Asthenosphere

The asthenosphere is the layer of Earth that lies at a depth 60–150 mi (100–250 km) beneath Earth's surface. It was first named in 1914 by the British geologist J. Barrell, who divided Earth's overall structure into three major sections: the lithosphere , or outer layer of rock-like material; the asthenosphere; and the centrosphere, or central part of the planet. The asthenosphere gets its name from the Greek word for weak, asthenis, because of the relatively fragile nature of the materials of which it is made. It lies in the upper portion of Earth's structure traditionally known as the mantle.

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. But generally speaking, 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.

Seismic studies have shown that a type of wave known as S-waves slow down significantly as they reach a 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 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 melt if its temperature is raised to a high enough temperature. However, the melting point of any rock is also a function of the pressure exerted on the rock. In general, as the pressure is increased on a material, its melting point increases.

The temperature of the materials that make up the asthenosphere tend to be just below 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 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 many thousands of miles wide. They are thought to be very rigid themselves but capable of flowing back and forth 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-sea rifts.

In order for plate tectonic theory to seem sensible, 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.

Geologists have now developed sophisticated theories to explain the changes that take place in the asthenosphere when plates begin to thin or 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 lava flow.

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 flowing toward each other, asthenospheric materials may also be exposed to reduced pressure and begin to flow upward. In this case, the lighter of the colliding plates slides upward and over the heavier of the plates, which dives down into the asthenosphere. This process is called subduction. Since the lithospheric material is more rigid than the material in the asthenosphere, the latter is pushed outward and upward. During this movement of plates, pressure on the asthenosphere is reduced, melting occurs, and molten materials flow upward to Earth's surface. 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.

See also Continental drift theory; Continental shelf; Crust; Earth, interior structure; Plate tectonics

asthenosphere The athenosphere (derived from the Greek for ‘weak sphere’) is the relatively weak, ductile layer in the upper mantle immediately underlying the lithosphere. Although solid at normal strain rates, like the rest of the non-lithospheric mantle it can deform slowly by solid-state creep. It has an effective viscosity of between about 10¹⁹ and 10²¹ Pa s (Pascal seconds), compared with an average of about 10²¹ for the mantle as a whole and 10⁻³ Pa s for water. The lowest viscosities tend to occur under volcanically active areas, and the highest under cratonic areas (which are stable regions within continents).

The top of the asthenosphere is a gradational boundary with the overlying lithosphere, and its depth varies according to the age and temperature of the lithosphere (see lithosphere). The base of the asthenosphere is also gradational, and is not well defined. Some consider that the base occurs around 300–400 km, whereas others consider it to extend to the base of the upper mantle at 670 km.

Evidence for the local existence of an asthenosphere is derived from long-term, regional changes in elevation, such as the postglacial uplift of Fennoscandia. There, historical uplift of about 10 mm per year is occurring as a result of isostatic rebound of the lithosphere following the removal (by melting) of the load represented by the glacial ice cap. When the ice cap first extended over Fennoscandia, its weight depressed the lithosphere, and the underlying asthenosphere gradually flowed away to the sides. As the load is released, the elastically flexed lithosphere ‘rebounds’ to its pre-deformation level, and the asthenosphere flows back into its former position. The rate of rebound is limited by the viscosity of the athenosphere, and modelling such movements provides estimates of the viscosity.

Although such local studies are important, it is seismology that has demonstrated the global presence of the asthenosphere. This is inferred partly from the almost ubiquitous ‘low-velocity zone’ for P- and especially S-waves that occurs between about 100 and 300 km below the Earth's surface, and partly from studies of the attenuation of seismic energy. The low-velocity zone occurs almost everywhere, except perhaps under cratonic areas. It consists of a 0.5–1 km s⁻¹ reduction in seismic velocity between about 100 and 300 km depth. This velocity reduction is best explained by the close approach of the mantle at this depth to its melting temperature, although the actual degree of melt in the asthenosphere is probably very low except under volcanically active areas such as mid-ocean ridge axes, hot spots, and the mantle above subducting slabs. Seismic energy is strongly attenuated in the asthenosphere, in contrast to the lithosphere and the rest of the mantle, further attesting to the near-melting temperature there.

The presence of a worldwide asthenosphere is an important part of the concept of plate tectonics (see plate tectonics), allowing the detailed movements of plates to be decoupled from the convection in the underlying mantle. This allows plates to move around relatively freely, interacting only at their edges. This is in strong contrast to pre-plate-tectonic ideas on continental drift, the acceptance of which was hampered by the apparent difficulty of moving continents through a rigid upper mantle.

Roger Searle

Bibliography

Kearey, P. and and Vine, F. J. (1996) Global tectonics. Blackwell Scienctific Publications, Oxford.

asthenosphere , 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 .

asthenosphere The weak zone within the upper mantle, underlying the lithosphere, where the mantle rocks deform by plastic flow in response to applied stresses of the order of 100 MPa. It is commonly considered to be coincident with the upper-mantle seismic low-velocity zone, but this is probably valid only for the oceanic sectors of the mantle. Viscosity is of the order of 10^21–22 poise, i.e. the same as the underlying mantle, but it is much more ‘fluid’ than the overlying lithosphere. Originally it was recognized as a possible explanation for isostatic behaviour, and it is generally recognized as a mantle zone within which convective motions take place. The depth of earthquake foci in subduction zones suggests that descending convection limbs penetrate to 700 km, just above the upper mantle–lower mantle boundary. Rising limbs of asthenospheric mantle convection are located under surface spreading centres (mid-oceanic ridges).

asthenosphere A weak layer within a planetary body, beneath the rigid outer layer known as the lithosphere. It is slightly weaker than the lithosphere and deformable over long periods, allowing continental drift to take place on the Earth. On other planets and satellites the asthenosphere allows the topography to smooth out slowly, highlands becoming lower and deep basins shallower. The depth of the asthenosphere varies according to the size, density, composition, and thermal structure of the planet or satellite. On Earth, the top of the asthenosphere is around 100 km below the surface, but on the Moon it lies 800 km below the surface, more than half-way to the centre.