Asthenosphere

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.