RocksdayAndesite vs Porphyry
How to tell them apart
An intermediate volcanic rock typical of explosive volcanoes above subduction zones.
Photo: Photographer: Siim Sepp, 2005, CC BY-SA 3.0
Any igneous rock with large crystals floating in a fine-grained background.
Photo: James St. John, CC BY 2.0
Key differences
- Andesite is a porphyritic rock too, but gray and duller.
Side by side
| Property | Andesite | Porphyry |
|---|---|---|
| Type | Igneous rock | Igneous rock |
| Mohs hardness | 6 | 6–6.5 |
| Luster | Dull | Dull to polished |
| Colors | Gray | Purple, Red, Gray, Green |
| Crystal system / structure | Porphyritic (crystals in fine groundmass) | Porphyritic (large crystals in fine groundmass) |
| Specific gravity | 2.5–2.8 | 2.6–2.8 |
| Found in | The Andes; Cascade Range, USA | Eastern Desert, Egypt; Älvdalen, Sweden |
Properties that are the same for both are grayed out.
How to identify andesite
- Medium gray.
- White feldspar crystals and dark specks scattered in a fine-grained base.
- Heavier than rhyolite, lighter than basalt.
- No visible quartz.
Structure: Porphyritic (crystals in fine groundmass)
Full Andesite guide →How to identify porphyry
- Distinct larger crystals scattered through a fine matrix.
- Often purple, red or green.
- Takes a high polish.
Structure: Porphyritic (large crystals in fine groundmass)
Full Porphyry guide →More comparisons
Could you tell them apart from a blurry close-up?
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