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Chapter 8
The Theory of Plate Tectonics
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Guiding Questions
• What is the evidence for Gondwanaland?
• How does paleomagnetism demonstrate that continents
have moved?
• How was paleomagnetism used to show that lithosphere
forms at and migrates away from mid-ocean ridges?
• How did features of the seafloor engender the concept of
plate tectonics?
• Why do faulting and volcanism occur along oceanic plate
margins?
• What causes lithospheric plates to move?
• How can geologists measure rates of plate movement?
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Plate Tectonics
• tectonics
– movement of Earth’s
crust
• plate tectonics
– movement of discrete
segments of Earth’s
cruise in relation to one
another
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hypothesis
theory
law
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hypothesis
A tentative explanation for an observation, phenomenon, or scientific
problem that can be tested by further investigation.
theory
1. Common usage-a proposed explanation whose status is still
conjectural, in contrast to well-established propositions that are
regarded as reporting matters of actual fact.
2. Scientific usage-a more or less verified or established explanation
accounting for known facts or phenomena: the theory of relativity.
law
paradigm-the generally accepted perspective of a particular discipline at a
given time
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Theory-a Skinnerism
The best explanation we have (so far) that
explains more of the facts than any other
idea, and for which there is no serious
evidence flaw.
Plate tectonics explains volcanoe and
earthquake belts, mountain ranges, etc.
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Continental Drift
• the idea that continents
move horizontally over
Earth’s surface
• scoffed at in the 40s
• revolutionary in the 60s
• proposed and defended
by
– Alfred Wegener
– Alexander DuToit
Puzzle-like fit of continents using 500 fathom coastline depth
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Evidence for Continental Drift
Early scientists recognized the
relationship between fossils
on continents separated by
sea
Glossopteris flora present only
in southern hemisphere
continents
– proposed land bridges
– consistent with the distribution
pattern
– so is the idea of united
continents
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Evidence for Continental Drift
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Evidence for Continental Drift
Early recognition that
Mid-Atlantic Ridge
was site of landmass
rupture and Atlantic
Ocean formation
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Evidence for Continental Drift
• Wegener’s Evidence
– continents fit together
• Pangaea
– geologic similarities
– floral and faunal similarities
Wegener’s reconstruction of ancient world
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Evidence for Continental Drift
• DuToit’s evidence
– expanded Wegener’s ideas
– Mesosaurus fossils
• found on Gondwanaland
(southern hemisphere)
continents
• a fresh/brackish water
species
– could not swim across
Atlantic
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Evidence for Continental Drift
• geologic similarities
– Brazil and South
Africa have nearly
identical geologic
sequences
• similar in Antarctica
and India
– Glacial sediments
– Coal
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Continuity of Geologic Structures
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Evidence for Continental Drift
• glaciers
– orientation of
glacial
markings on
all continents
suggest they
were linked
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Additional Evidence
• Lystrosaurus
– heavyset herbivore
– found in Africa and SE
Asia
– recently (1969) found
in Antarctica
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Paleomagnetism-the smoking gun
• magnetization of
ancient rocks at the
time of their formation
• declination
– angle that a compass
needle makes with the
line running to the
geographic north pole
• rocks lock this
orientation in at
formation
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Paleomagnetism
• apparent polar
wandering
– first studies indicated
poles had moved
– instead, plates had
moved
– North American and
European paths met
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Rise of Plate Tectonics
• Harry Hess, 1962
– Geopoetry
• continents didn’t plow
through seafloor
• entire crust moved
– Crust must be created and
destroyed
• sedimentary cover too thin
for four b.y. of accumulation
– driven by convective cells
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Rise of Plate Tectonics
• ridges
– site of crustal
formation
– hot rising mantle
material rises to top of
lithosphere, cools
– ocean crust is formed
– bends away from
center to form ridge
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Rise of Plate Tectonics
• Guyots
– had identified flattopped seamounts in
Pacific
– realized they were
volcanoes that had
been eroded by waves
at sea level
– postulated as crust
moved away from
ridge it cools and sinks
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Rise of Plate Tectonics
• crust is destroyed at subduction zones
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Test of Plate Tectonics
• Paleomagnetism
– Vine and Matthews,
1963
– measured
magnetization of rocks
across the Indian
Ocean central ridge
• found normal and
reversed “stripes”
• mirror image
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Processes at Plate Boundaries
• Normal faults-extensional motion
• Thrust faults-compressional motion
• Strike-slip faults-e.g., San Andreas fault
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Processes at Plate Boundaries
• Mid-Ocean Ridges
(MOR)
– graben
• valley bounded by
normal faults along
which a central block
has slipped downward
– pillow basalt
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Processes at Plate Boundaries
• transform faults
– offset MOR’s
– enormous strikeslip faults
– seismically active
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Processes at Plate Boundaries
• Subduction
– descending slab
undergoes partial
melting
– molten material is less
dense, rises
– common around
Pacific Rim
– Ring of Fire
• location of most of
the world’s trenches
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Processes at Plate Boundaries
• associated with:
– volcanoes-island arc
– deep-focus earthquakes >300 km depth
– forearc basin
• zone of intensely deformed rocks in belt between island arc and deep-sea
trench
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Processes at Plate Boundaries
• forearc Basin
– deep-ocean dark muds and
graywackes with ocean
crust mixed in
• mélange
– chaotic deformed mixture
of rocks
• accretionary Wedge
– body of rock that
accumulates as plate is
subducted
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Plate Motion
• Why plates move:
– drag on the base of the
plate
– elevation at ridge
pushes plate ahead of it
– plate is pulled into
subduction zone by
preceding parent plate
– broken plate segments
create additional forces
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Plate Motion
• eight large plates
• several small plates
• not all plates move at the same rate
– relative motion
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Plate Motion
• absolute plate motion
– establish using fixed point
– hot spot
• small geographic area where heating and igneous activity occur within the crust
• Yellowstone
• Hawaii
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Plate Motion
• Hawaiian hot spot
– thermal plume creates
volcano
– plate moves away from
plume
– stranded volcano cools,
leaves a chain
– chain indicates
direction and rate
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Plate Motion
• GPS-Global Positioning System
– Earth-orbiting satellites identify motion
• transmitter on satellite with ground-based receiver
• Average rate of movement: 5 cm/year
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Effect on Global Climate
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