1. Gravity: The downward force exerted by the mass of the Earth's crust and mantle.

2. Buoyancy: The upward force exerted by the less dense mantle on the denser crust.

Imagine a block of wood floating in water. The wood sinks until the buoyant force from the water pushing upwards is equal to the weight of the wood pushing downwards. Similarly, in isostasy, the crust sinks into the mantle until the upward buoyant force of the mantle balances the downward force of gravity acting on the crust.