Basalt

Physical Properties

Hardness: 6 (Mohs scale); Color: Dark grey to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often containing phenocrysts; Cleavage: None; Specific Gravity: 2.9 - 3.1

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 rock in the Earth's crust, primarily forming the ocean floors.

Uses & Applications

Used extensively in construction as crushed stone for road base, concrete aggregate, railroad ballast, and as a raw material for stone wool insulation. High-quality specimens are used in landscaping and architecture.

Geological Facts

Basalt is also found on the Moon and Mars. Most of the lunar maria are composed of basaltic lava flows. It is the primary rock type of oceanic islands like Hawaii and Iceland.

Field Identification & Locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows vesicular (bubbly) textures or columnar jointing. Found globally in volcanic regions and ocean basins.

Identified on: 4/20/2026

Mode: Standard