Question

The mean thickness of oceanic crust and continental crust is around:
A.5 kilometer and 15 kilometer respectively.
B.10 kilometer and 30 kilometer respectively.
C.5 kilometer and 30 kilometer respectively.
D.18 kilometer and 40 kilometer respectively.

Answer 1

Because the crusts are rock and the oceans are water…

Continental crust is thicker than oceanic crust because it is all of the accumulated sediment and felsic igneous rocks that are less dense than the oceanic crust. The old oceanic crust gets too dense to stay floating on the mantle and starts to sink (subduction). Sediments on the oceanic crust get scraped off and accumulate as continental crust. Repeat many times and the continental crust gets quite thick.

The oceanic crust forms at mid-ocean ridges and gets pulled away from the ridge by subduction zones; this prevents it from increasing in thickness the same way that continental crust does. Oceanic crust is also more dense than continental crust, and that density difference mean that while oceanic crust can be subducted, continental crust cannot.

This means that continental crust is raised in comparison to oceanic crust. Water runs downhill, so it flows from the continental crust onto the oceanic crust (for the most part - some of the continental crust is still under the oceans).

There is only so much liquid water on the Earth and the Earth is really quite big.