Plate Tectonics

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Plate Tectonics
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Continental Drift

• It was noticed that oceanic coastlines often fit
  together

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Continental Drift

• Wegener (early 20th century)
• Noticed same late Paleozoic (250 MYA) fossils in
• South America
• Africa
• India
• Australia
• Postulated that rather than these species
  (including plant species) having colonised 4
  continents (that were thousands of km apart),
  they actually lived when those 4 continents were
  together geographically

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Continental Drift

• Wegener put together existing continents like a
  puzzle
• Future scientists refined this model to the one
  seen here

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Pangaea

• Initially separated into two parts
• Laurasia
• Gondwanaland

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Continental Drift (more evidence)

• Late Paleozoic glaciation
• Deposits from this glaciation all over parts of
  Gondwanaland
• NO evidence of this glaciation in Laurasia
• In fact, in the present northern hemisphere, we
  find coal beds that were deposited in the late
  paleozoic
• Coal forms from the preservation and compaction
  of organic matter
• Usually requires a warm, swampy (i.e. NOT
  glaciated) environment

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Continental Drift (more evidence)
If these deposits were created with the
continents in their present positions, the
glacial event that affected almost all of India
would have affected most of the world
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Plate Tectonics

• The mechanism by which continents move
• Driven by mantle convection

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Plate Tectonics

• Rigid pieces of lithosphere (plates!) move atop
  the upper mantle
• Interact with each other

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Evidence for Plate Tectonics

• Paleomagnetism
• Magnetic poles reverse periodically for unknown
  reasons
• North becomes south, south becomes north
• Basalt contains magnetite (iron oxide, Fe3O4), a
  mineral that is magnetic, and that aligns itself
  with magnetic north upon crystallisation
• At a rift zone, new basalt is being generated
  constantly
• Magnetite crystals are aligned north in new
  basalt
• BUT

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• There is a pattern of magnetism on the ocean
  floor
• Areas of north alignment and south alignment are
  separate, and form stripes on the ocean floor
• Symmetrical around mid-ocean ridge

• Magnetic striping matches known record of pole
  reversal in rocks found elsewhere on Earth

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Age of rocks
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Plate BoundariesAn Overview

• Divergent boundaries
• Continental crust gt continental crust
• Oceanic crust gt oceanic crust
• Convergent boundaries
• Oceanic crust gt continental crust
• Oceanic crust gt oceanic crust
• Continental crust gt continental crust
• Transform boundaries

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Divergent Boundaries

• Places of new crust formation
• Ocean-ocean divergence driven by upwelling magma
  from the mantle
• Simplest model below

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Divergent Boundaries

• Continent-continent divergence
• Magma upwelling puts stress on continental crust
• Rifting occurs (crust tears apart)

e.g. East African Rift Valley
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Two examples
Eurasia
North America
Iceland a surficial expression of the
mid-Atlantic ridge
Western Africa
Eastern Africa
Kenya East African Rift Valley
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Convergent Boundaries

• Ocean-continent convergence
• Creates a subduction zone
• Denser oceanic crust will sink under buoyant
  continental crust
• Forms volcanic arcs (to be discussed in much more
  detail later)

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• Ocean-ocean convergence
• Also creates a subduction zone
• The older, colder (therefore denser) oceanic
  plate will subduct under the younger, more
  buoyant one
• Forms island arcs (also to be discussed in much
  more detail later)

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• Continent-continent convergence
• What happens when two equally buoyant, massive
  pieces of continental crust collide?

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Transform Boundaries

• Plates can slide against each other
• Motion is rarely smooth
• Stress can build up, and be released instantly
  with one plate moving centimetres or even metres
  at a time relative to another ? earthquakes
• Transform faults occur at many divergent and
  convergent scenarios
• We will discuss these later in the course

Transform Boundary