igneous
Basalt
Basalt (Aphanitic Mafic Volcanic Rock)
Hardness: 6.0 on Mohs scale; Color: Black, dark gray, or dark green; Luster: Dull to sub-metallic; Crystal Structure: Very fine-grained (aphanitic) due to rapid cooling; Specific Gravity: 2.8–3.0.
- Hardness
- 6
- Color
- Black, dark gray, or dark green
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6.0 on Mohs scale; Color: Black, dark gray, or dark green; Luster: Dull to sub-metallic; Crystal Structure: Very fine-grained (aphanitic) due to rapid cooling; 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 basalt formed during the Cenozoic era, though it has existed throughout Earth's history.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. It is also used as dimension stone for floor tiles and in the production of stone wool insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also famously found on the Moon and Mars, forming the 'maria' or dark plains visible from Earth.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and lack of visible crystals. Commonly found in volcanic regions like Hawaii, Iceland, and the Deccan Traps. Collectors look for vesicular basalt (with holes) or basalt containing olivine crystals.
