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Chapter 2: Plate
Tectonics
(The Unifying Theory)
 The Earth’s outermost rigid layer (lithosphere) is broken up into a many large
fragments called plates.
 They’re traced by distribution of earthquake epicenters and volcanism
 Lithosphere = Rigid, mechanically strong outermost layer of the Earth of 50100km thickness
 Asthenosphere = Soft, weak layer below the lithosphere identified by a decrease
in seismic wave velocities between ~80-200 km depth.
 The Earth’s crust comes from a range of composition with the mantle
(chemistry, mineralogy, rock type).
 The concept of the lithosphere is based on differences in mechanical strength
with the underlying asthenosphere.
 Lithosphere = Crust (continental and oceanic) + Upper Mantle
 Think snow,ice,water on lake
They are traced by the distribution of earthquake epicenters (shown above) and volcanism.
 Earth’s surface is a mosaic of 13 major plates of lithosphere and some smaller
plates that move slowly over the asthenosphere
 There are some places (between NA plate and Pacific plate) where the plates
are only moving by one another and not destoying/creating plates there.
Divergent Boundaries = Oceanic Plate separation
Mid-atlantic ridge (north american plate and eurasian plate)
Volcanoes and earthquakes are concentrated at the ridge.
Divergent Boundaries = Continental Plate Separation
East African Rift Valley ( African plate and Somali Subplate)
Parallel valleys (volcanoes and eathquakes)
Divergent Boundaries
Oceanic Plate Separation
Volcanoes and earthquakes concentrate
Mid-Atlantic
Ridge
Divergent Boundaries
Continental Plate Separation
Parallel valleys; volcanoes and earthquakes
East African
Rift Valley
 Tomographic image of oceanic slab penetrating the mantle based on seismic
tomography
 The chunks of rock does pass the 700km mark but only occurs only between
some plates.
 The chunks usually pile up in one way or another and then when they get old
they drop down under the 700km mark.
Convergent Boundaries
Ocean-Ocean Convergence
Deep-sea trench; volcanic island arc.
Marianas Trench
Mariana Islands
Convergent Boundaries
Ocean-Continent Convergence
A volcanic belt of
mountains forms.
Andes
Mountains
Peru-Chile Trench
South
American
Plate
Convergent Boundaries
Continent-Continent Convergence
Himalaya
Crust crumbles, creating high
mountains and a wide plateau.
Tibetan
Plateau
Main
thrust
fault
Eurasian
Plate
The Rockies near Great Falls Montana,
created by satellite radar interferometry
Whole-mantle convection
Mantle
Outer core
Inner core
Upper
mantle
700 km
Lower
mantle
2900 km
Outer core
Whole-mantle
Stratified convection
convection
Mantle
Outer core
Inner core
Upper
mantle
700 km
Boundary near
700 km separates
Lower
the two convection
mantle
systems.
2900 km
Outer core
Sam butler usask therm_histnoph (sab248)
homepage.usask.ca/~sab248/therm_histnoph.wmv
Transform-Fault Boundaries
Mid-Ocean Ridge Transform Fault
Spreading centers offset.
Transform-Fault Boundaries
Continental Transform Fault
Offset continental crust.
San Andreas Fault
As plates
move past
each other...
As plates
move past each
other...
…creek beds
are offset
As plates
move past
each other...
…creek beds
are offset
San
Francisco
Los Angeles
Convection in the outer core
creates the Earth’s magnetic
field
Somehow—changes in
convection regime cause
polarity reversals of the
Earth’s magnetic field, so that
the SOUTH pole becomes the
NORTH pole.
The polarity today is called
NORMAL, whereas the
opposite mode is called
REVERSED
The polarity of the
Earth’s magnetic field is
recorded in rocks
(thermal remnant
magnetization)
Magnetic mapping of the sea floor near Iceland
An oceanic survey over the Reykjanes Ridge, part of the MidAtlantic Ridge southwest of Iceland, showed an oscillating pattern
of magnetic field strength. This figure illustrates how scientists
worked out the explanation of this pattern.
