Plate Tectonics: The Science of Earth’s Moving Lithosphere

The lithosphere is the outer shell of the planet (crust and solid upper mantle), ranging from 0 to 100 kilometers deep. It is not a continuous, unbroken sphere; instead, it is fractured and composed of seven major plates—African, Antarctic, Eurasian, Indo-Australian, North American, Pacific, and South American—and dozens of smaller plates. Plates float over the viscous asthenosphere (upper mantle) at 80 to 200 kilometers in depth. Plate boundaries, both major and minor, are defined as regions where seismic activity occurs between adjacent plates due to friction. The governing scientific theory related to this plate movement is known as plate tectonics, from the Greek word tekton (tbktov), meaning carpenter or builder.

There are three well-defined plate boundary types: transform (conservative), convergent (destructive), and divergent (constructive). Transform faults occur where two plates move laterally against each other. This does not create or destroy plate material but can cause large seismic events due to enormous friction force. Transform boundaries, like the famous San Andreas Fault in California, are also commonly found as linking segments in mid- oceanic ridges. Convergent plate boundaries, also known as active margins, occur where two plates slide toward each other. This results in either a continental collision, as found in the Himalayas, or a subduction zone where one plate pushes under another, as found off the coast of Japan. Subduction zones can create very deep submarine canyons, such as the Marianas Trench. Convergent boundaries are known for high seismic and volcanic activity. Divergent boundaries are locations where plates are moving away from each other, creating areas of spreading from new magma pushing toward the surface. The Mid-Atlantic Ridge is a classic example of a divergent boundary.

The modern theory of plate tectonics attributes the movement of plates primarily to the dissipation of Earth’s internal heat from the mantle and the differential density of oceanic lithospheric rock with regard to age. Convection currents in the mantle form new basalt at divergent boundaries, which is initially less dense than the asthenosphere. As the basalt cools over time, it becomes denser, sinking lower into subduction zones and resorbing into the mantle. This simplification combines many underlying forces that fall broadly into three categories: mantle dynamics, gravitational force, and rotational force. The relative contributions and importance of each force are major ongoing research topics, but most geologists currently agree that mantle dynamics provide the primary force for plate movement. This occurs through a combination of basal drag between the convection currents in the fluid mantle and the solid overlying lithosphere and “slab suction,” where localized convection currents found at subduction zones force the lithosphere deeper. It is interesting to note that Earth is currently the only planet (or moon) known to have plate tectonics—potentially due to its large percentage of liquid water combined with the optimal thickness and rigidity of the crust.

The theory of plate tectonics is relatively recent, only really gaining acceptance and evidence of its driving forces in a series of publications during the 1960s. However, observations of the foundational theory of continental movement preceded this modern concept by nearly 400 years. Abraham Ortelius (1527-98), a Flemish geographer and cartographer, observed in his Thesaurus Geographicus (1596) the complementary shapes of the Americas and Africa/Europe and posited that these continents were separated due to earthquakes and floods. Many geologists in the 1800s, including Charles Lyell and Alfred Russel Wallace, noted these similarities as well. These observations were coalesced by German geophysicist and meteorologist Alfred Wegener in 1912 and further defined in his book The Origin of Continents and Oceans. Wegener coined the term “continental drift” to describe this movement, using fossil evidence and radiometric dating, and theorized there was an original supercontinent he called Pangea.

Despite a credible and evidence-based theory of continental drift, there was no immediately proposed mechanism for the process. Many discounted the theory for decades, assuming instead a permanence of continental positions. In the 1950s and 1960s, the invention and improvement of seismographs enabled scientists to observe that seismic activities were focused on particular boundaries, especially oceanic ridges and trenches. Concurrently, the new field of paleomagnetism and the invention of magnetometers allowed scientists to show magnetic striping of the sea floor, demonstrating sea-floor spreading and supporting the theory of plate movement and subduction. Harry Hammond Hess (1906-69) is considered one of the founders of the unifying theory of plate tectonics that accounted for plate drift and provided a mechanism of conveyance and subduction of lithospheric rock into the asthenosphere.

FURTHER READING: Bercovici, David, Yanick Ricard, and Mark A. Richards. 2000. “The Relation between Mantle Dynamics and Plate Tectonics: A Primer.” The History and Dynamics of Global Plate Motions. Geophysical Monographs Series, Vol. 121. Washington, D.C.: American Geophysical Union, 5-46.

Hess, Harry H. 1962. “History of Ocean Basins.” Petrologic Studies 4: 599-620.

Stadler, Georg, Michael Gurnis, Carsten Burstedde, Lucas C. Wilcox, Laura Alisic, and Omar Ghattas. 2010. “The Dynamics of Plate Tectonics and Mantle Flow: From Local to Global Scales.” Science 329 (5995): 1033-8.

Wegener, Alfred. 1915 The Origin of Continents and Oceans. New York: Courier Corporation.