Continental margins

geodynamics

1. Continental Margins
and
Ocean Basins

2. Continental Margins
Three 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
Steeply sloping compared to shelf
(5-25°)
Submarine canyons are major
features
 Extends from shelf break to rise

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

6. 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

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

8. Continental Margins
Types
 Passive Margin Active Margin

9. Continental Margin
Passive
No plate boundary
 Little tectonic activity
Thick sediment
accumulation
 Wide continental margin

10. •http://oceanexplorer.noaa.gov/okeanos/explorations/ex1304/logs/photolog/welcome.html

11. Continental Margin
Active
Convergent plate boundary
Trenches are boundaries
 Tectonically active
Thin accumulation of
sediments
 Narrow continental margin

12. 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

13. Active Continental Margin
Southern California
 Santa Monica Bay
3-D shaded relief map

14. Active Continental Margin
Southern California

15. Ocean Basin Floor
Features
 Seamounts and Guyots
 Coral Reefs and Atolls
 Ridges and Rises
 Abyssal Plain
 Trenches

16. 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

17. Ocean Basin Floor
Abyssal Plain
Flat, deep ocean floor
 Depth may be 2-3 miles or more
Thick sediment accumulation covers oceanic crust

18. 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)

19. Ocean Basin Floor
Ridges 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

20. Ocean Basin FloorOcean Basin Floor
Seamounts 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

21. 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