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Continents Adrift and Sea-Floors Spreading: The Revolution of Plate Tectonics Stephen Brusatte 1 As the mercury inched below -54 °C, the sense of panic and fear suddenly became palpable. Faced with the uncertainty of a long polar march, a group of 12 native Greenland guides decided they had had enough. If the two European scientists wanted to continue their trek to a remote weather station in the center of Greenland’s barren ice cap, they would have to do it alone. The leader of the team, a German-born earth scientist by the name of Alfred Lothar Wegener, was unfazed by the desertion of his guides. His friend and former student, Johannes Georgi, lay stranded at the station without supplies, and the long Arctic winter was quickly morphing into an agent of death. Wegener elected to proceed, and set out with two companions during the waning days of September 1930. If anybody could survive such an arduous trip it was Wegener. A Ph.D. who spent years scrutinizing ancient astronomical measurements, a world-record setting hot air balloonist, and a veteran of both World War I and several previous trips to Greenland, Wegener was noted for his determination and rock-solid mentality. As a scientist he was accustomed to the blinding cold of academia and the frostbite of scientific controversy, and as an intrepid explorer he was familiar with the dangers of the Arctic wasteland. A little over a month after their launch, Wegener’s team reached Georgi and delivered the necessary supplies. Knowing that the rations could only feed two men, Wegener found it necessary to return to the coast, and set out soon after. He never made it back alive. Today Alfred Wegener is entombed in the Greenland ice cap, a fitting resting place for a man whose life was gradually enveloped by the freezing gusts of scientific controversy. During his short 50-year lifespan, Wegener was the consummate polymath, a dabbler in meteorology, climatology, geology, paleontology, and physics whose refusal 2 to specialize long hampered his search for an academic position. At the time of his death Wegener embodied an ultimate paradox. He was both a climate specialist of international fame and an outsider geologist of international infamy. Fifteen years before the relentless Arctic winter snuffed out his young life, Wegener had outlined a provocative and controversial theory: continental drift. At the time of his death, Wegener was ridiculed and his theory was scorned. Today, it is renowned as an essential element of the unifying theory of earth science: plate tectonics. Long before Wegener pieced together the notion of continental drift, philosophers and geographers had recognized that the coasts of Africa and South American snugly fit together like a child’s jigsaw puzzle. As early as 1596 the geographer Abraham Ortelius even suggested that the Americas were once conjoined with Europe and Asia, but later “torn away” by earthquakes and other catastrophes. In recent years, historians of science have revealed nascent hints of continental drift in the writings of Francis Bacon, Scottish philosopher Thomas Dick, and the famous French scientist Comte de Buffon. It was the genius of Wegener, however, that assembled widely divergent lines of evidence into the first coherent theory of continental motion. Like the early geographers before him, Wegener was intrigued by the congruence of the South American and African coastlines as a young boy. It was only during the final days of 1910, after completing his Ph.D., finishing his first expedition to Greenland, and receiving his first lectureship, that Wegener seriously began to give currency to continental drift. In the autumn of 1911 his suspicions would be strengthened, when he came “quite by accident” upon a paper describing similar Paleozoic fossils from South America and Africa. This paper argued for a former land bridge connecting the two 3 continents, but Wegener frowned on such a suggestion. The issue ate at his mind so forcibly that the 31-year-old scientist soon after launched a massive literature search in the hopes of finding additional data to support continental drift. What he found surprised him, and gave birth to a revolution in the earth sciences. The data Wegener uncovered were varied and wide-ranging. Not only did the coastlines of South America and Africa match, but so too did the coasts of Newfoundland, England, parts of Greenland, and Scandinavia. Interestingly, the Paleozoic fossils shared by South America and Africa were only the tip of a paleontological iceberg of supporting observations. As it turned out, South America, Africa, Australia, Antarctica, and India shared a mélange of Paleozoic and Mesozoic fossils, including a unique tropical plant flora characterized by the fern Glossopteris and a reptile fauna that included the tusked, pig-like Lystrosaurus. Modern animals do not range across all continents, because it is often impossible to disperse across oceans and other barriers. However, the modern lands supporting these unique fossils are widely divergent today. This suggested to Wegener that they must have been connected in the past. Additionally, the presence of tropical fern fossils in Antarctica makes no sense if the continent has always occupied a polar position. Other evidence revealed by Wegener included closely matching rock units shared by Africa and South America; the distributions of former equatorial climate belts (as shown by coals and fossil reefs shared by the five forementioned lands); and the locations of past Paleozoic glaciations. Taken together, these facts suggested to Wegener that all of the continents had once been joined together into a supercontinent named Pangaea, which later split into two large fragments. The first included North America, Europe, and Asia, while the second consisted of Africa, South America, India, 4 Madagascar, Australia, and Antarctica, all of which separated