Plate Tectonics

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Plate Tectonics

• Evolution of the Earth and Our Restless Planet
  Chapter 33

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Review Interior
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33.1 The Theory of Continental Drift

• Scientists in the early 20th century believed
• Oceans and continents were fixed in place
• The surface is a skin spread over a cooling
  interior
• Cooling caused contraction of the skin wrinkles
  and bumps aka mountains and valleys

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Alfred Wagner Thought Differently!

• He believed
• The continents are slowly moving
• All the continents has once been attached in a
  supercontinent and this was called Pangaea all
  land
• Pangaea split and the continents moved apart
• The boundary of each continent is not at the
  shoreline but the continental shelf

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Alfred Wegener cont.
No one believed Wagner..at least not yet!
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Scientific Revolution

• Discovery of the Earths Magnetic Field gave a
  window into the past
• Paleomagnetism (magnetism from the geologic
  pastshows 3 major things)
• The position of the magnetic north versus the
  geographic north pole
• Direction to the magnetic pole at the time the
  rock was formed
• Magnetic latitude of the rocks location at the
  time it was formed
• The movement of the magnetic pole over time(500
  million years) suggests that either the pole
  moves around or the continents move

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• The Earths North and South pole have flipped many
  times
• These leaves magnetic stripes in rock
  containing iron minerals

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Seafloor Spreading H. H. Hess

• 1950s had detailed mapping of the ocean floor
• Figured out
• Some really deep parts are next to the continents
  and some shallower parts are out in the middle
  due to the mountains under the water
• Why?

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Finally Wagner's Ideas became the Theory of
Plate Tectonics
The earths outer shell, the lithosphere, is
divided into 8 large plates and a number of
smaller ones Most earthquakes, volcanoes, and
mountains occur along the borders
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All Based on Motion

• It turns out that the upper section of the mantle
  is stuck to the underside side of the crust to
  form what we call tectonic plates

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Evidence of Tectonic Plates
Our first evidence of tectonic motion is based on
similar fossils and rock types on opposing sides
of the ocean
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Tectonic Plates
Today plate boundaries are determined by
examining the location of volcanoes and
earthquakes. Volcanoes result from the friction
(heat) of the plates motion. Earthquakes occur
where plate rub against one another
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Tectonic Plates
Volcanoes
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Tectonic Plates
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Why do the Plates Move?

• No single idea explains everything but we can
  identify several forces that contribute to the
  movement of the plates.
• Slab pull
• The sinking of the cooled dense oceanic plates
  pulls on the rest of the plate
• Ridge rises
• The material deposited on the top of the ridge
  slides downs from the rise pushing on the plate
• Convection
• Movement within the mantle could be part of the
  driving force behind the motion of the plates.

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Why do the Plates Move?
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Why are they spreading?

• Why is the Atlantic still getting wider
• The plates are pulled apart by convection
  currents in the mantle below
• Caused by heat released from natural radioactive
  processes
• At the mid Atlantic ridge molten rock from below
  rises up to fill the gap with new basaltic rock

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Plate Boundaries

• Convergent plates move toward one another
• Divergent plates move away from each other
• Transform-Fault plate moves sideways from each
  other

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Plate Boundaries
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Convergent Plates
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Convergent Plates
Ocean-Continental Convergence
Ocean-Ocean Convergence
Continental-Continental Convergence
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Divergent Plates
Spreading centers on the ocean floor and on land
(create rifts) These create the beginning of a
new ocean basin
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Transform Plates
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Transform Plates
San Andreas Fault
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Mid-Plate Hotspots
Hawaii
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Pangaea
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Pangaea
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Pangaea
The break up of Pangaea
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Where are we going?
We appear to be headed for another super
continent as North America, South America, Asia
and Australia converge in the ever shrinking
Pacific Ocean