# Lithosphere

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A lithosphere (Ancient Greek : λίθος [lithos] for "rocky", and σφαίρα [sphaira] for "sphere") is the rigid, [1] outermost shell of a terrestrial-type planet, or natural satellite, that is defined by its rigid mechanical properties. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a rocky planet, the crust, is defined on the basis of its chemistry and mineralogy.

A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun, i.e. Mercury, Venus, Earth, and Mars. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, "Earth-like". These planets are located between the Sun and the Asteroid Belt.

A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet.

Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.

## Earth's lithosphere

Earth's lithosphere includes the crust and the uppermost mantle, which constitute the hard and rigid outer layer of the Earth. The lithosphere is subdivided into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the soil forming process is called the pedosphere. The lithosphere is underlain by the asthenosphere which is the weaker, hotter, and deeper part of the upper mantle. The Lithosphere-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 deforms viscously and accommodates strain through plastic deformation.

Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

The hydrosphere is the combined mass of water found on, under, and above the surface of a planet, minor planet or natural satellite. Although the Earth's hydrosphere has been around for longer than 4 billion years, it continues to change in size. This is caused by seafloor spreading and continental drift, which rearranges the land and ocean.

The biosphere also known as the ecosphere, is the worldwide sum of all ecosystems. It can also be termed the zone of life on Earth, a closed system, and largely self-regulating. By the most general biophysiological definition, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, geosphere, hydrosphere, and atmosphere. The biosphere is postulated to have evolved, beginning with a process of biopoiesis or biogenesis, at least some 3.5 billion years ago.

### History of the concept

The concept of the lithosphere as Earth's strong outer layer was described by A.E.H. Love 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". [2] [3] [4] [5] 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." [6] 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.

Augustus Edward Hough Love FRS, 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 his work on the structure of the Earth in Some Problems of Geodynamics won for him the Adams prize in 1911 when he developed a mathematical model of surface waves known as Love waves. Love also contributed to the theory of tidal locking and introduced the parameters known as Love numbers, which are widely used today. These numbers are also used in problems related to the tidal deformation of the Earth due to the gravitational attraction of the Moon and Sun.

Joseph Barrell was an American geologist who developed many ideas on the origins of the Earth, isostasy and ideas on the origins of sedimentary rocks. He suggested that they were produced by the action of rivers, winds, and ice (continental), as well as by marine sedimentation. He also independently arrived at the theory of stoping as a mechanism for igneous intrusion. He was elected a Fellow of the American Academy of Arts and Sciences in 1915.

Reginald Aldworth Daly was a Canadian geologist.

### Types

There are two types of lithosphere:

• Oceanic lithosphere, which is associated with oceanic crust and exists in the ocean basins (mean density of about 2.9 grams per cubic centimeter)
• Continental lithosphere, which is associated with continental crust (mean density of about 2.7 grams per cubic centimeter)

Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together constitute oceanic lithosphere.

Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in silicates and aluminium minerals and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate minerals and is denser. Changes in seismic wave velocities have shown that at a certain depth, there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character.

The thickness of the lithosphere is considered to be the depth to the isotherm associated with the transition between brittle and viscous behavior. [7] The temperature at which olivine begins to deform viscously (~1000 °C) is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle. Oceanic lithosphere is typically about 50–140 km thick [8] (but beneath the mid-ocean ridges is no thicker than the crust), while continental lithosphere has a range in thickness from about 40 km to perhaps 280 km; [8] the upper ~30 to ~50 km of typical continental lithosphere is crust. The mantle part of the lithosphere consists largely of peridotite. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity.

The mineral olivine is a magnesium iron silicate with the formula (Mg2+, Fe2+)2SiO4. Thus it is a type of nesosilicate or orthosilicate. It is a common mineral in Earth's subsurface but weathers quickly on the surface.

