Asthenosphere (World of Earth Science)
The asthenosphere is the layer of Earth that lies at a depth 6050 mi (10050 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