Continental Drift, SeaFloor Spreading, and Plate Tectonics

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Continental Drift, Sea-Floor Spreading, and Plate
Tectonics

• Topics to be Covered - 1
• Early Ideas about Continental Drift
• Lack of a Mechanism
• Paleomagnetism and the Revival of Continental
  Drift
• Evidence from the Ocean Basins

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Continental Drift, Sea-Floor Spreading, and Plate
Tectonics

• Topics to be Covered - 2
• Hypothesis of Sea-Floor Spreading
• Confirmation of Sea-Floor Spreading
• Plate Tectonics
• Mountain Building Part of the Story

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From Continental Drift to Plate Tectonics
Geological Revolution

• Early Ideas about Continental Drift
• Pre-1900 idea drifting of continents
• Breakup of a supercontinent
• Pangaea
• Drifting away from each other.

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Physical Features of the Earth
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Land of 200 Million Years Ago
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Alfred WegenerThe Courage to Be Right
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Continental Drift - 2

• Ideas of Alfred Wegener -1
• Fit of Africa and South America
• Features common to southern continents
• rock types and ages
• Glaciation of late Paleozoic age
• Similar Permo-Triassic fossils
• Plants - Glodsopteris
• Reptiles - Mesosaurus

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Fit of South America and Africa
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Permian Glaciation
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Long swim for Mesosaurus
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Why they didnt have to swim or float
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Continental Drift

• Ideas of Alfred Wegener -2
• Paleoclimatic evidence
• - Either continents had a different orientation
  with respect to the poles in the past.
• - Or, the poles have wandered.

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Wegeners Downfall Lack of a Mechanism

• Support from the southern continents
• Some acceptance in Europe
• Almost total skepticism in N.A.
• Condemnation by American geophysicists.
• NO MECHANISM!!!

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Alfred WegenerThe Courage to Be Right
Yes, but Watch my Comeback!
Yeah
W
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Paleomagnetism and the Revival of Continental
Drift

• Nature of Paleomagnetism
• Magnetism a property of iron- bearing minerals.
• If permitted to do so, these grains align
  parallel to earths magnetic field.
• Records position of earths magnetic poles.
• Determined in laboratory from field samples.

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Paleomagnetism, etc.Results of Paleomagnetic
Studies

• Plotting position of pole based on North
  American data shows an apparent wandering
  through time.
• 13,000 MILES TO PRESENT POSITION IN ARCTIC
  CIRCLE.

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Polar Wandering ?

• The symbol
• shows the
• path of the
• NMP from
• Precambrian
• To
• Cretaceous.

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Paleomagnetism, etc.Results - 2

• However, the position of the pole from European
  data does not match that of the data from North
  America.
• Indeed, each continent appears to have its own
  path for polar wandering.
• Because this is impossible, continents must have
  been moving about during this time.
• Only recently have they moved to the present
  position.

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Evidence from the Oceans

• World War II brought a spectacular advance in our
  abilities to study the ocean.
• Among the discoveries were
• Guyots
• Heat-flow measurements
• Thickness of the oceanic crust
• Thickness and age of sedimentary deposits on the
  ocean floor.
• Earthquakes along ridges, trenches.
• Very large fractures, most of them faults.

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Exploring the Ocean Floor
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Volcanoes and Guyots
Guyot
Volcano
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Significance of Guyots

• Flat-topped mountains believed to be eroded
  volcanoes.
• - Implies erosion at sea level.
• May occur in chains at right angles to
  mid-oceanic ridges.
• The farther they are from the ridges, the
• Deeper they are.
• Odder they are.

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Significance of Guyots

• Explanation
• Volcanoes formed as oceanic ridge and tops eroded
  by waves.
• Sea floor spreads out from ridges, slowly
  carrying guyots into deeper water as new ones
  form.

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Heat-Flow Measurements

• Areas of high heat flow
• - On continents young mountain ranges.
• -On ocean floor over oceanic ridges.
• Area of low heat flow over oceanic trenches.
• Over oceanic trenches.

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Heat-Flow MeasurementsA Puzzle

• Most of radioactive minerals are in rocks of the
  continental crust.
• Yet, on the average, heat flow from the oceans
  and continents is the same.
• Needed a mechanism to redistribute the heat
  beneath the continents and ocean floors.