A mid-ocean ridge (MOR) is an underwater mountain system formed by plate tectonics. It consists of various mountains linked in chains, typically having a valley known as a rift running along its spine. This type of oceanic mountain ridge is characteristic of what is known as an 'oceanic spreading center', which is responsible for seafloor spreading. The production of new seafloor results from mantle upwelling in response to plate spreading; this isentropic upwelling solid mantle material eventually exceeds the solidus and melts. The buoyant melt rises as magma at a linear weakness in the oceanic crust, and emerges as lava, creating new crust upon cooling. A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.

Peridotite is a dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica (SiO4−
4
). It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

#### Oceanic lithosphere

Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. 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 [9] 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.

Mafic is an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron, and is thus a portmanteau of magnesium and ferric. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar.

In geology, felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.

.

${\displaystyle \,h\,\sim \,2\,{\sqrt {\kappa t}}\,}$

Here, ${\displaystyle h}$ is the thickness of the oceanic mantle lithosphere, ${\displaystyle \kappa }$ is the thermal diffusivity (approximately 10−6 m2/s) for silicate rocks, and ${\displaystyle t}$ 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. This is because the chemically differentiated oceanic crust is lighter than asthenosphere, but 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. 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. [10] [11]

#### 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 2900 km to near the core-mantle boundary, [12] while others "float" in the upper mantle, [13] [14] while some stick down into the mantle as far as 400 km but remain "attached" to the continental plate above, [11] similar to the extent of the "tectosphere" proposed by Jordan in 1988. [15]

## Mantle xenoliths

Geoscientists can directly study the nature of the subcontinental mantle by examining mantle xenoliths [16] 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. [17]

## Related Research Articles

Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.

In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase.

The asthenosphere is the highly viscous, mechanically weak and ductilely deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between approximately 80 and 200 km below the surface. The Lithosphere–asthenosphere boundary is usually referred to as LAB. The asthenosphere is almost solid, although some of its regions could be molten. The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. However, the rheology of the asthenosphere also depends on the rate of deformation, which suggests that the asthenosphere could be also formed as a result of a high rate of deformation. In some regions the asthenosphere could extend as deep as 700 km (430 mi). It is considered the source region of mid-ocean ridge basalt (MORB).

Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced to sink due to gravity into the mantle. Regions where this process occurs are known as subduction zones. Rates of subduction are typically in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.

Isostasy is the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density.

Convergent boundaries are areas on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other causing a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Benioff Zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.

A craton is an old and stable part of the continental lithosphere, where the lithosphere consists of the Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by younger sedimentary rock. They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into the Earth's mantle.

Ridge push or sliding plate force is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.

A mantle is a layer inside a planetary body bounded below by a core and above by a crust. Mantles are made of rock or ices, and are generally the largest and most massive layer of the planetary body. Mantles are characteristic of planetary bodies that have undergone differentiation by density. All terrestrial planets, a number of asteroids, and some planetary moons have mantles.

In geophysics, delamination refers to the loss and sinking (foundering) of the portion of the lowermost lithosphere from the tectonic plate to which it was attached.

Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.

Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces created by faulting create subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.

The opening of the North Atlantic Ocean is a geological event that occurred over millions of years, during which the supercontinent Pangea broke up. As modern-day Europe and North America separated during the final breakup of Pangea in the early Cenozoic Era, they formed the North Atlantic Ocean. Geologists believe the breakup occurred either due to primary processes of the Iceland plume or secondary processes of lithospheric extension from plate tectonics.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

The Lithosphere–asthenosphere boundary (LAB) represents a mechanical difference between layers in Earth's inner structure. Earth's inner structure can be described both chemically and mechanically. The Lithosphere–asthenosphere boundary lies between Earth's cooler, rigid lithosphere and the warmer, ductile asthenosphere. The actual depth of the boundary is still a topic of debate and study, although it is known to vary according to the environment.

The subcontinental lithospheric mantle (SCLM) is the uppermost solid part of Earth's mantle associated with the continental lithosphere.

Effective elastic thickness of the lithosphere is the estimated thickness of the elastic plate to substitute for lithosphere in order to investigate observed deformation. It is also presented as Te.

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