By 1962, the idea that pieces of the Earth's surface moved around no longer seemed radical. The concepts of continental drift and seafloor spreading had revolutionized geology (see our module The Origins of Plate Tectonic Theory), and scientists excitedly began to revise their interpretations of existing data into a comprehensive theory of plate tectonics. For example, geologists had long recognized that earthquakes are not randomly distributed on the Earth (see Figure 1).
In fact, earthquakes are concentrated along the plate boundaries drawn by Harry Hess along mid-ocean ridges and subduction zones. Not all earthquakes occur at the same depth, however. Where Hess had postulated that the rocks of the ocean floor were diving down into subduction zones, earthquakes occur at shallow depths of 0 to 33 km below the surface near the trenches and at depths of almost 700 km below the surface further inland (illustrated in Figure 1 by different colored circles). On the other hand, only shallow earthquakes (depths of 0 to 33 km, shown in red in Figure 1) are recorded at the spreading ridges. These data helped geologists draw more detailed cross-sections showing that plates are thin at spreading ridges, and that subduction extends long distances, taking plates deep beneath the continents.
Similar to earthquakes, volcanoes are located preferentially on or near plate boundaries (see Figure 2).
Also similar to earthquakes, different kinds of volcanoes occur along different types of plate boundaries. Most of the volcanic eruptions that make the news, such as the 1980 Mount St. Helens eruption, take place near subduction zones. This type of volcano is represented by green triangles in Figure 2. These devastating, explosive eruptions reflect the composition of the magma - it is extremely viscous, or thick and resistant to flow, and thus results in tall, steep-sided volcanoes. In contrast, the volcanic eruptions that occur along spreading ridges are much gentler, in part because most of these eruptions occur under 2 to 3 kilometers of water, but also because the magma is far less viscous. This type of volcano is represented by blue triangles in Figure 2.
These observations about the distribution of earthquakes and volcanoes helped geologists define the processes that occur at spreading ridges and subduction zones. In addition, they helped scientists recognize that there are other types of plate boundaries. In general, plate boundaries are the scene of much geologic action - earthquakes, volcanoes, and dramatic topography such as mountain ranges like the Himalayas are all concentrated where two or more plates meet along a boundary.
There are three major ways that plates interact along boundaries: (1) They can move away from each other (diverge), (2) they can move toward each other (converge), or (3) they can move past each other, parallel to the boundary (transform). Each of these interactions produces a different and characteristic pattern of earthquakes, volcanic activity, and topography. The results of these interactions also depend on the type of crust involved, and there are two types of crust: oceanic and continental. Continental crust is thick and buoyant; oceanic crust is thin, dense, and forms at mid-ocean ridges.
The most common divergent boundaries are the mid-ocean ridges that launched the plate tectonics revolution, and the Mid-Atlantic Ridge is a classic example (see Figure 3). Shallow earthquakes and minor basaltic lava flows characterize divergent boundaries at mid-ocean ridges. The seafloor at the ridges is higher than the surrounding plain because the rocks are hot and thus less dense and more buoyant, riding higher in the underlying mantle. As the rocks move away from the spreading center, they cool and become more dense and less buoyant. Spreading has been occurring along the Mid-Atlantic Ridge for 180 million years, resulting in a large ocean basin - the Atlantic Ocean.
Convergent boundaries are the most geologically active, with different features depending on the type of crust involved. The activity that takes place at convergent boundaries depends on the type of crust involved, as explained next.
Oceanic meets continental
These are the subduction zones first imagined by Hess, where dense oceanic crust is diving beneath more buoyant continental crust. These boundaries are characterized by: (a) a very deep ocean trench next to a high continental mountain range, (b) large numbers of earthquakes that progress from shallow to deep, and (c) large numbers of intermediate composition volcanoes (see Figure 4). The Andes owe their existence to a subduction zone on the western edge of the South American plate; in fact, this type of boundary is often called an Andean boundary since it is the primary example.
Oceanic meets more oceanic
Where two plates converge along a boundary where the crust on both sides is oceanic, a subduction zone also occurs, but the result is slightly different than an Andean margin. Since the densities of the two plates are similar, it is usually the plate with the older oceanic crust that is subducted because that crust is colder and denser. Earthquakes progress from shallow to deep, moving away from the trench like in the oceanic-continental convergence, and volcanoes form an island arc, like the mountain range along the Tonga trench in the western Pacific (see Figure 5).
