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The Himalayas: Two continents collide
Among the most dramatic and visible creations of plate-tectonic forces are the lofty Himalayas,
which stretch 2,900 km along the border between India and Tibet.
This immense mountain range began to form between 40 and 50 million years ago,
when two large landmasses, India and Eurasia, driven by plate movement, collided.
Because both these continental landmasses have about the same rock density,
one plate could not be subducted under the other.
The pressure of the impinging plates could only be relieved by thrusting skyward,
contorting the collision zone, and forming the jagged Himalayan peaks.
About 225 million years ago, India was a large island still situated off the Australian coast,
and a vast ocean (called Tethys Sea) separated India from the Asian continent.
When Pangaea broke apart about 200 million years ago, India began to forge northward.
By studying the history — and ultimately the closing– of the Tethys,
scientists have reconstructed India’s northward journey.
About 80 million years ago, India was located roughly 6,400 km south of the Asian continent,
moving northward at a rate of about 9 m a century. When India rammed into Asia about 40 to 50 million years ago,
its northward advance slowed by about half.
The collision and associated decrease in the rate of plate movement are interpreted to mark the beginning of the rapid uplift of the Himalayas.
The Himalayas and the Tibetan Plateau to the north have risen very rapidly.
In just 50 million years, peaks such as Mt. Everest have risen to heights of
more than 9 km. The impinging of the two landmasses has yet to end.
The Himalayas continue to rise more than 1 cm a year — a growth rate of 10 km in a million years!
If that is so, why aren’t the Himalayas even higher?
Scientists believe that the Eurasian Plate may now be stretching out rather than thrusting up,
and such stretching would result in some subsidence due to gravity.
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Two continents collide
Fifty kilometers north of Lhasa (the capital of Tibet), scientists found layers of pink sandstone containing grains of magnetic minerals (magnetite) that have recorded the pattern of the Earth’s flip-flopping magnetic field. These sandstones also contain plant and animal fossils that were deposited when the Tethys Sea periodically flooded the region. The study of these fossils has revealed not only their geologic age but also the type of environment and climate in which they formed.
For example, such studies indicate that the fossils lived under a relatively mild, wet environment about 105 million years ago, when Tibet was closer to the equator. Today, Tibet’s climate is much more arid, reflecting the region’s uplift and northward shift of nearly 2,000 km. Fossils found in the sandstone layers offer dramatic evidence of the climate change in the Tibetan region due to plate movement over the past 100 million years.
At present, the movement of India continues to put enormous pressure on the Asian continent, and Tibet in turn presses on the landmass to the north that is hemming it in. The net effect of plate-tectonics forces acting on this geologically complicated region is to squeeze parts of Asia eastward toward the Pacific Ocean. One serious consequence of these processes is a deadly “domino” effect: tremendous stresses build up within the Earth’s crust, which are relieved periodically by earthquakes along the numerous faults that scar the landscape. Some of the world’s most destructive earthquakes in history are related to continuing tectonic processes that began some 50 million years ago when the Indian and Eurasian continents first met.
The Himalayan chain
The easternmost segment of the system begins at the western end of the Sunda island arc and continues into the arcuate chain of mountains that constitute the Himalayas, which contain the highest peaks on Earth. This chain was formed as the Indian subcontinent, a passenger on the same plate that currently underthrusts the Sunda arc, collided with the southern margin of Asia and subsequently penetrated some 2,000 kilometres into the rest of Asia.
As the leading edge of India, on which Paleozoic and Mesozoic sedimentary
rocks had been deposited, plunged beneath southern Tibet, these rocks were scraped off
the subcontinent and thrust back onto its more stable parts. With continued penetration of the Indian subcontinent,
slices of the metamorphic basement of its leading edge were scraped off the rest of it and thrust onto one another,
so that the rocks of the present-day Himalayan chain consist of slices of India’s ancient northern continental margin.
Physiographically, this chain can be subdivided into three parallel belts: the Lesser Himalayas, the Great Himalayas,
and the Tethys Himalayas. (Some authorities prefer a subdivision into four belts,
the additional one designated the Outer, or Sub-Himalayas.) The Great Himalayas are defined by an arcuate chain of the highest peaks.
To the south lie the Lesser Himalayas, a belt about 100 kilometres wide with
an average elevation of 1,000 to 2,000 metres that is dissected by the rivers emanating from the Great Himalayas and north of it.
To the north, the Tethys Himalayas form the southern edge of the Tibetan Plateau.
The rocks of the Lesser Himalayas consist primarily of mildly metamorphosed sedimentary rock largely of Precambrian age. At present, the remainder of the Indian subcontinent underthrusts the Lesser Himalayas on a very gently dipping thrust fault, so that the rocks forming this belt are sliding over the ancient top surface of India. As a result, the uplift of the Lesser Himalayas seems to be relatively slow.
The rate of uplift in the Himalayas seems to be rapid in two parallel zones: (1) at the very front of the range where the ancient metamorphic and sedimentary rocks of the Lesser Himalayas have been thrust up and onto the young sediments, and (2) beneath the Great Himalayas. The thrust fault that carries the Himalayas onto the intact part of India is a ramp overthrust, with the steep part of the ramp dipping north beneath the Great Himalayas. Slip on this steep part allows the rapid uplift of the Great Himalayas, which in turn creates the high peaks and carries rock from deep in the crust to the Earth’s surface.
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