The Field Description Of Igneous Rocks Apr 2026

Large crystals ( phenocrysts ) set in a fine-grained groundmass (indicates a two-stage cooling process).

Note patterns like columnar jointing (hexagonal columns) or cooling cracks. The Field Description of Igneous Rocks

Is it a dyke (cutting across layers), a sill (parallel to layers), or a massive pluton ? Large crystals ( phenocrysts ) set in a

Example: "A melanocratic, phaneritic, medium-grained gabbro with minor olivine phenocrysts, showing vertical columnar jointing." a sill (parallel to layers)