Magnetic mapping can measure the rate of seafloor spreading
An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic
Ridge southwest of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates how scientists worked
out the explanation of this pattern.
A sensitive magnetometer
records magnetic anomalies,…
Magnetic mapping can measure the rate of seafloor spreading
An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic
Ridge southwest of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates how scientists worked
out the explanation of this pattern.
A sensitive magnetometer
records magnetic anomalies,…
Iceland
MidAtlantic
Ridge
Magnetic mapping can measure the rate of seafloor spreading
An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic
Ridge southwest of Iceland, showed an oscillating pattern of
magnetic field strength. This figure illustrates how scientists worked
out the explanation of this pattern.
A sensitive magnetometer
…alternating bands of high
records magnetic anomalies,…
and low magnetism.
Iceland
MidAtlantic
Ridge
Symmetrical bands on both sides.
Why?
Seafloor magnetic stripes
Mid-ocean ridge
Million years ago (Ma)
4.0
3.0
2.0
Ocean
crust today
40K-40Ar
dating of basaltic lava flows allows for the
creation of the magnetic polarity reversal time scale
because the sea floor pattern of magnetic stripes can
be dated.
Oceans are spreading 5-10cm per year
Hawaiian Islands have formed over a stationary hot spot
Hotspots are like a jet (goes right up in a plump and not
spreading horizontally). The hotspot isn’t moving but
the lithosphere is moving and the volcanoes are created
(hotspot blows) and islands formed. All tectonic plate
activities are connected and what caused the change of
momentum (change of direction) between Hawaiian
chain and Emperor Seamount chain must have been
because of some event on the other side of the world.
Pangea
~250 million
years ago
Alfred Wegener and the Continental Drifters
"Scientists still do not appear to understand sufficiently that all
earth sciences must contribute evidence toward unveiling the
state of our planet in earlier times, and that the truth of the
matter can only be reached by combining all this evidence. . .
It is only by combining the information furnished by all the
earth sciences that we can hope to determine 'truth' here, that is
to say, to find the picture that sets out all the known facts in the
best arrangement and that therefore has the highest degree of
probability. Further, we have to be prepared always for the
possibility that each new discovery, no matter what science
furnishes it, may modify the conclusions we draw.”
1880-1930
From the “Origin of the Continents.” 1915
Alfred Wegener and the Continental Drifters
The evidence for drift—the mirror image coastlines, the similarities
between sedimentary rock successions, fossil types, and climate
indicators like evaporates and glacial deposits
 Type here
ASSEMBLY OF PANGAEA
RODINIA Late Proterozoic, 750 Ma
ASSEMBLY OF PANGAEA
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
ASSEMBLY OF PANGAEA
Late Proterozoic, 650 Ma
The pre-Pangean pattern
of continental drift.
ASSEMBLY OF PANGAEA
Middle Ordovician, 458 Ma
The pre-Pangean pattern
of continental drift.
ASSEMBLY OF PANGAEA
Early Devonian, 390 Ma
The pre-Pangean pattern
of continental drift.
ASSEMBLY OF PANGAEA
PANGAEA (a) Early Triassic, 237 Ma
ASSEMBLY OF PANGAEA
PANGAEA (a) Early Triassic, 237 Ma
Assembled by 237 Ma.
BREAKUP OF PANGAEA
(b) Early Jurassic, 195 Ma
BREAKUP OF PANGAEA
(b) Early Jurassic, 195 Ma
Signaled by the opening
of rifts from which lava
poured; relics can be
found today in volcanic
rocks from Nova Scotia
to North Carolina.
BREAKUP OF PANGAEA
(c) Late Jurassic, 152 Ma
BREAKUP OF PANGAEA
(d) Late Cretaceous, Early Tertiary, 66 Ma
THE PRESENT-DAY AND FUTURE WORLD
(e) PRESENT-DAY WORLD
THE PRESENT-DAY AND FUTURE WORLD
(e) PRESENT-DAY WORLD
The modern world has
been produced over the
past 65 million years.
THE PRESENT-DAY AND FUTURE WORLD
(f) 50 million years in the future