{"id":1865,"date":"2016-05-23T21:29:18","date_gmt":"2016-05-23T21:29:18","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/geologyxwaymakerxmaster\/?post_type=chapter&p=1865"},"modified":"2025-10-13T16:49:32","modified_gmt":"2025-10-13T16:49:32","slug":"reading-developing-a-theory","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/geo\/chapter\/reading-developing-a-theory\/","title":{"raw":"Reading: Developing the Theory","rendered":"Reading: Developing the Theory"},"content":{"raw":"In line with other previous and contemporaneous proposals, in 1912 the meteorologist Alfred Wegener amply described what he called continental drift, expanded in his 1915 book The Origin of Continents and Oceans<\/i>[footnote]Wegener, Alfred (1929). Die Entstehung der Kontinente und Ozeane<\/em> (4 ed.). Braunschweig: Friedrich Vieweg & Sohn Akt. Ges.[\/footnote], and the scientific debate started that would end up fifty years later in the theory of plate tectonics.[footnote]Hughes, Patrick (8 February 2001). \"Alfred Wegener (1880\u20131930): A Geographic Jigsaw Puzzle.\" On the shoulders of giants<\/em>. Earth Observatory, NASA. Retrieved 2007-12-26. ... on January 6, 1912, Wegener... proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography.[\/footnote]\u00a0Starting from the idea (also expressed by his forerunners) that the present continents once formed a single land mass (which was called Pangea\u00a0later on) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to \"icebergs\" of low density granite floating on a sea of denser basalt.[footnote]Wegener, Alfred (1966). The origin of continents and oceans<\/em>. Biram John (translator). Courier Dover. p. 246; Hughes, Patrick (8 February 2001). \"Alfred Wegener (1880-1930): The origin of continents and oceans<\/a>.\" On the Shoulders of Giants. Earth Observatory, NASA. Retrieved 2007-12-26. \"By his third edition (1922), Wegener was citing geological evidence that some 300 million years ago all the continents had been joined in a supercontinent stretching from pole to pole. He called it Pangaea (all lands),...\"[\/footnote]\r\n\r\nSupporting evidence for the idea came from the dove-tailing outlines of South America's east coast and Africa's west coast, and from the matching of the rock formations along these edges. Confirmation of their previous contiguous nature also came from the fossil plants Glossopteris<\/i> and\u00a0Gangamopteris<\/i>, and the therapsid or mammal-like reptile Lystrosaurus<\/i>, all widely distributed over South America, Africa, Antarctica, India and Australia. The evidence for such an erstwhile joining of these continents was patent to field geologists working in the southern hemisphere. The South African Alex du Toit\u00a0put together a mass of such information in his 1937 publication Our Wandering Continents<\/i>, and went further than Wegener in recognising the strong links between the Gondwana fragments.\r\n\r\n[caption id=\"attachment_1869\" align=\"aligncenter\" width=\"1024\"]\"Detailed<\/a> Figure 1. Detailed map showing the tectonic plates with their movement vectors. (Click on the image to open a larger version of the map.)[\/caption]\r\n\r\nBut without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted: the Earth might have a solid crust and mantle and a liquid core, but there seemed to be no way that portions of the crust could move around. Distinguished scientists, such as Harold Jeffreys and\u00a0Charles Schuchert, were outspoken critics of continental drift.\r\n\r\nDespite much opposition, the view of continental drift gained support and a lively debate started between \"drifters\" or \"mobilists\" (proponents of the theory) and \"fixists\" (opponents). During the 1920s, 1930s and 1940s, the former reached important milestones proposing that convection currents might have driven the plate movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements now incorporated in plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove.\r\n\r\nOne of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a variable magnetic field\u00a0direction, evidenced by studies since the mid\u2013nineteenth century. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north pole varies through time. Initially, during the first half of the twentieth century, the latter phenomenon was explained by introducing what was called \"polar wander\" (see apparent polar wander), i.e., it was assumed that the north pole location had been shifting through time. An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to the north pole, and each continent, in fact, shows its own \"polar wander path\". During the late 1950s it was successfully shown on two occasions that these data could show the validity of continental drift: by Keith Runcorn in a paper in 1956,[footnote]Runcorn, S.K. (1956). \"Paleomagnetic comparisons between Europe and North America\". Proceedings, Geological Association of Canada<\/i> 8<\/b> (1088): 7785.[\/footnote]\u00a0and by Warren Carey in a symposium held in March 1956.