Geology of Himalayas - Mountain Building
As India pushed relentlessly toward Eurasia, the Tethys Trench was compressed, folded, and faulted. The base of the trench consisted of sedimentary rocks that were weak to begin with. When they were broken by the colliding plates, weaknesses in these overlying materials allowed basalt and granite to intrude upward from Earth's underlying mantle. These materials were fresh and hard; when the Indian plate encountered the Tethys trench, the plate was sheared under, or subducted (curved and sucked down into the mantle) under the trench. The trench rose in elevation as it was pushed up by the subducting plate and the compressive forces, and the water drained away. The flat ocean bottom became today's Tibetan Plateau. The Trans-Himalayan Range formed the southern edge of the Plateau, and, as the mountains rose, new rivers were created, and their drainage changed the climate and the downslope topography. At this point in the development of the Himalayas, the mountains were impressive but had not reached the monumental elevations we know now.
From about 50-23 million years ago, the subduction began to slow (the Indian plate of rock was too buoyant to be drawn down into the mantle), and the plate corner intersected Asia and began to slide under Asia. During the Miocene Epoch (23 million years ago), the compression of the plates intensified and continued into the Pliocene Epoch (1.6 million years ago). As the Indian plate slid under the Asian plate, its upper layers were stripped off and curled back on the subcontinent. These layers, called nappes, were older metamorphic rock from the ancient Gondwana. As the mountains rose, the rivers steepened, the runoff and erosion increased, deposition of sediments similarly increased, and the weight of the sediments forced receiving basins downward so they could hold still more alluvium.
The creation of the nappes left a core zone. In the Pliestocene Epoch about 600,000 years ago, serious mountain building began in a time that is relatively recent in geologic terms. New granite and gneiss intrusions pushed up and rose to the astounding height of Everest. As Everest itself was uplifted, the crystalline rock dragged evidence of the older sediments including those from the Tethys Basin to the summit so the most ancient fossils in the region are present on the newest mountain tops. These uplift episodes again changed the climate and blocked rains from moving to the north. The mountains on the north (the Trans-Himalayas) and the Tibetan Plateau became deserts. Heavy rains to the south changed the line of the crest and shifted the direction of rivers to create a high midlands between the Greater Himalayas and the Lesser Himalayas to the south. High valleys filled with sediment to form lush valleys like the Kathmandu Valley. The Outer Himalayas including the Shiwalik Hills form the southern line of the Himalayas, and the Gangetic Plain (draining toward the Ganges River) lies below it for the full extent of the Indian subcontinent and Bangladesh.
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