later. There was no conceivable way for land bridges to explain these varied observations. The continents must have moved. Wegener first presented his hypothesis of continental drift in a series of lectures and journal articles in 1912. Three years later, after a successful expedition to Greenland and a stint as a solider in World War I, he outlined his ideas in a short, 94-page book, Die Entstehung der Kontinente und Ozeane. Today regarded as one of the rarest and most treasured books in the history of science, this short compendium is catalogued in only two libraries in the United States, the New York Public Library and the John Crerar Library. Fortunately, Wegener’s book would be revised three more times before his death, and issued in English and French in 1924. As would be expected, this book caused a considerable stir in the geological community. University of Chicago geologist R.T. Chamberlin ridiculed the theory as being “of the foot-loose type,” while Edward Berry bluntly labeled Wegener’s method as “unscientific.” While a good number of geologists accepted Wegener’s data, many geophysicists scoffed at Wegener’s failure to provide a plausible mechanism for continental motion. In his book, Wegener suggested that the centrifugal force resulting from the Earth’s rotation, or possibly the pull of gravity from the moon, drove the lighter, granite-rich continents through the denser, basalt-rich oceanic crust like a ship plowing through water. However, eminent Cambridge geologist Harold Jeffreys, one of the most respected scientists of his time, did the calculations and found these forces insufficient to move something as large as a continent. At the time of his death, Wegener was the subject of ridicule and contempt from the geological community. However, as a new age of science dawned in the shadow of 5 World War II, an avalanche of observations would drag continental drift back into the light. Although Wegener himself could never appreciate it, the new data strongly vindicated him. Over the course of the 1960s, a handful of young earth scientists from across the globe would mold the new observations into the theory of plate tectonics, which today explains everything from animal dispersal and mountain building to volcanism and earthquakes. One key observation involved the Earth’s magnetic field. As lava cools into solid rock, tiny crystals of a magnetic mineral called magnetite are “locked” into position, thereby recording the strength and direction of the Earth’s magnetic field at the time of the rock’s formation. Using relatively simple trigonometric equations, geologists can take this data and determine the latitude at which a certain rock formed. This procedure was applied to igneous rocks from across the globe, and it was discovered that the latitudes at which the rocks formed were different than the latitudes they occupy today. At first it was thought that the Earth’s magnetic pole simply wandered over time, but further research showed that the pole was essentially fixed. Instead, it was the continents that moved. Paleomagnetic data strongly supported continental drift, but current geologists view Wegener’s hypothesis as only half of the unifying theory of plate tectonics. The other contribution is sea-floor spreading, a mechanism that was long hinted at but only rigorously demonstrated in the 1960s. Leading the revolution was a Princeton geologist by the name of Harry Hess, a mild-mannered scientist who discreetly took measurements with a Fathometer while captaining a Navy ship during World War II. Hess’ measurements detailed the topography of the sea-floor, and revealed that a series of long 6 mountain ranges, deep trenches, and extinct volcanoes littered the deep abyss. Later evidence suggested that a large percentage of the world’s earthquakes were occurring in these submerged mountains, hinting that the ocean bottom was a dynamic place. Over time Hess became convinced that sea-floors were actually spreading in both directions away from the mid-ocean mountain ridges. He presented his data in a landmark 1962 paper, and was soon supported by a slew of additional observations. For example, Cambridge geologists Drummond Matthews and Fred Vine measured the magnetic fields in the rocks of the ocean floor, and found that the strength and direction of the fields followed striped patterns that were the same on both sides of the ridges. Later studies directly dated the rock of the sea-floor, and found that the absolute dates were also symmetrical around the ridges, with rocks becoming older with distance from the hypothesized spreading center. An influx of additional data only confirmed what Hess, Matthews, and Vine suspected: that new sea-floor was created in mid-ocean ridges and gradually spread outward in both directions. This picture of plate tectonics, a unification of continental drift and sea-floor spreading, was largely pieced together during an eight-year period in the 1960s. Today, geologists view the Earth as composed of two principle layers, the brittle, outer lithosphere and the denser, warmer asthenosphere. The lithosphere, akin to the fragile shell of an egg, is broken into some 20 distinct plates, which are rigid but deformable at their edges. When two plates meet one of three general interactions occur: they can move away from each other, move toward each other, or slide past each other. When plates move away from each other, at what are called divergent boundaries, sea-floor spreading takes place, and new oceanic crust is formed. When plates come together at a convergent 7 boundary, the result is more complicated. If denser oceanic crust meets lighter continental crust, the oceanic crust is subducted beneath the continent, which gives rise to volcanism, explaining the Ring of Fire. When two segments of continental crust meet they will collide and wrinkle, forming mountains. This is precisely what happened when India and Tibet collided, uplifting the