THE THEORY OF ISOSTASY

A.  BACKGROUND

       Today's lecture addresses the topic of isostasy, a word used to
       describe how the principle of buoyancy applies to blocks of the
       earth's crust as they rest on the mantle.  In addition to learning  
       about isostasy, there are two other purposes to today's lecture:

       (1) To trace the development of a theory from observations that
            initially could not be explained.
           
       (2) To see how two different models (multiple hypotheses) can
            both explain the same observations with equal validity.
  

B.  DISCOVERY

       1.  OBSERVATIONS

            When British engineers were attempting to make a map of
            northern India near the Himalayan foothills during the mid-
            1800s, they discovered that their plumb bobs did not hang
            straight down but were deflected towards the Himalayas.

       2.  INITIAL HYPOTHESIS

            J. H. Pratt hypothesized that the gravitational attraction of
            the mountain mass caused the plumb bob to be deflected.  

            However, Pratt's calculations suggested that the deflection
            from true vertical was only one third of what it should be for 
            the size of the mountains involved.  Why?

B.  ISOSTASY MODELS

       Using the same observations, two engineers developed slightly
       different models to explain the plumb bob deflection in India.
 

       1.  PRATT'S MODEL

            Pratt hypothesized that elevation is inversely proportional to
            density.  Therefore, the higher the mountain, the lower is its
            density (i.e., light rocks "float" higher).

       2.  AIRY'S MODEL

            Airy hypothesized that mountains have "roots" which extend
            down into the mantle.  Therefore, elevation is proportional to
            the depth of the underlying "root".

 

C.  TESTING THE MODELS

        For many years, no one knew which model is a more accurate
        depiction of reality.  But as geologists learned about the earth's
        interior (through studying shock waves created by earthquakes
        and nuclear explosions), it became apparent that Airy's model 
        is a better one for most continental mountain ranges.
 

        However, Pratt's model is better at explaining why mid-oceanic
        ridges rise above the surrounding ocean floor, so both models
        have application to the "real" world.

D.  APPLICATIONS

       The principle of isostasy suggests that the earth's crust should
       adjust to any changes in mass that occur at the earth's surface
       (we call these "isostatic adjustments").  There are basically two
       types of responses:
 

       1.  SUBSIDENCE

            a.  Definition:  the slow, sinking of the earth's crust 

            b.  Cause:  the addition of mass to the crust

            c.  Example:  the advance of glacial ice sheets 

      2.  REBOUND

            a.  Definition:  the slow, vertical rise in earth's crust

            b.  Cause:  the removal of mass from the earth's crust

            c.  Example:  post-glacial rebound (Scandinavia) (U.S.)

       It should also be noted that isostasy explains why continental
       plates cannot be subducted:  the forces that drive subduction
       cannot overcome the "buoyancy" of these low density plates.