Plate Tectonic Theory and the Unification of the Earth Sciences
Plate Tectonic Theory and the Unification of the Earth Sciences
It took nearly a century for scientists to accept the idea that continents were not forever fixed in their places, but had, in fact, slowly drifted to their current locations. In the 1960s plate tectonics, a further refinement of this concept bolstered by irrefutable geologic proof, burst into widespread acceptance in less than a decade.
Plate tectonic theory holds that continents ride atop thin plates of crust that are constantly moving across the face of the Earth. These plates break apart at midocean ridges, such as the mid-Atlantic Ridge; when they come together, one plate dives beneath the other to be recycled into the mantle—a process called subduction. These subduction zones, appearing as deep-sea trenches, are the sites of most of the world's earthquakes. As the plates descend into the Earth, they heat up and start to melt. The rising magma reaches the surface, forming volcanoes at the surface, usually within about 100 kilometers of the subduction zone.
Plate tectonic theory has become the single unifying factor in the earth sciences. In the words of John Tuzo Wilson (1908-1993), one of its founders: "The acceptance of continental drift has transformed the Earth sciences from a group of rather unimaginative studies based upon pedestrian interpretations of natural phenomena into a unified science that holds the promise of great intellectual and practical advances." This theory, for the first time, gave a single mechanism to explain the locations of mineral and ore deposits, the origins of volcanoes, the reason for the "Ring of Fire," the origin of many earthquakes, the origin of seafloor magnetic anomalies, the formation of mountains, and much more. Plate tectonics may not have the emotional and theological impact of evolutionary theory, but it helps explain phenomena that evolution alone could not. It is the workhorse of theories because it serves many sciences.
The impact of plate tectonics is not limited to the earth sciences, however. Paleontological evidence first helped to prove plate tectonics and, later, continental drift helped explain otherwise impossible fossil evidence, such as the presence of strictly land-based animals in Antarctica and South America. This theory was a true scientific revolution because of its wide-ranging applicability and the sweeping changes it forced in both scientists' and nonscientists' view of the Earth. Put more simply, before plate tectonics we believed that we lived on a static, dead world enlivened with only an occasional spasm. We discovered instead that we live on a vibrant, active world, constantly changing and in continual motion.
Ever since the world was mapped, scientists noticed that the coastlines of Europe, Africa, and the Americas seemed as if they could fit together like puzzle pieces. Eduard Suess (1831-1914) proposed in the late nineteenth century that large ancient continents had broken into smaller ones. He believed, however, that instead of drifting apart, large parts of these giant continents had sunk beneath the ocean. This, along with other land bridges that had disappeared, explained similarities between continents that were no longer connected. Not until the first part of the twentieth century, however, did anyone suggest that the continents had separated and moved to their present locations. Alfred Wegener (1880-1930), who held a Ph.D. in astronomy and worked as a meteorologist, presented his findings at a lecture in 1912; he published his studies in 1915 in Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans). Scientists of the day scoffed at Wegener and criticized his idea.
Wegener was not ridiculed simply because his idea was thought to be absurd, though that did play a part. His ideas were dismissed because he could not suggest a motivating force for continental motion and because the very concept of something as huge as a continent moving was simply incredible to his contemporaries. Geology in 1912 was far different than it is today. Its practitioners were not far removed from the great debates over evolution, whether or not fossils represented animals lost in the biblical flood, and similar concepts. Here are some other "facts" believed by early-twentieth century geologists:
Mountains and valleys were caused by the wrinkling of the Earth's crust as it cooled.
Similar animals in the Americas and Africa crossed from one continent to the other via land bridges that later sank into the ocean.
Continents are fixed and unmoving on the Earth's surface.
The Earth occasionally shifted on its axis, causing the poles to "wander."
Wegener had synthesized information from a number of fields, including stratigraphy, zoology, paleontology, and others to support his theory. Despite this, his theories were rejected. One reason is that he was an outsider dabbling in areas in which he was not trained. Unfortunately, specialists often focused only on their own particular problems and failed to see, as Wegener did, more than one possible answer. Another factor was that while no single scientist knew enough about all fields to criticize Wegener's theory as a whole, specialists knew more than he did about their particular fields, allowing them to carp about small inaccuracies. This combination of ridicule and scientific rejection caused continental drift theory to be discarded for nearly a half century.
After World War II, oceanographers began to conduct magnetic surveys of the ocean floor. They saw that when molten rocks cool they "freeze" into mineral crystals that reflect the pattern of the Earth's magnetic field. By studying magnetic fossils, geophysicists learned that the polarity of the Earth's magnetic field had undergone many past reversals. Oceanographers were surprised to discover that these magnetic field traces changed as they moved across the ocean floor. They encountered stripes running the length of the Atlantic Ocean in which the magnetic field polarity was identical. Then they found another stripe with opposite polarity, and yet another stripe with polarity reversed again. This pattern was repeated on the European and African side of the mid-Atlantic Ridge. This suggested that the oceanic crust was formed continuously and, as the molten rock forming the crust solidified, it froze into place whatever magnetic field existed at that time. The stripes simply revealed the history of oceanic crust formation over time.
