The tectonic plates of the lithosphere on EarthEarth cutaway from center to surface, the lithosphere comprising the crust andlithospheric mantle (detail not to scale)
Earth's lithosphere, which constitutes the hard and rigid outer vertical layer of the Earth, includes the crust and the lithospheric mantle (or mantle lithosphere), the uppermost part of themantle that is notconvecting. The lithosphere is underlain by theasthenosphere which is the weaker, hotter, and deeper part of the upper mantle that is able to convect. Thelithosphere–asthenosphere boundary is defined by a difference in response to stress. The lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deformsviscously and accommodates strain throughplastic deformation.[citation needed]
The thickness of the lithosphere is thus considered to be the depth to theisotherm associated with the transition between brittle and viscous behavior.[2] The temperature at whicholivine becomesductile (~1,000 °C or 1,830 °F) is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle.[3]
The concept of the lithosphere as Earth's strong outer layer was described by the English mathematicianA. E. H. Love in his 1911 monograph "Some problems of Geodynamics" and further developed by the American geologistJoseph Barrell, who wrote a series of papers about the concept and introduced the term "lithosphere".[4][5][6][7] The concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must exist a strong, solid upper layer (which he called the lithosphere) above a weaker layer which could flow (which he called theasthenosphere). These ideas were expanded by the Canadian geologistReginald Aldworth Daly in 1940 with his seminal work "Strength and Structure of the Earth."[8] They have been broadly accepted by geologists and geophysicists. These concepts of a strong lithosphere resting on a weak asthenosphere are essential to the theory ofplate tectonics.[citation needed]
The lithosphere can be divided into oceanic and continental lithosphere. Oceanic lithosphere is associated withoceanic crust (having a mean density of about 2.9 grams per cubic centimetre or 0.10 pounds per cubic inch) and exists in theocean basins. Continental lithosphere is associated withcontinental crust (having a mean density of about 2.7 grams per cubic centimetre or 0.098 pounds per cubic inch) and underlies the continents andcontinental shelves.[9]
Oceanic lithosphere consists mainly ofmafic crust andultramafic mantle (peridotite) and is denser than continental lithosphere. Young oceanic lithosphere, found atmid-ocean ridges, is no thicker than the crust, but oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. The oldest oceanic lithosphere is typically about 140 kilometres (87 mi) thick.[3] This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age. In fact, oceanic lithosphere is a thermal boundary layer for theconvection[10] in the mantle. The thickness of the mantle part of the oceanic lithosphere can be approximated as a thermal boundary layer that thickens as the square root of time.[citation needed]
Here, is the thickness of the oceanic mantle lithosphere, is thethermal diffusivity (approximately 1.0×10−6 m2/s or 6.5×10−4 sq ft/min) for silicate rocks, and is the age of the given part of the lithosphere. The age is often equal to L/V, where L is the distance from the spreading centre of mid-ocean ridge, and V is velocity of the lithospheric plate.[11]
Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere. While chemically differentiated oceanic crust is lighter than asthenosphere,thermal contraction of the mantle lithosphere makes it more dense than the asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that atsubduction zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old.[12][13]
Geophysical studies in the early 21st century posit that large pieces of the lithosphere have been subducted into the mantle as deep as 2,900 kilometres (1,800 mi) to near the core-mantle boundary,[14] while others "float" in the upper mantle.[15][16] Yet others stick down into the mantle as far as 400 kilometres (250 mi) but remain "attached" to the continental plate above,[13] similar to the extent of the old concept of "tectosphere" revisited by Jordan in 1988.[17] Subducting lithosphere remains rigid (as demonstrated by deepearthquakes alongWadati–Benioff zone) to a depth of about 600 kilometres (370 mi).[18]
Continental lithosphere has a range in thickness from about 40 kilometres (25 mi) to perhaps 280 kilometres (170 mi);[3] the upper approximately 30 to 50 kilometres (19 to 31 mi) of typical continental lithosphere is crust. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at theMoho discontinuity. The oldest parts of continental lithosphere underliecratons, and the mantle lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of cratons" helps to stabilize these regions.[12][13]
Because of its relatively low density, continental lithosphere that arrives at a subduction zone cannot subduct much further than about 100 km (62 mi) before resurfacing. As a result, continental lithosphere is not recycled at subduction zones the way oceanic lithosphere is recycled. Instead, continental lithosphere is a nearly permanent feature of the Earth.[19][20]
Geoscientists can directly study the nature of the subcontinental mantle by examining mantlexenoliths[21] brought up inkimberlite,lamproite, and othervolcanic pipes. The histories of these xenoliths have been investigated by many methods, including analyses of abundances of isotopes ofosmium andrhenium. Such studies have confirmed that mantle lithospheres below some cratons have persisted for periods in excess of 3 billion years, despite the mantle flow that accompanies plate tectonics.[22]
^Donald L. Turcotte, Gerald Schubert, Geodynamics. Cambridge University Press, 25 mar 2002 – 456
^Stein, Seth; Stein, Carol A. (1996). "Thermo-Mechanical Evolution of Oceanic Lithosphere: Implications for the Subduction Process and Deep Earthquakes".Subduction: Top to Bottom. Geophysical Monograph Series. Vol. 96. pp. 1–17.Bibcode:1996GMS....96....1S.doi:10.1029/GM096p0001.ISBN9781118664575.
^abcO'Reilly, Suzanne Y.; Zhang, Ming; Griffin, William L.; Begg, Graham; Hronsky, Jon (2009). "Ultradeep continental roots and their oceanic remnants: A solution to the geochemical "mantle reservoir" problem?".Lithos.112:1043–1054.Bibcode:2009Litho.112.1043O.doi:10.1016/j.lithos.2009.04.028.
