Chapter 3 Plate Tectonics: A Unifying Theory

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Chapter 3Plate Tectonics A Unifying Theory

• Introduction an earthquake in Turkey and Mt.
  Pinatubo eruption in the Philippines, both
  related to Plate Tectonics.
• Early ideas
• (1885) Edward Suess Noted similarities of plant
  fossils in India, Australia, South Africa,
  South America. Glossopteris flora (Figure 3.1,
  p. 33) in Southern Hemisphere coal layers
  different from Northern Hemisphere coal swamps.
  Named southern super-continent Gondwana in
  book The Face of the Earth. Land bridges
  connected the continents.

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• (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.
• These other early hypotheses concerning
  Continental Drift gained attention, but not
  acceptance, as mechanism for movement could not
  explained.

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• (1937) Alexander du Toit offered additional
  info by noting that the Permian reptile
  Mesosaurus (present in South America and southern
  Africa) lived in freshwater environments.

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• WWII technology used to map ocean floor.
  Included sonar and magnetometers. Variations in
  ocean floor magnetism led to study of terrestrial
  lava flows.
• Geologists discovered that orientations of
  iron-bearing minerals is affected by the strength
  and polarity of the Earths magnetic field (pp.
  37-38). Plotting of old lavas suggested
  wandering of Magnetic North (Figure 3.6).

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• Seafloor spreading proposed by Harry Hess in
  1962, by which continental and oceanic plates
  moved together. Driving mechanism is the thermal
  convection cells. Figure 3.7, 3.9, 3.10 (p. 39
  40) shows how parallel, symmetrical magnetic
  stripes in oceanic plates form. Measuring the
  Past Intensity of the Earth's Magnetic Field
  Magnetic Field weakening?
• More evidence for seafloor spreading Oldest
  oceanic crust lt180 m.y. vs. 3.96 b.y. oldest
  continental crust.

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• Basics of Plate Tectonics Theory
• Crust composed of irregular plates
• Plates float on asthenosphere Isostacy
• Plates move in relation to one another
• Crustal types Oceanic (basalt) 3 gm/cm3 and
  Continental (granite) 2.7 gm/cm
• Three types of plate boundaries
• Divergent (spreading)
• Convergent (subduction zones)
• Transform (lateral contact)

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Map showing major crustal plates and boundaries.
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Divergent margins Continental rift zone Rio
Grande Rift, East Africa Rift
Initiation 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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Divergent margins Oceanic rift zone
Mid-Atlantic Ridge, East Pacific Rise
Continued stretching opens a new ocean basin,
escaping mantle derived magma cools to form new
mafic ocean floor.
Continental crust/ Oceanic crust juncture.
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Old oceanic crust, preserved in areas of
conti-nental collision are called
Ophiolites. Jormua mafic-ultramafic com-plex in
Finland, about 1.96 billion years old. Oldest
ocean bottom is lt 180 m.y..
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When an oceanic plate is converging on another
oceanic plate or on a continental plate, this
creates a convergent zone (subduction zone).
Oceanic-oceanic con-vergent zones likely begin
when an oceanic rift zone loses its vertical
convection current upwhelling and one plate
starts to sink back into the mantle. Oceanic-conti
nental convergent zones may begin when sediment
loading at the conti-nental margin breaks the
oceanic plate loose from the continental plate
starts to sink back into the mantle.
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In the Subduction zones, the sinking oceanic
plate starts to melt, producing magma that rises
to form volcanic chains. Oceanic-Oceanic
Subduction Zone Volcanic Island Arc
System Examples Japan, Philipines, Aleutians,
Lesser Antilles Caribbean Oceanic-Continental
Subduction Zone Volcanic Continental Arc
System Examples - Cascade Mts., Andes Mts.,
Sierra Nevada Mts. (old), Central America, Mexico
USGS Cascades Volcano Observatory
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Convergent boundary Subduction zone
Subduction zone magma includes oceanic sediments,
water, melted oceanic crust explosive
intermediate lavas Andesites.
Back Arc Basin Subduction Zone
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• Melting oceanic crust, oceanic sediments, and
  sea water produces over-abundance of magma that
  rises to produce volcanism. Rising magma plutons
  may melt some of overlying crustal material.
  Increased quartz content causes more explosive
  volcanism. Examples include Mt. St. Helens, El
  Chichon, Mt. Pinatubo,
• Remember the Bowen Reaction Series Quartz
  last to solidify, first to melt.

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Other subduction zone materials include The
material scraped off the descending plate is
referred to as subduction complex (or mélange).
Occasionally, slices of oceanic crust and upper
mantle are included in the mélange. These
oceanic crust/mantle slices are referred to as
ophiolites or ophiolite suites see p. 45
slide 11. Additional info on Subduction Zones
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After oceanic plate is consumed by the
subduction zone, a continent to continent
collision occurs (Fig. 3.18, p. 47). Examples of
mountains resulting from continent to continent
collisions Alps, Appalachians, Himalayas,
Atlas. Transform Boundaries are present where
adjacent plates slide past one another San
Andreas Fault Zone, Anatoli Fault Zone.
Plumbing is not suitable for vulcanism in
Transform fault zones.
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• Unusually strong mantle plumes of magma
  occasionally break through crustal plates,
  forming Hot Spot volcanoes, Ex Hawaiian
  Islands/Emperor Seamounts, Yellowstone, San
  Francisco Volcanic Field (Flagstaff, AZ).
• Driving Force for Plate Movements are the
  vertical convection currents. Two models are 1)
  Shallow cells in asthenosphere and 2) Deep cells
  in entire mantle.
• Ridge-push at Divergent (Rift) zones and
  Slab-pull in Subduction zones may also contribute
  to plate motion.

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Plate Tectonics and Mountain Building
(Orogeny) Presence of ophiolite suites in
mountain interiors (Alps, Himalayas, Urals,
Appa-lachians) mark the sites of old subduction
zones. When subduction zones close, continents
collide to build mountains. Opening Closure of
an ocean basin is termed The Wilson Cycle,
Example is the Paleozoic Iapetus Ocean. Exotic
Terranes small microplates are accreted along
the continental margin.
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Plate Tectonics the Distribution of Life
(Ecosystems, Fossils, etc.) Distribution of
continents affects oceanic currents (which
redistribute tropical heat) and air currents
(land heats up faster than water). Mountain
ranges may act as biological barriers (to
migrations) and locally affect weather and
climate, which affects the distribution of
terrestrial ecosystems. The opening of an ocean
basin splits terrestrial ecosystems.
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The uplift of the Isthmus of Panama, 5 m.y.
ago, closed seaway between Caribbean and Pacific
separated marine organisms, while it joined
North America South America, allowing for the
migration of mammals. Mesozoic/early Cenozoic
separation of Australia (with its primitive
marsupials) saved marsupials from competition
with reproductively more efficient placental
mammals.
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• Plate Tectonics Natural Resources
• Plate tectonics may affect distribution of basin
  settings favorable for the deposition and
  preservation of organics (to form petroleum),
  then later on with the development of certain
  structural features to form oil reservoirs.
• Vulcanism related to rifting and subduction may
  introduce metallic-bearing hydro-thermal
  solutions from great depths.
• When a particular resource is found in a certain
  setting, other occurrences may occur elsewhere in
  similar settings.