Transcript Chapter 3 - earthjay science

The Dynamic
Geosphere and
Plate Tectonics
Chapter 3
© 2011 Pearson Education, Inc.
You will learn
• Origins of plate tectonic theory—how scientists
developed and tested it
• Driving mechanism for geosphere movements and
how plate tectonics work
• How continents have split apart, moved, and
reassembled
• How Earth system interactions at plate boundaries
are related to earthquakes, volcanoes, mountains,
and ocean basins
• How tectonic processes affect people by creating
both natural hazards and vital natural resources
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Continental Drift—An Early Idea
• Early evidence in 16th
century—continental
outlines fit (like Africa
and South America)
• Coal deposits and other
strata on separate
continents having similar
fossils such as
Glossopteris
• Suggestion that modern
day continents now
separated were joined in
Earth’s past.
Figure 3-2 Distinctive fossil seed fern
Glossopteris found in southern continents
or Gondwanaland.
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Continental Drift and
Alfred Wegener
• First to formulate a
detailed, global
explanation of how
continents assumed
their present locations
and shapes
• Laid groundwork for
theory of plate tectonics
Figure 3-3 Alfred Wegener
Hypothesized continental drift
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Wegener’s Evidence for Continental
Drift if Continents Reassembled
• Widely separated but
very similar sequences of
sedimentary rocks
containing same fossils
(Gondwana sequence)
• Glacial deposits
appeared to cluster
around Antarctica and
had associated ice flow
directions radiating out
from a central point
• Ages of rocks similar and
mountain belts matched
Figure 3-4 Fossil Evidence Examined
for Wegener’s Continental Drift Hypothesis
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The Supercontinent
Pangaea
• Pangaea = Greek for “all land”
• Collisions combined the
continents into one giant
landmass ≈ 250 million years
ago (MYBP) (at top)
• Since then, continents
positioned atop tectonic plates
have split and moved apart to
their present positions (at
bottom)
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Figure 3-5
The Saga of Alfred Wegener
• Wegener’s arguments for continental drift failed
because he did not present a viable mechanism for
moving the continents.
• Wegener erroneously proposed that tidal forces
generated by Earth’s spin tugged continents through
the oceanic crust, thereby moving them—basic
physics says this is impossible.
• Although Wegener’s continental drift idea was
deemed a failure and even ridiculed by the scientific
community, the idea, however, challenged
geologists to reconsider much of what they had
come to believe about how a dynamic Earth worked.
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Explaining “Moving Continents”—
The Theory of Plate Tectonics
• Wandering Magnetic Poles
• Exploring the Ocean Basins—
More Discovery:
• Seafloor Spreading
• Magnetic Stripes
• Earthquakes provide another
test
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Figure 3-6 Apparent
Polar Wander Curves
EXPLORING THE OCEAN BASINS
Mapping the Seafloor—Clues for Plate
Tectonics
Figure 3-7 Major
Ridges and Trenches
of the Seafloor
(a) Post World War II
mapping revealed that
the seafloor contains
features such as ridges
(mountain chains)
and deep trenches.
(b) A computergenerated map of part
of the East Pacific Rise,
a mid-ocean ridge.
Red = highest elevations,
blue = lowest.
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SEAFLOOR SPREADING—Harry Hess’
Contribution to Plate Tectonics
Figure 3-8 Seafloor Spreading and
Creation of New Ocean Crust
• Harry Hess, a Princeton Univ.
geologist, in 1962 published a
paper titled History of the Ocean
Basins—proposed hypothesis to
explain “continental drift.”
• Hess recognized a mechanism to
explain continents being separated
via spreading in one part of ocean
crust and a sinking in another part
(at trenches).
• At mid-ocean ridges (MORs), new
oceanic crust forms as lithosphere
pulls apart and magma from
mantle wells up, cools, and
solidifies.
• Volcanoes and a central rift valley
are common along fast-spreading
ridges like the Mid-Atlantic Ridge.
