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

Submarine Canyon  Origin of submarine canyons: river erosion turbidity currents

Submarine Canyon Turbidity 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

Monterey Canyon & Hudson Canyon 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) extends about 95 miles, terminates at Monterey fan reaching depths of 3,600 m (11,800 ft). canyon is about 1 mile deep

Continental Rise  At base of continental slope  Slope angle decreases  Caused by the accumulation of sediment

Continental Margins Types  Passive Margin  Active Margin

Continental Margin Aseismic ( Passive)  No plate boundary  Little tectonic activity  Thick sediment accumulation  Wide continental margin

Continental Margin Seismic ( Active)  Convergent plate boundary  Trenches are boundaries  Tectonically active  Thin accumulation of sediments  Narrow continental margin

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  Seamounts and Guyots  Coral Reefs and Atolls  Ridges and Rises  Abyssal Plain  Trenches

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

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  Contain central rift valleys km wide 500-1,500 m deep  Offset by fractures  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

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

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