RocksdayBasalt vs Hornfels
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 dense, very hard rock baked by the heat of nearby magma.
Photo: User Fed on ru.wikipedia, Public domain
Key differences
- Basalt often has gas holes and isn't near granite contacts.
Side by side
| Property | Basalt | Hornfels |
|---|---|---|
| Type | Igneous rock | Metamorphic rock |
| Mohs hardness | 6 | 6–7 |
| Luster | Dull | Dull |
| Colors | Black, Gray | Black, Gray, Brown |
| Crystal system / structure | Fine-grained (aphanitic) | Fine-grained, non-foliated |
| Specific gravity | 2.9–3.1 | 2.7–3 |
| Found in | Hawaii; Iceland | Skiddaw, England; Sierra Nevada, California |
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 hornfels
- Dark, fine-grained and very hard.
- Breaks with sharp, splintery edges.
- Rings when tapped.
Structure: Fine-grained, non-foliated
Full Hornfels guide →More comparisons
Could you tell them apart from a blurry close-up?
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