1. Gravity: This force pulls downwards on the crust and mantle, trying to compress them.

2. Buoyancy: This force pushes upwards on the crust, due to the less dense mantle material supporting it.

Isostasy is the state of gravitational equilibrium where the weight of a column of crust and mantle is equal to the weight of an equivalent column of mantle beneath it. This balance is achieved through vertical adjustments of the crust, either rising or sinking until the buoyant force and gravitational force are equal.