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What is the Seafloor like? The Sea floor is made of oceanic crust, which is both thinner and more dense than continental crust. Major features of the sea flor include the continetal margins and slopes, the abyssal plains, the mid-ocean ridges, seamounts and trenches. Near the edge of the continents most ocean sediments are sands, mud and gravels from the continent. The composition depends on latitude and climate: calcite in the tropics, sands in the midlatitudes and glacial deposits in the high latitudes. Out in the middle of the ocean, Sedimentation rates are very slow. Sediments are either biogenic oozes, wind-blown clays or authigenic minerals Biogenic oozes are the remains of calcareous (calcite) or siliceous (silica) shells of microorganisms living at the surface of the ocean Authigenic minerals are those formed in place and include manganese nodules and many other minerals that precipitate in the sediments Although most volcanic activity in the ocean is concentrated at the ridges, isolated volcanoes can form seamounts and guyots Basic terms and ideas Bathymetry — the measurement of water depths and mapping of sea floor features The sea floor has two distinct regions: continental margins and deep ocean basins. Continental margins are the relatively shallow areas of the ocean floor near shore. Geologically they are part of the adjacent continent. Passive vs. active continental margins are distinguished by their plate tectonic setting. Deep-ocean basins differ from the continental margins in tectonic origin, history, and composition. Most important features of the deep ocean basins were formed by plate tectonic processes. Major features: mid-ocean ridge, transform faults & fracture zones, hydrothermal vents, abyssal plain, abyssal hills, seamounts & guyots, oceanic trench & volcanic arc. Compositi on Physical Properties Crustal Properties Crust Density Composition Thickness continental ~2.8 g/cm3 Felsic Thick: 20-70 km ~3.2 g/cm3 Mafic Thin: 2-10 km oceanic Age Old: up to 4 Byrs Young: <200 Mys Bathymetry: Measuring Ocean Depths As late as 1870 bathymetric studies were often performed using a weighted line dropped to measure depth. Advances in Bathymetry • Echo sounding (1922) • Multibeam systems • Satellite altimetry Echo sounding is a method of measuring depth using powerful sound pulses. The time it takes for the sound pulse to travel to the sea bed and bounce back is a measure of the depth. Distance = Rate x Time Multibeam systems can provide more accurate measurements than echo sounders. Multibeam systems collect data from as many as 121 beams to measure the contours of the ocean floor. Fig. 4-17a, p. 90 Model of Oceanic Ridge Fig. 12-12, p. 370 Age of the sea floor Ridges subside and are covered with sediment Size comparison of various volcanic features Continental Margins Components: 1. Continental shelf = the shallow, submerged edge of the continent 2. Shelf break = the abrupt transition from continental shelf to the continental slope 3. Continental slope = the transition between the continental shelf and the deep-ocean floor 4. Continental rise = thick accumulations of sediment found at the base of the continental slope Continental Crust Oceanic Crust Continental margin = the submerged outer edge of a continent; really just an extension of the continental crust, which has an average composition of granite. Deep-ocean basin = the deep sea floor beyond the continental margin; made up of oceanic crust, which is composed mostly of volcanic basalt. Types of continental margins Passive margins (= ―Atlantic-type‖ margins) " They face the edges of diverging tectonic plates " Very little volcanic or earthquake activity Active margins (= ―Pacific-type‖ margins) " Located near the edges of converging plates, where one plate subducts beneath another at an oceanic trench " Extensive volcanic and earthquake activity Trenches Oceanic trenches are the deepest parts of the sea floor. They are formed by subduction. Notice that most trenches (and therefore most subduction) occur in the Fig. 12-11b, p. 368 Fig. 4-14, p. 87 Atlantic Coast Continental slope Continental rise Continental slope: California Fig. 4-11, p. 86 Fig. 4-10, p. 85 Submarine Canyons Underwater landslides or avalanches called turbidity currents commonly flow down submarine canyons. The debris settles out to build up a submarine fan at the base of the canyon. An underwa ter debris flow, i.e. a turbidite. Fig. 12-9b, p. 367 Turbidites graded bedding Turbidites graded bedding Turbidites Turbidites Ocean Sediments • The stuff that covers the basaltic ocean crust How do we collect sediment • Bottom dredges scrape the sediment and collect material in a wire or canvas bag. • Grab samplers take a “bite” out of the sediment covering the bottom. • Gravity and piston corers use a weight to drive a core barrel into a soft bottom. A piston corer takes a much longer core than a gravity corer because of the piston in the core barrel. Grab samples Drilling cores Deep Ocean Basins Major features: • Mid-ocean ridges • Fracture zones and transform faults • Hydrothermal vents • Shallow earthquakes • Abyssal plains • Abyssal hills • Seamounts and guyots • Oceanic trenches • Volcanic arcs • Deep earthquakes