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.
Lecture Outline
1. The discovery of plate tectonics
2. Plates and their boundaries
3. Rates and history of plate movements
4. The grand reconstruction
5. Mantle convection: the engine of plate
tectonics
Lecture Outline
6. Theory of plate tectonics and the
scientific method
1. Evolution of the Theory
Continental
drift:
“jig-saw
puzzle” fit of
continents
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
1. Evolution of the Theory
Seafloor
spreading:
geological
activity in midocean ridges
1. Evolution of the Theory
Seafloor
spreading:
new crust
formed there
Thought questions for this chapter
What mistakes did Wegener make in formulating his
theory of continental drift? Do you think the geologists of
his era were justified in rejecting his theory?
2. The Mosaic of Earth’s Crustal Plates
● mosaic of rigid plates
2. The Mosaic of Earth’s Crustal Plates
● three types of boundaries
2. The Mosaic of Earth’s Crustal Plates
● divergent, convergent, transform
2. The Mosaic of Earth’s Crustal Plates
● next: a detailed look at the above
1. Divergent Boundaries
(a) Oceanic plate separation
rifting, volcanoes, and earthquakes
MidAtlantic
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
crustal thickening, folded
mountains, and earthquakes
Himalaya
Mountains
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
Thought questions for this chapter
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?
3. Rates and History of Plate Motion
Ship towing a
sensitive magnetometer
Magnetic
anomalies:
seafloor areas
of high and low
magnetic
values
3. Rates and History of Plate Motion
Iceland
Mid-Atlantic
Ridge
● seafloor as a magnetic tape recorder
3. Rates and History of Plate Motion
● magnetic time scale developed
3. Rates and History of Plate Motion
Figure 2.15
● magnetic isochrons on the seafloor
3. Rates and History of Plate Motion
● velocity of seafloor spreading = d / t
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
Thought questions for this chapter
In Figure 2.15, the isochrons are symmetrically distributed
in the Atlantic Ocean, but not in the Pacific Ocean. For
example, seafloor as much as 180 million years old (in
darkest blue) is found in the western Pacific, but not in the
eastern Pacific. Why?
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 supercontinent 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
Plate recycling
extends to the
core-mantle
boundary.
700 km
Lower
mantle
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 this chapter
Would you characterize plate tectonics as a hypothesis, a
theory, or a fact? Why?
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?
Key terms and concepts
Continental drift
Covergent boundary
Divergent boundary
Geodesy
Island arc
Isochron
Magnetic anomaly
Magnetic time scale
Mantle plume
Mid-ocean ridge
Pangaea
Plate tectonics
Relative plate velocity
Rodinia
Key terms and concepts
Seafloor spreading
Spreading center
Subduction
Transform fault