igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 5-6 Mohs. Color: Dark grey to black, weathering to brown or green. Luster: Dull to sub-metallic. Structure: Fine-grained (aphanitic), often contains small white plagioclase crystals. Specific gravity: 2.8-3.0.
- Hardness
- 5-6 Mohs
- Color
- Dark grey to black, weathering to brown or green
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 Mohs. Color: Dark grey to black, weathering to brown or green. Luster: Dull to sub-metallic. Structure: Fine-grained (aphanitic), often contains small white plagioclase crystals. 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 on Earth, comprising most of the ocean floor and large volcanic provinces.
Uses & applications
Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality specimens are also used as 'hot stones' in massage therapy and for stone carvings.
Geological facts
Basalt is not unique to Earth; it is the most common rock type on the Moon (the dark lunar maria) and covers a significant portion of Mars and Venus.
Field identification & locations
Identified by its heavy weight (high density), dark color, and fine-grained texture. To identify in the field, look for hexagonal columnar jointing or vesicular (pitted) surfaces. Commonly found in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.
