Plate Tectonics Erin Johnson

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Plate TectonicsErin Johnson
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Brief History

• Since the Theory of Continental Drift was first
  proposed by Alfred Wegener in 1912, it has had a
  long, shaky history.
• The theory was strongly challenged and widely
  ignored, however with the discovery of sea-floor
  spreading in the late 1950's and early 60's, the
  idea was reinvigorated, this time as the Theory
  of Plate Tectonics.

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Tectonic Plate Boundaries

• The theory of Plate tectonics states that Earth's
  outermost layer, the lithosphere, is broken into
  7 large, rigid pieces called plates the African,
  North American, South American, Eurasian,
  Australian, Antarctic, and Pacific plates.
• Several minor plates also exist, including the
  Arabian, Nazca, and Philippines plates.

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Tectonic Plate Boundaries (cont.)

• The plates are all moving in different directions
  and at different speeds (from 2 cm to 10 cm per
  year.)
• The plates are moving around, which means they
  sometimes crash together, pull apart, or
  sideswipe each other.
• The place where the two plates meet is called a
  plate boundary.
• Boundaries have different names depending on how
  the two plates are moving in relationship to each
  other -crashing Convergent Boundaries, pulling
  apart Divergent Boundaries, or sideswiping
  Transform Boundaries.

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Layers of the Earth

• Earth is divided into three chemical layers the
  core, the mantle and the crust.
• The core is divided into two layers a solid
  inner core and a liquid outer core.
• The middle layer of the Earth, the mantle, is
  made of minerals rich in the elements iron,
  magnesium, silicon, and oxygen.

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Layers of the Earth (cont.)

• The crust is rich in the elements oxygen and
  silicon with lesser amounts of aluminum, iron,
  magnesium, calcium, potassium, and sodium.
• There are two types of crust. Oceanic crust is
  made of relatively dense rock called basalt.
  Continental crust is made of lower density rocks,
  such as andesite and granite.
• The outermost layers of the Earth can be divided
  by their physical properties into lithosphere and
  asthenosphere.

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Layers of the Earth (cont.)

• The lithosphere (from the Greek, lithos, stone)
  is the rigid outermost layer made of crust and
  uppermost mantle. The lithosphere is the "plate"
  of the plate tectonic theory.
• The asthenosphere (from the Greek, asthenos,
  devoid of force) is part of the mantle that
  flows, a characteristic called plastic behavior.

• A good example of a solid that flows, or of
  plastic behavior, is the movement of toothpaste
  in a tube.
• The flow of the asthenosphere is part of mantle
  convection, which plays an important role in
  moving lithospheric plates.

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Continental Drift

• Continental drift was originally proposed by
  Alfred Wegener, a German meteorologist, in 1912.
• He used the fit of the continents, the
  distribution of fossils, a similar sequence of
  rocks at numerous locations, ancient climates,
  and the apparent wandering of the Earth's polar
  regions to support his idea.
• He used his observations to hypothesize that all
  of the present-day continents were once part of a
  single super continent called Pangaea.

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Continental Drift - Fossils

• Fossils of the same species were found on several
  different continents.
• Wegener proposed that the species dispersed when
  the continents were connected and later carried
  to their present positions as the continents
  drifted.

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Continental Drift Fossils (cont.)

• For example, Glossopteris, a fern, was found on
  the continents of South America, Africa, India,
  and Australia. If the continents are reassembled
  into Pangaea, the distribution of Glossopteris
  can be accounted for over a much smaller
  contiguous geographic area.
• The distribution of other species can also be
  accounted for by initially spreading across
  Pangaea, followed by the breakup of the super
  continent, and movement of the continents to
  their present positions.

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Continental Drift - Rock Sequences

• Rock sequences in South America, Africa, India,
  Antarctica, and Australia show remarkable
  similarities.
• Wegener showed that the same three layers occur
  at each of these localities.
• The bottom (oldest) layer is called tillite and
  is thought to be a glacial deposit.

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Continental Drift - Rock Sequences (cont.)

• The middle layer is composed of sandstone, shale,
  and coal beds. Glossopteris fossils are in the
  bottom and middle layers.
• The top (youngest) layer is lava flows.
• The same three layers are in the same order in
  areas now separated by great distances.
• Wegener proposed that the rock layers were made
  when all the continents were part of Pangaea.
  Thus, they formed in a smaller contiguous area
  that was later broken and drifted apart.

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Summary

• The plate tectonic model has been rigorously
  tested since the mid-1960s. Because the model has
  been successfully tested by numerous methods, it
  is now called the plate tectonic theory and is
  accepted by almost all geologists.

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Sources

• http//volcano.und.edu/vwdocs/vwlessons/plate_tect
  onics/introduction.html
• http//www.cotf.edu/ete/modules/msese/earthsysflr/
  plates1.html