The outer shell of the Earth is broken up into 12 major plates and a few smaller ones. The boundaries between those plates, according to plate tectonic theory, come in three varieties: midocean ridges, where plates diverge; deep-sea trenches and other subduction zones, where two plates converge and one dives under the other; and transform faults like the San Andreas, where plates slide past each other. In 1995, however, this neat scheme stood for some revision: the 12 major plates became 13, and a new type of plate boundary–convergent and divergent at the same time–was discovered in the Indian Ocean.
According to geophysicist James Cochran of the Lamont-Doherty Earth Observatory and his colleagues, India and Australia are going their separate ways: the giant Indo-Australian plate that was once thought to carry both has actually been breaking apart for roughly seven and a half million years. The most likely cause was the stress suffered by the giant plate as it slammed into Asia and pushed up the Himalayas and the Tibetan Plateau. As the mountains got higher, Cochran says, the resistance against the pushing became greater, and then the stresses built up to the point that the plate could not support them, and it began to buckle and break.
The break zone, Cochran’s team reported last July (confirming an earlier proposal by researchers at Northwestern University), is centered on a point roughly 600 miles south of the tip of India. The Australian plate is pivoting counterclockwise around that point, pushing into the Indian plate to the east, and pulling away from it to the west. Cochran and his colleagues found evidence for this by bouncing sound waves off the seafloor and measuring how much it has been compressed–and thereby thrust up along east-west trending faults–to the east of the pivot point. (To the west the terrain is too rough for good measurements.) The compression was greater farther from the pivot point, indicating that the Australian plate was indeed rotating into the Indian one.
The bizarre boundary between the two plates is a fault zone hundreds of miles wide, but Cochran expects it will eventually evolve into a narrower, more traditional boundary–or rather two of them. If we look ahead a few million years, he says, it may well become a midocean ridge to the west, and to the east it will most likely become a deep-sea trench and a subduction zone. My guess is that it will be Australia diving under India, but there is really no reason to favor one over the other.
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