Plate Tectonics

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Plate Tectonics

• Prepared by Betsy Conklin for
• Dr. Isiorho

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Plate Tectonics

• plate tectonics the idea that the earths
  surface is divided into a few large, thick plates
  that move slowly and change in size
• continental drift the idea that continents move
  freely over the earths surface, changing their
  positions relative to one another
• sea-floor spreading a hypothesis that the sea
  floor forms at the crest of the mid-oceanic
  ridge, then moves horizontally away from the
  ridge crest toward an oceanic trench

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The Early Case for Continental Drift

• continents can be made to fit together like
  pieces of a picture puzzle
• Alfred Wegener purposed that the continents were
  originally one giant supercontinent which he
  called Pangaea. Pangaea then split into two parts
  - Laurasia which is now North America and
  Eurasia, and Gondwanaland which is now the
  southern-hemisphere continents and India
• polar wandering an apparent movement of the
  earths poles
• the distribution of fossils on various continents

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Skepticism about Continental Drift

• fossil plants could have been spread from one
  continent to another by winds or ocean current
• polar wandering might have been caused by moving
  poles rather than by moving continents

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Sea-Floor Spreading

• subduction the sliding of the sea floor beneath
  a continent or island arc
• convection a circulation pattern driven by the
  rising of hot material and\or the sinking of cold
  material

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Plates and Plate Motion

• plate a large, mobile slab of rock that is part
  of the earths surface
• lithosphere the relatively rigid outer shell of
  the earth of which the plates are a part
• asthenosphere a zone of low seismic-wave
  velocity that behaves plastically because of
  increased temperature and pressure
• transform fault the portion of a fracture zone
  between two offset portions of a ridge crest

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Divergent Plate Boundaries

• divergent plate boundary a boundary between
  plates that are moving apart

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Transform Plate Boundaries

• transform plate boundary a boundary between
  plates that are moving horizontally past one
  another

Transform boundaries between two ridges
Transform boundaries between a ridge and a
trench
Transform boundaries between two trenches
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Convergent Plate Boundaries

• convergent plate boundary a boundary between
  plates that are moving toward each other
• ocean-ocean convergence when two plates capped
  by sea floor converge and one plate subducts
  under the other
• island arc a curved line of volcanoes that form
  a string of islands parallel to the oceanic trench

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Convergent Plate Boundaries (cont.)

• ocean-continent convergence when a plate
  captured by oceanic crust is subducted under the
  continental lithosphere and an accretionary wedge
  and forearc basin form an active continental
  margin between the trench and the continent
• magmatic arc a broad term used both for island
  arcs at sea and for belts of igneous activity on
  the edges of continents

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Convergent Plate Boundaries (cont.)

• continent-continent convergence when two
  continents approach each other an collide

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What causes Plate Tectonics

• mid-oceanic ridge crests are hot and elevated,
  while trenches are cold and deep
• ridge crests have tensional cracks
• the leading edges of some plates are subducting
  sea floor, while the leading edges of other
  plates are continents (which cannot subduct)

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Mantle Plumes and Hot Spots

• mantle plumes narrow columns of hot mantle rock
  that rise through the mantle, much like smoke
  rising from a chimney

Continental breakup caused by a mantle plume. (A)
A dome forms over a mantle plume rising beneath
a continent. (B) Three radial rifts develop due
to outward radial flow from the top of the
mantle plume. (C) Continent separates into two
pieces along two of the three rifts, with new
ocean floor forming between the diverging
continents. The third rift becomes an inactive
failed rift (or aulacogen) filled with
continent sediment.
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Pictures

• All pictures used in this power point
  presentation were taken from the following
• Carlson, Diane H., David McGeary and Charles C.
  Plummer. Physical Geology Updated Eighth
  Edition. New York City, McGraw-Hill Higher
  Education, 2001.