Oceanography Chp 3

• Bathymetry


  Discovery and study of ocean floor contours

  
• Posidonius in 85 B.C.


  • Earliest to use a rope and a stone to measure depth
• Echo Sounding


  • First was the Iceberg detector and Echo Depth Sounder
  • First echo sounding not perfect as speed of sound depends on temperature, pressure and salinity
  • Multibeam systems cover wider area

  
• Multibeam Systems


  • Up to 121 beams
  • Fan out at right angles to direction of travel
  • Cover 120o arc
• Seafloor near East Pacific Rise


  Sail back and forth in systematic pattern to gather information on ocean floor

  
• Satellite Altimetry


  • Measure variations in sea surface levels
  • Take waves, tides and currents out of the equation
  • Found ocean surface can vary in height by as much as 200 m
  • Measured by Geostat satellite

  
• Lighter continental crust floats on top of the heavier ocean basins
• More than half of Earth's solid surface is at least 10,000 feet below sea level

  
• Transition of basalt marks the true edge of the continent and the ocean floor

  
• Distribution of Elevations and Depths

  • Majority of solid earth is below sea level due to greater density of seabed
  • Average depth of ocean is 4.5 times greater than average of height of continents

  
• Passive Continental Margins


  • Continental margins on facing edge of the diverging plate
  • Little earthquake or volcanic activity
  • Surround the Atlantic Ocean

  
• Active Continental Margins


  • Near edge of converging plates
  • Earthquake and Volcanic Activity is prevalent
  • Common in Pacific Ocean

  
• Continental Margins Divisions


  • Flat continental shelf close to shore
  • Steeply sloped continental slope that is seaward

  
• Continental Shelves


  • Extension of adjacent continents
  • Underlain by granite (continental crust)
  • More like the continent
  • Covers 7.4% of Earth's ocean area
  • Width dependent on tectonics and marine processes

  
• Shelf Width


  • Active margin is narrow as on edge of converging plate
  • Passive margin is broad as in the middle of a plate
• Continental Shelves


  • Gently sloped
  • Influenced by changes in sea level
  • During last ice age sea level was 410ft lower and exposed continental shelves
  • Surface area of continents was 18% greater than present

  
• Continental Slopes


  • Transition between the gently descending continental shelf and deep ocean floor
  • Made of sediments that are transported over the side of the shelf
  • Average inclination is 4o
  • Bottom of continental slope is the true edge of a continent

  
• Shelf Break


  Abrupt transition from continental shelf to continental slope
• Submarine Canyons


  • Form at right angles to shoreline and shelf edge
  • Formed by underwater avalanches & turbidity currents

  
• Turbidity Currents


  Cause mass movement of sediments
• Continental Rises


  Transition between continental slope and deep ocean basin

  
• Deep Ocean Basins


  • Seafloor up to 5 km thick
  • Make up more than 50% of Earth's surface
  • Sediments reflect age of basins, history of nearby continent, and biological activity in water above

  
• Oceanic Ridges


  • Mountainous chain of young basaltic rock
  • At active spreading center of ocean
  • 22% of world's solid surface
  • Widest where they are most active
  • East Pacific Rise spreads 6 times faster than Mid-Atlantic Ridge
• Transform Faults


  • Fractures along plates that slide horizontally past each other
  • A fault is a fracture in the lithosphere where movement has happened
  • Shallow earthquakes are common along transform faults
  • Transform faults are ACTIVE part of fracture zones
  • Transform faults common on the Mid-Atlantic Ridge
• Hydrothermal Vents


  • Rocky Chimneys ("Black Smokers")
  • Blasts dark mineral water out at 660 oF
  • Common on oceanic ridges especially in zones of rapidly spreading seafloors
  • Important for seawater composition, atmosphere and formation of mineral deposits
  • Water is more acidic
  • Enriched with metals
  • Higher dissolved gas than seawater
• Abyssal Plains


  • Flat, featureless expanses of sediment-covered ocean floor
  • Common in Atlantic, less in Indian, rare in the Pacific
  • Periphery of all oceans

  
• Abyssal Hills


  • Associated with seafloor spreading
  • Form when newly formed crust moves away from spreading center of a ridge, stretches and cracks
  • Parallel to oceanic ridges
  • Abundant where rate of seafloor spreading is fastest
  • Small sediment covered extinct volcanoes
  • Less than 200 m (650 ft) high

  
• Seamounts


  • Volcanic projections
  • Circular or Elliptical
  • Do not rise above sea surface
  • More than 1 km (0.6 miles) in height
  • Steep Slopes
  • Can form over hot spots or spreading centers

  
• Guyots


  • Flat topped seamounts
  • Once tall enough to penetrate sea surface
  • Generally confined to west-central Pacific
  • Flat top suggests peak was eroded away by wave action

  
• Trenches


  • V-shaped arc depression in deep ocean floor
  • Caused when converging oceanic plate subducts
  • Earthquakes and Tsunamis originate here
  • 3-6 km (1.9-3.7miles) deeper than adjacent basin floor

  
• Island Arcs


  • Curving chain of volcanic islands and seamounts
  • Found parallel to concave edge of trench
  • Aleutian islands, Mariana Islands and most of Caribbean islands
  • Ocean sedimentary history is short
  • Seafloors are recycled by tectonic forces

  
• Sediments


  Particles of organic or inorganic matter that accumulate in a loose, unconsolidated form
• Origins of Sediments


  • Weathering and erosion of rocks
  • Activity of living organisms
  • Volcanic eruptions
  • Chemical processes within water
  • Space
• Marine Sediments


  • Come in a variety of colors depending on origins
  • Sizes and distribution depend on physical and biological influences
• Affects of currents


  • Consistent bottom currents created this scour moat on the side of this seamount
  • Deep sediments have buried base of seamount

  
• Classifying Sediments-Size


  • Use particle size
  • The smaller the particle size the easier it is to transport
  • Sediments are sorted by size when transported
  • Most marine sediments are considered sand, silt or clay
• Sorting Sediments


  • Well sorted sediments are mostly one size
  • Energy fluctuates within narrow limits
  • E.g. calm deep ocean floor
  • Poorly sorted sediments are a mixture
• Origins of Sediments


  • Weathering and erosion of rocks
  • Activity of living organisms
  • Volcanic eruptions
  • Chemical processes within water
  • Space
• Marine Sediments


  • Come in a variety of colors depending on origins
  • Sizes and distribution depend on physical and biological influences
• Sediment beneath Antarctic Circumpolar Current