Hi, robal-ga:
Let's start with the breakup of Pangea, a supercontinent that combined
what are now North and South America, Africa, Europe and Asia until
the end of the Triassic period, 200 million years ago:
[The Breakup of Pangea - Animation from 200 mya to Present]
(Paleomap Project, Christopher R. Scotese)
http://www.scotese.com/pangeanim.htm
"This animation shows the motions of the continents during the last
200 million years."
If we can agree that was a point in time when the configuration of
continents was definitely different, then we can move forward in time
and observe some milestones at which various features of the present
configuration evolved.
If like me, you find it different to keep the chronological
terminology straight, here's a nice chart of the hierarchical
timescales used by geologists:
[Detailed Geological Timescale]
http://www.palaeos.com/Timescale/timescale.html
As you will see there, the end of the Triassic period brought the
beginning of the Jurassic period, the middle period of the Mesozoic
era, itself the middle era of the Phanerzoic eon which lasts until the
present time.
As you drag the mouse from left to right across the "virtual reality"
animation above, the map moves backward in time. Note that during the
late Cretaceous period, a lot of North America was covered by a
shallow "inland" sea, which receds to the north as we approach the
present and disappeared during the Cenozoic era.
It seems clear from the way you've asked your Question that the
Isthmus of Panama is an important feature of the "present geographical
formation". As the animation of the breakup of Pangea shows, the
North and South American land masses maintained a measure of proximity
throughout, but the actual connection to South America formed
comparatively recently, some 3-4 million years ago.
Since the size of the graphics in the above animation make it hard to
see this, additional images found here may be helpful:
[Plate Tectonic Reconstructions at UTIG]
http://www.ig.utexas.edu/research/projects/plates/
Anthony G. Coates, Staff Scientist Emeritus at the Smithsonian
Tropical Research Institute, is a leading expert on this subject:
[Anthony G. Coates - Smithsonian Tropical Research Institute]
http://www.stri.org/english/scientific_staff/staff_scientist/scientist.php?id=42
An account of a lecture he gave about this is here:
[Panama's geological history explained in Smithsonian lecture]
http://www.thepanamanews.com/pn/v_08/issue_13/science_01.html
"On July 2 [2002] the speaker was geologist Tony Coates, who described
himself as "one of only a couple of geologically-oriented people at
STRI." He has been researching Panama's geological history, using a
number of techniques to construct a "temporal template of the rise and
fall of the isthmus" over the past 20 million years or so.
"As Coates explained, the isthmus came into place about four million
years ago, as the result of a complicated set of movements among four
distinct tectonic plates which find their confluence in Panama. He
bases this conclusion on research at sites around the isthmus, in
which layers of rocks are mapped and dated by their composition, their
magnetic properties and the fossils they contain."
...
"What Coat[e]s found in Bocas was that the sea bottom that was to rise
up and become part of the Isthmus of Panama rose to become a shallow
sea, then fell to a depth of about 200 meters some six million years
ago, then rose again to emerge as dry land. He dates the Valiente
Peninsula as about 20 million years old, and Popa Island at about
eight million years."
...
"After the lecture, Coates explained that in more recent geological
times, a cycle of ice ages and warming periods that is not connected
to the tectonic forces that created Panama has caused sea levels to
rise and fall. This has made the isthmus rise and fall relative to sea
level, and widen and narrow correspondingly as shallow seas became
land and were then inundated again, on a recurring cycle of about
130,000 years.
"Panama's geological history in turn becomes important for other
disciplines whose scholars come down here to study under STRI's
auspices. For example, it explains to biologists how South America's
wildlife evolved separately from Central America's until four million
years ago, and to archaeologists looking for the traces of human
migration from North America to South America how they may have
difficulty finding traces on the present isthmus because the
coastlines that the migrants probably followed are now underwater."
* * * * * * * * * * * * * * * * * * *
So, although a reasonable Answer to your Question based on these
considerations is 3-4 million years ago, a number of changes important
to paleogeography have continued to happen since then. The water
levels rise and fall, primarily driven by the "ice ages", and perhaps
of greatest importance is the opening of the Bering Strait (previously
the Bering Land Bridge connecting North America to Asia) perhaps as
recently as 10,000 years ago. This page quotes a 1997 dated article
but covers the basics:
[The Bering Strait and The Land Bridge]
http://www.cabrillo.edu/~crsmith/bering.html
Let me know if futher Clarification on my part would be useful!
regards, mathtalk-ga |
