Transcript 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
 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)
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
Passive
 No plate boundary
 Wide continental margin
 Thick sediment
accumulation
 Little tectonic activity
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Continental Margin
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
Active Continental Margin
Southern California
3-D shaded relief map
 Santa Monica Bay
Active Continental Margin
Southern California
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)
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