Continental meets more continental
When two pieces of continental crust converge, the result is a great pileup of continental material. Both pieces of crust are buoyant and are not easily subducted. Continental convergence is exemplified by the Himalayan mountain range, where the Indian plate runs into the Asian plate (see Figure 6). Numerous shallow earthquakes occur, but there is very little volcanism.
Most boundaries are either convergent or divergent, but transform boundaries occur in a few places to accommodate lateral motion, where plates move horizontally past one another. This type of boundary is very rare on continents, but they are dramatic where they do occur. For example, the San Andreas Fault in California is a continental transform boundary. Along this boundary, frequent, shallow earthquakes occur (like the famous 1906 and 1989 San Francisco earthquakes), but there is little associated volcanic activity or topographic relief (see Figure 7). The Alpine Fault in New Zealand is very similar. Most transform boundaries occur not on land, however, but in short segments along mid-ocean ridges.
A few boundaries defy simple classification and are referred to as "plate boundary zones." For example, a complicated earthquake pattern is produced by a wide, poorly understood plate boundary zone between the Eurasian and African plates in the Mediterranean region.
Geologic activity away from plate boundaries
The plate boundaries described above account for the vast majority of seismic and volcanic activity on Earth. The more data that began to fit into the plate tectonics scheme, however, the more the exceptions stood out. What could account for Hawaii, for example, a scene of long-lived volcanic activity in the middle of the Pacific plate where there is no subduction or spreading to generate magma?
There had to be something else. In 1963, J. Tuzo Wilson, a Canadian geophysicist, theorized that the mantle contained immobile hotspots, thin plumes of hot magma that acted like Bunsen burners as plates moved over them (Wilson, 1963). The Hawaiian Islands form a long, linear chain, with ongoing volcanic eruptions on the island of Hawaii and extinct, highly eroded volcanic islands to the northwest. According to Wilson's hotspot theory, the chain of islands represents the northwestward motion of the Pacific plate over a mantle plume.
One important implication of Wilson's theory was that because hotspots were stationary, hotspot tracks could be used to trace plate motion history. For example, the track of the Hawaiian chain continues to the northwest as an underwater chain of progressively older, no longer active volcanoes. Once the volcanic eruptions stop, ocean waves begin to take their toll, eroding the islands down to just below sea level, at which point they are called seamounts. The islands and seamounts associated with the Hawaiian hotspot provide a history of motion for the Pacific plate, which appears to have taken an eastward turn around 42 million years ago (see Figure 10). Other hotspot tracks around the world can be used in a similar manner to reconstruct a global plate tectonic history.
What are the driving forces?
Hotspots added further proof to confirm that plates move constantly and steadily. Ironically, however, the question that incited ridicule for Wegener continues to launch heated debate today: What ultimately drives plate motion? Plates are constantly shifting and rearranging themselves in response to each other. Eventually, a new Pangaea (or single supercontinent) will form, break apart, and form again on Earth. What keeps these plates moving?
Hess assumed that mantle convection was the main driving force - hot, less dense material rises along mid-ocean ridges, cools, and subsides at subduction zones, and the plates "ride" these convection cells (see our Density module for more information). Though there is little doubt that convection does occur in the mantle, current modeling suggests that it is not so simple. Many geologists argue that the force of convection is not enough to push enormous lithospheric plates like the North American plate. They suggest instead that gravity is the main driving force: Cold, dense oceanic crust sinks at subduction zones, pulling the rest of the plate with it. According to this theory, magmatic intrusions at spreading ridges are passive - the magma merely fills a hole created by pulling two plates apart.
Undoubtedly, gravity and convection both supply energy to keep plates moving. Their relative contributions, however, are a matter of debate and ongoing research.
The strength of plate tectonic theory lies in its ability to explain everything about the processes we see both in the geologic record and in the present. Our understanding of the subtleties continues to evolve as we learn more about our planet, but plate tectonics is truly the foundation upon which the science of geology is built.
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