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 1A).
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 1A, 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 1B, 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 1B 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.