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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. Plate Tectonics: The Unifying Theory. Peter W. Sloss, NOAA-NESDIS-NGDC. - PowerPoint PPT Presentation
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Classroom presentations Classroom presentations to accompany to accompany
Understanding EarthUnderstanding Earth, 3rd edition, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 20Chapter 20Plate Tectonics: The Unifying Theory
Plate Tectonics:The Unifying Theory
Peter W. Sloss, NOAA-NESDIS-NGDC
Plate TectonicsPlate Tectonics
• Fundamental concept of geoscience
• Integrates from many branches
• First suggested based on geology and paleontology
• Fully embraced after evidence from geophysics
Fig. 20.3
Mosaic of Earth’s Plates
Peter W. Sloss, NOAA-NESDIS-NGDC
PlatesPlates
• Group of rocks all moving in the same direction
• Can have both oceanic and continental crust or just one kind.
Types of plate boundariesTypes of plate boundaries
• divergent: mid-ocean ridges
• convergent: collision zones volcanic arcs
• strike-slip: San Andreas fault Alpine fault, N.Z.
Divergent plate boundariesDivergent plate boundaries
Usually start within continents—
grow to become ocean basin
Features of Mid Ocean RidgesFeatures of Mid Ocean Ridges
• Central rift valley (width is inversely proportional to the rate of spreading)
• Shallow-focus earthquakes
• Almost exclusively basalt
Continental Continental RiftsRifts
• 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
Rifting and Seafloor Spreading
Fig. 20.4a
Fig. 20.4a
Rifting and Seafloor
Spreading Along the
Mid-Atlantic Ridge
Peter W. Sloss, NOAA-NESDIS-NGDC
Inception of Rifting Within a Continent
Fig. 20.4b
Fig. 20.4b
Inception of Rifting
Along theEast African Rift System
Peter W. Sloss, NOAA-NESDIS-NGDC
Fig. 20.5a
Nile Delta
Gulf ofSuez
Gulf of‘Aqaba
Red Sea
Earth Satellite Corp.
Fig. 20.5b
The Gulf of California Formed by
Rifting of Baja California
from Mainland Mexico
Worldsat International/Photo Researchers
Fig. 20.1
“Fit” of the Continents
Anomalous Distribution of Fossils
Fig. 20.2
Convergent Convergent boundariesboundaries• New crust created at MOR—old 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
Convergent Convergent boundariesboundaries
Three types:
ocean–ocean Philippines
ocean–continent Andes
continent–continent Himalaya
Ocean–OceanOcean–Ocean
Island arcs:
• Tectonic belts of high seismic ?????
• High heat flow arc of active volcanoes (andesitic)
• Bordered by a submarine trench
Fig. 20.6b
Ocean–Ocean Subduction Zone
Ocean–ContinentOcean–Continent
Continental arcs:
• Active volcanoes (andesite to rhyolite)
• Often accompanied by compression of upper crust
Fig. 20.6a
Ocean-ContinentSubduction Zone
Continent–Continent–ContinentContinent• In ocean–continent boundaries
convergence, collision convergence is taken up by subduction (± thrusting).
• Continent–continent boundaries, convergence is accommodated by• Folding (shortening and thickening)
• Strike-slip faulting
• Underthrusting (intracontinental subduction)
Fig. 20.6c
Continent-Continent Collision
Himalayas and Tibetan PlateauHimalayas 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.
Himalayas and Tibetan PlateauHimalayas and Tibetan Plateau
Models
• Underthrusting
• Distributed shortening
•Strike-slip faulting
Spreading Centers Offset by Transform Boundary
Fig. 20.7
Wilson cycleWilson 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 Earth’s history.
Examples of Plate Boundaries
Fig. 20.8a,b
O-Oconvergent
O-Odivergent
O-Cconvergent
O-Cconvergent
O-Odivergent
C-Cdivergent
O-Odivergent
Ocean–Continent Convergent Boundaries
Fig. 20.8c
Continent–Continent Convergent Boundary
Fig. 20.d
Rates of plate motionRates of plate motion
Mostly obtained from magnetic
anomalies on seafloor
Fast spreadingFast spreading: 10 cm/year
Slow spreadingSlow spreading: 3 cm/year
Fig. 20.9
Magnetic Anomalies
Fig. 20.10
Formation of
Magnetic Anomalies
Fig. 20.11
Age of Seafloor Crust
R. Dietmar Muller, 1997
Relative Velocity and Direction of Plate Movement
Fig. 20.12Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990
Fig. 20.13
Opening of the
Atlantic by Plate Motion
After Phillips & Forsyth, 1972
Rock assemblages and Rock assemblages and plate tectonicsplate 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.
Fig. 20.14
Idealized Ophiolite Suite
Peridotite
Gabbro
Pillow basalt
Deep-sea sediments
Model for Forming Oceanic Crust at Mid-ocean Ridges
Fig. 20.15
Fig. 20.16
Precambrian Ophiolite Suite
Pillow basalt
M. St. Onge/Geological Survey of Canada
Volcanic and Nonmarine sediments are deposited in rift valleys
Fig. 20.17a
Cooling and subsidence of rifted margin allows sediments to be
deposited
Fig. 20.17b
Carbonate platform develops
Fig. 20.17c
Continental margin continues to grow supplied from erosion of the
continent
Fig. 20.17d
Fig. 20.18
Parts of an Ocean–Ocean Convergent Plate Boundary
Fig. 20.19
Parts of an Ocean–ContinentConvergent Plate Boundary
Continued Subduction
Fig. 20.20a
Fig. 20.20b
Continent– Continent Collision
Approaching Arc or Microcontinent
Fig. 20.21a
Collision
Fig. 20.21b
Accreted Microplate Terrane
Fig. 20.21c
Fig. 20.22
Microplate terranes Added to Western
North America Over the Past
200 Million Years
After Hutchinson, 1992-1993
Fig. 20.22
After Hutchinson, 1992-1993
Tectonic reconstructionsTectonic reconstructions
A variety of evidence traces the motion of continents over time:
• Paleomagnetism
• Deformational structures
• Environments of deposition
• Fossils
• Distribution of volcanoes
Fig. 20.23
Assembly of Pangaea
I.W.D. Dalziel, 1995
Fig. 20.24a
Breakup of Pangaea
200 million years ago
After Dietz & Holden, 1970
Fig. 20.24b
Breakup of Pangaea
140 million years ago
After Dietz & Holden, 1970
Fig. 20.24c
Breakup of Pangaea
65 million years ago
After Dietz & Holden, 1970
Fig. 20.24d
Breakup of Pangaea
Today
After Dietz & Holden, 1970
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 4–6 times.
Other factors
• Trench pull
• Ridge push
Fig. 20.25a
Fig. 20.25b
Fig. 20.25c
Fig. 20.25d
Cross Section of Western Canada
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
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?
Examining Deep-sea Drill Cores
Texas A&M University
After map by Sclater & Meinke
Age of the Ocean BasinsAge of the Ocean Basins