Plate tectonics: Journeys to the deep
The convergence of continental tectonic plates may push surface material to depths greater than 200 km into the mantle
Introduction
© (2000) Nature
Researchers at the Chinese Academy of Sciences in Beijing found evidence that material from the earth's crust travels deeper down into the molten rock of the mantle than was previously thought. This process, known as subduction, may occur where two continental plates collide. Subduction, which can destroy crust and build mountains, is important for understanding the long-term growth of continents.
Kai Ye and colleagues1 studied crystallized garnet in very dense rocks called eclogites. The eclogites were from Yangkou in the Sulu belt, China, where the South China plate subducted under the North China plate during the Triassic period (about 230 million years ago). Sodium and silicon oxide concentrations, which have a positive dependence on pressure, were higher than in garnet synthesized in basalt systems at known pressures of around 7 GPa. This suggests that the Yangkou samples originated at even greater pressures, implying subduction depths greater than 200 km.
When continental material is pushed to such depths, its density increases to levels greater than the surrounding mantle. It may not return to the surface under its own buoyancy and could sink even further. Therefore the rocks found at Yangkou must have been exhumed by a raft of more buoyant material pushing from underneath.
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