavior of tectonic plates on the Earth's surface.
The hypothesis has generated debate because it proposes a new cause of tears in tectonic plates.
Christopher Small and Dallas Abbott, geophysicists at Columbia's Lamont-Doherty Earth Observatory in Palisades, NY, have observed that the narrow 800-kilometer valley does not have characteristics of other extinct "spreading centers," where tectonic plates once pulled apart.
The Small-Abbott hypothesis answers questions raised by earthquake data and satellite imagery from the area, and it asks new questions about plate tectonics.
"What we see has broad implications for plate boundaries and old plates," said Small. "Old, strong plates might not be as strong as we think."
At Lamont's remote sensing computer laboratory, Small pointed to the colorful satellite image of the valley, located about 1,600 kilometers northwest of New Zealand.
"The valley is very straight, without offsets associated with slow spreading centers," he said, also indicating that the walls of the valley are steeper than those on spreading centers. "We thought there were enough inconsistencies that we should consider an alternative to the spreading center hypothesis."
Small and Abbott unofficially named the valley the Louisville trough, after the Louisville seamount chain, a line of seafloor volcanoes that meets the westernmost end of the trough.
Their paper, which was published in the September issue of Geology, identifies one very large seamount in the chain, tentatively named Godzilla, that may be the cause of the tear. The chain of seamounts are on the Pacific plate, which is sliding underneath the Australian-Indian plate in a process called subduction. One of the Earth's deepest chasms, the Tonga-Kermadec Trench, is at the margin of the two plates. Using measurements of seafloor topography derived from satellite gravity maps and shipboard surveys, Small and Abbott proposed that the largest seamount in the chain has become stuck in the trench. The accumulated stress where the trench is clogged would need an outlet.
This is why they think the valley is a recent tear rather than an extinct spreading center. To support their theory, they also point to an unusual absence of earthquakes at the place where the seamount meets the Tonga-Kermadec Trench and where the tear is located. The lack of earthquakes at the margin, which Small and Columbia seismologist Christopher Scholz reported last year in Geology, suggests that the tension might be released through a different mechanism.
The hypothesis offered by Small and Abbott provides a possible explanation for the release of this built-up stress.
"It would be an unbelievable coincidence that the valley intersects the seamount," said Abbott, referring to the opposing view that the valley and seamount coexist independently of each other. "However, there is a possibility that both hypotheses are correct-something that happens a lot in scientific processes."
Geologists have traditionally thought that new rifts in the seafloor occur where plates are young and weak-at mid-ocean ridges, where plates pull away from each other and rising molten rock creates new plate material. The oldest, strongest plate material is thought to be at the edges of the plates, where they become subducted into trenches.
The new hypothesis, in which a relatively small (4-km) seamount is the cause of a gash in thick ocean crust (100-km), might change how geoscientists think about and model the creation of tectonic plates and the evolution of continents and ocean basins.
Another scientist, based in California, is skeptical about the new hypothesis even when faced by the image and earthquake data. Some who have studied the area believe that the valley was created in the Cretaceous period, when the seafloor was young and weak. A kind of rock that is high in sodium and titanium is characteristic of slow spreading centers, so scientists can sample rocks from the ocean floor to acquire further data in the quest for the chasm's origin.
At the very least, Small and Abbott's hypothesis "gets the idea out there," said Small. "It's worth considering because the existing explanation has so many inconsistencies. This gets people questioning the assumptions we make about how the Earth works."