The Earth's mantle is a solid, but it behaves like a very viscous fluid over long periods. This means it can flow, and the flow is what allows tectonic plates to move. However, it's not about things "floating" in the sense of being on the surface of something.

Here's a better way to think about it:

* Tectonic plates "float" on the asthenosphere: The asthenosphere is a layer within the upper mantle that is partially molten and behaves like a very viscous fluid. The tectonic plates, which are the rigid outer layer of the Earth (crust and uppermost mantle), sit on top of this layer.

* Density differences drive the movement: The tectonic plates are less dense than the asthenosphere, which is why they "float" on top. However, the movement of the plates is driven by convection currents within the mantle itself. Hotter, less dense material rises, while cooler, denser material sinks, creating a circular flow. This flow drags the plates along with it.

So, while the concept of "floating" isn't entirely accurate, the density differences between the tectonic plates and the asthenosphere are crucial for the movement of the Earth's tectonic plates.