Tectonic Plates and Their Motions

1
Tectonic Plates and Their Motions

• How did the idea of continental drift lead to
  plate tectonics?
• How do we know that plates move apart at
  divergent plate boundaries?
• How do we know that subduction occurs at
  convergent plate boundaries?
• How do we know that plates slide past one another
  at transform plate boundaries?
• What does the mantle-plume (hot spot) hypothesis
  explain that plate tectonics cannot?
• What forces cause plate motions?

2
How did the idea of continental drift lead to
plate tectonics?

• Alfred Wegener Noted matching continental
  boundaries by shape and geologic features (1912,
  1928). Developed the idea of continental drift -
  continents move over geologic time.
• Good evidence for fitting continents together.
• However, no good mechanism for movements.
• Wegener suggested they move like icebreakers
  through the oceanic crust.
• This idea was dismissed by most of the geologic
  community at the time, in fact Wegener was
  ridiculed!
• However, it started a search for a mechanism that
  would explain the motion of the continents.

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Continental Drift The Geologic Evidence
The plants and animals (fossils), the ice, the
rocks all match.
Alfred Wegner proposed Continental Drift in
1912 because.
but the climates and poles do not.
The shapes of continents match,
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Fig 12.1
Evidence From The Rocks
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Evidence From Fossils
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Evidence From Paleoclimate Studies
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How did the idea of continental drift lead to
plate tectonics?
Wegener saw the continents as a giant jigsaw
puzzle that fit together into a single continent
in the past - Pangea.
Fig 11.2
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How do we know that plates move apart at
divergent plate boundaries?

• Matching the edges of Africa and South America
  requires three conditions
• A plate containing the two existed before the
  Atlantic Ocean existed.
• That plate split and separated the two
  continents, producing the Atlantic Ocean.
• The Atlantic Ocean grows wider with time.
• Plate tectonics predicts that oceanic crust forms
  where continents separate. Is the geology of the
  seafloor consistent with this?

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How do we know that plates move apart at
divergent plate boundaries?
A divergent plate boundary creation of new
oceanic crust and lithosphere from the upwelling
mafic magmas as the ocean basin opens up by
extension. Can we directly observe this
happening anywhere on Earth?
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How do we know that plates move apart at
divergent plate boundaries?
Iceland - a mid-ocean ridge is present above sea
level. The island is widening at the rate of 4
mm/year due to the divergent plate boundary.
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We know the Earths magnetic field has reversed
many times in the past. Stacks of lava flows on
land record this. If the seafloor spreads at
divergent boundaries, the periodic magnetic
reversals should be captured in the cooling mafic
rocks and carried away on both sides and rocks
should get older away from the spreading center.
This was verified by geologists in the 1960s
aboard ships taking magnetic readings.
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The magnetic record of ocean crust The normal
and reversed fields are recorded as a set of
matched strips on either side of the divergent
boundary. Thus, the time periods of Earths
magnetic intervals is recorded in rock - younger
in the center of ocean basins, and older towards
the edges. Animation.
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How do we know that plates move apart at
divergent plate boundaries?
Hundreds of isotopic dates on seafloor basalts
have confirmed this.
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How do we know that plates move apart at
divergent plate boundaries?
Heat flow data measured on the ocean floor
Mid-ocean ridges are hot areas and heat flow is
progressively lower away from the ridge.
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Mid-Ocean Ridges are Seismically Slow Areas Due
to Heat
Mantle tomography provides images similar to
ultrasound.
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Movement of GPS Stations
GPS station positions change as plates move.
Will the position of these stations change
relative to one another? Beginning in the early
1990s GPS stations were set up globally, they
now number in the thousands worldwide. We have
nearly 20 years of measurements of plate motions.
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Continental Rifts Are the Precursors to Ocean
Basins
When tensional forces act on a continent, rift
valleys form.
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Continental Rifts Are the Precursors to Ocean
Basins
With time the rift widens, mafic crust forms, it
is covered with eroding sediment, and sinks into
the asthenosphere due to its high density,
becoming an ocean. Animation.
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Continental Rifts Are the Precursors to Ocean
Basins
Fig 12.19
The East African Rift may eventually form a new
ocean basin.
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How do we know that subduction occurs at
convergent plate boundaries?

