As far as I recall from the OU and various evening classes, there is a layer called the asthenosphere which underlies the lithosphere, which is solid rock and includes the crust and the upper part of the mantle. The asthenosphere is more plastic and permits the movement of the tectonic plates. It's not a sea of magma, though. The mantle below it is as Steve describes.

I think work has been done on whether lunar behaviour influences volcanic eruptions - if one is building up, is it more likely to happen as the Moon passes overhead or not? I haven't seen the results.