Ocean sediment is thickest over continental margins and thinnest over active oceanic ridges. Based upon water depth, the ocean can be divided into 2 major depositional environment s: the shelf, which is shallow and near a terrigenous source, and the deep ocean basin, which is deep and far from a terrigenous source. Shelf sediments Geologic controls of continental shelf sedimentation must be considered in terms of a time frame. • For a time frame up to 1000 years, waves, currents and tides control sedimentation. • For a time frame up to 1,000,000 years, sea level lowered by glaciation controlled sedimentation and caused rivers to deposit their sediments at the shelf edge and onto the upper continental slope. • For a time frame up to 100,000,000 years, plate tectonics has determined the type of margin that developed and controlled sedimentation. Relict Sediments and glacial/interglacial cycles Shelf sediments are controlled by latitude and climate. • Calcareous biogenic sediments dominate tropical shelves. • River-supplied sands and muds dominate temperate shelves. • Glacial till and ice-rafted sediments dominate polar shelves. High latitudes Midlatitudes Tropics High latitudes Midlatitudes Tropics Equator Deep Sea Sediments Sedimentation in the Deep Sea Sources of Deep-Sea Sediment: Terrigenous (Rivers and Wind) Biogenic (Organisms) Authigenic (Minerals forming in the mud Biogenic "oozes‖ The remains of ‘plants’ and ‘animals’ Plankton - floaters Zooplankton and phytoplankton –Calcareous ooze (CaCO3) –Siliceous ooze (SiO2) Coccolithophores: calcareous phytoplankton (photosynthetic/autotrophs) Foramifera: calcareous (CaCO3) heterotrophs http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html Foramifera: calcareous (CaCO3) heterotrophs http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html Foramifera: calcareous (CaCO3) heterotrophs http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html Diatoms: siliceous (SiO2) phytoplankton (photosynthetic/autotrophs) http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html Diatoms: siliceous (SiO2) phytoplankton (photosynthetic /autotrophs) http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html Radiolarians: siliceous (SiO2) heterotrophs http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html Radiolarians http://www.ucl.ac.uk/GeolSci/micropal/calcnanno.html Chalk (Coccolithophores) Biogenic "oozes‖ > 30% of biogenous sediment –Calcareous ooze (CaCO3) –Siliceous ooze (SiO2) seawifs-6year.mpg GlobalProd_Seawifs.mpg OceanProd_SeaWifs.mpg Other sources of sediment to the deep sea 1. Volcanic ash 2. Rivers 3. Wind blown dust 4. Glaciers 5. Cosmogenic microtektites The distribution of sediments in the deep ocean reflects latitude, distance from landmasses, and the calcium carbonate compensation depth. • Terrestrial sediments close to shore • Glacial marine sediments occur in the high latitudes. • Pelagic clays occur far from land and in the deepest water. • Calcareous oozes occur above the calcium carbonate composition depth over areas of moderate productivity, (warm and no dilution). • Siliceous oozes where productivity is the greatest (cool and no dilution) Fig. 12-20a, p. 377 Biogenic "oozes‖ > 30% of biogenous sediment –Calcareous ooze (CaCO3) –Siliceous ooze (SiO2) seawifs-6year.mpg GlobalProd_Seawifs.mpg OceanProd_SeaWifs.mpg Oceanic sediment patterns Areas of the seafloor dominated by calcareous (shallow spots) and siliceous (high productivity spots) biogenous ooze. The remaining deep seafloor accumulates (very slowly) only pelagic clay. Finally, areas near continents may be dominated 1 by terrigenous or periglacial clastic sediments. Authigenic sediments: Manganese nodules Fig. 12-19a, p. 376 Fig. 12-19b, p. 376 SUMMARY Distribution of sediment on the sea floor •Terrigenous: continental margins and adjacent abyssal plains. •Calcareous oozes: wide-spread in relatively shallow areas of the deep sea. •Siliceous oozes: polar and equatorial bands where surface waters are rich in nutrients due to upwelling. •Manganese nodules: deep basins, especially the Pacific. •Red Clay: deep ocean regions where not diluted The Atlantic basin contains a ―two-layercake‖ stratigraphy–a thick basal layer of carbonate ooze overlain by a layer of mud. Stratigraphy of the Atlantic Basin Distribution of calcite sediments on a globe http://geosci.uchicago.edu/~archer/classes/GeoSci238/ Glaciers are dirty Fig. 12-18, p. 375 Normal marine sediment: calcareous (CaCO3 i.e. Calcite) microfossils Marine sediments during Heinrich events: only lithic fragments, i.e. gravel Dropstones Norway 5. ‘Cosmogenic’ Shocked grains Abyssal Plains Abyssal plains = broad flat areas of sedimentcovered ocean floor found between the continental margins and the mid-ocean ridges • Typically 4-6 km below sea level • The flattest surface on Earth • Sedimentation rates are very slow -millimeters/1000 years! • Mostly very fine clay, windblown dust, and shells of microscopic organisms Passive Continental Margin Thick layers of sediment on top of submerged mountains Fig. 12-13, p. 371 Formation of Guyots and submerged mountain chains Fig. 4-23a, p. 95 Fig. 12-17a, p. 374 Fig. 12-17b, p. 374 Fig. 12-17c, p. 374 Fig. 12-17d, p. 374 Concept Art, p. 380 Fig. 12-7, p. 365 Fig. 12-11a, p. 368