igneous
Basalt
Basalt (composition of plagioclase, pyroxene, and olivine)
Hardness: 6 (Mohs scale), Color: Dark grey to black or brownish-grey, Luster: Dull to sub-metallic (if mineral grains visible), Crystal Structure: Aphanitic (fine-grained), Specific Gravity: 2.8–3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black or brownish-grey, Luster: Dull to sub-metallic (if mineral grains visible), Crystal Structure: Aphanitic (fine-grained), Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or moon. Most basalt on Earth is formed by decompression melting of the mantle at mid-ocean ridges.
Uses & applications
Extensively used in construction for aggregate, road base, concrete, and railroad ballast. Exceptionally durable varieties are used for floor tiles and sculpture.
Geological facts
Basalt is the most common rock type in the Earth's crust; almost all of the ocean floor is basalt. It is also common on the Moon, Mars, and Venus in the form of 'maria' or vast plains.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often shows small vesicles (gas bubbles) or phenocrysts of olivine that appear as translucent green spots.
