Planetary tectonics

A tectonic process is any process that causes movement or distortion of a solid planetary surface. The gas giant planets do not have solid surfaces, so tectonics does not apply to them. The other planets and satellites in the solar system, however, have varying types of tectonic activity, which can include volcanic activity, activity or movement along fault lines, uplifting, and crust folding.

Overview

The contours of the eastern coasts of North and South America fit nicely into those of the western coasts of Europe and Africa, suggesting that these continents were at one time a single landmass. Various geological and fossil patterns also match. On the basis of this evidence, Alfred Wegener proposed the theory of “continental drift” in 1912. At the time, few geologists took Wegener’s ideas seriously; the idea that continents might move, even if only a few centimeters a year, seemed too far-fetched. Furthermore, Wegener was unable to suggest a mechanism that might cause this motion.

Evidence favoring Wegener’s ideas continued to accumulate, however. By the 1960s, geologists had begun to embrace the theory of plate tectonics. On Earth, the crust is divided into several large plates that float on the mantle, which is in a plastic-like state between liquid and solid. Slow convection currents in the mantle cause the plates to drift very slowly. Hence, the theory of plate tectonics confirms Wegener’s idea of continental drift, provides a mechanism for the drift, and explains Earth’s geological structures and evolution. The theory of plate tectonics was the major geological advance of the twentieth century.

On Earth, most earthquakes and volcanic activity occur at the boundaries between plates. Plate boundaries are of three types: divergent, where plates move apart; convergent, where plates move together; and transform, where plates slide horizontally past each other.

Plates diverge along the ocean ridge-rift system. An example is the Mid-Atlantic Ridge, which runs roughly north-south along the floor of the Atlantic Ocean midway between Europe and Africa to the east, and North and South America, to the west. As the plates on each side of the Mid-Atlantic Ridge diverge, hot magmawells up from below and oozes out along a series of fissures, adding new ocean crust to each plate. The new crust is elevated into an undersea ridge with a central rift. At spots where the volume of extruded lava is especially large, volcanic islands such as Iceland can build up above sea level.

There are three types of convergent boundaries: oceanic-continental, oceanic-oceanic, and continental-continental. Where an ocean plate collides with a continental plate, such as along the west coast of South America, a subduction zone forms. The oceanic plate, composed of slightly denser rock, subducts (descends) below the continental plate, sinking back into the mantle. The continental plate remains on the surface, sliding over the subducting ocean plate, and its leading edge is crumpled and buckled into a mountain range, such as the Andes. When two oceanic plates converge, one of them subducts under the other, and a volcanic island arc develops along the upper oceanic plate. Subduction zones (at both oceanic-continental and oceanic-oceanic boundaries) are marked by deep-sea trenches, formed where the descending oceanic plate bends downward. The deepest of these is the Mariana Trench in the western Pacific Ocean, which drops to more than eleven kilometers below sea level. When two continental plates collide, neither plate subducts. Instead, the leading edges of both plates are compressed and crumpled, forming along the suture a high mountain range, such as the Himalayas, which developed when India collided with the rest of Asia.

Most transform boundaries are short and offset segments of divergent boundaries. A few transform boundaries are long, like the San Andreas fault, which cuts across California from its border with Mexico to north of San Francisco. The land to the west of the fault is part of the Pacific Plate, and it is sliding northwest past the North American Plate, which forms the eastern side of the fault.

Earth is the only known planet with plate tectonics, but scientists have confirmed that other planets and satellites have had different types of tectonic activities. As a general principle, planets or satellites with very heavily impact-cratered surfaces have had relatively little tectonic activity. The heaviest era of crater formation in the solar system was shortly after its formation when there was still plenty of debris to crash into solid planets or satellites to form impact craters. If a planet or satellite renews its surface by some type of tectonic activity, the renewal obliterates the craters. Hence, a planetary surface with many impact craters has not been renewed and has undergone little tectonic activity, whereas one that shows evidence of surface renewal may have been subject to tectonic activity.

