The solid crust rides on the fluid mantle and is comprised of two layers: the contninental crust and the oceanic crust. The outer continental crust is comprised of mostly light (comparitively) granite, while the oceanic layer is heavier basalt.
Fractured into plates, these are of varying dimensions and all move under the influence of sub-crustal convection currents in the mantle. This gives cause to continental drift.
SEE P.145 IN THE COLLINS GEM BASIC FACTS GEOGRAPHY BOOK
Last Updated: 29th February 2004