Easter 2011
The mountains ooze outwards under their own weight onto the lowlands, which are made of more rigid rock
Alex Copley is a Research Fellow in Pembroke College and the Department of Earth Sciences
Alex copley
Rajash Himalayan Trails
composed of these minerals to behave as viscous fluids, with a viscosity over 1020 times greater than honey. Mountain ranges can therefore be imagined as piles of very thick custard left out to cool. Earthquakes become the tearing of the cool skin on the surface. Given this fluid-like behaviour of the majority of the mountain ranges, we can use the rich body of fluid dynamics research to understand how the ranges behave, and shed light on the forces that cause the earthquakes. Such research has recently been conducted on the Tibetan Plateau. The figure shows that the southwestern (Himalayan) and northern margins of the Tibetan plateau are steeply sloping, whilst the northeastern and southeastern sides have much more gentle gradients. Because of their geological histories over the past few billion years, the rocks that form peninsular India and those in the lowlands on the northern side of the Tibetan Plateau are very hard and inert. This is because in previous episodes of mountain building, the rocks have been heated to extreme temperatures. Although they have since cooled down, all of the volatile constituents, which serve to weaken the rocks as a whole, have been melted out of them. In these regions the convergence between the Indian and Asian plates forces the rigid lowlands underneath the mountains. The mountains ooze outwards under their own weight, over-riding the lowlands like a dollop of custard spreading under its own weight across a slice of toast, known as a ‘gravity current’.
Fluid dynamics tells us that in this situation the flow forms a very distinctive shape with a flat top and a steeply sloping front, as we see on the northern and southwestern sides of Tibet. The gentle slopes on the northeastern and southeastern sides of Tibet show that the story here is clearly different. In these regions, there are no hard rocks being pushed under the edges of the mountains, as is the case in India, so the mountains spread out over hotter and weaker underlying material. Our custard is now spreading over the surface of a tank of olive oil, rather than a slice of toast. In this case fluid dynamics tells us that gentle slopes should form, as we see on the eastern edges of the mountain range. The notable exception is the location of the devastating magnitude 8 earthquake that occurred in the Sichuan province of China in 2008. Here we see steep slopes because the Sichuan Basin is a region of hard rock (much like India), with the mountains oozing out over it. The earthquake was a manifestation of this motion in the brittle upper layer of the Earth—the skin on the top of our custard. Treating the Tibetan Plateau as a pile of viscous fluid has shown that we can understand the shape of the mountain range and the earthquakes within the region by applying our existing knowledge of fluid dynamics. So the next time you’re eating Marmite on toast, take a moment to place a large dollop in the middle of the slice and look at how it spreads out under its own weight; by doing this you are recreating southern Tibet and the Himalayan mountains in miniature. This story, however, carries a sobering postscript: the same processes which are occurring in the Himalayas are also occurring in the region of the devastating Sichuan earthquake. This means it is only a matter of time until a similar or larger earthquake rocks the very densely populated Ganges River Valley—an event which, as historical records show, has happened many times in the past.
The Himalayan and northern margins of the Tibetan Plateau are steeply sloping, whilst the northeastern and southeastern sides have much more gentle gradients
Mountains: Go with the Flow 9