1. Gravity: This force pulls the mass of the Earth's crust and mantle downwards.

2. Buoyancy: This force pushes upwards on the crust and mantle, caused by the pressure exerted by the denser material beneath it.

Think of it like an iceberg. The part of the iceberg above the water is analogous to the mountains, while the submerged part is like the root of the mountain extending into the mantle. The iceberg floats because the weight of the ice is balanced by the upward force of the water (buoyancy). Similarly, mountains "float" on the denser mantle because the downward force of gravity on the mountain is balanced by the upward force of buoyancy from the mantle.