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:
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
Look for xenoliths (pieces of "foreign" country rock caught in the magma). 5. Field Relationships Observe how the rock body relates to its surroundings: Distinguish between the fresh surface (broken with a
Large crystals ( phenocrysts ) set in a fine-grained groundmass (indicates a two-stage cooling process). Texture (The Cooling History) Texture is the most
If porphyritic, describe the shape, color, and size of the large crystals specifically. 4. Fabric and Structure Look for patterns in how the rock is arranged: Massive: Uniform throughout with no preferred orientation.
Identify the minerals present and estimate their relative abundance: