mercurio said:
Ophioliolite, undoubtedly the other factors you mention like internal drainage twist the figures. On the other hand, high levels of precipitation and life also seem to go hand in hand, so the one does not rule out the other.
Naturally, since the bulk of all living things are made of water, we find more life where there is more water. So you have two dependent variables (biomass and erosion/transport/sedimentation) and one independent variable (precipitation).
Obviously this is something of a simplification: thus, plant cover will
reduce erosion [not increase as you are trying to suggest]; the presence, or absence, of plant cover will effect precipitation; but these are generally effects of smaller magnitude.
mercurio said:
On rereading I see I gave the impression there are four factors, I did not mean to do that, only to indicate those things precede life or are not influenced by life and therefore life could never be their sole cause.
I understand your meaning here.
mercurio said:
The mechanism I mentioned earlier: pure and simple weight of sediment (wedging):
Again,
sedimentation is not the motive force for plate tectonics. I take it that you are referring to accretionary wedges. These are a
consequence and not a
cause of plate tectonics.
mercurio said:
If mantle convection was the only cause of plate tectonics, why does it not stop, like on other planets
Of the other terrestrial planets Mercury is too small and is almost ‘all core’; Mars is too small and cooled early; Venus, as you noted, is just plain different. Why?
There is a very good chance that the answer is visible to you if you step outside today and look up. The moon likely formed when a Mars sized planetoid collided with the proto-Earth. This stripped away much of the early sialic crust.
Now, as you are aware, sialic crust (that forms the continents) is a lot thicker and harder to push around than simatic crust (that floors the oceans). If Earth had not lost that proportion of sial at its birth the plates might well have locked up a couple of billion years ago: the result could well have been a true sister for Venus.
mercurio said:
Nature does not prefer anything, but evolution decides what stays or not. If there is a feedback between prolonged plate tectonics and greater opportunity for the abundance of life, it will be exploited, I think.
You appear to be changing your argument, switching between cause and effect. Earlier your argument was that life promoted/initiated/powered plate tectonics. This is erroneous. In this last statement you are saying the reverse, that life may be favoured by plate tectonics. That is absolutely true. Without plate tectonics the sedimentation rates would have eliminated most land mass. Plume volcanics would have produced large islands (picture Olympus Mons), but no continents.
So , I am now confused as to what you are saying.
[There is one way in which life might just have saved, though not caused, plate tectonics. Without life to remove carbon dioxide from the atmosphere, Earth could have suffered the greenhouse fate of Venus. Without water to lubricate subduction zones the whole process might have locked up.] .
