Re: Pangaea

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>I have always had a problem with the concept of Pangaea. (is that
>spelled right?)  I didn't want to through this out without some
>careful forethought, so here it is after several days of revising.

A good place to check on more details on this subject would be a good
recent (post 1975) historical geology or structural geology or intro to
physical geology textbook.  The local libraries should have several.  I'll
try to answer you questions here, as well.

>>1) The concept of making "puzzle pieces" out of today's continents
>and fitting them into a picture of the original continent seems,
>well, childish.  Considering what I have heard about all of the
>other significant changes in the earth over the millions of years
>that scientists have, as yet, figured out, it seems inconceivable
>for the above-sea-level land masses to remain largely unchanged.
>( This is the most insupportable of my objections to the theory of
>Pangaea.  I do understand that supporting the theory is a lot of
>research showing similarities between coastal areas of different
>continents. )

It is not the current coastlines of the continents that match, it is the
configuration of the continental shelves and slopes (a couple of hundred
meters or so under water).  Find a good bathymetric map and look at the fit
between South America and Africa, or between the northern Canadian shelf
and the western edge of Greenlands shelf.

>>2) The "continental drift" as it applies to the theory of Pangaea.
>I am not a geologist or whatever "-ist" deals with the movement of
>the earth's plates.  I only understand that there is a certain
>amount of "drift" of the plates that moves them potentially apart
>from each other.  ( I can't remember if this is why California is
>sliding into the ocean or not.  Maybe just a modern myth. )  Is
>it really feasible for the plates containing the major land
>masses to move as dramatically as they have?  How big are the
>plates?  How many plates made up Pangaea?

The plates range from a few hundred thousand acres (microplates) to
gigantic masses thousands of kilometers across.  The continents are not
pushed along the sea floor, but instead are "floating" on deeper
structures.

Incidentally, the oceanic crust is also moving.  The magnetic signature of
the ocean floor forms mirror images across the mid-oceanic ridges.  Also,
the current sea floor is much younger than the continents.  Whereas some
continental rocks are datable near 4 billion years now, the oldest ocean
crust is from the Late Jurassic, and most of it is younger than the
Cretaceous.  The oceanic crust, being heavier than the continents, gets
pushed underneath the continental crust at collisional plate margins.  This
collision forms what is called a subduction zone, where the ocean crust
dives down (forming the deep marine trenches), gets melted in the mantle,
and "bubbles" up through the coastal margin of the continents, forming
volcanic mountain ranges such as the Andes.  (The collision itself also
forms mountain ranges as a sort of crumple zone).

Of course, these motions are very slow form our standpoint.  These
movements have been measured at about the rate of fingernail growth (only a
few centimeters a year).

(And you are correct - the California doom is a myth.  The San Andreas
Fault system is working to move the western half of the coast northward
relative to the rest of the continent.)

>3) What is the theory for the creation of Pangaea?  What would
>cause a planet to form with all its higher land mass to form on
>one side?  Was it on the side or was it formed at one of the poles?
>Is a planet stable with this deformity?  Is that the reasoning
>for the breakup of Pangaea?  These are questions I have never
>seen discussed when I have read about Pangaea.

Most evidence suggests that the Earth goes through cycles of continental
accretion and continental break-up.  The Permo-Triassic Pangaea was the
most recent accretion, when the mid-Panthalassa ridge (basically equivalent
to the mid-Pacific ridge) was dominant, pushing the various paleocontinents
together to this side of the world.  For several possible reasons, the
presence of Pangaea generated a new ridge system down its middle, breaking
up the supercontinent.  This new ridge, the mid-Atlantic ridge, is now
dominant, so that through time the Atlantic is getting bigger and the
Pacific smaller.

>>4) About the breakup, this has been blamed/credited for dividing
>the dinosaur families according to the area of Pangaea.  Is this
>an over-generalization?  Now that dinosaurs are being found in
>areas thought to be out of reach due to the breakup, suddenly
>we have a "land-bridge".

There is still abundant evidence of the effect of Pangaea's break-up on
various animal and plant groups.  A variety of books and papers have been
written (and are being written ;-)) on the subject.

>Forgive me for being sacrilegious, but
>this *sounds* like someone's pet theory was being disproved and
>everyone rushed to save it.  It seems that only the surface of
>dinosaur fossils has been touched.  What will happen if more
>out-of-bounds dinosaurs are found?  Will Pangaea start to look
>like a quilt with "land-bridges" and other patches criss-
>crossing the dinosaur map?  ( This is the list's area of
>expertise.  Feel free to be offended and refute my assertions. )

Actually, land bridges are the older theory, back when there was no good
model for continental drift.

>5) On more earth-formation issue: I recall reading that the
>surface of the earth is constantly rising and falling.

You are, for the most part, out of date.  This is part of the theory which
proceeded plate tectonics (the geosyncline theory).  Sea level has risen or
fallen over time, islands have been built up, but for the most part the
continental and the oceanic plates have been distinct.

>Land
>masses pop up in the middle of the ocean causing islands to
>form.  I also remember reading of how the highest mountains
>were once thriving forests and plains.  ( This was in a story
>about amber and why it is found in the mountains. )  It seems
>that if the earth's surface is going up and down on a steady
>basis ( we won't talk about Atlantis  8-D ) then what are the
>continents now weren't necessarily continents then.  Was
>Pangaea pieced together using the plates or the above-sea-level
>continents?  Using the plates would easily defeat this question.
>Using the continents would not.

There is a major difference between uplift on a continental plate (which
can raise lowland or coastal marine sediments up as mountains) and true
oceanic crust being brought to the top of the mountain.

>>OK, I'm done using up your bandwidth.  I just had to get this
>out in order to fully understand some of the discussions going
>on here.

My best advice to you is:

1) Find an introductory textbook on physical, historical, or structural
geology.  These would be able to answer your questions much better than a
couple of paragraphs on the net.

2) If possible, sign up for an introductory physical, historical, or
structural geology course at a local community college/college/whatever.
They will address precisely these issues.

Hope this helps.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.