Plate Tectonics 2 Making oceans and continents

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Plate Tectonics 2Making oceans and continents
http//www.ucmp.berkeley.edu/geology/tectonics.htm
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Pangea seen at about225 mya
Collision of Laurasia and Gondwana
Sir Francis Bacon 1620
Benjamin Franklin 1782 The crust of the earth
must be a shell floating on a fluid interior.
Thus the surface of the globe would be broken
by movements of the fluids.
Wegener 1912 evidence
Breakup begins about 200 mya, floods about 190
mya
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Alfred Wegener 1912

• Continental drift hypothesis
• Continents "drifted" to present positions
• Evidence used in support of continental drift
  hypothesis
• Fit of continents
• Fossil evidence
• Rock type and mountain belts
• Paleoclimatic evidence

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Evidence Precise Matching of Continental
Shelves of Circum-Atlantic Continents
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Ranges of Triassic Reptiles
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Similar Rocks on opposite shores
Example, NJ and Morocco
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Why wasnt it popular?

• Objections to drift hypothesis
• Inability to provide a mechanism capable of
  moving continents across globe
• Wegener suggested that continents broke through
  the ocean crust, much like ice breakers cut
  through ice

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Attitudes change

• During the 1940s and 1950s technological strides
  permitted extensive mapping of the ocean floor
• Seafloor spreading hypothesis was proposed by
  Harry Hess in the early 1960s

http//pubs.usgs.gov/gip/dynamic/HHH.html
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Seafloor spreading in detail

• Harry Hess Convection currents in mantle
• Seafloor spreading occurs along relatively narrow
  zones, called rift zones, located at the crests
  of ocean ridges
• As plates pulled apart, magma moves into
  fractures and makes new oceanic lithosphere

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Hesss Seafloor spreading (cont)

• New lithosphere moves from the ridge crest in a
  conveyor-belt fashion
• Newly created crust at the ridge is elevated
  because it is hot and therefore occupies more
  volume than the cooler rocks of the deep-ocean
  basin
• Area also seems to be pushed up by mantle
  upwelling

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Vine and Matthews Test

• Magnetic North and South exchange places at
  irregular intervals average 100K years
• Very high variance
• Dates when polarity of Earths magnetism changed
  were determined from radiometric dating of lava.

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Vine and Matthews Test

• Geomagnetic reversals are recorded in the ocean
  crust
• Spreading predicts matching bands of lava
  polarity on either side of ridge.
• Vine and Matthews looked for symmetric magnetic
  stripes in the ocean crust
• Tow magnetometers record N or S

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IdeasEarth's Convection Cells
1963
This is the part we have been discussing. But the
hottest area is the core.
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Ideas Earth's Convection Cells
Jason Morgan (1971) In this model, Plumes and
convection cells are aspects of the same thing
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Ideas Earth's Convection Cells
Hawaii
MOR
Andesitic
Basaltic
Combination
A combined model.
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Hot Spots and Hawaii
Hot Spots are rising magmas from deep in the
mantle. As plates move over them, new volcanic
Seamounts are formed. Any that stick up above the
oceans surface as islands are eroded away, and
as they move away from the Hot Spot, they cool,
contract, and are submerged. They are then called
Guyots Their lavas are datable
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Hot Spots and Hawaii
Speeds vary from 1 to 10 centimeters per year
Initial Flood Basalt was partly subducted
Hey look, the direction changed!
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Hot Spots Plate Motions
Average 5 centimeters/year
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LAGEOS and GPS satellites determine that plates
move 1-10 cm per year, avg 5
Global Positioning Satellites
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Active Rifting of A Continental Plate
Note 3-D Triple Junction
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Active Rifting of A Continental Plate
Inactive Branch AulocogenSubsided Passive
Margins
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East African Rift Zone
Active Red Sea and Gulf of Aden. Failed Arm
Great Rift Valley (aulocogen)
Zagros Mts
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Mid-Ocean Ridge System Motion
Fracture Zones and Transform Faults
Shallow weak earthquakes
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Subduction-Zone Features
Note sequence from land to trench
Note ocean plate rocks that dont get subducted
in a collision
If a continent converges from the left, what
rocks will fold in the collision? Himalayas
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Mélange from California Coast
Sea-floor and land-derived sediments, some
volcanics. When stuffed down trench into Low
Temperature- High Pressure zone, with plenty of
ion-rich water, result is Blueschist
Metamorphic Facies
Source Betty Crowell/Faraway Places
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Continent Shield
Canadian Shield, North Americas crystalline
core exposed by glaciers
Terms Shield, Platform, Craton, Coastal Plain
Continental Platform
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Exotic (Displaced) Terrains
Collisions with Volcanic Island Arcs and
microcontinents
Continental Crust is thick and silica- rich, so
low density. Buoyant and hard to subduct. Also
Erosion resistant.
Sutures
Made of volcanic island arcs, backarc basins and
microcontinents Moved by transform faults, then
accreted
Anecdote Western California
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Paleogeography Reconstructions

