_03_PlateTectOne

_03_PlateTectOne - GEL 1: Lecture 3: Drifting Continents...

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1 GEL 1: Lecture 3 : Drifting Continents and Spreading Seas Continental Drift - precursor to plate tectonics First serious ideas about shifting continents and the existence of an ancient supercontinent proposed by Alfred Wegener , a German meteorologist/geologist. - 1915 book suggested that the continents were once combined into one huge continent that he called Pangea . Continents subsequently drifted apart to their current configuration. - his ideas of continental drift were met with resistance by the establishment of the day because no one could conceive of a way for continents to plow through the oceans which were thought to be immobile and fixed through time. - idea of shifting continents wasn't accepted till 1960s when a better understanding of the oceans was achieved - Wegener's ideas were based on data completely from continents , but a mechanism for moving continents across ocean basins couldn't be determined - It wouldn't be until WWII and the Cold War when the oceans were more completely explored that a mechanism could be found to explain ancient continental configurations. - original ideas of continental drift were modified and evolved into the comprehensive theory of plate tectonics Exploration of the Oceans topographic features of the seafloor: precursor for understanding plate tectonics WWII and the Cold War precipitated a need for understanding the world's oceans. Echo sounding (recording the echo of a sound wave bounced off the ocean floor) provided information about the bathymetry of the ocean bottom. Bathymetry is simply the depth and topography of the sea floor. - - ships sailed back and forth along traverses, recording echoes and converting the time of the sounding and the time of the echo to a vertical distance, in the process creating bathymetric profiles of the sea floor. These profiles were combined to make maps of the sea floor. - average depth of the world ocean revealed to be ~ 4 km The first maps revealed the presence of elongate mountain chains that extended beneath all the world's oceans - called mid-ocean ridges (about 2-2.5 km beneath the surface)
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2 - mid-ocean ridges occur in the mid-Atlantic, the East Pacific, the Indian Ocean and between Australia and Antarctica in the Southern Ocean - mid-ocean ridges are roughly symmetrical - bathymetry on one side is a mirror image of bathymetry on the other - most mid-ocean ridges have a deep, narrow valley extending along the crest of the ridge - called an axial trough (aka rift valley ) - mid-ocean ridges are bisected at right angles by steep-walled fracture zones that parallel one another, effectively segmenting ridges into smaller pieces - between the mid-ocean ridges and the continents are broad, relatively flat regions of the ocean called abyssal plains (about 4-5 km deep) Other parts of the oceans, especially regions adjacent to chains of active volcanic islands, exhibit narrow, deep-ocean trenches that may reach depths of 8-11 km (~34,000' deep!) Margins of continents slope gently seaward along a relatively smooth
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_03_PlateTectOne - GEL 1: Lecture 3: Drifting Continents...

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