Basalt

Physical Properties

Hardness: 6.0; Color: Dark grey to black, often weathering to green or brown; Luster: Dull to earthy; Crystal structure: Aphanitic (fine-grained); 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 surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth, making up most of the ocean floor.

Uses & Applications

Used extensively in construction as an aggregate (crushed stone) for roads, railroad ballast, and concrete. It is also used as a raw material for cast basalt liners and building veneers.

Geological Facts

Basalt is a major component of the lunar mare (the dark spots on the moon) and is found on Mars. In its molten state, basalt has a low viscosity, allowing lava to flow long distances before cooling.

Field Identification & Locations

Identify by its dark color, fine-grained texture, and high density. In the field, look for volcanic flow indicators or columnar jointing. It is commonly found in oceanic crust and volcanic hotspots like Hawaii or the Pacific Northwest.

Identified on: 4/14/2026

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