Re: Pangaea

[Stan Friesen <swf@ElSegundoCA.NCR.COM>]

>From: Joe Average <71573.2027@compuserve.com>
 > 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.
 > 
 > 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. )

There is certainly some basis for this.  That is one of the reasons
why more evidence than that was required before the concept was
accepted.

However, the answer, in this specific case, is that in the relatively
short time since the breakup of Pangea, the originally connected
coasts have been what are called "passive margins", which remain
relatively stable in form.  The old *outer* coasts are what have
changed drastically since then - since they are the sites of active
subduction and mountain building.  [In the Americas, the Rockies,
Sierras, and Andes all date from *after* the breakup of Pangea,
while the east coast mountains, the Appalachians, date from the
*formation* of Pangea].

One thing to remember is that the Pangea episode is only the most
recent round of continental drift, which has been going on for much
longer than Pangea existed.  Pangea formed near the end of the
Paleozoic, and very quickly started to break up again.

There is now reason to suspect that at least twice previously
there has been a similar union of almost all continental crust.
This has lead to the suggestion that the formation and subsequent
breakup of a universal supercontinent is a normal geological cycle.
 > 
 > 2) The "continental drift" as it applies to the theory of Pangaea.
 > ...  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?

Yes, such movement is possible.  Not only that, but we can *measure*
it happening today - directly!

The California bit is a mix of reality and myth.  The coastal
portion of California is on the Pacific Plate, not the North
America Plate.  [In fact I am sitting on a bit of the Pacific
Plate right this moment].  The motion is northwest, so that
in a few million years Los Angeles will be west of San Francisco.
Since continental crust "floats" on ocenaic crust, none of this
land is going to "fall off" into the sea - it will just gradually
seperate from the rest of California, until eventually the whole
area is a seperate island, or at least peninisula, much like Baja
California already is.  [In fact Baja California was *formed*
by the very same drift process that is now causing so many
earthquakes in California].

Plates vary greatly in size, from vast masses like Eurasia,
to small zones, called microplates, like some parts of the East
Indies.  The *major* paltes are: North American, South America,
North Pacific, Australian, Indian, African, Antarctican,
and Eurasian.

How many plates made up Pangea?  Do you mean how many did it
break up into, or how many merged to form it?  The answer is
different.
 > 
 > 3) What is the theory for the creation of Pangaea? 

Many seperate plates collided, just like the Indian Plate is
now doing to the Eurasian Plate.  This process of collision
formed massive mountin ranges, just like India is doing today.
India is making the Himilayas, the original formation of Pangea
made the Appalachians, the Atlas Mountains, and certain highlands
elsewhere (perhaps the Scottish Highlands).

 > What would
 > cause a planet to form with all its higher land mass to form on
 > one side? 

It didn't.  It formed at first with *no* continental land masses
at all.  How the continents formed is stillnot entirely worked
out, but there is some reason to suspect that they were seeded
by the volcanic arcs along the plate margins (plates need not have
continents on them - most microplates do not).  Good examples of
such island arcs today are Japan, and the East Indies, where
Borneo is a good example of what a protocontinent *may* have looked
like.

What is clear is that at first there were quite a number of these,
and that the larger continents formed, at least in part, by the
collision and merger of several of these smaller domains.  The
remnants of that stage are to be found in places like the Canadian
Shield, and the comparable shield areas in Africa, Asia, and South
America (and maybe elsewhere - my memory is uncertain on that).

The famous Pangea formed *much*, *much*, *much* later.  Contienents
of some sort apparently existed by 2 billion years ago, if not
earlier, Pangea only formed about 300 million years ago.

[There may have been several earlier "pangeas", but the evidence is
only really clear for the one in the very latest Precambrian].

 >  Was it on the side or was it formed at one of the poles?

See above.

 > Is a planet stable with this deformity?  Is that the reasoning
 > for the breakup of Pangaea?  

No. The break up, like all of contiental drift, is driven by
vast, excruciatingly slow, convection currents in the Earth's
mantle.  The relatively brittle crust is simply carried along
for the ride.  Because it is relatively brittle, it breaks up
into plates as it moves about.  Pangea broke up due to a major
upwelling of the mantle in what is now the middle of the Atlantic.

Some suggest that the upwelling may have formed there due to
Pangea acting like a blanket and retaining mantle heat underneath.
[This is part of the hypothesis that pangeas form and break up in a
cyclic manner].
 > 
 > 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".  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? 

No, the main evidence for Pangea is geological, and so is the
evidence for its breakup.

I think that Afrovenator is less significant in this respect than
has been suggested in the press.  For one thing, at the time
Europe was a chain of islands bridging much of the gap between
North America and Africa, so a certain amount mixing could occur
by island hopping, in much the same way as the polynesian islands
were populated by mammals.  I seriously doubt a more substantial
land bridge is really necessary to explain the discovery.
[Also, Afrovenator is part of a relatively early, very conservative
lineage, so it may simply have been a late survival].
 > 
 > 5) On more earth-formation issue: I recall reading that the 
 > surface of the earth is constantly rising and falling.  Land
 > masses pop up in the middle of the ocean causing islands to
 > form. 

That sounds like a very old model to me.  As far as we can tell
today, almost all land mass forms from one of two processes - the
melting and subsequent eruption as volcanos of pieces of oceanic
crust subducted on the leading edges of the various plates (e.g.
Japan, the East Indies, the Aleutians, and so on), or the eruption
of bubbles of hot material from deep below the surface of the
Earth (called hot spots - examples of which include the Hawaiian
Islands, Yellowstone, and Iceland).

These are all *volcanic* processes, not simple rising of the surface.

The main exception I know of are a few small island in the middle
of the Atlantic which are the very tops of the spreading ridge
where new oceanic crust is being formed - thus pushing the Americas
away from Eurasia and Africa.  These are likely to be short-lived,
and of no long term consequence.

The volcanic islands, on the other hand, are the source of new
continental crust.  Almost the entire western coast of North
America consists of a fair number of these oceanic islands that
have been swept up onto the continent as it drifts westwards.
[Everything west of the Sierras and the equivalent ranges in
Canada are "new" additions to North America - indeed it is the
collisions with these island arcs that probably *formed* the
Sierras, and maybe the Rockies as well]

 >  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. 

Again, mountain formation is related to contenental drift - it
does not just happen spontaneously.  Continental mountains form
when two bits of continental crust collide.  A good example of
this is the Himalayas, which used to be the south coast of Eurasia,
and the north coast of India.  Coastal mountains form where a
ocenaic crust is being subducted (pushed down into the mantle)
by continental crust.  Such ranges are invariably highly volcanic.
The Andes are one of the best examples, but the Coast Ranges of
North America are another example.

The fact is that continental crust is too "light" to be lost
back into the mantle, unlike oceanic crust, which tends to
flow back down into the mantle when pressured.  [When two
section of oceanic crust collide, one goes under, and the
other slides on top, with the melted remnants of the first
erupting through it as volcanos - the East Indies were formed
this way].

Now, in addition to this, the oceans do seem to have risen and
fallen a number of times.  The most recent time being due to the
formation and melting of the continental glaciers - which locked
up lots of the ocean into ice, dropping the sea level.  Also,
the great weight of the ice depressed the continents slightly,
and they are *still* rebounding back to the equilibrium level.
[The continents effectively float in the mantle, like a boat,
and like a boat they must displace their own weight].

There are also other causes of changes in ocean level which are
still poorly understood, but are clearly indicated because the
ocean levels are known to have changed during time when no massive
glaciers existed.

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.