Tectonic Plate Theory

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Tectonic Plate Theory

• Understanding Basic Principles of Earth Science
  Related to Geology

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Earths Crust

• Earths crust is the outermost layer.
• The rigid crust floats on the molten part of
  the mantle called the asthenosphere.

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Pangea

• Pangea was the supercontinent from which all
  continents have split.
• In 1915 Wegener proposed Earths continents
  looked this way about 225 million years ago.
• The theory of continental drift was used to
  explain the current positions of the continents.
• Wegener could not come up with a mechanism for
  continental drift.

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Evidence that Pangea Existed

• Fit of the continents
• Fossil similarities
• Rock similarities
• Mountain chains
• Paleoclimatic evidence
• Paleomagnetism
• Theory of Plate Tectonics

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Fit of the Continents

• North and South America match almost perfectly
  with Africa and Europe

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Fossil Similarities

• Mesosaurus, a reptile similar to an alligator
  once lived in the shallow waters of both South
  America and Africa.

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Rock Similarities

• Rocks of same age are found in locations of South
  America and Africa that were once joined in
  Pangea.

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Termination of Mountain Chains

• The map shows location of Pangean mountain chains
  (dark blue) that correspond with current mountain
  chains.

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Paleoclimatic Evidence

• Glacial deposits at the equator
• Coral reefs in Antarctica
• Therefore the landmasses must have been in
  different locations in the past.

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Magnetism

• Earth is a bar magnet
• Magnetic north and south poles
• These are not the same location as the geographic
  north and south poles
• Exact location varies year-to-year

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Paleomagnetism

• This is magnetism frozen in the rock at the
  time it was formed.
• Geologists can determine pole reversals by
  studying rock all over the globe.

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Sea Floor Spreading

• Surveys of the ocean basins showed a system of
  ridges and trenches.
• This led geologists to come up with the idea of
  Sea Floor Spreading.
• Symmetrical magnetic stripes were discovered on
  either side of the Atlantic Mid-ocean ridge.

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Sea Floor Spreading
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Magnetic Reversals along a Mid-Ocean Ridge
(stripes)
Youngest, newest rock is along the ridge.
Oldest rock is farthest from ridge.
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Theory of Plate Tectonics

• A rigid piece of lithosphere (outer crust) is
  floating on a partially plastic (molten, soft)
  asthenosphere
• Seven Major Plates
• 3 types of Plate Boundaries

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Plate Boundaries

• Divergent-Spreading Center- Ocean ridges and
  seafloor spreading (Atlantic Ocean)
• Convergent-Creates trenches and island arcs
  (Pacific Ocean)
• Transform-Plates move past one another (San
  Andreas Fault in California)

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Divergent Boundaries

• Magma from deep within Earth forces its way
  between two plates, pushing them apart.
• The magma cools and forms new seafloor.
• More magma emerges and pushes apart the cooled
  rock.
• Seafloor Spreading/mid-oceanic ridges

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Rift Valley

• A rift valley forms when a continent is slowing
  splitting apart along a divergent boundary.
• The African Rift Valley is an example.

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Locations of Seafloor Spreading
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Convergent

• Plates move together
• Oceanic crust slides under continental crust.
• Subducting crust slides under.
• Andes Mountains Chain in South America is an
  example.
• Mountains formed from volcanoes along the
  convergent boundary.

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Convergent Boundaries

• The Pacific Ocean by several continental plates.
• The ocean basin is sliding under the continental
  plates along the plate boundaries.
• As a result, the Pacific Ocean is growing smaller.

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Why the Pacific is Shrinking and the Atlantic is
Growing
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Hawaiian Islands are an island arc.
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More Examples of Convergent Boundaries

• Japanese Islands (Pacific Ocean)
• Cascade Mountains (Northwestern U.S.A.)
• Himalaya Mountains (Nepal-Asia)

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Transform Plate Boundary

• Plates move past one another along strike-slip
  faults

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What Drives the Tectonic Plates??

• Convection Cells
• Hot Spots