Margins & Basins

1. Margins & Basins • Ocean depths versus elevations above sea-level. • Difference between a continental margin and ocean basin. • Passive versus active continental margins. • Margin Features • Basin Features

2. Compare Land versus Ocean Topography: The ocean cover over 75% of Earth’s surface area. Mean depth of ocean (3,790 m) is about 4-times that for mean land elevation (840 m). Furthermore, the total relief (difference between high and low points) is slightly more for the ocean (11 km deep trenches) than on land (8.8 km).

3. Defining Continental Margins and Ocean Basins: Margins are the submerged outer edge of the continental crust. Consisting of the “shelf “and “slope”. The continental margin extends to where the granitic continental crustal rock stops and the ocean crust basalt begins. This point is associated with the “rise”.

4. Oceans cover Continental Crust at the margins. This area is shelf and slope and represents 11.4% of the Earth’s area. Ocean basins are about evenly divided in surface area between ridges and plains. Ocean plains 29.8%

5. ????? T/F Continental margins are where sea meets the land, i.e. the shoreline. Continents are made mostly of the rock _____. T/F On average the oceans are deeper than land is high. About how much of the ocean basins are plains?

6. Continental shelf: Extends from coast to ~200km on average.  It’s narrow for active margins associate with subduction.  Shelf width also varies with sea level (+5 to -120 m over last 2.8 My) and marine processes (like scouring and sediment movement, especially for passive margins).  Average slope is 0.2º, pretty flat!  Shelf area represents 7.4% of oceans, but has some of the highest biological productivity.  Max depth is 150m at shelf break.

7. Continental slope lies just beyond the shelf and has a large slope average of 4º, but steeper slopes of active margins have been measured as steep as 25º.  Ends at the rise. Continental rise begins at bottom of the continent.  Basalt underlies sediments of the rise.   Slope is 0.5º. Continental rise grades into the abyssal plains (flat) which extend to the base of the mid-ocean ridge system.

8. Continental Margins: Active:These are margins with intense earthquake and/or volcanic activity due to their intimate association with convergent and transform plate boundaries (e.g. west coast of South America).Short shelf; steep slopes. Passive:These are often distant from a divergent plate boundary (e.g. east coast of North America). Long shelf; gradual slope. Recall that convection currents of mantle are the primary force that drives plate movement.

9. Active margin of Oregon. Note crinkled feature created by sediments scraped off during subduction of the Jaun de Fuca Plate (view from north to south)

10. ??????? Choose: slope, shelf, rise, abyssal plain. • Which is steepest? • Which are over basalt? • Which stops at a ridge? • Which typically has high biological productivity in the surface seawater above? Which margin type typically has a divergent plate boundary far away and nothing convergence of faulting in between?

11. Submarine Canyons are important features of the margin During last glacial period sea level was ~120m lower so rivers cut through the upper parts of the continental shelf to deposit their sediment load at shelf break. Canyons are maintained by intense and frequent turbidity currents.

13. Evidence for the formation of submarine canyons: The 1929 event on the Grand Banks. Severed telecommunication cables one after another toward deeper depths further offshore. A large are of abyssal plain was covered with the sediments from off the Grand Banks.

14. Ocean Basins: Dominated by abyssal plains and mid-ocean ridges (spreading centers).

15. Mid-Ocean (Oceanic) Ridges             Spreading centers are associated with intense volcanism, creating new basaltic crust.  They are of immense distance (65,000 km), elevation (2km from abyssal plain), and global area (22%).  Running perpendicular to breaks in the ridge axis are fracture zones, the active area between ridge segments is a transform fault. 

17. Adjacent to the ridge axis seawater gets sucked into fissures in the basalt.  This seawater becomes super-heated by the magma, and now contains extremely high amounts of dissolved minerals.  When the mineral laden super-heated water “vents” to the surface it can create dramatic chimney structures that bellow “black smoke” (actually mineral precipitates).  These chimneys and other hot spring features of the ridge axis are collectively referred to as hydrothermal vents. 

18. There are only a few manned submersibles in the world that can explore hydrothermal vent communities (e.g. Alvin)

19. Mid-Atlantic Ridge seafloor with little sediment among rocky outcroppings

21. Abysal Plains and Hills             Moving away from the ridge system the basalt cools and thins, becoming denser.  Sediments accumulate and the topography flattens as sediments cover the rough features observed at the ridge (see echogram of younger seafloor).

22. By depths of 3,500 to 5,500 m the plains are mostly flat.  Abyssal hills and valleys at the edge of the ridge and beyond are due to more dramatic faults and volcanoes (seamounts and guyots). Sediments kilometers deep, particularly those at the continental rise, will form into sedimentary rock due to intense pressure from above, a process called lithification.

23. Sediments at 4,010 m under Drake Passage. Rippled due to influence of the strong Antarctic Circumpolar Current.

24. Active margins are associated with subduction of seafloor under continental crust, forming a deep-sea trench. Trenches also form from subduction of old ocean crust under younger ocean crust, on whose margin arc islands form.

26. Moving away from the mid-ridge axis: T/F ocean crust gets older. T/F ocean crust gets cooler. T/F ocean crust gets less dense. T/F ocean crust gets covered with thicker sediments that can bury seamounts. Abyssal plains have deeper depths than ridges for two reasons. What are they? Thick (> km) old sediments form rock by _____________. Deepest ocean depths are due to what tectonic process, to form what kind of basin feature?