Asthenosphere

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The asthenosphere shown at a subduction boundary

The asthenosphere (from Greek ἀσθενής asthenḗs 'weak' + "sphere") is the highly viscous, mechanically weak^[1] and ductile region of the upper mantle of the Earth. It lies below the lithosphere, at depths between approximately 80 and 200 km (50 and 120 miles) below the surface. The lithosphere–asthenosphere boundary is usually referred to as LAB.^[2][3] The asthenosphere is almost solid, although some of its regions could be molten (e.g., below mid-ocean ridges). 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,^[4]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).^[5]

Contents

• 1Characteristics
• 2Historical
• 3References
• 4Bibliography
• 5External links

Characteristics[edit]

The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. The lithosphere-asthenosphere boundary is conventionally taken at the 1300 °C isotherm, above which the mantle behaves in a rigid fashion and below which it behaves in a ductile fashion.^[6] Seismic waves pass relatively slowly through the asthenosphere^[7] compared to the overlying lithospheric mantle, thus it has been called the low-velocity zone (LVZ), although the two are not exactly the same.^[8][9] This decreasing in seismic waves velocity from lithosphere to asthenosphere could be caused by the presence of a very small percentage of melt in the asthenosphere. The lower boundary of the LVZ lies at a depth of 180–220 km,^[10] whereas the base of the asthenosphere lies at a depth of about 700 km.^[11] This was the observation that originally alerted seismologists to its presence and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity.

In the old oceanic mantle the transition from the lithosphere to the asthenosphere, the lithosphere-asthenosphere boundary (LAB) is shallow (about 60 km in some regions) with a sharp and large velocity drop (5–10%).^[12] At the mid-ocean ridges the LAB rises to within a few kilometers of the ocean floor.

The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the temperature and pressure conditions in the asthenosphere, rock becomes ductile, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a convection current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, allowing the movement of tectonic plates.^[13][14]

Historical[edit]

Although its presence was suspected as early as 1926, the worldwide occurrence of the asthenosphere was confirmed by analyses of seismic waves from the 9.5 M_w Great Chilean earthquake of May 22, 1960.

Both the lithosphere and asthenosphere are part of Earth and are made of similar material. Lithosphere is made up of Earth's outermost layer, the crust, and the uppermost portion of the mantle. In comparison, the asthenosphere is the upper portion of Earth's mantle (which is also the middle layer of Earth). The lithosphere lies over the asthenosphere. In fact, if any material from the asthenosphere were to solidify, it would become part of the lithosphere.

Being closer to the Earth's core, the asthenosphere is a higher temperature as compared to the lithosphere and hence its rocks are plastic and can flow. In comparison, the lithosphere's rocks are more rigid. The asthenosphere is more dense and viscous in comparison to the lithosphere. The lithosphere is comprised of a large number of fragments, each of which is known as the tectonic plate. These tectonic plates are in constant motion and are floating over the plastic material underneath.

$\text{The asthenosphere}$ is a layer (zone) of Earth’s mantle lying beneath the lithosphere.

• It is a layer of solid rock that has so much pressure and heat the rocks can flow like a liquid.
• The rocks are also less dense than the rocks in the lithosphere
• It is believed to be much hotter and more fluid than the lithospher
• Asthenosphere extends from about 100 km (60 miles) to about 700 km (450 miles) below Earth’s surface

The asthenosphere is the ductile part of the earth just below the lithosphere, including the upper mantle. The asthenosphere is about 180 km thick.

Asthenosphere

asthenosphere ăsthēn´əsfēr [key], region in the upper mantle of the earth's interior, characterized by low-density, semiplastic (or partially molten) rock material chemically similar to the overlying lithosphere . The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the earth's crust move about (see plate tectonics ). The asthenosphere is generally located between 45–155 miles (72–250 km) beneath the earth's surface, though under the oceans it is usually much nearer the surface and at mid-ocean ridges rises to within a few miles of the ocean floor. Although its presence was suspected as early as 1926, the worldwide occurrence of the plastic zone was confirmed by analyses of earthquake waves from the Chilean earthquake of May 22, 1960. The seismic waves, the speed of which decreases with the softness of the medium, passed relatively slowly though the asthenosphere, thus it was given the name Low Velocity zone, or the Seismic Wave Guide (see seismology ). Deep-zone earthquakes, i.e., those that occur in the asthenosphere or below it, may be caused by crustal plates sinking into the mantle along convergent crustal boundaries. See earth .

