Ancient history and settlement.
The Himalayas may be more than 450 million years old – nine times older than previously estimated – according to a controversial new dating study.
The mountain range is generally thought to have arisen from the collision of India and Asia 55 million years ago. But George Gehrels, of the University of Arizona in Tucson, and his colleagues are proposing that the world’s loftiest peaks may owe some of their height to an earlier continental crash.
“We’ve come to the conclusion that there was an older mountain range in place before the current Himalayas,” says Gehrels.
Gehrels’ team analysed radioactive uranium in garnet and zircon grains from Himalayan rocks to determine their age. The amount of uranium that had decayed to thorium and lead indicated that many grains are between 450 million and 500 million years old.
The granites and schists containing these garnets were formed during a collision between India and a section of what is now Asia, the researchers suggest. The rocks were forced down to great depths, where intense heat and pressure melted and changed them.
As the collision continued, the mountains grew, pushing some of the rocks back up to the surface, the team argues. Thereafter, tons of sediment eroded from the mountains and settled on their flanks. Zircon grains in some of these ancient sediments are just slightly younger than the garnets.
Sometime after the collision, the idea goes, India pulled back. Then, 55 million years ago, it plowed into Asia once more, and a similar episode of mountain-building followed. India is still moving northwards, and the Himalayas grow about a centimetre taller each year – very fast by geological standards.
Ancient history and settlement.
The Himalayan range is one of the youngest mountain ranges on the planet and consists mostly of uplifted sedimentary and metamorphic rock. According to the modern theory of plate tectonics, its formation is a result of a continental collision or orogeny along the convergent boundary between the Indo-Australian Plate and the Eurasian Plate. The Arakan Yoma highlands in Myanmar and the Andaman and Nicobar Islands in the Bay of Bengal were also formed as a result of this collision.
During the Upper Cretaceous, about 70 million years ago, the north-moving Indo-Australian Plate (which has subsequently broken into the Indian Plate and the Australian Plate was moving at about 15 cm (5.9 in) per year. About 50 million years ago this fast moving Indo-Australian Plate had completely closed the Tethys Ocean, the existence of which has been determined by sedimentary rocks settled on the ocean floor and the volcanoes that fringed its edges. Since both plates were composed of low density continental crust, they were thrust faulted and folded into mountain ranges rather than subducting into the mantle along an oceanic trench. An often-cited fact used to illustrate this process is that the summit of Mount Everest is made of marine limestone from this ancient ocean.
Today, the Indian plate continues to be driven horizontally at the Tibetan Plateau, which forces the plateau to continue to move upwards. The Indian plate is still moving at 67 mm per year, and over the next 10 million years it will travel about 1,500 km (930 mi) into Asia. About 20 mm per year of the India-Asia convergence is absorbed by thrusting along the Himalaya southern front. This leads to the Himalayas rising by about 5 mm per year, making them geologically active. The movement of the Indian plate into the Asian plate also makes this region seismically active, leading to earthquakes from time to time.