Planet Earth/6b. Plate Tectonics: You are a Crazy Man, Alfred Wegener
Alfred Wegener and Plate Tectonics
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It was a great surprise when Alfred Wegener appeared at the mid-ice station in the center of the vast snowy landscape in the frozen heart of Greenland. His dog sled pulled through the Arctic cold near the onset of winter on October 19, 1930, carrying supplies that would allow the 3-person crew to remain through the winter at the weather station. Their mission had been to record the year-round weather on the great ice sheet, but supplies had dwindled during the summer, and the arrival of fresh supplies was delayed until late fall. They were going to have to abandon the weather station, but the arrival of Alfred Wegener and his younger assistant Rasmus Villumsen, changed their plans. The two travelers suffered through the bitter cold, which reached lows of −60 °C (−76 °F) to arrive at the weather station, and they were only protected by thick furs and parkas. Rasmus had lost his toes to frost bite on the journey, and they looked to the 3-person crew at the station, as if on death’s solemn door. They had only brought enough food and fuel to feed and keep warm the 3-person crew, with the hopes of returning back to their main camp. But the weather had become colder and colder, as winter was setting in. Originally, they had carried more supplies, but at the risk of losing men, only Wegener, and his assistant pushed onward with the bare minimum of supplies to staff the 3-person weather station for the winter. If Wegener refused to make the journey, there would not be enough food for the crew stationed at the isolated outpost on the ice sheet and they would starve, so he and Rasmus Villumsen drove their dog sled onward across the ice and snow into the center of Greenland. After arriving at the station and emptying their supplies for the grateful crew, the two solemn men disappeared across the ice cover world.
Fifteen years before, a much younger Alfred Wegener taught physics, meteorology and applied astronomy at the University of Marburg in Germany, a small town dominated by the Marburger Schloss, an 11th century castle on the hill top above the college. It was an ideal place to teach, and Wegener was a well-respected professor during the peaceful years before World War I. His research for the past decade was focused on the temperature of the atmosphere, and with his brother’s help, he had been sending weather balloons high into the air to record air temperatures at different levels in the atmosphere. While the Wright Brothers were attempting to develop flying machines in the United States, Wegener was launching the first weather balloons, equipped with thermometers to measure the changes in temperature with elevation. His research was published in a well-received book, Thermodynamik der Atmosphäre (Thermodynamics of the Atmosphere), which also reported on his measurements of the atmospheric temperatures at different altitudes during a brief expedition to Greenland in 1906. The trip introduced him to arctic exploration, a fascination with which he would never lose interest in.
During all this research on global atmospheric circulation Wegener would stare at geographic maps of the entire Earth, and notice the odd way that the continents seemed to fit together. The eastern coast of South America appears to mirror the western coast of Africa, while North America and Europe seem to fit against each other with Greenland between them. If you could cut out the oceans, the continents appeared like a jig-saw puzzle, locking together to form a single super continent.
In 1912, Wegener submitted his crazy and wild idea at a geology conference at the famous Senckenberg Museum in Frankfurt, home to a recently installed giant Diploducus dinosaur from the United States. His lecture argued that the continents had drifted apart and their split had given rise to the ocean basins. One of his strongest and most vocal critics was a geologist named Max Semper, who studied fossil shells to understand the history of ocean currents, his research strongly suggested that the continents did not move, and the distribution of fossils implied that sea level rose and fell through Earth’s long history, but the continents stayed fixed. However, to many other geologists in the audience Wegener’s idea was an interesting one, a hypothesis that needed much testing and evidence gathering to prove whether the continents had indeed drifted apart. Wegener returned to Greenland the next year, where he crossed the Greenland ice sheet between the Denmark Havn and Kangersuatsiaq, a distance much longer than the Norwegian explorer Fridtjof Nansen had done during his crossing of Greenland. Although the expedition garnished him some fame for the young scientist, he nearly died during the trip. On his return home, he married the daughter of the famous Russian geographer and climate scientist Wladimir Köppen, who was also his own mentor and teacher. In 1913 to 1915, Wegener did not give up on his hypothesis of drifting continents and continued to write about his ideas of an ancient super continent (Pangaea) and how it broke up over time.
