Topic 4 Plate TectonicsPlate Theory

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Topic 4Plate TectonicsPlate Theory

• GEOL 2503
• Introduction to Oceanography

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Two Parts

• Topic 4. The Birth of a Theory
• Topic 5. Plate Dynamics
• See videos on Learner.org under the Earth
  Revealed series
• See the U. S. Geological Survey on-line
  publication This Dynamic Earth at
• http//pubs.usgs.gov/gip/dynamic/dynamic.html

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Interior of the Earth

• Four main layers
• Inner core
• Outer core
• Mantle
• Crust
• Other layers
• Lithosphere
• Asthenosphere

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Inner Core

• Mostly iron
• Magnetized and very hot (4000-5500 ?C)
• Radius of 1222 km. (759 mi.)about the same size
  as the moon
• Solid due to intense pressure and heat

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Outer Core

• Mostly iron
• 3200 ?C, slightly cooler than Inner Core
• Radius of 2258 km. (1402 mi.)
• Has fluid or liquid characteristics

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Mantle

• Largest layer
• 2866 km. (1780 mi.) thick
• Cooler than core (1100-3200 ?C)
• Composed of magnesium/iron

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Crust

• Cold and rigid
• Oceanic crust
• Thin 10 km (6 mi)
• Basalt
• Denser
• Continental crust
• Thick 100 km (60 mi)
• Granite
• Less dense

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Lithosphere

• Crust and very upper mantle
• Cold and rigid
• Brittlecan crack and break
• Floats on asthenosphere
• Forms the plates in plate tectonics

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Asthenosphere

• Upper part of mantle below lithosphere
• Weaker than lithosphere, behaves plastically

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Evidence for Internal Structure

• From seismic waves
• P-waves (primary waves) are fast and can
  penetrate all states of matter
• S-waves (secondary waves) are slow and cannot
  pass through liquid outer core
• Patterns of seismic wave travel give information
  on earth internal structure
• Earthquakes and nuclear bomb testing

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Dont worry about the details, just know that
this is how we have learned of Earths internal
structure.
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Density

• Measure of mass per unit volume (m/v)
• Mass versus weight
• Units grams per cubic centimeter (g/cm3) or
  kilograms per cubic meter (kg/m3)
• Average density of Earth 5.51 (g/cm3)
• Pure water has a density of 1 g/cm3 which is
  equal to 1,000 kg/m3

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Theory of Continental Drift

• Continents move about on the surface of the Earth
• Proposed by Alfred Wegener in Early 1900s
• Evidence
• Fit of continents
• Rock and mountain formations
• Fossils
• Climatic evidence in fossils and rocks

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Where is Central America?
?
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The Glossopteris fauna
http//geomaps.wr.usgs.gov/parks/pltec/pangea.html
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Sea Floor Spreading

• Proposed by Harry Hess in 1960
• Proposed convection cells in the mantle as the
  mechanism for continental drift
• Oceanographic observations during the years after
  World War II provided evidence

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Theory of Sea Floor Spreading
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Evidence of Sea Floor Spreading

• Earthquakes along spreading and subducting areas
• Radiometric dating of rocks
• (youngest at ridge oldest, farther from ridge)
• Magnetism of rocks/polar reversals

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Not randomly distributed, but mostly restricted
to narrow belts
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Ocean crust is young (Y) at spreading centersand
older (O) in both directions away from spreading
centers
spreading centers
O
Y
O
O
Y
O
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Iceland is one place where the spreading center
comes up on land.
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Paleomagnetism

• New oceanic crust is formed as sea floor
  spreading continues
• Earths magnetic field reverses often as the
  ocean crust is formed
• Earths magnetism is recorded in the new crust
• The result is magnetic stripes in the sea floor
  parallel to ridges recording the magnetic
  reversals

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