Plate Tectonics: A Scientific Theory Unfolds Chapter 5

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Plate Tectonics A Scientific Theory
UnfoldsChapter 5
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Continental Drift An idea before its time

• Before the 1960s geologists believed the ocean
  basins and continents were fixed
• During the 1600s the jigsaw puzzle idea came
  about, but the idea was never pushed
• Edward Suess (1831-1914) conceived the first idea
  of a large land mass
• Alfred Wegener (1880-1930) followed with the same
  idea and had followers

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Continental Drift An idea before its time

• Alfred Wegener
• First proposed his continental drift hypothesis
  in 1915- breakup of sea ice
• Published The Origin of Continents and Oceans
• Continental drift hypothesis
• Supercontinent called Pangaea (all lands) began
  breaking apart about 200 million years ago (early
  part of the Mesozoic era)
• Continents drifted to present positions
• Pangaea Breakup

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• Evidence used in support of continental drift
  hypothesis
• Fit of the continents
• Fossil evidence
• Rock type and geologic features
• Paleoclimatic evidence

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Matching mountain ranges
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Paleoclimatic evidence
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The great debate

• Objections to the continental drift hypothesis
• Lack of a mechanism for moving continents
• Wegener suggested the same gravitational forces
  from the Moon and Sun that cause tides caused
  continental drift
• Wegener incorrectly suggested that continents
  broke through the ocean crust, much like ice
  breakers cut through ice
• Strong opposition to the hypothesis from the
  scientific community

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• Continental drift and the scientific method
• Wegeners hypothesis was correct in principle,
  but contained incorrect details
• A few scientists considered Wegeners ideas
  plausible and continued the search

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• Continental drift and the scientific method
• Following WWII, the oceanic ridge system was
  discovered
• Dredging of the sea floor did not bring up any
  oceanic crust older than 180 MY
• Sediment accumulations in the deep-ocean trenches
  were found to be thin
• By 1968, these new developments led to the theory
  known as Plate Tectonics

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Plate Tectonics A modern version of an old idea
Cracked egg shell
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Plate Tectonics A modern version of an old idea

• Earths major plates
• Associated with Earths strong, rigid outer layer
• Known as the lithosphere
• Consists of uppermost mantle and overlying crust
• Overlies a weaker region in the mantle called the
  asthenosphere

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• Earths major plates
• The crust is broken up into oceanic and
  continental
• Oceanic crust is cooler and denser (3.3g/cm3)
• Continental crust is warmer and less dense
  (2.7g/cm3)

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• Earths major plates
• Seven major lithospheric plates
• Pacific, North American, Eurasian, African,
  Australian-Indian, South American, and Antarctic
• Other minor lithospheric plates
• Caribbean, Nazca, Philippine, Arabian, Cocos,
  Scotia, and Juan de Fuca
• Plates are in motion and continually changing in
  shape and size
• Largest plate is the Pacific plate
• Several plates include an entire continent plus a
  large area of seafloor

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• Earths major plates
• Plates move relative to each other at a very slow
  but continuous rate
• About 5 centimeters (2 inches) per year
• Cooler, denser slabs of oceanic lithosphere
  descend into the mantle

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• Plate boundaries
• Interactions among individual plates occur along
  their boundaries
• Types of plate boundaries
• Divergent plate boundaries (constructive
  margins)
• Convergent plate boundaries (destructive margins)
• Transform fault boundaries (conservative margins)
  

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Divergent plate boundaries

• Most are located along the crests of oceanic
  ridges
• Oceanic ridges and seafloor spreading
• Along well-developed divergent plate boundaries,
  the seafloor is elevated forming oceanic ridges
• Seafloor spreading occurs along the oceanic ridge
  system

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• Spreading rates and ridge topography
• Ridge systems exhibit topographic differences
• These differences are controlled by spreading
  rates

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Sea floor spreading
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• Continental rifting
• Splits landmasses into two or more smaller
  segments along a continental rift
• Examples include the East African rift valleys
  and the Rhine Valley in northern Europe
• Produced by extensional forces acting on
  lithospheric plates

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Continental rifting
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Convergent plate boundaries

• Older portions of oceanic plates are returned to
  the mantle in these destructive plate margins
• Surface expression of the descending plate is an
  ocean trench (deepest places on earth)
• Also called subduction zones
• Average angle of subduction 45 degrees

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Worlds oceanic trenches and ridge system
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• Types of convergent boundaries
• Oceanic-continental convergence
• Denser oceanic slab sinks into the asthenosphere
• Along the descending plate partial melting of
  mantle rock generates magma
• Resulting volcanic mountain chain is called a
  continental volcanic arc (Andes and Cascades)

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Oceanic-continental convergence
Animation
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• Types of convergent boundaries
• Oceanic-oceanic convergence
• When two oceanic slabs converge, one descends
  beneath the other
• Often forms volcanoes on the ocean floor
• If the volcanoes emerge as islands, a volcanic
  island arc is formed (Japan, Aleutian islands,
  and Tonga islands)

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Oceanic-oceanic convergence
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• Types of convergent boundaries
• Continental-continental convergence
• Less dense, buoyant continental lithosphere does
  not subduct
• Resulting collision between two continental
  blocks produces mountains (Himalayas, Alps, and
  Appalachians)

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Continental-continental convergence
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Transform fault boundaries

• Plates slide past one another and no new
  lithosphere is created or destroyed
• Transform faults
• Most join two segments of a mid-ocean ridge along
  breaks in the oceanic crust known as fracture
  zones
• A few (the San Andreas Fault and the Alpine Fault
  of New Zealand) cut through continental crust

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Transform faults
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San Andreas Fault near Taft, California
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Testing the plate tectonics model

• Evidence from ocean drilling
• Some of the most convincing evidence confirming
  seafloor spreading has come from drilling
  directly into ocean-floor sediment
• Age of deepest sediments
• Thickness of ocean-floor sediments verifies
  seafloor spreading

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• Hot spots and mantle plumes
• Caused by rising plumes of mantle material
• Volcanoes can form over them (Hawaiian Island
  chain)
• Mantle plumes
• Long-lived structures
• Some originate at great depth, perhaps at the
  mantle-core boundary

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The Hawaiian Islands
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Snake River Plain
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• Paleomagnetism
• Iron-rich minerals become magnetized in the
  existing magnetic field as they crystallize
• Rocks that formed millions of years ago contain a
  record of the direction of the magnetic poles
  at the time of their formation

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Tampa airport Magnets
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• Apparent polar wandering
• Lava flows of different ages indicated several
  different magnetic poles
• Polar wandering paths are more readily explained
  by the theory of plate tectonics

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Polar Wandering paths for Eurasia and North
America
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• Geomagnetic reversals
• Earths magnetic field periodically reverses
  polaritythe north magnetic pole becomes the
  south magnetic pole, and vice versa
• Dates when the polarity of Earths magnetism
  changed were determined from lava flows
• Geomagnetic reversals are recorded in the ocean
  crust
• In 1963 Vine and Matthews tied the discovery of
  magnetic stripes in the ocean crust near ridges
  to the concept of seafloor spreading

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Paleomagnetic reversals recorded in oceanic crust
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What drives plate motions?

• Researchers agree that convective flow in the
  mantle is the basic driving force of plate
  tectonics
• Forces that drive plate motion
• Slab-pull
• Ridge-push

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Some of the forces that act on plates
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