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February 20, 2005

Geologists Find: An Earth Plate Is Breaking in Twohttp://www.columbia.edu/cu/pr/95/18688.htmlScientists at Columbia University's Lamont-Doherty Earth Observatory reportdirect evidence that one of the Earth's great crustal plates is cracking intwo.In a report published in the most recent issue of Earth and Planetary ScienceLetters (vol. 133), the scientists say they have confirmed that theIndo-Australian Plate--long identified as a single plate on which both India andAustralia lie--appears to have broken apart just south of the Equator beneaththeIndian Ocean. The break has been underway for the past several million years,and now the two continents are moving independently of one another in slightlydifferent directions.Scientists have known that for some 50 million years,

the Indian subcontinenthas been pushing northward into Eurasia, forcefully raising the TibetanPlateau and the Himalayan Mountains. The new research suggests that startingabout8 million years ago, the accumulated mass became so great that theIndo-Australian Plate buckled and broke under the stress."The result of this critical stage in the collision between India and Asia isthe breakup of the Indo-Australia Plate into separate Indian and Australianplates," Jeffrey Weissel, a scientist at Lamont-Doherty, Columbia's earthsciences research institute in Palisades, N.Y., said in an interview."This is a newly observed way of creating a new boundary between plates,"said Lamont-Doherty scientist James Cochran, who co-authored the report with Dr.Weissel, Florence Jestin of the Ecole Normale Supérieure in Paris, France, andJames Van Orman, now a graduate student at the Massachusetts Institute ofTechnology. Mr. Van Orman, the report's

lead author, was an undergraduate atFlorida State University in 1993 when he began the research with Drs. WeisselandCochran at Lamont-Doherty as part of a summer internship program sponsored bythe National Science Foundation (NSF)."In the Central Indian Ocean, Nature is conducting a large-scale laboratoryexperiment for us, showing us what happens to the oceanic lithosphere (Earth'souter layer) when force is applied," Dr. Weissel said in an interview.Essentially pushed into an immovable object, "it can buckle like a piece oftin," hesaid.A fundamental tenet of plate tectonics theory is that the Earth's surface isdivided into rigid plates that move together and apart like pieces of a jigsawpuzzle. Scientists have long recognized 12 major plates. Now there are 13.In the 1970's, scientists first discovered a broad zone, stretching more than600 miles from east to west where the equatorial Indian Ocean floor

wascompressed and deformed. Drilled samples had shown that the zone had begun tobuckle and crack about 8 million years ago at the same time that the TibetanPlateau had reached its greatest height. Dr. Cochran was chief scientist of thedrilling cruise that collected this data.More recently, researchers at Northwestern University, led by Richard Gordonand Seth Stein, used data on how newly created seafloor had spread outwardfrom mid-ocean ridges to the west and south of the deformed region in the IndianOcean. They theorized that the movements of the newly created seafloor couldbe accommodated only if a distinct plate boundary existed between separateIndian and Australian plates across the equatorial Indian Ocean.In relation to the Indian plate, the Australian Plate is movingcounterclockwise, the Northwestern University scientists calculated. In thewestern part ofthe new plate boundary, the plates are moving away from

each other. To theeast, the Australian Plate is converging on the Indian Plate, they said.If the theory was correct, the ocean floor in the eastern part of the newplate boundary should be compressed, buckled, cracked and eventually thrustupward along the cracks. More critically, if a separate Australian Plate wererotating counterclockwise in relation to a separate Indian Plate, the amount ofcompression should increase rapidly and systematically from west to east acrossthe Central Indian Ocean.To test the theory, the Lamont-Doherty team took actual measurements of howcompressed the Indian Ocean floor has become in the region believed to be thenew plate boundary. Using sound waves to probe oceanic rock layers, theycreated images of subseafloor structures.The images were collected during two separate research voyages that eachspanned the entire deformed zone from north to south. Dr. Weissel and Dr. Jestinwere

aboard the 1991 "Phedre" cruise of the French research vessel MarionDufresne. In 1986, Lamont-Doherty's former research vessel, the Robert D.Conrad,obtained images along a north-to-south line 185 miles farther west.The images showed scores of systematically aligned cracks, or faults, in theoceanic lithosphere--created as the once-whole plate buckled and cracked. Asthe now-distinct plates continued to converge, slabs of ocean floor slid upwardalong the faults to alleviate the strain. The more the two plates converged,the farther the slabs slid upward."Van Orman's summer job," Dr. Weissel said, "was to very carefully measurehow far vertically the blocks of crust were thrust upward along more than 200faults."The measurements clearly showed that two separate plates were converging.More importantly, the thrusting observed on the "Phedre" seismic line was abouttwice that found along the Conrad's line. That proved that

compression was moreintense to the east--confirming the Northwestern group's prediction from thedata on spreading rate and direction at the mid-ocean ridges."Our result therefore provides direct evidence from the deformation itselfthat the compression of oceanic lithosphere in the Central Indian Ocean,originally regarded as 'intraplate,' is better described as constituting part ofabroad boundary zone between distinct Indian and Australian plates," thescientists wrote in Earth and Planetary Science Letters.Dr. Cochran said the research "gives insight into how strong and rigid platesare, how they respond to stress and what their limits are before they break."Dr. Weissel said, "This is an important piece of work that came out of theNSF's Research Experiences for Undergraduates program. It was basically anundergraduate's summer intern project."The NSF supported the entire research

project.7.7.9518,688