Re: Why is the plate that contains Australia called the Indo-Australian plate?

Some people draw the plate boundaries in a way that the Indian subcontinent 
is also a part of the plate carrying Australia. In that case, the name 
Indo-Australian is appropriate.  However, in this plate, there is a slow 
moving spreading center called the Ninety-East Ridge (named for its 
longitude). The presence of this ridge leads some scientists to distinguish 
the Australian plate from the Indian plate.  

How can scientists disagree over such a simple question?  Plate boundaries 
and plate names do not come painted on the earth.  They are interpretations 
of data collected by scientists working on the study of tectonics.  
Sometimes, we can make sense of our data in several different ways, 
sometimes different scientists use different sorts of data and reach 
different conclusions, and sometimes scientific understanding changes and a 
map made ten years ago will be different from a map made last year.  
Finally, if your map is from a textbook or encyclopedia, rather than a 
scientific paper, there is another person interpreting the interpretations. 
 The book's author or editor may have even simplified a map just to make 
the list of plates shorter (there are a whole buch of small plates that get 
left out of most textbooks).

There is an excellent web site at Rice University with maps of the major 
plate boundaries and also of the data that scientists (and you) and use to 
locate those boundaries.  It's at: http://zephyr.rice.edu/plateboundary/  
Be sure to really explore the maps and to think about how you would decide 
what is a boundary and what is not.  Maybe you can do it with your science 
teacher.

David Smith
Associate Professor, Geology & Environmental Science
La Salle University, Philadelphia, PA