RocksdayBasalt vs Gabbro
How to tell them apart
The most common volcanic rock on Earth. Dark, fine-grained lava that forms the ocean floor and vast lava plains.
Photo: James St. John, CC BY 2.0
A dark, coarse-grained rock made of pyroxene and plagioclase — basalt that cooled slowly underground.
Photo: Darla Sondrol, CC0
Key differences
- Gabbro has the same chemistry but coarse, visible crystals.
- Basalt's crystals are too small to see.
Side by side
| Property | Basalt | Gabbro |
|---|---|---|
| Type | Igneous rock | Igneous rock |
| Mohs hardness | 6 | 6–6.5 |
| Luster | Dull | Granular |
| Colors | Black, Gray | Black, Gray, Green |
| Crystal system / structure | Fine-grained (aphanitic) | Coarse-grained (phaneritic) |
| Specific gravity | 2.9–3.1 | 2.9–3.1 |
| Found in | Hawaii; Iceland | Duluth Complex, Minnesota; Cuillin, Isle of Skye |
Properties that are the same for both are grayed out.
How to identify basalt
- Dark gray to black and heavy.
- Grains too small to see.
- Often has gas bubbles (vesicles).
- May show six-sided columns in outcrops.
Structure: Fine-grained (aphanitic)
Full Basalt guide →How to identify gabbro
- Dark gray to black with visible crystals.
- Heavy.
- Little or no quartz.
- Gray plagioclase may show faint parallel lines.
Structure: Coarse-grained (phaneritic)
Full Gabbro guide →More comparisons
Could you tell them apart from a blurry close-up?
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