Ancient cooling may not have raised mountain crests


The drastic change in the Earth's surface caused by the creation of the Himalayas and Tibetan plateau is thought by some researchers to have induced global cooling in the Cenozoic period, 64 million years ago.

Other researchers have argued the opposite: that a cooler and more erosive climate would raise mountain crests, which could be misinterpreted as late Cenozoic tectonic uplift. Now an MIT researcher who studies long-term interaction of climate and tectonics says it is unlikely that Cenozoic cooling could have substantially increased the height of mountain peaks.

At most, alpine glacial erosion caused by global cooling could raise the height of a peak only a few hundred meters, which is not a significant increase, said Kelin Whipple, assistant professor of earth, atmospheric and planetary sciences, in a study that appeared in a recent issue of Nature.

PEAKS AND VALLEYS

As part of an effort to understand why the Earth has become progressively cooler over the last 50 million years, Professor Whipple and colleagues at MIT are studying how relief, or the differences in height between peaks and valleys, changes in response to a change in climate.

The peaks and valleys of the Earth's surface are caused by the giant tectonic plates of the Earth's crust crashing into each other and the continually wearing forces of weather, water and glacial ice. These uneven surfaces, in turn, cause changes in weather patterns.

Although it seems that erosion would cause mountains to get shorter, it can in some circumstances actually lead to uplift of mountain peaks. Because the low-density crust of the Earth floats on the mantle like an ice cube on water, the mean elevation of mountain ranges is set by crustal thickness, just as larger ice cubes protrude farther above the surface. Erosion thins the crust and generally reduces mean elevation. However, if erosion increases relief, then mountain peaks tend to rise as valleys deepen.

Professor Whipple does not argue with the theory that Cenozoic climate change has enhanced erosion rates. However, he explains that under most circumstances, enhanced erosion can be expected to "reduce relief as much as it produces relief, so it doesn't actually drive the peaks up."

The trick is that you can't separate what happens in the channel from what happens at the ridge top. If the main valley is lowered by rapid incision, the tributary valleys and the hillslopes will follow, he said. This results in an overall decrease in relief.

"Since 1990, there has been much interest in a debate over a 'chicken-and-egg' problem regarding preliminary evidence that tectonic uplift of large mountain belts may have influenced global climate," Professor Whipple said. "Key to the debate has been the question of whether the reverse might actually be true: that global cooling and glacial erosion produced an increase in topographic relief in many glaciated mountain ranges that was then mistaken as evidence of accelerated tectonic uplift.

"Our paper tackles one aspect of this: how a shift to a 'more erosive' climate state affects topographic relief in tectonically active mountain ranges. We show that contrary to many researchers' intuition, in most cases, a shift to a more erosive climate will reduce topographic relief, partially resolving the 'chicken-and-egg' debate," he said.

This work is supported by the National Science Foundation.

A version of this article appeared in MIT Tech Talk on October 27, 1999.


Topics: Earth and atmospheric sciences

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