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Chapter 3
Plate Tectonics
Plate Tectonics
In 1855, Antonio Snider published a sketch showing
how the two continents could fit together, jigsaw-puzzle
fashion
In 1912, Alfred Wegener published the concept of
continental drift
Continental drift is just one aspect of a broader theory
known as plate tectonics, which has evolved over the
last several decades
Tectonics is the study of large-scale movement and
deformation of the earth’s outer layers
Plate tectonics relates such deformation to the
existence and movement of rigid “plates” over a weak
or partly molten layer in the earth’s upper mantle
Figure 3.6
Continental Drift and Plate Tectonics
• Jigsaw-puzzle fit of continents observed a few
centuries ago
• Mechanism to describe how continental masses
moved was not easily visualized for decades
• Later half of 20th century the concept of
continental drift was incorporated into a broader
concept of Plate Tectonics
– Mechanisms and processes of continent scale
movement detailed
– Evidence based on physics, chemistry, mathematics,
and geology used to explain how rigid plates move
relative to each other
Figure 3.1
Figures 3.2 a and b
Rock Response to Plate Tectonics
• Stress –force applied on a rock
– Compressive stress – squeeze or compress
an object
– Tensile stress – pull or stretch an object
– Shearing stress – different parts of an object
move in different directions or at different
rates
• Strain – results from stress; is the change
in shape or size of an object because of
the stress it experienced
Strain
• Temporary or permanent
• Elastic deformation – temporary strain,
object recovers original size and shape
once the stress is removed
– Elastic limit – strain that becomes permanent
in an object once limit of recoverable strain
has been exceeded
– Plastic deformation occurs in materials once
elastic limit has been exceeded
– Brittle deformation occurs at the limit of
strength of the material, a rupture or a break
occurs
Figure 3.3
Lithosphere and Asthenosphere
• Earth’s crust and upper most mantle are
solid and compose the lithosphere
– Stresses cause brittle and elastic deformation
• Beneath the lithosphere is a plastic layer
called the asthenosphere
• Lithospheric plates can move over this
plastic layer; plate tectonics plausible
• Boundaries of the plates are active with
earthquake and some with volcanic activity
Figure 3.4
Evidence for Plate Tectonics
• Earthquakes and volcanoes
• Sea Floor topography
– Trenches
– Ridges
• Paleomagnetism
•
•
•
•
Magnetic patterns imprinted on oceanic crust
Curie temperature
Magnetic reversals
Magnetic polar wandering curves
• Sea Floor Spreading
• Age of the sea floor
• Other evidence
– Fit of continents, GPS data, and more …
Figure 3.5
Figure 3.7
Figures 3.8 a and b
Figure 3.9
Figure 3.10
Other Evidence for Plate Tectonics
• Distribution of rocks representing ancient
deserts, sea shores, tropical areas,
glaciated areas, swamps, and equatorial
regions
• Location of fossils that were originally
restricted in their distribution but now
separated by oceans and on separate
continents
• Fit of continents reveal super continent of
Pangaea
• Recognition of plate boundaries
Figure 3.13
Figure 3.14
Figure 3.6
Figures 3.15 a, b, and c
Plate Boundaries
• Divergent Plate Boundary
– Lithospheric plates move apart; form oceanic
ridges
– Upwelling of asthenosphere injects magma
forming oceanic ridges and new oceanic crust
– Forces plates apart
– Sea floor spreading occurs
• Transform Boundaries – short segments of
a ridge
– Transform faults offset ridge
– San Andreas Fault – transform fault under
continental crust
Figures 3.16 a and b
Plate Boundaries
• Convergent Plate Boundaries
– Lithospheric plates move toward each other
– Higher density oceanic crust overridden by
low density continental crust
– Subduction zone forms and produces a trench
– Subduction of older oceanic crust balances
the spreading seafloor equation
– Subduction zones are active geologic places
• Volcanism
• Earthquakes
• Island arc formation
Figures 3.18 a, b, and c
Figure 3.19
Tectonics
• Convection cells operate in mantle
• Upwelling of heat and magma occurs at
divergent plate boundaries
– New oceanic crust formed
– Oceanic crust pushed away from spreading centers
• Hot spots located independent of plate
boundaries
– High heat flow radiate from them
– Volcanic activity associated with them
• Hawaiian Islands
• Yellowstone
Figure 3.20
Figure 3.21
Figure 3.22
Figure 3.23