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Frank Press, Raymond Siever, John Grotzinger, and Thomas H. Jordan Understanding Earth Fourth Edition Chapter 17: Earth Beneath the Oceans Copyright © 2004 by W. H. Freeman & Company Marine Geology •Most of the world is under water -- 71% •Plate Tectonics came about mainly from the study of marine geology Major Differences between the Ocean Floor and the Surface of Continents Continental surfaces are a source of sediment, that is they are regions where erosion and weathering prevail. Continents are ~4 by old Ocean floors are regions where net sedimentation and volcanism prevail. Oceans are no more that 200 Myr old About Oceanic Geology •Volcanism dominates near oceanic ridges and hot spots. •Sedimentation dominates elsewhere •Older volcanic crust usually has a thicker pile of overlying sediments How have earth scientists learned about the depths of the world’s oceans? HMS Challenger, 1872 throughout a 4year voyage, used rope lines to estimate ocean depths. Today we use echo sounders, devices that emit a pinging sound and record its return later in time. Knowing the speed of sound and the time of flight of these echoes can be used to calculate water depths. How have earth scientists learned about the geology of the world’s oceans? •Submersibles-- e.g. Jhohnson SeaLink, Alvin -- biology, petrology •Academic Drilling- DSDP, ODP, IODP •Commercial drilling and seismic exploration for oil and gas •Swath Mapping--Beams of sound (many pings in many directions received at many stations on the underside of a vessel) -bathymetry •seismometer stations around the world How have earth scientists learned about the geology of the world’s oceans? Satellites data can be used to make bathymetric maps •Satellites can access the most remote parts of the world •Satellites can detect ocean surface elevation. •Ocean height depends on waves,currents, and the underlying topography (bathymetry) A “typical” oceanic bathymetric profile across two plates that are diverging ….. The Atlantic Ocean •is 5000 km across •is shallowest in the center (2500 m deep) and deeper along its flanks in the abyssal plain Key Features across the Atlantic Seafloor •a continental shelf ( < 200 m water depth) that is often cut by submarine canyons •A continental slope ( > 200 m, 4 degrees dip, i.e. 70 m drop per 1 km run) •A continental rise (2000-3000 m) •An abyssal plain (4000-6000 m) •Seamounts - extinct underwater volcanoes •Mid-oceanic Ridges - active underwater mountain chain Key Features across the Pacific Seafloor •Trenches •seamounts •guyots - flat-topped extinct marine volcanoes, flat-topped because they were once above sea-level and were eroded A Great Discovery •Life does not only derive from sunlight energy! Sources of Deep-Sea Sediments Derive from (1) the dead skeletons of foraminifera (single-cell animals) that live in the (2) wind-borne or underwater-borne clays Carbonate Compensation Depth (CCD) Below a certain depth (CCD), the skeletons dissolve back into sea water and do not make it into the sediment! Wave Action •Waves in general, are created by wind •Wind velocity, area affected by winds and length of time that the wind blows are all factors that help to increase the height of a wave •Wave particle motion is circular at the surface to elliptical at the bottom •Waves that hit the coast at an angle created a longshore drift of sediments along the coast Wave Action •Waves that hit the coast at an angle created a longshore drift of sediments along the coast Tides •Tides can leave you high and dry, twice a day and can be intensified by local bathymetry •The largest tides occur when the sun and moon are aligned (spring tides) and the smallest are when the sun and moon are aligned at right angles to each other.