Oceans as thick as continents

[Enthalpy]

Hello you all!

 

It's just puzzling me... The Oceanic bed has some rifts and trenches, but these are rather local; on most area, the bed is rather flat.

Continents have mountains and valleys, but these are local as well, with most area fitting within 0 to 300m.

 

Now, I wonder: when observed from the Oceanic bed, the Ocean is about 3000m thick, and continents are only 100-200m higher (not thicker).

 

Is that pure chance that lasted for eons? Or is there some deep reason for it?

 

Thanks!

Marc Schaefer, aka Enthalpy

Edited September 30, 2012 by Enthalpy

[iDevonian]

When you say "thick", do you mean wide?

 

If you mean thick in the sense of elevation from the center of the earth, then id say erosion has gotten a hold of mountain features. Which gives continents similar elevations to oceans (with respect to the center of the earth).

 

Also there are relationships between buoyancy of continental plates over oceanic. A continental mass isn't just going to sky rocket into space.

Edited October 7, 2012 by iDevonian

[Ophiolite]

The oceanic plates are thin, temporary features of the Earth's crust, being generated at mid-ocean ridges and returned to the mantle at subduction zones. Compositionally oceanic crust is made of basic igneous rocks with a superficial smattering of sedimentary deposits.

 

The continental plates are largely permanent, though they have likely been growing to some extent throughtout Earth history. They are composed of lighter rocks that we might simplify as being of 'granitic' composition. They tend to 'float' on the underlying mantle, often with deep roots beneath mountain belts.

 

I believe this answers your question as to permanence of features, but I'm not to sure what you mean by some of your other points.