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Phys. Geology Chapter 2 - Plate Tectonics

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Phys. Geology Chapter 2 - Plate Tectonics

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    1. Phys. Geology – Chapter 2 - Plate Tectonics Early ideas on “Continental Drift” – (1885) Edward Suess – Noted similarities of plant fossils in India, Australia, South Africa, & South America. (1910) Frank Taylor – suggested that the mid-Atlantic ridge was related to continental drift. (1915) Alfred Wegener suggested the name Pangaea for the single large continent. Used geologic, paleontologic, and climatic evidence to show joining of continents prior to their breakup. He couldn’t explain the movement mechanism.

    2. (1937) Alexander du Toit noted that the Permian reptile Mesosaurus (S. America & southern Africa) lived in freshwater.

    3. WWII technology used to map ocean floor. Included sonar and magnetometers. Sonar (sound waves) detected variable topography in ocean floor. Post-WWII sonar mapping found the edges of the continental shelves (p. 41). Magnetometer detected variations in ocean floor magnetism. After WWII, magnetic orientation of terrestrial lava flows was conducted.

    4. Geologists discovered that orientations of iron-bearing minerals is affected by the strength and polarity of the Earth’s magnetic field. Plotting of old lavas suggested “wandering” of Magnetic North (Figure 2.10, p. 48). Mapping of ocean floor magnetism continued. Seafloor spreading was proposed by Harry Hess in 1962, by which continental and oceanic plates moved together. Driving mechanism is the thermal convection cells (see pp. 48 – 49).

    5. Mapping of ocean floor showed polarity changes (stripes) and “mirror images” on opposing sides of mid-ocean ridges (see pp. 50 - 51). More evidence for seafloor spreading – Radiometric age dating suggests oldest oceanic crust <180 m.y. vs. 3.96 b.y. oldest continental crust. Basics of Plate Tectonics Theory Crust composed of irregular plates Plates float on asthenosphere – “Isostacy” Plates move in relation to one another Plates driven by vertical convection curr.

    8. Tensional strain – Brittle deformation –– continental rift – crustal stretching

    9. Oceanic crust Continental crust

    14. Oceanic – Continental plate collision = subduction of denser (mafic) oceanic plate & partial melting of plate to form magmas – Continental Arc System. Ex: Cascades, Andes, Central America – composite volcanoes.

    16. Figure 2.23 (p. 59) illustrates the movement of the Indian “subcontinent” towards Asia, by subduction and the collision uplift of the Himalayan Mts.. This is similar to the collisional events that uplifted the Appalachian Mts.. Within the collision zone, there may be “fault slices” of accretionary wedge material and ophiolites (old oceanic crust). The “suture zone” is the boundary between the two continental masses. May be obscured by metamorphism and deformation.

    18. Transform Fault zones – lateral movement of adjacent plates and also in offset zones assoc. with Divergent zones (see pp. 60 – 61, Figure 2.24). Most of the seismic activity in the “lower 48” states is in the San Andreas Fault Zone of California, where a small portion of the Pacific Plate is sliding past the North American Plate. Earthquakes are constant, but volcanism is absent. Basaltic eruptions, a la the movie “Volcano” are more likely in El Paso or Albuquerque, than in Los Angeles. “Dante’s Peak” – Continental Arc System.

    19. Divergent Zone – rising man-tle plume causes plates to separate, spread. Rising magma = earthquakes & volcanoes. Convergent Zone – denser oceanic crust sinks into mantle beneath other plate margin. Sinking plate melts to form magma. Rising magma = volcanoes. Friction = earthquakes Transform Zone – Plates slide past one another. Friction = earthquakes.

    20. Additional evidence for Plate Tectonics: Oceanic deposition begins after creation of new oceanic crust. Basal sediments and oceanic plate become progressively older as you move away from rift zone.

    21. More additional evidence – Hot Spots and Mantle Plumes & Earthquake clustering. Individual mantle upwhellings (plumes) break through overlying crust. Hot Spot is stationary, plate moves over the Hot Spot. Best example – Hawaiian Islands/Emperor Seamount Chain (Fig. 2.29, p. 64). Oldest Emperor Seamount is approx. 65 m.y., & is close to Aleutian subduction zone. Angle between the Emperor and Hawaiian chains is due to change in plate direction. Earthquakes cluster along plate boundaries.

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