Introduction to Geology GEO101004 Class 3: Plate Tectonics

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Introduction to GeologyGEO-101-004Class 3
Plate Tectonics
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Last Classes Question of the Day

• What was Pangea?
• Answer Pangea was a single superconcontinent
  that existed about 200 million years ago. It
  broke up to form the current continents.

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Question of the Day

• Name the three major plate boundary types and
  define the process that occurs at each

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A scientific revolution begins
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Paleomagnetic reversals recorded in oceanic crust
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The Last Piece of the Puzzle

• The 1960s were a period of chaos for
  geoscientists.
• Some geoscientists even still believed in the
  expanding Earth hypothesis.
• In 1965 J. Tuzo Wilson suggested that large
  faults connected the global mobile belts into a
  continuous network that divides the Earths outer
  shell into several rigid plates.
• Wilson defined three types of plate boundary
• Oceanic ridges plates are moving apart.
• Deep ocean trenches plates are moving together.
• Transform faults Plates slide past one another.
• By the 1968 the concepts of continental drift and
  seafloor spreading were united into the theory
  known as plate tectonics.

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A scientific revolution begins

• Geomagnetic reversal
• Paleomagnetism was the most convincing evidence
  set forth to support the concepts of continental
  drift and seafloor spreading

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Plate tectonics The new paradigm

• Earths major plates
• Associated with Earth's strong, rigid outer layer
• Known as the lithosphere
• Consists of uppermost mantle and overlying crust
• Lithosphere thickness ranges from a few km under
  spreading ridges to 100 km in the deep ocean
  basins to 100-150 km thick under the oceans

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Plate tectonics The new paradigm

• The lithosphere Overlies a weaker region in the
  mantle called the asthenosphere.
• In the upper asthenosphere the pressure/temperatur
  e regim is such that the rocks are near their
  melting point, allowing the asthenosphere to
  detach from the lithosphere.
• The lithospheric plates can thus move over the
  asthenosphere.

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The lithosphere is ____________
02.02

• the central portion of the Earth below 2900 km
  depth
• the weaker portion of the Earth below about 100
  km depth on which the plates move
• the upper 7-40 km of the Earth
• the strong and rigid outer shell of the earth

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Plate tectonics The new paradigm

• Earths major plates
• Seven major lithospheric plates
• Plates are in motion relative to one another and
  continually changing in shape and size
• North America, South American, Pacific, African,
  Eurasian, Australia-Indian, Antarctic
• Largest plate is the Pacific plate
• Several plates include an entire continent plus a
  large area of seafloor

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Earths plates
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Earths plates
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Plate tectonics The new paradigm

• Earths major plates
• Plates move as coherent units
• Some small internal deformation
• Plates move relative to each other at a very slow
  but continuous rate
• About 5 centimeters (2 inches) per year
• Cooler, denser slabs of oceanic lithosphere
  descend into the mantle
• Driven by unequal distribution of heat in the
  Earth that causes convection which in turn
  drives plate tectonics.
• Movement of continents causes earthquakes to
  occur, volcanoes to form.

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Plate tectonics The new paradigm

• Plate boundaries
• Interactions among individual plates occur along
  their boundaries
• Types of plate boundaries
• Divergent plate boundaries (constructive margins)
  
• Convergent plate boundaries (destructive margins)
• Transform fault boundaries (conservative margins)
  

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Plate tectonics The new paradigm

• Plate boundaries
• Each plate is bounded by a combination of the
  three types of boundaries
• New plate boundaries can be created in response
  to changing forces
• The total surface area of Earth does not change
  one plate grows at the expense of another or
  itself
• Africa and Antarctica are getting larger the
  boundaries are moving away from the center of the
  continents
• The Pacific is shrinking

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Divergent plate boundaries

• Most are located along the crests of oceanic
  ridges
• Oceanic ridges and seafloor spreading
• Along well-developed divergent plate boundaries,
  the seafloor is elevated forming oceanic ridges
• Fractures that form as the plates separate are
  filled with molten rock that wells up from the
  hot mantle below.
• These boundaries are occasionally found on
  continents.

