GEOL: CHAPTER 9

1. GEOL: CHAPTER 9 The Seafloor

3. Learning Outcomes LO1: Examine the history and methods of oceanic exploration LO2: Describe the structure and composition of the oceanic crust LO3: Identify the continental margins LO4: Discuss the features found in the deep-ocean basins

4. Learning Outcomes, cont. • LO5: Discuss sedimentation and sediments on the deep seafloor • LO6: Explore coral reefs • LO7: Recognize the types of natural resources found in the oceans

5. Introduction • Oceanic crust is thinner and denser than continental crust • Gabbro at depth and basalt at surface • Oceanic crust produced continually • Largest part of Earth’s surface • Varied topography of seafloor

7. Exploring the Oceans • Interconnected body of saltwater • Oceans and seas cover 71% Earth’s surface • Oceans are large; seas are smaller • 1400s and 1500s: voyages of exploration: • Columbus 1492

8. Exploring the Oceans, cont. • Scientific voyages later: • Captain Cook in 1768, 1772, 1777 • HMS Beagle 1831-1836 with Charles Darwin – evolution of organisms and coral reefs • HMS Challenger 1872: voyage to sample seawater, determine depths, collect seafloor sediments, and classify organisms

9. Exploring the Oceans Today • Echo sounder: sound waves used to determine ocean depths • Seismic profiling: seismic waves penetrate seafloor and reflect from layers; helps determine structure of oceanic crust • Ocean ships drill into seafloor • R/V Chikyu • Submersibles: Alvin

10. Oceanographic Research Vessels The R/V Chikyu (“Earth”), a research ship in the Integrated Ocean Drilling Program

11. Oceanic Crust Structure and Composition • Ophiolites: sections of oceanic crust and upper mantle emplaced by subduction zones and thrust faults in mountain ranges • Top to bottom: • Pillow lava and sheet lava flows • Sheeted dike complex, basaltic • Gabbro

13. Continental Margins • Continental margin: area separating continent portion above water from the deep seafloor • Continental shelf • Gently sloping; 1 degree or less • Between shore and steeper continental slope • Shelf-slope break averages 135 m deep • Pleistocene (1.8 million years ago to 10,000 years ago): much of shelf above sea level

14. Continental Margins, cont. • Continental slope: begins at shelf-slope break • Continental rise: gently sloping area between continental slope and abyssal plain • Absent in Pacific (oceanic trenches) • Present in most of Atlantic

15. Continental Margins, cont. • Shelf-slope break: • Landward: sediments are affected by waves and tidal currents • Seaward: gravity transports and deposits sediments • Much of land-derived sediment is seaward of shelf-slope break and covers the continental slope and continental rise

16. Features of Continental Margins A generalized profile showing features of the continental margins. The vertical dimensions of the features in this profile are greatly exaggerated, because the vertical and horizontal scales differ.

17. Turbidity Currents • Underwater flows of sediment/water mix; denser than seawater alone • Reaches relatively flat seafloor • Deposits sediments in graded beds, with largest particles first and smallest particles last • Forms overlapping submarine fans • Evidence: 1929 event breaks North Atlantic cables

18. Submarine Canyons • Best developed on continental slopes, but also found on continental shelves • Some connect to land rivers, but most don’t • Turbidity currents move through submarine canyons, and likely are the primary agents of their formation

21. Active Continental Margins • At leading edge of continental plate where oceanic lithosphere is subducted • Narrow continental shelf • Continental slope descends to trench, so no continental rise • South America • Pacific Northwest • Earthquakes and volcanoes

22. Passive Continental Margins • Broad continental shelves • Well-developed continental slopes and rises • Abyssal plains extend from continental rise • Within a plate • Overlapping submarine fans at continental rise

25. Features of the Deep-Ocean Basins • Average 3.8 km deep; dark, cold • Abyssal plains • Oceanic trenches • Oceanic ridges • Hydrothermal vents • Seafloor fractures • Seamounts, guyots, aseismic ridges

27. Abyssal Plains • Beyond continental rises of passive continental margins • Flat and cover large areas • A few peaks up to 1 km • Flatness from sediment deposition that covers topography • Not found near active margins: sediments are trapped in oceanic trenches

28. Oceanic Trenches • Long steep-sided depressions • Near convergent boundaries • Subduction: cool, dense oceanic lithosphere is consumed • Common in Pacific Ocean basin • Earthquakes along Benioff zones • Volcano chain on overriding plate

29. Oceanic Ridges • Mostly submarine mountain system composed of basalt and gabbro • Found in all ocean basins • Divergent boundary: new crust formation • May have rift along crest • Mid-Atlantic Ridge • East Pacific Rise

31. Submarine Hydrothermal Vents • At oceanic ridges • Cold seawater seeps below crust, is heated at depth, discharges as plumes up to 400ºC • Black smoker: from dissolved minerals • Community of organisms: bacteria, crabs, mussels, starfish, tube worms • Chemosynthesis: bacteria oxidize sulfur compounds

32. Submarine Hydrothermal Vents, cont. • Economic potential: • Heated seawater reacts with crust • When discharged into ocean it cools, and iron, copper, and zinc sulfides and other minerals precipitate

35. Seafloor Fractures • Oceanic ridges terminate at fractures that run at right angles to the ridges • Several hundred kilometers long • Shallow-focus earthquakes • Transform faults (active) • Fracture zone (inactive)

37. Seamounts and Guyots • All are volcanic in origin • Seamount: at least 1 km height • Guyot: • Volcano originally above sealevel • Plate carries it away from ridge and into deeper waters • Waves eroded top and made it flat • Abyssal hills: 250 m high

39. Aseismic Ridges • Ridge or broad area rising up to 2-3 km above seafloor; lacks seismic activity • Some are microcontinents • Form as linear succession of hot-spot volcanoes near oceanic ridges • Can also form in the interior of plates • Hawaiian Islands/Emperor Seamount chain

40. Deep Seafloor Sediments • Silt- and clay-sized particles • Sources: • Windblown dust and volcanic ash • Shells of microscopic plants and animals from near-surface waters • Particles from chemical reactions in seawater • Cosmic dust

41. Deep Seafloor Sediments, cont. • Pelagic clay: particles from continents and islands • Calcareous ooze: calcium carbonate skeletons of marine organisms • Siliceous ooze: silica skeletons of some marine organisms

43. Reefs • Skeletons of marine organisms: corals, mollusks • Shallow tropical seas with clear water and water temperature above 20ºC • Corals with symbiotic photosynthetic algae: 50 m deep maximum

45. Reefs, cont. • Fringing reefs • Attached to island or continent • Rough tablelike surface • Slope steeply to seafloor • Barrier reefs • lagoon separates reef from shore • Atoll • Oval/round reef surrounds lagoon • Form around subsiding volcanic islands

47. Barrier Reef The white line of breaking waves marks the site of a barrier reef around Rarotonga in the Cook Islands in the Pacific Ocean. The island is only about 12 km long.

48. Resources from Oceans • Evaporation of seawater: sodium chloride • Seafloor deposits • Ownership questions • U.S. Exclusive Economic Zone – 200 nautical miles • Oil production • Methane hydrate • Manganese nodules • Sulfide deposits at hydrothermal vents