Phys' Geology Chapter 2 Plate Tectonics

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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
  couldnt explain the movement mechanism.

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

Various authors attributed distribution to land
bridges, though Wegener disagreed.
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• 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. 38).
• Magnetometer detected variations in ocean floor
  magnetism. After WWII, magnetic orientation of
  terrestrial lava flows was conducted.

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• Geologists discovered that orientations of
  iron-bearing minerals is affected by the strength
  and polarity of the Earths magnetic field.
  Plotting of old lavas suggested wandering of
  Magnetic North (Figure 2.11, p. 45). 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. 46 47).

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• Mapping of ocean floor showed polarity changes
  (stripes) and mirror images on opposing sides
  of mid-ocean ridges (see pp. 48-49).
• More evidence for seafloor spreading
  Radiometric age dating suggests oldest oceanic
  crust lt180 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.

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Simplified map showing major plates 3 types
of plate boundaries Divergent (split-ting),
Convergent (subduction) and Transform fault
zones.
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Divergent margins Continental rift zone Rio
Grande Rift, East Africa Rift
Initiation (p. 56) begins with stretching and
splitting of continental crust. Extension
results in formation of graben basins.
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Tensional strain Brittle deformation
continental rift crustal stretching
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Graben basin
High angle Normal fault (gravity fault)
Rio Grande Valley from El Paso northward into S.
Colorado consists of a series of graben basins,
resulting from stretching of crust. Basins
in-fill with sediments basalts.
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• Oceanic crust Continental
  crust

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The basaltic flows and cinder cones of the Rio
Grande Rift are typical. Mafic (dark)
composition is due to thinned crust.
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Divergent zone becomes an Oceanic rift zone
Mid-Atlantic Ridge, East Pacific Rise
Escaping mantle derived magma cools to form new
mafic ocean floor (gabbro).
Continental crust/ Oceanic crust juncture.
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Old oceanic crust, preser-ved in areas of
continental collision - Ophiolites. Ophiolites
in Finland, about 1.96 b.y. old. Oldest ocean
bottom is lt 180 m.y. old.
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Convergent boundary Subduction zone
Oceanic Oceanic plate collision subduction of
one plate melting to form magmas Volcanic
Island Arc System.
Ex Japan, Philipines, Aleutians, Indonesia
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• 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.

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Eventually, subduction pulls the ocean basin
closed, resulting in continental collisions
orogenies. Ex Appalachians, Alps, Himalayas.
Heat and pressure regional metamorphism
magmatism.
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• Figure 2.24 (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.

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There are some mountain ranges that are
uplifted by non-collisional compression in
Continental Arc settings. Subduction zone
friction triggers back arc thrust faulting
uplift Ex Rocky Mts., Andes Mts..
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• Transform Fault zones lateral movement of
  adjacent plates and also in offset zones assoc.
  with Divergent zones (see pp. 60 - 61).
• 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. Dantes Peak
  Continental Arc System.

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• 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.

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• 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.

Basal sediments the first to be deposited.
Older crust, older basal sediments
Older crust, older basal sediments
Newest oceanic crust
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• 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. 2.20, p. 52). 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.