AS SUMMITEERS gather in Nepal to celebrate the 50th anniversary of the first ascent of Everest, a British geologist has announced the completion of a geological map of the mountain. As well as providing the most detailed record of the rocks exposed across a vast swathe of northern Nepal and southern Tibet, the map reveals that the forces behind the world鈥檚 highest peak were more powerful than anyone imagined.
Everest was already known to be complicated. It formed about 50 million years ago, when the Indian plate ploughed northwards into the Asian plate, closing the Tethys Ocean basin in the process. The bulk of the mountain consists of metamorphic rocks and granite generated by high temperatures and pressures. But it is topped by layers of unaltered limestone that still contain marine fossils.
The process continues to this day, as the two vast slabs of continental crust are forced into each other, pushing the Asian plate upwards while the Indian plate is forced down. To complicate things, two fault lines that slice through the Himalayan range have moved some of the deeper rocks to the high peaks.
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The map created by Mike Searle of the University of Oxford shows the mountain-building process in unprecedented detail. 鈥淲e can match rocks from 100 kilometres beneath the Tibetan plateau to those at the base of Everest,鈥� he says. And by analysing the characteristic signatures of minerals in the rocks, he has seen rock layers that have eroded from the top of the Himalayas in sediments being laid down in the Bengal basin, Indian Ocean and Siwalik basin.
But Searle got a surprise when he combined his Everest findings with his data from the rest of the Himalayas. He found that the rock types and structures were remarkably similar across the entire range, encompassing thousands of kilometres. 鈥淭he age of the collision between India and Asia was the same all the way from Zanskar and Ladakh to south-east Tibet,鈥� he says. The granite also formed at the same time, about 20 million years ago, all the way down the line of the collision. Rather than various parts of the plates moving at different speeds or colliding at different times, Searle鈥檚 map reveals that India moved as one when it collided with Asia, exerting unrelenting pressure along its entire length in one of the most powerful events in the Earth鈥檚 geological history.
Mapping Everest was no mean feat. A climber himself, Searle has visited the mountain six times over the past 22 years, collecting hundreds of samples. Back in the lab, he analysed, dated and cross-referenced the rocks against satellite and aerial photographs to deduce information about areas too remote to access.
Searle now plans to drape his geological data onto a three-dimensional map compiled from satellite data. 鈥淭he idea eventually is to make a detailed comparison of every profile across the Himalaya, to relate the timing of processes such as collision, metamorphism and uplift into a single theory for mountain building in the region,鈥� he says. 鈥淏ut that is a lifetime鈥檚 work.鈥�
How geology affects the climb
When the British Everest team first set out to bag the world鈥檚 highest peak, they had little idea of the geology of the mountain. The only maps of the region鈥檚 rocks had been sketched from a distance, so the climbers had to rely on aerial photographs and information gathered by a Swiss team during their failed attempt on the summit in 1952.
We now understand exactly how Everest鈥檚 geology dictates the nature of its treacherous terrain. The lower parts of the mountain comprise steep cliffs of granite that give way to the notorious Khumbu Icefall, where the Khumbu Glacier has carved a chasm between the south face of Everest and the neighbouring peak, Nuptse. One of the most dangerous parts of the climb, the icefall is a jumble of shifting ice blocks. Above the icefall the terrain flattens out, providing relatively stable ground where climbers pass from granite to layers of softer black shale. The steep climb up to the South Col peak is mostly on similar snow-covered slate until near the summit, when the infamous 鈥測ellow band鈥� limestone appears.
On the North Ridge that leads to the summit there are three bands of this hard limestone, forming the three steps where British mountaineers George Mallory and Andrew Irvine disappeared in 1924. The last and most perilous of these is the Hillary step, named after Edmund Hillary who, with Sherpa Tenzing Norgay, scaled it for the first time on 29 May 1953.