Plates Plates Plate tectonics deals with the study of the motion and deformation of the Earth's crust. Plate tectonic theory tries to account for the movement of the crust throughout geologic time. Motion of the crust is that of a rigid body , which accounts for the style of deformation we see. Plates are rigid bodies of rock that essentially float atop a region of partial melt called the asthenosphere . The crust is of two basic types, continental and oceanic, that differ on the basis of composition.
Continental crust is mostly of granitic composition.
Oceanic crust, on the other hand, is of basaltic composition.