[ īsə-grăd′ ]

A line on a map connecting points on the Earth where metamorphism of rocks occurred under the same pressure and temperature conditions. These lines are established on the basis of the distribution of index minerals and are useful in reconstructing the tectonic history of a given region.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.