[footnote]Carey, S. W. (1958). \"The tectonic approach to continental drift.\" In Carey, S.W. Continental Drift\u2014A symposium, held in March 1956<\/em>. Hobart: Univ. of Tasmania. pp. 177\u2013363. Expanding Earth from p. 311 to p. 349.[\/footnote]\r\n\r\nThe second piece of evidence in support of continental drift came during the late 1950s and early 60s from data on the bathymetry of the deep ocean floors and the nature of the oceanic crust such as magnetic properties and, more generally, with the development of marine geology\u00a0which gave evidence for the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals, published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley.[footnote]Korgen, Ben J. (1995). \"A voice from the past: John Lyman and the plate tectonics story\"<\/a> (PDF). Oceanography (The Oceanography Society) 8<\/b> (1): 19\u201320; Spiess, Fred; Kuperman, William (2003). \"The Marine Physical Laboratory at Scripps\"<\/a> (PDF). Oceanography<\/i> (The Oceanography Society) 16<\/b> (3): 45\u201354.[\/footnote]\r\n\r\nSimultaneous advances in early seismic imaging techniques in and around Wadati-Benioff zones along the trenches bounding many continental margins, together with many other geophysical (e.g. gravimetric) and geological observations, showed how the oceanic crust could disappear into the mantle, providing the mechanism to balance the extension of the ocean basins with shortening along its margins.\r\n\r\nAll this evidence, both from the ocean floor and from the continental margins, made it clear around 1965 that continental drift was feasible and the theory of plate tectonics, which was defined in a series of papers between 1965 and 1967, was born, with all its extraordinary explanatory and predictive power. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology.\r\n

Continental Drift<\/span><\/h2>\r\n[caption id=\"attachment_1870\" align=\"alignright\" width=\"400\"]\"Alfred Figure 2. Alfred Wegener in Greenland in the winter of 1912-13.[\/caption]\r\n\r\nIn the late nineteenth and early twentieth centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as basin development and mountain ranges could be explained by vertical crustal movement, described in what is called the geosynclinal theory. Generally, this was placed in the context of a contracting planet Earth due to heat loss in the course of a relatively short geological time.\r\n\r\nIt was observed as early as 1596 that the opposite coasts of the Atlantic Ocean\u2014or, more precisely, the edges of the continental shelves\u2014have similar shapes and seem to have once fitted together.[footnote]Kious, W. Jacquelyne; Tilling, Robert I. (February 2001) [1996]. \"Historical perspective<\/a>.\"\u00a0This Dynamic Earth: the Story of Plate Tectonics<\/i><\/a> (Online ed.). U.S. Geological Survey. Retrieved 2008-01-29. \"Abraham Ortelius in his work Thesaurus Geographicus... suggested that the Americas were 'torn away from Europe and Africa . . . by earthquakes and floods. . . . The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents].'\"[\/footnote]\r\n\r\nSince that time many theories were proposed to explain this apparent complementarity, but the assumption of a solid Earth made these various proposals difficult to accept.[footnote]Frankel, H. (1987). \"The Continental Drift Debate.\" In H.T. Engelhardt Jr and A.L. Caplan. Scientific Controversies: Case Solutions in the resolution and closure of disputes in science and technology<\/em>. Cambridge University Press.[\/footnote]\r\n\r\nThe discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of the Earth.[footnote]Joly, John (1909). Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History<\/em>. London: Archibald Constable. p. 36.[\/footnote]\u00a0This had previously been estimated by its cooling rate and assumption the Earth's surface radiated like a black body.[footnote]Thomson, W (1863). \"On the secular cooling of the earth<\/a>.\" Philosophical Magazine 4 (25): 1\u201314 (inactive 2015-01-09).[\/footnote]\u00a0Those calculations had implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists realized that the Earth would be much older, and that its core was still sufficiently hot to be liquid.\r\n\r\nBy 1915, after having published a first article in 1912,[footnote]Wegener, Alfred (6 January 1912). \"Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage\" (PDF). Petermanns Geographische Mitteilungen<\/em> 63<\/strong>: 185\u2013195, 253\u2013256, 305\u2013309.[\/footnote]\u00a0Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans<\/i>.[footnote]Wegener 1929.[\/footnote]\u00a0In that book (re-issued in four successive editions up to the final one in 1936), he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener was not the first to note this (Abraham Ortelius, Antonio Snider-Pellegrini, Eduard Suess, Roberto Mantovani and Frank Bursley Taylor preceded him just to mention a few), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as Alex du Toit). Furthermore, when the rock strata of the margins of separate continents are very similar it suggests that these rocks were formed in the same way, implying that they were joined initially. For instance, parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountainsof North America are very similar in structure and lithology.\r\n\r\nHowever, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930.\r\n

