igneous
Basalt
Aphanitic mafic volcanic rock (primarily plagioclase feldspar and pyroxene)
Hardness (Mohs scale): 6; Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic (if minerals are visible); Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0.
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull to sub-metallic (if minerals are visible)
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Physical properties
Hardness (Mohs scale): 6; Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic (if minerals are visible); Crystal Structure: Aphanitic (fine-grained) to porphyritic; Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity magnesium-rich and iron-rich lava. Common in oceanic crust, mid-ocean ridges, and volcanic hotspot regions. Ages range from modern eruptions to billions of years old.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used as stone 'wool' for insulation and in high-end floor tiles or statues.
Geological facts
Basalt is the most common rock on Earth's surface and also covers vast areas of the Moon (the 'Maria') and Mars. It forms hexagonal columns (columnar jointing) when thick lava flows cool and contract slowly.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and lack of visible large crystals unless it is porphyritic. It is found globally in volcanic fields, notably in the Pacific Northwest USA and Iceland.
