Topic 6 The Seafloor

1. Topic 6The Seafloor GEOL 2503 Introduction to Oceanography

2. See the video “The Sea Floor” on Learner.org under the Earth Revealedseries

3. Measuring the Depths • Posidonius made first measurements • Old methods used weighted ropes or cables—used for 2000 years (refer back to Topic 1) • Problems were stretch and drift of ropes and cables • Echo sounder developed in 1920s

4. Key Vocabulary • Sounding—any depth measurement • Fathom—old length measure still in use, equal to 6 feet • Bathymetry—study and measurement of the sea floor and the variations of water depth; the topography of the sea floor

5. Bathymetry of the Sea Floor • Just as rugged and complex as the land • Erosion is very slow • Main areas: • The Continental Margins • The Ocean Basin Floor • The Ridges, Rises, and Trenches

8. Example of an ocean basin: the Atlantic profile

9. Example of an ocean basin: the Pacific profile

10. Vertical Exaggeration (VE) • The distortion of the vertical scale of a topographic profile to emphasize the relief and slope of the land • VE = horizontal scale (distance) divided by vertical scale (elevation or depth)

11. Vertical Exaggeration Example • The horizontal scale is 1:12,000 which means 1 inch equals 12,000 inches, or 1 inch equals 1,000 feet • The vertical scale is 1 inch equals 200 feet • 1,000 ft  200 ft = 5 • VE = 5

12. The Continental Margin • Edges of the continents • Sea-level changes affect margins • Main components • Continental shelf • Continental shelf break • Continental slope • Continental rise

13. Passive Continental Margins Submarine canyons Deep-sea fan Coastal plain Abyssal plain Continental shelf Continental slope Continental crust Oceanic crust Mantle Continental rise

14. Active Continental Margins: Convergent Continental volcanic belt Offshore trench Continental crust Oceanic crust Mantle Accretionary wedge

15. The bottom diagram is at true scale. What is the vertical exaggeration of the top diagram?

17. Remember this graphic? The bottom diagram is at true scale. What is the vertical exaggeration of the top diagram?

18. Marianas Trench

19. Sigsbee Deep Deepest spot in the Gulf of Mexico 3787 meters

20. Recent sea-level changes have been the right magnitude to have alternately flooded and exposed the continental margins.

21. There have been many changes in sea level throughout Earth’s history. Look at this curve for the last 30 million years. The curve on the previous page shows only the last 40 thousand years. You can’t even see that little bit of time on the curve at left. Eustatic refers to a uniform global sea-level change. http://strata.geol.sc.edu/exerices/seismic/Sealevel.jpg

25. Continental Shelf • Part of the continents, underlain by granite • Covered with shallow water • Slope less than one degree • Widths average 65 km (40 mi) • Narrower along active margins than passive • Water depth varies 20-500 m (65-1640 ft) • Average about 130 m (427 ft)

27. Continental Shelf Break • Boundary between shallow shelf and beginning of deep ocean • Zone of abrupt change in slope

28. Continental Slope • Steep slope extending to ocean basin floor • Water depth increases from less than 200 m to average of 4,000 m • Occurs over very narrow width

29. Continental Rise • At base of continental slope • Accumulation of sediment on seafloor • Lessens the steepness of the slope • Similar to alluvial fans on land

30. Submarine Canyons • A dominant feature of continental slopes • Steep-sided with V-shaped cross section • Many associated with modern river systems on land, probably cut into shelf during lower sea levels • Evidence suggests most were formed and/or maintained by turbidity currents

31. Submarine canyons are a dominant feature of continental margins

33. Offshore Monterey Bay, California. Note all the submarine canyons.

34. Turbidity Currents • A fast-moving avalanche of mud, sand, and water • Flow down slopes by gravity • May travel at 90 kph (56 mph) • Erode sea bottom and excavate canyons • Caused by earthquakes or overloading of sediments on steep slopes

36. Turbidite • Deposit of sediment from a turbidity current • As current passes, largest grains settle out first • Successively smaller grains settle • Called a “graded bed” • Have scoured bottoms from erosion by passing current

38. Image of a turbidity current moving down the continental slope off California 

39. This is how we first learned of the erosive potential in the deep sea

40. A sand fall in a submarine canyon off Baja California

41. The Ocean Basin Floor • Abyssal plain—flattest places on earth • Abyssal hill—small undersea volcano • Seamount—undersea volcano 1,000+ m • Guyot—flat-topped seamount (see plate tectonics) • Atoll—circular reef around shallow lagoon (Charles Darwin)

43. Deepest areas are mostly older parts of ocean and abyssal plains

44. The Salt Water

45. As you go from an oceanic ridge outwards, the lithosphere cools and compacts, causing islands that were once above water to be below the water.

46. Volcanic islands weather and erode rapidly in tropical areas. Eventual submergence will lead to formation of an atoll.

47. Charles Darwin was the first to realize how atolls are formed As the former volcanic island continues to sink, eventually a guyot will form

48. Fringing reef  Barrier reef  Atoll • Caused by sinking of volcanic island as it moves away from oceanic ridge

49. Each of these Pacific atolls began as a volcanic island

50. Guyot Seamounts Abyssal Plain