Himalayas. And, when two plates slide past each other, earthquakes frequently occur, as manifested by the San Andreas Fault of California. Despite this resolution, the mechanism of continental drift is still somewhat murky. However, it is thought that convective currents in the wax-like asthenosphere drive the motion of the overriding lithosphere. The development of plate tectonics, stretching from Wegener’s initial observations to the more rigorous experiments of Hess and cohorts, constituted nothing less than a revolution, a complete transformation of the practice of geology. Unlike most traditional sciences, geology deals with large-scale patterns and processes that operate over unthinkable lengths of time. Early in the formation of geology as a scientific discipline, practitioners found it necessary to describe the geological past of Earth based on processes that occur in the present. Although intuitively rational, this method of “uniformitarianist” thinking disabled many geologists from recognizing complex processes that are difficult to observe. This often led to rampant speculation and frankly unscientific methods when geologists were faced with explaining complicated issues, such as earthquakes or mountain building. For a long period it was widely accepted that mountains formed as the warm surface of the Earth gradually cooled and contracted, much as the skin of an apple wrinkles as it rots. There was little rational evidence to support this suggestion, but geology was unable to do much better. 8 Today, geologists have a much easier time explaining how mountains form. The new answer is not only simpler, but also more scientific. It is based on numerous observations, a healthy gathering of collected data, and years of testing and refinement. This new answer is based on plate tectonics, a beautiful and elegant theory that unifies centuries of divergent observations into a complete, understandable, working theory of the Earth. Mountains form not via contraction, but when continents collide. Earthquakes are not a mysterious force of nature, but instead a consequence of plate motion. Volcanism doesn’t reflect the wrath of the gods, but rather a chaotic and vibrant Earth whose sea-floors spread and continents drift. This is the beauty and importance of plate tectonics. It is the grand unifying theory of earth science, the geological equivalent to Darwinian evolution or Newtonian and Einsteinian physics. Plate tectonics has truly delivered geology from a historical science based solely on modern observation to a pulsating discipline rooted in a solid framework. Although critical for those studying the past evolution of life or the changing nature of the Earth’s magnetic field, plate tectonics saves its most important contributions for modern, industrial societies. Finding the Higgs Boson will have few practical applications, but understanding plate tectonics has already saved innumerable lives and promises to better our standard of living. It is near impossible to understand the frequency and potential destruction of earthquakes if we view these tremors as mysteries of nature or the forces of a god. However, now that we have a general understanding of why and where earthquakes occur we can better predict their damage, and better survive their wrath. The study of earthquakes has allowed governments to update building codes, construct safer freeways and power lines, and enact detailed responses to a tremor. The 9 same is true of volcanism. Knowing where volcanism occurs and how potentially dangerous an eruption can be has allowed for safe and easy evacuations of potential hot spots. Not only does an understanding of plate tectonics help save lives, but it also fosters a deep respect for the power of nature and the mind-numbing length of geologic time. Human time, measured on the span of days and years, rarely coincides with geologic time, which is enumerated by epochs, periods, and eras. People have little appreciation for the vast history of our planet, and the slow, gradual geologic forces that can build a mountain, split a continent, or close an entire ocean. We are using our natural resources at an alarming rate, exhausting within decades supplies of oil, coal, and ores that took millions of years to form. Similarly, biodiversity is rapidly decreasing, as the extinction rates of species have risen with the evolution of the human race. Much of this isn’t directly our fault. We simply find it difficult to understand how we, a single species existing for hundreds of thousands of years, fit into the larger picture of a planet that has been around for 4.5 billion years. Plate tectonics puts the enigma of deep time into perspective. Sea-floors spread at 2-4 centimeters per year, a rate comparable with the growth of our fingernails. However, as is obvious by a quick glance at the topography of Earth, these slow forces can create incredibly significant results. Plate tectonics is exhibit A in the case that the Earth is ancient and ever-changing, and that we are only a small part of the history of such an amazingly complicated and dynamic planet. As a student of geology with an interest in fossils, learning the workings of plate tectonics has given me a new perspective on the history of our planet. Earth isn’t static, it is dynamic. The present state of affairs is just a temporary moment in time, subject to 10 change with the next earthquake or volcanic eruption. I will never perceive a sea-floor spreading or a continent drifting during my lifetime. Human lifetimes are simply too short. But, extended over millions of years, the very same forces in operation today constantly reshape our planet. Understanding these forces will not only allow me to be a better scientist, but will also certainly aide society in coping with nature and humanity in respecting its true place in the cosmos. BIBLIOGRAPHY Bacon, F. 1855. Novum Organum. 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