* Crustal Thickness is Determined by Tectonic Processes: The depth of the crust is primarily determined by the forces of plate tectonics. When tectonic plates collide, the denser plate subducts (sinks) beneath the less dense plate. This process creates mountains and deepens the crust.

* Erosion is a Surface Process: Erosion is a surface process that wears down existing rock formations. It removes material from the surface of the Earth but does not directly change the thickness of the crust.

* Isostatic Equilibrium: The Earth's crust floats on the denser asthenosphere. As mountains erode, the weight on the crust decreases. This causes the crust to rise slightly, maintaining a state of isostatic equilibrium. However, this rise is relatively small and does not significantly alter the overall depth of the crust.

Think of it like this: Imagine a large iceberg floating in the ocean. If you carve away some ice from the top of the iceberg, the iceberg will rise slightly but its overall depth below the waterline won't change much. Similarly, erosion removes material from the top of a mountain, but it does not alter the overall thickness of the crust below.

In summary, erosion is a surface process that affects the shape of mountains but does not change the fundamental relationship between the crust and the asthenosphere.