In the meantime, other observations continued to trouble and puzzle geologists. First, there was the still-nagging presence of nearly identical fossils on opposite sides of oceans, something that had led Wegener to propose his continental drift theory. Specifically, the plant Glossopteris, a fern, was found in South America, Africa, India, and Australia. A number of nonmarine animals were also found in similar locations. In addition, geologists were becoming increasingly uncomfortable with the concept of a land bridge a few thousand miles long that just happened to vanish without a trace. Plus, there were well-documented matching rock formations in England and New England, Antarctica and Australia, South America and Africa, and other places; too many places to be simple coincidence. Some earlier geologists had postulated a mid-Atlantic continent, now vanished, that could explain some of these similarities in rock type, but this, again, stretched credulity to the breaking point.
Next came the realization that the Hawaiian Islands were likely formed by a single "hot spot" in the Earth's mantle, and that the Emperor Seamounts, a chain of submarine mountains, were probably a continuation of the Hawaiian Island chain that had eroded beneath the waves. Scientists also discovered that no oceanic crust on Earth was more than a few hundred million years old, yet 3 billion-year-old rocks could be found on the surface. Geologists were also puzzled by unmistakable evidence of ancient glaciers in Australia and Africa; they also found evidence of swamps in the Antarctic. The more they looked, the more they realized that continental drift must be causing these, and perhaps other, phenomena. The question then became how?
The motivating force for continental drift—the explanation that Wegener lacked—turned out to be plumes of hot rock in the Earth's mantel. Although a solid material, hot rock can deform and flow, just as hot glass can. Over geologic time, the rock of the mantel forms rising plumes that can move the continental crust. Under this scenario, hot mantle pushes its way up at the midocean ridges, where convection cells rise to the bottom of the crust. Here, it melts the oceanic crust, perhaps contributing some mantel rock to the mix. The crust buckles up and is forced to the sides, pushing the entire plate with it, away from the ocean ridge—at the break-neck rate of an inch (2.5 cm) or so per year. That translates to about 25 kilometers (16 miles) every million years—enough to open the Atlantic Ocean to its present width in about 100 million years. Geologically speaking, this is about 2% of the age of the Earth, so the Atlantic could have opened to its present width and closed again 25 times over the history of the Earth.
If oceans open up, they must close, too. This means that at various times in the Earth's past, the continents must have been assembled into supercontinents—huge single landmasses. This explains how the land plant, Glossopteris could appear simultaneously in so many locations and how worms could move from South America to South Africa. Scientists now envision a "supercontinent cycle" in which, over hundreds of millions of years, the continents assembled themselves into supercontinents that stayed together for awhile and then broke up again. The last supercontinent, Pangaea, broke up into Gondwanaland and Laurasia about 200 million years ago. These two continents then broke up to form the seven present continents beginning about 120 million years ago.
By 1971 all of these pieces had come together into a coherent theory that, while not fully accepted, had won acceptance by a large number of prominent geologists, paleontologists, and others. From that point on, rather than fighting for acceptance, adherents to plate tectonic theory refined and expanded their theory, finding it ever more valuable in explaining the workings of the Earth. It would be fairly accurate to say that by the 1980s the theory of plate tectonics was almost universally accepted.
The impact of plate tectonic theory on science has been enormous and may not yet be fully realized. The impact on the general public has been somewhat less obvious, but significant nonetheless. Plate tectonics has allowed us to:
- Better understand volcanoes and earthquakes, giving us insight into their causes, mechanisms, and risks.
- Improve theories for locating mineral deposits, allowing more efficient prospecting and recovery of mineral resources.
- Reconstruct earlier climates and land positions at those times, giving more detailed information about the Earth's climate history.
- Extend our knowledge of the Earth to other planets and moons in the solar system as we try to better understand them.
- Develop a unified theory of the Earth, instead of a number of piecemeal and ad hoc theories explaining individual features.
Volcanoes and Earthquakes
Most earthquakes and virtually all volcanoes originate at the margins of tectonic plates. As slabs of oceanic crust descend into the lower crust and mantel, they catch, build up stress, and then slip. As plates move past one another, they catch and release. This is what causes earthquakes. As one moves further from a subduction zone, the earthquakes move steadily deeper, following the descending plate into the Earth. Further from the surface, the earthquakes cause less damage at the surface and the area becomes seismically safer.
Most volcanoes are formed when molten rock rises from descending plates to the surface. Again, this means that living further from subduction zones generally reduces the hazards associated with volcanic eruptions. Notable exceptions to this general rule are "hot spot" volcanoes such as the Hawaiian Islands or the volcanic caldera beneath Yellowstone. In these cases, a plume of mantel material is apparently hitting the bottom of the crust as it moves past, melting crustal rocks and causing volcanoes to form. However, knowing that this is the cause leads us to predict that, while Yellowstone will likely erupt again someday, Cincinnati will probably not do so because it is not over any known hot spot (the mantel hot spots have been pretty well mapped by measuring the temperature and heat flow rate of the crust over the Earth).
