The Field Description Of Igneous Rocks 〈RECENT - 2025〉

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

Is it a dyke (cutting across layers), a sill (parallel to layers), or a massive pluton ? 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

Distinguish between the fresh surface (broken with a hammer) and the weathered surface . Weathering can hide the rock's true color or highlight certain mineral textures. 2. Texture (The Cooling History) Texture is the most critical diagnostic feature: Large crystals ( phenocrysts ) set in a

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

Crystals are too small to see (indicates rapid cooling at the surface).

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: