Lecture Two Jan. 7 - PHY 395S: Lecture 2: History of Plate...

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PHY 395S: Lecture 2: History of Plate Tectonics -for a nice general overview of the story of plate tectonics, see the online publication “This Dynamic Earth: the story of plate tectonics” at: http://pubs.usgs.gov/gip/dynamic/dynamic.html -the link can also be found under “External links” on the course webpage Courtesy USGS
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What needed explaining Snider-Pellegrini 1858 -first maps trying to fit the continents back together •1800’s, early 1900’s: mountains, earthquakes… all explained by a contracting earth •Shrinking surface wrinkles like a desiccating apple •Problem: can’t explain many features of the Earth’s surface •What needed explaining : •presence of ancient beaches and fossils of sea creatures in mountains 1000’s of feet above sea level •complementarity of continent coastlines surrounding the Atlantic •separation of unusual geologic structures source. People tried to explain this in terms of “land bridges” that linked the continents across the Atlantic, but had now sunk into the ocean. (Note: in the 1950’s, although it was no longer believed true, the mapping of the ocean floor proved that there were no land bridges) Courtesy USGS
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Continental Drift •the theory of continental drift was first put forward by Alfred Wegener in 1915 to explain the matching coastlines, fossils etc. •he suggested that about 200Ma (Ma=million years ago), a ‘supercontinent’ called Pangea began to split apart. •It first split into 2 pieces: -Laurasia (in the northern hemisphere) and -Gondwanaland (in the southern hemisphere). Courtesy USGS
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Animations: from 650Ma to the future http://www.break.com/index/650-million-years-in-1-min-20-sec.html http://www.ucmp.berkeley.edu/geology/tectonics.html 4
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Continental Drift •Notice how the Southern supercontinent “Gondwanaland” with the coasts fit back together suddenly links regions where specific fossils are found and regions with similar geologic units Biological evidence Geological evidence Courtesy USGS Courtesy USGS
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The Problem •A fatal weakness in Wegener's theory was that it could not satisfactorily answer the most fundamental question raised by his critics: •What kind of forces could be strong enough to move such large masses of solid rock over such great distances? •Wegener suggested that the continents simply plowed through the ocean floor, but Harold Jeffreys, a noted English geophysicist, argued correctly that it was physically impossible for a large mass of solid rock to plow through the ocean floor without breaking up. •Wegener’s idea was not accepted in his lifetime
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Lecture Two Jan. 7 - PHY 395S: Lecture 2: History of Plate...

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