igneous
Basalt with Plagioclase Phenocrysts (Porphyritic Basalt)
Porphyritic Basalt (primarily SiO2, Al2O3, FeO, MgO, CaO)
Hardness: 6-7 on Mohs scale. Color: Dark grey to black groundmass with light-colored (tan or white) rectangular inclusions. Luster: Dull to sub-vitreous. Texture: Porphyritic (large crystals in a fine-grained matrix). Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark grey to black groundmass with light-colored (tan or white) rectangular inclusions. Luster: Dull to sub-vitreous. Texture: Porphyritic (large crystals in a fine-grained matrix). 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 larger crystals (phenocrysts) formed slowly underground before the final eruption. Common in volcanic fields and oceanic crust.
Uses & applications
Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally durable specimens are used for garden landscaping or decorative 'river rocks.'
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 in abundance on the Moon and Mars. The contrasting light crystals are often plagioclase feldspar.
Field identification & locations
Identify by looking for a dense, dark, heavy rock with distinct rectangular or lath-shaped crystals embedded within it. Commonly found in areas with historic volcanic activity or in glacial till and riverbeds where they have been transported and rounded by water.
