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A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
, it is composed of the crust and the portion of the upper
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
that behaves elastically on time scales of up to thousands of years or more. The crust and upper mantle are distinguished on the basis of chemistry and
mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proce ...
.


Earth's lithosphere

Earth's lithosphere, which constitutes the hard and rigid outer vertical layer of the Earth, includes the crust and the uppermost mantle. The lithosphere is underlain by the
asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is ...
which is the weaker, hotter, and deeper part of the upper mantle. The
lithosphere–asthenosphere boundary The lithosphere–asthenosphere boundary (referred to as the LAB by geophysicists) represents a mechanical difference between layers in Earth's inner structure. Earth's inner structure can be described both chemically ( crust, mantle, and core) a ...
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 deforms viscously and accommodates strain through
plastic deformation In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strai ...
. The thickness of the lithosphere is thus considered to be the depth to the isotherm associated with the transition between brittle and viscous behavior. The temperature at which
olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...
becomes ductile (~) is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle. The lithosphere is subdivided horizontally into tectonic plates, which often include terranes accreted from other plates.


History of the concept

The concept of the lithosphere as Earth's strong outer layer was described by
A. E. H. Love Augustus Edward Hough Love FRS (17 April 1863, Weston-super-Mare – 5 June 1940, Oxford), often known as A. E. H. Love, was a mathematician famous for his work on the mathematical theory of elasticity. He also worked on wave propagation and hi ...
in his 1911 monograph "Some problems of Geodynamics" and further developed by Joseph Barrell, who wrote a series of papers about the concept and introduced the term "lithosphere". 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 the asthenosphere). These ideas were expanded by Reginald Aldworth Daly in 1940 with his seminal work "Strength and Structure of the Earth." 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 of
plate tectonics Plate tectonics (from the la, label= Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large t ...
.


Types

The lithosphere can be divided into oceanic and continental lithosphere. Oceanic lithosphere is associated with oceanic crust (having a mean density of about ) and exists in the ocean basins. Continental lithosphere is associated with continental crust (having a mean density of about ) and underlies the continents and continental shelfs.


Oceanic lithosphere

Oceanic lithosphere consists mainly of mafic crust and
ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are compos ...
mantle ( peridotite) and is denser than continental lithosphere. Young oceanic lithosphere, found at mid-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 thick. 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 the
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...
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. h \, \sim \, 2\, \sqrt Here, h is the thickness of the oceanic mantle lithosphere, \kappa is the
thermal diffusivity In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end. It has the SI ...
(approximately ) for silicate rocks, and t 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-oceanic ridge, and V is velocity of the lithospheric plate. 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 at subduction 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.


= Subducted lithosphere

= Geophysical studies in the early 21st century posit that large pieces of the lithosphere have been subducted into the mantle as deep as to near the core-mantle boundary, while others "float" in the upper mantle. Yet others stick down into the mantle as far as but remain "attached" to the continental plate above, similar to the extent of the "tectosphere" proposed by Jordan in 1988. Subducting lithosphere remains rigid (as demonstrated by deep
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
s along Wadati–Benioff zone) to a depth of about .


Continental lithosphere

Continental lithosphere has a range in thickness from about to perhaps ; the upper approximately of typical continental lithosphere is crust. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity. The oldest parts of continental lithosphere underlie cratons, 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. Because of its relatively low density, continental lithosphere that arrives at a subduction zone cannot subduct much further than about 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.


Mantle xenoliths

Geoscientists can directly study the nature of the subcontinental mantle by examining mantle
xenolith A xenolith ("foreign rock") is a rock fragment ( country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in i ...
s brought up in kimberlite, lamproite, and other volcanic pipes. The histories of these xenoliths have been investigated by many methods, including analyses of abundances of isotopes of osmium and rhenium. 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.


See also

* Carbonate–silicate cycle * Climate system * Cryosphere * Geosphere * Kola Superdeep Borehole *
Pedosphere The pedosphere (from Greek ''pedon'' "ground" or "earth" and ''sphaira'' "sphere") is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, ...
* Solid earth * Vertical displacement *
Mohorovičić discontinuity The Mohorovičić discontinuity ( , ), usually referred to as the Moho discontinuity or the Moho, is the boundary between the Earth's crust and the mantle. It is defined by the distinct change in velocity of seismic waves as they pass through ch ...


References


Further reading

*


External links


Earth's Crust, Lithosphere and Asthenosphere

Crust and Lithosphere
{{Authority control Plate tectonics Earth's mantle Systems ecology