How do the different densities of the oceanic and continental crust affect earth's outer shape?

Question

I am currently reading a textbook and I am slightly confused at a paragraph that deals with some general facts about earth's outer shape.

It mentions that the density of the continental crust is less than the density of the oceanic crust. Then it concludes (somehow?) that the continental lithospheres weigh less than the oceanic lithospheres. Using this then, it says that the continental lithospheres are "floating more up" on the asthenosphere than the oceanic lithospheres. And then it continues with other facts.

I feel like there's something missing here. I understand that the denser material will sink in more. But how does this affect earth's outer shape? I mean sure, it means that the continental crust will appear more higher than the oceanic, but I fail to see this. All pictures I have seen don't show this clearly. Could someone maybe explain this in more detail with an appropriate image or animation?

Answer 1

The two confusions have everything to do with Isostasy as mentioned by Gimelist. I advise you to read the Wikipedia page because it will explain everything to you! But maybe this helps:

1. The continental crust is made up of relatively light-weight material compared to oceanic crust. That's really all that the authors of your book should have said.

2. The fundamental idea of isostasy is that from a certain depth onwards to the center of the earth, the pressure is identical no matter where you are. This 'compensation point' is usually taken at about 100 km depth. This is an assumption about the physical state of the Earth -- it is thus an assumed reality!

3. This means that you can take any single column from 100 km depth (say, a cylinder, or a square meter) and extend it all the way to the surface, and all the columns that you could choose must weigh exactly the same (such that they cause the same pressure at 100 km depth). Mathematically, this is an integral over the densities(!) that you encounter as you move towards the Earth's surface: $\text{Pressure}=\text{mass}/\text{area}=\int_{100 \text{km depth}}^{\text{surface}(x)}\rho(x,z)\Delta z=\text{constant}$. It is the density that varies with $x$, i.e. your location on the earth, that makes the exact location of the $\text{surface}$ variable with $x$ also!

4. So you're moving up in your column and summing all the densities you encounter every meter. Density is always positive, so your sum is always growing. You also know that the solution has a fixed, constant, solution. We could thus just keep integrating from 100 km depth towards the surface, and when our integral reaches the constant, we know that we are at the earth's surface! That's a useful bit of information, because...

5. If the material in your column is relatively dense, the distance from 100 km depth until the $\text{surface}$ must then also be relatively small, because you're reaching the integrated constant faster! And vice versa, if the material in your column is relatively light you know that the surface extends out relatively far. It then follows that the oceanic crust has its surface at a lower level than the continental crust, because it is about 20% denser.

6. Effects of buoyancy only make the matters worse, as continental crust tends to be much thicker than the oceanic crust, and correspondingly it 'floats' higher on the fluid-y astenosphere below it. This is much like an iceberg floating on the sea: a larger iceberg both sticks into the sea more and sticks out of the sea more. The continental crust does the same: it has a deeper base and higher surface than the continental crust, because it 'floats' on the more dense 'fluid' underneath.

There are thus roughly two effects (although of very similar physical principles) that make the continental crust rise above the ocean crust: it is less dense and it is thicker! Therefore, continental crust rises above continental crust to produce the same pressure at depth, and its base lies below the oceanic crust because it is thicker (and floating like an iceberg).

So the missing piece of information in your book was the perspective of a constant pressure. I hope that helps :-)

Answer 2

It mentions that the density of the continental crust is less than the density of the oceanic crust. Then it concludes (somehow?) that the continental lithospheres weigh less than the oceanic lithospheres.

Density = mass / volume. For a given volume of substance (like lithosphere), the less dense one (continental) will weigh less than the other (oceanic).

I understand that the denser material will sink in more. But how does this affect earth's outer shape? I mean sure, it means that the continental crust will appear more higher than the continental, but I fail to see this. All pictures I have seen don't show this clearly.

Islands and continents sit higher than the bottoms of oceans.