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Igneous Rock Classification

Igneous rocks are classified by their texture crystal size and composition mineral content . Mastering this classification system is essential for field geology. Texture Classification Texture reflects cooling history: | Texture | Crystal Size | Cooling Rate | Setting | |---------|--------------|--------------|---------| | Phaneritic | Visible 1mm | Slow | Intrusive | | Aphanitic | Microscopic | Fast | Extrusive | | Porphyritic | Mixed sizes | Two-stage | Either | | Glassy | No crystals | Very fast | Extrusive | | Vesicular | Has holes | Gas escape | Extrusive | | Pyroclastic | Fragments | Explosive | Extrusive | Porphyritic Texture Large crystals phenocrysts in a fine-grained groundmass matrix indicate two-stage cooling: 1. Initial slow cooling at depth → large crystals 2. Rapid cooling at surface → fine matrix Composition Classification Based on silica content and mineral assemblage: Felsic 65% SiO₂ - Light colored white, pink, light gray - Quartz + K-feldspar + Na-plagioclase ± micas - Granite phaneritic / Rhyolite aphanitic Intermediate 52-65% SiO₂ - Medium gray to gray-green - Plagioclase + amphibole + pyroxene - Diorite phaneritic / Andesite aphanitic Mafic 45-52% SiO₂ - Dark colored dark gray to black - Ca-plagioclase + pyroxene ± olivine - Gabbro phaneritic / Basalt aphanitic Ultramafic <45% SiO₂ - Dark green to black - Olivine + pyroxene no feldspar - Peridotite phaneritic / Komatiite rare aphanitic Quick Field ID Color Index : Estimate the percentage of dark minerals mafic minerals : - Felsic: <15% dark minerals - Intermediate: 15-40% dark minerals - Mafic: 40-70% dark minerals - Ultramafic: 70% dark minerals

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • How rocks are classified — U.S. National Park Service. Rock-group classification using composition, texture, and geologic processes.
  • Collecting rocks — U.S. Geological Survey. Field context, specimen observation, and responsible rock-collecting guidance.

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