igneous
Basalt
Basalt (Aphanitic Mafic Volcanic)
Hardness (6 on Mohs scale), dark grey to black color, dull to sub-metallic luster, fine-grained (aphanitic) texture, no clear cleavage, specific gravity of 2.8–3.0
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Physical properties
Hardness (6 on Mohs scale), dark grey to black color, dull to sub-metallic luster, fine-grained (aphanitic) texture, no clear cleavage, specific gravity of 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. Common in volcanic fields and oceanic crust.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast; also used for stone carving and as an industrial thermal insulator (basalt fiber).
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 found on the Moon and Mars.
Field identification & locations
Identify by its dark color, heavy weight for its size, and the lack of visible large crystal grains without a magnifying glass. Commonly found in areas with historic volcanic activity.
