igneous
Basalt
Extrusive mafic igneous rock
Hardness of 6 on the Mohs scale, typically dark gray to black in color, dull to sub-with a fine-grained (aphanitic) crystal structure, poor cleavage, and a high specific gravity around 2.8-3.0 due to high iron and magnesium content.
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Physical properties
Hardness of 6 on the Mohs scale, typically dark gray to black in color, dull to sub-with a fine-grained (aphanitic) crystal structure, poor cleavage, and a high specific gravity around 2.8-3.0 due to high iron and magnesium content.
Formation & geological history
Formed from the rapid cooling of low-viscosity, iron- and magnesium-rich lava exposed at or very near the Earth's surface, such as from volcanic eruptions or ocean floor spreading centers.
Uses & applications
Commonly used in construction as crushed stone for road bases, concrete aggregate, railroad ballast, and in the production of stone wool insulation.
Geological facts
Basalt is the most common rock type in the Earth's crust underlying the ocean basins, and it is also found abundantly on the Moon (forming the dark lunar maria) and Mars.
Field identification & locations
Identify in the field by its dark color, heavy feel, fine-grained texture where individual crystals are barely visible to the naked eye, and sometimes vesicular texture with gas cavities.
