Geography and Oceanography - Chapter 4 ppsx

Today’s Tunes
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Fathoms Below / Under the Sea
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From soundtrack of Disney’s The Little Mermaid
Continental shelf
Submarine canyons
(cut into the c. slope)
Abyssal
plain
Continental rise
Continental
slope
Seafloor Features:
Continental Margins
Abyssal
plain
Seamounts
Abyssal Hills
(linear hills)
Seafloor spreading center
(e.g., East Pacific RISE or
Mid-Atlantic RIDGE)
Deep Ocean
Seafloor Features: Deep Ocean
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Plate Tectonic Boundaries
Ridges or Rises
Trenches
Transform Faults, Fracture Zones
The Shifting Crust

Continental Drift
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Alfred Wegener -1912
–
large “supercontinent” (Pangea) existed and
then split into pieces
–
fossil & glacial deposit evidence
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Wegener not able to provide MECHANISM for his theory
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Major mechanism later found in the OCEANS
Seafloor Spreading &
Plate Tectonics
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Harry Hess - 1960s
–
new ocean basins form from volcanism
–
ocean floor forms IN BETWEEN pieces that
have split
–
SEAFLOOR SPREADING
Seafloor Spreading &
Plate Tectonics
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Theoretical breakthrough
–
PLATE TECTONICS
–

surface of earth composed of “plates”
(LITHOSPHERE) that move on a “conveyor
belt” (ASTHENOSPHERE)
Evidence for Continental Drift
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Fit of the continents
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Correlation of mountain belts
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Diversity of species
–
correlation of dinosaur species (Pangea)
–
isolation of mammal species (after breakup)
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Glaciers
Important Quote
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“Relationships known since early in the century yet the
validity of continental drift was not generally recognized
until the late 1960’s”
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It took ocean floor data to solidify ideas and convince
scientific community!
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The oceans rule!
One Mechanism
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seafloor-spreading
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lithosphere
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asthenosphere
Another Mechanism
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subduction
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world’s most explosive volcanos are formed over
subduction zones
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Oregon’s Cascades Mtns. over Cascadia Subduction Zone
–
Juan de Fuca plate under N. American plate
“Munch and Crunch”
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“MUNCH” - subduction of oceanic plate under continent or
ocean
–
oceanic crust is thin and dense
–
dives
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“CRUNCH” - collision of continental plates
–
India into Tibet and China
–
continental crust is thick and light

Sliding By
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Transform faults
–
plates neither created nor destroyed
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Transform faults are active
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Fracture zones are inactive extensions of transforms
–
“fossil transforms”
Young & Old
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Oldest seafloor - 200 million years
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Oldest land - billions of years
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With seafloor spreading, is the earth expanding?
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Why is seafloor so young relative to continents?
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SUBDUCTION
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“law of conservation of ocean floor”
Convergent
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Convergent - subduction

–
trenches
–
Tonga Trench, Cascadia Subduction Zone
Divergent Plate Boundaries
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Divergent - seafloor spreading
–
mid-ocean ridges or rises
–
Mid-Atlantic RIDGE, East Pacific RISE
Translational
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Translational - strike-slip faulting
–
transform faults (active) - San Andreas Fault
–
fracture zones (inactive) - Mendocino FZ
Plate Boundaries
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How do we know where these boundaries are?
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bathymetry
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earthquakes
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deepest earthquakes at CONVERGENT boundaries
(subduction zones/trenches)
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downgoing plate breakage causes earthquake

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like potato chip breaking off in bean dip