Powerpoint Presentation Physical Geology, 10e

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Plate TectonicsChapter 18-20
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Plate Tectonics

• Plate tectonics - Earths surface composed of
  thick plates that move
• Intense geologic activity is concentrated at
  plate boundaries
• Combination of continental drift and seafloor
  spreading hypotheses proposed in late 1960s

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Review Three Types of Plate Boundaries
But how do we know that plates move at all ?
Transform Convergent
Divergent
(strike-slip)
(subduction) (spreading)?
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Early Case for Continental Drift

• Puzzle-piece fit of coastlines of Africa and
  South America has long been known
  

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Early Case for Continental Drift

• In early 1900s, Alfred Wegner noted South
  America, Africa, India, Antarctica, and Australia
  have almost identical rocks and fossils

• Glossopteris (plant), Lystrosaurus and
  Cynognathus (animals) fossils on five continents
  
• Mesosaurus (reptile) in Brazil and South Africa
  only

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Early Case for Continental Drift

• Wegner reassembled continents into the
  supercontinent Pangaea
• Late Paleozoic glaciation patterns on southern
  continents explained reconstruction into
  (Pangaea) Gondwanaland

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Early Case for Continental Drift

• Coal beds of North America and Europe indicate
  Laurasia super continent
  
  
  
  
• Continental Drift hypothesis initially rejected
• Wegener could not come up with viable driving
  force
• continents should not be able to plow through
  sea floor rocks

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The Earth's Magnetic Field Can Give Us Clues
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Paleomagnetism and Continental Drift

• Paleomagnetism magnetic minerals align and dip
  with Earth's internal magnetic field lines
• Steeper dip angles indicate rocks formed closer
  to the magnetic poles
  
  
• Rocks with increasing age point to pole locations
  increasingly far from present magnetic pole
  positions

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Paleomagnetism and Continental Drift Revived

• Apparent polar wander curves for different
  continents indicate plate movement !
• Wegner was right!

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Plate Tectonics and the Scientific Method
How does an idea become a theory ?
When Wegner first suggest the idea of Plate
tectonics what was it ? a) theory b)
hypothesis c) data d) proof What data was
later acquired to test this ? When did plate
tectonics become an accepted theory ?
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The Scientific Method
Step 1 A question is asked Step 2 Gather
data Step 3 Use data to propose a
hypothesis (Has this hypothesis been
tested yet ? NO! Step 4 Make a prediction
(based on hypothesis)? Step 5 Test prediction
(with new data) Step 6 Hypothesis becomes a
theory ! Yeah! (if predictions
and tests are good)
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Banded rocks on the seafloor
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Seafloor Spreading
Magnetic anomalies found on the seafloor parallel
to the spreading axis with alternating
polarity. At the spreading center magma melted
above Curie Temp, cooled, crystalized and
realigned magnetic rocks with the Earth's
magnetic field. The plates move apart, then
began melting again....
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a
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Southern California Coast Continental Shelf
Continental shelf is cut by submarine canyons
that channel sediments to the deep ocean.
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Does the Earth Breath ?
Earthquakes recorded along the Juan de Fuca
spreading ridge indicate that microseismic eq.
events coincide each day with low
tide. Gravitational attraction of the Moon and
Sun on the Earth disturbs ocean water (very
fluid). Which tides give you the best surf ?
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Mountain Belts

• Mountain belts are chains of mountain ranges
  1000s of km long
• Located along the edges of continents
• As mountains grow higher and steeper, erosion
  rates increase (from running water and ice )

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Continents Grow During the Subduction Process
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Continental Cratons

• Ancient mountain belts have eroded nearly flat to
  form the stable core of a continent (craton or
  shield)
  
  
• Every continental plate has a central, old,
  craton.

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Growth of Continents

• Continents grow larger as mountain belts evolve
  along their convergent margins
• New accreted terranes can be added to older
  cratons with each episode of convergence

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