1. Gravity: This force pulls downward on the Earth's crust and mantle, trying to compress and flatten the Earth.

2. Buoyancy: This force pushes upward on the Earth's crust, caused by the less dense crust floating on the denser mantle.

Imagine a block of wood floating in water. The wood sinks until it displaces an amount of water equal to its own weight. This is analogous to the Earth's crust floating on the mantle. The crust will sink until it displaces an amount of mantle equal to its own weight.

The principle of isostasy states that the Earth's crust is in a state of hydrostatic equilibrium, where the downward force of gravity is balanced by the upward force of buoyancy. This equilibrium is dynamic, meaning the crust can adjust its elevation in response to changes in its mass, like the accumulation of ice sheets or erosion of mountains.