THE THEORY OF PLATE TECTONICS

A.  BACKGROUND

        The continental drift hypothesis was initially rejected; but as new
        information concerning the earth's interior was discovered during
        the 1950s and 1960s, it was re-examined and eventually modified
        to become what is now known as the theory of plate tectonics.  

        The purpose of today's lecture is to learn the evidence supporting
        "plate tectonics" and to understand why this hypothesis eventually
        became an accepted theory.

B.  EVIDENCE

       In addition to the circumstantial evidence presented previously by
       Wegener, several new lines of evidence that emerged in the 1950s
       and 1960s supported the idea of "drifting continents".
 

       1.  EARTH'S INTERNAL STRUCTURE  (Close Up View)

            a.  Continental Crust
 

            b.  Oceanic Crust
 

            c.  Mohorovicic Discontinuity
 

            d.  Upper Mantle
 

            e.  Asthenosphere 
 

             f.  Lithosphere

       2.  MAJOR TOPOGRAPHIC FEATURES

            a.  Coastal mountain ranges
 

            b.  Offshore ocean trenches (map)
 

            c.  Mid-oceanic ridges
 

       3.  DISTRIBUTION OF EARTHQUAKES

            a.  Most earthquakes occur in narrow zones
 

            b.  Benioff Zone (descending band of earthquakes)
 

C.  HYPOTHESIS

       1.  LITHOSPHERIC PLATES

       2.  PLATE MOVEMENT

       3.  PLATE BOUNDARIES  (Cross Section)

            a.  Divergent ("Rift Zones"):  examples: Iceland; East Africa
 

            b.  Subduction:  example of the Andes Mountains
 

            c.  Collision:  example of Himalayas, (Mt. Everest)
 

            d.  Transform:  example of the San Andreas Fault
 

D.  HYPOTHESIS TESTING

       1.  AGE OF THE OCEAN FLOOR

       2.  DIRECT MEASUREMENTS 

       3.  DIFFICULTIES

            b.  How are large, brittle lithospheric plates able to withstand
                 the enormous forces necessary to move them?