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; also see the links from the online syllabus for
additional information)
- Plate tectonics as a unifying theory for explaining observed patterns of geological
phenomena
- Physical features:
- continents (p.539-544)
- 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 (p.435-448)
- 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 (p.470-472) and volcanoes (p.121-125)
- "fit" of the continents; Wegener's continental drift hypothesis and its
rejection (p.506-508)
- 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 (p.540-542)
- oceanic crust generally < 200 million years old, distributed in symmetrical age bands
around mid-ocean ridges (p.516-518)
- Paleomagnetism (p.514-516)
- Polarity reversals in earth's magnetic field
- Magnetic "stripes" on the ocean floors and Vine and Matthews' hypothesis 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 (p.508-510, 520-523)
- Transform faults (p.512-513)
- Trenches and subduction zones (p.510-513, 524-528):
- 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 (p.526-528, 539-544)
- 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