12_marine_SFspread_09_post

12_marine_SFspread_09_post - 12. Marine Geology &...

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Unformatted text preview: 12. Marine Geology & Seafloor Spreading Seafloor Spreading: 1 Why this is important • The topographic features of the ocean basins are prime evidence of plate tectonics. Seafloor Spreading: 2 Continental margins • Portions of ocean floor extending from shoreline to landward edge of abyssal plain Continental shelf: broad, very gently sloping platform underlain by marine sediments (commonly very thick) overlying continental crust; may be exposed during glacial periods Continental margins Continental shelf: broad, very gently sloping platform underlain by marine sediments (commonly very thick) overlying continental crust; may be exposed during glacial periods Continental slope: inclined at 2-4°; marine sediments overlie transitional crust Continental rise: more gently inclined region adjacent to the continental slope overlying oceanic crust Rise Fig. 18.5 Fig. 4.3 Seafloor Spreading: 3 Seafloor Spreading: 4 Continental margins • Trench: large topographic depression, commonly partially filled with sediments, adjacent to many continental margins Ocean basins Fig. 3.7b • Abyssal plains: relatively flat parts of ocean floor (between continental margins and midocean ridge system) Fig. 18.5b Seafloor Spreading: 5 Fig. 18.6c Seafloor Spreading: 6 Ocean basins • Mid-ocean ridge: large-scale, laterally continuous positive topographic feature, sometimes capped by an axial graben (depression), associated with basaltic pillow lavas, normal faults, and black smokers Ocean basins Fig. 3.9 Fig. 3.10 Map by: Marie Tharp & Bruce Heezen Seafloor Spreading: 7 Seafloor Spreading: 8 • Mid-ocean ridge: large-scale, laterally continuous positive topographic feature, sometimes capped by an axial graben (depression), associated with basaltic pillow lavas, normal faults, and black smokers Ocean basins Ocean basins • Mid-ocean ridge: large-scale, laterally continuous positive topographic feature, sometimes capped by an axial graben (depression), associated with basaltic pillow lavas, normal faults, and black smokers • Seamounts: positive topographic features underlain by volcanic rock; not Seamounts: laterally continuous. Fig. 3.9 • Guyot: flat-topped seamounts Guyot: Black smokers: eruptions of hot water containing abundant dissolved minerals • Many seamounts form linear chains (e.g., Hawaiian chain) Fig. 4.10 Seafloor Spreading: 9 Seafloor Spreading: 10 Review Questions Review Questions 12-1. M is a ___. A. continental shelf C. spreading center M Seafloor Spreading: 11 N1 P Q N1 N2 B. guyot or seamount D. trench 12-3. P is a ___. A. continental shelf C. spreading center B. guyot or seamount D. trench 12-4. Q is a ___. A. continental shelf C. spreading center M B. guyot or seamount D. trench 12-2. N is a ___. A. continental shelf C. spreading center Lower figure shows a topographic profile across the Pacific Ocean (red line gives location in map). B. guyot or seamount D. trench P Fig. 4.22 Photo: D.B. Foster Videos Q N2 12-5. Which of the following features is generally found at the deepest depth in the oceans? A. abyssal plain B. continental shelf C. mid-ocean ridge D. trench 12-6. Which feature in the oceans is the least laterally continuous? A. continental rise B. mid-ocean ridge C. seamount D. trench 12-7. Refer to Figure 3.9 on slide 8. The difference in elevation between the crest of a mid-ocean ridge and the abyssal plain is: A. 1 km B. 3 km C. 5 km D. 7 km Seafloor Spreading: 12 Review Questions 12-8. How deep is the deepest part of the ocean (measured down from sea level)? [Hint: see slide 8.] A. ~ 0.3 km B. ~ 3 km C. ~ 10 km D. ~ 30 km Seafloor-spreading hypothesis 1. Oceanic crust separates at mid-ocean ridges 2. Magma rises to surface to form new oceanic crust 3. Oceanic crust moves away from ridge 4. Old crust is recycled into mantle, descending at trenches 12-9. Which topographic feature is typically located farthest from the shoreline? A. continental shelf B. continental slope C. continental rise 12-10. If you were sailing on a cruise ship out of New York Harbor, which features would you cross going from west to east? A. Continental