Here's why:

* Crust: The Earth's crust is made up of mostly lighter elements like silicon, oxygen, aluminum, and sodium. It's the outermost layer and relatively thin.

* Mantle: The mantle is a thick layer below the crust composed of denser minerals like iron, magnesium, and calcium.

This difference in density is crucial for plate tectonics. The less dense crust "floats" on the denser mantle, allowing the plates to move and interact with each other.