Chpt13Oceanfloorppt

Chpt13Oceanfloorppt - DivergentBoundaries:Origin...

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Divergent Boundaries: Origin  and Evolution of the Ocean  Floor Chapter 13
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   Mapping the ocean floor The HMS Challenger, 1876 used weighted lines that were lowered overboard to measure depth Some type of sonar – sound navigation and ranging, is used to measure water depth Echo sounder Invented in the 1920s Reflects sound from ocean floor Speed of sound in water is 1500 m/s; by knowing the time for a reflection, the depth can be calculated
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 Mapping the ocean floor Multibeam sonar developed in 1990’s Employs an array of sound sources and listening devices Obtains a profile of a narrow strip of seafloor Collects highly accurate bathymetric data This method is slow therefore most of the sea floor has not been mapped
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Echo sounder (A) and  multibeam sonar (B)
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 Mapping the ocean floor Seismic Reflection profiles Depth charges produce low-frequency sounds that bounce off of rock layers and send back reflections of geologic strata below sea floor Viewing the ocean floor from space Satellites use radar altimeters to measure subtle differences of the ocean surface Small variations reflect the gravitational pull of features on the seafloor Gravity pulls water to underwater seamounts and ridges Gravity is lower over trenches so that the water surface is depressed
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Mapping the ocean floor Three major provinces of the ocean floor Continental margins Deep-ocean basins Oceanic (mid-ocean) ridges
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Major topographic divisions of the north  Atlantic Ocean Figure 13.6
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Continental margins- Passive Passive continental margins Found along most coastal areas that surround the Atlantic ocean Not associated with plate boundaries Experience little volcanism and few earthquakes
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Features comprising a passive  continental margin Continental shelf Submerged extension of the continent Varies greatly in width From nonexistent to 1500 km Gently sloping, about 0.1 o about 10ft/mi Contain important deposits Oil, natural gas, sand and gravel Important fishing areas Some areas are mantled by extensive glacial deposits Shelf valleys, ( submarine canyons ) extensions of river valleys during times of lowered sea levels Shelf break is the place where the cont. shelf changes to the cont. slope.
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Features comprising a passive  continental margin Continental slope Marks the seaward edge of the continental shelf Relatively steep structure, 5-25 o slope, 20 km in width. Boundary between continental crust and oceanic crust Continental rise Found in regions where trenches are absent Continental slope merges into a more gradual incline – the continental rise Thick accumulation of sediment At the base of the continental slope turbidity currents deposit sediment that forms deep-sea fans
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Provinces of a passive continental  margin Figure 13.7
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Chpt13Oceanfloorppt - DivergentBoundaries:Origin...

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