Download Plate Tectonics: The Unifying Theory

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
Plate Tectonics
• Fundamental concept of
geoscience
• Integrates from many branches
• First suggested based on geology
and paleontology
• Fully embraced after evidence from
geophysics
Mosaic of Earth’s Plates
Peter W. Sloss, NOAA-NESDIS-NGDC
Fig. 20.3
Plates
• Group of rocks all moving in the
same direction
• Can have both oceanic and
continental crust or just one kind.
Types of plate boundaries
• divergent:
mid-ocean ridges
• convergent: collision zones
volcanic arcs
• strike-slip:
San Andreas fault
Alpine fault, N.Z.
Divergent plate boundaries
Usually start within continents—
grow to become ocean basin
Features of Mid Ocean Ridges
• Central rift valley (width is
inversely proportional to the rate
of spreading)
• Shallow-focus earthquakes
• Almost exclusively basalt
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
Rifting and Seafloor Spreading
Fig. 20.4a
Rifting and
Seafloor
Spreading
Along the
Mid-Atlantic
Ridge
Peter W. Sloss, NOAA-NESDIS-NGDC
Fig. 20.4a
Inception of Rifting Within a
Continent
Fig. 20.4b
Inception of
Rifting
Along the
East African
Rift System
Peter W. Sloss, NOAA-NESDIS-NGDC
Fig. 20.4b
Nile Delta
Gulf of
Suez
Gulf of
‘Aqaba
Red Sea
Earth Satellite Corp.
Fig. 20.5a
The Gulf of
California
Formed by
Rifting of Baja
California
from Mainland
Mexico
Worldsat International/Photo Researchers
Fig. 20.5b
“Fit” of the
Continents
Fig. 20.1
Anomalous Distribution of Fossils
Fig. 20.2
Convergent boundaries
• 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 boundaries
Three types:
ocean–ocean
Philippines
ocean–continent
Andes
continent–continent
Himalaya
Ocean–Ocean
Island arcs:
• Tectonic belts of high seismic
?????
• High heat flow arc of active
volcanoes (andesitic)
• Bordered by a submarine trench
Ocean–Ocean
Subduction Zone
Fig. 20.6b
Ocean–Continent
Continental arcs:
• Active volcanoes (andesite to
rhyolite)
• Often accompanied by
compression of upper crust
Ocean-Continent
Subduction Zone
Fig. 20.6a
Continent–Continent
• 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
Continent-Continent
Collision
Fig. 20.6c
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.
Himalayas and Tibetan Plateau
Models
• Underthrusting
• Distributed shortening
•Strike-slip faulting
Spreading Centers Offset by
Transform Boundary
Fig. 20.7
Wilson cycle
Plate tectonics repeats itself: rifting,
seafloor 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
O-C
convergent
O-O
convergent
O-O
divergent
C-C
divergent
O-O
divergent
O-O
divergent
O-C
convergent
Fig. 20.8a,b
Ocean–Continent Convergent
Boundaries
Fig. 20.8c
Continent–Continent Convergent
Boundary
Fig. 20.d
Rates of plate motion
Mostly obtained from magnetic
anomalies on seafloor
Fast spreading: 10 cm/year
Slow spreading: 3 cm/year
Magnetic
Anomalies
Fig. 20.9
Formation
of
Magnetic
Anomalies
Fig. 20.10
Age of Seafloor Crust
R. Dietmar Muller, 1997
Fig. 20.11
Relative Velocity and Direction
of Plate Movement
Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990
Fig. 20.12
Opening
of the
Atlantic
by Plate
Motion
After Phillips & Forsyth, 1972
Fig. 20.13
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.
Idealized Ophiolite Suite
Deep-sea sediments
Pillow basalt
Gabbro
Peridotite
Fig. 20.14
Model for Forming Oceanic Crust
at Mid-ocean Ridges
Fig. 20.15
Precambrian Ophiolite Suite
Pillow basalt
M. St. Onge/Geological Survey of Canada
Fig. 20.16
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
Parts of an Ocean–Ocean
Convergent Plate Boundary
Fig. 20.18
Parts of an Ocean–Continent
Convergent Plate Boundary
Fig. 20.19
Continued Subduction
Fig. 20.20a
Continent–
Continent
Collision
Fig. 20.20b
Approaching Arc or Microcontinent
Fig. 20.21a
Collision
Fig. 20.21b
Accreted Microplate Terrane
Fig. 20.21c
Microplate terranes
Added to Western
North America Over
the Past
200 Million Years
After Hutchinson, 1992-1993
Fig. 20.22
After Hutchinson, 1992-1993
Fig. 20.22
Tectonic reconstructions
A variety of evidence traces the motion
of continents over time:
• Paleomagnetism
• Deformational structures
• Environments of deposition
• Fossils
• Distribution of volcanoes
Assembly of Pangaea
I.W.D. Dalziel, 1995
Fig. 20.23
Breakup of Pangaea
200 million years ago
After Dietz & Holden, 1970
Fig. 20.24a
Breakup of Pangaea
140 million years ago
After Dietz & Holden, 1970
Fig. 20.24b
Breakup of Pangaea
65 million years ago
After Dietz & Holden, 1970
Fig. 20.24c
Breakup of Pangaea
Today
After Dietz & Holden, 1970
Fig. 20.24d
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
Age of the Ocean Basins
After map by Sclater & Meinke