Principles of Geology

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Principles of Geology

• Chapter 2
• Plate Tectonics - overview

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Alfred Wegener

• 1880-1930, German meteorologist
• proposed the hypothesis of continental drift in
  "On the Origin of Continents and Oceans" (1912)

• proposed that a supercontinent, named Pangea,
  started to split apart 200 million years ago
• Died while crossing the Greenland ice cap in 1930.

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Continental drift An idea before its time

• Evidence used to support continental drift
  hypothesis
• Fit of the continents
• Fossil evidence
• Rock type and structural similarities
• Paleoclimatic evidence

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Geological Evidence for Continental Drift

• geological matches
• mountain belts (Appalachian-Caledonides)
• age similarities (matching Archean cratons)
• stratigraphic similarities (e.g., Nova Scotia and
  Morocco)
• igneous provinces (Mesozoic basalts, Precambrian
  anorthosites)

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Paleoclimate evidence
Ice sheets covered big areas of southern
hemisphere 220-300 million years ago
Glacial striations
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The great debate

• Objections to the continental drift hypothesis
• Lack of a mechanism for moving continents
• Wegener incorrectly suggested that continents
  broke through the ocean crust
• Strong opposition to the hypothesis from all
  areas of the scientific community
• Continental drift and paleomagnetism
• Renewed interest in continental drift initially
  came from rock magnetism
• Magnetized minerals in rocks
• Show the direction to Earths magnetic poles
• Provide a means of determining their latitude of
  origin

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Polar-wandering paths for Eurasia and North
America
Figure 2.11
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Plate tectonics The new paradigm

• Earths major plates
• Associated with Earth's strong, rigid outer layer
• Known as the lithosphere
• Consists of uppermost mantle and overlying crust
• Overlies a weaker region in the mantle called the
  asthenosphere

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Plate tectonics The new paradigm

• Earths major plates
• Plates move relative to each other at a very slow
  but continuous rate
• About 5 centimeters (2 inches) per year
• Cooler, denser slabs of oceanic lithosphere
  descend into the mantle

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Plate tectonics The new paradigm

• Plate boundaries
• Interactions among individual plates occur along
  their boundaries
• Types of plate boundaries
• Divergent plate boundaries (constructive margins)
  
• Convergent plate boundaries (destructive margins)
• Transform fault boundaries (conservative margins)
  

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We will come back later