Question

18. Which of the following must be balanced for a portion of the lithosphere to reach isostatic equilibrium?

A. The densities of the lithosphere and the asthenosphere
B. The compressive forces of two converging plates
C. The buoyant force from the asthenosphere and gravity
D. The isostatic uplift and the crustal rebound

Answer 1

The answer above is incorrect, I am sure. The correct answer is C.

Step-by-step explanation:

The Basin and Range Province in the western United States and the Himalayas in Asia are two areas with changing elevation. Once a region of the lithosphere stops rising, this region is said to be in isostatic equilibrium. This condition is reached when the buoyant force from the asthenosphere balances gravity.

Its not A because the density of the lithosphere isn't the same everywhere. Therfore they cannot be balanced.

Answer 2

A. The densities of the lithosphere and the astenosphere

Step-by-step explanation:

The isostatic equilibrium occurs when enormous tectonic plates composed of crustal and upper mantle material are able to float on top of the astenosphere, which in turn is composed of denser rock material. In order for such a thing to be achieved the lithosphere and the astenosphere have to have balanced densities between each other. In other words, the density of the material on top, has to be in balance with the density of the material below it, thus to not have any uplift or sinking occurring between them.