igneous
Porphyritic volcanic rock (likely porphyritic basalt)
Porphyritic basalt; fine-grained groundmass with larger mineral phenocrysts, composition not confirmable from the photo
AI-generated identification · may be incorrectDark gray to brown, rough and irregular, with conspicuous pale rounded crystals or patches in a darker matrix. Likely plagioclase phenocrysts; hardness and exact mineral content cannot be determined visually, and no clear cleavage is visible.
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Physical properties
Dark gray to brown, rough and irregular, with conspicuous pale rounded crystals or patches in a darker matrix. Likely plagioclase phenocrysts; hardness and exact mineral content cannot be determined visually, and no clear cleavage is visible.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew first, followed by rapid cooling of the remaining melt. This texture occurs in volcanic rocks of many geological ages.
Uses & applications
Commonly collected as a geological specimen; basalt is also widely used as crushed aggregate and building stone.
Geological facts
The contrasting crystal sizes are called porphyritic texture and record a change in cooling rate.
Field identification & locations
Look for distinct larger crystals embedded in a much finer dark matrix; a fresh surface and simple hardness or streak tests can help distinguish basalt from other porphyritic rocks. The specimen’s exact identity cannot be established from the image alone.
