By Sid Perkins of Nature magazine
Evidence of historical irregularities in the motions of both the Indian and African tectonic plates bolsters the contention that plumes of hot rock rising from deep within Earth's mantle can drive the planet's tectonic plates.
About 68 million years ago, the tectonic plate that includes the Indian subcontinent--which, at that time, had yet to slam into southern Asia--lay northeast of Madagascar and was moving north-eastward at a tectonically typical few centimeters per year. Then, it suddenly surged forward, racing along at an average annual speed of more than 10 centimeters for about 15 million years, before gradually slowing to its earlier pace.
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This extended period of rapid motion has been recognized since the late 1960s, says Steven Cande, a marine geophysicist at the Scripps Institution of Oceanography in La Jolla, California. But what most scientists have overlooked, he notes, is that during the same interval, the eastward motion of southern Africa slowed drastically and almost ceased.
In today's issue of Nature, Cande and his Scripps colleague Dave Stegman suggest that the two anomalies are linked to another geophysical event, the arrival at Earth's surface of the Réunion mantle plume. This mass of hot, buoyant rock rose beneath what is now southern India about 67 million years ago, spewing out massive amounts of basaltic lava, and eruptions plagued the region for at least another 5 million years.
As the rising Réunion plume hit the lower surface of Earth's crust, it spread in all directions, much like the flow of water from a power hose slamming into a wall. Cande and Stegman speculate that tectonic forces generated by the plume's spread acted as a 'tailwind' for the Indian plate, propelling it forwards, and simultaneously acted as a 'headwind' for the southern end of the African plate, slowing it considerably. Then, as the plume waned, so did the forces driving the plates, and the Indian and African plates subsequently resumed their previous rates of motion.
Baffling bends
The jostling of tectonic plates triggered by the Réunion plume may also explain other geophysical anomalies that occurred during the same period, including the previously enigmatic bends in the fracture zones along portions of the mid-ocean ridge southeast of Africa.
These anomalies, along with the inferred motions of the Indian and African tectonic plates, "are like the skid marks at a crash scene", says Cande. "Maybe no one saw what actually happened, but the clues can help you figure it out."
However, Dietmar Müller, a geophysicist at the University of Sydney in Australia, says that some recent studies suggest that the plate-driving forces that can be generated by mantle plumes are too modest to explain the acceleration of the Indian plate proposed by Cande and Stegman. In addition, he adds, the boost in speed seems to have lasted several million years longer than the surface eruptions triggered by the Réunion plume. Nevertheless, Müller notes, the duo's findings do offer an intriguing solution to several longstanding geophysical puzzles.
This article is reproduced with permission from the magazine Nature. The article was first published on July 6, 2011.
