Basalt with Porphyritic Texture

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black matrix with lighter phenocrysts; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) groundmass with visible phenocrysts; 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 visible crystals (phenocrysts) formed slowly at depth before the lava erupted and the rest cooled quickly.

Uses & Applications

Commonly used in construction as aggregate, road base, and railroad ballast. Exceptionally durable varieties are used for floor tiles and monuments.

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 extensively on the Moon and Mars.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (high density), and the presence of small, distinct crystals embedded in a very fine-grained dark background. Found in volcanic regions and oceanic crustal outcrops.

Identified on: 4/29/2026

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