Crustal plates float on the mantle, specifically the asthenosphere, which is a semi-molten layer of the Earth's upper mantle.

Here's a breakdown:

* Earth's Structure: The Earth is made up of several layers, including the crust (outermost layer), the mantle (middle layer), and the core (innermost layer).

* The Mantle: The mantle is a thick layer of hot, dense rock.

* The Asthenosphere: Within the mantle, there's a zone called the asthenosphere. This layer is partially molten and behaves like a very viscous fluid, allowing the rigid lithospheric plates (which include the crust) to move on top of it.

So, the plates don't actually float in the same way a boat floats on water. Instead, they are constantly moving due to convection currents within the mantle. Think of it as a giant conveyor belt where the plates are the "packages" being transported.