igneous
Basalt (Porphyritic Basalt)
Porphyritic Basalt (Mafic volcanic rock)
Hardness: 6 (Mohs scale); Color: Dark grey to black with distinct darker phenocrysts; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix with visible large crystals (phenocrysts); Cleavage: None/Irregular; Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black with distinct darker phenocrysts
- Luster
- Dull to sub-metallic
Identified More igneous →
Explore Basalt (Porphyritic Basalt) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black with distinct darker phenocrysts; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix with visible large crystals (phenocrysts); Cleavage: None/Irregular; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The 'porphyritic' texture indicates two stages of cooling: a slow cooling period underground allowing for larger crystals to grow, followed by a rapid eruption and quick cooling of the remaining melt.
Uses & applications
Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Larger slabs are used for architectural cladding and floor tiles. It is occasionally used as 'hot stones' in spa therapy due to its heat retention.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus. The dark spots visible in this specimen are likely pyroxene or olivine phenocrysts.
Field identification & locations
Identify by its heavy weight (density), dark color, and fine-grained texture. To identify this specific specimen in the field, look for the 'salt and pepper' look or clear larger crystals embedded in a dark matte background. Often found in volcanic regions, riverbeds, and coastal areas.
