* Isostasy: The Earth's crust and upper mantle are in a state of isostatic equilibrium. This means that the crust "floats" on the denser mantle, much like an iceberg floats on water.

* Buoyancy: The lighter continental crust "floats" higher than the denser oceanic crust. Mountains are formed by uplift, often due to tectonic plate collisions, which push the crust upwards. The root of the mountain, extending deep into the mantle, acts like a large buoyant object, keeping the mountain elevated.

* Strength of the Crust: The crust, despite being less dense than the mantle, is surprisingly strong. It can withstand the weight of mountains and resist sinking.

Think of it this way:

Imagine a block of wood floating on water. The wood floats because it's less dense than water. The wood has a "root" that extends below the waterline, keeping the wood afloat. Mountains are similar - they are less dense than the mantle and have a "root" that extends deep into the mantle, keeping them from sinking.

Erosion: Mountains are also constantly eroded by wind and water. This erosion actually helps maintain their height. As the mountain is eroded, the "root" is also shortened, reducing the weight and maintaining isostatic balance.

So, in summary, it's not a single force but a combination of:

* Isostasy: The balance between crust and mantle.

* Buoyancy: The force that keeps the mountain elevated due to its density compared to the mantle.

* Strength of the Crust: The ability of the crust to resist sinking.

* Erosion: The process that helps maintain the height of the mountain by removing weight.