In 1914, World War I brought him to the front, as he served in the war effort, and despite the war, he remained dedicated to formulating his ideas. During the war he published a comprehensive book of his ideas, entitled Die Entstehung der Kontinente und Ozeane (The Origin of the Continents and Oceans). Wegener immersed himself into the study of isostasis theory; that is the study of the brittle strong lithosphere resting on a ductile weak asthenosphere, and how those layers may play a role in the motion of continents over longer periods of geologic time, as plates of lithosphere carried by the motion in the asthenosphere below. He wrote, as translated into English, “one can assume that the continental crust is capable of isostatic compensation in movement, not only in the vertical direction (as in the rise and fall of mountains), but also in the horizontal direction (with the movement of continents).” Wegener took his ideas beyond just a hunch or idea, to a formally argued scientific hypothesis, filled with numerous examples of how it could be further tested. But World War I would continue to rage across Europe for another three more years. His book and ideas were not widely read, and published only in German, so they did not make it to England or the Americas. At the end of the war, Wegener did not give up on his ideas, but moved onto new scientific interests including working with the now freed Serbian scientist Milutin Milanković on Earth’s past ice ages and Earth’s oscillating orbit. In 1926, he traveled to the United States to present a lecture on his ideas in New York City, and although by now he had expanded out his ideas more fully into a theory of plate tectonics, that is the motion of thick plates of the lithosphere forming the continents and how they floated across a weak asthenosphere. His ideas were still considered radical, even among American geologists, and were not well received. He returned to Germany where he was awarded funding by the government to return to Greenland to further his climate research, and to establish a weather station in the very center of Greenland’s ice sheet. He would be put in charge of the entire expedition to capture temperatures through the 1930-1931 winter from the center of the ice sheet.
Departing from the weather station located in the heart of Greenland, the two men pushed onward over the frozen landscape. They had saved the expedition from disaster, the crew would record the temperatures through the coming winter, but they had put themselves at great risk in crossing the ice sheet this late in the year. Winter was closing inward on them. With dog sleds and the bare minimum of food and supplies for themselves they pushed onward back to the main base camp. Wegener was fifty years old, and had loved his tobacco pipe smoke. He was not ready for the excursion that came when the last of the dogs perished in the cold, and he had to drag the sled himself. Half way across the ice sheet, in the frozen landscape of Earth’s vast ice sheets, he died. Rasmus Villumsen hastily buried him in the ice, and pressed onward, never to be seen again. With Alfred Wegener’s death, the most prominent promoter for the idea of plate tectonics was gone. The idea was shelved by geologists for several decades. It would take a new generation of scientists to unearth the evidence for the motion of the lithosphere, and advance the idea of plate tectonics.
The Geology of the Ocean Floor
[edit | edit source]One of the great limitations toward the pursuit for the scientific gathering of evidence to test the idea of plate tectonics stemmed from the poor understanding of the actual geology of the ocean floor. The ocean floor is hidden under the dark deep ocean waters, and while out of view, offers the most important source of information about the motion of continents. The ocean floor under the theory of plate tectonics is young thin crust, generated by the cooling of molten magma that rises up, splitting the ocean floor wider and pushing the continents apart. The advent of World War II established to the public a widespread lack of awareness of the topography of the ocean floor. The United States Government and U.S. Navy rushed to fund programs to map the ocean floor. World War II was fought as much at sea, as it was on land. Knowledge of the ocean floor, would be useful in the search of enemy submarines lurking in deep ocean waters off the coasts. New underwater surveys were carried out using echo sounders that transmitted sound waves to the ocean bottom and record the length of time they took to be retrieved by the ship floating above. The seafloor depth could be mapped by the passage of these ships, this is known as bathymetry; the measurement of water depth. The other advanced tool developed during World War II was magnetometers that could be dragged behind a ship to record magnetic anomalies along the sea floor. Submarines are made mostly with iron, so their presence in the ocean below would cause the Earth’s magnetic field to distort, like a compass brought close to a hulk of iron. Magnetic anomalies could signal a submarine below the ship. During and after World War II ended these two technologies were used to begin the comprehensive mapping of the Earth’s ocean floor—most of this task was undertaken by a single woman named Marie Tharp.
Marie Tharp arrived in New York in 1947. She was hired to draft maps based on a new project to map the world’s ocean floor at the Lamont Geological Laboratory at Columbia University. Tharp was well educated in geology and mathematics. During the war years, there was need of women scientists to study geology in pursuit of petroleum to fuel the war, and she was recruited to study geology at the University of Michigan, graduating with a master’s degree, and went on to study mathematics in Oklahoma. Arriving at the laboratory, Marie Tharp was teamed up with a geologist named Bruce Heezen, on a project to find ships and aircraft lost during the war years. As the sole woman on the team, Marie Tharp was unable to travel on any of the naval ships, which meant that she had to stay behind to work at her drafting table, while Bruce Heezen traveled the world recording information about the sea floor. Despite the lack of first-hand experience, Marie Tharp would use the data gathered by other members of the team to map the ocean floor. It was a historic opportunity for her to fill in the last blank unmapped region of the Earth’s surface, a place never explored before. Compiling huge sets of data that recorded each ship’s geographic location and bathymetry was tedious work. But by the 1950s, she revealed a mid-ocean ridge that ran down the central axis of the Atlantic Ocean.
