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Name: Alex S.
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: September 12, 2002


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?



Replies:
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

Anonymous



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