4.2: Plate Tectonics

Over the geological timescales of the Earth’s history, oceans and continents have formed and disappeared to be replaced by others through the movements of the lithospheric plates. We now have a basic understanding of how and why the lithospheric plates move and of how plate tectonics has continually shaped and changed the face of the planet.

Driving Forces of Plate Tectonics

The movements of the lithospheric plates are thought to be driven by heat energy transferred through the mantle by convection (CC3). The mantle has areas where its constituent material is upwelled and downwelled. As discussed later in this chapter, the convection processes in the mantle are complex, probably chaotic (CC11), and as yet not well understood. However, we do know that the upper mantle loses some of its heat by conduction outward through the Earth’s cooler crust. As the upper material cools and contracts, its density increases. When its density exceeds the density of the mantle material below, the cooled material will tend to sink (downwell). This sinking mantle material is balanced by upwelling mantle material that has been heated by the Earth’s core and radioactive decay within the mantle itself. As a result, this heated mantle material is slightly less dense than the material through which it rises. The precise nature and locations of the convection cells in the Earth are not yet known (Fig. 4-6). However, we believe that the convection process extends from the base of the lithosphere all the way to the core-mantle boundary, almost 3000 km below Earth’s surface. We do know that flow of mantle material vertically does not take place at a uniform rate as there are discontinuities in the physical properties (that affect density) of mantle material at some depths, notably at about 650 km. Thus, sinking and rising mantle materials slows and speeds up along its path through the mantle and this causes a back up and lateral spreading of subducting slabs and ascending heated mantle material at various depths between the core-mantle boundary and the lithosphere (Fig. 4-6). 

Figure 4-6. A generalized depiction of convection processes thought to occur in the Earth’s mantle. The view here is from below the Southern Hemisphere. Not quite half of the Earth has been removed by slicing across the Earth at an angle to the equator. This slice removes much of the Southern Hemisphere, including Antarctica, Australia, much of the southern Atlantic and Indian Oceans, and most of the Pacific Ocean as far north as Hawaii and Indonesia. Look at a globe to see this better. It is known that the convection processes beneath some hot spots extend to the core–mantle boundary, and some subducted blocks of lithosphere may sink to this boundary, but the details of mantle convection circulation are not yet known. Convection cells must exist in the liquid outer core, but almost nothing is known about this circulation. The solid inner core rotates a little faster than the rest of the Earth, making an additional revolution about every 400 years. Scientists believe the faster rotation of the inner core may explain the periodic reversal of the Earth’s magnetic field and plate tectonic processes that cause the continents to periodically gather together and then spread apart. However, much research is needed before these processes can be understood. Inset: We know little about superplumes but there are two superplumes at present; in the south Pacific Ocean south of Hawaii (in the part of Earth that is cut off in main figure); and under the African continent (further north than the tip of Africa visible in the main figure). These superplumes are thought to be a “blob” with plumes rising from it as depicted.

The existence of deep convection all the way down to the core-mantle boundary is known primarily from computer tomography. For example, the plumes of higher-temperature mantle material beneath Hawaii and several similar hot spots (discussed later in this chapter; Fig. 4-10) have been traced through the entire mantle to the core-mantle transition. In addition, cool, rigid slabs of lithosphere, which apparently have been subducted at deep-sea trenches, have been detected deep in the lower mantle close to the core–mantle boundary. It is believed that the slabs of lithosphere sink into the lower mantle, where they are eventually heated and mixed with mantle material, and that some of this mixed material then rises back into the upper mantle at different locations. A single cycle of cooling, sinking, warming, and rising probably takes several hundred million years.

The areas where downwelling is thought to occur in the mantle are beneath the deep-ocean trenches, which surround most of the Pacific Ocean, and are found less extensively in other oceans (Fig. 3-4, Fig. 4-10). The areas under which mantle upwelling is thought to occur include large low shear velocity provinces (LLSVPs). In these areas, superplumes the size of continents “swell” more than 100 times Mount Everest’s height upward from the core–mantle boundary into the mid-mantle. Two such areas are thought to lie under much of Africa and a large region of the southwestern and central Pacific Ocean, respectively (Fig. 4-6). There are also a number of volcanic hot spots in the crust, some of which are the locations of upwelling from deep within the mantle, and some of which may originate from superplumes. As we will see later in this chapter, there are also other hot spots and areas along the oceanic ridges where magma upwells through the lithosphere. However, this magma is thought to originate from shallow depths within the asthenosphere or upper mantle and be formed by melting of upper mantle material. 

Oceanic ridges (also called “mid-ocean ridges”) mark the boundaries between two tectonic plates that are moving apart. At these oceanic ridges, the lithosphere is pulled apart as the rigid tectonic plates are moved in response to subduction at other boundaries of each plate. As the plate edges separate, the weight of the lithosphere on the upper mantle at the ridge is reduced, which reduces the pressure on the upper mantle below. That upper mantle material is already hot and close to its melting point. Reduced pressure causes the melting point of the mantle material to become lower, and the upper mantle material melts and rises to fill the gap between the two plates. This results in volcanic eruptions along the ridge crest. The erupted material is cooled and solidifies as it erupts at the seafloor, and this solid material is added to the edges of both plates as new oceanic crust.

