* Gravity: The downward pull of the Earth on the lithosphere (the rigid outer layer of the Earth).

* Buoyancy: The upward force exerted by the asthenosphere (the partially molten layer beneath the lithosphere) on the lithosphere.

Imagine a block of wood floating in water. It sinks until the weight of the wood is balanced by the upward buoyant force of the water. Isostasy is similar. The lithosphere "floats" on the asthenosphere, and its depth is adjusted until the gravitational force pulling it down is balanced by the buoyant force pushing it up.