Plate Tectonics

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Plate Tectonics
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lithosphere
asthenosphere
mesosphere
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Lithosphere (hard)
Asthenosphere (soft)
Mesosphere
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Earth formed 4.6 bya

• Inner Core- 4300oC
• mostly iron core
• inner part is so compressed that it is solid
• Outer Core- 3700oC
• iron and sulfur
• liquid

• Crust
• floats on top of lithosphere
• continental crust (granite)
• 20 to 70 km thick
• oceanic crust (basalt)
• 8 km thick

• Mantle- 1000oC
• mesosphere
• Solid
• 2300 km thick
• asthenosphere
• Soft
• 3000 km thick
• lithosphere
• hard
• 100 km thick

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Principles of plate tectonics

• The Earth is composed of a mosaic of thin rigid
  plates (pieces of lithosphere) that move
  horizontally with respect to one another
• Plates interact with each other along their plate
  boundaries
• Plate boundaries associated with tectonic
  activity (mountain building, earthquakes, active
  volcanoes)

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Continental Drift Theory
Alfred Wegener

• Proposed Theory of Continental Drift (1915)
• Failed to provide a mechanism

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• Evidence for Continental Drift
• continental shape
• similar geology
• fossil evidence (animal and plant)
• volcano and earthquake zone
• paleomagnetism

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Objections to the continental drift model

• Wegener envisioned continents plowing through
  ocean basins
• Wegener did not provide a plausible mechanism to
  explain how the continents could have drifted
  apart
• Most Earth scientists rejected continental drift
  because it was
• Too far-fetched
• Contrary to the laws of physics

Lacked technology
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Jigsaw Puzzle
Evidence for continental drift

• Matching coastlines on different continents

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Similar Geology
Evidence for continental drift

• Matching mountain ranges across oceans

300 million years ago
Today
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Evidence for continental drift

• Glacial ages and climate evidence

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Fossil Evidence
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Fossil Evidence
Distribution of fossils such as Mesosaurus
Mesosaurus
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Triassic 200 mya
Permian 225 mya
Jurassic 135 mya
Cretaceous 65 mya
Present Day
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Marie Tharp
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Marie Tharp's "World Ocean Floor Map
1977
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Evidence for plate tectonics

• Pattern of worldwide earthquakes (left) matches
  plate boundaries (right)

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Plate Tectonics
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Hess- Convection Cell Theory
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The 3 types of plate boundaries

• Divergent
• Convergent
• Transform

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Divergent plate boundaries

• The Mid-Atlantic Ridge is a divergent plate
  boundary where sea floor spreading occurs

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Divergent plate boundaries

• Iceland sits atop a divergent plate boundary
  where continental rifting occurs

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Divergent plate boundaries

• Formation of an ocean basin by rifting and sea
  floor spreading

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Convergent plate boundaries
a. Ocean-continent

• Convergent plate boundaries vary depending on the
  type of crust

c. Continent-continent
b. Ocean-ocean
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Convergent plate boundaries

• An ocean-continent convergent plate boundary
  produces the Cascadia subduction zone and Cascade
  Mountains

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Convergent plate boundaries

• A continent-continent convergent plate boundary
  produces the Himalaya Mountains

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Transform plate boundaries

• Transform plate boundaries occur between segments
  of the mid-ocean ridge
• Can also occur on land (ex San Andreas Fault)

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The world as it may look 50 million years in the
future
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Glomar Challenger (1960s) Deep sea ocean drilling
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Earth's Magnetic Field
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Mid-Ocean Ridge (Atlantic Ocean)






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There have been 170 reversal in the last 76
million years. The earths present orientation
has existed for the past 60,000 years.
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Age of the Atlantic
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Inquiry

• What layer of the mantle do the plates move on?
• What is the convection cell theory?
• Wegeners evidence for continental drift includes
  ______.
• Why was his theory disregarded?
• What did Marie Tharp discover?
• How are trenches formed?