Basalt with Plagioclase Phenocrysts

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to black matrix with white speckled phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic matrix with visible crystals (porphyritic texture); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the Earth's surface. The visible white crystals (phenocrysts) formed slowly underground before the lava erupted and the rest of the rock cooled quickly.

Uses & Applications

Used primarily in construction as crushed stone for aggregate, road base, and railroad ballast. Porphyritic varieties are sometimes used as decorative stones or in landscaping.

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 and Mars.

Field Identification & Locations

Identify by its heavy weight (density), dark color, and the presence of small, distinct white mineral grains set in a fine-grained dark background. Often found in volcanic regions, riverbeds, and coastal areas.

Identified on: 4/26/2026

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