Basalt

Physical Properties

Hardness: 6-7 (Mohs). Color: dark grey to black, weathering to brown or rust. Luster: dull to sub-metallic. Crystal structure: fine-grained (aphanitic), often contains microcrystals of plagioclase feldspar and pyroxene. 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 can account for more than 90% of all volcanic rock on Earth and is found in ocean basins and continental flood basalt provinces.

Uses & Applications

Extensively used in construction for aggregate, road base, railroad ballast, and concrete. It is also used in high-quality floor tiles, monument stone, and occasionally in stone wool insulation production.

Geological Facts

Basalt is the most common rock type in the Earth's crust. It also makes up most of the surface of the Moon (the lunar maria) and is the primary rock type on Mars and Venus.

Field Identification & Locations

Identify by its dark color, heavy weight (density), and very fine-grained texture that lacks visible crystals to the naked eye. Often shows columnar jointing in large outcrops. Common in volcanic regions such as Iceland, Hawaii, and the Pacific Northwest.

Identified on: 5/5/2026

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