ES 106 2007 April 23 27 Sea Floor Tectonics Ocean chemistry physics of ocean water oceanic life oceanic circulation I Continental margins A Two types depending on position with respect to edge of lithospheric plate 1 passive not on edge of lithospheric plate 2 active edge of lithospheric plate usually convergent plate boundary B 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 a seaward edge of continental shelf b boundary of continental to oceanic crust c 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 c massive underwater landslide 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 C Active continental margins 1 convergent lithospheric plates 2 subduction zone 3 accretionary wedge or not depending on rate age 4 narrow continental shelf margin 5 trench catches sediment before it reaches deep sea floor II 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 III Oceanic Ridge divergent plate boundary A Description 1 70 000 km interconnected 2 20 of Earth s surface a 1000 to 4000 km wide b 2 to 3 km 2000 to 3000 m high B Features 1 Earthquake activity common shallow 2 high heat flow 3 volcanic activity basaltic pillow lava 4 rift valleys parallel to ridge axis C new oceanic crust created at ridge older is drawn away from ridge IV Physics of sea water A Composition 1 Salinity a parts per thousand permil sea water is 35 permil b salt from dissolved mineral material and volcanic gases c dissolved matter removed by 1 organisms secreting hard parts and 2 chemical precipitation 2 variations in salinity due to variations in water a additional water from 1 precipitation 2 melting ice 3 runoff b water removed by 1 evaporation 2 formation of sea ice polar sea salinity varies seasonally c variation from 33 permil to 38 permil although some marginal areas 1 to 42 permil Persian Gulf or 2 10 permil Baltic B Temperature 1 Varies from equator to pole and from top to bottom 2 Variation with depth a in low latitudes it is warm at top by sun energy b declines sharply at about 300 m to 1000 m the thermocline c below about 1000 m constant to sea floor about 2o C d in high latitudes cold and constant from surface to sea floor e mid latitudes may have seasonal thermoclines 3 change in temperature over time would affect stability of life in the sea C Density 1 Affected by salinity and temperature a temperature has inverse relation to density b salinity has direct relation to density c because sea water salinity varies only slightly temperature has a greater affect on density of sea water 2 density zones with depth pycnocline is change in density with depth a surface mixed zone 2 1 to 300 meters 2 depth varies with latitude and season b intermediate transition zone 18 includes thermocline and pycnocline c high density zone 80 1 below 1000 m in mid latitudes 2 somewhat more shallow in equatorial seas 3 all high latitude sea water is high density D desalinization of sea water 1 Expensive a minor source of drinking water b unlikely source of agricultural water 2 Processes a distillation b membrane processes use semi permeable membranes c freezing chemical catalyst demineralization V Ocean life A Basis of sea life is algae photosynthesis 1 need sunlight 2 provides food and oxygen for other organisms B classifications 1 plankton float a phytoplankton plants algae etc b zooplankton consumers animals c most of Biosphere is plankton 2 nekton swim a confined by environment constrains of 1 temperature 2 salinity 3 depth 4 density of water 5 food sources b fish 1 most abundant near shore in cold water 2 some migrate between rivers and sea for reproduction
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