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lithosphere The top of the mantle and the crust of the Earth is a relatively cold and rigid boundary layer called the lithosphere, which is typically about 60 miles (100 km) thick. Heat escapes through the lithosphere largely by conduction, transport of heat in igneous melts, and 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 (250 km) 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 five miles per second (8 km/s). 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 from which the crustal rocks were extracted. 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, or line in two dimensions and plane in three dimensions, along which temperatures have the same value.
Since the lithosphere is rigid, it cannot convect. 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 profile. The lithosphere thus forms a rigid, conductively cooling thermal boundary layer riding on mantle convection cells, becoming convectively recycled 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 miles (or kilometers) at the ocean ridges. The lithosphere increases in thickness proportionally to the square root of age to about a 35-mile (60-km) thickness at an age of 160 million years.
One can also measure the thickness of the lithosphere by the wavelength and amplitude of the flex-ural 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. The load of mountains on the edges of continents tends to deflect the underlying lithosphere into a bulge with an amplitude of typically 1,000 feet (300 meters) and a wavelength several hundred miles long. In contrast, oceanic crust exhibits shorter wavelength and higher amplitude bulges, because oceanic lithosphere is not as stiff or flexurally rigid as oceanic crust. 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; craton; ophiolites; plate tectonics.
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