ES 106 Sea Floor Sediments Sea Floor Tectonics I Water planet A 71 covered by sea B All oceans are interconnected C Less land exposed in southern hemisphere II Ocean basins four main basins A Pacific 1 of all ocean surface 2 greater than area extent of all continents 3 average depth 3940 m 4 has deepest ocean trench in western portion Mariana Trench B Atlantic 1 size of Pacific 2 Narrow bounded by continents with similar shoreline shape 3 less average depth than Pacific C Indian 1 depth nearly same as Atlantic 2 slightly smaller than Atlantic 3 bounded by land on north mostly in southern hemisphere D Arctic 1 small 7 of Pacific 2 shallow average depth of other oceans E Average elevation Continents vs Oceans 1 continents 840 m above sea level 2 oceans 3730 m below sea level 4 times elevation above sea level III Ocean Floor A Mapping 1 Challenger 127 500 km voyage in 1870s a Depth by weighted line b Salinity temperature clarity 2 Modern depth measurements by SONAR a SOund Navigation And Ranging b Ping emitted is received c Distance calculated by travel time of ping d Sidescan instruments show details not depth 3 Seismic Reflection Profiles a Explosions produce sediment penetrating sound waves b Shows information about character of sea floor 4 Satellilte Radar Altimiters a Measure height of sea surface b Gravity causes irregularities 1 Higher surface over undersea mountains 2 Lower surface over deep sea floor c Ability to detect small scale differences B Provinces of the sea floor 1 continental margin a continental shelf 1 part of continental crust flooded by ocean 2 narrow gently sloping zones to about 130 m depth b drops off at continental slope 1 very narrow 2 markedly steeper than shelf c submarine canyons 1 cut outer shelf and slope 2 deliver sediment to ocean basin floor 2 ocean basin floor a most is abyssal plains flat featureless surface b deep ocean trenches small portion at lithosphere plate convergence c seamounts volcanic peaks d plateaus volcanic plains 3 oceanic ridge a tectonic feature of divergent lithospheric plates b 1000 to 4000 km wide 2000 to 3000 m high not topographic marvel in appearance IV Two types of crust continental and oceanic A Continental 1 enriched in silica 75 2 less dense than oceanic crust a not subducted in tectonic processes b older thicker because it remains at surface B oceanic 1 about 50 silica about 50 iron magnesium 2 greater density than continental crust a plate convergence results in subduction b none over 180 million years old C shoreline NOT the division between them some ocean water has flooded edges of continental crust 1 amount of water in ocean basin is variable presently 3 water is ice 2 volume of ocean basin is variable V Continental margins A Two types depending on position with respect to edge of lithospheric plate 1 active edge of lithospheric plate usually convergent plate boundary 2 passive not on edge of lithospheric plate B Active continental margins 1 convergent lithospheric plates 2 subduction zone 3 narrow continental shelf margin 4 trench catches sediment before it reaches deep sea floor 5 accretionary wedge or not depending on rate age C Passive Continental Margins 1 Continental Shelf a Gently sloping from shoreline to ocean basin floor 1 Slopes 1 10 of one degree 2 m km 2 Would look flat to observer some exceptions a Glacial deposits from time of lowered sea level b Submarine canyons also formed in lowered sea level b Part of continental crust flooded by ocean c Various widths worldwide up to 1500 km in places 1 Average width 80 km 2 Average depth of outer edge 130 m a Shallow enough for exploitation b Petroleum sand and gravel fishing d 7 5 of world ocean e Gradual subsidence of shelf results in thick sediment deposits 2 continental slope seaward edge of continental shelf a boundary of continental to oceanic crust b narrow and steep 1 20 km wide 2 Average slope 5o to 25o in places 3 continental rise only forms where shelf is not terminated by trench a wedge of sediment beyond continental slope on deep sea floor b slightly more slope than shelf surface c 100s of km wide d Composed of coalescence of deep sea fans deposited by flows from submarine canyons 4 submarine canyons a extensions of shelf valleys cut into continental margin from shelf to deep sea floor b river valleys extended onto shelf during glacial ages 1 additional runoff during melting enhanced erosion 2 sediment laden water could be dense enough to flow below sea water c undersea erosion continues with sediment laden sea water 1 deposition of sediment on canyon slopes continental shelf 2 episodic downslope movements of unstable deposit a overcomes oversteepened slope b massive underwater landslide c may have trigger such as earthquake 3 flow capable of scouring canyon further a Turbidity current creates turbidite deposit i High density due to sediment load ii Flows down the submarine canyon to deep sea floor iii Spreads and slows on exit from canyon mouth a Deposits its coarse load first b Gradually drops finer and finer material c Single bed with coarse to fine grain character graded bedding b May erode canyon head closer to shore create distinctive scour marks in surface it flows over VI Ocean Basin Floor A Deep sea trenches Atlantic has only two 1 At convergent lithospheric plates a Subduction zone takes oceanic plate into mantle 1 Earthquakes deeper toward trench 2 Volcanic activity above subducted plate a Water lowers melting temperature of hot rock b Magma volcanic arc continental or oceanic island b Accretionary wedge may be massive or absent 2 Mariana Trench in western Pacific a is 11 022 m deep b explored in 1960 1 by Picard and Walsh in Trieste 2 reached 10 912 m 3 saw flatfish jellyfish shrimp B Abyssal plains 1 surface is flat deposits of abyssal clay 2 subsurface often more rugged 3 more abyssal plains where there are no deep ocean trenches along continental margin C Seamounts guyots oceanic plateaus 1 volcanic features of deep ocean floor 2 Seamounts Guyots a Hotspot or oceanic ridge volcanism 1 may emerge from sea surface leads to formation of Guyot a subject to wave erosion b eventual subsidence below sea surface b seamount keeps conical shape due to lack of wave erosion 3 oceanic plateaus a topographically high ocean floor composed of volcanic rock b mantle plume or deep sea rifts discharge massive amounts of basaltic lava VII Oceanic Ridge divergent plate boundary A Description 1 70 000 km interconnected 2 20 of Earth s surface a 1000
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