if continental crust were thinner than its average thickness of 40 km, would it depress the mantle more or less than it does now? Explain..
If the continental crust were thinner than its average thickness of 40 km, it would depress the mantle less than it does now. To understand why, let's consider a basic principle of physics known as isostasy.
Isostasy is the concept that Earth's lithosphere (which includes both the crust and a portion of the upper mantle) floats on the denser, more fluid asthenosphere beneath it. The lithosphere adjusts its elevation to maintain equilibrium with the underlying mantle. The thicker the lithosphere (such as the continental crust), the deeper it will penetrate into the underlying mantle to achieve isostatic balance.
So, when the continental crust is thicker, it pushes deeper into the mantle, displacing more mantle material and causing the mantle to be depressed further. Conversely, if the continental crust is thinner, it exerts less downward force and displaces less mantle material, resulting in less depression of the mantle.
In other words, a thinner continental crust would have less effect on the underlying mantle, causing it to be depressed to a lesser extent compared to the current average thickness of 40 km.