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The scientific development of the hypothesis (and then theory) of Plate Tectonics Earthquakes: Seismology and Society PHY 131 Mr. M. Castner Fall 2014 History What did we know about the cause of earthquakes in the 1950’s ? History What did we know about the cause of earthquakes in the 1950’s ? Click here: Leet-Physical Geology 1954-1963.pdf History What did we know about the cause of earthquakes in the 1950’s ? Click here: Longwell-Physical Geology - 19321960 Maps of epicenters before WW II Maps of epicenters before WW II Physical Geology Leet & Judson Maps of epicenters before WW II Earthquake History of the United States, Ed. by J L Coffman & C A vonHake, U.S. Department of Commerce, Pub. 41-1, 1973 A problem During the first half of the 20th century, seismologist mapped thousands of earthquakes and discovered elastic rebound as the immediate cause of earthquakes. But they had no definitive cause for the buildup of strain leading to elastic rebound and they had no unifying theory for the real cause of earthquakes. See excerpts from historic textbooks. An answer In the mid-1960’s seismologists brought together many different types of data to produce a new theory. This new theory, plate tectonics, explains the mid-ocean ridges, the close fit of widely separated continents, and the concentration of earthquakes in long narrow bands. Adding up the pieces: Maps of the ocean floor. live Adding up the pieces: Maps of the ocean floor. Magnetic reversals found in ocean floor rocks. A Short History of Planet Earth J. D. Macdougall Natural Disasters 4th edition, Patrick Abbott Natural Disasters 4th edition, Patrick Abbott Adding up the pieces: Maps of the ocean floor. Magnetic reversals found in ocean floor rocks. Maps of world seismicity: the distribution of earthquakes around the world. Maps of epicenters after WW II Maps of epicenters after WW II Shallow earthquakes, magnitude 5 & larger, 1963-1988 Earthquakes 5th edition, Bruce Bolt World seismicity map from IRIS Incorporated Research Institutions for Seismology Live Magnitude greater than 5.5 Lamont-Doherty Earth Observatory of Columbia University, copyright©1999, all rights reserved See IRIS or USGS for a map of today’s Earthquakes. Adding up the pieces, continued: Similar fossils (identical even) found on continents separated by an ocean. Similar rock formations on continents separated by an ocean. This information was not new. It was discovered by geologists in the 19th and 20th centuries Field Guide to the San Andreas Fault, D. Lynch, Thule Scientific, 2006, p. 2 Fossils in New York and Morocco In March of 2007, Dr. Gordon Baird of SUNY Fredonia was a member of an expedition that spent two weeks in Morocco mapping the rock strata in the Anti Atlas Mts and collecting trilobites and other fossils. The work of the expedition confirmed earlier work, showing that the limestone/shale strata in Morocco is identical to the strata in Western New York. In particular, the Onondaga limestone (which can be seen from the Kensington Expressway between Grider St. and the Scajaquada Expressway) is identical to the limestone in Morocco from the lower Middle Devonian period, including trilobites and other fossils. Fossils in New York and Morocco In addition, a study of the trilobites on the two different continents shows that they are not only similar in the two locations, they are identical genus and species. New York Phacops rana Morocco = Greenops boothi = Phacops speculator Hollardrops mervcristata 70% of the trilobites in both locations are identical Phacops and 25-30% are identical Greenops/Hollardrops. Thanks to Walter Drabek, paleontologist and staff member in ITS for his help with fossil and strata identification. Greenops boothi from 18 Mile Creek, Hamburg, NY Hollardrops mervcristata, Morocco Fossils in New York and Morocco are one of thousands of pieces of evidence that eventually lead to this description of how the continents have moved. 350 million years ago (mya) most of the land mass on Earth was concentrated in two large continents: 1. Near the equator was Laurentia which contained present day North America, Greenland, and parts of northern Europe. 2. Just south of Laurentia was Gondwanaland which contained present day South America, Africa, and Arabia. Africa was separated from present day eastern North America by a shallow sea called the Tethys Sea. The predominant life forms in this sea were trilobites, brachiopods, and corals. From: The Historical Atlas of the Earth, R. Osborne and D. Tarling, Holt and Co. 1996 Description of how the continents have moved Over tens of millions of years, trilobites and other sea creatures died and fell to the sea floor where they were covered by sediment. The sediment slowly turned into either limestone or shale, depending on the chemical composition of the water in the Tethys Sea and the sediment that fell to the bottom with the trilobites and other sea creatures. By 300 mya the Tethys Sea was closing up due to the northward movement of Gondwanaland. Present day Africa ran into present day eastern North America and mountains began to form. Today we call those mountains the Appalachians in the U.S. and the Anti Atlas Mountains in Morocco. From: The Historical Atlas of the Earth, R. Osborne and D. Tarling, Holt and Co. 1996 Description of how the continents have moved By 250 mya the Tethys Sea was gone and the Appalachian/Anti Atlas Mountains were becoming a major mountain range. The limestone and shale layers were pushed up and tilted, carrying the fossils with them. From: The Historical Atlas of the Earth, R. Osborne and D. Tarling, Holt and Co. 1996 The Grand Conclusion Ocean