Why do plates of lithosphere float?

Answer 1

I'm guessing here so don't take this as a reliable answer, however I would guess that it is because they move about due to the movements of the liquid magma in the mantle moving about (probably convectional currents controlling it)

Answer 2

The main force that shapes our planet's surface over long amounts of time is the movement of Earth's outer layer by the process of plate tectonics.

This picture shows how the rigid outer layer of the Earth, called the lithosphere, is made of plates which fit together like a jigsaw puzzle. These plates are made of rock, but the rock is, in general, lightweight compared with the denser, fluid layer underneath. This allows the plates to "float" on top of the denser material.

Movements deep within the Earth, which carry heat from the hot interior to the cooler surface, cause the plates to move very slowly on the surface, about 2 inches per year. There are several different hypotheses to explain exactly how these motions allow plates to move. Interesting things happen at the edges of plates. Subduction zones form when plates crash into each other, spreading ridges form when plates pull away from each other, and large faults form when plates slide past each other.

Answer 3

The Earth's lithosphere is made up of light rocks which makes it able to float on the asthenosphere which is made up of molten rock, which is a little bit heavier.