Understanding how volcanoes erupt, earthquakes occur, and why they are where they are can help in many ways. Knowing where to find them lets us construct maps showing the seismic and volcano risk of various parts of the world. These maps are used to determine insurance rates, to help find locations for hazardous and radioactive waste disposal sites, to calculate the risk of damage to power plants, and so forth. This knowledge can also help people to decide where to live. On the other hand, many geologists purposely move toward such zones to be closer to the events they study.
Continental rifting creates stress in rocks and is accompanied by the movement of magma and hot liquids (geothermal fluids). Volcanoes are often associated with mineral deposits that vary depending on the chemistry of the subducting slab and the overlying "country" rock. Much work has been done to correlate seismically and tectonically active regions and mineral deposits, leading to a greater potential for economically viable mineral extraction.
In addition, many rock formations were simply ripped asunder during continental rifting. A rich mineral deposit on one side of an ocean or sea implies a corollary deposit on the other side, if one can determine continental motion in the intervening millions (or tens or hundreds of millions) of years. A careful examination of the rock record near good mineral deposits may indicate areas of similar geology several thousand miles away that would be worth exploring for economic mineral deposits.
Despite the arguments against environmental degradation, maintaining and (for less-developed nations) building a modern industrial society requires ready access to large amounts of relatively inexpensive energy and raw materials. Much human progress has depended on the use these commodities. The lack of raw materials can even push a country into war, as Japan did in World War II to assure continued access to raw materials and energy sources. Understanding the location of valuable mineral and energy deposits can help the world to continue progress towards goals of greater wealth and a higher standard of living for all.
Earth sciences can also help study past climates. For example, the composition of rocks formed at the Earth's surface indicate the temperature at which they formed and the rough chemical composition of the atmosphere at that time. In addition, certain plants and animals, which may be found as fossils, are known to have been associated with specific types of climate. However, simply knowing what the weather was like at some time in the past is not sufficient to reconstruct a past climate. For example, a scientist 30 million years from now, finding the fossil of a palm tree beneath arctic ice may conclude that, at this time, the polar caps were melted and the poles were warm. However, that same scientist, looking at continental motions over this time period, would realize that the palm tree grew near the equator and had been subsequently carried to its frigid new home.
There is great interest in studying ancient climates because of the debate currently raging over whether and by how much humans are changing the Earth's climate. To know if we're having an effect, we must know what's "normal." To do that, paleoclimate records must be adjusted for ancient position on the globe. This, in turn, will help us better understand our effect on the world. As it turns out, the past few million years have been uncommonly cool. Ironically, the situation known as "global warming" more closely resembles the typical climate on Earth. However, that must be balanced by the fact that, in the more recent past, we have been in an ice age that does not seem to be over yet. Therefore, although our current climate is "unseasonably" cold, any warming trends can't yet be said to reflect what the Earth would be doing without our help, or if it a departure from the expected. Research in this area continues.
In our exploration of the other planets and moons in our solar system we have seen evidence of tectonic activity in a number of places. Io, a moon of Jupiter, is the most volcanically active body known. Ganymede, another Jupiter moon, shows evidence of "ice tectonics" that suggests an underlying ocean of liquid water. Mars, probably devoid of tectonic activity at present, has the largest known volcanoes and rift valley, possible signs of early tectonic activity. Venus appears to have been subject to plate tectonics at one point, if not still today. Other worlds are not yet sufficiently well explored, but we are likely to find more tectonic activity in the future as we learn more about our planetary neighbors.
This is important for a number of reasons. First, it confirms that plate tectonics is a viable and important theory. In addition, we can observe other planets and virtually all of their features because, with the possible exception of Ganymede, they are devoid of the oceans that hide so many of Earth's tectonics. By studying other worlds, we can better understand our own. And, finally, we are learning more about some of the mechanisms that can cause plate tectonics. Plate movement requires energy. On Earth, this energy comes from the radioactive decay of uranium, thorium, and an isotope of potassium in rocks. On Io, the energy likely comes from tidal flexing of the moon as it moves through Jupiter's intense gravitational field. Mars probably lost its internal heat long ago, which is why it's now tectonically dead. As we learn more about the "styles" of tectonic activity elsewhere, again, we learn more about our Earth.
Interdisciplinary Unification of the Sciences
As mentioned above, plate tectonics was a unifying theory. It helped bring together many disparate parts of the earth sciences, uniting them with evolutionary theory, paleontology, and some aspects of biology. In fact, this may be its single-biggest contribution to science because, more than anything, it shows how a few well chosen tools can solve seemingly unrelated problems in a number of disciplines. The more we look, the greater the number of interconnections we see, and we start to realize that all problems are interdisciplinary to some degree, so that the answer to any problem lies with no single discipline or person. It both broadens and unifies science as few other theories have ever done.
P. ANDREW KARAM
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