According to epirogenesis, if the crust decreases its mass, then the mantle.?

Explanation:

Epirogenesis explains the movements that the cortex undergoes vertically as a function of the variation in its mass or volume and the gravitational anomalies associated with these variations. These movements take place looking for the isostatic balance of the lithosphere on the mantle.

If the crust decreases in mass, then the mantle increases in volume and pushes the crust up

Explanation:

The mantle is the mostly-solid bulk of Earth's interior. The mantle lies between Earth's dense, super-heated care and its thin out layer, the crust. The mantle is about 2,900kilometers (1,802metres) thick, and makes up a whooping 84% of earth's total volume.