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Continental Margins and Ocean Basins. Bathymetry. February 2000 a map of the surface of our planetary neighbor mars was completed. Bathymetry: is the discovery and study of our ocean floor. (Bathy= deep, Meter = measure).

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Bathymetry

  • February 2000 a map of the surface of our planetary neighbor mars was completed.

  • Bathymetry: is the discovery and study of our ocean floor. (Bathy=deep, Meter= measure).

  • The earliest known bathymetry studies were carried out in the Mediterranean by a Greek named Posidonius in 85 B.C.

    • he tied a rope that was 2 kilometers to a stone and let it down until the stone touched the bottom.


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Bathymetry

  • Sir James Clark Ross South obtained soundings of 4,893 meters in Atlantic in 1818.

  • The Challenger crew made 492 bottom soundings and confirmed Matthew Maury’s earlier discovery of the Mid-Atlantic Ridge.


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Echo Sounding

  • Reginald A. Fessenden

    • developed the Echo Depth Sounder

      • directed a powerful sound pulse ahead of the ship & listened for echo from submerged portion of iceberg

  • The USS Stewart a U.S Navy Vessel made 14 profiles using an echo sounder from 1925-1927.

  • A comprehensive sounder track chart was made by 1959.


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Echo Sounding

  • Multi Beam Systems

    • bounces sound off the seafloor to measure the ocean depth.

      • A pulse of sound of energy is sent toward the seabed every 10 seconds.

    • may have as many as 121 beams radiating from the ship’s hull.

    • With that researchers can build a complete map of the area they are testing.


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Satellite Altimetry

  • Satellite can measure small variations in elevation of surface water can’t measure ocean depth directly

  • uses about 1000 radar pulses each second

  • Researchers have found that the ocean surface can vary from smooth shape by as much as 200 meters. (because of the pull of gravity)

  • An undersea mountain or ridge “pulls” water toward it from the sides forming a mound of water over itself.


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Topography of the Ocean Floor

  • Earth’s surface is not a static arrangement of continents and ocean, but a dynamic mosaic of jostling lithospheric plates.

  • The lighter continental lithosphere floats in isostatic equilibrium above the level of the heavier ones.

  • Continental crust is thicker than oceanic crust; continental lithosphere is less dense than oceanic lithosphere.


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Topography of the Ocean Floor

  • Near shore the features of the ocean floor are similar to those of the adjacent continents. ( they share the same granitic basement.)

  • Continental margin: the submerged outer edge of a continent

  • Ocean basin: deep seafloor beyond the continental margin.


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Continental Margins

  • Continental margins facing the edges of divergingplates are called passive margins also known as Atlantic-type margins

  • Continental margins near the edges of the of converging plates are called activemargins also know as Pacific-type margins


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Continental Margins

  • Three main divisions:

    • continental shelf: shallow, nearly flat, close to shore

    • continental slope: seaward, more steeply sloped

    • continental rise: blends the continental margins into deep sea basins


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Continental Shelves

  • Def: the shallow, submerged extension of a continent

  • Underlain by granite continental crust

  • Area of shelf is 74% o earth’s area

  • Most of material that makes up shelf comes from erosion of continental mass

  • Width of shelf is usually determined by its proximity to a plate boundary


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Continental Slope

  • Def: the transition between the gently descending continental shelf and the deep-ocean floor

  • Slopes are formed of sediments that reach the built-out edge of the shelf and are transported over the side

  • Inclination of a typical slope is about 4 degrees

  • Slopes average about 12 mi. wide and end at continental rise

  • Shelf break marks the abrupt transition from continental shelf to slope


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Continental Rises

  • def: an apron of accumulated sediment

  • sediments from the cont. shelf slowly descend to ocean floor along the whole cont. slope

  • most sediments that form are transported by turbidity currents

  • Deep boundary currents held against the con. Slopes by the Coriolis effect pick up volcanic debris and sediments transported by turbidity currents and sweep them along the ocean floor


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Submarine Canyons

  • Submarine canyons cut into the con. Shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment

  • canyons generally trend at right angles to the shoreline, sometimes beginning very close to the shore

  • epicenter is the point on the surface of earth directly above the focus of an earthquake

  • turbidity currents are avalanche-like sediments movements

  • turbidity currents are caused when turbulence mixes sediments into water above a sloping bottom


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Deep-Ocean Basins

  • Consists of oceanic ridge systems and adjacent sediment-covered plains.

  • May be rimmed by trenches or masses of sediments.

  • Flat expanses are interrupted by islands, hills, active & extinct volcanoes, and active zones of seafloor spreading.

  • Sediments reflect the history of the surrounding continents, biological productivity of the water & the age of the basins.


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Oceanic Ridges

  • Oceanic Ridges- chain of mountains of young basaltic rock at the active spreading center of an ocean

    • devoid of sediments

    • rise about 2km above seafloor

    • may form islands

    • account for 22% of the world’s solid surface area


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Oceanic Ridges

  • associated with rift zones

  • widest & youngest where most active

    • EXAMPLE= Mid-Atlantic Ridge

  • Transform Faults- plane along which rock masses slide horizontally past one another

    • Offsets the Mid-Atlantic Ridge in almost regular intervals

    • Earthquakes are common

    • Fracture Zones- area of irregular, seismically inactive areas that show evidence of once-active transform faults


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    Hydrothermal Vents

    • Hydrothermal Vents- spring of hot, mineral- & gas-rich seawater found on some oceanic ridges in zones of active seafloor spreading

      • Black smokers- water descends through fissures & cracks in ridge floor, comes in contact with very hot rocks (assoc. with active seafloor spreading), superheated & chemically active water dissolves minerals & gases, escapes upwards through vents b/c of convection


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    Abyssal Plains & Abyssal Hills

    • Abyssal Plains- flat, featureless expanses of sediment covered ocean floor

      • Most common in Atlantic & Rare in Pacific

      • Lie between oceanic ridges & continental margins

      • Sediments mostly from shallow-water origins

  • Abyssal Hills- small, sediment-covered extinct volcanoes or intrusions of once-molten rock (< 200 meters or < 650 feet)

    • Associated with seafloor spreading

    • almost all the area of the deep-ocean floor not part of oceanic ridge system


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    Seamounts & Guyots

    • Seamounts

      • volcanic projections that don’t rise above sea surface

    • Guyots

      • flat-top seamounts once tall enough to approach/penetrate sea surface


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    Trenches & Island Arcs

    • Trenches- arc-shape depression in deep-ocean floor

      • Occur where converging oceanic plate is subducted

      • Among Most active geological feature on earth

      • Earthquakes & tsunamis often originate in trenches

      • Deepest places in earth’s crust

        • Mariana Trench- Pacific ocean

    • Curving chains of V-shaped indentions


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    Trenches & Island Arcs

    • Island Arcs- curving chain of volcanic islands and seamounts

      • Almost always parallel to concave edges of trenches

      • Formed by tectonic & volcanic activity associated with subduction

        • Aleutian Islands, Mariana Islands, & most Caribbean Islands


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