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The Tibetan Plateau |
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The largest highest place on Earth is a showcase of plate tectonics.
In all the world's continents, there are five exceptional places that have strong, outsized effects on the whole globe. Two of them, the Antarctic and Greenland ice caps, act as great cooling engines for the ocean and atmosphere. The other three are high places, including the North American mountain belt and the Altiplano of South America. But the third is the greatest by far: the Tibetan Plateau.
The Tibetan Plateau is an immense upland, some 3500 by 1500 kilometers in size, that averages more than 5000 meters in elevation. It includes almost all of the world's territory higher than 4000 meters. Its southern rim, the Himalaya-Karakoram complex, contains not just Mount Everest and all 13 other peaks higher than 8000 meters, but hundreds of 7000-meter peaks each higher than anywhere else on Earth.
The Tibetan Plateau is not just the largest, highest area in the world today; it may be the largest and highest in all of geologic history. That's because the set of events that formed it, and continue to build it, appears to be unique: a full-speed collision of two continental plates.
Nearly 100 million years ago, India was a separate continent much like Australia is today. It had just separated from southern Africa as part of the breakup of the supercontinent Gondwanaland. From there the Indian plate moved to the north at speeds of around 150 millimeters per yearmuch faster than any plate is moving today.
What made the plate move so fast? Our best guess is that the Indian plate was being pulled from the north as the oceanic crust making up that part of it was being subducted beneath the Asian plate. The oceanic crust was old, which means that it was cold and denser than the mantle beneath it. Once you start subducting this kind of crust, it wants to sink fast. The "slab pull" that results is especially strong. Today, the Pacific plate is moving relatively fast for the same reason (
see its present-day motion on this map).Another reason the Indian plate moved so fast may have been "ridge push." This force comes from the crustal spreading zonethe midocean ridgeon the other edge of the plate, where new hot crust is created. The new crust in spreading zones stands higher in elevation than old ocean crust, and the difference in elevation results in a downhill gradient. In India's case, the mantle beneath Gondwanaland may have been especially hot. Thus the ridge push may have been more significant than usual too.
Whatever the reasons, India was moving north and, beginning around 55 million years ago, began to plow directly into the Asian continent. (
See an animation here.) Now when two continents meet, neither one can be subducted under the other. Continental rocks are too light. Instead, they pile up. The continental crust beneath the Tibetan Plateau is the thickest on Earth, some 70 kilometers thick on average. And under the Pamir mountains, at the northwest end of the plateau, the thickness reaches nearly 100 km.Besides piling up, continental rocks can also be shoved aside. This is what's happening to the north of the Tibetan Plateau: great chunks of Asian rock are being pushed eastward. This is why the large Chinese earthquake in November 2001 was a strike-slip event, like those on California's San Andreas fault, and not a thrust quake. A French team including Paul Tapponnier, who first described this process 30 years ago, has put up
a preliminary analysis of that earthquake.This is just a bare outline of the Tibetan Plateau. Today's researchers are finding this region a showcase of collision-related geology, and they're asking questions that may pay off everywhere else on our planet. Part 2 will get into some of the specifics: things like Nanga Parbat, Pleistocene granites, super erosion, and eduction.
