1. Structure of Ocean Basins Chapter 4

3. Continental shelves  Part of continents  Exposed or not exposed depending on sea level  Cut by submarine canyons  Vary in width depending on type of margin  Accumulate sediment from the continents (including rivers)  Fun fact: 25 % of global continental shelf is in the Bering Sea (very productive fisheries)

4. Continental Slope  Separated from the shelf at the shelf break (~ 130 m).  Continues to an average depth of about 4000 m (range is 3000-8000 m)  Most prominent topographic feature on earth  Marks the end of continents and the beginning of ocean crust  Average width of 20 – 100 km  Average slope of 4 o (road grade) - 70 m/km or 370 feet/mile

5. Continental Rise  Sediments from the shelf move down the continental slope and accumulate on the continental rise.  Most are transported there by turbidity currents or through riverine-derived canyons.  Width of the rise varies and slope is gradual.  Deep sea currents shape the rises.  Slope is about 1/8 that of the Continental slope (about 0.5 o or < 10 m/km or < 50 feet/mile)  Rise absent when slope terminates in a trench  Conspicuous features of passive (tectonically inactive) margins so rare in the Pacific

7. Trenches  Act as sediment traps!  Peru-Chile trench – oldest at the S end (shallower because its full of sediments) and vice versa  Feature of active margin/subduction zone

8. Deep Ocean Basins  Mainly, abyssal plains and ocean ridges  Also, trenches, seamounts and guyots  Include more than half the earth’s surface

9. Abyssal plain  True sea floor  Flat, sediment-covered ocean floor  Sediments often more than 1000 m thick (not so featureless underneath sediments)  Begins at the base of the continental rise  Very flat except for abyssal hills (which are low, < 1 km high, origin unclear), seamounts and guyots and islands  25% of the earth’s surface and 42% of the ocean bottom is plain  Most common in the Atlantic and rare in the Pacific where trenches trap sediments  Average of about 4000 – 5500 m depth

10. Abyssal hills  Not very high and steep  Protruding ocean floor topography?

11. Abyssal plain sediments bury old mountains (image generated by Echo sounding)

12. Important features of the sea floor  Trenches –Extremely deep –Found in association with plate margins (converging); mainly in the Pacific because of plates there  Seamounts and guyots –Guyots are flat-topped seamounts (eroded) –Islands are seamounts that break the sea surface  Coral reefs –Most common in Pacific with range 40 o N and 40 o S –Require warm water, light (deepest living corals at 100 m), salinity (no freshwater forms) and low sediment load. –Fragile ecosystems

13. Trenches  Convergence zones  Cold at the bottom  3 to 6 km deeper than adjacent seafloor  Trenches curved because of geometry of plate interactions on a sphere  Trench steeper on the island/continent side than the seaward side  Tectonically active  Big sediment traps

15. Island arcs  Parallel to trenches  Chain of islands behind a trench

17. Seamounts and guyots  Circular or eliptical  More than 1 km in relief  Alone or in groups  Old hot spots or extinct volcanoes formed at spreading centers  Guyots have flat tops

19. Coral reef formation  Atolls – around volcanoes  Fringe reefs  Corals grow up to keep in the sunlight  Evidence of old reefs at 1400 m

20. Oceanic Ridges Includes diverging and converging boundaries. 15 cm/yr at Iceland

21. Ocean ridge system  Describe plate boundaries  Largest and longest mountain range on earth  Made up of young, basaltic rock at active spreading centers  Ridges devoid of sediment  Oceanic ridges and associated structures account for 22% of the world’s surface (compare to all land which is 29%!)  < 60% of their length is at the center of basins  Youngest rocks at spreading centers  Steeper ridges at slower spreading centers

22. Lavas  Lava cooling underwater forms pillow lavas –Solidifies rapidly because of contact with cold seawater; this also slows flow –Water pressure keeps gases in magma “solution” during cooling  Lave extruded on land –Cools more slowly; flows longer –Releases gases (sometimes explosively)

24. Transform faults  Fractures in the lithosphere along which movement has occurred  Characterized by shallow earthquakes  A result of the earth being a sphere  Transform faults are the active parts of fracture zones  Fracture zones extend further away from ridge axis and are evidence of past transform faulting.  Lithospheric plate on either side of transform faults move in opposite directions  Outward sections of fracture zones move in the same direction

26. Structure of the mid-Atlantic Ridge between FL and W. Africa Depressed central rift valley is in blue

27. Further North and West Seismic profile of ridge being buried

28. Hydrothermal vents  Discovered only in 1977 along the East Pacific Rise (now found on mid-Atlantic Ridge, in the Sea of Cortez and on the Juan de Fuca Ridge)  Really hot (~ 350 o C), mineral-rich water (black smokers). Can form chimneys, deposit minerals, affects ocean chemistry, etc.  Seawater descends through fissures to meet hot rock. Superheated seawater dissolves minerals and escapes upward.  Found in Lake Baikal – future ocean?  Average water temperature is 8 – 16 o C (bottom ocean water is 4 o C)

31. Broad continental shelves Trenches Ridges GBR Red Sea Galapagos Iceland

32. Take home points  Continental shelf, break, slope and rise (relative slopes)  Abyssal plains  Mid-ocean ridge system (describes plate boundaries)  Hydrothermal vents, coral reefs, seamounts and guyots

33. Opening since the Miocene (65 million years ago)