Week 21: Plate Tectonics

Plate Tectonic Theory is relatively new. It was developed by synthesizing the ideas of Continental Drift and Sea Floor Spreading.

  1. Continental Drift
  2. Proposed by Alfred Wegener in 1912
  3. Wegener compiled the evidence to suggest all the landmasses were once connected as a supercontinent he called PANGEA.
  4. Continents seem to fit like pieces of a jigsaw puzzle
  5. Fossil evidence: Mesosaurus fossils found on S. America and Africa.
  6. Matching Mountain Ranges: Appalachians of N. America match the Caledonians of Scandinavia
  7. Glacial deposits in locations that now have a tropical climate – Striations on bedrock in central Africa.
  8. Tropical deposits in locations that now have a polar climate – Coal and oil in Alaska.
  9. Problem with Wegener’s theory was he couldn’t explain what could cause the continents to drift (NO MOTIVE). No known force at the time could be strong enough to move entire landmasses.

  1. Sea Floor Spreading – Toward the end of WWII a lot of information about the sea floor was being gathered by submarines.
  2. Sonar was used to determine the depths of the seafloor.
  3. Large underwater volcanic mountain ranges were discovered MID OCEAN RIDGES.
  4. Harry Hess a geologist with the US Navy was able to collect and date rocks from the bottom of the Atlantic on either side of the Mid Atlantic Ridge. The youngest rocks were nearest the ridge and as distance from the ridge increased so did the age of the bedrock. The age of the sea floor was symmetrical on either side of the ridge.
  1. Paleomagnetism (ancient magnetism) also suggested Sea Floor Spreading. Bands of normal and reversed polarity rocks were also symmetrical on either side of the ridge.

  1. The driving force behind the spread is CONVECTION CELLS IN THE EARTH’S MANTLE (asthenosphere).
  2. In the 1930’s seismic wave analysis led to the distinction between the Earth’s Inner and Outer cores. The interior was known to be hot. And the heat of the Earth’s interior causes the Mantle material to convect, dragging the lithospheric plates along.

  1. Sea Floor Spreading provided a possible mechanism for Wegener’s theory of Continental Drift.
  1. Plate Tectonic Theory – Combining Continental Drift and Sea Floor Spreading theories led to the current Plate Tectonic Theory
  1. If new ocean floor is being created at the Mid-Ocean Ridges as Harry Hess suggested, and the Earth isn’t getting any larger, then somewhere in the world old ocean crust must be getting destroyed  SUBDUCTION.
  1. The mid-ocean ridges are Divergent boundaries ().
  1. Convergent boundaries () could result in subduction as long as oceanic (mafic & dense) crust is involved.
  1. Transform boundaries exist where the plates are moving past each other (not colliding or separating, just slipping by one another). The San Andreas Fault in California is a great example here in the US.
  1. Hot Spots are locations of rising magma that is independent of the mantle convection cells. Yellowstone and Hawaii are Hot Spots.

Know all the features associated with each type of boundary. Study the chart below.

Transform Plate Boundary / Divergent Plate Boundary
San Andreas Fault – Shallow EQ’s / Mid-Atlantic Ridge – New oceanic crust is formed as mafic magma rises due to convection currents in the mantle.
Convergent Plate Boundary
Continent & Continent / Continent & Ocean / Ocean & Ocean
Because both plates are felsic (low density) the plates rise up in an orogeny instead of subduction. Earthquakes are moderate and shallow depth. / The oceanic (mafic) plate is more dense then the continental (felsic) plate, so the oceanic plate subducts beneath the continent. A trench forms. Deep, shallow & moderate EQ occur. Volcanoes also are present. / Since both plates are mafic, the cooler (older) plate is the one that subducts under the other. Volcanoes form here too. Chains of volcanic islands created by this kind of subduction are called Island Arcs. EQ’s are of moderate and deep depths.
Hot spots
/ The Hot Spot doesn’t change location. The plume of rising magma is independent of the convection of the mantle. The lithospheric plate moves over the hot spot carrying volcanic islands away from the plume. Over time the volcanoes disconnect from the plume and become extinct. The islands farther from the hot spot are older and smaller. If they erode below the sea surface they are called sea mounts.
  1. The pattern of where earthquakes occur and their depths along with the locations of volcanoes led to identifying all the plate boundaries.

  1. Putting everything together gives us a complex and dynamic crust, with regions of separation creating new ocean floor, regions of destruction were old ocean floor returns back down into the mantle, zones of frequent earthquake activity where plates slide past each other, and other areas away from the boundaries called passive margins.

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