igneous
Porphyritic Basalt
Porphyritic Basalt (containing Plagioclase phenocrysts)
AI-generated identification · may be incorrectHardness: 6-7 on Mohs scale; Color: Black or dark grey groundmass with light-colored (white to tan) crystals; Luster: Dull to sub-vitreous; Texture: Porphyritic (fine-grained matrix with large visible crystals); Specific Gravity: 2.8-3.0
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Black or dark grey groundmass with light-colored (white to tan) crystals; Luster: Dull to sub-vitreous; Texture: Porphyritic (fine-grained matrix with large visible crystals); Specific Gravity: 2.8-3.0
Formation & geological history
Formed from volcanic lava that cooled in two stages: first, large crystals (phenocrysts) formed slowly underground, followed by a rapid eruption where the remaining liquid cooled quickly into a fine-grained matrix. Often found in volcanic arcs and oceanic crust.
Uses & applications
Commonly used as crushed stone for aggregate in construction, road base, and railroad ballast. Distinctive specimens are often kept by collectors or used as decorative garden stones.
Geological facts
The large crystals in this rock are called phenocrysts. In this specific specimen, the elongated tan shapes are likely laths of plagioclase feldspar. This texture is a geological 'time capsule' showing the changing cooling rates of magma.
Field identification & locations
Look for a dark, dense, heavy rock with distinctively shaped, lighter-colored mineral inclusions. Commonly found in glacial till, on beaches in volcanic regions (like the Pacific Northwest or Great Lakes), and near ancient volcanic sites.
