Igneous Textures
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In plain terms
In igneous rocks, crystal size is a cooling-rate clock.
Common igneous textures
- Aphanitic: crystals too small to see (fast cooling; basalt/andesite).
- Phaneritic: visible crystals (slow cooling; granite/gabbro).
- Porphyritic: big crystals in fine groundmass (two-stage cooling).
- Glassy: no crystals (obsidian).
- Vesicular: gas bubbles (scoria, pumice).
- Pyroclastic: ash and fragments welded/cemented (tuff).
Key takeaways
- Texture is often more diagnostic than color.
- Porphyritic texture indicates changing cooling conditions.
- Vesicles record gas content and eruption style.
- Glassy rocks indicate extremely rapid cooling.
- Use texture + context to name the rock confidently.