Question

what features do you use to classify igneous rocks as extrusive or intrusive

Answer 1

You classify igneous rocks as extrusive (volcanic, formed at or near the surface) or intrusive (plutonic, emplaced below the surface) primarily by features that record their cooling environment and emplacement. Key diagnostic features:

- Texture / grain size
- Intrusive: phaneritic (coarse-grained, crystals visible to the eye — mm to cm), because slow cooling allows large crystals (e.g., granite, gabbro).
- Extrusive: aphanitic (fine-grained, crystals too small to see) or glassy, from rapid cooling (e.g., basalt, rhyolite, obsidian).

- Glass and devitrified glass
- Glassy surfaces (obsidian) or devitrified textures indicate very rapid quenching at the surface → extrusive.

- Vesicles and pumice
- Presence of vesicles (gas bubbles), scoria, pumice indicate degassing during eruption → extrusive.

- Porphyritic texture
- Phenocrysts in a fine-grained groundmass: indicates two-stage cooling (crystals formed slowly at depth, then erupted and the melt cooled quickly). That rock is extrusive (porphyritic lava) even though it has large crystals.

- Fragmental / pyroclastic textures
- Tuffs, breccias and other fragmental textures are produced in explosive volcanic eruptions → extrusive.

- Chilled margins and flow structures
- A fine-grained or glassy rim against a coarser interior (chilled margin), flow banding, vesicle concentration near one side → extrusive or shallow intrusive dike/sill.

- Pegmatitic texture
- Extremely large crystals (pegmatite) indicate very slow cooling, late-stage intrusive bodies.

- Field relationships / emplacement
- Cross-cutting bodies (dikes, sills, plutons, batholiths) and contact effects (baked country rock, contact metamorphism) indicate intrusive emplacement.
- Lavas, ash beds and layering, or a volcanic edifice indicate extrusive origin.

- Jointing and cooling structures
- Columnar jointing commonly forms in cooling lava flows (extrusive) but can occur in shallow intrusions, so interpret with other evidence.

- Mineral zoning and xenoliths
- Xenoliths of country rock enclosed in a cooled body often indicate intrusive emplacement; xenocrysts/phenocrysts can occur in extrusives.

Practical identification checklist
1. Look at grain size: visible crystals → intrusive; very fine or glassy → extrusive.
2. Look for vesicles, pumice, or pyroclastic fragments → extrusive.
3. Look for chilled margins, flow banding → extrusive (or shallow intrusion).
4. Look for large pegmatitic crystals, contact baking, cross-cutting relationships → intrusive.
5. If ambiguous, thin-section microscopy and field mapping (contacts) or geologic context will resolve.

Notes
- Composition (felsic/mafic) doesn’t by itself determine intrusive vs extrusive — both compositions occur in either setting.
- Some bodies are shallow intrusions and show mixed features (e.g., fine-grained dikes with chilled margins), so use a combination of features and field relations.