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Earth
Chapter 15 Part 2
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Limestone
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Breccia
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Shale
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Oil Shale
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Siltstone
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Sandstone
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Sandstone
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Coal
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Metamorphic rocks
• Previously existing
rocks changed by
heat, pressure or hot
solutions into distinctly
different rock
• Causes associated
with geologic events
– Movement of the crust
– Heating and hot
solutions from magma
intrusion
– Temperatures must be
high enough to cause
recrystallization, but not
melting
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Metamorphic rocks
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Gneiss
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Slate
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Marble
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Marble
Colorado
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Seismic Waves
• Remember the earth is not solid rock
• All rocks and rock materials can be made to flow
– Behaves as semi-solid like wax or putty
– Under extreme temperature & pressure
– Moves slowly upward toward the surface
• Earth’s internal heat and rock movement is related
to what's happening on the surface
• To get a look at the Earth’s interior
– Study Earthquakes
– These create Seismic Waves radiating outward through
the solid and semi-solid interior
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Seismic Waves
Epicenter
Hypocenter
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Seismic Waves
• P-wave
– Longitudinal (compressional)
– Fastest waves
– Move through surface rocks and
interior solid and liquid materials
• S-wave
– Transverse (shear) wave
– Second fastest
– Do not travel through liquids
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Seismic Waves
– Surface waves
• Much like water waves
• Slowest of the three
– Occur where S- or P-waves
reach the surface
– Two types of surface waves
• Love waves
– horizontal S-waves
– move side to side
• Rayleigh waves
– Like rolling water waves
– More destructive
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Seismic Waves
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Seismic Waves
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Theory of Plate Tectonics
• Individual continents
shift positions on
Earth’s surface
• Patterns between
continental shapes
can be fit together
• “Pangaea”
• Original concept:
“Continental drift”
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Pangaea
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Theory of Plate Tectonics
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Evidence from the Ocean
• Oceanic ridges
– Coincide with
submarine
earthquakes
– Rift along crest
– Large amount of heat
escapes from crest
– Mid-Atlantic Ridge
• Oceanic trenches
– Long, narrow, and
deep troughs with
steep sides
– Always run parallel to
continental edges
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Seafloor Spreading Hypothesis
• Hot, molten rock moves
up from Earth’s interior
emerging along a rift
• Outflow in both
directions creates new
rocks
• Drilling evidence
– Thin sediment layers
near ridge, becoming
thicker toward continents
– Older fossils and rocks
near continents,
becoming younger near
rifts
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Plates and Boundaries
• Basis of Plate Tectonics theory
• Lithosphere broken into fairly rigid plates
• Plates move on asthenosphere
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• Earthquakes, volcanoes and other rapid changes in
Earth’s crust occur most often at plate edges
• Three plate motions:
– Divergent
– Convergent
– Transform
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Divergent Boundaries
• Occur between two
plates moving away
from each other
• Molten material
from mantle rises
to fill fissures
• New crust zone
• Often accompanied
by volcanic activity
• Example: MidAtlantic Ridge
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Convergent Boundaries
• Occur when two
plates move toward
each other
• Old crust destroyed
in the process
• Subduction zone
– Belt with one crust
subducting under
another
– Subducted material
partially melts and
joins mantle
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Convergent Boundaries
• Three possibilities
1. Converging continental and oceanic plates
2. Converging oceanic plates
3. Converging continental plates
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Ocean-Continent Plate
Convergence
• Oceanic plate of
denser basaltic
material subducted
under less dense
granite-type
continental shelf
• Marked by oceanic
trench, deep-seated
earthquakes and
volcanic mountains
• Example:
convergence of South
American Plate with
Nazca Plate
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Ocean-Ocean Plate
Convergence
• Trench and
underwater ridge
created by
Subduction
• Associated with
deep-seated
volcanoes
• Island arcs form
where melted,
subducted material
rises up above sea
level through the
overriding plate
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Continent-Continent Plate
Convergence
• Less dense, granitetype materials resist
Subduction
• Colliding plates pile
up, producing a
deformed and thicker
crust of lighter
materials
• Example: Tibetan
Plateau and
Himalayan Mountains
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Transform Boundaries
• Occur when two plates
slide by each other
• Crust is neither created
nor destroyed in the
process
• Irregularities in
movement along
boundary
• Sudden jerks produce
earthquakes
• Example: San Andreas
Fault along California
coastline
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Present-day Understandings
• Major remaining question:
What drives the plates?
• Current working hypothesis:
convective cells in
asthenosphere
– Hot fluid materials rise at
diverging boundaries
– Some escapes to form new
crust
– Remainder spreads beneath
the lithosphere, dragging
overlying plates with it
– Problem: little supporting
evidence
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