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WOU ES 106 - Sea Floor Tectonics

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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 slopeb) 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 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 surfaceb. 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 thermoclines3. 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.


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