Question

WHAT FEATURES DUD YOU USE TO CLASSIFY IGNEOUS ROCKS AS EXTRUSIVE OR INTRUSIVE

Answer 1

I assume you mean “What features do you use to classify igneous rocks as extrusive or intrusive?” Key observable features and what they indicate:

Main features
- Texture / crystal size
- Phaneritic (coarse-grained, crystals visible to naked eye) → intrusive (slow cooling underground; e.g., granite, gabbro).
- Aphanitic (fine-grained, crystals too small to see) → extrusive (fast cooling at/near surface; e.g., basalt, andesite).
- Glassy or very fine groundmass
- Glassy (obsidian) or extremely fine → rapid quenching at the surface → extrusive.
- Vesicles / porosity
- Presence of gas bubbles (vesicles), pumiceous texture → extrusive (lava erupted with trapped gas).
- Phenocrysts / porphyritic texture
- Large crystals (phenocrysts) in a fine-grained groundmass indicate two-stage cooling: crystals formed slowly at depth, then erupted and cooled quickly. Can be extrusive (porphyritic lava) or near-surface intrusive—interpret context.
- Pegmatitic texture
- Very large crystals (pegmatite) → very slow cooling, deep intrusive or late-stage intrusive bodies.
- Chilled margins
- Glassy or fine-grained rim where magma met cooler rock → suggests intrusion into cooler rock or lava chilled at the surface; a fine chilled margin on a dike/lava flow typically indicates an extrusive contact.
- Contact relationships / field position
- Cross-cutting dikes, sills, or plutons cutting country rock and associated baked zones → intrusive. Lava flows, pyroclastic deposits at or above the surface → extrusive.

Examples to link features and rock types
- Granite: phaneritic, coarse grains → intrusive.
- Gabbro: coarse-grained mafic → intrusive.
- Basalt: aphanitic, may be vesicular → extrusive.
- Obsidian/pumice: glassy/vesicular → extrusive.
- Andesite porphyry: phenocrysts in fine groundmass → erupted (extrusive) with earlier crystal growth.

Quick field checklist
1. Can you see individual mineral grains? Yes → likely intrusive. No → likely extrusive.
2. Are there vesicles or glass? Yes → extrusive.
3. Is there a chilled margin or baked contact? Chilled margin at the edge of a body → rapid cooling (surface or near-surface); baked country rock adjacent to an intrusion → intrusive.

If you want, tell me a specific rock sample (photo or description) and I can help classify it.