igneous
Basalt
Basalt (aphanitic mafic volcanic rock)
Hardness: 6 (Mohs); Color: Dark gray to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Fine-grained/Aphanitic (crystals often microscopic); Specific gravity: 2.8-3.0.
- Hardness
- 6 (Mohs)
- Color
- Dark gray to black, often weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs); Color: Dark gray to black, often weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Fine-grained/Aphanitic (crystals often microscopic); 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. Most common rock type in Earth's crust, making up most of the ocean floor and large continental flood plains.
Uses & applications
Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality dense basalt is used for floor tiles and monumental sculpture.
Geological facts
Basalt is the most common volcanic rock on Earth. It also covers large parts of the Moon (the Lunar Maria), Mars, and Venus. On Earth, the Giant's Causeway in Northern Ireland is a famous example of columnar basalt.
Field identification & locations
Identify by its dark color, heavy weight (density), and fine-grained texture lacking visible crystals. Commonly found in former volcanic regions, oceanic islands like Hawaii, and near rift valleys.
