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Igneous Rock Identification Chart

Igneous rocks form when molten material cools and solidifies. A useful hand-sample classification begins with texture and then composition. Coarse phaneritic rocks have visible interlocking grains and commonly cooled slowly below the surface; simplified felsic, intermediate, mafic, and ultramafic examples are granite, diorite, gabbro, and peridotite. Fine aphanitic rocks commonly cooled rapidly at or near the surface; their simplified counterparts are rhyolite, andesite, basalt, and rare komatiite. Porphyritic texture records larger phenocrysts in a finer groundmass. Glassy texture indicates rapid quenching. Vesicles record gas bubbles but can be confused with weathering cavities or industrial slag. Pyroclastic rocks consist of volcanic ash, crystals, pumice, or rock fragments. Composition should be estimated from identifiable minerals and their proportions rather than colour alone. Obsidian is an important dark-looking glass that is commonly felsic. Alteration, oxidation, metamorphism, and fine grain size can hide the original mineralogy. GeoMiner's private selector accepts only bounded texture and composition groups and sends no specimen name, free text, photo, location, exact colour, mineral percentages, or measurements. Treat the result as a provisional field or classroom shortlist. Formal names for fine-grained, altered, mixed, or borderline rocks may require thin-section petrography, modal mineral counts, or whole-rock chemistry.

  • Quick ID for rocks — Carry a broad texture into the account-free property key.
  • Granite vs basalt — Compare composition, visible grain size, minerals, and intrusive versus extrusive formation.
  • Basalt vs gabbro — Use groundmass grain size and cooling context to separate two mafic equivalents.
  • Granite vs rhyolite — Use groundmass grain size and cooling context to separate two felsic equivalents.
  • Granite vs diorite — Compare quartz, feldspar, dark-mineral proportions, and overall texture.
  • Gneiss vs granite — Separate metamorphic foliation and banding from a massive intrusive texture.
  • Vesicular rocks — Compare pumice, scoria, vesicular basalt, and industrial look-alikes.
  • Complete rock and mineral chart — Test whether an igneous family is plausible before choosing a name.

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • undefined. Igneous origin, intrusive versus extrusive cooling, grain size, and the role of magma composition.
  • undefined. Classification by chemical and mineral composition, texture, and structure, including porphyritic texture.
  • undefined. Intrusive and extrusive origins and the relationship between rapid cooling and fine-grained or glassy texture.
  • undefined. Cooling-rate controls, plutonic mineral proportions, and compositional equivalence between volcanic and plutonic rocks.
  • undefined. Simplified felsic, intermediate, mafic, and ultramafic classification and intrusive–extrusive rock pairs.
  • undefined. Classroom observation of origin, texture, grain size, colour index, and composition.
  • undefined. Texture, mineralogy, compositional groups, and common coarse-, fine-, glassy-, vesicular-, and pyroclastic names.
  • undefined. Common identification examples and the combined use of texture, composition, colour, and mineralogy.

Read GeoMiner's editorial methodology.

Common questions

How do I identify an igneous rock?
Describe grain size, glass, vesicles, fragments, and whether two grain sizes occur. Then estimate composition from minerals and colour index and use the texture–composition intersection as a provisional name.
What is the difference between intrusive and extrusive igneous rocks?
Intrusive rocks crystallize below the surface and commonly grow visible crystals. Extrusive rocks form at or near the surface and commonly have fine-grained, glassy, vesicular, or fragmental textures.
Are granite and rhyolite the same?
They share a broad felsic compositional family, but granite is coarse-grained and intrusive while rhyolite is fine-grained and volcanic.
Can I classify volcanic rock by colour?
Not reliably. Weathering, alteration, glass, vesicles, and a fine groundmass can make colour misleading. Obsidian is commonly felsic despite often looking dark.
Is a hand-sample name a formal classification?
No. Fine-grained and altered rocks may require thin-section petrography, modal mineral counts, or whole-rock chemistry.

JavaScript is loading the interactive GeoMiner experience. The identification tools provide educational candidates, not laboratory confirmation.