Classroom presentations to accompany Understanding Earth, 3rd edition

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Classroom presentations to accompany
Understanding Earth, 3rd edition

• prepared by
• Peter Copeland and William Dupré
• University of Houston

Chapter 20 Plate Tectonics The Unifying Theory
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Plate TectonicsThe Unifying Theory
Peter W. Sloss, NOAA-NESDIS-NGDC
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Plate Tectonics

• Fundamental concept of geoscience
• Integrates from many branches
• First suggested based on geology and paleontology
• Fully embraced after evidence from geophysics

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Mosaic of Earths Plates
Fig. 20.3
Peter W. Sloss, NOAA-NESDIS-NGDC
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Plates

• Group of rocks all moving in the same direction
• Can have both oceanic and continental crust or
  just one kind.

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Types of plate boundaries

• divergent mid-ocean ridges
• convergent collision zones volcanic
  arcs
• strike-slip San Andreas fault Alpine fault,
  N.Z.

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Divergent plate boundaries

• Usually start within continents
• grow to become ocean basin

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Features of Mid Ocean Ridges

• Central rift valley (width is inversely
  proportional to the rate of spreading)
• Shallow-focus earthquakes
• Almost exclusively basalt

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Continental Rifts

• East Africa, Rio Grande rift
• Beginning of ocean formation (may not get that
  far)
• Rifting often begins at a triple junction (two
  spreading centers get together to form ocean
  basin, one left behind).
• Rock types basalt and sandstone

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Rifting and Seafloor Spreading
Fig. 20.4a
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Rifting and Seafloor Spreading Along the
Mid-Atlantic Ridge
Fig. 20.4a
Peter W. Sloss, NOAA-NESDIS-NGDC
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Inception of Rifting Within a Continent
Fig. 20.4b
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Inception of Rifting Along theEast African Rift
System
Fig. 20.4b
Peter W. Sloss, NOAA-NESDIS-NGDC
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Nile Delta
Gulf of Aqaba
Gulf of Suez
Red Sea
Fig. 20.5a
Earth Satellite Corp.
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The Gulf of California Formed by Rifting of Baja
California from Mainland Mexico
Fig. 20.5b
Worldsat International/Photo Researchers
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Fit of the Continents
Fig. 20.1
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Anomalous Distribution of Fossils
Fig. 20.2
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Convergent boundaries

• New crust created at MORold crust destroyed
  (recycled) at subduction zones (i.e., the Earth
  is not expanding)
• Relative important densities
• continental crust 2.8 g/cm3
• oceanic crust 3.2 g/cm3
• asthenosphere 3.3 g/cm3

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Convergent boundaries

• Three types
• oceanocean Philippines
• oceancontinent Andes
• continentcontinent Himalaya

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OceanOcean

• Island arcs
• Tectonic belts of high seismic ?????
• High heat flow arc of active volcanoes
  (andesitic)
• Bordered by a submarine trench

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OceanOcean Subduction Zone
Fig. 20.6b
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OceanContinent

• Continental arcs
• Active volcanoes (andesite to rhyolite)
• Often accompanied by compression of upper crust

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Ocean-ContinentSubduction Zone
Fig. 20.6a
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ContinentContinent

• In oceancontinent boundaries convergence,
  collision convergence is taken up by subduction
  ( thrusting).
• Continentcontinent boundaries, convergence is
  accommodated by
• Folding (shortening and thickening)
• Strike-slip faulting
• Underthrusting (intracontinental subduction)

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Continent-Continent Collision
Fig. 20.6c
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Himalayas and Tibetan Plateau

• Product of the collision between India and
  Asia.
• Collision began about 45 M yr. ago, continues
  today.
• Before collision, southern Asia looked
  something like the Andes do today.

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Himalayas and Tibetan Plateau

• Models
• Underthrusting
• Distributed shortening
• Strike-slip faulting

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Spreading Centers Offset by Transform Boundary
Fig. 20.7
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Wilson cycle

• Plate tectonics repeats itself rifting, sea-
• floor spreading, subduction, collision,
• rifting,
• Plate tectonics (or something like it)
• seems to have been active since the
• beginning of Earths history.

