PLATE TECTONICS

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PLATE TECTONICS
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• Alfred Wegner hypothesized that the continents
  were once joined into a single continent called
  Pangaea (meaning all land). He further
  suggested that this supercontinent later broke
  into smaller pieces and drifted to their present
  positions.

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• He referred to his hypothesis as continental
  drift. To support his hypotheses, Wegner and
  others collected evidence from around the world.
  Some of the evidence to support continental drift
  include

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• The coastlines of some of present day continents
  seem to fit together like a jigsaw puzzle

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• Identical fossil organisms are found on
  continents separated by thousands of miles of
  ocean

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• Rock types and mountain ranges match up when the
  continent puzzle is put together
• Glacial deposits are found in areas which are now
  located in the tropics

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• The one thing that Wegner could not do was
  explain a mechanism that could move continents!

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• Scientists now have seismic data to support
  Wegners hypotheses. Todays theory is referred
  to as plate tectonics.

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• Plate tectonics describes plate motion and the
  effects of the motion. Plate tectonics says that
  Earths outer shell is made of about 20 plates.
  The plates are rigid and in constant motion
  relative to one another.

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• The largest plates consist of two different kinds
  of crust oceanic and continental. Many of the
  smaller plates contain only oceanic crust.

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• Continental crust is thicker and less dense than
  oceanic crust. Continental crust is composed
  mostly of the igneous rock granite, while ocean
  crust is mostly the igneous rock basalt.

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• The plates sit on the hot asthenosphere. While
  the rock in the asthenosphere is solid, it is
  less rigid than the plates, and therefore moves.
  The movement within the asthenosphere is caused
  by heat from the core of the earth.

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• The heat rises slowly through the earth creating
  convection cells within the asthenopsphere.
  Scientists do not fully understand the forces
  that move the plates.

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• The most accepted theory is the idea that the
  material carried by convection cells in the
  asthenosphere drag the plates along.

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• The interaction between plates results in
  different effects on the earths surface. One of
  the effects of plate motion is the Ring of Fire.
  The Ring of Fire is an area of volcanoes that
  form a ring around the Pacific Plate.

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• Along these areas, plates are colliding and ocean
  crust is subducted, or pulled into the mantle,
  and melted. This process destroys plate
  material.

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• Along the Mid-Atlantic ridge, molten material
  wells up between plates that are moving apart.
  This magma cools and hardens to form new crust.

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• By destroying and creating plate material, plate
  tectonics constantly recycles Earths materials.

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• Each plate moves as a distinct unit. The borders
  between plates are called plate boundaries.

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Divergent - plates move away from one another

• New crust material is created
• Shallow earthquakes along the plate boundary

• Ocean/ocean crust

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• Landforms
• Creates Mid-ocean Ridge
• Rift Valleys
• Places
• Mid-Atlantic Ridge, Atlantic Ocean

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Convergent plates move toward one another
Ocean / Continental
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• Ocean crust is pulled beneath the continental
  crust into the mantle and melts
• Old crust is destroyed
• Tight lines of volcanoes form on the continental
  crust
• Deep Earthquakes on one side of the plate
  boundary and shallow on the

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• Landforms
• Deep Ocean Trenches
• High Volcanic Mountains

• Places
• Cascade Mountain Range, Oregon and Washington
• Mt. Saint Helens

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Ocean / Ocean Crust

• One plate descends beneath the other (subduction)
• Tight lines of Volcanoes on the ocean floor
• Earthquakes

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• Landforms
• Island and Volcanic Arcs
• Ocean Trench
• Places
• Mariana Trench (Pacific Ocean)
• Japan

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Continental / Continental Crust

• Neither plate is subducted
• Plates crash into one another
• No volcanic activity

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• Landforms
• Mountain Ranges
• Places
• Himalaya Mountains, India

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Transformplates grind past one another

• Shallow Earthquakes
• Some volcanoes, but spread out

Mostly Ocean Crust Some Continental
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• Landforms
• Faults (breaks in rock)
• Places
• San Andreas Fault, California