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Mountains and Red Herrings

Peter Molnar
Professor, Geological Sciences
CIRES, University of Colorado

Climate Change, Uplift of Mountain Ranges, and Evolution of Humans: Which is chicken, which - egg, and which - red herring?

Many of us were taught, in elementary school if not later, that mountain ranges go through an evolution of "youth," characterized by dramatic relief and rugged topography, to "maturity," in which mountains and ridges become gentler, and finally to "old age," where hills rather than mountains dominate the landscape.

Although this notion makes qualitative sense, its converse - that rugged mountain ranges must necessarily have formed recently in geologic time - does not logically follow. Imbued with the association of rugged topography with youthful mountain belts, geologists throughout the world have attributed rugged landscapes to geologically recent uplift (since late Pliocene and Quaternary time or since 3-4 Million years ago), regardless of whether or not other geologic evidence suggests recent growth. Said in other words, not only regions of geologically active mountain building, like the Himalayas, but also tectonically dead ranges, like the Rocky Mountains of the USA, (and geologically senile ranges, like the Alps) are treated as young.

I challenge this general view and suggest that the evidence commonly used to infer recent "uplift" of mountain ranges is better understood to result from global climate change, with an increase in erosion one consequence of such a change. To call attention to the absurdity of the resulting implication that mountain ranges grew simultaneously on all continents, I first note the correspondence in time of such alleged "uplift" with the evolution of humans, which could be taken to suggest that the rise of mountain ranges is responsible for the evolution of Homo Sapiens. Then I analyze evidence of different kinds that have been used to infer uplift of mountain ranges, beginning with misuse of the word "uplift" itself, and I try to show that all such evidence can be explained by climate change. For instance, paleobotanical evidence of plants that today live in warm climates but that left fossils in regions that today are high and cold can be understood as the result of global cooling, without the need to postulate elevation changes. Similarly, the cooling increased glacial activity and hence glacial erosion, which has left an obvious imprint on the modern landscapes of mountain ranges.

We live in an environment that is very different from that characterizing most the past 300 million years, and most of the earth's entire 4.5 billion year history. If we had evolved only 3 million years earlier, before the cooling that led to the Ice Ages, it is quite likely that there would be no winter Olympics, skiing would not be a sport except on Greenland and Antarctica, Colorado would be little more than a hilly Kansas, and Boulder county would be just as conservative politically as the rest of the state.