Why Are Old Mountains High?

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Science  28 Mar 2003:
Vol. 299, Issue 5615, pp. 1947-1949
DOI: 10.1126/science.299.5615.1947e

Many ancient mountain belts are still mountainous. For instance, the last major orogenic activity in the Appalachians, Urals, and parts of Canada and Australia was several hundred million years ago, yet these regions still have mountains and locally high relief (though not as imposing as the younger ranges of the Himalayas, Alps, or Rockies). Simple calculations imply that, at reasonable erosion rates, rivers alone should level this topography in a few tens of millions of years.

Baldwin et al. explore this longstanding conundrum in a series of river topography models. Their analysis identifies processes that may serve to extend the times required for removing mountains. First, some uplift or rebound of the crust after the initial erosion (as a result of decreased mass) helps to preserve mountain height. Two factors reduce the efficiency of river erosion: The channel bed becomes protected by alluvium, and increasingly larger flows, which require major floods, are required to entrain significant sediment as relief drops even slightly. These additive factors can explain the persistence of mountains in Paleozoic orogens, and perhaps the absence of even older Precambrian ranges. — BH

J. Geophys Res.108, 2158 (2003).

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