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Examples of Plate Boundaries
O-C convergent
O-O divergent
C-C divergent
O-O divergent
O-O convergent
O-O divergent
O-C convergent
Fig. 20.8a,b
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OceanContinent Convergent Boundaries
Fig. 20.8c
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ContinentContinent Convergent Boundary
Fig. 20.d
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Rates of plate motion

• Mostly obtained from magnetic
• anomalies on seafloor
• Fast spreading 10 cm/year
• Slow spreading 3 cm/year

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Magnetic Anomalies
Fig. 20.9
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Formation of Magnetic Anomalies
Fig. 20.10
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Age of Seafloor Crust
Fig. 20.11
R. Dietmar Muller, 1997
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Relative Velocity and Direction of Plate Movement
Fig. 20.12
Data from C. Demets, R.G Gordon, D.F. Argus, and
S. Sten, Model Nuvel-1, 1990
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Opening of the Atlantic by Plate Motion
Fig. 20.13
After Phillips Forsyth, 1972
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Rock assemblages and plate tectonics

• Each plate tectonic environment produces a
  distinctive group of rocks.
• By studying the rock record of an area, we can
  understand the tectonic history of the region.

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Idealized Ophiolite Suite
Deep-sea sediments
Pillow basalt
Gabbro
Peridotite
Fig. 20.14
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Model for Forming Oceanic Crust at Mid-ocean
Ridges
Fig. 20.15
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Precambrian Ophiolite Suite
Pillow basalt
Fig. 20.16
M. St. Onge/Geological Survey of Canada
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Volcanic and Nonmarine sediments are deposited in
rift valleys
Fig. 20.17a
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Cooling and subsidence of rifted margin allows
sediments to be deposited
Fig. 20.17b
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Carbonate platform develops
Fig. 20.17c
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Continental margin continues to grow supplied
from erosion of the continent
Fig. 20.17d
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Parts of an OceanOcean Convergent Plate Boundary
Fig. 20.18
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Parts of an OceanContinentConvergent Plate
Boundary
Fig. 20.19
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Continued Subduction
Fig. 20.20a
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Continent Continent Collision
Fig. 20.20b
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Approaching Arc or Microcontinent
Fig. 20.21a
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Collision
Fig. 20.21b
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Accreted Microplate Terrane
Fig. 20.21c
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Microplate terranes Added to Western North
America Over the Past 200 Million Years
Fig. 20.22
After Hutchinson, 1992-1993
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After Hutchinson, 1992-1993
Fig. 20.22
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Tectonic reconstructions

• A variety of evidence traces the motion of
  continents over time

• Paleomagnetism
• Deformational structures
• Environments of deposition
• Fossils
• Distribution of volcanoes

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Assembly of Pangaea
Fig. 20.23
I.W.D. Dalziel, 1995
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Breakup of Pangaea
200 million years ago
Fig. 20.24a
After Dietz Holden, 1970
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Breakup of Pangaea
140 million years ago
Fig. 20.24b
After Dietz Holden, 1970
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Breakup of Pangaea
65 million years ago
Fig. 20.24c
After Dietz Holden, 1970
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Breakup of Pangaea
Today
Fig. 20.24d
After Dietz Holden, 1970
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Driving mechanism of plate tectonics

• Thought to be convection of the mantle.
• Friction at base of the lithosphere transfers
  energy from the asthenosphere to the lithosphere.
• Convection may have overturned asthenosphere 46
  times.

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Other factors

• Trench pull
• Ridge push

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Fig. 20.25a
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Fig. 20.25b
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Fig. 20.25c
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Fig. 20.25d
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Cross Section of Western Canada
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What tectonics theory explains

• Distribution of earthquakes and volcanoes
• Relationship of age and height of mountain
  belts
• Age distribution of oceanic crust
• Magnetic information in rocks

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Questions about plate tectonics

• What do we really know about convection cells
  in the mantle?
• Why are some continents completely surrounded
  by spreading centers?
• Why are tectonics in continental crust and
  oceanic crust so different?

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Examining Deep-sea Drill Cores
Texas AM University
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Age of the Ocean Basins
After map by Sclater Meinke