Chapter 13: the ocean floor

• Physical characteristics of the ocean floor:
  
• passive and active continental margins (figures 13.7, 13.8)
  
• topographic features: continental shelf, continental slope, continental rise, abyssal plain, mid-ocean ridge, fracture zones, deep-sea trenches, accretionary wedges, forearc basins, seamounts and guyots, coral atolls
  
• relative age of ocean floor vs. continents
• technlogy for remote sensing of the ocean floor
• relation of major topographic features to plate tectonics
• Features of the mid-ocean ridge
• relation between ridge form and spreading rate
• ophiolite suite: vertical sequence of layers within oceanic crust, formed at mid-ocean ridge: deep-sea sediment, pillow basalts, sheeted dike complex, layered gabbro, peridotite of upper mantle
  
• hydrothermal vents and black smokers
• Rifting of continents and formation of new ocean floor (figures 13.18-13.21)
• possible role of mantle plumes and hot spots as a cause of rifting
• slab pull and slab suction
  
• not all rifts are successful in forming a new ocean; failed rifts include e.g. figure 13.20, or Reelfoot Rift under Mississippi valley
• Convergent plate boundaries, trenches and subduction
• as oceanic lithosphere ages and cools, it gets denser and eventually becomes gravitationally unstable and starts to sink
• younger lithosphere (closer to mid-ocean ridge) is not as cold and not as dense, hence it descends at a shallower angle
• older lithosphere (farther from mid-ocean ridge) is colder, denser, descends at a nearly vertical angle
• whether the subducted slab follows a shallow or a steep path affects interaction with the overlying lithospheric plate
• History of the plate  boundary bordering the west coast of the U.S. (figure 13.24)
  
• Farallon plate bordering subduction zone on east and mid-ocean ridge on west
• plate was subducted faster than the rate of sea-floor spreading, eventually parts of the ridge reached the trench and the plate was split into several smaller plates
• San Andreas fault formed when the ridge disappeared and was replaced by a transform fault boundary; direction of plate motion changed even though subduction continues along the Juan de Fuca plate to the north and the Cocos and Nazca plates to the south
• The supercontinent cycle (figure 13.25)
  
• repeated episodes of formation and breakup of "supercontinents"
  
• formed as oceanic plates are subducted and continental landmasses are sutured at convergent boundaries
• Pangaea is the most recent (230 million years); before that was Rodinia (~1 billion years ago)
• Rodinia split apart 750-550 million years ago
• trapping of heat beneath large continental land masses may cause buildup of forces tending to cause uplift, stretching, and rifting apart (also see above for other causes)
• extrapolation of present plate movements to predict future configuration of continents (figures 13.26, 13.27)

        Chapter 14: the continents and mountain building

• Terrain types on continents
• active (young) mountain or orogenic belts: high relief, tectonically active, areas where uplifted rocks are exposed by erosion; aligned paralle to continental margins and plate boundaries
• old mountain belts: no longer active, lower relief, may be aligned with continental margins or may be in continental interior at site of ancient suture zones
• most mountain belts associated with tectonic activity near convergent plate boundaries either active or ancient
  
• stable platforms: low relief, sedimentary cover over igneous/metamorphic basement
• continental shields: deeply eroded roots of ancient mountain belts; metamorphic rock exposed at surface, very low relief, earth's oldest exposed rocks
• Subduction, convergent plate boundaries and continental landscapes
• features associated with subduction zones: volcanic island arcs or continental volcanic arcs (Andean-type plate margins); trench, forearc basin and accretionary wedge; backarc basin (in association with volcanic island arcs) (figures 14.4, 14.6)
• accretionary wedges formed as sediments and sedimentary rocks are scraped off the descending or subducting slab as it enters the trench; can build up thick accumulations of jumbled, mashed up rock at the leading edge
• accumulation of sediment in forearc basins derived from volcanic deposits and (in some cases) from rivers draining the continent
• backarc basins sometimes form due to spreading between the volcanic island arc and the continent, caused by tensional forces e.g. slab suction and trench rollback (figure 14.6); analogous to effect on continents described in figure 13.24
• compression in the vicinity of the subduction zone may cause shortening and thickening of an adjacent continental margin (e.g. west margin of Andes/South American plate)
• growth of continental mountain belts at subduction zone; transition from passive to active continental margin, growth of accretionary wedge and forearc basin, continental volcanic arc and emplacement of batholiths beneath the volcanoes; compression may cause uplift of accretionary wedge and forearc basin above sea level (figure 14.7)
• relate the above to formation of the Sierra Nevada, Great Valley and Coast Ranges of California (figures 14.8, 5.32)
• evolution of plate boundary along California coast over the last 180 million years with formation and recent destruction of subduction zone (discussed in class) - alternating compression followed by uplift and stretching inland to form the fault-block mountains of the Basin and Range province east of the Sierra Nevada (p. 435-436, figure 14.18; figure is lacking some information, note additional figures shown in class)
  

• Suture zones and mountain belts at locations of continental collisions (figure 14.9)
  
• examples: Himalayas, Appalachians (figures 14.11-14.13)
• steps in evolution of the Appalachians: accretion of microcontinens and volcanic arcs in early episodes of orogenesis; eventual closing of proto- or ancestral Atlantic Ocean as part of the formation of Pangaea
• rifting of Pangaea 200-170 million years ago to form the Atlantic Ocean
• possible connection between rifting on Atlantic side and westward drift of continent over subduction zone on Pacific side of North American plate
  
 
• Terranes, continental accretion, and orogenic belts
  
• importance of crustal fragments: micro-continents or "microplate terranes," rafted together by subduction-zone conveyor belt and sutured together to make "patchwork" continents, e.g. Alaska and Pacific northwest (figures 14.15, 14.16)
  
• also related to history of Appalachians (see above, figures 14.12-14.13)
• Isostasy, roots of mountains and isostatic adjustment (p. 437-439, figures 14.20-14.22)
• buoyancy of thicker continental crust causes uplift
• as material erodes off he top, isostatic adjustment causes uplift to compensate just like an iceberg that floats upward as the surface melts
• uplift eventually slows down and stops once crust reaches "normal" thickness
  

• Continents as remnants of collisions and accretion of terranes: North America as an example (figure 14.23)
  
• Stable interior craton (including shields and platforms) flanked by orogenic belts, with oldest rocks in the center and younger rocks at the margins
• were there continents in early earth history or did they form originally as volcanic island arcs that were gradually sutured together to form continental fragments at subduction zones? alternate hypotheses (early evolution vs. gradual evolution of continents); North America seems to provide an example of gradual evolution