Here's a breakdown of what this means:

* Thinner Crust: The thinner crust, around 8km thick, is found under the oceans and is called oceanic crust. It's primarily composed of denser, heavier rocks like basalt and gabbro.

* Thicker Crust: The thicker crust, around 40km thick, is found under the continents and is called continental crust. It's mostly made of lighter, less dense rocks like granite and andesite.

This variation in thickness highlights the following:

* Different geological processes: Oceanic crust is constantly being formed at mid-ocean ridges and recycled back into the mantle at subduction zones. Continental crust is older and much more stable.

* Different densities: The density difference between oceanic and continental crust is why continents rise above sea level.

* Plate tectonics: The movement of tectonic plates is responsible for the creation and destruction of both oceanic and continental crust.

In summary, the varying thickness of the Earth's crust is a key indicator of the dynamic and ever-changing nature of our planet.