The Sea Floor

Two thirds of the Earht's surface is covered by oceans.

The main zones of submarine topography:

• continental shelf: average 0.1° slope; 10s of meters to 100s of kms wide
• slope: average 4° slope; may be cut by canyons; turbidity currents flow down and across
• rise
• abyssal plain: average depth nearly 4000 meters
• ocean ridge
  • Mid-Atlantic ridge is comparable in height and width to the mountainous western portion of the U.S.
• trenches

Other features:

• volcanic islands
• seamounts (submarine volcanoes that do not reach the ocean surface
• guyots (seamounts that did reach the surface and have been eroded flat by waves and have since sunk below the surface by contraction during cooling)
• abyssal hills
• reefs and atolls: corals must live near the surface in the photic zone - as volcano cools and seafloor subsides the coral may suceed in growing at the same rate, leaving a ring of coral with a shallow lagoon in the center.
  • seamounts do not sink *into* the seafloor, they ride downward *with* the seafloor as it cools
• submarine canyons: often associated with the mouth of a major river system, caarved principally by tubidity currents (which may be triggered by earth quake

Marine Sediments:

• Mode of deposition:
  • Pelagic - settled down through the water column
  • Transport along bottom by waves, currents, and particularly turbidity currents
  • Formation in place
• Sources:
  • Land - products of weathering and erosion
    • wind-blolwn dust
    • ice raflting
    • turbidity currents - leads to graded bedding
  • Marine life - principally shells of microscopic plants and animals
    • diatoms - siliceous-shelled palnts
    • radiolaria - siliceous-shelled animals
    • foraminifera - calcareous-shelled animals
  • Chemicals in ocean water and uppermost sediments - chemical precipitation leads to *manganese nodules*
• Factors affecting distribution:
  • distance from shore: i.e. distance from continental sources
  • distance from spreading center: age of ocean floor affects total thickness of sediments
  • temperature - what kinds of marine life flourish and die
  • total depth of water: calcium carbonate dissolves at depths greater than 4500-5000 meters so will be absent at great depths
  • submarine topography

Key Evidence Leading To or Supporting Plate Tectonic Theory:

• Magnetic anomalies: near symmetrical distribution on the 2 sides of the mid-ocean ridges
• Inclination and declination of the magnetic field: may be preserved and measured in some rocks
• Magnetic reversals: preserved in rocks - paleomagnetism
  • normal vs reversed polarity

Composition of Ocean Crust

Evidence comes from dredging, drilling, and examination of ophiolites on land

• Sediments: see above
• Pillow lavas: result of extruding basalt into seawater
• Sheeted dikes: the cracks or feeders through which the basalt reaches the seafloor
• Gabbro: residual material left in the magma chamber which fed the basalts
• Peridotite: oceanic lithosphere