shelf, then the continental rise, and finally the continental slope B. Continental rise, then the continental slope, and finally the continental shelf C. Continental shelf, then the continental slope, and finally the continental rise Fig. 3.12 12-11. A. True / B. False: Black smokers are typically found in deep-sea trenches. Seafloor Spreading: 13 Seafloor Spreading: 14 Seafloor spreading--evidence • Existence of mid-ocean ridges (spreading centers) & trenches Seafloor spreading--evidence • Existence of spreading centers & trenches Fig. 3.7b Fig. 3.9 Seafloor Spreading: 15 Seafloor Spreading: 16 Seafloor spreading--evidence • Existence of spreading centers & trenches Seafloor spreading--evidence • Existence of spreading centers & trenches and associated earthquakes Fig. 3.9 Seafloor Spreading: 17 Seafloor Spreading: 18 Seafloor spreading--evidence • Normal faults at mid-ocean ridges: indicate stretching • High heat flow at ridge axis: rising hot material at depth Seafloor spreading--evidence • Composition of oceanic crust: uniformly basaltic; all crust produced by the same process Fig. 18.3 Fig. 3.10 Seafloor Spreading: 19 Seafloor Spreading: 20 Seafloor spreading--cause Heat transfer • Conduction: heat is flowing; mass does not move • Convection currents in mantle • Convection: heat transfer with movement of mass • Rising currents at ridges • Descending currents at trenches Fig. a.19 Fig. 4.27 Seafloor Spreading: 21 Seafloor Spreading: 22 Seafloor spreading--tests • Thickness of sediment: increases away from ridge Fig. 18.6c Seismic-reflection profile across the Mid-Atlantic ridge at 44°N. From Hamblin & Christiansen Seafloor Spreading: 23 Seafloor spreading--tests • Age of sediments directly overlying basaltic rocks (based on drilling and fossil content) increases away from ridge crest JOIDES Resolution. Photo: Ocean Drilling Program Seafloor Spreading: 24 Seafloor spreading--tests Seafloor spreading--tests • Age of seafloor • Paleomagnetic evidence • Stripes of different magnetic intensity parallel to ridge Fig. 4.11 145-200 Seafloor Spreading: 25 Youngest Magnetic reversals • Some rocks are magnetized in direction of current magnetic field; others are reversed. • Magnetic field reverses itself. Oldest Seafloor Spreading: 26 Magnetic reversals Normal polarity: magnetic field points down at North pole (present-day situation) Fig. 3.15 Fig. 3.14 Seafloor Spreading: 27 Reversed polarity: magnetic field points up at North pole Seafloor Spreading: 28 Magnetic reversals Seafloor spreading: tests • Sequence of reversals determined by radiometric dating of thick sequences of lava flows • Magnetic stripes: bands of oceanic crust w/ normal & reversed polarities • “Bar code” for dating, correlating rocks Fig. 3.18 Fig. 3.17 Seafloor Spreading: 29 Seafloor Spreading: 30 Seafloor spreading Review Questions • New crust is magnetized at ridge crest in direction of magnetic field. 12-12. What type of fault is most commonly present at the axis of a mid-ocean ridge? A. normal fault B. oblique-slip fault C. reverse fault D. strike-slip fault • 1/2 of crust moves to one side of ridge; other 1/2 moves to other side • Reversals of magnetic field through time produce stripes. 12-13. A. True / B. False: According to the seafloor-spreading hypothesis, mid-ocean ridges are associated with the rising parts of convection cells. 12-14. A. True / B. False: Oceanic crust becomes progressively younger with increasing distance from the mid-ocean ridges. 12-15. A. True / B. False: Sediment thickness increases with increasing distance from the mid-ocean ridge. 12-16. A. True / B. False: All oceanic crust at the crests of mid-ocean ridges is reversely magnetized. 