The Field Description Of Igneous Rocks Guide

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

Estimate the percentage of dark minerals (Mafic Index).

Streaks or layers caused by the movement of molten rock. The Field Description of Igneous Rocks

No crystals (extremely rapid quenching, like obsidian). Vesicular: Contains gas bubbles (common in lavas).

Note patterns like columnar jointing (hexagonal columns) or cooling cracks. Look for xenoliths (pieces of "foreign" country rock

Identify the primary minerals (e.g., Quartz, Feldspar, Olivine, Pyroxene).

Example: "A melanocratic, phaneritic, medium-grained gabbro with minor olivine phenocrysts, showing vertical columnar jointing." Pyroxene). Example: "A melanocratic

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