Floating Continents, Paleomagnetism, and Seismicity Zones<\/span><\/h2>\r\nAs it was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt, the prevailing concept during the first half of the twentieth century was that there were two types of crust, named \"sial\" (continental type crust) and \"sima\" (oceanic type crust). Furthermore, it was supposed that a static shell of strata was present under the continents. It therefore looked apparent that a layer of basalt (sial) underlies the continental rocks.\r\n\r\n[caption id=\"attachment_1871\" align=\"aligncenter\" width=\"800\"]\"Quake Figure 3. Global earthquake epicenters, 1963\u20131998[\/caption]\r\n\r\nHowever, based on abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer had deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had \"roots\" was confirmed by George B. Airy a hundred years later, during study of Himalayan gravitation, and seismic studies detected corresponding density variations. Therefore, by the mid-1950s, the question remained unresolved as to whether mountain roots were clenched in surrounding basalt or were floating on it like an iceberg.\r\n\r\nDuring the 20th century, improvements in and greater use of seismic instruments such as seismographs enabled scientists to learn that earthquakes tend to be concentrated in specific areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40\u201360\u00b0 from the horizontal and extended several hundred kilometers into the Earth. These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN)[footnote]Stein, Seth; Wysession, Michael (2009). An Introduction to Seismology, Earthquakes, and Earth Structure<\/em>. Chichester: John Wiley & Sons.[\/footnote]\u00a0to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide.\r\n\r\nMeanwhile, debates developed around the phenomena of polar wander. Since the early debates of continental drift, scientists had discussed and used evidence that polar drift had occurred because continents seemed to have moved through different climatic zones during the past. Furthermore, paleomagnetic data had shown that the magnetic pole had also shifted during time. Reasoning in an opposite way, the continents might have shifted and rotated, while the pole remained relatively fixed. The first time the evidence of magnetic polar wander was used to support the movements of continents was in a paper by Keith Runcorn in 1956,[footnote]Runcorn, 1956[\/footnote]\u00a0and successive papers by him and his students Ted Irving (who was actually the first to be convinced of the fact that paleomagnetism supported continental drift) and Ken Creer.\r\n\r\nThis was immediately followed by a symposium in Tasmania in March 1956.[footnote]Carey, S. W. (1958). \"The tectonic approach to continental drift\". In Carey, S.W. Continental Drift\u2014A symposium, held in March 1956<\/em>. Hobart: Univ. of Tasmania. pp. 177\u2013363. Expanding Earth from p. 311 to p. 349; see also Quilty, Patrick G.; Banks, Maxwell R. (2003). \"Samuel Warren Carey, 1911\u20132002.\" Biographical memoirs<\/em>. Australian Academy of Science. Retrieved 2010-06-19. This memoir was originally published in Historical Records of Australian Science<\/em> (2003) 14<\/strong> (3).[\/footnote]\u00a0In this symposium, the evidence was used in the theory of an expansion of the global crust. In this hypothesis the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this was unsatisfactory because its supporters could offer no convincing mechanism to produce a significant expansion of the Earth. Certainly there is no evidence that the moon has expanded in the past 3 billion years; other work would soon show that the evidence was equally in support of continental drift on a globe with a stable radius.\r\n\r\nDuring the thirties up to the late fifties, works by Vening-Meinesz, Holmes, Umbgrove, and numerous others outlined concepts that were close or nearly identical to modern plate tectonics theory. In particular, the English geologist Arthur Holmes proposed in 1920 that plate junctions might lie beneath the sea, and in 1928 that convection currents within the mantle might be the driving force.[footnote]Holmes, Arthur (1928). \"Radioactivity and Earth movements.\" Transactions of the Geological Society of Glasgow<\/em> 18<\/strong>: 559\u2013606; see also Holmes, Arthur (1978). Principles of Physical Geology<\/em> (3 ed.). Wiley. pp. 640\u2013641; Frankel, Henry (July 1978). \"Arthur Holmes and continental drift.\" The British Journal for the History of Science<\/em> 11<\/strong> (2): 130\u2013150. doi: 10.1017\/S0007087400016551<\/a>.[\/footnote]\u00a0Often, these contributions are forgotten because:\r\n