Describe plate–mantle convection and explain two of the primary driving forces of plate motion.
Researchers are in general agreement that some type of convection—where hot mantle rocks rise and cold, dense oceanic lithosphere sinks—is the ultimate driver of plate tectonics. Many of the details of this convective flow, however, remain topics of debate in the scientific community.
Forces That Drive Plate Motion
Geophysical evidence confirms that although the mantle consists almost entirely of solid rock, it is hot and weak enough to exhibit a slow, fluid-like convective flow. The simplest type of convection is analogous to heating a pot of water on a stove (Figure 1).
Heating the base of a pot causes the water to become less dense (more buoyant), causing it to rise in relatively thin sheets or blobs that spread out at the surface. As the surface layer cools, its density increases, and the cooler water sinks back to the bottom of the pot, where it is reheated until it achieves enough buoyancy to rise again. Mantle convection is similar to, but considerably more complex than, the model just described.
Geologists generally agree that subduction of cold, dense slabs of oceanic lithosphere is a major driving force of plate motion (Figure 2).
This phenomenon, called slab pull, occurs because cold slabs of oceanic lithosphere are more dense than the underlying warm asthenosphere and hence “sink like a rock”—meaning that they are pulled down into the mantle by gravity.
Another important driving force is ridge push (see Figure 2). This gravity-driven mechanism results from the elevated position of the oceanic ridge, which causes slabs of lithosphere to “slide” down the flanks of the ridge. Ridge push appears to contribute far less to plate motions than slab pull.
The primary evidence for this is that the fastest-moving plates—Pacific, Nazca, and Cocos plates—have extensive subduction zones along their margins. By contrast, the spreading rate in the North Atlantic basin, which is nearly devoid of subduction zones, is one of the lowest, at about 2.5 centimeters (1 inch) per year.
Although the subduction of cold, dense lithospheric plates appears to be the dominant force acting on plates, other factors are also at work. Flow in the mantle, perhaps best described as “mantle drag,” is also thought to affect plate motion (see Figure 2).
When flow in the asthenosphere is moving at a velocity that exceeds that of the plate, mantle drag enhances plate motion. However, if the asthenosphere is moving more slowly than the plate, or if it is moving in the opposite direction, this force tends to resist plate motion. Another type of resistance to plate motion occurs along some subduction zones, where friction between the overriding plate and the descending slab generates significant earthquake activity.
Models of Plate–Mantle Convection
Although convection in the mantle has yet to be fully understood, researchers generally agree on the following:
• Convective flow in the rocky 2900-kilometer- (1800 mile-) thick mantle—in which warm, buoyant rock rises and cooler, denser material sinks—is the underlying driving force for plate movement.
• Mantle convection and plate tectonics are part of the same system. Subducting oceanic plates drive the cold downward-moving portion of convective flow, while shallow upwelling of hot rock along the oceanic ridge and buoyant mantle plumes are the upward-flowing arms of the convective mechanism.
• Convective flow in the mantle is a major mechanism for transporting heat away from Earth’s interior to the surface, where it is eventually radiated into space.
What is not known with certainty is the exact structure of this convective flow. Several models have been proposed for plate–mantle convection, and we will look at two of them.
One group of researchers favor some type of whole- mantle convection model, also called the plume model, in which cold oceanic lithosphere sinks to great depths and stirs the entire mantle (Figure 3A).
The whole-mantle model suggests that the ultimate burial ground for these subducting lithospheric slabs is the core–mantle boundary. The downward flow of these subducting slabs is balanced by buoyantly rising mantle plumes that transport hot mantle rock toward the surface.
Two kinds of plumes have been proposed—narrow tube-like plumes and giant upwellings, often referred to as mega-plumes.
The long, narrow plumes are thought to originate from the core–mantle boundary and produce hot-spot volcanism of the type associated with the
Hawaiian Islands, Iceland, and Yellowstone. Scientists believe that areas of large mega-plumes, as shown in Figure 3A, occur beneath the Pacific basin and southern Africa. The latter structure is thought to explain why southern Africa has an elevation much higher than would be predicted for a stable continental landmass. In the whole-mantle convection model, heat for both types of plumes is thought to arise mainly from Earth’s core, while the deep mantle provides a source for chemically distinct magmas. However, some researchers have questioned that idea and instead propose that the source of magma for most hot spot volcanism is found in the upper mantle (asthenosphere).
Layer Cake Model
Some researchers argue that the mantle resembles a “layer cake” divided at a depth of perhaps 660 kilometers (410 miles) but no deeper than 1000 kilometers (620 miles). As shown in Figure 3B, this layered model has two zones of convection—a thin, dynamic layer in the upper mantle and a thick, larger, sluggish one located below. As with the whole-mantle model, the downward convective flow is driven by the subduction of cold, dense oceanic lithosphere. However, rather than reach the lower mantle, these subducting slabs penetrate to depths of no more than 1000 kilometers (620 miles).
Notice in Figure 3B that the upper layer in the layer cake model is littered with recycled oceanic lithosphere of various ages. Melting of these fragments is thought to be the source of magma for some of the volcanism that occurs away from plate boundaries, such as the hot-spot volcanism of Hawaii.
In contrast to the active upper mantle, the lower mantle is sluggish and does not provide material to support volcanism at the surface. Very slow convection within this layer likely carries heat upward, but very little mixing between these two layers is thought to occur.
Geologists continue to debate the nature of the convective flow in the mantle. As they investigate the possibilities, perhaps a hypothesis that combines features from the layer cake model and the whole-mantle convection model will emerge.