Basalt

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull to submetallic; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; 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 surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth.

Uses & Applications

Extensively used in construction for aggregate, road base, railroad ballast, and concrete. Also used for making floor tiles, monuments, and stone carvings.

Geological Facts

Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. It also makes up the lunar maria on the Moon.

Field Identification & Locations

Identify by its dark color, heavy weight, and fine-grained texture. It often occurs as massive flows or columnar joints. Common in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.

Identified on: 4/29/2026

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