Introduction to Plate Tectonics

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Introduction to Plate Tectonics

• A Revolution in the Earth Sciences -
• Peter Wyllie
• GLY 2010 , Lecture 3

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Alfred Wegener, 1880-1930

• German meteorologist
• Published a book in 1915 whose
• (translated) title is
• The Origin of Continents and Oceans
• Wegener proposed the idea of Continental Drift

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Continental Drift

• Pangaea supercontinent containing all land
• Based on evidence available in early 1900s

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Wegeners Evidence

• Fit of continents when map cut apart and
  rearranged
• Climate similarities in adjacent areas

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Lithologic (Rock) Evidence

• Unusual rocks found only where continents fit
  together
• Cratons cores of continents, strongly showed
  this pattern
• Karoo (South Africa) and Santa Catarina (Brazil)
  formations appear identical

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Fossil Evidence

• Glossopteris had left leaf remains in large areas
  of Southern Hemisphere
• Wegener concluded that southern continents must
  have been joined

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Fate of Continental Drift Hypothesis

• Biggest objection How to move a continent?
• Wegener died in Greenland in 1930 before most
  people accepted his ideas
• Wegeners ideas languished until the end of WWII
• Use of submarines during the war spurred research
  after the war
• This lead to oceanographic exploration

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Oceanographic Exploration

• Ocean floors were mapped to add submarine
  navigation
• Knowledge gained revitalized Wegeners ideas

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What Does Plate Tectonics Mean?

• Plate Large, Rigid slab of rock
• Tectonics comes from Greek root meaning to
  build

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Plates

• The earths surface is divided into about a dozen
  major plates
• Composed of lithosphere - crust plus the extreme
  outer mantle
• Lithosphere comes from lithos, meaning stony, and
  sphere - hard and rigid
• Lithosphere extends from the surface to the top
  of the mantle

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Map of Major Tectonic Plates
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Mid-ocean Ridge Map
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MOR Video
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Alvin

• Jan Morton entering Alvin

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East Pacific Rise SegmentComputer Generated Image

• Yellow to red shows high elevation
• Green to blue shows lower elevation
• Latitude 9 north

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Asthenosphere

• Behaves as a plastic - a solid that may deform
  slowly
• Plastic because it is hot and under pressure
• Extends a few hundred kilometers below the
  lithosphere
• It is entirely in the upper mantle

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What Supports the Plates?

• Lithospheric plates float on the asthenosphere,
  which is denser than the lithosphere

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Sea-floor Spreading

• Concept came from oceanographic investigations
• Uses Convection cells, an idea Wegener would have
  been familiar with

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Convection Cell

• Heat beaker
• Water expands and rises
• It spreads and cools at the top
• Cool water sinks

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Harry Hess, 1906-1969

• In Navy during WWII
• Rear Admiral in Naval intelligence
• Commented that geologists make good intelligence
  officers because they can work with incomplete
  data sets

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Hess in WWII

• Keenly interested in geology of ocean basins
• Used time between battles to collect data
• Collected echo-sounding surveys of ocean depths

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Hess at Princeton

• After WWII, Hess became Professor of Geology at
  Princeton University
• Used WWII data to publish a paper called History
  of the Ocean Basins in 1962
• Paper outlined idea of sea-floor spreading
• Robert Dietz, working independently, proposed a
  very similar concept

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Hess-Dietz Hypothesis

• Asthenosphere contains numerous convection cells
• Cells cause molten rock (magma) to rise
• Some magma erupts on surface
• Most magma stays beneath the surface and spreads,
  carrying lithospheric plates with it, and slowly
  cooling

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Hess-Dietz Hypothesis, Cont.

• Cooling magma sinks, completing convection cell
• Mobile sea-floor helped to answer several puzzles

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Mid-Ocean Ridge

• Click to start

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Spreading Center

• Click to start

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Mantle Convection Cells

• New crust created by magma hardening at the
  Mid-Ocean Ridge (MOR)
• From other data, we know the earth is not
  expanding
• Crust must be destroyed somewhere

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Puzzles Solved

• Why is there so little sediment on ocean floor?
• What are the rock ages so young?

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Seismic Evidence

• In 1935, K. Wadati showed earthquakes occur at
  greater depths toward the interior of the Asian
  continent
• Earthquakes further toward the Pacific Ocean
  occurred at shallower depths
• H. Benioff later observed the same distribution
  in other regions

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Age of Ocean Fossils

• Continental fossils are at least 3.5 billion
  years old
• Oldest marine fossils are about 180 million years
• Since life is though to originate in the oceans,
  why arent ocean fossils older?

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Subduction Zones

• The key to subduction is the density of the rock
  types involved
• Density mass/unit volume

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Rock Densities

• Continental lithosphere is about 3.00 grams/cubic
  centimeter
• Oceanic lithosphere gradually increases in
  density as it ages, reaching a maximum value of
  about 3.28 grams/cubic centimeter

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Converging Plates

• When two plates collide, the denser plate will
  sink (subside) beneath the less dense plate
• Density differences as small as 1 are enough to
  cause subduction

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Subduction
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Plate Movement

• Plates move slowly (up to 15 cm/yr)
• Plates may collide, move apart, or slide past
  each other
• Friction during plate movement often generates
  earthquakes

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Asthenosphere Density

• The density of the asthenosphere is about 3.3
  g/cm3
• Density increases with depth below the surface