- 2062 Baisakh (old course) · 2 marks
Gabbro versus basalt
Short answer
Gabbro is a coarse-grained plutonic mafic rock (plagioclase, pyroxene, olivine) with visible grains; basalt is a fine-grained volcanic mafic rock with the same composition but grains invisible. Gabbro cools slowly deep, basalt rapidly at surface.
Differentiate between gabbro and basalt.
| Point | Gabbro | Basalt |
|---|---|---|
| Class | Plutonic (intrusive) igneous | Volcanic (extrusive) igneous |
| Cooling | Slow, deep in crust | Rapid, at the surface |
| Texture | Coarse grained, phaneritic | Fine grained, aphanitic (sometimes vesicular) |
| Composition | Basic (mafic): plagioclase, pyroxene, olivine | The same basic (mafic) composition |
| Colour | Dark, grains visible | Dark, grains not visible |
✓ Answer checked by Claude Opus 5.5 (high effort)
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