Asthenosphere

Properties of the asthenosphere

The asthenosphere in plate tectonic theory

Resources

The asthenosphere is the ductile layer situated beneath Earth’s rigid lithosphere. It was first named in 1914 by the British geologist Joseph 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 derives its name from the Greek word asthenis, meaning weak, because its strength is much lower than that of the overlying lithosphere. The lithosphere and asthenosphere are defined on the basis of their mechanical properties, whereas the crust and mantle are defined on the basis of their chemical composition. As such, the lithosphere includes both the crust and the upper portion of the mantle, in which temperatures are less than 2,372°F (1, 300°C). The asthenosphere includes the portion of the mantle with temperatures above 2,372°F. The depth of the top of the asthenosphere ranges from a few miles near mid-ocean ridges to 62-93 miles (100-150 km) beneath old oceanic crust (far removed from mid-ocean ridges) and 155-186 miles (250-300 km) beneath continental cores or cratons.

The top of the asthenosphere is marked by a change in the velocity with which certain kinds of seismic waves, known as S-waves, move through Earth. The velocity of the S-waves is inversely proportional to the temperature of the rock through which they are moving, and the top of the asthenosphere corresponds to a low velocity zone near the top of the mantle.

Properties of the asthenosphere

The asthenosphere is ductile and deforms easily compared to the overlying lithosphere because of its temperature and pressure. Any rock will melt if its temperature is raised high enough. However, the melting point of any rock or mineral is also a function of the pressure exerted on the rock or mineral. In general, as the pressure is increased on a material, the melting point increases. The temperature of the rocks that constitute the asthenosphere is below their melting point. As temperature or pressure on increases, the material tends to deform and flow. If the pressure reduced, so will be its melting point and the material may begin to melt. The melting point and pressure balance in the asthenosphere have led geologists to infer that as much as 10% of the asthenospheric material may be molten. The rest is so close to being molten that relatively modest changes in pressure or temperature may cause further melting.

The asthenosphere is heated by contact with hot materials that make up the mesosphere beneath it, but the temperature of the mesosphere is not constant. It is hotter in some places than others. In those regions where the mesosphere is warmer than average, the extra heat may increase the extent to which the asthenosphere is heated and local melting may occur.

The asthenosphere in plate tectonic theory

The asthenosphere is thought to play a critical role in the movement of Earth’s tectonic plates. According to plate tectonic theory, the lithosphere consists of a small number of rigid and relatively cool slabs known as plates. Although the plates are comparatively rigid, they can move along the top of the plastic asthenosphere. The collision, lateral sliding, and separation of plates is responsible for geologic features and events such as volcanoes and lava flows, episodes of mountain building, and deep crustal faults and rifts.

Geologists have developed theories to explain the changes that take place in the asthenosphere when plates begin to diverge or converge. If a region of weakness has developed in the lithosphere, the

KEY TERMS

Lithosphere —The rigid outer layer of Earth that extends to a depth of about 62 mi (100 km).

Magma —Molten rock beneath Earth’s surface.

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

pressure exerted on the asthenosphere beneath it is reduced, melting begins to occur, and the asthenosphere begins to flow upward. If the lithosphere has not separated, the asthenosphere cools as it rises and becomes part of the lithosphere. If there is a break in the lithosphere, magma may escape and flow outward. Depending on the temperature and pressure in the region, that outflow of material (magma) may occur in a violent volcanic eruption or a quiescent lava flow. In either case, the plates spread apart in a process known as rifting.

In zones of convergence, where two plates are moving toward each other, the asthenosphere may be exposed to increased pressure and begin to flow downward. In this case, the lighter of the two colliding plates slides up and over the heavier plate, which is subucted into the asthenosphere. Because the cooler and denser lithosphere is more rigid than the asthenosphere, the asthenosphere is pushed outward and upward. During subduction, the downward moving plate is heated, melting occurs, and molten rock flows upward to Earth’s surface. Such mountain ranges as the Urals, Appalachian, and Himalayas were formed as products of plate collisions. Island arcs such as the Japanese or Aleutians Islands and deep sea trenches are also common products of plate convergence.

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

Resources

BOOKS

Tarbuck, E. J., F. K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2004.

David E. Newton