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Aphanitic identification and geology

Describing an igneous rock texture in which individual mineral grains are too small to be distinguished with the unaided eye, typically less than 1 mm in diameter. Aphanitic texture indicates rapid cooling at or near the Earth's surface, which prevented crystals from growing large. The term is applied to the groundmass of volcanic rocks such as basalt, andesite, and rhyolite. Field: petrology. Example: A basalt hand specimen from a Hawaiian lava flow displays an aphanitic texture—the dark groundmass appears uniform and fine-grained, though olivine phenocrysts may be visible as larger green spots.. Related terms: Phaneritic, Porphyritic, Groundmass, Volcanic.

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
2026-03-30T15:19:41.970235+00:00
Review status
Published; not independently peer-reviewed
  • USGS Thesaurus — U.S. Geological Survey. Controlled earth-science terminology used to describe USGS information resources.
  • Glossary of Geologic Terms — U.S. National Park Service. Geologic definitions prepared by the NPS Geologic Resources Inventory.

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Common questions

What does Aphanitic mean in geology?
Describing an igneous rock texture in which individual mineral grains are too small to be distinguished with the unaided eye, typically less than 1 mm in diameter. Aphanitic texture indicates rapid cooling at or near the Earth's surface, which prevented crystals from growing large. The term is applied to the groundmass of volcanic rocks such as basalt, andesite, and rhyolite.
What is an example of Aphanitic?
A basalt hand specimen from a Hawaiian lava flow displays an aphanitic texture—the dark groundmass appears uniform and fine-grained, though olivine phenocrysts may be visible as larger green spots.
Which earth-science field uses the term Aphanitic?
Aphanitic is used in petrology.
Which terms are related to Aphanitic?
Related terms in this reference are Phaneritic, Porphyritic, Groundmass, Volcanic.

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