History and Etymology for asthenosphere

Greek asthenḗs "weak" + -O- + -SPHERE — more at ASTHENIA

NOTE: Term introduced by the American geologist Joseph Barrell (1869-1919) in "The strength of the earth's crust. Part VI. Relations of isostatic movements to a sphere of weakness—the asthenosphere," Journal of Geology, vol. 22 (1914), p. 659: "The theory of isostasy shows that below the lithosphere there exists in contradistinction a thick earth-shell marked by a capacity to yield readily to long-enduring strains of limited magnitude…To give proper emphasis and avoid the repetition of descriptive clauses it needs a distinctive name. It may be the generating zone of the pyrosphere; it may be a sphere of unstable state, but this to a large extent is hypothesis and the reason for choosing a name rests upon the definite part it seems to play in crustal dynamics. Its comparative weakness is in that connection its distinctive feature. It may then be called the sphere of weakness—the asthenosphere…"

Asthenosphere

The Asthenosphere In Plate Tectonic Theory

The asthenosphere is now thought to play a critical role in the movement of plates across the face of Earth's surface. According to plate tectonic theory, the lithosphere consists of a relatively small number of very large slabs of rocky material. These plates tend to be about 60 mi (100 km) thick and in most instances many thousands of miles wide. They are thought to be very rigid themselves but capable of being moved on top of the asthenosphere. The collision of plates with each other, their lateral sliding past each other, and their separation from each other are thought to be responsible for major geologic features and events such as volcanoes, lava flows, mountain building, and deep crustal faults and rifts.

In order for plate tectonic theory to make any sense, some mechanism must be available for permitting the flow of plates. That mechanism is the semi-fluid character of the asthenosphere itself. Some observers have described the asthenosphere as the 'lubricating oil' that permits the movement of plates in the lithosphere. Others view the asthenosphere as the driving force or means of conveyance for the plates.

Geologists have now developed theories to explain the changes that take place in the asthenosphere when plates begin to diverge from or converge toward each other. For example, suppose that a region of weakness has developed in the lithosphere. In that case, the pressure exerted on the asthenosphere beneath it is reduced, melting begins to occur, and asthenospheric materials begin to flow upward. If the lithosphere has not actually broken, those asthenospheric materials cool as they approach Earth's surface and eventually become part of the lithosphere itself. On the other hand, suppose that a break in the lithosphere has actually occurred. In that case, the asthenospheric materials may escape through that break and flow outward before they have cooled. Depending on the temperature and pressure in the region, that outflow of material (magma) may occur rather violently, as in a volcano, or more moderately, as in a lave flow. Both these cases produce crustal plate divergence, or spreading apart. Pressure on the asthenosphere may also be reduced in zones of divergence, where two plates are separating from each other. Again, this reduction in pressure may allow asthenospheric materials in the asthenosphere to begin melting and to flow upward. If the two overlying plates have actually separated, asthenospheric material may flow through the separation and form a new section of lithosphere.

In zones of convergence, where two plates are moving toward each other, asthenospheric materials may also be exposed to increased pressure and begin to flow downward. In this case, the lighter of the colliding plates slides upward and over the heavier of the plates, which dives down into the asthenosphere. Since the heavier lithospheric material is more rigid than the material in the asthenosphere, the latter is pushed outward and upward. During this movement of plates, material of the downgoing plate is heated in the asthenosphere, melting occurs, and molten materials flow upward to Earth's surface. Mountain building is the result of continental collision in such situations, and great mountain chains like the Urals, Appalachian, and Himalayas have been formed in such a fashion. When oceanic plates meet one another, island arcs (e.g., Japan or the Aleutians) are formed. Great ocean trenches occur in places of plate convergence. In any one of the examples cited here, the asthenosphere supplies new material to replace lithospheric materials that have been displaced by some other tectonic or geologic mechanism.

