Andesite: Formation & Identification

Q: How can you identify andesite?

Fine-grained, medium gray to greenish-gray Often has small visible crystals of feldspar scattered in the fine groundmass Named for the Andes, where it's extremely common

01 / Origin story

How does andesite form?

Andesite is extrusive, chemically intermediate between basalt and rhyolite — common in volcanoes formed at subduction zones, where oceanic crust melts as it's pushed under a continental plate.

02 / Read the clues

How can you identify andesite?

Use several clues together.
One photograph shows one specimen, not every variation.

Enlarged crop of the Andesite specimen — Texture study · enlarged crop of this specimen

01Fine-grained, medium gray to greenish-gray
02Often has small visible crystals of feldspar scattered in the fine groundmass
03Named for the Andes, where it's extremely common

03 / Commonly confused with

Similar at first glance.

Compare the materials side by side.
Photographs are not shown to the same scale.

Andesite specimen for comparison — Andesite ↗
Fine-grained, medium gray to greenish-gray

Basalt specimen for comparison — Basalt ↗
Fine-grained — crystals are too small to see individually, often looks almost glassy or dull

Basalt — andesite tends to be lighter gray rather than basalt's near-black, and slightly less dense, but the two can genuinely be hard to tell apart by eye alone without a closer mineral analysis.

04 / Out in the world