1. Key Points 1. The ocean floor is mapped by bathymetry.
2. Ocean-floor topography varies with location.
3. Continental margins are “active” or “passive”.
4. The topography of deep-ocean basins differs from that of the continental margin.

2. Bathymetry = study of ocean floor contours
The early, simplest methods involved lowering a weight on a line.
2 km
Posidonius
conducted the first bathymetric studies
85 B.C.

3. HMS Challenger
( )
made the first systematic attempt to chart the basins of the world ocean
made 492 bottom soundings
confirmed the discovery of the Mid-Atlantic Ridge

4. Reginald Fessenden (1866-1932)
Canadian inventor
in 1914, developed a type of sonar system for locating icebergs
“Iceberg Detector and Echo Depth Sounder”

5. Reginald Fessenden (1866-1932)
Canadian inventor
in 1914, developed a type of sonar system for locating icebergs
“Iceberg Detector and Echo Depth Sounder”

6. Echo sounders sense the contour of the seafloor by beaming sound waves to the bottom and measuring the time required for the sound waves to bounce back to the ship.
V = speed of sound in water
(about 1.5 km/sec)
T = time

7. used to detect enemy submarines
During World War I ( )
used to detect enemy submarines
Meteor expedition ( )
used to study the seabed

8. World Ocean Floor

9. Pacific Ocean

10. Two new techniques improved studies of the seafloor:
1) multibeam echo sounders
2) satellite altimetry

11. Multibeam systems combine many echo sounders.
up to 121 beams
signal sent every 10 secs
<200 research vessels are equipped with multibeam systems

12.  Seabed contours can be mapped using satellites.
Satellites cannot measure ocean depths directly
but, they can measure sea surface height

13. Sea surface
Seafloor

14. ？
Sea surface
Seafloor

15. Gravitational attraction “pulls” water
Sea surface
Gravitational attraction “pulls” water
Over a 2000 m seamount, water rises about 2 m
Seafloor

16. Seafloor topography inferred from sea surface height measurements
Mapped by: Geosat, TOPEX/Poseidon, and Jason-1

17. Oceans can be divided into two major provinces: 1) continental margin
2) ocean basin (deep ocean floor – Basalt)

18. Continental margins are “active” or “passive”.
Pacific-Type Atlantic-type
no earthquakes
no volcanic activity
earthquakes
volcanic activity

19. Three main parts of the continental margin:
1. Shelf 2. Slope 3. Rise

20. Continental shelf: shallow submerged extension of a continent
Average width: km (800 miles)
Granitic rock covered by sediments
Methane compounds

21. most material comes from erosion of continent
up to 350 km
most material comes from erosion of continent

22. Atlantic

23. The shelf width is usually determined by its proximity to a plate boundary.
active margin – often very narrow
passive margin – broad

24. Continental shelves are greatly influenced by changes in sea level (Ice Ages)
Sea level rise

25. Bering Strait

26. Bering Strait

27. Allowed human migration 12,000 years ago
Bering Strait

28. Continental slopes connect continental shelves to the deep-ocean floor
shelf break

29. Submarine canyons form at the junction between continental shelf and continental slope. (Edge of ocean basins)

30. Monterey Bay canyon

31. Monterey Bay canyon
2000 m

32. How do submarine canyons form?
Submarine canyons cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment.
How do submarine canyons form?

34. an underwater “avalanche” of sediment
turbidity current
an underwater “avalanche” of sediment
Most geologists believe that submarine canyons have been formed by abrasive turbidity currents plunging down the canyons.

35. Continental rises form as sediments accumulate at the base of the continental slope
much sediment
usually along passive margins

36. The topology of deep-ocean basins differs from that of the continental margin
Deep-ocean basins comprise mainly:
oceanic ridge systems
sediment-covered plains

37. Oceanic ridges circle the world
underwater mountain ranges
stretch 65,000 km
often covered with little sediment

38. Mid-Atlantic Ridge

39. Mid-Atlantic Ridge fracture zones transform faults

40. Hydrothermal vents are hot springs on active oceanic ridges
350 degrees Celsius
discovered in 1977 by Robert Ballard and J. F. Grassle
Alvin

41. Alvin manned submersible 6,500 m can carry 3 people can dive to 4000 m
1964 – 2007
>4000 dives
unmanned submersible
11,000 m

43. “black smokers” 350oC 20 m 2,800 m depth
solutions exiting vents are acidic (pH = ~3.5) and contain up to 300 ppm hydrogen sulfide (H2S)
a highly reduced molecule, so much energy can be obtained when it is oxidized

44. includes snails, shrimps, crabs, tube worms, fishes
hydrothermal vent community
deep-sea vent mussels
includes snails, shrimps, crabs, tube worms, fishes
depends on chemosynthetic bacteria for food
chemosynthesis
Tube worms

45. Abyssal plains and abyssal hills cover most of Earth’s surface.
Flat
Abyssal plains
40% of the ocean floor
common in the Atlantic
rare in the Pacific
covered by sediment
cold
Abyssal hills
small sediment-covered extinct volcanoes or rock
> 200 m (650 ft)

46. Volcanic seamounts and guyots project above the seabed
>1 km in height
important fishing areas
seamount
about 30,000
about 10,000 in the Pacific
Emperor Seamounts

47. Guyot: flat-topped seamount that once reached the surface

48. Trench: arc-shaped depression on the deep-ocean floor
occur near subduction zones
deepest places in the ocean
most in the Pacific

49. Peru-Chile trench Puerto Rico trench

50. Japan Trench
10,595 m
Mariana Trench
11,022 m

51. Trieste
reached the bottom of the Mariana Trench in 1960 – Challenger Deep

52. Kaiko Japanese deep-sea submarine
sampled bacteria from the bottom (10,897 m) of the Mariana Trench in 1996

53. Sampling of the world's deepest sea sediment by "Kaiko" at the Mariana Trench, Challenger Deep
Bacteria collected from the Mariana Trench

54. Nereus

55. Key Points 1. The ocean floor is mapped by bathymetry.
2. Ocean-floor topography varies with location.
3. Continental margins are “active” or “passive”.
4. The topography of deep-ocean basins differs from that of the continental margin.