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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 Shelf ends at shelf break – boundary between shelf and steeper slope Continental Slope Extends from shelf break to rise Boundary between continental and oceanic crust Steeply sloping compared to shelf (5-25°) Submarine canyons are major features Submarine Canyon Origin of submarine canyons: • river erosion • turbidity currents Submarine Canyon Turbidity Currents Downslope movement of dense mixture of clay, silt, sand and water Deposits are called turbidites Turbidites are layered and exhibit graded bedding (decrease in sediment grain size from bottom to top) Monterey Canyon & Hudson Canyon •extends about 95 miles, •terminates at Monterey fan •reaching depths of 3,600 m (11,800 ft). •canyon is about 1 mile deep •2,200 m (7,217 ft) deep at base of continental slope. •As much as 12 km (7.5 miles) wide (from east rim to west rim) Continental Rise At base of continental slope Slope angle decreases Caused by the accumulation of sediment Continental Margins Types Active Margin Passive Margin Continental Margin Aseismic (Passive) No plate boundary Wide continental margin Thick sediment accumulation Little tectonic activity Continental Margin Seismic (Active) Convergent plate boundary Trenches are boundaries Narrow continental margin Thin accumulation of sediments Tectonically active Continental Margin Passive vs. Active Passive Margins • Major rivers drain into ocean • Sediment transported by river builds out shelf Active Margins • Large rivers uncommon • Irregular shelves Southern California Seafloor Extensive, complex Consist of basins & ridges Widest shelf in California NW-SE structural grain Active Continental Margin Southern California 3-D shaded relief map Seafloor instability – Santa Barbara Basin Active Continental Margin Southern California Southern California Borderland--Origin Ocean Basin Floor Features Abyssal Plain Trenches Ridges and Rises Seamounts and Guyots Coral Reefs and Atolls 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 Ocean Basin Floor Abyssal Plain Flat, deep ocean floor Depth may be 2-3 miles or more Thick sediment accumulation covers oceanic crust Ocean Basin Floor Deep 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) Marianas Trench Ocean Basin Floor Ridges and Rises Ridges = steep slopes Rises = gentle slopes Underwater volcanic mountain chain • Extends for 65,000 km • 1,000 km wide • 1,000-2,000 m high Contain central rift valleys • 15-50 km wide • 500-1,500 m deep Offset by fractures Ocean Basin Floor Seamounts 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 Ocean Basin Floor Coral 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