Re: How many earth plates are there?

That depends on who is counting them and how they decide what is a plate.

There are 15 generally agreed upon large plates and there is a very good 
map available at the United States Geological Survey web site:
 http://geology.er.usgs.gov/eastern/plates.html

There is also an Adobe Acrobat file with a more detailed reportby clicking 
on the link (on the map picture):  http://pubs.usgs.gov/pdf/planet.html

The book, "The Dynamic Earth" is a very good introduction to plates and is 
available on-line at: http://pubs.usgs.gov/publications/text/dynamic.html

Another wonderful web page, that has data used to define plate boundaries 
and a set of exercises for students is at Rice University:
 http://zephyr.rice.edu/plateboundary/

The problems in counting plates come in when we start to look at plates in 
more detail and in their historical context.  For an example, refer to 
the diagrams at this site:
 http://pubs.usgs.gov/publications/text/Farallon.html

which shows the western margin of North America over the past 30 million 
years.  Thirty million years ago, there was a single large plate off the 
margin of North America, called the Farallon plate.  Since then, plate 
boundary changes have produced the San Andreas fault and have broken the 
Farallon into a number of much smaller plates.  The Juan da Fuca plate and 
the Cocos plate are big enough to be counted in the fifteen I reported 
above, but if you look in the last picture, you will see the tiny Rivera 
plate.  This piece of oceanic lithosphere meets the definition of a plate, 
but is too small to count at the global scale.  There are other similar 
"microplates" around the world.  Should the Rivera plate be counted as 
separate, or should it be counted as a piece of the Farallon plate.  What 
about Juan da Fuca and Cocos?  Are the separate or just pieces of Farallon, 
too?  It depends on who you ask and on why you ask.

As with many questions in science, the point of the answer here isn't to 
find a fixed number (I doubt any working scientists really care if there 
are 16 and not 15 plates, we don't spend our time counting them)but to 
engage in a conversation about what it takes to be a plate.  What if two 
pieces are geographically separate but have a shared motion? Should they 
count as one plate or two?  What about plates that are breaking apart, such 
as is happening at the East African rift zone?  Is Africa one plate or two? 
 There aren't any definite answers to these questions, but the questions 
produce more learning than an answer would.

David Smith, Ph.D.
Associate Professor, Geology and Environmental Science
La Salle University, Philadelphia, PA USA