continentaldrift

continentaldrift - Monday Sept. 15, 2008 Plate Tectonics...

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Unformatted text preview: Monday Sept. 15, 2008 Plate Tectonics Plate Tectonics •Theory of Plate Tectonics was developed in 1960’s •outer layer of Earth’s crust (lithosphere) consists of separate plates that move •Explains locations of •earthquakes •volcanoes •mountain belts 1820’s version of the world 1820’s version of the world Oceans and continents don’t move Oceans are old Mountains from contraction of earth like grape to raisin ­ too much wine? http://www.lib.utexas.edu/maps/historical/world_cyclopedia_1820.jpg Antonio Snider­Pellegrini Antonio Snider­Pellegrini 1858 fit of continents Alfred Wegener (1880­1930) first proposed the Hypothesis of Continental Drift: The Origin of the Continents and Oceans (1915) ­ continents have moved and are still moving across Earth. Alfred-Wegener-Institute for Polar & Marine Research What evidence supported Wegener’s What evidence supported Wegener’s continental drift hypothesis? 1. Obvious fit of continents 2. Paleoclimate data (glaciers, climate belts) 3. Similar fossils separated by oceans 4. Rock type and structural similarities What evidence did What Wegener use? Wegener Pangaea 250 Ma 1. Obvious fit of continents continents now W. W. Norton. Modified from Wegener A, The Origin of the Continents and Oceans. The What other evidence did Wegener use? Glacial striations Glacial striations •Evidence for distribution of glaciers Glaciation (260­280 Ma) striations­direction till deposits­perimeter If continents If continents were connected, one large ice cap explains record Support for continental drift Support for continental drift Climate belts seem to match across continents Equator Support for Continental Drift 3. Distribution of fossils …similar fossils …oceans apart Modern Distributions Each continent has unique assemblage of terrestrial species “Isolated” by large ocean basins Mesosaurus Mesosaurus freshwater reptile lived ~286 Ma Support for Continental Drift Distribution of fossils Support for Continental Drift Distribution of fossils Otherwise, fossil Otherwise, fossil distribution must be explained by migrations across land bridges Castaway? Feasible…but need Feasible…but Sufficient #’s Sufficient WWII Support for Continental Drift 4. Alignment of ancient 4. Alignment of ancient mountain belts & rock types Continental drift problems……. Continental drift problems……. But how did the continents move? Evidence for continental drift was compelling .... but how could weak continents ‘sail’ through the strong ocean crust? continental crust mantle Wegener’s hypothesis was largely unaccepted and untested for 40 years Alfred Wegner- Father of Continental Drift Theory of Continental Drift mechanism: low density continents “floating” on higher density ocean floor rocks and “sailing” through them Arthur Holmes (1890­1965) in 1928 proposed convection cells in the mantle as a mechanism for moving continents However, he warned his ideas were "purely speculative" and could "have no scientific value until they acquire support from independent evidence." Harry Hess (1960) built on Holmes’ ideas, & proposed sea floor spreading New crust forms at ridges Old crust disposed at trenches But how to prove it? Remember about Remember about Earth’s internal structure Earth’s magnetic field • Earth’s rotation­­>circulation of liquid iron alloy in outer core ­­> creates an electric current ­­> generates a magnetic field • Earth behaves like a giant electromagnet (dynamo) • Earth’s magnetism creates magnetic lines of force in space (magnetic field) Magnetic pole ≠ geographic pole! • 11° difference today (=declination) • moves ~0.2°/yr • in 10,000 yrs. average magnetic pole position equals geographic pole W. W. Norton Magnetic Magnetic Inclination • Angle between magnetic field and horizontal • = 0° at magnetic equator • = 90° at magnetic north pole • latitude-dependent • Is NOT measured by a standard compass Is Paleomagnetism - how do rocks Paleomagnetism acquire a magnetic signature? acquire Magnetite • common magnetic mineral • provides weak magnetization to provides basalt rocks basalt • >550°C, thermal chaos prevents thermal organized magnetic effect in lava organized Below 550°C • thermal motion slows down • magnetite grains align with Earth’s magnetite field, providing weak magnetization • direction of Earth’s magnetic field is locked into cold basalt field Analogy to Concrete! Analogy Deposition Cooling Solid Sedimentary rocks can also record a magnetic signature also Remember that the Earth’s magnetic field reverses in time Time between reversals is fairly random with shortest at 20,000 to 30,000 years and the