Mystery of the Himalayas solved

Mystery of the Himalayas solved
The world's highest mountains shot up by 2km when a massive slab of rock anchoring them fell away

The mystery of why the Himalaya mountains and the Tibetan plateau are the highest in the world has at last been answered, with the discovery of a gigantic chunk of rock slowly sinking towards the centre of the Earth.

When the massive slab - up to eight times the area of the UK and as thick as a dozen Everests on top of each other - dropped off, the lighter crust above it rebounded upwards like a cork released under water, geophysicists say. This "sudden uplift" would have raised the Himalayas by as much as 2km (1.24 miles) to their present height.

If not for the surge, Sir Edmund Hillary and Sherpa Tenzing Norgay might have found themselves reaching the "roof of the world" by conquering Aconcagua (6,962m) in Argentina while Everest languished at a mere 6,848m above sea level, 2,000m below its actual peak.

The discovery of the missing mantle - the cold, heavy rock beneath the crust - was revealed last week by Professor Wang-Ping Chen at the University of Illinois, whose team used more than 200 super-sensitive seismometers strung across the Himalayas, from India deep into Tibet.

"While attached, this immense piece of mantle under Tibet acted as an anchor, holding the land above in place," said Professor Chen, whose results are to be published in the Journal of Geophysical Research. "Then, about 15 million years ago, the chain broke and the land rose."

By pushing the Himalayas to their current altitude, more than 8,000m above sea level, and raising the Tibetan plateau to 5,000m, the detachment of the block was responsible for both the monsoon rains that make south Asia so fertile and the Gobi desert in central Asia. Warm winds blowing from the Pacific Ocean cool as they rise over the mountains, releasing the moisture they contain as torrential rains, leaving almost no water to fall on the arid interior of the continent.

Some scientists have even suggested that the rise of the Himalayas could have triggered the Ice Age by increasing the total amount of global rain and removing vast amounts of carbon dioxide from the air.

The gigantic slab from the Indian continental plate - now 670km below the Earth's surface and sinking through the hotter, softer rock surrounding it at a rate of some 10cm a year - was predicted in 1989 by the Oxford geophysicist Philip England, who welcomed Professor Chen's paper, calling it "a good piece of forensic seismology".

The region covered by the Himalayas and the Tibetan plateau should have the thickest part of the Earth's lithosphere - the crust plus mantle - due to the collision between the Indian and Asian continental plates 55 million years ago, which continues today with India inching north at a rate of 5cm a year. Yet surprisingly, volcanoes - usually found where the crust is thin - feature on the Tibetan plateau, and rifts have formed as the raised crust spreads eastwards into China, suggesting that it is far higher than can be supported by the underlying mantle.

Professor England's explanation for this - that the lithosphere is thinner than expected due to the loss of a huge portion of the Indian mantle - was supported by little hard evidence until now.

Geologists trying to establish how high mountains were in the distant past have only one measuring stick: fossil leaves. "Leaf shapes vary according to the temperature; the colder the ambient temperature, the more spiky," said Nigel Harris, professor of tectonics at the Open University, who has examined fossil leaves from the Himalayas. "We couldn't tell the difference between present day and 15 million years ago."

Scientists can't even be sure how high the land was before India crashed into Asia, obliterating the Tethys Ocean which used to separate them. Like western South America, the coast could have been lined by mountains.

But some scientists remain sceptical. One geologist at Cambridge, who wanted to remain anonymous because he hadn't yet read Professor Chen's paper, suggested that the slab could be the remains of the Tethys Ocean plate. Professor England counters that both the Asian and Indian plates have moved north since then. "Tethys would be 2,000km to the south," he said. The hunk of mantle found by Dr Chen is directly below Tibet.