Both Earth's Moon and Mercury are covered with impact craters, telling us that they have had relatively little tectonic activity in their geologic histories. This fact is related to their relatively small sizes. Smaller objects cool faster; a spoonful of hot soup, for example, cools faster than a large bowl. Small worlds have less interior heat to drive tectonic activity. Both Mercury and the Moon have fractures and fault valleys that formed from meteorite impacts. On the Moon, basins that formed from large impacts were partially filled in by lava flows to form the maria, or so-called “seas.” Rilles, such as Hadley’s rille near the Apollo 15 lunar landing site, were once rivers of flowing lava. The lunar lava flowed much more easily than lava on Earth, so it did not form extensive volcanoes. Mercury, by contrast, has intercrater plains rather than maria; these plains probably are lava flows from billions of years ago. Mercury’s geologic history is, however, less well understood than the Moon’s. Mercury also has many scarps, which are cliffs stretching for hundreds of kilometers. They probably formed when Mercury’s interior cooled and shrank, causing the crust to buckle.

Venus has an extremely large number of volcanoes. There are approximately sixteen hundred large volcanoes on Venus and perhaps as many as hundreds of thousands of smaller volcanic features. Most of the large volcanoes are shield volcanoes, which are similar to those on the Hawaiian Islands. Shield volcanoes form when lava flows up through a tube in the center of the volcano. Venus has much larger volcanic features called coronae (the singular is corona) and smaller volcanic structures called either pancake or volcanic domes. Both of these types of structures formed when hot lava flowed up from the mantles and pushed the crust upward, causing it to swell. In addition to these volcanic features, about 80 percent of Venus’s surface is covered with solidified lava flows. Geologic evidence suggests that the surface of Venus was renewed by tectonic activity as recently as a few hundred million years ago. Venus does not have the types of tectonic features that are associated with boundaries between tectonic plates on Earth, and thus, Venus does not have plate tectonics, as Earth does. Rather than causing crustal plates to move horizontally as on Earth, convection currents in Venus’s mantle cause the crust to move vertically.

Mars has what are probably the most impressive tectonic features in the solar system. The Valles Marineris is a large tectonic canyon on Mars formed by a crack in the crust that is roughly as deep as the Grand Canyon. It is, however, a few hundred kilometers wide, about as wide as the Grand Canyon is long. It is long enough that, if the Valles Marineris were placed in the continental United States, it would stretch from about Cape Hatteras, North Carolina, to Los Angeles. Valles Marineris probably formed when tectonic uplifting from the Martian interior cracked the crust. Mars’s volcanic mountains are equally impressive. Olympus Mons, which is the largest volcanic mountain known in the solar system, towers about twenty-five kilometers above the Martian surface. Its base is about seven hundred kilometers wide. The Tharsis Ridge volcanoes are almost as large. These Martian volcanoes are shield volcanoes but are much larger than shield volcanoes on Earth. Shield volcanoes form above volcanic hotspots. On Earth, the plates drift while the hotspots remain stationary. During dormant periods on Earth, the portion of the drifting tectonic plate above the hotspot moves. A chain of smaller volcanic mountains results. On Mars, however, the plates do not seem to drift during dormant periods, so the volcano repeatedly erupts at the same location, and a single, huge mountain results. Martian volcanoes are now extinct.

In August 2012, UCLA scientist An Yin determined from his research of Mars that the planet may indeed have plate tectonics, though it was at a primitive stage of the process. Yin concluded that the Velles Marineris was a boundary between two plates. The scientist also believed that "Mars-quakes" likely occurred on the planet. Yin and other scientists continued to work on proving (or disproving) this theory. In 2019, scientists announced the discovery of the first active fault zone on Mars.

Lacking solid surfaces, the Jovian planets (the gas giants) do not have tectonic activity, but some of the Jovian satellites do. For example, Io, the innermost satellite of Jupiter, is perhaps the most volcanically active world in the solar system, with about sixty active volcanoes observed by the Galileo orbiter. Io also has lava flows and other features associated with volcanic activity. The volcanoes on Io are distributed randomly rather than along tectonic plate boundaries, suggesting that Io does not have plate tectonics similar to Earth’s. Jupiter’s second-closest moon, Europa, has an icy crust covering a water mantle. The crust has many cracks caused by shrinking as the watery layers melted. The tectonic activity on both Io and Europa is largely driven by tidal forces from Jupiter heating their interiors. There is also volcanic activity on Saturn’s Enceladus and Titan and on Neptune’s Triton.

