RocksdayBasalt vs Greywacke
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 hard, dark 'dirty' sandstone of mixed grains, laid down by underwater avalanches.
Photo: James St. John, CC BY 2.0
Key differences
- Basalt has no sand grains.
Side by side
| Property | Basalt | Greywacke |
|---|---|---|
| Type | Igneous rock | Sedimentary rock |
| Mohs hardness | 6 | 6–7 |
| Luster | Dull | Dull |
| Colors | Black, Gray | Gray, Black, Green |
| Crystal system / structure | Fine-grained (aphanitic) | Clastic, poorly sorted |
| Specific gravity | 2.9–3.1 | 2.6–2.8 |
| Found in | Hawaii; Iceland | Southern Alps, New Zealand; Wales |
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 greywacke
- Dark gray and very hard.
- Mix of sand grains and rock fragments in fine matrix.
- Breaks into sharp, angular blocks.
Structure: Clastic, poorly sorted
Full Greywacke guide →More comparisons
Could you tell them apart from a blurry close-up?
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