Geography 111 - Principles of Geology Plate tectonics and the evolution of the continents and ocean basins (note: the sequence of topics is not necessarily in the same order as the book; and not all topics are discussed in the book)

• Plate tectonics as a unifying theory for explaining observed patterns of geological phenomena

 
• Physical features:
• continents
• mountain or orogenic belts: high relief, tectonically active, areas where uplifted rocks are exposed by erosion; aligned paralle to continental margins and plate boundaries
• 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
• ocean floors
• passive continental margins: continental shelf, slope, and rise; ancient normal faults beneath sediment cover at passive margins
• abyssal plains
• mid-ocean ridges and transform fault/fracture zones; note symmetrical pattern of topography on opposite sides of ridges
• trenches and subduction zones, volcanic island arcs, and tectonically active continental margins
• seamounts and guyots (see p.446-448)
• global distribution of earthquakes and volcanoes
• "fit" of the continents; Wegener's continental drift hypothesis and its rejection
• additional evidence from fossils and distribution of rock types; correlation between South America and Africa; Appalachian and Caledonian mountain belts; Paleozoic glaciations

 
• Other geologic evidence for sea-floor spreading (first proposed by Hess in 1962):
• Age of rocks:
• continental rocks up to 4 billion years old, oldest rocks in shield areas
• oceanic crust generally < 200 million years old, distributed in symmetrical age bands around mid-ocean ridges
• Paleomagnetism
• Polarity reversals in earth's magnetic field
• Magnetic "stripes" on the ocean floors and Vine and Matthews' hypothesis (p.514-515) of sea-floor as a recording "strip chart" of the earth's changing magnetic field
• stages of rifting and ocean-floor development: East African rift valleys, Red Sea and Gulf of Aden

 
• Transform faults and their origins
  
  
• Trenches and subduction zones:
• spatial distribution of volcanic activity in relation to trenches
• subduction zones and island-arc mountain chains
• accretionary wedge of material scraped off the subducting plate: formation of melange
• suture zones and mountain belts at locations of continent-continent collision

 
• 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 (p.526-528), e.g. Alaska and Pacific northwest
• history of Appalachians (p.544-547)
• multiple orogenies over several hundred million years, associated with accretion of terranes followed by final continental collision

 
• Paleogeography of Pangaea: a supercontinent and its breakup (p. 529-532)

 
• Competing hypotheses about the driving mechanisms of plate tectonics: p. 532-533

 
• Continents as remnants of collisions and accretion of terranes: North America as an example
• Stable interior craton (including shields and platforms) flanked by orogenic belts (p.540-544)

• Relation of plate-tectonic motion to mountain building (orogeny):
• subduction zones and Andes- or Cascade-type mountain chains on land
• interior California considered as an inactive Andean-type setting:
• Coast Ranges as accretionary wedge
• Great Valley as marginal basin
• Sierra Nevada as eroded roots of Andes-type volcanic mountains with batholith exposed after uplift
• continent-continent collisions and Himalayan-type orogeny
• note similar origins for Appalachians, Alps, Urals
• subsequent denudation of mountain belts to produce thick wedges of marine sediment deposited along continental margins with associate isostatic adjustment

 
• Possible role of hot spots as major upwelling sources: hypothesis of hot spot role in breakup of supercontinents; the Wilson cycle (not covered in textbook, mentioned briefly in class)
• stationary supercontinent, poor conductor of heat
• buildup of heat, uplift, doming, rifting, splitting, sea-floor spreading
• cooling of oceanic lithosphere with age, negative buoyancy, development of subduction zones along passive continental margins
• contraction of ocean floor, recombination of continents
• elapsed time about 500 million years (?) per cycle