For Immediate Release
Steve McGregor, UTD, (972) 883-2293, [email protected]
Geoscientists from Israel, Texas Offer Theory
Calling the origin of the 35,000-foot-deep gash in the sea floor near the island of Guam in the Western Pacific Ocean “one of the great unaddressed puzzles in the earth sciences,” Dr. Robert J. Stern, head of the Geosciences Department at The University of Texas at Dallas (UTD), believes that he and a colleague, Dr. Zohar Gvirtzman of Geological Survey of Israel, now know how the greatest depth on our planet’s solid surface formed.
In an article to be published in the online version of the journal Tectonics, the pair states that the trench lies on a “convergent plate boundary,” where two huge plates of the earth’s lithosphere – in this case, the Pacific Plate and the Philippine Sea Plate – converge. The Pacific Plate descends, or is “subducted,” below the Philippine Sea Plate, making a trench on the ocean floor.
Previously, some experts believed that the segment of the Mariana Trench where the Challenger Deep is located was part of what is know as a “transform boundary,” where two plates slide horizontally past one another.
Recognizing that the Challenger Deep lies within a subduction trench, however, still does not explain why it is located where it is and not hundreds of miles to the north or in other oceanic trenches. According to Gvirtzman and Stern, up until now it was commonly cited in the scientific literature that the depth of oceanic trenches is mostly determined by the length and age of the lithosphere involved and by the rate of subduction – generally, the older and longer that portion of the earth’s outer skin and the faster it sinks, the deeper the trench. However, at the Challenger Deep, the subducting lithosphere is quite short and the subduction rate is relatively slow.
What explains the apparent dichotomy? The hydrodynamic shape of the subducting slab and its ability to sink and move in the viscous underlying mantle may be the answer, Gvirtzman and Stern said.
“It’s Dr. Gvirtzman idea that lithospheric fragments that are relatively narrow can bend down into the earth’s mantle much more easily than a broader slab, much like a spade plowing through the soil,” Stern said. “The lithospheric segment that is being subducted along the plate boundary where the Challenger Deep lies is relatively narrow and able to sink easily into the mantle.”
“We believe that the unusual depth of the Challenger Deep is related to an exceptionally narrow plate coupling zone, where the heavy descending slab hangs almost entirely on the overriding oceanic plate with a very weak attachment to the upper plate,” the authors concluded. “As a result of the weak coupling, the descending plate is relatively free to sink and steepen, resulting in an unusually deep trench.”
The Challenger Deep is located at the southern end of the Mariana Trench, near the Mariana Islands Group. It takes its name from the British ship Challenger II, which surveyed the trench in 1951.
In 1960, the U.S. Navy Bathyscaphe Trieste descended to 35,813 feet in the trench. In 1984, a Japanese survey vessel using advanced sonar equipment measured the depth of the trench as 35,838 feet.
Stern described the trench as being “so deep you could put Mt. Everest in there and still have a mile of water left over.”
Stern is currently on a research trip to the Mariana Islands, but not to visit the Challenger Deep. He and a UTD graduate student are part of a three-week National Oceanic and Atmospheric Administration cruise to study seafloor hydrothermal vents using a remotely operated vehicle. According to Stern, the expedition is one of the first to look systematically at submarine hydrothermal systems associated with convergent plate margins.
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This page last updated August 03, 2013