Continental Marginsand

Ocean Basins

Continental MarginsThree 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 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

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 MarginsTypes

Passive Margin Active Margin

Continental MarginAseismic (Passive)

No plate boundary

Little tectonic activity

Thick sediment accumulation

Wide continental margin

Continental MarginSeismic (Active)

Convergent plate boundary

Trenches are boundaries

Tectonically active

Thin accumulation of sediments

Narrow continental margin

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

Southern California Seafloor

Extensive, complexConsist of basins & ridgesWidest shelf in CaliforniaNW-SE structural grain

Active Continental MarginSouthern California

3-D shaded relief map

Seafloor instability – Santa Barbara Basin

Active Continental MarginSouthern California

Southern California Borderland--Origin

Ocean Basin FloorFeatures

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 FloorAbyssal Plain

Flat, deep ocean floor

Depth may be 2-3 miles or more

Thick sediment accumulation covers oceanic crust

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)

Marianas Trench

Ocean Basin FloorRidges and Rises

Contain central rift valleys• 15-50 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 FloorOcean Basin FloorSeamounts and GuyotsSeamounts 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 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