Teaching Physics with the Physics Suite

Edward F. Redish

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Drifting Continents

In 1912, Alfred Wegner suggested that continents actually moved. One reason for his suggestion was the remarkable fit between the coastlines of South American and Africa (as shown in the figure at the right*). This idea was rejected as implausible for half a century.

As part of the moon program, a corner reflector was placed on the moon. A laser beam bounced off this reflector would come right back in the direction it came in from. A laser put through a telescope at Goddard Space Flight Center in Greenbelt, Maryland (picture at the right below**) was able to reflect the beam off the moon and detect its relfection back again through the telescope. Accurate measurements of the emission and reception time lead to very accurate measurements of position.


Careful analysis of data from experiments like these lead to a measurement of the drift rates of continents. Drift rates of a few cm per year have now been measured for some points.

If we assume that the South American continent drifted away from Africa at a constant velocity of 5 cm/year, estimate how many years it would have taken to reach its current position.

* From http://www.calstatela.edu/faculty/acolvil/plates.html.

** From http://kids.earth.nasa.gov/archive/pangaea/nasa.html.

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Page last modified 20. October 2007:G25