Basalt

Physical Properties

Hardness: 6.0; Color: Dark grey to black (weathering to brown or green); Luster: Dull to sub-metallic when wet; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0; Vesicular or scoriaceous texture often present.

Formation & Geological History

Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava (mafic lava) at or very near the Earth's surface. Most commonly formed at mid-ocean ridges and hotspots.

Uses & Applications

Extensively used in construction as aggregate (crushed stone) for road base, railroad ballast, and concrete; used in high-quality floor tiles and as a component in rock wool insulation.

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.

Field Identification & Locations

Identify in the field by its high density, dark color, and fine-grained texture. It often shows vesicles (gas bubbles). Check for magnetism with a strong magnet as it contains magnetite. Found in volcanic regions globally.

Identified on: 4/27/2026

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