Ocean penetrating the earth

[Reg007]

If earth absorbs water then what percentage of water is the ocean floor made out of. How deep does the ocean drain down to the earth�s crust? If this is the case, then surly the earth�s crust is drifting on one huge water table!

[Moog]

Oceanic crust is made up of igneous rocks which are crystalline and therefore impervious to water. Where water does penetrate the oceanic crust through cracks and fissures the rapid temperature gradient between the crust and mantle causes it to be heated and rise back through the crust to be vented though underwater geysers. The upper mantle does have a certain percentage of water in it [I'd have to look it up for an exact figure - it's estimated to be about the same percentage as in meteorites] but this water doesn't come from the oceans and is stored in "wet" silicates rather than molecules of H2O.

[New Forester]

Soil contains many air spaces. When it absorbs water, the water is soaking into these spaces. Rock has fewer spaces, so it absorbs less water. The so-called "water table" is actually just the level in porous rock where the spaces are full of water -- but it's still mainly rock. Rock is very much denser than water. If water and rock (or sand) are mixed, the water "floats" up, and the rock settles down. As rock layers build up, the pressure increases and more water is squeezed out. All this means that the mud and rock under the ocean can't be all that wet -- and of course at the very high pressures down there, there are no air spaces for water to drain into. In parts of the world where the ocean-floor rock is being pushed down towards the lower, hotter layers of the crust, some water goes too. I believe this eventually comes out in hot springs and volcanic eruptions, and I think it's also thought to lubricate the slippage of rock faults. As the high-pressure, water passes through hot rock, it dissolves minerals, moving them through the rock and often concentrating them. This is where many valuable mineral ores have come from.

[Reg007]

Moog & New Forester, I must applaud and thank you for your outstanding answers. It is all so clear now and makes sense. Does the earth�s rotation around the sun and the moon play a part in all of this?

[Moog]

That's no problem Reg007 the only way I know that the Earth's rotation and the moon influence the oceans is in terms of tides.

[Moog]

Just to add another point - the amount of sediment on the ocean floor isn't as great as you might initially think, because of sea floor spreading oceanic crust is constantly being destroyed by being subducted back into the mantle and replaced by new crust at mid ocean ridges. So the maximum age of the ocean floor is around 200 million years [which is less than 5% of the age of the Earth] so sediment only has a relatively short amount of time to accumulate on the ocean floor so are never more than a couple of thousand metres thick at the edges of oceans