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A Magnetic Signature on the
Seafloor—A “Giant Barcode”
• In addition to Hess’ discovery,
Vine and Matthews, two
geophysicists from
Cambridge Univ., in 1963
proposed magnetic stripes
corresponded to past when
Earth’s magnetic field was
either normal or reversed
• Additional data from magnetic
mapping of the seafloor also
further helped to unravel the
mystery of how plates moved
• Magnetic field stripes are
arranged symmetrically about
the center of MORs
Figure 3-9 Magnetism of the Ocean Crust
Along Mid-Atlantic Ridge south of Iceland.
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Matching Magnetic Signatures—Stripes
on Land and on the Seafloor
• Analysis of well-dated rocks on land, like these 5-million-yearold basalt flows on Kauai, a Hawaiian island (a), made it
possible to construct a timetable of magnetic polarity reversals.
• The timing and duration of these reversals (b) could be
correlated with the magnetic stripe patterns on the seafloor (c).
Figure 3-10
Volcanic Rock
Sequences Were
Used to Date
Magnetic Polarity
Intervals
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Age of Seafloor
Figure 3-11 Age of the Seafloor
MORs are extremely young by geological
standards—no more than 2 million years
old. Rocks of the seafloor become
progressively older as one travels away
from MORs.
• Not only is there symmetry of
the magnetic signature on
the seafloor but there is also
a similar symmetry with
respect to age of the volcanic
rocks making up the seafloor
relative to the MOR
• Youngest ocean crust is at
MOR (red) and the oldest
ocean crust is generally
furthest from the MOR (blue,
green, and cooler colors)
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Earthquakes—Another Test For
Plate Tectonics
FIgure 3-12 Transform Faults (a) At MidAtlantic Ridge, long linear breaks connect
spreading movements on adjacent MOR
segments. (b) Motion of plates toward or away
from each other transformed into motion of
plates sliding past each other.
• Earthquake studies of oceanic
ridges and trenches provided
key evidence needed to further
support the emerging theory of
plate tectonics.
• In 1965, a Canadian
seismologist, J. Tuzo Wilson,
made a major contribution by
noting lateral sliding movement
at faults near MORs connecting
the spreading movements
taking place at adjacent ridge
segments (Fig. 3-12b) =
transform faults.
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Plate Tectonics Today
Modern tectonic
plates move slowly,
averaging several
cm (a few inches)
per year ≈ fast as
your fingernail
grows.
Figure 3-13 The Geosphere’s Tectonic Plates
Tectonic plates, many of which include both oceanic and
continental lithosphere, come in many shapes and sizes.
Arrows indicate some relative motions between plates.
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East Africa-Arabia: An Example of
Modern Rifting
• Upwelling in asthenosphere is
causing lithosphere to thin and
weaken. As a result, the
African continent is being
separated from Arabia along
the Red Sea and split apart
along the East African rift
valleys.
• Africa is splitting apart, in the
same way Pangaea did,
starting some 250 million
years ago (turns out Wegener
was correct on how continents
initially split apart).
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Figure 3-14 Africa Is Rifting
Apart Today
Rifting—A Continent Is Split
• Rifting starts when hot asthenosphere
material begins to rise.
• Upwelling in asthenosphere heats and
weakens crust, resulting in lithospheric
thinning and normal faulting.
• Crustal extension proceeds until the
continental crust becomes thin enough
to split apart.
• Pieces of continent migrate away from
each other as mantle upwelling
continues and oceanic crust forms
where continents once joined.
Figure 3-15 Rifting Splits
Continents
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Divergent Plate Boundaries in the
Ocean—Spreading Centers
New seafloor is formed at
spreading centers
Figure 3-17 Hot Springs Form on the
Seafloor Near Spreading Centers
Mineral-rich hot waters from hydrothermal
vents form dark plumes called “black
smokers.”
Figure 3-16 Normal Faults
Common at divergent plate boundaries.
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Convergent Plate Boundaries—Subduction
Zones and Reverse Faults
• Convergent plate boundaries = two
plates move toward each other.
• Boundaries can be between 2 oceanic
plates, an oceanic plate and continental
plate, or 2 continental plates.
