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Andesite identification and geology

Andesite rock reference specimen

Gray to dark gray fine-grained volcanic rock named after the Andes. The extrusive equivalent of diorite. Texture: Aphanitic to porphyritic. Rock type: Igneous Extrusive. Grain size: Fine. Minerals present: Plagioclase, Hornblende, Pyroxene, Biotite. Formation: Erupts from stratovolcanoes at convergent plate boundaries, forming from intermediate magma 52-63% SiO₂ . Common geological settings: Andes, Cascades, Japan, Indonesia; the defining rock of continental volcanic arcs. Common look-alikes and related entries: Basalt, Dacite, Diorite.

Sources, scope & corrections

These authoritative resources support category-level nomenclature, terminology, identification methods, and safety. When a specimen-specific safety warning appears, the public-health source listed here directly supports that precaution. They are not line-by-line citations for every other property on this entry, and natural specimens vary.

Editorial owner
GeoMiner
Last updated
2026-03-30T14:38:11.256378+00:00
Review status
Published; not independently peer-reviewed
  • Rocks — U.S. National Park Service. Authoritative overview of rock groups, the rock cycle, and geologic processes.
  • Can you identify my rock or mineral? — U.S. Geological Survey. Identification limitations and routes to in-person professional verification.

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Common questions

What is Andesite?
Gray to dark gray fine-grained volcanic rock named after the Andes. The extrusive equivalent of diorite.
How can you identify Andesite?
Compare several properties rather than relying on colour alone. This entry records rock type: Igneous Extrusive; texture: Aphanitic to porphyritic; grain size: Fine; common minerals: Plagioclase, Hornblende, Pyroxene, Biotite. Confirm an important identification with additional field tests or qualified laboratory work.
How does Andesite form?
Erupts from stratovolcanoes at convergent plate boundaries, forming from intermediate magma 52-63% SiO₂ .
Where is Andesite commonly found?
Andes, Cascades, Japan, Indonesia; the defining rock of continental volcanic arcs.
What is Andesite commonly confused with?
Andesite may be confused with Basalt, Dacite, Diorite. Compare multiple diagnostic properties because colour and surface appearance can overlap.

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