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A ridge that rose from the ocean floor, jagged cracked and shifted along canyons representing smaller perpendicular ridges and valleys. Seismic surveys revealed that the Moho discontinuity was extremely shallow under these mid-ocean ridges, the crust thin and elevated from the ocean floor. It was further confirmed by Bouguer gravity anomalies over these mid-ocean ridges which showed a drop indicating high topography beneath the ocean, and less dense and warmer rock layers composing these ridges. These mid-ocean ridges were like cracks on egg shell splitting the oceans into two halves. Survey of magnetic anomalies of the northeastern pacific coast, revealed a zebra pattern of magnetic reversals that baffled geologists at the time. But to Marie Tharp all this evidence she had revealed and gathered from her maps of the ocean floor supported Wegener’s theory of plate tectonics—that the ocean floor was growing and expanding from these mid-ocean ridges, pushing the continents further apart. They arose up from the ocean floor, driven by thermal convection in the underlying asthenosphere. Here the mid-ocean ridges mark the spreading of the ocean floor, as molten magma is brought to the sea floor as long series of underwater volcanoes and volcanic vents. These mid-ocean ridges were the divergent plate boundaries that pushed the ocean basin’s wider. Their higher topography was a result of hot magma moving upward, and as the crust diverged from the high ridge, cooled and sank down into the deep abyssal plains. The perpendicular ridges and canyons were a result of the different rates and speed of this spreading ridges on the ocean floor, resulting in transverse faults.
The magnetic anomalies in the sea floor of the Pacific Ocean appeared as bands of reverse polarities in the orientation of the magnetic fields of the iron-rich rocks on the ocean floor, they would later be found across nearly the entire ocean floor. Ribbons that appeared to record a history of growth and movement away from these spreading ridges.
The Paleomagnetist
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Allan Cox was not very good in school, and struggled in his chosen program of study, chemistry. After his first semester he failed out of the University of California, and joined the United States Merchant Marines. As a sailor he traveled the oceans, and became an avid reader while at sea, he wanted to return to school and learn more about the Earth. In 1950 he was hired by a geologist Clyde Wahrhaftig to help him map and study the glaciers of Alaska. The two men embarked on the work in the Alaska wilderness. Over the course of their close working relationship, the two men fell in love. Both Allan Cox and Clyde Wahrhaftig were closeted homosexuals, a secret they kept closely guarded. Clyde Wahrhaftig pushed Cox to return to school. Cox returned to the University of California to finish his degree in chemistry, but again failed, and was drafted into the army. In the army, Cox kept up his correspondence with Clyde Wahrhaftig, and after 2 years switched his major to geology, and spent more summers in Alaska with Clyde Wahrhaftig. In 1955, Cox realized that the geology faculty were not teaching him the full story, Alfred Wegener’s writings on continental drift were forbidden text, and only one professor, Dr. John Verhoogen even acknowledged its existence in the lecture halls of the school. Cox with fellow students formed a geology cub, to meet at a local pub and drink beer and discuss the forbidden field of geology; continental drift and plate tectonics.
As a magma or lava cools and solidifies, the magnetic iron minerals it contains become magnetized parallel with the Earth’s magnetic field. The orientation of a rock’s magnetism is fixed at the time of this cooling. Earth’s magnetic field has changed over time, with complete reversals in the past where the magnetic pole becomes the south pole and vice versa. These polarity reversals preserved in the once molten rock became the basis of Cox’s research as he believed they might offer a way to test the theory of continental drift and plate tectonics.
In 1959 he graduated with a PhD in geology and was hired by the United States Geological Survey to work with a team to develop a geomagnetic polarity time scale. Dating of sedimentary rock layers using magnetostratigraphy was not yet possible because, while it was known that the polarity of Earth’s magnetic field reversed over the course of its history, as the inner and outer core rotated inside the Earth, the nature of this record of polarity reversals was not chronologically understood. Allan Cox teamed up with Richard Doell and Brent Dalrymple. The research required measuring magnetic reversals in the once molten rock layers as well as calculating the radiometric age of each layer to make a workable geomagnetic polarity time scale. Their work proved very influential in the dating of rock layers for the United States Geological Survey, and became known as the Cox-Doell-Dalrymple calendar.
The observed magnetic anomalies along the sea floor of the Pacific Ocean which appeared as zebra strips where held up to the pattern revealed by the Cox-Doell-Dalrymple calendar, like two matching bar codes, the record across the mid-ocean ridge exhibited on the sea floor matched the same carefully worked out chronological pattern in rock layers elsewhere. In 1973, the theory of plate tectonics became the standard model in geology with the publication of Allan Cox’s Plate Tectonics and Geomagnetic Reversals and Marie Tharp’s 1973 map of the entire ocean floor showing the distribution of mid-ocean ridges. Textbooks were changed and both became celebrated scientists for gathering the evidence for a radical theory first formulated by Alfred Wegener.
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