Figure 3-19
Reverse Faults
Figure 3-18 Consuming Lithosphere at Convergent Plate Boundaries
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Making of the Himalayas—
Convergence
• Where continents converge,
neither plate is dense enough
to sink into the mantle—they hit
and form a suture zone.
• When two continental plates (in
this case the Indian and
Eurasian plates) come together
in great collisions they form the
world’s highest mountains.
• Thrust faults are common in
crustal shortening areas like at
convergent plate margins or
boundaries.
Figure 3-20 The Making of the Himalayas
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Convergent Margin—Aleutian
Trench, Alaska
Figure 3-21 The Alaska-Aleutian
Convergent Plate Boundary
Plate convergence gives rise to
deep-sea trenches, long volcanic
arcs, island arcs, and earthquakes.
At Alaska-Aleutian convergent
boundary (a), oceanic Pacific plate is
sinking into mantle beneath North
American plate and earthquakes
originate in the inclined subduction
zone (b).
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Strike-Slip—Faults at Transforms
• If two plates are not
diverging or
converging, they are
sliding past one
another along a
transform plate
boundary.
• Most common fault
along transforms is
the strike-slip fault.
Figure 3-22 Strike-Slip Faults
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Surface Offset on San Andreas Fault
• Nearly instantaneous
movement of the land’s
surface during earthquakes
marks plate boundary as
along the San Andreas
Fault
• Fence line (upper right)
displaced dramatically
during the Great San
Francisco Earthquake of
1906
Figure 3-23
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Fault Movements at Plate
Boundaries
San Andreas
Fault, California
Note offsets of
stream valley (far
left) and linear
lakes along fault.
Figure 3-24 Effects of Earthquakes and Fault Movements at
Plate Boundaries
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Plate Tectonics and Earthquakes
Figure 3-25 Global Distribution of
Earthquakes
• Distribution of earthquakes is a
good guide to location of
tectonic plate boundaries.
• Most earthquakes occur along
convergent and transform plate
boundaries—the most
devastating are around the
Pacific Ocean rim.
• Offshore quakes displace
ocean waters generating
seismic sea waves, or
tsunamis (e.g. Indonesian
tsunami that killed 250,000
people in 2004).
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Plate Tectonics and
Volcanoes
Figure 3-26 Global Distribution of Volcanoes
Figure 3-27 Mt. Pinatubo
A Volcano at a Convergent
Plate Boundary
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Plate Tectonics
and Mountain Building
• Plate tectonics plays a role in
building mountains in a number
of ways such as by:
1. Accretion (adding pieces of the
geosphere) along the
continental margin (Fig 3-28).
2. Compression at convergent
plate boundaries.
3. Collision of continents at
convergent plate boundaries.
4. Magmatic processes also help
form large mountains and long
mountain ranges where
oceanic plates converge and
subduct beneath continental
margins.
Figure 3-28 The Chugach Mountains of
Alaska Offscraping (accretion) of seafloor
sediments against overriding plate at a
convergent margin produced mountains.
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Plate Tectonic Influence on Climate
and Resources—Some Examples
Figure 3-29 The Andes Mountains
Rainshadow Rainshadow effect ultimately the
result of subduction zone volcanism in satellite
photo of South America. Moist air from Pacific
Ocean moves eastward up slopes of the Andes
and cools, condenses, and results in precipitation
as it climbs the mountains.
Figure 3-30 A Copper Mine in Utah
Mineral deposits form in settings shaped by
plate tectonics. The Bingham Canyon copper
mine in Utah formed within subductionrelated volcanoes.
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SUMMARY
• Persistent research, applications of new technology, and
syntheses of diverse observations of 3+ generations of
scientists during the 20th century developed theory of plate
tectonics.
• Earth’s rigid lithosphere is broken into pieces called
tectonic plates.
• Plates move apart (divergent boundary—creating new
seafloor or rifting), come toward each other (convergent
boundary—creating volcanic mountains and major
earthquakes), or slide horizontally past one another
(transform—creating earthquakes).
• Plates localize volcanic and seismic activity and plate
motion influences the biosphere, climate, and availability of
resources.
© 2011 Pearson Education, Inc.