• Orientation of magnetic minerals gives latitude
  (north or south of equator)
• Radiometric dates of ocean floor basalts, plus
  distance from ridge, gives paleolatitude for last
  200 million years
• Explains continental movements

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Origin of Pangaea
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Origin of Pangaea
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http//www.odsn.de/odsn/services/paleomap/animatio
n.html
Last 150 Million Years
Note Exotic terranes western N. Am. Opening of
the Atlantic Separation of Australia and
Antarctica India hits Asia forming
Himalayas Formation Central Am. isolates N.
Atlantic
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Mapping the ocean floor

• Three major topographic units
• of the ocean floor
• Continental margins (few 100s of meters
  below surface)
• Deep-ocean basins (Abyssal Plain 4 km below
  surface)
• Mid-ocean ridges ( top is 1 km below surface)
• Trenches (may be 11 km
  below surface)

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A passive continental margin
Found along coastal areas that surround large
oceans. Coast is far from central MOR Not near
active plate boundaries Little volcanism and few
earthquakes Example East Coast of US
(turbidites, graded bedding)
(Microfossil Ooze)
sand
Very thick sediments
Carbonate and silica ooze
muds
High-angle Normal Faults Of Divergent Margin
(Initial Rifting)_
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An active continental margin
Continental slope descends abruptly into trench
Example Pacific Ocean margin. Note Accretionary
Wedge at trench
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Ocean surface depths
Trenches 11 km deep. Abyssal Plain 4 km deep,
MORs average 3 km high so 1 km deep
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Continent
Back-Arc Basin
Trench Accretionary wedge Fore-Arc Basin Volcanic
Arc

• Abyssal plains
• Can be sites of thick accumulations of sediment
• Sediments thickest away from MOR
• Abyssal plains found in all oceans
• Studded by old cold seamounts and MORs

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Seafloor sediment

• Abyssal Plain is mantled with sediment
• Sources
• Turbidity currents on continent margins
• Sediment that slowly settles to the bottom from
  above fine mud and plankton
• Thickness varies
• Thickest in trenches accumulations may exceed 9
  kilometers.
• Thinnest at MOR new ocean floor

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Foraminifera (a.k.a. Forams)
http//www.geomar.de/zd/labs/stab-iso/forams.jpg
Form deepwater carbonate oozes, depths less than
4 km
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Chert sample
Only Silica below carbonate line
So Depth of pure Chert gt4 km
Small Phytoplankton (tiny floating plants)
Diatoms (siliceous Ooze SiO2)
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Bathymetry of the Atlantic Ocean
A few hundreds
1 km average
Abyssal Plain
Abyssal Plain
4 km
Atlantic Passive Margins Note no trench no
subduction
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The structure of oceanic crust
Black Smoker on cracks near magma
MORs characterized by an elevated ridge Closely
spaced normal faulting Mantle flow below pulls
the crust apart Newly formed basalt ocean floor
fills in cracks
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Black Smokers
http//collections.ic.gc.ca/geoscience/images/deta
il/F92S0220.jpg
Circulation of hot water in cracks at mid-ocean
ridge dissolves metals which are reprecipitated
as sulphide ores
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Ophiolite Suite
2 km
Some Serpentine due to hot water circulation
3-6km
3-6km
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End Plate Tectonics 2
Outcrop of pillow lava