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Divergent plate boundaries

• The crest of the ridge is typically 2-3 km higher
  than the surrounding seafloor
• Hotter rock is less dense
• As it cools it becomes more dense and therefore
  subsides
• Takes about 80 million to cool completely
• The ridges range in width from 1000-4000 km.
• Many ridges have a central rift valley
• Spreading rates are typically about 5 cm per year
  but do vary

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Divergent plate boundaries

• Spreading rates and ridge topography
• Ridge systems exhibit topographic differences
• These differences are controlled by spreading
  rates and/or the thermal regime

From http//ocean-ridge.ldeo.columbia.edu/general/
html/home.html
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From http//ocean-ridge.ldeo.columbia.edu/Other_st
uff/PAR_Movie/parmovie.html
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Divergent plate boundaries

• Continental rifting
• Splits landmasses into two or more smaller
  segments along a continental rift
• Examples include the East African rift valleys
  and the Rhine Valley in northern Europe
• Produced by extensional forces acting on
  lithospheric plates
• If the continent is stretched enough seafloor
  spreading can start examples of where this is
  happening today are the Red Sea, Gulf of Aden,
  Gulf of California, and the Woodlark Basin.

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Continental rifting
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The modern-day Red Sea is explained by plate
tectonics theory because it is ____________.
02.02

• a tiny remnant of a once immense ocean that was
  closed as Africa moved Asia
• the site of a transform fault along which Arabia
  is moving away from Africa
• a rift zone that may eventually open into a major
  ocean if Arabia and Africa continue to separate
• a rare example of a two continent subduction zone
  where the African continental plate is sinking
  under the Arabian continental plate

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Convergent plate boundaries

• The Earth is not growing larger to balance the
  addition of new lithosphere, older parts must be
  subducted at destructive plate boundaries
• Older portions of oceanic plates are returned to
  the mantle in these destructive plate margins
• As the leading edge of one plate approaches
  another, one plate is bent down allowing it to
  slide under the other
• Surface expression of the descending plate is an
  ocean trench
• Trenches may be thousands of kilometers long,
  50-100 km wide and 8-10 km deep
• Also called subduction zones
• Average angle of subduction 45?

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Convergent plate boundaries

• Types of convergent boundaries
• Oceanic-continental convergence
• Denser oceanic slab sinks into the asthenosphere
• Along the descending plate partial melting of
  mantle rock generates magma
• Resulting volcanic mountain chain is called a
  continental volcanic arc (Andes and Cascades)
• Volcanoes sit about 100 km above the subducting
  slab
• How does cool oceanic lithosphere cause mantle
  rocks to melt? Voltiles (water) lower the melting
  temperature of the asthenosphere.

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Convergent plate boundaries

• Types of convergent boundaries
• Oceanic-oceanic convergence
• When two oceanic slabs converge, one descends
  beneath the other
• Volcanoes grow up from the ocean floor
• If the volcanoes emerge as islands, a volcanic
  island arc is formed (Japan, Aleutian islands,
  Tonga islands)

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Convergent plate boundaries

• Types of convergent boundaries
• Continental-continental convergence
• Continued subduction can bring two continents
  together
• Less dense, buoyant continental lithosphere does
  not subduct
• Resulting collision between two continental
  blocks produces mountains (Himalayas, Alps,
  Appalachians)

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Largest Earthquakes in the World Since 1900
From http//neic.usgs.gov/neis/eqlists/10maps_worl
d.html
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Transform fault boundaries

• Plates slide past one another and no new
  lithosphere is created or destroyed
• In 1965 J. Tuzo Wilson proposed that transform
  faults connect the active belts (convergent,
  divergent and transform boundaries) into a
  continuous network that divides the Earth into
  plates
• Transform faults
• Most join two segments of a mid-ocean ridge along
  breaks in the oceanic crust known as fracture
  zones approximately 100 km spacing
• A few (the San Andreas fault and the Alpine fault
  of New Zealand) cut through continental crust

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