The forces that actually move the lithospheric plates across the Earth’s surface act in response to the mantle convection, but they are far from fully understood. The plates are believed to be dragged across the mantle’s surface as slabs of old, cold, dense lithosphere at the plate edges sink at the deep-sea trenches. The deep-sea trenches mark boundaries along which different plates meet and are called subduction zones. The mechanism that moves the plate is somewhat like that of an anchor (the old cold dense subducting edge of the plate) that, when dropped into the water, will drag down the line attached to it (the remainder of the rigid plate), even if, by itself, this line would float. The descending slabs of lithosphere create an additional effect similar to the vortex created as a ship sinks, which drags floating materials down with it. Finally, the plates may also move in response to gravity. Oceanic crust near the oceanic ridges, where new crust is formed, is less dense and floats higher than older oceanic crust. The oceanic crust cools, becomes more dense, and sinks as it moves away from the oceanic ridges toward the subduction zones. Thus, the oceanic crust on each plate lies on a slope, albeit a very shallow one, between the oceanic ridges and subduction zones. The plate thus has a tendency to “slide” downhill (in response to gravity), with newer, higher crust “pushing” older, lower crust down the slope. 

Present-Day Lithospheric Plates

Seven major lithospheric plates are generally recognized—the Pacific, Eurasian, African, North American, South American, Indo-Australian, and Antarctic Plates—as well as several smaller plates (Fig. 4-7). However, some plate boundaries are not yet fully defined, and additional small plates that currently are considered to be parts of larger plates undoubtedly exist.

Figure 4-7. The major lithospheric plates and their current directions of motion relative to each other. Continental North America lies on the North American Plate, which extends to the center of the Atlantic Ocean. At the Pacific coast boundary of this plate, the North American and Pacific Plates are sliding past each other. The directions of motion show that the Atlantic Ocean is becoming wider, while the Pacific Ocean is becoming smaller.

All plates are in motion and, over geological time, both the direction and speed of movement can vary. Figure 4-7 shows the directions in which the plates are moving relative to each other at present. The movement of the plates is extremely slow, just a few centimeters per year, or about the rate at which human fingernails grow. Despite their slowness, plate movements can completely alter the face of the planet in a few tens of millions of years. As they move, plates collide, separate, or slide past each other, and sometimes fracture to form smaller plates. At plate edges, the motion is not smooth and continuous; it occurs largely by a series of short, sharp movements that we feel as earthquakes. Most earthquakes occur at plate boundaries (Fig. 4-8).

Figure 4-8. Locations of the world’s earthquakes. Compare this figure with Figure 4-7. Earthquakes are concentrated along plate boundaries and appear to be more frequent at the plate boundaries where plates are converging.

Interactions between the moving plates are responsible for most of the world’s topographic features. Most major mountain ranges, both on land and undersea, and all the deep-ocean trenches are aligned along the edges of plates. How plate movements create topographic features is examined later in the chapter.

Spreading Cycles

About 225 million years ago, most continental crust on the Earth was part of one supercontinent called “Pangaea.” Since then, Pangaea has broken up and the fragments have spread apart to their present locations. Continental crust breaks apart from a supercontinent, spreads out, and then rejoins to form a supercontinent in a process called a spreading cycle. The relative motions of the fragments of Pangaea during the past 225 million years, the current spreading cycle, have been investigated by studies of the magnetism and chemistry of rocks, fossils, and ocean sediments, which have revealed subtle clues as to when and where on the planet they were formed. From such studies we can determine from where and how fast the continents have drifted.

The history of the current spreading cycle is quite well known (Fig. 4-9). Initially the landmasses now known as Eurasia and North America broke away from Pangaea as a single block. North America later broke away from Eurasia, and South America and Africa separated from Antarctica, Australia, and India. Only much later were North and South America joined, as were Africa and Eurasia. Following the initial breakup of Pangaea, India and later Australia broke away from Antarctica. Since its break from Antarctica, India has moved northward to collide with Asia. This northward movement has been much faster than the rate of movement of other plates.

Figure 4-9. A reconstruction of the movements of the continents over the past 225 million years, which is the length of the present spreading cycle.

Although the history of the continents before the breakup of Pangaea is much less understood, Pangaea is known to have been assembled from a number of smaller continents that came together several hundred million years ago. The largest piece of Pangaea, called “Gondwanaland,” appears to have remained intact for more than a billion years. The other pieces of continental crust probably broke apart, spread out, and then re-joined to form a supercontinent several times before the formation of Pangaea. Therefore, we refer to the past 225 million years as the “current spreading cycle.”

As many as 10 spreading and assembling cycles may have occurred during the Earth’s history. Each continent is itself a geological jigsaw puzzle, consisting of pieces assembled and broken apart during previous spreading cycles. Today, for example, many areas in the interior of continents show geological evidence that they were subduction zones during earlier spreading cycles (Fig. 4-10). Subduction zones form at plate boundaries where oceanic crust is downwelled into the asthenosphere, so these ancient subduction zones could not have been formed where they now lie in the middle of continents. There are several types of plate boundaries, each with its own characteristic topographic features. The various types are identified in the next section, and each type is then further described in subsequent sections of this chapter.

Figure 4-10. Sutures and active subduction zones. Sutures are the remnants of subduction zones where plates once converged. The oldest sutures that have been identified are in the interior of continents and are more than 570 million years old.