DEFINITION OF LITHOSPHERE

The top of the mantle and the crust is a relatively

cold and rigid boundary layer called the lithosphere,

about 60 miles (100 km) thick. Heat escapes through the

lithosphere largely by conduction, transport of heat in

igneous melts, and in convection cells of water through midocean

ridges. The lithosphere is about 75 miles (125 km)

thick under most parts of continents, and 45 miles (75 km)

thick under oceans, whereas the asthenosphere extends to

about a 155-mile (250-km) depth. Lithospheric roots, also

known as the tectosphere, extend to about 155 miles beneath

many Archean cratons.

The base of the crust, known as the Mohorovicic discontinuity

(the Moho), is defined seismically and reflects the

rapid increase in seismic velocities from basalt to peridotite at

5 miles per second (8 km/s). However, some petrologists distinguish

between the seismic Moho, as defined above, and the

petrologic Moho, reflecting the difference between the crustal

cumulate ultramafics and the depleted mantle rocks that the

crustal rocks were extracted from. This petrological Moho

boundary is not recognizable seismically. In contrast, the base

of the lithosphere is defined rheologically as where the same

rock type on either side begins to melt, and it corresponds

roughly to the 2,425°F (1,330°C) isotherm.

Since the lithosphere is rigid, it cannot convect, and it

loses its heat by conduction and has a high temperature contrast

(and geothermal gradient) across it compared with the

upper mantle, which has a more uniform temperature pro-

file. The lithosphere thus forms a rigid, conductively cooling

thermal boundary layer riding on mantle convection cells,

becoming convectively recycled back into the mantle at convergent

boundaries.

The elastic lithosphere is that part of the outer shell of

the Earth that deforms elastically, and the thickness of the

elastic lithosphere increases significantly with the time from

the last heating and tectonic event. This thickening of the

elastic lithosphere is most pronounced under the oceans,

where the elastic thickness of the lithosphere is essentially

zero to a few kilometers at the ocean ridges. This thickness

increases proportionally to the square root of age to about a

35-mile (60-km) thickness at an age of 160 million years.

The thickness of the lithosphere may also be measured

by the wavelength and amplitude of the flexural response to

an induced load. The lithosphere behaves in some ways like a

thin beam or ruler on the edge of a table that bends and

forms a flexural bulge inward from the main load. The wavelength

is proportional to, and the amplitude is inversely proportional

to, the thickness of the flexural lithosphere under

an applied load, providing a framework to interpret the

thickness of the lithosphere. Natural loads include volcanoes,

sedimentary prisms, thrust belts, and nappes. Typically, the

thermal, seismic, elastic, and flexural thicknesses of the lithosphere

are different because each method is measuring a different

physical property, and also because elastic and other

models of lithospheric behavior are overly simplistic.

See also ASTHENOSPHERE; CONTINENTAL CRUST; CRATONS;

OPHIOLITES; PLATE TECTONICS.

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