12-17. A. True / B. False: All oceanic crust is younger than ~200 million years old. Animation! Seafloor Spreading: 31 Fig. 3.19 12-18. In which of the following locations would you most likely find rocks of basalt on the ocean floor? A. on an abyssal plain B. on the crest of a mid-ocean ridge C. on the continental shelf D. on the continental slope Seafloor Spreading: 32 Seafloor spreading: Transform faults Fracture zones: fault zones generally oriented perpendicular to mid-ocean ridges; ridge axes are offset across the fault zones. Seafloor spreading: Transform faults • Old idea: ridge forms first, then is offset by fault Transform fault: that part of a fracture zone present between the offset ridge axes. Fig. 4.17 • New idea: ridge and transform form at same time; ridge was never continuous. Sense of slip on fault only makes sense if seafloor spreading occurs. Seafloor Spreading: 33 Seafloor Spreading: 34 Seafloor spreading: Transform faults Transform faults: Review • Sense of motion along transform is opposite of sense of ridge offset 3 • No shearing occurs beyond ridge segments --> plates move in same direction 2 6 8 1 5 4 Plate X Plate Y 7 N Fig. 4.17 Seafloor Spreading: 35 Top view Seafloor Spreading: 36 Q1. Oceanic crust at 1 is ___ oceanic crust at 3. A. older than B. same age as C. younger than Q2. Oceanic crust at 3 is ___ oceanic crust at 6. A. older than B. same age as C. younger than Q3. Oceanic crust at 1 is ___ oceanic crust at 4. A. older than B. same age as C. younger than New global tectonics Transform faults: Review 3 2 6 8 1 5 4 Plate X Plate Y 7 N Top view Q4. Plate X is moving to the ___ and plate Y is moving to the ___. A. north, north B. north, south C. south, north D. south, south Q5. The sense of ridge-axis offset is ___, and the sense of slip on the transform is: A. left lateral, left lateral B. left lateral, right lateral C. right lateral, left lateral D. right lateral, right lateral Q6. Earthquakes occur: A. everywhere along the blue line B. only between points 7 and 8. Seafloor Spreading: 37 Plates rotate around a pole; velocity decreases toward pole Plate motion: • always parallel to transform faults • generally perpendicular to spreading centers Seafloor Spreading: 38 Plate tectonics Plate tectonics--boundaries • Earth's lithosphere is divided into several rigid plates • Plates move with respect to another and ride over asthenosphere • EQ / volcanic belts coincide with plate boundaries Divergent Convergent Fig. 4.6 Conservative (transform) Fig. 4.4 Seafloor Spreading: 39 Seafloor Spreading: 40 Plate tectonics (PT) vs. continental drift (CD) Plate tectonics (PT) vs. continental drift (CD) CD: Continents plow through oceans PT: Entire lithosphere moves CD: Continental margins are plate boundaries Fig. 18.3 Passive margin: relatively broad margin far from a plate boundary margin: Active margin: relatively narrow margin situated on or very near a plate margin: boundary Seafloor Spreading: 41 Review Questions 12-19. A. True / B. False: If a mid-ocean ridge exhibits a right-lateral offset, the sense of motion (slip) along the transform fault is right-lateral. CD: “Plates” are the continents PT: Some plates are entirely oceanic (or continental); many have both continental & oceanic crust C&O C&O O O Fig. 4.4 Seafloor Spreading: 43 12-20. A. True / B. False. Earthquake activity occurs only on the portion of a fracture zone between two offset mid-ocean ridge segments. 12-21. For the ridge-ridge transforms shown below, the ridges are the double lines; the transform is the single, thick line. Which transforms experience right-lateral strike-slip movement? [Hint: Draw in the spreading direction for the ridges first; then determine sense of slip on the transform faults.] A. a and d B. b and c C. All of them. D. None of them. O O O O C&O Fig. 4.4 Seafloor Spreading: 42 Plate tectonics (PT) vs. continental drift (CD) C C&O PT: Only active margins are plate boundaries C&O C&O C&O O a Seafloor Spreading: 44 b c d Review Questions 12-22. Which statement is most correct? A. Plate motions are always parallel to the trend of mid-ocean ridge segments. B. Plate motions are always parallel to the trend of transform faults. C. Plate motions are always perpendicular to the trend of trenches. 12-23. A. True / B. False: According to plate tectonics, continental margins always correspond to plate boundaries. 12-24. A. True / B. False: According to plate tectonics, all plates are exclusively composed of either oceanic lithosphere or continental lithosphere. 12-25. A. True / B. False: The New Jersey continental margin is a passive continental margin. 12-26. A. True / B. False: The California continental margin is a passive continental margin. Seafloor Spreading: 45 ...
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