Geoscience Reference
In-Depth Information
something with many of the properties of a liquid. And it is a liquid rather like non-drip
paint - the harder you push on it, the easier it becomes to deform it. Models of this sort can
account for the patterns of mountain ranges in central Asia but not for the high plateau of
Tibet.
The rise of Tibet
The pile-up of relatively low-density crustal rocks could not simply be accommodated by
downward thrusting, and the entire region began to float upwards. The dense lithospheric
root beneath Tibet detached and sank back into the underlying asthenosphere. The remain-
ing thickened continental rocks floated upwards, lifting the Tibetan plateau by as much as
8 kilometres. At the same time, parts of Asia tried to slide out of the way, with Indoch-
ina heading eastwards. This sideways motion stretched the continent further north, causing,
among other features, lake-filled rifts in Tibet and the deep rift of lake Baikal in Russia.
With some of the underlying cold, dense lithosphere removed, the hot asthenosphere was
sufficiently close to the Tibetan crust to cause localized melting and account for the recent
volcanic rocks found in parts of that country. There is also seismic evidence for a vast pool
of partially molten granite about 20 kilometres under the southwestern part of the Tibetan
plateau. That would also help to explain how Asia absorbed the impact of India and why the
Tibetan plateau has remained relatively flat though surrounded by high mountain ranges. It
seems that, overall, the Tibetan plateau is unlikely to get any higher than its present aver-
age of 5,000 metres. Any additional uplift would be balanced by a flow of material away
to the sides. The more mountainous areas too have an average elevation of no more than
about 5,000 metres above sea level. Here, heights are kept in check by erosion. Although a
great deal of material has already been eroded from the Himalayas, regions such as Nanga
Parbat in the north of Pakistan are still rising today by several millimetres per year, making
slopes unstable and prone to landslides.
Monsoon
The Himalayas reach almost as high as passenger aircraft normally fly, and the mountains
pose a significant barrier to atmospheric circulation. The result is that central Asia to the
north remains cold in winter and dry for most of the year. In the summer, warm air rising
above the Tibetan plateau holds back moist air from the southwest, so that the clouds build
up and release their moisture in the torrential rain of the Indian monsoon. The monsoon
stirs up the Arabian Sea, bringing nutrients to the surface and resulting in an annual plank-
ton bloom. This in turn leaves its trace in the sediments beneath. Sediment cores show that
this sequence began around 8 million years ago, perhaps corresponding to the end of the
major uplift of the Tibetan plateau and the origin of the monsoon weather pattern. Wind-
