if continental crust were thinner than its average thickness of 40 km would it depress the mantle more or less

If the continental crust were thinner than its average thickness of 40 km, it would actually depress the mantle less.

To understand why, let's first discuss the concept of isostasy. Isostasy refers to the state of equilibrium between the Earth's lithosphere (which includes the crust and a portion of the upper mantle) and the asthenosphere (the partially molten and ductile region of the mantle below the lithosphere). The lithosphere "floats" on the asthenosphere due to differences in density.

In isostatic equilibrium, the buoyant force acting on the lithosphere is balanced by the weight of the material above it. The thicker the continental crust, the higher the weight exerted on the underlying mantle. Consequently, it compresses or depresses the asthenosphere to some extent.

Now, if the continental crust were thinner, there would be less weight pressing on the mantle. This would result in less compression or depression of the asthenosphere. In other words, a thinner continental crust would lead to a lesser degree of mantle depression.

It's important to note that this explanation simplifies a complex geological process, but it gives a general understanding of how the thickness of the continental crust relates to mantle depression.