Thursday, February 4, 2016

Himalayan Mountains - A Convergent Boundary

Originally written 5/1/2015

The Himalayan mountains are one of the most impressive terrestrial landforms on present day Earth. They are also relatively young, which means that they are very geologically active. This activity, while exciting to geologists, has tragic consequences for the people that live in the region.

The Himalayan mountains are actually only a symptom of a much larger geological process. Starting about 71 mya the Indo-Australian continental plate began subducting under the Eurasian plate in a classic convergent plate boundary, in which the denser oceanic crust sinks below the lighter continental crust. Normally this process creates volcanic island arcs in the overlying plate, but this time something different was in store. As the ocean that had separated the continents disappeared the continents themselves started to collide, each refusing to sink beneath the other. Around 50 mya the last bits of the ocean basin, squeezed like Play-Doh in your fingers, was elevated to create mountains of limestone.

As the rocks closest to the actual boundary are pressed ever tighter together by the formidable forces of plate tectonics, their individual mineral identities are distorted, folded, and interwoven into the geologic equivalent of a suture in flesh. Hundreds of millions of years from now, when those tortured rocks are exposed by erosion to a new generation of geologists, they will display distinctive narrow stripes of minerals in tight folds, and it will be impossible to tell which mineral grains came from which plate.

Convergent boundaries are always marked by earthquakes, and usually also by volcanoes. This particular boundary does not have volcanoes, since there is no actual subduction occurring anymore, and thus no melting rock to provide magmatic fuel for explosions. The Himalayas are therefore built entirely of pre-existing rock, not magma as the Hawaiian islands are. Despite the lack of magma, the Himalayas are far from benign. On April 25th, 2015, India jerked a little farther into Asia, a regular spasm in the ongoing collision. The resulting loss of life and property on the surface has yet to be totally added up, but as of May 1st, 2015, the death toll is over 6,000 people, and many historical sites and temples have been flattened. The last earthquake of comparable strength was about 80 years ago, a long time ago to humans but a mere eyeblink ago to plate tectonics.

Essentials of Geology, section 2.12

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