The Lithosphere

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-

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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.