|MadSci Network: Earth Sciences|
Eric - You are right that plants create new organic matter which builds up the soil. This process can be incredibly slow - people estimate that it can take several hundred years to generate a centimeter of topsoil. Thus, in areas like the Great Plains, soil slowly accumulates over thousands of years if the system is left undisturbed. However, over longer time periods (hundreds of thousands to millions of years) other processes take over that can increase or decrease the amount of material in any given spot. Glaciers or floods can scrape or wash away soil and sediment. Receding glaciers can dump entire ridges of soil. Uplift and volcanism can generate mountains. Like many questions in geology it depends about the amounts of time and the size you're talking about (the scale). Over long time periods, the surface of the earth is constantly changing (increasing and decreasing) independent of what humans can do. Human activity can definitely rapidly remove material (meters of soil in steep areas with bad management processes) but even that amount is tiny compared to the total thickness of the earth. Still, it is that tiny, tiny skin of soil that supports most of the plants and animals on earth (including us!) - so at the scale of _our_ lifetimes, it can be a very important (and usually preventalbe) loss. http://e arthsci.org/teacher/basicgeol/weasoil/weasoil.html
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