Origin of the Ocean

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The Sea FloorPhysical Geology, Chapter 18

• Origin of the Ocean
• Methods of Studying the Sea Floor
• Continental Shelves Continental Slopes
• Submarine Canyons
• Passive Continental Margins
• Active Continental Margins
• Mid-Oceanic Ridges
• Fracture Zones
• Seamounts, Guyots Aseismic Ridges
• Reefs
• Sediments of the Sea Floor
• Oceanic Crust Ophiolites
• The Age of the Sea Floor
• The Sea Floor Plate Tectonics

form?
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Origin of the Ocean

• Early formation 4 b.y.
• Degassing of Earths interior
• Comet impacts

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Methods of Studying the Sea Floor

• Rock samples
• Rock Dredge
• Corer
• Sea Floor Drilling
• Submersibles
• Echo Sounder
• Multibeam Sonar
• Sidescan Sonar
• Seismic Reflection Profiler
• Magnetic Surveys
• Gravity Surveys
• Seismic Refraction Surveys

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Features of the Sea Floor

• Passive continental margin (Fig. 18.5)
• Continental shelf
• Continental slope
• Continental rise
• Abyssal Plain
• Active continental margin (Fig. 18.5)
• Continental shelf
• Continental slope
• Trench
• Mid-Oceanic Ridge
• Seamounts
• Fracture Zones
• Submarine canyons
• Aseismic ridges

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Continental Shelves Continental Slopes

• Continental shelf shallow, submarine platform,
  0.1º seaward dip
• Continental slope 4-5º steep slope from a depth
  of 100-200 m at edge of continental shelf

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Submarine Canyons

• Submarine canyons V-shaped valleys that run
  across continental shelves slopes
• Abyssal fans fan-shaped deposits of sediment at
  base of submarine canyons
• Turbidity currents masses of sediment-laden
  water pulled downhill by gravity

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Passive Continental Margins

• Passive continental margin
• Continental shelf, continental slope, and
  continental rise.
• Extends to abyssal plain at 5 km depth.
• Continental rise at base of continental slope,
  wedge of sediment from continental slope to
  deep-sea floor, slopes 0.5º.

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Passive Continental MarginsContinental Rise
Types of deposition

• Turbidity currents flowing down slope
• Contour currents flowing along slope

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Passive Continental MarginsAbyssal Plains

• Abyssal plains
• Very flat regions at base of continental rise.
• Composed of horizontal sediment layers probably
  deposited by turbidity currents.
• Flattest features on the Earth.

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Active Continental Margins

• Active continental margin
• Earthquakes, young mountain belt, and volcanoes.
  
• Consists of continental shelf slope, and
  oceanic trench.
• Lacks continental rise and abyssal plain.
• Associated with convergent plate boundaries.

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Active Continental MarginsOceanic Trenches

• Oceanic trench
• Narrow, deep trough parallel to edge of continent
  or island arc
• Continental slope steepens to 10-15º
• Benioff seismic zone
• Volcanoes landward
• Low heat flow
• Negative gravity anomaly

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Mid-Oceanic Ridges

• Mid-oceanic ridge
• Undersea mountain range
• Basalt
• 80,000 km long
• 1500-2500 km wide
• 2-3 km above ocean floor
• Rift Valley
• Crust extension
• Along ridge crest
• 1-2 km deep
• Several km wide
• Present in Atlantic Indian Ocean, absent in
  Pacific Ocean

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Mid-Oceanic RidgesGeologic Biologic Activity
at the Ridges

• Geologic Activity at the Ridges
• Shallow focus (0-20 km) earthquakes
• High heat flow decreasing away from the ridge
• Basaltic eruptions
• Hot springs (Black smokers)

• Biologic Activity at the Ridges
• Mussels
• Crabs
• Starfish
• Giant white clams
• Giant tube worms
• Thermophilic bacteria

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Europe
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India
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North America

• Fracture zones
• Major lines of weakness of the Earths crust
• Cross MOR at right angles
• Rift valley is offset
• May extend onto continents

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South America

• Seamounts
• Conical undersea mountains that rise 1000 m above
  the sea floor
• Some are islands
• 10,000 in W. Pacific

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Seamounts, Guyots, Aseismic Ridges

• Guyots Flattopped seamounts
• Aseismic ridges Submarine ridges not associated
  with earthquakes.

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Reefs

• Reefs
• Wave resistant ridges of coral, algae, other
  calcareous organisms
• Warm, shallow, sunlit, clean water
• Reef types
• Fringing reefs Flat table-like, attached
  directly to shore
• Barrier reefs parallel to shore, detached by
  lagoons
• Atolls Circular reefs rimming lagoons,
  surrounded by deep water

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Sediments of the Sea Floor

• Basaltic oceanic crust
• Terrigenous sediment
• Land-derived sediment.
• Turbidity contour currents
• Pelagic sediment
• Fine-grained clay skeletons of microscopic
  organisms.
• Absent on ridge crests.

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Oceanic Crust Ophiolites

• Oceanic crust is thinner (7 km) and denser (3.3
  g/cm3) than continental crust.
• Layer 1 Marine sediment (variable thickness
  composition).
• Layer 2 1.5 km, pillow basalts overlaying basalt
  dikes (closely spaced, parallel, vertical).
• Layer 3 5 km sill-like gabbros.
• Ophiolite Slivers of oceanic crust emplaced on
  land represented by distinctive rock sequences

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Oceanic Crust Ophiolites

• Pillow basalt from a northern California ophiolite

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The Age of the Sea FloorThe Sea Floor and
Plate Tectonics

• The age of the sea floor
• Younger than 200 m.y.

• The sea floor plate tectonics
• Origin of most sea floor features related to
  plate tectonics (Chapter 19)

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End of Chapter 18