Therefore, whether scientists are considering the origin of compressed mountain ranges like the Himalayas, or the origin of the great ocean trenches (like the Peru-Chile trench), they also consider the activity of the asthenosphere, which keeps Earth's plates continually geologically active.

See also Continental drift; Continental margin; Continental shelf; Planetary geology; Plate tectonics.

Resources

Books

Press, Frank, and Raymond Sevier. Understanding Earth. San Francisco: Freeman, 2000.

Tarbuck, Edward. J., Frederick K. Lutgens, and Dennis Tassa, eds. Earth: An Introduction to Physical Geology, 7th ed. Upper Saddle River, NJ: Prentice Hall, 2002.

Fuchs, Karl, and Claude Froidevaux. Composition, Structure, and Dynamics of the Lithosphere and Asthenosphere System. Washington, DC: American Geophysical Union, 1987.

David E. Newton

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Lithosphere

—The outer layer of Earth, that extends to a depth of about 60 mi (100 km).

Magma

—Molten material exuded from below Earth's surface, generally consisting of rock-like materials rich in silicon and oxygen.

Seismic wave

—A disturbance produced by compression or distortion on or within the earth, which propagates through Earth materials; a seismic wave may be produced by natural (e.g. earthquakes) or artificial (e.g. explosions) means.

The material of which the asthenosphere is composed can be described as plastic-like, with much less rigidity than the lithosphere above it. This property is caused by the interaction of temperature and pressure on asthenospheric materials. Any rock will, of course, melt if its temperature is raised to a high enough temperature. However, the melting point of any rock (or of any material) is also a function of the pressure exerted on the rock (or the material). In general, as the pressure is increased on a material, its melting point increases.

Materials that make up the asthenosphere tend to be slightly cooler than their melting point. This gives them a plastic-like quality that can be compared to glass. As the temperature of the material increases or as the pressure exerted on the material increases, the material tends to deform and flow. If the pressure on the material is sharply reduced, so will be its melting point, and the material may begin to melt quickly. The fragile melting point/pressure balance in the asthenosphere is reflected in the estimate made by some geologists that up to 10% of the asthenospheric material may actually be molten. The rest is so close to being molten that relatively modest changes in pressure or temperature may cause further melting.

In addition to loss of pressure on the asthenosphere, another factor that can bring about melting is an increase in temperature. The asthenosphere is heated by contact with hot materials that make up the mesosphere beneath it. Obviously, the temperature of the mesosphere is not constant. It is hotter in some places than in others. In those regions where the mesosphere is warmer than average, the extra heat may actually increase the extent to which asthenospheric materials are heated and a more extensive melting may occur. The results of such an event are described below.

Evidence For The Existence Of The Asthenosphere

Geologists are somewhat limited as to the methods by which they can collect information about Earth's interior. For example, they may be able to study rocky material ejected from volcanoes and lava flows for hints about properties of the interior regions. Generally speaking, however, the single most dependable source of such information is the way in which seismic waves are transmitted through Earth's interior. These waves can be produced naturally as the result of earth movements, or they can be generated synthetically by means of explosions, air guns, or other techniques.

In any case, seismic studies have shown that a type of waves known as S-waves slow down significantly as they reach an average depth of about 62 mi (100 km) beneath Earth's surface. Then, at a depth of about 155 mi (250 km), their velocity increases once more. Geologists have taken these changes in wave velocity as indications of the boundaries for the region now known as the asthenosphere.

The Earth's Crust, Lithosphere and Asthenosphere

Crust, the upper layer of the Earth, is not always the same. Crust under the oceans is only about 5 km thick while continental crust can be up to 65 km thick. Also, ocean crust is made of denser minerals than continental crust.

The tectonic plates are made up of Earth’s crust and the upper part of the mantle layer underneath. Together the crust and upper mantle are called the lithosphere and they extend about 80 km deep. The lithosphere is broken into giant plates that fit around the globe like puzzle pieces. These puzzle pieces move a little bit each year as they slide on top of a somewhat fluid part of the mantle called the asthenosphere. All this moving rock can cause earthquakes.

