Tectonic Plates
1) Two basic types of LITHOSPHERE: continental and oceanic.
CONTINENTAL lithosphere has a low density because it is made of
relatively light-weight minerals. OCEANIC lithosphere is denser than
continental lithosphere because it is composed of heavier minerals. A
plate may be made up entirely of oceanic or continental lithosphere,
but most are partly oceanic and partly continental.
2) Beneath the lithospheric plates lies the ASTHENOSPHERE, a layer
of the mantle composed of denser semi-solid rock. Because the
plates are less dense than the asthenosphere beneath them, they are
floating on top of the asthenosphere.
3) Deep within the asthenosphere the pressure and temperature are so
high that the rock can soften and partly melt. The softened but dense
rock can flow very slowly over geologic time. Where temperature
instabilities exist near the core/mantle boundary, slowly moving
convection currents may form within the semi-solid asthenosphere.