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Key Concepts
— earthquakes and volcanoes occur primarily along plate boundaries; the frequency and type of events vary with the type of boundary.
— plates interact with one another at boundaries in one of three ways: they diverge, converge, or slide past one another.
— plates are made up of two types of crust
oceanic and continental;oceanic crust is thinner and denser that continental crust.
A single plate can have both continental and oceanic crust.
— gravity and mantle convection are two driving forces for the movement of plates.
By 1962, the idea that pieces of the earth's surface moved around no longer seemed radical. The concepts of continental drift and seafloor spreadinghad revolutionized geology, and researchers excitedly began to revise their interpretations of existing data (see our Plate Tectonics I module). For example, geologists had long recognized that earthquakes are not randomly distributed on the earth.
In fact, earthquakes are concentrated along the plate boundaries drawn by Harry Hess. Not all earthquakes occur at the same depth, however. Where Hess had postulated that the rocks of the ocean floor were diving down intosubduction zones, earthquakes occur at shallow depths of 0 to 33 km below the surface near the trenches, and depths of almost 700 km below the surface further inland. On the other hand, only shallow earthquakes (depths of 0 to 33 km) 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.
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. These devastating, explosive eruptions reflect the composition of the magma - it is extremely viscous and thus cannot flow easily. 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.
These observations about the distribution of earthquakes and volcanoes helped geologists define the processes that occur at spreading ridges andsubduction 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: they can move away from each other (diverge), they can move toward each other (converge), or they can slide by each other (transform). Each of these interactions produces a different and characteristic pattern of earthquakes, volcanism, and topography.
Divergent boundaries are the mid-ocean ridgesthat launched the plate tectonics revolution - the Mid-Atlantic Ridge is a classic example. Shallow earthquakes and minor lava flows characterize mid-ocean ridges. The seafloor at the ridges is higher than the surrounding abyssal plain because the rocks are hot and thus less dense and more buoyant. As they move away from the spreading center, they cool and become denser 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. 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 activity that takes place at convergent boundaries depends on the type of crust involved, as explained next.
These are the subduction zones first imagined by Hess, where denseoceanic 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. 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 margin.
Where two oceanic plates converge, 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 older oceanic crust that is subducted because it is colder and slightly denser. Earthquakes progress from shallow to deep like in the oceanic-continental convergence, and volcanoes form an island arc, like Mt. Fuji in Japan and Pinatubo in the Philippines. These volcanoes are slightly different from those that form the Andes because themagma is produced from the melting of oceanic crust rather than the melting of continental crust.
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. 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, in which plates slide 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 - frequent, shallow earthquakes occur (like the famous 1906 and 1989 San Francisco earthquakes), but there is little associated volcanism or topographic relief. 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.
The boundaries described above account for the vast majority of seismic and volcanic activity on the 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. 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 28 million years ago. Other hotspot tracks around the world can be used in a similar manner to reconstruct a global plate tectonic history.
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 the 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 atsubduction 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.
Note that arrows on convection cells and overlying plate are going in the same direction. Figure modified from This Dynamic Earth, a publication from the U.S. Geological Survey. 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.