Researchers are in general agreement that some type of convection—with hot mantle rocks rising and cold, dense oceanic lithosphere sinking—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 34). Heating the base of a pot warms the water, making it less dense (more buoyant) and 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 35). 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 (refer to Figure 35). 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. Despite its importance, ridge push contributes far less to plate motions than does slab pull. The primary evidence for this is that the fastest-moving plates—the 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 4.5 centimeters (about 2 inches) per year.

Models of Plate–Mantle Convection

Although convection in the mantle is not yet fully understood, researchers generally agree on the following:

• Convective flow—in which warm, buoyant mantle rocks rise while cool, dense lithospheric plates sink—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.

Whole-Mantle Convection Model

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 36A). 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 in the whole-mantle model—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 36A, occur beneath the Pacific basin and southern Africa. These mega-plumes are 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 the narrow plumes and the mega-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).

Double-Layer Model

Some researchers argue that the mantle is divided at a depth of perhaps 660 kilometers (410 miles) but no deeper than 1000 kilometers (620 miles). As shown in Figure 36B, the double-layer 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 in the double-layer model is driven by the subduction of cold, dense oceanic lithosphere. However, rather than reach the lower mantle, as in the whole-mantle model, these subducting slabs penetrate to depths of no more than 1000 kilometers (620 miles) in the double-layer model. Notice in Figure 36B that the upper layer in the double-layer model is littered with recycled oceanic lithosphere of various ages. According to this model, melting of these fragments would 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 in the double-layer model 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 occurs between these two layers.

Geologists continue to debate the nature of the convective flow in the mantle. As they investigate the possibilities, perhaps a widely accepted hypothesis that combines features from the double-layer model and the whole-mantle convection model will emerge.

• In general, convection (upward movement of less dense material and downward movement of more dense material) appears to drive the motion of plates.

• Slabs of oceanic lithosphere sink at subduction zones because the subducted slab is denser than the underlying asthenosphere. In this process, called slab pull, Earth’s gravity tugs at the slab, drawing the rest of the plate toward the subduction zone. As oceanic lithosphere slides down the mid-ocean ridge, it exerts a small additional force, called ridge push.

• Convection may occur throughout the entire mantle, as suggested by the whole-mantle model. Alternatively, it may occur in two layers within the mantle—an active upper mantle and a sluggish lower mantle—as proposed in the double-layer model.

• convection: The transfer of heat by the movement of a fluid mass or substance.

• ridge push: A mechanism that contributes to plate motion, which involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity.

• slab pull: A mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along.