Here's why:
* Isostasy: The concept of isostasy explains how the Earth's crust floats on the denser mantle. Mountains and other topographic highs are essentially "floating" on the mantle.
* Buoyancy: To maintain equilibrium, the mass of the mountain range, including its deep root, must be equal to the mass of the displaced mantle. This means the root must be denser than the surrounding mantle, but less dense than the crust above it.
* Compensation: The root acts as a compensation mechanism for the weight of the mountain range, ensuring that the pressure exerted on the mantle is balanced.
Think of it like an iceberg: the part of the iceberg that's above water is just the tip, while the majority of its mass lies below the surface. Mountains are similar in that they have a large root that extends deep into the mantle.
Let me know if you'd like more detail on any of these concepts!