The asthenosphere is ductile and can be pushed and deformed like silly putty in response to the warmth of the Earth. These rocks actually flow, moving in response to the stresses placed upon them by the churning motions of the deep interior of the Earth. The flowing asthenosphere carries the lithosphere of the Earth, including the continents, on its back.

Flow in the asthenosphere drags tectonic plates along by Rice University A 3D computer model of the asthenosphere by Rice University geophysicists finds that the convective cycling and pressuredriven flow can sometimes cause the asthenosphere to move even faster than the tectonic plates riding atop it. This 2D slice of data from the model shows stronger, faster moving sections of the asthenosphere (yellow) bracketed above and below by slower, more fluid regions (orange). Credit: A. Semple/Rice University New simulations of Earth's asthenosphere find that convective cycling and pressure-driven flow can sometimes cause the planet's most fluid layer of mantle to move even faster than the tectonic plates that ride atop it. That's one conclusion from a new study by Rice University geophysicists who modeled flow in the 100- mile-thick layer of mantle that begins at the base of Earth's tectonic plates, or lithosphere. The study, which is available online in the journal Earth and Planetary Science Letters, takes aim at a much-debated question in geophysics: What drives the movement of Earth's tectonic plates, the 57

interlocking slabs of the lithosphere that slip, grind and bump against one another in a seismic dance that causes earthquakes, builds continents and gradually reshapes the planet's surface every few million years? "Tectonic plates float on top of the asthenosphere, and the leading theory for the past 40 years is that the lithosphere moves independently of the asthenosphere, and the asthenosphere only moves because the plates are dragging it along," said graduate student Alana Semple, lead co-author of the new study. "Detailed observations of the asthenosphere from a Lamont research group returned a more nuanced picture and suggested, among other things, that the asthenosphere has a constant speed at its center but is changing speeds at its top and base, and that it sometimes appears to flow in a different direction than the lithosphere." Computational modeling carried out at Rice offers a theoretical framework that can explain these puzzling observations, said Adrian Lenardic, a study co-author and professor of Earth, environmental and planetary sciences at Rice. "We've shown how these situations can occur through a combination of plate- and pressure-driven flow in the asthenosphere," he said. "The key was realizing that a theory developed by former Rice postdoc Tobias Höink had the potential to explain the Lamont observations if a more accurate representation of the asthenosphere's viscosity was allowed for. Alana's numerical simulations incorporated that type of viscosity and showed that the modified model could explain the new observations. In the process, this offered a new way of thinking about the relationship between the lithosphere and asthenosphere." Though the asthenosphere is made of rock, it is under intense pressure that can cause its contents to flow. "Thermal convection in Earth's mantle generates dynamic pressure variations," Semple said. "The weakness of the asthenosphere, relative to tectonic plates above, allows it to respond differently to the pressure variations. Our models show how this can lead to asthenosphere velocities that exceed those of plates above. The models also show how flow in the asthenosphere can be offset from that of plates, in line with the observations from the Lamont group" The oceanic lithosphere is formed at mid-ocean ridges and flows toward subduction zones where one tectonic plate slides beneath another. In the process, the lithosphere cools and heat from Earth's interior is transferred to its surface. Subduction recycles cooler lithospheric material into the mantle, and the cooling currents flow back into the deep interior. Semple's 3-D model simulates both this convective cycle and the asthenosphere. She credited Rice's Center for Research Computing (CRC) for its help running simulations—some of which took as long as six weeks—on Rice's DAVinCI supercomputer. Semple said the simulations show how convective cycling and pressure-driven flow can drive tectonic movement. "Our paper suggests that pressure-driven flow in the asthenosphere can contribute to the motion of tectonic plates by dragging plates along with it," she said. "A notable contribution does come from 'slab-

pull,' a gravity-driven process that pulls plates toward subduction zones. Slab-pull can still be the dominant process that moves plates, but our models show that asthenosphere flow provides a more significant contribution to plate movement than previously thought."