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Additional Sea-Floor Discoveries

• Oceanic crust (lithosphere) 5 km , much thinner
  than for continental crust.
• Furthermore, of uniform thickness.
• Sediments on sea floor no more than 1000 feet
  thick.
• No rocks on ocean floor older than Jurassic.
• Earthquakes along the ridges, trenches, large
  fractures.
• Faults with movements of over 1000 km.

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Hypothesis of Sea-Floor Spreading

• Convection currents in mantle rise under oceanic
  ridges and spread.
• Driving force is here transferred from core to
  mantle.
• Oceanic crust (basaltic) created at ridges.
• Crust plus upper mantle (lithosphere) move
  laterally away going along for the ride.

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Hypothesis of Sea-Floor Spreading
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Sea-Floor Spreading - 2

• Lithosphere plunges into oceanic trenches.
• Does this explain the anomalies of ocean floor
  heat distribution?
• Continents dont drift through the mantle but are
  passengers.
• Oceanic crust has to be young because older rocks
  have been
• Plastered onto the edge of continents.
• Thrust down into the mantle.
• Why are the sedimentary materials on the ocean
  floor so thin?

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Confirmation of Sea-Floor Spreading

• Earths magnetic field periodically reversed.
• The age of these reversals determined by
  radiometric dating.
• Magnetic anomalies found on both sides of oceanic
  ridges.
• Preserved in basalt formed in center of rise.

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Magnetic anomalies
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Confirmation of Sea-Floor Spreading
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Confirmation of Sea-Floor Spreading

• Through sea-floor spreading, sea floor has served
  as a tape recorder working in stereo.
• Preserved record of reversals and their time of
  occurrence.
• Can also determine rate of spreading.
• ½ inch per year in North Atlantic
• 2 inches per year in South Pacific

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Plate TectonicsBasics

• Earths lithosphere broken into large plates.
• Upwelling of magma creates new oceanic crust.
  Added to edge of plate.
• Plates diverge from ridge.
• Elsewhere, plates
• Pass into oceanic trenches (subduction).
• Collide with each other.
• Push past each other along great strike-slip
  faults.

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Plate Boundaries
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Convergent Plate Boundaries
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Alfred WegenerThe Courage to Be Right
How sweet it is!!
Yeah
W
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Plate Tectonics

• Collisions can involve an oceanic plate and a
  continental plate, two continental plates, or two
  oceanic plates.
• Continents do not drift, but are rafted about.
• Some oceanic basins are steadily widening others
  are closing.
• Driving mechanism believed to be convection
  currents of some type.
• Shallow or deep?

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Convection Cells
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Hypotheses About Convection Cells
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Plate TectonicsOther Considerations

• Mantle plumes doming, splitting, radial flow.
• Transform faults
• Strange movements what movements would you
  predict?

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Transform Faults
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Plate Tectonics

• Benioff zones long narrow earthquake zones
  dipping into the lithosphere.
• Created by rupture of subducting plate.
• Hot spots where plumes of magma rise from the
  asthenosphere.
• Remain fixed
• Plate migrates over, creating chain of volcanoes.
• Hawaii, Yellowstone

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• Continental Rifting

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Terminology Used in the Study of CD, SFS, and PT

• Apparent polar wandering
• Continental drift
• Guyot
• Magnetic anomaly
• Paleomagnetism
• Pangaea

• Plate tectonics
• Polar reversals
• Polar wandering
• Rifting
• Sea-floor spreading
• Transform fault

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Student responsibilities with regard to CM, SFS,
and PT

• Know the two parallel stories, the scientific and
  the human-interest histories as continental drift
  eventually evolved to sea-floor spreading and
  then on to plate tectonics. Understand how
  paleomagnetism played not one, but two, major
  roles in this story. Know other lines of evidence
  used for and against these ideas. Be able to
  concisely state the basic aspects of modern plate
  tectonics.

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Alfred WegenerThe Courage to Be Right
Goodbye now. Overcome adversity.
Yeah
W
49
Alfred WegenerThe Courage to Be Right
And kiss my ass!!
Yeah
W