Transcript Document

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How do we explain the geological
activity of the earth?
Many pieces of information had to come together...
Age of the Earth
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• uniformitarianism vs
catastrophism (1800’s)
• about 4.6 billion years old
Continental Drift
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• Alfred Wegener, 1912
• based on Amazing fit of
Africa & South America
& fossils evidence
Figure 2.2
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Figure 2.6
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Figure 2.5
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Continental Drift
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• single super continent = Pangea
– split 200 million years ago & are still
drifting
• Wegener’s proposed mechanism =
centrifugal force of spinning earth
and tidal drag of moon & sun
– He was WRONG about this
• His idea was criticized through his
death in 1930
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Discovery of
Mid-Atlantic Ridge
in 1925
Fig. 3-18, p. 63
Patterns of Earthquakes and
Volcanoes
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• Plotted by Wadati & Benioff in late
1930’s
– Related to continental drift?
– Pacific Ring of Fire
• a world wide pattern that followed
orderly lines
– many corresponded to oceanic ridges
(first plotted in 1925)
Figure 2.13a
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Youth of Ocean Floor
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• Maximum age of ocean floor was 200
million years
• centers of continents were much
older (3.9 billion years)
Figure 2.12
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Mantle Studies
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• Seismographic evidence showed that
the upper mantle was deformable and
plastic
• perhaps the continents could move
Sea Floor Spreading
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• 1960, Harry Hess and Robert Dietz
• new seafloor (basaltic crust) develops
at mid-oceanic ridges and then
spreads outward
• continental drift would be caused by
the same forces
Sea Floor Spreading
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• powered by convection currents in
the asthenosphere
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Fig. 3-11, p. 57
Figure 2.10
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Sea Floor Spreading
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• explained...
– why ridges were hot
– why ocean was deeper away from ridges
– why sediments were thicker and older
away from ridges
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Since the earth is not
increasing in diameter...
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Fig. 3-13, p. 59
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Subduction Zones
• Subduction zones (Wadati-Benioff
zones) were discovered where the
crust plunges into the mantle
• crust is destroyed here
• explains why the ocean floor is so
young
Figure 2.20a
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Plate Tectonics
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• 1965, primarily by Wilson
• lithosphere is divided into plates that
float & drift on top of the plastic
asthenosphere
Figure 2.13b
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Plate Tectonics
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• plates move about 5 cm per year
• Plate movement is powered by:
– downward pull of descending plates
leading edge
– friction of asthenosphere convection
currents
– outward push of new seafloor at
spreading centers
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Fig. 3-11, p. 57
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Plate Tectonics
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• plates interact at boundaries
– diverge, converge or slip past each
other
Divergent Plate Boundaries
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• spreading centers, mid-oceanic ridges
• a line along which two plates are
moving apart
• new oceanic crust forms
Figure 2.17
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Figure 2.15
Divergence along
Mid-Atlantic Ridge
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Figure 2.16
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Rift Valley
of Iceland
Figure 2.18a
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East African
Rift Valley
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The Red Sea = a divergent plate boundary
Fig. 3-16d, p. 61
Convergent Plate Boundaries
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• aka subduction zones, Wadati-Benioff
zones
• regions where plates are coming
together
• crust is destroyed
Convergent Plate Boundaries
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• Oceanic-continent
– denser oceanic crust is subducted
Figure 2.20a
Figure 2.21
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Convergent Plate Boundaries
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• Oceanic-oceanic
– older crust is more dense & is
subducted
– deep oceanic trenches & volcanic
islands
Convergent Plate Boundaries
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• Continent-continent
– neither plate is completely subducted
– they compress & uplift forming
mountains
Figure 2.22
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Transform Plate Boundaries
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• Plates slide laterally past each other
• Crust is neither created nor destroyed
Figure 2.23
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Table 2.1
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Confirmation of Plate
Tectonics
• Paleomagnetism
– Fossil magnetic field
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Fig. 3-27, p. 69
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Age of the
ocean floor
Fig. 3-28, p. 70
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Confirmation of Plate
Tectonics
• Hot Spots
– Surface expressions of plumes of
magma rising from stationary heat
sources in the mantle
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Fig. 3-30, p. 71
Figure 2.25
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Figure 2.24
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Fig. 3-12, p. 58
Figure 2.33
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Figure 2.31a
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Figure 2.31b
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Figure 2.31c
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Figure 2.31d
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Figure 2.31e
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Figure 2.31f
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Figure 2.31g
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Figure 2.31h
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Figure 2.31i
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Figure 2.31j
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Figure 2.32
What Earth may look like in 50 million years…
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