This is only tangentially-relevant, but I recently read that the earth's mantle is mostly solid! (It does still flow, but as a solid.)

The state of the rock beneath the crust is subject to two competing influences:

* High temperature [promotes liquid state]

* High pressure [promotes solid state]

and apparently the latter influence (mostly) wins.

Of course, when such rock reaches the surface, number 1 wins so we see liquid rock.