Ask A Scientist Environmental Earth Science Archive

name         Alex S.
status       student
age          15

Question -   I have often heard about continental drift.  However,
most books have show diagrams of lithospheric plates back only to the
time of Pangaea, or around 250 million years ago, around the time of
Cambrian explosion.  my question is this: what were the configurations of
the continental plates BEFORE 250 million years ago?

Logic tells me that the configuration can be found, through measuring
magnetic anomalies, and measuring the offset of magnetic poles in ancient
rocks relative to the earth's poles.  If this process can be used in rocks
as old as 250 million years ago, why not even older? Perhaps we can even
map continental plates as far back as the formation of the cratons?
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I cannot say it any better than the US Geological Survey's web site, which I
have quoted below.

"Plate-tectonic movements since the break-up of the supercontinent Pangaea
are now fairly well understood. Most scientists believe that similar
processes must also have occurred earlier. However, the pre-Pangaea history
of plate tectonics is very difficult to decipher, because nearly all of the
evidence has been obscured by later geologic and plate-tectonic processes,
including the subduction of older oceanic crust, which carried with it the
record of magnetic reversals and hotspot traces.

The clues to past plate tectonics can only be found on the present-day
continents-in rocks, fossils, and structures older than about 200 million
years. This is because the average age of the present-day oceanic crust is
about 55 million years; the oldest parts are about 180 million years old,
indicating that oceanic crust is entirely recycled every 150 million years
or so. By contrast, the average age of the present-day continental crust is
about 2.3 billion years, with the oldest known rocks (other than meteorites)
dating back 3.96 billion years; these oldest rocks in turn contain minerals
(zircons) derived from older rocks, possibly as old as 4.3 billion years."

To learn more, visit USGS's site at the following address:
http://pubs.usgs.gov/publications/text/Pangaea.html
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