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Groundmass identification and geology

The fine-grained or glassy matrix of a porphyritic igneous rock that surrounds the larger, more prominent phenocrysts. The groundmass crystallized relatively rapidly during eruption or shallow emplacement, while the phenocrysts grew more slowly at depth before eruption. The texture of the groundmass—whether aphanitic, glassy, or trachytic—provides information about the cooling history of the rock. Field: petrology. Example: A porphyritic andesite from Mount Rainier displays euhedral plagioclase phenocrysts up to 5 mm long set in a dark gray aphanitic groundmass of intergrown plagioclase, pyroxene, and glass.. Related terms: Phenocryst, Porphyritic, Aphanitic, Matrix.

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
2026-03-30T15:19:41.999576+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 Groundmass mean in geology?
The fine-grained or glassy matrix of a porphyritic igneous rock that surrounds the larger, more prominent phenocrysts. The groundmass crystallized relatively rapidly during eruption or shallow emplacement, while the phenocrysts grew more slowly at depth before eruption. The texture of the groundmass—whether aphanitic, glassy, or trachytic—provides information about the cooling history of the rock.
What is an example of Groundmass?
A porphyritic andesite from Mount Rainier displays euhedral plagioclase phenocrysts up to 5 mm long set in a dark gray aphanitic groundmass of intergrown plagioclase, pyroxene, and glass.
Which earth-science field uses the term Groundmass?
Groundmass is used in petrology.
Which terms are related to Groundmass?
Related terms in this reference are Phenocryst, Porphyritic, Aphanitic, Matrix.

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