EARTH MOVEMENTS

 

I want to end this story at a specific moment:  Monday, 12 May, 2008, at exactly 2.28 pm local time. The place is 942 miles west southwest of Beijing, in the capital of China's Sichuan province. Chengdu was a modern city of sky scrappers and ancient temples, universities and industry, home to 7 million people.  And one and one half seconds later almost all of that was destroyed. At the end.

The earth underfoot jerked and vibrated for four interminable minutes. Office and apartment towers were whipsawed back and forth until they crumbled. Eighty percent of the structures in Chengdu collapsed.

Some 90,000 humans were killed during those four minutes, including five thousand  three hundred children, most killed when their schools collapsed. Five million in all were left homeless. 

As such things are measured, the event was recorded as a 7.9 on the modified Richter scale, "Very Destructive" on the European Macro-Seismic scale,  and "Extreme" on the Modified Mercalli Intensity Scale. The shaking produced 200,000 landslides across 170,000 square miles spread across three Chinese provinces. That was the end, which was a product of the middle of this story.

Some 66 million years ago, about the time the Chicxulub impact killed off the dinosaurs,  a 140 mile long single cohesive slab of magma was exposed near the surface of the Euro-Asian tectonic plate.  It cooled to 850 degrees Fahrenheit so quickly it's crystals were locked rigidly together. Over eons it was then buried under layer upon layer of sediment, until it was 12 miles below the surface.

Then, beginning about 10,000 years ago, new pressure from the north and west against the Chengdu Plain, bulldozed  up "...one of the world's most remarkable continental escarpments",  the 250 mile long Longmen Shan, or "Dragons Gate", Mountain range. (above). 

This northeast by southwest escarpment piled up 16,000 feet high over less than 30 miles of width, directly above the long buried slab, while the pressure from the northwest grew and grew.

And when that 140 mile long slab of bedrock could no longer withstand the pressure, it snapped like a twig, fracturing at exactly 2:28pm and one and one half seconds, 2008, releasing the equivalent of 25,000 nuclear bombs, its destructive waves racing outward through the surrounding bedrock at 7,200 miles an hour, covering the 50 miles to Chengdu in one second , destroying that city and every town and village  in between. 

What was squeezing the Dragon's Gate escarpment upward against the Chengdu Plain was the rising of the Tibetan Plateau, described by Andrew Alden, of About.com as what maybe   "...the largest and highest in all of geological history…."  It is an area five times the size of modern day France. 

The average elevation of the Plateau is 14,800 feet above sea level. It's 46,000 glaciers are the third largest supply of fresh water on the planet, which gives birth to seven of the world's greatest rivers;  the 2,000 mile long Indus in Pakistan, the 1,600 mile long Ganges in India, the 1,400 mile long Irrawaddy in Myanmar , the 2,700 mile long Mekong of Thailand and Vietnam, the 3,900 mile long Yangtse and 3,400 mile long Yellow rivers, both in China. 

The Tibetan Plateau has bulged because the entire Indian tectonic plate, 6,000 miles long by  2,000 miles wide and sixty-two miles thick, encompassing four billion six hundred thousand square miles of rock and earth has been slamming into the Eurasian plate at a speed of about 2 inches a year for the last fifty-seven million years.  

Like the crush zone on a automobile,  the front bumper of India has crumpled and thickened on impact, producing first the Karakoram - or black gravel - mountains (above, center left) and secondly the Himalayan - or abode of snow - mountain range, including Mount Everest (above, left center). And that is why there are fossils of ocean creatures just below the peak of the highest mountain on earth.

This collision was so fast it forced the thinner and thus lighter Indian plate to subduct under the Tibetan Plateau, shoving in up rather than aside. 

It appears the Indian plate  did not "drift" into a collision with the Eurasian plate, but rather was pulled north 3,700 miles by two smaller plates carrying volcanic island arcs which India ran over. As those plates were shoved into the mantle they pulled the Indian plate northward, speeding it up in the process.   

By about 550 million years ago some 20% of the earth's surface - 39 millions square miles - was joined together into a supercontinent called Gondwana (above). It included the tectonic cratons of South America, Africa, Antarctica, Australia,  Zeelandia, Arabia and India (with the Red Dot).  This chunk of the dry surface remain joined until the start of the Jurassic Age -  about 180 million years ago - when heat trapped beneath it's thick surface finally drove the individual cratons to break apart.   

And like the last act of our story, the first act began with an earthquake, some 180 million years ago along the border of the Madagascar plate and the Indian Plate, the very first moment they began to pull apart from Gondwana, some 3,500 miles south of where the story was to end, along the southern border of the Eurasian plate. Millions were thus condemned to suffer through the great Chengdu Earthquake of 12 May, 2008, because of the break up of Godwana 180 million years earlier. 

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