Plate Tectonics

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Plate Tectonics

• Chapter 4

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Section 4.1 Objectives
1. Explain Wegeners hypothesis of continental
drift
2. List evidence for Wegeners hypothesis of
continental drift
3. Describe seafloor spreading
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4.1 Continental Drift

• In 1912, Alfred Wegener proposed his continental
  drift hypothesis.
• It stated that the continents once formed part of
  a single landmass, which he named Pangaea, which
  means all lands.
• Wegener thought that Pangaea began breaking up
  into smaller continents about 200 million years
  ago, and drifted to their present locations.

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• EVIDENCE
• Fit of the shapes of the continents
• Fossils of plants and animals found on different
  continents that are separated by oceans

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Evidence 1 Geologic Fit of the continents

• 1. Age and type of rocks match in West Africa
  and Brazil

• 2. The Appalachian mountains match up with the
  mountain range that runs through Scotland and
  North Europe.

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Mesosaurus fossils date back to 270 million years
ago in South America and West Africa. Did it swim
that far? There is not evidence of a land bridge.
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EVIDENCE 3. Glacial grooves show that glaciers
looked like they moved from sea to land
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EVIDENCE 4. Climatic patterns shown by rock
layers Some rock types only form in certain
climates, for example coal, which forms in warm,
very wet (rainy) environments. If coal is found
in a place that is not warm and rainy, then
either the climate has changed or the rock has
moved.
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The Dance of the Continents
GOgt
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• The evidence that Alfred was looking for was
  found On the Ocean floor!

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Seafloor Spreading Hypothesis

• In 1947, a group of scientists set out to map the
  Mid-Atlantic Ridge. This is part of an 80,000
  km.-long system of mid-ocean ridges.
• The oldest rocks found on the sea floor were less
  than 175 million years old. The oldest
  continental rocks are about 4 billion years old.
  WHY IS THIS INFORMATION SURPRISING?

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Seafloor Spreading Hypothesis

1. The valley at the center of the MOR is a rift (a
   long fracture in the crust)
2. Magma is coming up from inside the earth into the
   rift.
3. This magma can find space in the crust because
   the ocean floor is moving away from both sides of
   the MOR

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PALEOMAGNETIc REVERSALS

• As magma solidifies to form rock, the magnetic
  fields of iron-rich minerals align with the
  earths magnetic field, just like a compass.
• Scientists have discovered that this is not
  always the case. they have found minerals that
  point south.

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PALEOMAGNETIc REVERSALS

• Throughout the earths history, the magnetic
  field has reversed itself many times.
• Such reversals have come at irregular intervals,
  averaging about every 300,000 years the last one
  was 780,000 years ago. Are we overdue for
  another? No one knows

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PALEOMAGNETISM of THE OCEAN FLOOR

1. Scientists discovered magnetic patterns locked
   into the rocks of the ocean floor
2. These patterns showed alternating bands of normal
   and reversed magnetism
3. As molten rock rises from the rift in an MOR, it
   quickly cools and hardens and its magnetic
   orientation becomes fixed.

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Paleomagnetism of the ocean floor
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Sea Floor Spreading
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Section 4.2 Objectives
1. Summarize the theory of plate tectonics.
2. Compare the characteristic geologic activities
that occur along the three types of plate
boundaries.
3. Explain the possible role of convection
currents in plate movement.
4. Summarize the theory of microplate terranes.
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Plate Tectonic Theory Summary

1. Earths outer layer is broken into about 30
   sections called plates.
2. The plates are composed of uppermost mantle and
   either continental crust or oceanic crust
   (lithosphere).
3. The plates ride on the asthenosphere.
4. The plates move because of convection currents
   flowing in the mantle below the plates.

.
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Earths Tectonic Plates
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Types of Crust

• 1. oceanic crust 2. continental crust

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Lithosphere/Asthenosphere

• The oceanic and continental crust and the rigid
  upper mantle make up the lithosphere.
• The lithosphere forms a thin outer shell that
  lies above the plastic rock of the asthenosphere.

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Divergent Boundary

1. The Red Sea occupies an area that contains a
   divergent boundary.
2. Seafloor is pushing the African and Arabian
   plates away from each other.
3. A rift valley runs down the center of the sea.

Arabian Plate
African Plate
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Divergent Boundary
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Convergent Boundary

• convergent boundary the direct collision of one
  plate with another.
• Three types of convergent boundaries
• ocean to ocean
• continent to ocean
• continent to continent

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Convergent Boundary

• 3. Ocean To Ocean oceanic plate subducting
  under another oceanic plate

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Convergent Boundary
OCEAN TO OCEAN
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Convergent Boundary

• Continent To Ocean
• when the oceanic crust slides beneath continental
  crust.
• this is a process called subduction.
• subduction zones create deep trenches as the
  plate is subducted (tucked under) heat is
  released, the mantle partially melts, and magma
  rises to the surface VOLCANOES

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Convergent Boundary
1. Continent to Continent Neither plate is
subducted because they have the same density.
Instead the land is crumpled and uplifted
creating MOUNTAIN RANGES!
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Transform Boundary

• Transform boundary two plates are grinding
  side-by-side past each other

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Transform Boundary These boundaries have sudden
spurts of activity and then periods of no motion
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Plate Boundaries
How many plates? Convergent boundaries? Divergent
boundaries? Transform boundary?
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2
1
3
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Causes Of Plate Motion

• Scientists think that the movement of
  lithospheric plates is due to convection.

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Microplate Terranes

• Theory of Microplate Terranes continents are
  actually a patchwork of crustal blocks, called
  terranes. Each block has its own distinct
  geological history.
• Terranes are regions that are bounded by faults
  and have rocks of different character (age, type,
  fossils) than in surrounding regions.
• Terranes form in another part of the world and
  are moved by plate motion to their present
  locations

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Microplate Terranes of Virginia