According to the theory of isostasy, which statement must be true?
a. the asthenosphere has the same density as the lithosphere.
b. the lithosphere is denser than the asthenosphere.
c. the asthenosphere is denser than the lithosphere.
d. the lithosphere has the same density as the core.

A

Explanation:

Isostatic equilibrium is used to state why some regions of the lithosphere have greater elevation than others. The crust ‘floats’on the‘liquid’ mantle below and different regions  of the crust submerge into the mantle differentially depending on their local densities or weight. Heavier regions such as in mountainous region (like the Himalayas) are sunken deeper into the mantle that other regions.  Every local region has its own local isostatic equilibrium.

The correct option is Option C

i.e. Asthenosphere is denser than lithosphere.

Explanation:

Asthenosphere is the layer situated below the lithosphere.

Theory of isostasy says that the layers above the other layer must have lesser density. This has two explanations. Firstly, if its not hold, then the denser above layer would sink below the less denser bottom layer. Secondly, as we go inwards, the pressure increases. This leads to the lower layers have greater density. So, as Asthenosphere is situated below the lithosphere, it has higher density.