Knowledge Gained

Wegener’s continental drift theory was held in poor regard by geologists for several decades. By the 1960s, however, undersea mapping had revealed the Mid-Atlantic Ridge, a chain of undersea volcanic mountains marking the plate boundaries. Scientists also discovered matching patterns of magnetism on either side of the ridge. These lines of evidence, which were not available in Wegener’s time, led to the theory of plate tectonics, which encompassed and provided a mechanism for the continental drift theory.

With the advent of the space age, exploration of the solar system provided evidence of tectonic activity on other worlds. The Apollo missions sent astronauts to directly study the lunar surface and bring back samples for additional study. Hence, our knowledge of the Moon is second only to our knowledge of Earth. Radar maps of Venus made from Earth using the Arecibo radio telescope and from spacecraft orbiting Venus revealed the considerable volcanic activity on Venus. The most detailed radar maps of Venus were made by the Magellan spacecraft in the early 1990s. The impressive tectonic features on Mars were first discovered by the early Mariner missions, launched in the late 1960s. Each subsequent successful mission to Mars has increased our understanding of the tectonic forces that shaped the Martian surface. Rovers, such as the Perseverance, launched in the early 2020s, continued to expand knowledge about Mars.

The Voyager missions to the outer solar system discovered many tectonic surprises including the tectonic and volcanic activity on Europa and Io. In addition, Neptune’s largest satellite, Triton, is so cold that it has volcanic and tectonic activity related to solid and liquid nitrogen. Smaller satellites should have cooled too much to be tectonically active, but Saturn’s 500-kilometer-diameter satellite Enceladus shows signs of tectonic activity discovered by the Voyager mission and further studied by the later Cassini mission. The launching of the James T. Webb Space Telescope in 2021 has also expanded scientists' knowledge of the solar system and produced images conducive to the study of planetary tectonics. As humanity continues to explore the outer solar system, it is likely that more unexpected evidence of tectonic activity will be discovered.

Context

Because we live on Earth and have easy access to its tectonic features, planetary scientists understand tectonic activity on Earth more fully than for any other world in the solar system. Even the best photos from a spacecraft cannot substitute for a geologist’s ability to stand on and explore one of Earth’s many tectonic features. This knowledge of tectonics on Earth, however, helps planetary scientists understand and interpret tectonic features that spacecraft have revealed on other planets. Similarly, planetary scientists who discover new types of tectonic features on other worlds and study how tectonic processes work on other worlds help geologists understand tectonic activity on Earth.

Tectonic activity on Earth often manifests itself in natural disasters such as earthquakes and volcanoes. With an increased understanding of planetary tectonics, it may eventually be possible to predict when tectonic events will occur on Earth or at least to forecast the conditions that may lead to these disasters. The ability to predict earthquakes, volcanic eruptions, and tsunamis caused by undersea tectonic events could save many lives, just as advances in atmospheric science and computer modeling have increased our ability to predict and track major storms and hurricanes, which now saves many lives. Space exploration has contributed to this ability to predict major storms and may similarly make it possible to predict natural disasters caused by tectonic events.

Bibliography

Chaisson, Eric, and Steve McMillan. Astronomy Today. 7th ed. New York: Addison-Wesley, 2011.

Freedman, Roger A., and William J. Kaufmann III. Universe. 9th ed. New York: W. H. Freeman, 2011.

Hartmann, William K. Moons and Planets. 5th ed. Belmont, Calif.: Thomson Brooks/Cole, 2005.

Hester, Jeff, et al. 21st Century Astronomy. 4th ed. New York: Norton, 2013.

Morrison, David, Sidney Wolf, and Andrew Fraknoi. Abell’s Exploration of the Universe. 7th ed. Philadelphia: Saunders, 1995.

Tomecek, Steve. Plate Tectonics. New York: Chelsea House, 2009.

Wei, Maya. “First Active Fault Zone Found on Mars.” National Geographic, 24 Dec. 2019, www.nationalgeographic.com/science/article/first-active-fault-system-found-mars2. Accessed 16 Sept. 2023.

Wolpert, Stuart. "UCLA Scientist Discovers Plate Tectonics on Mars." UCLA Newsroom. UC Regents, 9 Aug. 2012.

Zeilik, Michael. Astronomy: The Evolving Universe. 9th ed. Cambridge: Cambridge UP, 2002.

Zeilik, Michael, and Stephen A. Gregory. Introductory Astronomy and Astrophysics. 4th ed. Fort Worth: Saunders, 1998.