Andesite is a fine-grained volcanic rock intermediate in composition between basalt and rhyolite. It frequently exhibits a porphyritic texture, with distinct larger crystals of plagioclase or amphibole embedded in a finer groundmass. Collectors look for this rock primarily within subduction zone volcanic chains where it forms significant lava flows and volcanic edifices.
Is this andesite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch andesite with a known reference. Andesite sits at Mohs 6-7, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Aphanitic to porphyritic. Grain size: Fine-grained.
- 3Pick out the mineralsComposition: Mostly sodium-rich plagioclase (typically andesine) with pyroxene or hornblende; 57 to 63 percent silica. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: dull.
- 5Match the color rangeCompare against the expected color range: gray, dark gray, brownish-gray, greenish-gray.
Often confused with
Andesite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside andesite
Minerals reported to co-occur with andesite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 6-7
- Density
- 2.1-2.8 g/cm³
- Luster
- Dull
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Construction aggregate, Road metal, Decorative stone
- Host rock
- volcanic arcs
- Texture
- Aphanitic to porphyritic
- Grain size
- Fine-grained
- Composition
- Mostly sodium-rich plagioclase (typically andesine) with pyroxene or hornblende; 57 to 63 percent silica
Where rockhounds find andesite
2 mapped spotsClassic worldwide localities
- Andes Mountains, South America
- Cascade Range, USA
- Japan
- Indonesia
- New Zealand
Field-hunting tip
Where it forms: volcanic arcs. If you start seeing plagioclase, pyroxene, hornblende in float, you are in the right ground. In the U.S., the densest reported localities are in Oregon, Utah. Start trip planning there.
Common questions
How do you identify andesite?+
Where is andesite found?+
Can I find andesite in the United States?+
What rocks look like andesite?+
What minerals are found with andesite?+
What kind of rock does andesite form in?+
What is andesite used for?+
Find andesite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