• Convergent boundaries are characterized by the
  following (contrast with divergent boundaries)
• Plates move towards one another (rather than
  apart).
• Subduction destroys lithosphere at convergent
  boundaries (rather than creating it).
• Convergent boundaries are very asymmetrical, with
  one plate angled downward beneath another
  (divergent boundaries are generally symmetrical).

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How do we know that subduction occurs at
convergent plate boundaries?
Mountain building and volcanoes are the expected
result of convergence and subduction of one plate
beneath another.
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How do we know that subduction occurs at
convergent plate boundaries?

• Characteristics of subduction zones
• Deep-sea trenches mark the plate boundary.
• Plate collision causes compressional stress, and
  related folding, faulting, and earthquakes.
• Deep earthquakes occur within the subducted
  plate, which retains its brittle character at
  depth because it is much cooler than the mantle
  surrounding it.
• Addition of water to the mantle above the
  downgoing slab causes melting and volcanic
  activity over the subduction zone.

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How do we know that subduction occurs at
convergent plate boundaries?
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How do we know that subduction occurs at
convergent plate boundaries?
Calculated depths to earthquake foci show
progressive increase of depth to the subducted
slab as distance increases from the trench.
on next slide
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How do we know that subduction occurs at
convergent plate boundaries?
Earthquakes in relation to a subducting plate -
The planar area of earthquakes is known as the
Wadati-Benioff zone.
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Mantle tomography hot material in red (slower
seismic wave velocity), cold material in blue
(faster seismic wave velocity). Earthquake
locations shown by white dots. Clearly shows the
subducting oceanic lithosphere (cold) beneath the
Japan volcanic arc system.
Zhao, 2004, Global tomographic images of mantle
plumes and subducting slabs Insight into deep
Earth dynamics, Physics of the Earth and
Planetary Interiors, v.146, p.3-34.
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More detailed image of subduction zone beneath
Japan.
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Types of Convergent Plate Boundaries1)
Oceanic-Continent
The western margin of South America is this type
of boundary. Denser oceanic lithosphere subducts
under the less dense, much older, continental
crust. Density of the continental crust
2.7-2.8 g/cm3, density of oceanic crust 3.1-3.3
g/cm3.
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Types of Convergent Plate Boundaries2)
Oceanic-Oceanic
Most of the western Pacific typifies this type of
boundary, e.g. the area around Japan. Older,
colder, denser ocean crust will subduct beneath
younger, hotter, more bouyant crust.
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Types of Convergent Plate Boundaries2)
Continent-Continent
The Himalayas are this type of boundary, due to
the collision of India with SE Asia.
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How do we know that plates slide past one another
at transform plate boundaries?
Strike-slip faults show obvious and measurable
displacement over time. The rate of motion on the
San Andreas, 5.6 cm/yr, is similar to spreading
ridge velocities.
32
How do we know that plates slide past one another
at transform plate boundaries?
Transform boundaries occur where there are
strike-slip faults between spreading ridges.
Earthquakes occur dominantly along the transform
between spreading ridge segments.
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Plate motions and velocities are directly
measured with GPS
Confirms plate tectonic motions over periods of
just a few years.
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What does the mantle-plume (hot spot) hypothesis
explain that plate tectonics cannot?

• The mantle-plume hypothesis has two main
  components
• Hot spots form where narrow columns (plumes) of
  unusually hot mantle convectively rise from the
  core-mantle boundary, and
• 2) Plume locations are considered to be
  stationary in the mantle.

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What does the mantle-plume (hot spot) hypothesis
explain that plate tectonics cannot?
Fig 12.33
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What forces cause plate motions?

• Plate motions and plumes are caused by convection
  in Earths mantle.
• Downwelling at convergent boundaries
• Controls the speed and direction of plate
  motions.
• Gravity pulls the dense, subducted slab down,
  this is called slab pull.
• Mid-ocean ridges are high areas, plates created
  here slide downhill away from the divergent
  boundary, this is called gravity sliding.
• Convection cells in the mantle may also act to
  drive plate motion via traction forces at the
  base of the lithosphere.

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Forces Which Drive Plate Motion

• Slab Pull at trenches of convergent boundaries.
• Ridge push at divergent boundaries.
• Convection cells in the mantle.