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Igneous and Metamorphic Rock
Forming Minerals
Department of Geology
Mr. Victor Tibane
1
SGM 210_2014
Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 2:
PLATE TECTONICS:
The Unifying Theory
© 2011 by W. H. Freeman and Company
Chapter 2:
Plate Tectonics:
The Unifying
Theory
About Plate Tectonics
• It is the movement of plates and the forces
acting on them.
• It explains the distribution of volcanoes,
earthquakes, folded mountain chains, rock
assemblages, and seafloor structures.
• The forces that drive plate motions arise from
the mantle convection system.
1. Evolution of the Theory
Continental drift:
“jig-saw puzzle” fit of continents
Alfred Wegener 1880-1930
German meteorologist
1. Evolution of the Theory
Continental drift:
similarity of rock
assemblages
and
ages across oceans
1. Evolution of the Theory
Continental drift:
distribution of certain fossils
Alexandre du Toit 1878-1948
South African Geologist
1. Evolution of the Theory
Seafloor spreading:
geological activity in
mid-ocean ridges
1. Evolution of the Theory
Seafloor
spreading:
new crust
formed there
1. Divergent Boundaries
(a) Oceanic plate separation
rifting, volcanoes,
and earthquakes
Mid- Atlantic
Ridge
1. Divergent Boundaries
(b) Continental plate separation
rift valleys, volcanoes,
and earthquakes
East African
Rift Valley
2. Convergent Boundaries
(a) Ocean-ocean convergence
oceanic trench, volcanic island arc, and
deep earthquakes
Mariana Islands
Marianas Trench
2. Convergent Boundaries
(b) Ocean-continent convergence
volcanic mountain chain,
folded mountains, and
deep earthquakes
Andes
Mountains
Peru-Chile Trench
South
American
Plate
2. Convergent Boundaries
(c) Continent-continent convergence
Himalaya
Mountains
crustal thickening, folded
mountains, and earthquakes
Tibetan
Plateau
subduction
Eurasian
Plate
3. Transform-Fault Boundaries
(a) Mid-ocean ridge transform fault
lateral (transform) faults and earthquakes
3. Transform-Fault Boundaries
(b) Continental transform fault
lateral (transform) fault and earthquakes
3. Rates and History of Plate Motion
magnetic time scale developed
3. Rates and History of Plate Motion
example area: mid-ocean ridge, south of Iceland
3. Rates and History of Plate Motion
Velocity = 60 km/3.3 mil. yr. = 18 km / mil. yr. (or 18 mm/yr)
3. Rates and History of Plate Motion
Example relative plate velocities:
East Pacific Rise (Pacific and Nazca plates) – 138 to 150 mm/yr
South Atlantic (Mid-Atlantic Ridge) – 34 to 35 mm/yr
Southern Ocean, south of Australia – 70 to 75 mm/yr
Southern Ocean, south of Africa – 14 mm/yr
4. The Grand Reconstruction
Reconstructing the history of plate motions:
1. Assembly and breakup of the
supercontinent Rodinia
2. Assembly and breakup of the
supercontinent Pangaea
4. The Grand Reconstruction
The Earth’s geography one billion years ago. Let’s see continental
motion!
ASSEMBLY OF RODINIA
Late Proterozoic (750 Ma)
Formed about 1.1
billion years ago;
began to break up
about 750 million
years ago
ASSEMBLY OF PANGAEA
Late Proterozoic (650 Ma)
The distribution of
continents and oceans
between Rodinia and
the
assembly
of
Pangaea
ASSEMBLY OF PANGAEA
Middle Ordovician (458 Ma)
The
distribution
of
continents and oceans
about 458 million years
ago
ASSEMBLY OF PANGAEA
Early Devonian (390 Ma)
The distribution of
continents and
oceans about 390
million years ago
ASSEMBLY OF PANGAEA
Early Triassic (237 Ma)
The distribution of
continents and
oceans about 237
million years ago;
Pangaea is formed
BREAKUP OF PANGAEA
Early Jurassic (195 Ma)
The breakup of the
super-continent
about 195 million
years ago;
Pangaea is being
rifted
BREAKUP OF PANGAEA
Late Jurassic (152 Ma)
The distribution of
continents and
oceans about 152
million years ago
BREAKUP OF PANGAEA
Late Cretaceous-Early Tertiary (66 Ma)
The
distribution
of
continents and oceans
about 66 million years
ago; much like today in
some ways
PRESENT DAY
The distribution of
continents and
oceans as we
know them today
5. Mantle Convection: The Engine of Plate Tectonics
Upper
mantle
Theory 1: whole
mantle convection
700 km
Lower
mantle
Plate recycling extends to the
core-mantle boundary.
2900 km
Outer core
5. Mantle Convection: The Engine of Plate
Tectonics
Theory 2: stratified
convection
The lower mantle convects
more sluggishly than the
upper mantle.
Boundary near 700 km
separates the two different
convection systems.
5. Mantle Convection: The Engine of Plate Tectonics
spreading centers
and hot spots
6. Theory of Plate Tectonics and the Scientific
Method
• Plate tectonics is not a dogma, but a confirmed theory whose
strength lies in its simplicity, its generality, and its
consistency with many types of observations.
• This theory has survived so many attempts to prove it wrong
and has been so important in explaining and predicting so
many phenomena that geologists treat the theory as fact.
• Reasons why proof and acceptance took so long: very
cautious approach of many scientists studying this issue;
global scale of the problem; and specialized technology
required to gain data took time to develop.
Thought questions for you!
• What mistakes did Alfred Wegener make in formulating his
theory of continental drift? Do you think the geologists of his era
were justified in rejecting his theory?
• Why are there active volcanoes along the Pacific coast in
Washington and Oregon but not along the east coast of the United
States?
• How do the differences between continental and oceanic crust
affect the way lithospheric plates interact?
• Would you characterize plate tectonics as a hypothesis, a theory,
or a fact? Why?
Thought questions for you!
• The theory of plate tectonics was not widely accepted until
the banded patterns of magnetization on the ocean floor
were discovered.
• In light of earlier observations – the jigsaw-puzzle fit of the
continents, the occurrence of fossils of the same life-forms
on both sides of the Atlantic, and the reconstruction of
ancient climate conditions – why are these banded patterns
of magnetism such key pieces of evidence?