Continental Margins and Ocean Basins

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Continental Margins and Ocean Basins
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The Oceans

• Ocean waters released early in Earths history
  via outgassing
• Interconnected oceans cover 71 of Earths
  surface
• Pacific is the largest, contains 53 of Earths
  water
• Oceans are the largest marine water bodies seas
  are smaller bodies, often a marginal part of an
  ocean (Caribbean Sea/Atlantic Ocean)

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The Seafloors

• Oceanic crust composed of basalt and gabbro,
  continuously formed at spreading centers,
  continually consumed at subduction zones
• Oceanic crust is young oldest 180 Ma, oldest
  continental crust 4 Ba

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The Seafloors

• Seafloor exploration methods include
• Seismic profiling of seafloor topography
• Deep Sea Drilling Project (DSDP) by Glomar
  Challenger, 1968-1985 drilled over 1000 holes up
  to 6000m deep in seafloor to examine composition
  others followed
• Manned deep sea submersibles such as Alvin
• Remote submersibles such as Argo, which found
  Titanic

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Oceanic Crust

• Oceanic crust layers
• deep sea sediments
• pillow lavas and sheet lava flows
• sheeted dike complexes
• massive gabbros formed in magma chambers
• peridotites from the upper mantle
• Ophiolites, wedges of sediment scraped from
  oceanic plates at subduction zones and welded to
  the overiding continental crust contain
  accessible deep sea rocks (ophilolite complex,
  accretion wedge)

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Oceanic Crust Layers
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Continental margins

• Continental margin is not at the shoreline, but
  at the point where continental crust changes to
  oceanic crust
• Continental margin is divided into
• shelf the continuation of the gentle slope of
  the continental surface below the edge of the
  ocean that is affected by waves and tidal
  currents
• slope the steep drop off toward the ocean floor,
  begins at the shelf/slope break at 135 m depth
• rise, the angled slope base where the oceanic
  crust is joined (composed of sediments slumped
  off of the continents)

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Continental Margins
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Continental margins

• Active margin margin where oceanic lithosphere
  is being subducted, example West coast of South
  America, Andes Mountains
• Passive margin margins with broad continental
  shelves that have abundant sediment deposits, are
  not at plate boundaries, example East coast of
  United States-Merritt Island National Wildlife
  Refuge

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Continental Margins

• Features may include
• Turbidity currents and turbidites
• Submarine fans
• Submarine canyons

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Deep-ocean basins

• Average depth of ocean is 3.8 km with no light,
  no plant life, temperatures to below freezing,
  and tremendous pressures
• Abyssal plains flat areas of seafloor formed by
  sediment deposition adjacent to passive margins
• Oceanic trenches linear, steep-sided depressions
  formed adjacent to active margins as oceanic
  crust is subducted (Marianas Trench gt 11,000m
  deep)

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Oceanic Ridges

• Oceanic ridges Linear volcanic ridges formed at
  plate divergent boundaries, the site of new
  oceanic crust formation Iceland, the Azores, and
  Easter island are all islands formed at spreading
  centers
• Tensional stresses form crest rifts along oceanic
  ridges that indicate where seafloor spreading is
  taking place and new crust is being formed
• Unequal spreading rates produce stresses that
  cause fracturing perpendicular to ocean ridges,
  these are transform faults and are formed by
  shear stresses along ridge segments

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Seamounts

• A variety of seafloor hills and mountains cover
  many areas of the ocean floor, all of volcanic
  nature, including
• Seamounts underwater volcanoes taller than one
  kilometer
• Guyots Seamounts with flat, plateau-like crests,
  worn so by subaerial erosion before submersion
• Abyssal hills small seamounts (250m or less)
• Aseismic ridges Remnants of continents separated
  during plate tectonic movements and not related
  to volcanism

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Emperor Seamounts
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Deep sea sediments

• Pelagic clays sediments carried in suspension
  from the continents
• Deepsea ooze remnants of microscopic marine
  organism shells (or tests), may be calcareous or
  silaceous
• Eratics clasts carried away from the continents
  by icebergs from glacial calving

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Coral reefs and atolls

• Reefs are mound-like, wave-resistant structures
  created by the calcium carbonate skeletons of
  colonial marine animals
• Reefs form in shallow, warm marine waters (to
  about 40-50 m depth and gt 20 degrees C.
• Fringing reef reef attached to the margins of
  islands or coastlines
• Barrier reef separated from the shoreline by a
  body of water, example the Great Barrier Reef
• Atoll circular reef formed by the subsidence and
  erosion of an island

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Seafloor resources

• Seawater contains minerals in solution, and thus
  produce evaporite mineral deposits such as table
  salt (NaCl)
• Mineral deposits are deposited by precipitation
  directly on the seafloor, example manganese
  nodules, which can be recovered by dredging
• Fossil fuels reservoirs are located beneath the
  ocean in many regions
• These and other resources make control and
  exploration of ocean floors profitable and
  important, and provide incentive for Exclusive
  Economic Zones (EEZ)