if continental crust were thinner than its average thickness of 40 km, would it depress the mantle more or less than it does now?
To determine whether a thinner continental crust would depress the mantle more or less than its current state, we need to understand the concept of isostasy.
Isostasy refers to the balance between the lithosphere (which includes the crust and uppermost part of the mantle) and the asthenosphere (the partially molten, ductile portion of the mantle below the lithosphere). The lithosphere "floats" on the asthenosphere, similar to an iceberg floating in water.
The thickness of the crust impacts the isostatic equilibrium of the lithosphere. Generally, thicker crust will "float" higher in the asthenosphere compared to thinner crusts.
Therefore, if the continental crust were thinner than its average thickness of 40 km, it would depress the mantle less than it does now. This is because the decreased thickness of the crust would result in a lower elevation in the asthenosphere.
To summarize: a thinner continental crust would cause less depression of the mantle compared to its current state.