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Continental Margins and Ocean Basins

Continental Margins and Ocean Basins. Continental Margins Three Main Divisions. Continental shelf. Continental slope. Continental rise. Continental Shelf. Submerged part of the continent. Slopes gently toward ocean basin (<1°). Composed of continental crust.

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Continental Margins and Ocean Basins

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  1. Continental MarginsandOcean Basins

  2. Continental MarginsThree Main Divisions • Continental shelf • Continental slope • Continental rise

  3. Continental Shelf • Submerged part of the continent • Slopes gently toward ocean basin (<1°) • Composed of continental crust • Shelf ends at shelf break – boundary between shelf and steeper slope

  4. Continental Slope • Boundary between continental and oceanic crust • Extends from shelf break to rise • Steeply sloping compared to shelf (5-25°) • Submarine canyons are major features

  5. Submarine Canyon • Origin of submarine canyons: • river erosion • turbidity currents

  6. Submarine CanyonTurbidity Currents • Turbidites are layered and exhibit graded bedding (decrease in sediment grain size from bottom to top) • Downslope movement of dense mixture of clay, silt, sand and water • Deposits are called turbidites

  7. Continental Rise • At base of continental slope • Slope angle decreases • Caused by the accumulation of sediment

  8. Continental MarginsTypes • Passive Margin • Active Margin

  9. Continental MarginPassive • No plate boundary • Wide continental margin • Thick sediment accumulation • Little tectonic activity

  10. Continental MarginActive • Convergent plate boundary • Trenches are boundaries • Narrow continental margin • Thin accumulation of sediments • Tectonically active

  11. Continental MarginPassive vs. Active • Passive Margins • Major rivers drain into ocean • Sediment transported by river builds out shelf • Active Margins • Large rivers uncommon • Irregular shelves

  12. Active Continental MarginSouthern California • Santa Monica Bay 3-D shaded relief map

  13. Active Continental MarginSouthern California

  14. Ocean Basin FloorFeatures • Seamounts and Guyots • Abyssal Plain • Trenches • Ridges and Rises • Coral Reefs and Atolls

  15. Ocean Basin Floor • Covers about 30% of Earth’s surface • Begins at base of continental rise • Sedimentation: Passive and turbidity currents • Contain abyssal plains, deep sea trenches, and seamounts

  16. Ocean Basin FloorAbyssal Plain • Flat, deep ocean floor • Depth may be 2-3 miles or more • Thick sediment accumulation covers oceanic crust

  17. Ocean Basin FloorDeep Sea Trenches • Occur at subduction zones where oceanic crust is forced downward into mantle • Associated with earthquakes and volcanoes • Deepest is Mariana Trench (11,020 m) • Longest is Peru-Chile trench (5,900 km)

  18. Ocean Basin FloorRidges and Rises • Underwater volcanic mountain chain • Extends for 65,000 km • 1,000 km wide • 1,000-2,000 m high Ridges = steep slopes Rises = gentle slopes • Contain central rift valleys • 15-50 km wide • 500-1,500 m deep • Offset by fractures

  19. Ocean Basin FloorSeamounts and Guyots • Seamounts are underwater volcanoes formed along ocean ridges or over hot spots • May emerge as an island • May be eroded flat on top and called Guyots

  20. Ocean Basin FloorCoral Reefs and Atolls • Volcanic islands (from seamount) form in warm latitudes • Fringing coral reefs form in shallow, sunlit waters • Dormant volcano subsides and flattens (becomes a guyot) • Actively-growing reef becomes a barrier reef and then an atoll

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