Here's why:

* Lithosphere: This is the rigid outermost layer of Earth, consisting of the crust and the uppermost part of the mantle. It's about 100 km thick and is relatively less dense than the material beneath it (the asthenosphere). This density difference is key to plate tectonics, as the lithosphere "floats" on the asthenosphere.

Let me know if you have any other questions about Earth science!