Here's why:

* Lithosphere: This is the rigid outermost layer of the Earth. It's composed of the crust and the uppermost part of the mantle.

* Brittle Behavior: Brittle materials break or fracture when subjected to stress. The lithosphere is relatively cold and rigid, making it prone to fracturing under pressure, which is why we see earthquakes and the formation of mountains.

Other layers:

* Asthenosphere: The layer below the lithosphere is more ductile, meaning it can deform and flow under pressure.

* Mesosphere: The deeper part of the mantle is very hot, and its behavior is more plastic.

Let me know if you'd like to know more about the Earth's layers!