How to understand Mohr diagram? Structural geologists use Mohr diagram to evaluate stress on rock's surface, but I find it difficult to understand how this diagram works (Though I am good at math). Where can I find a good explanation of Mohr diagram?
Here is an excellent explanation of the math as related the stresses represented by the Moh'r circle.

http://www.geology.sdsu.edu/visualstructure/vss/htm_hlp/mor_cir.htm