1. GEOL: CHAPTER 9 The Seafloor

3. 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 Learning Outcomes

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

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

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

8. Scientific voyages later: 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 Exploring the Oceans, cont.

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

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

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

13. Continental margin: area separating continent portion above water from the deep seafloor Continental margin: area separating continent portion above water from the deep seafloor Continental shelf 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 Continental Margins

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

15. Shelf-slope break: 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 Continental Margins, cont.

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

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

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

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

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

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

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

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

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

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

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

37. All are volcanic in origin All are volcanic in origin Seamount: at least 1 km height Seamount: at least 1 km height Guyot: 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 Abyssal hills: 250 m high Seamounts and Guyots

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

40. Silt- and clay-sized particles Silt- and clay-sized particles Sources: Sources: 1.Windblown dust and volcanic ash 2.Shells of microscopic plants and animals from near-surface waters 3.Particles from chemical reactions in seawater 4.Cosmic dust Deep Seafloor Sediments

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

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

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

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