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OCE 2001 - Introduction to Oceanography
Chapter 3 – Plate Tectonics
I. The Earth's Interior
A. Density stratification
1. Distinct layers with densest at center
2. Rock 'behavior' determined by temperature, density
and stress
 from brittle to plastic ("deformable") to elastic
B. Core is 90% iron with nickel
1. 5,500-6,600OC (9900-12000OF)
2. Solid inner core and viscous liquid outer core
3. Differential spin of core  Earth's magnetism
C.Mantle
1. Iron and magnesium silicates
2. 2500OC (4500OF)
3. Inner mantle is rigid
4. 'Upper' mantle (Asthenosphere) partially molten 
plastic
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D.Lithosphere = outermost 'upper' mantle + crust
1. Crust is rigid, 'cold' (500OC, 900OF)
a. Continental crust is thick (30-40 km)
 Granite (rich in aluminum & silica), less dense
b. Oceanic crust is thin (5-7 km)
 Basalt (rich in magnesium and iron) is denser
2. Isostasy
a. Lithosphere 'floats' on the plastic asthenosphere
b. Thicker, less dense continental crust floats higher
II. Historical perspective of continental drift & plate
tectonics – the big picture
A. Early observations
1. Continents fit together – Leonardo da Vinci
2. Similar fossils on separate southern continents
3. Earthquake patterns similar to oceanic ridges
 Ring of Fire
B. Alfred Wegener (1912) – proposed continental drift
hypothesis
 Celestial mechanism incorrect –lost favor by 1930's
C.Later observations
1. Echo-sounding  ocean ridges match continental
margins
2. Radiometric dating  ocean crust < 200 my; cont.
> 3.9 by
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D.Seafloor spreading proposed by Harry Hess &
Robert Dietz (1960)
1. New seafloor created at mid-ocean ridges 
enlarged oceans
2. Creation of seafloor balanced by subduction
E. Integrated Plate Tectonic theory
1. Proposed by J. Tuzo Wilson (1965)
2. Powered by three forces…
a. Gravity's pull of descending plate's dense
leading edge
b. Convection current's friction on bottom of plate
moves plates
c. Outward push of new seafloor at spreading
centers
3. Satellite data verifies process – shows continents
moving few cm/yr
F. Major paradigm shift in Geology and Oceanography
1. Helps explain mountain building
2. Helps explain distribution of plants and animals,
and their evolutionary history
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III. Plate tectonic theory – the process
A. 3 plate boundaries - Divergent, Convergent,
Transform-fault
B. Divergent boundaries
1. Form on thicker continental crust  traps heat
a. Crust rises and fractures  rift valley & lakes
 African Rift Valley & lakes
b. Deepens  ocean water invades to form
oceanic ridge
2. Magma breaks through to surface and solidifies
a.  new crust pushes away from valley - currently
~5 cm/yr
b. Ridges are centers of seafloor spreading
c. Mid-Atlantic Ridge split Pangea
1) Laurasia  North America and Europe
2) Gondwanaland  South America and Africa
3. Seafloor spreading continues to enlarge basin
a. Cools & shrinks  denser rock sinks deeper
away from ridge
b. Sediment gradually builds up, deeper toward
basin margins
4. Forms passive ('Atlantic-type') basin margins
a. No mountain building or subduction
b. Thick sediment and older oceanic crust
c. Important economically - oil, gas
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C.Convergence zones – two major types
1. Subduction zones  form trenches at plate
boundaries
a. Gravity-driven conveyor belt
1) Thinner, denser crust moves beneath less
dense layer
a) Pressure makes leading edge even denser
b) Gravity pulls edge deeper
  causes earthquakes as it moves deeper
c) Crust mixes with mantle down to core 
resorbed
2) Volcanoes form continental crust at
convergence downstream of subduction
a) Denser crust moves under  easily melted
(less dense) rock rises
b) H2O, gases, rock erupt as island arcs or
mountains
b. Oceanic-continental convergence
1) Oceanic crusts move below continents
2)  deep trenches offshore of N. and S. America
c. Oceanic-oceanic convergence
1) One plate is older, and hence denser
2) Forms deep oceanic trenches in W. Pacific
(island arcs)
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2. Compression zone  continental-continental
convergence
a. Plates of equal density collide
1) Uplifting causes mountain-building
2) Any oceanic crust between plates uplifted with
mountains
b. Examples:
1) Himalayas  Indian plate moving north against
Eurasian plate (some oceanic crust uplifted)
2) Appalachians formed during formation of
Pangea
3. Forms active ('Pacific-type') basin margins
a. Seismically active
b. Pacific's Ring of Fire
D.Transform plate boundaries is 3rd type of boundary
1. Major plates shear laterally past one another
2. Crust is neither created or destroyed
3. Seismically very active (e.g. San Andreas fault)
IV. Hot Spots
1. Surface expressions of plumes of magma  Shield
volcanoes
2. Lithospheric plate move across hot spot
a. creates chain of volcanoes
b. Hawaiian Is., Emperor Seamounts, Yellowstone
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V. History of the Ocean Basins – review
A. Pacific Ocean
1. Oldest ocean, formed from earlier spreading event
2. Surrounded by subduction zones  Ring of Fire
3. Large ridges in east and south
a. Seafloor betw. Asia & Australia is complex from
collisions
b. W. Pacific is oldest seafloor  mountainous &
deepest
c. Emperor Seamounts & Hawaiian Is. Indicate
direction change in plate movement
B. Atlantic Ocean
1. Expanding east-west at Mid-Atlantic Ridge
2. Early in its formation, Atlantic was scattered,
stagnant basin
a. Evaporation caused salt deposits
b. Anoxic seas accumulated organic matter  fossil
fuels
3. Subduction zones: east of West Indies & South
Sandwich Tr.
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C.Indian Ocean
1. Youngest ocean, formed by breakup of
Gondwanaland 125 mya
2. Most active – 3 active ridges with N-S spreading
a. Indian plate  under SE Asia at Java trench
b. Indonesian arc from collision of Australia & Asia
c. India moving north colliding w/ Asia 
Himalayas ~50 mya
D.Southern Ocean
1. Formed by spreading of ridges encircling Antarctic
a. Began ~50 mya when Australia separated from
Antarctica
b. Antarctica & S. America split ~20 mya
2. As seaway widened  cold, deep water formed
circumpolar currents
a. Changed warm climate to current Ice Age
b. Antarctic ice sheet formed ~14 mya
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