Re: Erosional rates

rjtrembgeologist@xxxxxxxxx wrote:

Your question depends on many complex geologic variables. However, I
think its safe to say that over geologic time the denudation rate and
sedimentation rate is equal. When you think about it, there is no
other conclusion that would be valid. Matter can neither be created or
destroyed.

Sediments just get shifted around.

I would disagree with this for four reasons: (not the first law of thermodyamics, but the conclusion)

1)Matter is being added to the planet from space,(meteorites, comets, space dust) and also being ablated from the planet into space. (escaping gases, earth dust, space probes). Therefore the earth is not an entirely closed system, and there is no reason this needs to be in equilibrium.

2)Sediments on earth can undergo phase change as minerals and chemicals break up--especially those minerals containing H,C,O, S and N, which can dissolve and/or gas off. This is not simply limestone dissolving and calcite being deposited somewhere else, but changing compositions of water and air. While the entire system is always striving towards chemical equilibrium, the amount of sediments does not remain constant.

3) Climatological change affects the phase state of water, hydrogen and oxygen on the planet. One might argue that total crust does not vary much, but the amount of dry land exposed does, which in turn affects mountain-building/sedimentation/erosion/dissolution etc.

4) Cycling of materials from the mantle to the crust and vice versa is not necessarily at a constant rate and mountain-building vs subduction has an effect on the amount of crust at any given time. (Not sure if this has ever been quantified, but fairly sure that the amount of volcanism/subduction are only in equilibrium over long time scales.)

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