igneous
Porphyritic Basalt
Basalt (Porphyritic texture)
Hardness: 5-6 Mohs; Color: Mottled green, grey, and brown with white crystals; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Specific gravity: 2.8-3.0.
- Hardness
- 5-6 Mohs
- Color
- Mottled green, grey, and brown with white crystals
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 5-6 Mohs; Color: Mottled green, grey, and brown with white crystals; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the gradual cooling of mafic lava, where larger phenocrysts (likely plagioclase feldspar) crystallized first underground before the rest of the magma erupted and cooled rapidly at the surface. Common in volcanic regions and oceanic crust.
Uses & applications
Commonly used as crushed stone in construction, road base, and concrete aggregate. Smooth river-worn specimens are popular in decorative landscaping and garden paths.
Geological facts
The term porphyry comes from the Ancient Greek word for purple, though it now refers to the 'spotted' texture rather than color. Basalt is the most common rock type in the Earth's crust and also covers large areas of the Moon and Mars.
Field identification & locations
Identify by looking for distinct, light-colored 'spots' (phenocrysts) set in a much finer-grained, darker matrix. Commonly found on beaches or in riverbeds in volcanic areas like the Pacific Northwest or volcanic islands.
