TOPIC 14-SFS & Plate Tectonics

TOPIC 14-SFS & Plate Tectonics - Sea Floor...

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Sea Floor Spreading The missing link in the search for a new paradigm.
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EARLY 1900’S VIEW OF EARTH What do those mysterious ocean basins look like?
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A mid 20 th  Century Perspective  (1940-1965): A paradigm shift in action! 1945  Arther Holmes (Geologist,  University Edinburgh).  He recognized that the main question  is whether oceans and continents  stable or not?
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Evidence for and against stable  continents and oceans Three ways to change one into the other : 1. Former continents sink, oceans rise. 2. Continental drift. 3. Both ocean floor and continents pulled apart. Two distinct elevations with very different densities (denser stuff  lower), so can’t change by vertical motions. Distinct sedimentary record in oceans vs continents These rule out #1 Geologic features truncated at  continental margins. Above suggests #2 - continental drift Not much direct evidence for testing #3
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Holmes’ Important step:  Ignore problem of mechanism  and just look at data. Explored both Taylor’s and Wegener’s ideas for continental drift. Clearly swayed that CD occurred. Although he talks about the “strange  maps of Wegener.” But to him the alternatives were even worse, for  example the need for huge land- bridges, now gone, between continents.   But Holmes could not accept polar wandering & external  (from earth) driving mechanism.
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Holmes Proposes idea of  mantle  convection  to drive CD. Upflowing = rifting or pulling apart. Downflowing =  trenches and zones of  collision, orogeny, earthquakes,  magmatism.
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Holmes - 1945 quote “It must be clearly realized, however, that purely  speculative ideas of this kind, specifically invented to  match the requirements, can have no scientific value until  they acquire support from independent evidence.”  He is clearly not sure whether to believe his own model!
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New Discoveries Harry Hess During World War II: lots of submarine travel so topography, or  bathymetry of oceans very important.  Hess was a submarine commander. Used sonar (sound waves) to record  bathymetry.
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TOPIC 14-SFS & Plate Tectonics - Sea Floor...

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