Re: A Question about Plate Tectonics And Continental Drift

You are a good observer, India and Australia have indeed changed their relative positions over the past 200 million years. Your question implies however, that you may believe them to be on the same plate. They are not, or at least have not always been (present-day boundaries in the Indian Ocean are a bit obscure). Continents are fixed to their plates and cannot move independently of the rest of the plate. To see more about the plates and their interactions I highly recommend that you browse through the United States Geological Survey on-line book on plate tectonics:

http:// pubs.usgs.gov/publications/text/dynamic.html
or go straight to their plate map:
http:// pubs.usgs.gov/publications/text/slabs.html

The first part of your question refers to the continents beginning all in one place and then spreading apart. The breakup of Pangea and the subsequent growth of the Atlantic ocean is certainly one of the most accessible images of plate tectonics, however, it is important to remeber that those, events, thought they span an almost incomprehensible period of 200 million years, only encompass five percent of earth's history. Careful geologic work by hundreds of scientists in dozens of countries has revealed that groups of continents have collided and rifted apart repeatedly during earth's history. The first person to describe a cyclic crunch and then breakup was J. Tuzo Wilson, a key figure in the early development of plate tectonic theory, and this cycle is often referred to as the "Wilson cycle."

I hope this answers your question. If you have more questions please feel free to send them back to Mad Scientist Network (or directly to me, but if you use the network, others can benefit as well)

By the way, I love the name of your city. Is it really in a "handsome desert"?

David L. Smith
Assistant Professor of Geology, Environmental Science, and Physics
La Salle University
Philadelphia, Pennsylvania, USA
dsmith@lasalle.edu