Origin and Evolution of the Ocean Floor

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1 Origin and Evolution of the Ocean Floor Outline Mapping the Ocean Floor Continental Margins Origin of Oceanic Lithosphere Structure of Ocean Crust Mapping the ocean floor Depth originally measured by lowering weighted lines overboard Echo sounder (aka sonar ) Invented in 1920s Primary instrument for measuring depth Reflects sound from ocean floor Multibeam sonar Employs array of sound sources, listening devices Profiles of narrow strip of seafloor Viewing ocean floor from space Satellites use radar altimeters to measure subtle differences of ocean surface Small variations reflect gravitational pull of features on seafloor Three major provinces of ocean floor Continental margins Deep-ocean basins Oceanic (mid-ocean) ridges Passive Continental margins Found along most coastal areas that surround Atlantic ocean Not associated w/plate boundaries Have little volcanism, few earthquakes Features: Continental shelf Flooded extension of continent Varies greatly in width Gently sloping Contains important mineral deposits Some areas mantled by extensive glacial deposits

2 Continental slope Marks seaward edge of continental shelf Relatively steep structure Boundary between continental crust & oceanic crust Continental rise Found in regions where trenches are absent Continental slope merges into more gradual incline = continental rise Thick accumulation of sediment Base of continental slope turbidity currents deposit sediment, forms deep-sea fans Active Continental margins Continental slope descends abruptly into deep-ocean trench Located primarily around Pacific Ocean Accretionary wedges=accumulations of deformed sediment, scraps of ocean crust Features of deep-ocean basins Deep-ocean trench Long, relatively narrow features Deepest parts of ocean Most located in Pacific Ocean Sites where moving lithospheric plates plunge into mantle Associated w/volcanic activity Earth s deep-ocean trenches Abyssal plains Seamounts Most level places on Earth? Sites of thick accumulations of sediment Found in all oceans Isolated volcanic peaks Many form near oceanic ridges May emerge as island May sink, form flat-topped seamounts (guyots) Oceanic plateaus=vast outpourings of basaltic lavas on ocean floor, creating extensive volcanic structures Anatomy of oceanic ridge Oceanic ridges =broad, linear swells along divergent plate boundaries

3 Occupy elevated positions Extensive faulting, earthquakes High heat flow Numerous volcanic structures Longest topographic feature on Earth s surface Over 70K km (43K miles) in length 23% of Earth s surface Winds through all major oceans Ridge = misleading widths of 1K to 4K km give appearance of broad, elongated swells Axis of some ridge segments have rift valleys (deep down-faulted structures) Portions of mid-atlantic ridge have been studied in detail Origin of oceanic lithosphere Seafloor spreading Concept formulated in early 1960s by Harry Hess Occurs along narrow zones (rift zones), crests of ocean ridges Plates move apart Magma wells up into fractures generates new slivers of oceanic lithosphere New lithosphere moves from ridge crest ( conveyor-belt fashion) Zones of active rifting =20-30 km wide Primary=newly created oceanic lithosphere= hot & occupies more volume than cooler rocks Basaltic crust travels away from ridge cools by seawater thermally contracting becomes more dense Spreading rates and ridge topography Ridge systems exhibit topographic differences controlled by spreading rates slow rates (1-5 cm/year)= prominent rift valley develops along crest, ~30-50 km across, meters deep Intermediate spreading rates (5-9 cm/year), valleys = shallow, less than 200 meters deep Rates greater than 9 cm/year no median rift valley develops, areas=usually narrow, extensively faulted Structure of the oceanic crust

4 Four distinct layers Layer 1 unconsolidated sediments Layer 2 pillow lavas (basaltic) Layer 3 interconnected dikes (sheet dikes) Layer 4 gabbro Together=ophiolite complex Structure of oceanic crust Formation: Basaltic magma originates from partially melted mantle peridotite Molten rock injected into fractures above magma chambers, creates sheeted dike complex Submarine lava flows chill quickly Congealed margin = forced forward Produces pillow basalts (large tube-shaped protuberances) Interactions seawater + oceanic crust Seawater circulates downward through fractured crust Basaltic rock altered by hydrothermal metamorphism Hydrothermal fluids dissolve ions of various metals, precipitate them on seafloor as black smokers (particle-filled clouds) Continental rifting: The birth of a new ocean basin Evolution of ocean basin A new ocean basin begins w/continental rift Splits landmasses = 2+ segments Examples: East African Rift Baikal rift Rhine Valley Rio Grand Rift Basin and Range Red Sea = rift valley lengthened, deepened into narrow, linear sea If continues, Red Sea will grow wider, develop oceanic ridge similar to Atlantic Ocean Not all rift valleys develop into full-fledged spreading centers Destruction of oceanic lithosphere Why oceanic lithosphere subducts: its overall density > underlying mantle

5 Subduction of older, colder lithosphere results in descending angles of nearly 90º Younger, warmer oceanic lithosphere buoyant, angles of descent small Moves horizontally beneath block of continental lithosphere This phenomenon= buoyant subduction Subduction changed when oceanic crust unusually thick (seamounts) Subducting plates: demise of ocean basin Plate movement reconstruction for past 200 million years uses magnetic stripes on ocean floor Research indicates parts, or even entire oceanic basins, have been destroyed along subduction zones Destruction of oceanic lithosphere Farallon plate once occupied much of eastern Pacific basin ~ 180 mya, plate subducted beneath Americas faster than generation Plate got continually smaller, only fragments of original plate remain: Juan de Fuca Cocos Nazca plates Opening and closing basins: The supercontinent cycle Rifting, dispersal of one supercontinent followed by long period as fragments are reassembled The supercontinent cycle Before Pangaea Rodinia=earliest documented supercontinent formed ~1 bya split apart between 750 and 550 mya Some fragments formed Gondwana others = continental landmasses in Northern Hemisphere Most landmasses reassembled into Pangaea