RocksdayBasalt vs Phonolite
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 silica-poor volcanic rock that splits into plates and rings when struck.
Photo: No machine-readable author provided. Siim assumed (based on copyright claims)., CC BY-SA 3.0
Key differences
- Basalt is darker and heavier.
Side by side
| Property | Basalt | Phonolite |
|---|---|---|
| Type | Igneous rock | Igneous rock |
| Mohs hardness | 6 | 5.5–6 |
| Luster | Dull | Dull to greasy |
| Colors | Black, Gray | Gray, Green |
| Crystal system / structure | Fine-grained (aphanitic) | Fine-grained, often platy |
| Specific gravity | 2.9–3.1 | 2.4–2.6 |
| Found in | Hawaii; Iceland | Devils Tower, Wyoming; Mount Teide, Canary Islands |
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 phonolite
- Gray-green, fine-grained, slightly greasy.
- Splits into flat slabs.
- Rings like a bell when hit.
Structure: Fine-grained, often platy
Full Phonolite guide →More comparisons
Could you tell them apart from a blurry close-up?
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