Over the course of Earth history, many parts of the crust have undergone multiple episodes of metamorphism.  Modern methods of dating and measuring trace-element abundances are now able to tease out the timing and conditions of the individual episodes.  But new techniques were needed before these methods could be scaled up to unravel regional tectonic events such as the formation of mountain belts and subduction zones and continental rifting.  In the podcast, John Cottle describes one such technique that he and his group developed and that ushered in a revolution in the study of metamorphic rocks.  He discusses how the technique was used to resolve the multiple phases of metamorphic history in the Himalaya, Antarctica, and New Zealand.   John Cottle is a Professor in the Department of Earth Science at the University of California, Santa Barbara. Go to geologybites.com for illustrations that support the podcast and to learn more about Geology Bites.