An essential feature of plate tectonics is that only the outer shell of the
Earth, the lithosphere, remains rigid during intervals of geologic time. Be-
cause of their low temperature, rocks in the lithosphere do not significantly
deform on time scales of up to 109 years. The rocks beneath the lithosphere
are sufficiently hot so that solid-state creep can occur. This creep leads to
a fluidlike behavior on geologic time scales. In response to forces, the rock
beneath the lithosphere flows like a fluid.
The lower boundary of the lithosphere is defined to be an isotherm (surface
of constant temperature). A typical value is approximately 1600 K. Rocks
lying above this isotherm are sufficiently cool to behave rigidly, whereas
rocks below this isotherm are sufficiently hot to readily deform. Beneath the
ocean basins the lithosphere has a thickness of about 100 km; beneath the
continents the thickness is about twice this value. Because the thickness of
the lithosphere is only 2 to 4% of the radius of the Earth, the lithosphere
is a thin shell. This shell is broken up into a number of plates that are in
relative motion with respect to one another. The rigidity of the lithosphere
ensures, however, that the interiors of the plates do not deform significantly.
The rigidity of the lithosphere allows the plates to transmit elastic stresses
during geologic intervals. The plates act as stress guides. Stresses that are
applied at the boundaries of a plate can be transmitted throughout the
interior of the plate. The ability of the plates to transmit stress over large
distances has important implications with regard to the driving mechanism
of plate tectonics.