longest was 50 million years, Time the actual reversal averages ~7000 yrs but depends on latitude Collection of oriented rock samples in the field Collection of oriented rock samples in the field Why Igneous or Volcanic Rocks? Why We can date them!!! • measured magnetic orientation in rock layers • • sharp magnetic reversals • Conclusion: polarity of Earth’s magnetic field suddenly reverses periodically Normal Reversed Finding paleopoles measure direction of ‘locked in’ magnetism ­declination points to paleo­north (you don’t know how far, just which way) ­inclination provides paleo­magnetic latitude (distance from you to paleopole) Interpreting paleo­magnetic poles •inclination, declination, location of magnetic pole through time •hypothetical continent ­ ‘polar wander’ ends at today’s magnetic pole But has the magnetic pole really moved this much ??! Apparent polar wander paths ­ the last 300 million years Apparent North America & Europe Four continents Plotted on present day continent positions Only explanation: continents drifted, pole stationary..? Only explanation: continents drifted, pole stationary..? • rotate Europe’s polar wander curve west • curves overlap from 280­180 Ma­­>continents adjacent = time span Wegener proposed for Pangea ­ before & after this time, continents were separate DEEP BLUE SEA SEA How the exploration of our ocean basins revolutionized our understanding of the Earth So far we have….. 1920s & 1930s ­ ocean studies expanded 1920s & 1930s ­ ocean studies expanded 1940s ­ greatly increased in the war years; new equipment yielded new view of oceans 1950s & 1960s ­ rise of Plate Tectonics Matthew Fontaine Matthew Fontaine Maury the first textbook of modern oceanography 1920s & 30s ­ sonar developed for military & industry; 1920s & 30s ­ sonar developed for military & industry; belatedly used for iceberg detection….. 1912 Radio­Acoustic­Ranging (RAR) navigation ­ invented by the Canadian Reginald Fessenden was a means to transmit and receive sound waves in the sea was the seed that led to the invention of depth­finding sonars and other types of sonars for looking ahead of a vessel and out to the sides of a vessel. Echo sounder circa 1925­27 Old days threw rope over the side Measure velocity & travel time to calculate distance to seafloor Beginning in the 1940’s: bathymetric profiles & magnetic surveys of the ocean floor distance = velocity x time All data put together to make a new ocean map Heezen & Tharp map Heezen & Tharp map earthquake locations, 1953 earthquake locations, 1953 Tharp noted that all epicenters located over deep valleys in Mid­Ocean Ridge Needed more data of ocean Dredge or grab samples Dredge or grab samples Composition of the seabed Composition of the seabed basalt sediment Launching an expendable bathythermograph, circa 1985 Heat flow Heat flow Highest heat flow at mid­ocean ridges, Highest heat flow at mid­ocean ridges, lowest at old oceanic crust. wwwrses.anu.edu.au/~uli/Teaching/ Heat/FouriersLaw.html Harry Hess (1906­1969) Assembled wide ranging information: • existing knowledge • his own observations • Wegener’s hypothesis 1962 ‘Essay in Geopoetry’ • • • • Sediment thickens above older crust Earthquakes + central rift result of splitting of the crust at MOR High heat flow due to magma rising to cool + form crust Ocean crust consumed in trenches ... continents carried by ­ not moving through ­ spreading seafloor Hess’s concept of sea­floor spreading Hess’s concept of sea­floor spreading Spreading: New oceanic crust produced at ridges. Subduction: Old oceanic crust sinks under continents into mantle in deep­sea trenches Continents: Along for the ride A mobile Earth – so far ­ just HYPOTHESIS Magnetometer ­ evidence for SFS (sea floor spreading) • • • measures strength of total magnetic field (Earth+magnetic rocks) > expected field = positive anomaly < expected field = negative anomaly Magnetometer Magnetometer circa 1964 Early magnetic stripe map of the California coast from 1971 that helped to decipher the nature of the ocean floor ocean Magnetic field recorded in oceanic crust is Magnetic field recorded in oceanic crust is symmetric about Mid­Ocean Ridge (MOR) A symmetric pattern of polarity stripes develops. A symmetric pattern of polarity stripes develops. positive anomaly ­ crust formed when Earth had normal polarity negative anomaly ­ crust formed when Earth had reverse polarity The Ultimate Proof ­ DSDP Leg 3 The Ultimate Proof ­ DSDP Leg 3 Test: does the basalt get older away from the ridge? • drilled through sediments and into basalt • determined age of oldest sediment at each site using fossils • age vs. distance from ridge proved seafloor spreading ...
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