Rigid, rocky outer layer of the Earth, consisting of the crust and the solid outermost layer of the upper mantle .
It extends to a depth of about 60 mi (100 km). It is broken into about a dozen separate, rigid blocks, or plates (see plate tectonics ). Slow convection currents deep within the mantle, generated by radioactive heating of the interior, are believed to cause the lateral movements of the plates (and the continents that rest on top of them) at a rate of several inches per year.