igneous
Porphyritic Basalt (Drill Core)
Porphyritic Basalt with Plagioclase Phenocrysts
Hardness: 6 (matrix) to 6.5 (crystals); Color: Dark grey to black groundmass with light green/white phenocrysts; Luster: Glassy to dull; Crystal structure: Porphyritic (large crystals in fine-grained matrix); Specific gravity: 2.8-3.0
- Hardness
- 6 (matrix) to 6
- Luster
- Glassy to dull
Identified More igneous →
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Physical properties
Hardness: 6 (matrix) to 6.5 (crystals); Color: Dark grey to black groundmass with light green/white phenocrysts; Luster: Glassy to dull; Crystal structure: Porphyritic (large crystals in fine-grained matrix); Specific gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of basaltic lava at or near the Earth's surface, where large crystals (phenocrysts) began forming at depth before a final eruption quenched the remaining melt into a fine-grained matrix.
Uses & applications
Used in geological surveying to analyze subsurface volcanic structures, petroleum exploration, mineral prospecting, and as high-quality construction aggregate.
Geological facts
This specimen is a 'drill core,' a cylindrical section of rock obtained by a specialized drill. The large green-tinted crystals are likely plagioclase feldspar that achieved their size through slow cooling before the rock was fully extruded.
Field identification & locations
Identify by its cylindrical shape (from drilling), the contrast between large rectangular crystals and the dark matrix. Common in volcanic regions such as the Pacific Northwest or oceanic islands.
