I.  Introduction



     A.  Tectonics -- study of the large-scale movements that are
          responsible for the gross structure of the crust and mantle. 


     B. Pre-1960's Notions



          1.  The crust of the earth was believed to be immobile 
               (fixed); continents and ocean basins were fixed and 
               unchanging.



          2.  Large-scale structures were produced by vertical forces
               (sagging and warping)



          3.  Problems



               a.  Geologists were never able to put forth an all 
                    encompassing picture to explain volcanism, 
                    earthquakes, faulting, heat flow, folding,
                    metamorphism, etc.


               b.  Geometric fit of continents --> Sir Francis 
                    Bacon


     C.  Early ideas on continental drift
 	

          1.  Proposed by Alfred Wegener (1912)


          2.  Evidence


               a.  paleoclimatology
               b.  fossils
               c.  geometric fit of continents
               d.  continuation of truncated structures and rocks

                    across oceans


          3.  Supercontinent----->Pangaea 



               a.  180 Ma it later split up into: 

                    Gondwanaland (southern continent) 
                                   and 
                    Laurasia (northern continents)


               b.  South America and Africa split approximately 70 
                    Ma as the southern Atlantic Ocean opened



          4.  Discounted by physicists who said that from what they

               knew of the physical properties of the earth that drifting of the 

               continents across the earth was impossible.


          5.  Idea died as Wegner died in 1930
	

     D.  1950-1960


          1.  Revival of interest in hypothesis



          2.  Exploration of seafloor

               a.  Discovery of Mid-Ocean ridge
                    (1000 km wide; 65,000 km long)
               b.  Subsea topography
               c.  Heat flow measurement
                    1.  Mid-Ocean ridge--hot zone of upwelling magma
                    2.  Heat flow values decline away from ridge
                    3.  Polar wandering
                         a. Remember-- volcanic rocks when they cool
                              have minerals (magnetite) that align
                              themselves in the direction of the 
                              magnetic field of the earth.
                         b.  If continents and ocean basins were
                              fixed in place then the magnetic poles
                              must have wandered
                         c.  But we know that the magnetic axis is
                              essentially fixed, so the continents
                              must move with respect to one another
	

          3.  Magnetic Stripes discovered in 1963


     E.  1967--Beginning of a Revolution


II.  Statement of Plate Tectonics Theory


     A.  Plate tectonics Jargon


          1.  Lithosphere (relatively rigid);
               a.  crust and upper mantle     
               b.  thickness of plate 

                      Continental areas 100-150 km
                      Oceanic regions     60-80 km

               c.  bottom defined by 1200°C isotherm
               d.  Lateral boundaries defined by seismicity


          2.  Asthenosphere (relatively plastic)

               ---bottom at ~700 km

               ---low-velocity zone  upper 200 km

                    (partially melted)


          3.  Mesophere  700-3000 km


     B.  Lithosphere broken into 10 large plates and many smaller ones


          1.  Interaction of plates produce and explain the 

               distribution of:
               a.  Volcanos
               b.  Earthquakes
               c.  Igneous rocks
               d.  Distribution of ocean and continents (continents
                    embedded in the lithosphere) like "chips in a
                    chocolate chip cookies)


     C.  Plate Boundaries (defined by process)


          1.  3 types  (with gradations)


          2.  Divergent boundary
               a.  Ridges
               b.  where plates grow
               c.  new lithosphere formed by the upwelling of mafic
                    magmas from the asthenosphere
               d.  plates are under tension at ridge
               e.  shallow earthquakes  0-70 km
               f.  basalts, gabbro and peridotite cool to form
                    lithosphere

          3.  Convergent boundary
               a.  Because the earth is not expanding,formation of new
                    plates must be balanced by plate destruction


                    Process = subduction


                    Place where it happen = subduction zone


               b.  topographic expression = trenches
               c.  earthquakes of all focal depths


                     shallow (0-70 km),
                     intermediate (70-300 km)
                     deep (>300-700 km)


                        Benioff Zone

               d.  associated with island arcs and intermediate 

                    composition volcanism

               e.  "wet" lithosphere heated as it goes down

                    subduction zone by shear friction and conduction

                    from asthenosphere
               f.  "Ring of Fire"
               g.  Thrust belts and extensional belts


          4.  Transform boundaries
               a.  Conservative plate boundary
               b.  No addition or subtraction of plates occurs
               c.  San Andreas fault; Pacific plate moving north


     D.  Implications of Plate Tectonics


          1.  Formation of mountains
          2.  Distribution of continents and oceans
          3.  Growth and evolution of continents
          4.  Predictions for the future


     E.  Movement of Plates


          1.  What drives them
               a.  part of the heat dispersive system of the earth
               b.  plates probably move due to  gravity
          2.  How fast do plates move?
               a.  subduction

                    ---Nazca-South America 18 cm/yr
               b.  mid-ocean ridges spreading
                    ---Atlantic ocean  3 cm/yr
               c.  transform faults
                    ---SAF  6 cm/yr

     F.  Relative motion vs. absolute motion