Friday 23 May 2014

Geology 101, by Ian Neuss

Today (Saturday 17 May 2014) we were in Tiger Leaping Gorge near the headwaters of the Yangtse River, between two famous rugged snow capped mountain ranges, both over 5000m, or 3000m above us. Here you can recognize that India is moving north. About 55 million years ago it began to collide with Asia at the rate of about 5cm a year. The Indian Plate is too thick and its density too low to go beneath Asia so it’s pushing the old Tethys Sea sediments up about 6 miles, forming the Tibetan Plateau just to the west of us here. India going north has forced Asia east and south east and has penetrated some 1200 miles, pushing the rocks into western China and Indochina in front of it. It’s still happening and that's why there are so many earthquakes in the region. More on these later.

To illustrate this, put your hand (India) on a table cloth (Asia) and push it. The cloth folds about your hand. The troughs near your fingertips represent the rivers and the folds. The mountains we are now seeing are broken by faults, or sliding over each other when the stresses are too great, and causing earthquakes.

The Tibetan Plateau is a basin bounded by mountains being pushed up by India coming north. Six of the largest rivers draining the Plateau, the Indus, the Brahmaputra, the Salween, the Mekong, the Yangtze and the Yellow are the home of over half the worlds population.

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