🚀 go-pugleaf

>About ten years ago, while I was in college, I read a book that presented
>a theory to explain many of the phenomenon used to back up the theory of
>plate tectonics.
>The theory went something like this...
>
>The earth consists of a large gob of semi-liquid stuff(mantle) that is coated
     
     The mantle is very much solid, in fact it is more dense than
     most any of the rocks seen on the surface.  The core of the
     earth is partial molten (the center part is not however).

>with an extremely thin shell of hardened goop, ie crust. Occasionally, 

       Basically correct.

>for unknown reasons, the thin shell 'slips' over the surface of the 
>semi-liquid stuff. After a while, the shell movement reaches some sort
>of equilibrium(sort of like Punctuated Revolution? :-} ).
>
     This 'thin shell' is constantly moving at a slow rate, on what
     is known as the mid-atlantic ridge about 2 cm per year.  Now
     this may not seem like a lot but for geology this is very rapid
     movement.

>This theory 'explains' the following phenomenon:
 
>1. magnetic reversals		- since the magnetic pole is in the mantle,
>				  when the crust shifts relative to the
>				  mantle, the apparent direction of the 
>				  earths magnetic field shifts.

     Magnetic reversals are believed to be caused by convection
     currents in the core of the earth, which acts as kind of a 
     magnet.  Some directional changes are caused by the movement
     of the crust over the mantle also, but only in the crust, not
     the rest of the earth.

>2. sea floor expansion		- since the earth is not a perfect sphere,
>				  when the crust shifts, cracks open as the
>				  crust gets stretched of the bulgy parts. 
>				  Stuff leakes out from the interior.

     This happens in that were places like the mid-atlantic ridge
     occur the drop in pressure caused by the crust being stretched
     causes the rock in the mantle to melt, and because this melted
     rock is lighter than what surrounds it, it works its way to the
     surface and creates the 'stuff that leaks out'.  This stuff 
     (melt) also hardens to form new crustal material because not 
     near all of it reaches the surface.  The distance that this
     melt must travel is about 10 km.

>3. weird fossil records	- the author sited examples of fossils of 
>				  tropical plants in Siberia. Siberia used
>				  to be on the equator. 

     Using the fact that the crustal 'plates' have moved in the past
     as well as they are currently moving also produces changing 
     environments.  Indiana, for example, during one period in 
     geologic time also was near the equator.  Coral reefs, abundant
     limestone (Indiana University and many of the buildings in
     Washington D.C. were made of one very pure type of limestone,)
     and coal were all formed under tropical conditions.  The
     northward movement of the North American plate has brought
     cooler weather to this once Bahama-type environment.  Traces
     of glaciers can be found in very dry areas of Africa.



>BTW, according to the Grab Bag column in the S. F. Examiner, the crust is
>much thinner relative to the inside then an egg shell is to the egg.

     It is.  That is why it is so difficult to study the mantle and
     core, because the crust, even though it is relatively thin, is
     on land up to 30 km thick, and in the oceans about 6-10 km
     thick.  A project known as the Deep Sea Drilling Project has
     drilled a hole 1.5 km deep, barely scratching the surface.
     The mantle and core are studied using siesmic procedures 
     using such things as earthquakes and planned and controlled
     explosions.  This subject of plate tectonics touches all
     areas of geology to some degree.   I am a geology under-
     graduate here at Purdue University and am currently taking
     a course about this very subject.  All the current literature
     I am familier with is very technical.  This theory has been
     greatly refined since it was first proposed.  Perhaps another
     geologist out there can come up with a good non-technical
     book on the subject.  Any protests or other questions can
     be sent to this account. 

				    Ellen Meadows

Disclaimer :
    The views stated herein are the views of the author and in
    no way should be connected to the organization to which I
    or to me for that matter.
-- 

In Real Life:           Jerry L. Bloomfield  

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