Description Of Igneous Rocks - The Field

Look for xenoliths (pieces of "foreign" country rock caught in the magma). 5. Field Relationships Observe how the rock body relates to its surroundings:

Describe the boundary between the igneous rock and the surrounding "country rock." Look for a chilled margin (finer grains at the edge) or a baked zone (heat damage to the host rock). Summary Naming Once you have the data, combine it into a field name: The Field Description of Igneous Rocks

Large crystals ( phenocrysts ) set in a fine-grained groundmass (indicates a two-stage cooling process). Look for xenoliths (pieces of "foreign" country rock

Note the overall color. Use terms like leucocratic (light-colored/felsic), mesocratic (medium), or melanocratic (dark-colored/mafic). or melanocratic (dark-colored/mafic).