PLATE TECTONICS

Continental Drift

Alfred Wegener first proposes Continental Drift in his book published in 1915. Suggests that 200 million years ago there existed one large supercontinent which he called Pangaea (All Land)(Figure). This was not really a new idea, but Wegener offered several lines of evidence in support of his proposal.


  1. Fit of the Continents - Noted the similarity in the coastlines of North and South America and Europe and Africa. Today the fit is done at the continental shelf and it is nearly a perfect match.

  2. Fossil Similarities - Mesosaurus, (Figure) reptile similar to modern alligator which lived in shallow waters of South America and Africa.

  3. Rock Similarities

    1. Rocks of same age juxtaposed across ocean basins. (Figure)

    2. Termination of mountain chains. (Figure)

  4. Paleoclimatic Evidence

    1. Glacial deposits at equator

    2. Coral reefs in Antarctica

Idea was rejected by North American geologists because Wegener couldn't come up with a mechanism for continental drift. Suggested tidal forces, but physicists showed this to be impossible. Wegener dies in 1930 and his idea dies with him.

Magnetism and Paleomagnetism

Earth is a bar magnet with a magnetic north and south. At poles a compass needle dips vertically. Downward at the north pole, upward at the south pole and horizontal at the equator. Magnetic poles do not correspond with geographic poles. Variation is termed the magnetic declination. It is 16 degrees east in California. However, it has been found that even though the magnetic and geographic poles do not correspond today when the location of the magnetic north pole is averaged over a 5,000 year period it does correspond with geographic north. Magnetic pole moves as much as 25 km per year.

Causes of Earth's Magnetism

Paleomagnetism

In the 1950's scientists discover how to measure paleomagnetism (magnetism frozen in the rock at the time it formed). With this knowledge scientists could tell the direction and latitude of geomagnetic pole at the time the rock formed. Europeans were the first to extensively study paleomagnetic pole locations and found that by 500 MY ago magnetic north was located near Hawaii. At first it was assumed the poles were free to wander (Apparent Polar Wandering). North American geologists attempted similar studies largely to disprove the Europeans and found that 500 my ago North American rocks showed the magnetic north pole to be in the East Pacific, 3000 miles to the west of the European magnetic north at that time. (Figure)


At the same time a series of bathymetric surveys of the ocean basins revealed a system of ridges and trenches with high heat flow over the ridges. H. Hess (1962) rushes to print with the idea of Sea Floor Spreading. Postulates convection cells beneath ocean basins to drive the spreading.(Figure)


Fred Vines supports Hess with his explanation of symmetrical magnetic stripes on either side of the Atlantic Mid-ocean ridge (Figure).


Plate Tectonics

Theory of Plate Tectonics is born. Plate - is a rigid piece of lithosphere floating on a partially plastic substrate (asthenosphere).

Seven Major Plates (See Figure in your Text)

  1. Pacific
  2. North American
  3. South American
  4. African
  5. Eurasian
  6. Antarctic
  7. Indo-Australian

Types of Plate Boundaries (Figure)


  1. Divergent (Figure)- Spreading Center - Constructive Margin. Characterized by ocean ridges and sea floor spreading.

  2. Convergent - (Figure) Characterized by trenches and island arcs

    1. Ocean - Ocean (Japanese Islands)
    2. Ocean - Continental (Cascade Mountains)
    3. Continent - Continent (Himalayas)

  3. Transform - Plates moving past one another along strike- slip faults. (Figure)

Additional Evidence in Support of Plate Tectonics

Driving Force (Figure)

  1. Convection Cells

  2. Hot Spots