floor spreading and plate tectonics. The theory of plate tectonics asserts that the earth’s crust is made up of a puzzle of large, moving crustal blocks of solid rock (plates) floating on denser, more plastic rocks below. Ocean floor spreading makes the plates move. Plate Tectonics The global distribution of earthquakes, mid-ocean ridges, ocean floor spreading, magnetic striping of the ocean floor rocks, the fit of the continental margins, fossil records, and other evidence mentioned above all lead to the theory of plate tectonics -- and the maps that follow. Live http://pubs.usgs.gov/gip/dynamic/slabs.html US Geological Survey Natural Disasters 4th edition, Patrick Abbott Natural Disasters 4th edition, Patrick Abbott Moving plates lead to earthquakes. Most plates move at a rate of 2 to 10 cm. per year. 6 cm. per year for 100 years = 6 meters When friction can no longer hold back the strain, the plates move past each other and produce seismic waves. We feel an earthquake. Field Guide to the San Andreas Fault, D. Lynch, Thule Scientific, 2006, p. 7 Summary - evidence for plate tectonics: The continents fit together. Similar rocks, fossils, and geologic structures are found on opposite sides of oceans. Discovery of the mid-ocean ridges. Evidence of ocean floor spreading. Summary - evidence for plate tectonics: Discovery of parallel bands of magnetized rock on the ocean floor, spreading out in mirror-like fashion on both sides of the mid-ocean ridges. Summary - evidence for plate tectonics: Increasing age of the sea floor with increasing distance from the mid-ocean ridges. Ocean depths increasing systematically with sea floor age. Summary - evidence for plate tectonics: Earthquake epicenters that outline and define the edges of tectonic plates! Earthquake epicenters that systematically increase in depth to explain ocean trenches and the destruction of crust. Summary - evidence for plate tectonics: The existence of ocean trenches at the boundaries of tectonic plates. The increasing depth of earthquakes corresponding to increasing distance from ocean trenches (subduction zones and Wadati-Benioff zones). By about 1970, scientists had accepted Plate Tectonics as a scientific theory. All evidence over the past 45 years has supported the theory of Plate Tectonics with only minor adjustments. Alternate summary David T. Sandwell. “Plate Tectonics: A Martian View”. Plate Tectonics, An Insider’s History of the Modern Theory of the Earth. Westview Press; Boulder, CO; 2001. PP 344-345. The path to discovery and confirmation of plate tectonics would have been smoother if we only had the advantage of exploring Earth from another nearby planet. Nevertheless, the outcome would be the same. Today, the most important observations related to plate tectonics are provided by space geodesy, seismology, ship surveys, and geological investigations. I would rank them as follows: 1. radar altimeter measurements of marine gravity, fit of the continents 2. space geodetic measurements of plate motion 3. shallow earthquakes to define plate boundaries 4. deep earthquakes to prove that slabs penetrate into the deep mantle 5. magnetic reversals at sea to provide plate speed 6. mid-ocean ridge axis topography and symmetric deepening about the ridge 7. dating of reversals on land 8. fossil evidence 9. glacial striations 10. matching of rock types on conjugate continental margins. You see: this is exactly the reverse of the order in which things actually occurred. Ocean floor spreading Sea floor spreading Before World War II, we knew more about the Moon's surface than we did the ocean floor! The need for better maps of the seafloor for submarine navigation led to a variety of studies of the seafloor that led to several amazing discoveries about the ocean floor: From careful bathymetric measurements the following were discovered: The mid-ocean ridge system deep sea trenches fracture zones John Encarnación, SLU http://www.eas.slu.edu/People/JPEncarnacion/easa133/outline.htm Ocean floor spreading The need for better maps of the seafloor for submarine navigation led to a variety of studies of the seafloor that led to several amazing discoveries about the ocean floor: The ocean floor is made up almost entirely of the volcanic rock basalt (whereas the continents are made up of lighter-colored "granitic" rocks), and the basalt is very young at the ridges. Sediments resting on the basalt are nil on the mid-ocean ridges and became thicker away from the ridges. The same sediments resting directly on the basalt are older away from the ridges. Measurements of heat flow from the ocean floor show that the ridges are hotter and heat flow declines away from the ridges. Magnetic anomalies form symmetrical, striped patterns parallel to the ridges. This observation was one of the most perplexing of them all. John Encarnación, SLU http://www.eas.slu.edu/People/JPEncarnacion/easa133/outline.htm Ocean floor spreading Harry Hess proposed a process called seafloor spreading and Vine & Matthews explained the striped magnetic anomalies by combining seafloor spreading and reversals of the Earth's magnetic field. (Note: these reversals are a phenomenon different from polar wander, but they are occurring during polar wander--north pole becomes south pole and vise versa. They lead to an ambiguity in whether a continent was in the northern or southern hemisphere.) A website giving a simple explanation of sea floor spreading can be found here. John Encarnación, SLU http://www.eas.slu.edu/People/JPEncarnacion/easa133/outline.htm Written in Stone Earthquake Country Los Angeles Video by Dr. Pat Abbott and UCSD. See chapters 2 & 9 in Canisius iTunes U for related information. Questions? end ~