Young-earth "proof" #16: Topsoil takes only a few thousand years to form. The present thickness of topsoil indicates a young earth.
16. Those "scientific" creationists who trot this plum about must be delirious! Do they really believe that we should wind up with x miles of topsoil (or some such nonsense) after billions of years? Geologically speaking, any given patch of land is seldom in equilibrium for long. Either it is collecting sediment or being eroded away, usually the latter. Suppose it collects sediment. Water-borne sediment will be washed in from higher ground, perhaps hills and mountains hundreds of miles away. Such sediment, even if from nearby hills, would normally carry very little organic material as the weathering slopes, themselves, would not have much to begin with. Sediment, in the form of dust, would normally come from very dry areas where organic material would be quickly oxidized. The sediment added to our patch of land may be great for building new soil, but if it accumulates too quickly it will merely bury the existing soil. The soil-making process would have to start over. In any case, the old topsoil, now compressed and deeply buried by sediment and soil, is no longer turned over by earthworms or small animals. It is deprived of oxygen and fresh organic material, such as rotting leaves. It is no longer a paradise for bacteria, and fungi. What organic material it did have is often lost by decay and slow oxidation. (Peat bogs and coal-forming swamps are an exception, but we would not count them as topsoils. Under unusual conditions a layer of topsoil can be "fossilized," even to the point of preserving the three-dimensional shape of tree leaves, as is the case at Yellowstone National Park.) In the long run, buried sediments are usually cemented into sedimentary rock, which brings us back to the beginning of this cycle. Thus, topsoil does not accumulate like most sediment, by simply piling up.
In the case of erosion, the topsoil, of course, is removed. This is usually the fate of every plot of land which remains above sea level long enough. Large areas of Canada, for instance, have been eroded down to the Precambrian basement rock! The geologic history of the strata making up the Grand Canyon is as much a history of erosion as it is of deposition! Consequently, a patch of soil cannot be older than the last local erosion--whenever that might have been. Forget about billions of years of soil accumulation!
Where sediment is neither being collected nor eroded, soils necessarily take their mineral components from the underlying parent rock. As more and more of that rock is weathered by the mechanical effects of freezing and thawing, the chemical and mechanical action of roots, or by other means, the soil is deepened. However, the deeper that soil gets, the more insulated the parent rock becomes to weathering. Fewer roots now reach the parent rock, and, in the bottom layers, the organic content of the soil is greatly reduced. That means less chemical weathering from bacteria and fungi. Sudden changes in temperature will have a smaller effect on the deeper parent rock. With the exception of the organic content, drawn mostly from the atmosphere, and the larger volume that broken rock takes up (which may raise the soil some distance above the original surface), in situ soils build downwards. Consequently, there is a practical limit to how deep the soil can get even if erosion never occurs. The accumulating humus will also reach an equilibrium, when new material balances that lost by decay and oxidation. (Topsoil is full of microbes that love to munch away on organic material, and don't forget the earthworms. Those earthworms don't get their calories from rock and clay!)
Just because a patch of topsoil takes x centuries to build up doesn't mean that the land is x centuries old. Most likely, that topsoil began to build up only recently, geologically speaking, and has either reached a practical limit to its depth or has been subject to erosion. Take the soil in my mother's backyard, for example. After about 18 inches the soil grades into a two-foot matrix of solid, smooth clay mixed with boulders. At about the three-foot level (in the center of the yard) the red-brown clay is abruptly terminated by a reddish conglomerate we call hardpan. A few sickly-looking roots, long dead for all I can tell, do penetrate the clay, usually by hugging the surfaces of the boulders, before being stopped cold by the hardpan. I suspect that most of them belong to plants which were chopped down years ago. There's not much down there in that clay to completely rot them away. Whatever damage is done to the clay by the few penetrating roots may, for all I know, be patched up by clay particles sifting down through the soil. The yard is located, along with much of San Diego, on a plateau, and meandering streams over thousands or millions of years have brought rocks down from the hills and rounded them into boulders. The numerous boulders in the clay and soil testify to much erosion since the limestone or chalk (just beneath the hardpan) was laid down. Further erosion is evident in the deeply etched edges of the plateau. Given the geologically recent erosion of the area and the difficulty of weathering the hardpan, is there any wonder that the soil is not thicker?
In summary, we're dealing with a dynamic and continuing cycle of topsoil formation and destruction, including periods of equilibrium, not a one-way accumulation of topsoil. Is that really so difficult to figure out? The whole idea of using topsoil-formation rates to prove that the earth is young just boggles my mind!
Last updated: Thursday, 10-Mar-2011 15:57:17 CST
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