Are the Himalayas Growing Shorter?

The recent studies on the question “Are the Himalayas growing shorter?”, has brought in to light a few new dimensions about the mechanism of mountain building.

Geologist know a lot about how mountains are built. But occasionally some new piece of evidence turns up that requires them to think again.

The latest awkward fact concerns a series of fruits high in the Himalayan mountains which do just the opposite of what geologists expected. They also shed some light on a possibly perverse question: why isn’t Mount Everest even taller than it is?

The story of the Himalayas begins about 45 millions years ago when the Indian and Eurasian continents first collided. Instead of both continents buckling together like an accordion, the Indian continental plate, a great slab of the earth’s crust and mantle, started sliding under the Euiasian plate.

Since then the Indian plate has been plunging under-neath the Eurasian one, but not without protest. The lower part, which is more dense, has split off from the top so that it can continue to sink. Meanwhile, the upper part has been breaking up.

A series of roughly parallel fractures called “thrust faults” have divided it into a series of wedges. Because the two continents are still converging, the wedges have gradually been pushed upwards and southwards, piling upon one another and making the earth’s crust in the region shorter and thicker. Hence the Himalays.

Recently, researchers working high in the Himalays found a series of “normal faults” which seem to spoil that story. Unlike thrust faults, which move the lower parts of the earth’s crust upwards, normal faults move younger, higher material downwards.

Thrust faults are found where the crust is shortening, making it thicker; normal faults occur where it is stretching, making it longer and thinner.

When geologists see a thrust fault and a normal fault in the same mountain range, they usually take it that they date from different periods. But detailed studies in the Himalayas have convinced Dr. Clark Burchfiel, Dr. Leigh Royden, and Dr. Kip Hodges of the Massachusetts Institute of Technology (MIT) that the Himalayan thrust faults and normal faults were active simultaneously.

They think this is because the Himalayas were getting too tall. They liken the earth’s crust floating on the mantle to a rubber duck floating on water. If the duck were to become taller, it would float higher in the water until finally it became unstable and wobbled. When the 102 TEXT BOOK OF PHYSICAL GEOLOGY

earth’s crust gets thick enough, it too becomes unstable. But mountains cannot easily lip over, because they are, as mountainers will have noticed, wider at the bottom than they are at the top. Instead they limit their height. Generally, mountains are worn away by erosion; the taller the mountain, the more rapid the erosion.

But the rate of erosion of the higher Himalayas is probably not fast enough to keep the height of the mountains under control. The Indian and Eurasian continental plates are converging and thrust faults are piling up too fast

According to the three geologists at MIT, the normal faults may be the result of a growth rate that erosion cannot handle, Because of the normal fault, a wedge part-way up the moutain range is being squeezed out towards the south, allowing the top to become lower as the wedge -moves out from beneath the range.

This is happening just where the researchers’ mathematical studies of stress say it should- along the mountain’s steepest slope. Thus although the Himalayas are still being built up from the bottom, they get no taller because they are collapsing at the top.

This adds a new twist to traditions’ theories of mountain-building. Geologists are now scattering over the globe to look for thrust faults and normal faults that happened at the same time; and they are rethinking the faults they observed in the past.

On closer examination, it may turn out that mountains elsewhere have, like the Himalayas, reached their peaks.