High & rising: Mike Searle describes how Everest and the Himalaya were formed 50 million years ago when the Indian subcontinent collided with Asia.
The story of Everest's geology begins down in the southern hemisphere, when the great continental landmass of Gondwana--the fusion of South America, Africa, India and Antarctica--began to crack. After the Indian plate broke away from Africa, Madagascar and the Antarctic about 200-150 million years ago (mya), it began to drift northwards across what is now the Indian Ocean at the relatively rapid rate of 15-20 centimetres a year. About 50 mya, when it was close to the equator, it collided with Asia, closing the wide ocean that separated them. Slowing to about five centimetres a year, it ploughed about 2,000 kilometres northwards into Asia, crumpling and lifting both the Himalaya and the Tibetan plateau in the process. The collision zone lies along the Indus and Yarlung Tsanpo valleys in southern Tibet, where geologists have discovered marine rocks formed in the relict Tethys Ocean.
The Himalaya lie along the northern margin of the Indian plate, which is still being thrust under southern Tibet, doubling the normal thickness of the continental crust. This thickening has resulted in an increase in temperature and pressure deep below the surface, which has caused the sedimentary rocks transform into crystalline metamorphic rocks. At temperatures of around 700[degrees]C, these rocks began to melt, forming granite.
As the Indian plate continued to move northwards, sliding beneath the Himalaya, large faults developed. The plate movement causes stresses along these faults to slowly build up and when the rocks finally give way and fracture, it frequently causes large earthquakes.
Mount Everest lies along the axis of the Greater Himalaya. The rocks at the mountain's summit are marine limestones that still contain fragments of tiny marine organisms, notably conodonts, from the Ordovician Period (505-440 mya). Throughout the late Palaeozoic and Mesozoic eras, the rocks that form the summit of Everest and much of southernmost Tibet were formed along a stable continental margin, where a great thickness of limestone was deposited close to sea level. Following the collision of India with Asia, these rocks crumpled into folds, as the continental plates converged with one another. The sedimentary rocks north of the Rongbuk Valley in Tibet show spectacular examples of this.
As the convergence continued and the Indian plate thickened, rocks in the middle and lower crust were transformed from sediments into schists and gneisses. These metamorphic rocks are made of minerals such as micas, garnet, kyanite, sillimanite and cordierite. As temperatures increased during compression and thickening, the first granitic melts began to form. Larger granites coalesced into horizontal layers, or sills, and were injected for large distances along planes of weakness. These granites--which contain the characteristic black, needle-shaped mineral tourmaline and some red garnets and blue aquamarine--form the huge cliffs above Everest Base Camp on the Khumbu Glacier, as well as most of Everest's neighbour, Nuptse.
The Himalayan chain was probably high mountain range 30-20 mya. Radioactive uranium and lead isotope dating suggests that thickening of the crust, metamorphism and melting reached a peak at this time. The Tibetan plateau was also high, but exactly how high it is impossible to say. Gravitational forces acting on the high plateau of Tibet forced the hot and ductile deforming crust beneath Tibet to flow outwards towards the south of the plateau along the Greater Himalaya. Seismic evidence from Tibet reveals the presence of a layer of partially molten rocks, with pockets of liquid melt or granite magma forming today beneath southern Tibet, in a similar position to that where the Everest granites formed 20 mya.
The Everest granites have been traced along the main Himalayan range west as far as the mountains of Lingtren, Pumori, Cho Oyu and Shisha Pangma, and east as far as Makalu and Chomolonzo, and form spectacular vertical cliffs. During the southward expulsion of the layer of hot, partially molten rocks 20-17 mya, major faults formed, bounding this extruding layer of `mush'.
Two of the largest faults in the Himalaya cut right through the Everest massif, and each one places higher-level rocks onto deep, hotter rocks. The upper fault cuts right across the summit pyramid of Everest and can be traced along the flanks of the Rongbuk and Kharta valleys for a distance of more than 50 kilometres. The lower fault cuts across the Southwest Face and across the Lhotse-Nuptse ridge, and places thin-bedded black schists above granites and deep crustal metamorphic rocks exposed around 84Base Camp.
The uplift of the Himalaya and Tibet undoubtedly caused major climatic change in the northern hemisphere. The timing of surface uplift in Tibet is poorly known, but there is good evidence that the plateau has been high for at least 17 million years. Rapid cooling during exhumation of deeply buried rocks from below Everest also provides evidence that the Himalaya were high mountains during the period 20-17 mya. Fossils of tropical leaves and palm trees found in 13-million-year-old rocks at 4,500 metres in Tibet suggest that the climate, and the altitude, was very different in the past. The strengthening of the monsoon around eight mya led to a decrease in carbon dioxide in the atmosphere, and increased chemical weathering in the Himalaya.
The timing of the onset of the monsoon system in southern Asia is poorly understood, but certainly the uplift of Tibet would have resulted in a heat source in the atmosphere above the plateau with a fixed low-pressure area centred over northern India. This effectively draws warm, moist air in from the Indian Ocean, causing the northeasterly flowing monsoon. The Himalaya forms an impressively abrupt climatic barrier. The southern slopes of Everest and the Khumbu Himalaya are deeply eroded, with lush forests and raging rivers carving deep gorges, whereas north of Everest, the Tibetan plateau lies in a rain shadow and is relatively flat, dry and barren.
Everest may currently be the highest mountain on our planet, but this is a transient phenomenon. The geological record goes back at least 4.4 billion years, but Everest has probably only been the highest peak for a few hundred thousand of them.
GPS data tell us that the current convergence rate between northern India and Tibet is about 45 millimetres a year. As tectonic forces continue to push Everest upwards, glacial, wind and water erosion continues to tear it down. As long as the spreading centres in the Indian Ocean continue to push the Indian Plate northwards and India continues to be thrust under the Himalaya and Tibet, then Everest and the Himalaya will continue to rise.
A senior research fellow at the University of Oxford's Department of Earth Sciences and an active rock climber, Mike Searle's main geological interests are in structural, metamorphic and igneous geology of mountain belts. The tectonic evolution of the Himalaya in particular is a specialist subject for Mike, who has carried out ongoing research in the region since 1981. He has taken part in more than 16 mountaineering expeditions to the range, including several to Everest--he visited advanced base camp on the East Rongbuk Glacier in 2000. On page 64, Mike, who gives regular talks to various organisations including the Alpine Club, takes us back to the very birth of Mount Everest. His explanation of when and how the mountain was formed, and (importantly to mountaineers) of what rock types it is comprised, gives a fascinating insight into the world's highest mountain.
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|Title Annotation:||geological formation of Mt. Everest|
|Date:||May 1, 2003|
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