Plate Tectonics

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• Plate Tectonics

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I. Continental Drift

• A. Alfred Wegener
• 1. First proposed the hypothesis, (1915)
• 2. German Meteorologist Geophysicist

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• 3. Published The Origin of Continents and
  Oceans

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• B. Continental Drift Hypothesis
• Super continent called Pangaea began breaking
  apart about 200 million years ago.
• Continents drifted to present positions
• Continents broke through oceanic crust

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• C. Evidence for Continental Drift
• 1. Map Fit- some of the continents seem to
  fit together like a jigsaw puzzle.

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• Ex S. America Africa

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• 2. Fossils-
• Fossils of certain species of ancient organisms
  were found on separate continents throughout the
  world. Mesosaurus

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• The continents must have been as ONE in order for
  these species to be found on different,
  widespread landmasses.
• Organisms were either too large or small to swim
  or fly to other continents without them being
  linked.

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• Land bridges could not have been the solution,
  because the remnants are nowhere to be found.

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• 3. Rock Structures
• Mountain chains of comparable age that appear to
  be severed by the oceans (Appalachians)

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• When these landmasses are reassembled they form a
  continuous mountain belt.
• N. America Europe
• S. America Africa
• S. America Antarctica

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• Ancient Climates
• Glacial deposits show that most of the Southern
  Hemisphere was covered by ice sheets.

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• Rock deposits left behind by moving glaciers
  were found in all southern continents India.
• Coal fields produced by ancient tropical plant
  fossils indicate that the Northern Hemisphere was
  once a tropical environment.

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• Main objection to Wegeners hypothesis was its
  inability to provide a mechanism for the movement
  of the continents.

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II. Plate Tectonics

• A. What is the Theory?
• The crust is broken into large moving sections
  called plates
• Lithosphere rigid layer made of the crust
  upper mantle
• Asthenosphere weaker upper part of the mantle

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• Explains the formation, movements, collisions,
  destruction of the Earths crust.
• Seven major plates
• Pacific (largest) N. American
• S. American African
• Eurasian Australian
• Antarctic

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• Plates include a continent mostly ocean floor.
• Plates are slowly moving continuously.

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• B. Plate Boundaries
• 1. What occurs at plate boundaries?
• a. Seismic activity
• b. Volcanism
• c. Mountain building

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• 2. Types of Plate Boundaries
• a. Divergent Boundaries
• 1. Where plates move apart
• (Constructive)

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• 2. Seafloor Spreading As divergent plates
  spread apart creating a fissure, magma flows
  through the fissure (rift valley.)

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• The lava then cools crystallizes to form new
  seafloor mountains (midocean ridges.)
• Younger rock is at the midocean ridges the
  older rock is at the continental margins.

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• b. Convergent Boundaries
• 1. Where plates move together, resulting
  in the subduction of oceanic lithosphere
  into the mantle.

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• 2. Types of Convergent Boundaries
• a. Oceanic-Continental
• Forms a subduction zone producing a
  deep-ocean trench.

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• Continental volcanic arcs are mountains formed
  by igneous activity associated with the
  subduction of oceanic lithosphere
• beneath a continent.
• Ex Andes, Cascades, Sierra Nevada

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• b.) Oceanic-Oceanic
• Occurs when two oceanic plates converge,
  one plate subducts below another.

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• Volcanic island arc forms as volcanoes
  emerge from underwater.
• Ex Mariana Islands
• Aleutian Islands

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• c. Continental-Continental
• Neither plate will subduct
• Causes the continents to fold buckle
  upwards forming mountains.
• Ex Himalayas, Alps, Appalachians,Urals

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• c. Transform Fault Boundary
• 1. Where plates grind past one another
  without the
• production or destruction of lithosphere

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• 2. Most transform faults are located
  within the ocean basins, a few cut
  through continental crust
• San Andreas Fault

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• C. Evidence
• 1. Paleomagnetism
• Study of ancient magnetism preserved in
  rocks
• Shows the location of the magnetic poles
  at the time of the rocks formation.

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• a. Polar Wandering
• Study showing that the magnetism of older
  rocks point in different directions.
• Showed that the magnetic poles either moved
  or the continents moved.

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• The Earths magnetic poles should remain
  close to the geographic poles
• If the magnetic poles dont move, the
  apparent movement must have been produced by
  the continents moving.

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• b. Magnetic Reversals
• Earths magnetic field reverses its polarity
  periodically.
• These reversals have been shown in the crust
  of the ocean floor
• Proves seafloor spreading.

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• 2. Earthquake Patterns
• If earthquake epicenters are plotted on the
  globe, it shows the outline of the plates.

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• 3. Ocean Drilling
• Drilling showed that the youngest rock is near
  the ridges the oldest is further from the ridge
• Proves seafloor spreading

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• 4. Hot Spots
• Rising plumes of magma
• As the plate moves above a hot spot, it will
  create volcanoes that produce underwater
  mountains and volcanic islands
• Hawaiian Midway Islands
• Used to track the direction a
• plate is moving.

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Hawaiian Islands
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• D. Breakup of Pangaea
• 1. Fragmented landmasses collided 500 to 225
  million years ago to form Pangaea
• 2. Pangaea began to break apart 200 million
  years ago during the Jurassic Period

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• 3. Break up formed two subcontinents
  Laurasia Gondwanaland
• 4. Those subcontinents began dividing about
  165 100 mya

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• E. Mechanisms Methods
• 1.Convection Current Hypothesis
• Hotter magma rises in the mantle

The magma cools and spreads
The spreading magma moves the plates laterally
The cooled magma then sinks back into the
mantle reheats
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• 2. Slab-Pull Slab-Push Hypotheses
• Colder oceanic plate will sink/subduct,
  pulling the rest of the plate with it.
• As new crust is formed at a rift valley, it
  pushes the plate forward.

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• 3. Hot Plumes Hypothesis
• Plumes of hot magma rise through the mantle,
  striking and pushing the plates in different
  directions