plate tectonics

Plate-driving mechanisms and the role of the mantle

By the late 1960s, details of the processes of plate movement and of boundary interactions, along with much of the plate history of the Cenozoic Era (the past 65.5 million years), had been worked out. Yet the driving forces that bedeviled Wegener continue to remain enigmatic because there is little information about what happens beneath the plates.

Mantle convection

Most agree that plate movement is the result of the convective circulation of Earth’s heated interior, much as envisaged by Arthur Holmes in 1929. The heat source for convection is thought to be the decay of radioactive elements in the mantle. How this convection propels the plates is poorly understood. In the western Pacific Ocean, the subduction of old, dense oceanic crust may be self-propelled. The weight of the subducted slab may pull the rest of the plate toward the trench, a process known as slab pull. Some geologists, however, argue that the westward drift of North America and eastward drift of Europe and Africa may be due to push at the spreading ridge, known as ridge push, in the Atlantic Ocean. Hot mantle spreading out laterally beneath the ridges or hot spots may speed up or slow down the plates, a force known as mantle drag. However, the mantle flow pattern at depth does not appear to be reflected in the surface movements of the plates.

The relationship between the circulation within Earth’s mantle and the movement of the lithospheric plates remains a first-order problem in the understanding of plate-driving mechanisms. Circulation in the mantle occurs by thermal convection, whereby warm, buoyant material rises, and cool, dense material sinks. Convection is possible even though the mantle is solid; it occurs by solid-state creep, similar to the slow downhill movement of valley glaciers. Materials can flow in this fashion if they are close to their melting temperatures. Several different models of mantle convection have been proposed. The simplest, called whole mantle convection, describes the presence of several large cells that rise from the core mantle boundary beneath oceanic ridges and begin their descent to that boundary at subduction zones. Some geophysicists argue for layered mantle convection, suggesting that more vigorous convection in the upper mantle is decoupled from that in the lower mantle. This model would be supported if it turned out that the boundary between the upper and lower mantle is coincident with a change in composition. A third model, known as the mantle plume model, suggests that upwelling is focused in plumes that ascend from the core-mantle boundary, whereas diffuse return flow is accomplished by subduction zones, which, according to this model, extend to the core-mantle boundary.

Seismic tomography

A powerful technique, seismic tomography, is providing insights into this problem. This technique is similar in principle to that of the CT (computed tomography) scan and creates three-dimensional images of Earth’s interior by combining information from many earthquakes. Seismic waves generated at the site, or focus, of an earthquake spread out in all directions, similar to light rays from a light source. As earthquakes occur in many parts of Earth’s crust, information from many sources can be synthesized, mimicking the rotating X-ray beam of a CT scan. Because their speed depends on the density, temperature, pressure, rigidity, and phase of the material through which they pass, the velocity of seismic waves provides clues to the composition of Earth’s interior. Seismic energy is absorbed by warm material, so that the waves are slowed down. As a result, anomalously warm areas in the mantle are seismically slow, clearly distinguishing them from colder, more rigid, anomalously fast regions.

Tomographic imaging shows a close correspondence between surface features such as ocean ridges and subduction zones to a depth of about 100 km (60 miles). Hot regions in the mantle occur beneath oceanic ridges, and cold regions occur beneath subduction zones. However, at greater depths, the pattern is more complex, suggesting that the simple whole mantle-convection model is not appropriate. On the other hand, subduction zones beneath Central America and Japan have been tracked close to the core-mantle boundary, suggesting that transition between the upper and lower mantle is not an impenetrable barrier to mantle flow. If so, convection is not decoupled across that boundary, again casting doubt upon the layered mantle model. Imaging the mantle directly beneath hot spots has identified anomalously warm mantle down to the core-mantle boundary, providing strong evidence for the existence of plumes and the possibility that the mantle plume hypothesis may indicate an important mechanism involved in mantle convection.