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9.4: How Fast Are the Continents Worn Down

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    13513
  • Geomorphologists use the term denudation for the overall, regional lowering of a continental land surface by processes of weathering, erosion, and transportation of bedrock material to the oceans. Rivers, of course, are by far the most important agents of denudation, although ice sheets have been very effective at certain times and places in geologic history.

    For a long time, geoscientists have been trying to develop estimates of rates of denudation. There are several ways of doing this—none of them perfect.

    Measuring the sediment load of rivers. Because almost all of the products of denudation are carried by rivers, it should seem natural to you to measure the sediment loads of all of the word’s rivers to get a yearly total of the sediment delivered to the oceans, and then imagine that volume to be spread over the area of the continents to get an annualized denudation rate. Many such estimates have been made. There are some serious problems, however. One is that it’s notoriously difficult to measure the sediment load of rivers, especially bed load. Moreover, systematic measurements of the total load (suspended load, bed load, and dissolved load) are common on relatively few rivers around the world. Abroaderproblemis:howrepresentativeislastyear’ssedimentload,say,of denudation on time scales of millions of years? It’s generally agreed that humankind has greatly accelerated denudation, owing largely to agriculture. And then there’s the matter of natural fluctuations in climate, on scales ranging from centuries to millions of years. Despite all of that pessimistic commentary, however, here are some figures. Worldwide estimates are in the range of several centimeters per millennium, but values range widely, from just a few centimeters per millennium, for tectonically stable regions without high relief, to far higher values, even up to a few meters per millennium, in tectonically active areas with high relief and abundant rainfall (a widely cited example is the eastern mountains of Taiwan).

    Deposition rates in the ocean. The obvious alternative to measuring the sediment loads of rivers is to estimate the volume of sediment that has been deposited in the oceans over some period of geologic time. Most of the solids load of rivers is deposited along the continental margins (but dissolved load is not easily taken into account in this way). Geophysical methods of measuring sediment thickness, together with coring to define the stratigraphy, lead to regional estimates of denudation. The advantage of this method is that it should provide estimates of denudation over geologically long times and also show changes in rates through time. Estimates for the eastern seaboard of North America are a few centimeters per millennium, in the same ballpark as estimates made on the basis of sediment loads of rivers.

    Rates of erosion of land areas. If the thickness of continental rocks removed in some interval of geologic time can be measured or estimated, that gives the most direct indication of denudation rates. In some places it has been possible to make a direct measurement—for example, where a basalt flow of known age covered a preexisting land surface and then has been removed along with some thickness of the underlying material. In many regions, study of metamorphic facies and thermal history has led to estimates of the thickness of rock unroofed owing to long-continued uplift, especially in orogenic regions. Again, rates vary widely but are generally in the range of a few centimeters per millennium to a few tens of centimeters per millennium.

    A striking conclusion from data of the kind noted above is that, given denudation rates even of just a few centimeters per millennium, which seems to be a conservative estimate, a continent like North America would be worn down to low-lying terrain in geologically short times, of the order of ten million years!

    Just divide the average elevation of North America, of the order of five hundred meters (\(5 \times 10^5\, cm\)), by a denudation rate of, say, five centimeters per millennium (\(5 \times 10^{-3}\, cm/yr\)). The result: ten million years. Why, then, are all of the continents not just low-lying plains? (One factor is that rates slow way down as elevation and relief decrease.) We are forced to appeal to long-continued uplift: orogenic, in the case of uplift of mountain ranges, which is relatively rapid and relative restricted in area, or epeirogenic, which is relatively slow but covering relatively large areas. (The adjective orogenic is used to describe building of mountains in a local region; the adjective epeirogenic, less easy to grasp, is used to describe broad vertical movements that affect large parts of a continent.)

    That leads to the issue of the competition between uplift rates and denudation rates. You might think in terms of the two end-member cases:

    • Uplift rates are much greater than denudation rates. This would be the case in an tectonically active but arid region with rapid uplift of the crust: the land is coming up fast, but rainfall to weather the rock and remove the weathering products is scarce. The maximum elevation in such an area would then be controlled by rock strength. There seems to be a self-limiting process at work here: when the uplifted mass becomes sufficiently high, it literally flows out sideways, at depth, on a regional scale of many hundreds of kilometers, by a complex of tectonic processes we need not deal with here, thus limiting the maximum elevation. That seems to be happening in the Altiplano, in the Andes, and in the Tibetan plateau, in southern Asia.
    • Denudation rates are as great as uplift rates. Owing to high rainfall in a humid climate conducive to rapid rock weathering, denudation rates can keep up with even very high rates of uplift, limiting mountain elevations to just a few thousand meters despite the very rapid uplift. One of the classic areas of that kind is the eastern mountains of Taiwan.
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