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Intrusive versus extrusive igneous rocks
Short answer
Intrusive rocks (granite, gabbro) cool slowly underground, are coarse-grained, and strong; extrusive rocks (basalt, rhyolite) cool rapidly on the surface, are fine-grained or glassy, and often vesicular. Intrusives make good foundations; extrusives vary in strength.
Differentiate between intrusive (plutonic) and extrusive (volcanic) igneous rocks with examples.
| Basis | Intrusive (plutonic) | Extrusive (volcanic) |
|---|---|---|
| Place of cooling | Deep inside the Earth, below the surface | On or near the surface |
| Cooling rate | Very slow | Rapid |
| Texture | Coarse-grained, phaneritic | Fine-grained, glassy, vesicular |
| Crystal size | Large, visible | Small or none |
| Types | Plutonic (deep), hypabyssal (shallow) | Lava flows, pyroclastic |
| Structures | Batholith, stock, sill, dyke | Lava flow, volcanic cone, vesicles |
| Colour | Depends on composition: light (granite) to dark (gabbro) | Depends on composition: light (rhyolite) to dark (basalt) |
| Examples | Granite, gabbro, diorite, syenite | Basalt, rhyolite, andesite, obsidian |
| Engineering | Strong, good foundations | Variable; basalt strong but jointed, vesicular rocks weak |
✓ Answer checked by Claude Opus 5.5 (high effort)
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