Basalt

Physical Properties

Hardness: 5-6 (Mohs scale); Color: Dark gray, black, or dark greenish; Luster: Dull to sub-metallic; Crystal Structure: Fine-grained (aphanitic), often contains microscopic crystals of plagioclase, pyroxene, and olivine; Specific Gravity: 2.8-3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. It is the most common volcanic rock and can form in various geological ages ranging from billions of years ago to current volcanic eruptions.

Uses & Applications

Extensively used in construction as an aggregate for roads, railroad ballast, and concrete. It is also used for floor tiles, monuments, and occasionally in stone carving.

Geological Facts

Basalt is the primary rock type of the Earth's oceanic crust. Large-scale basaltic lava flows are known as 'Flood Basalts,' and the rock is also found in significant quantities on the Moon and Mars.

Field Identification & Locations

Identify in the field by its dark color, fine-grained texture, and high density. It often exhibits columnar jointing in large outcrops. It is ubiquitous in volcanic regions like Hawaii, Iceland, and the Pacific Northwest.

Identified on: 4/20/2026

Mode: Standard