igneous
Basalt
Mafic extrusive igneous rock
Hardness: 6 (Mohs), Color: Charcoal grey to black, Luster: Dull/Submetallic, Crystal structure: Fine-grained (Aphanitic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 6 (Mohs), Color: Charcoal grey to black, Luster: Dull/Submetallic, Crystal structure: Fine-grained (Aphanitic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. It is the most common rock type in the Earth's crust, making up most of the ocean floors.
Uses & applications
Extensively used in construction as aggregate for roads and concrete, railway ballast, and sometimes as a building stone or decorative garden ornament.
Geological facts
Basalt is common on other planetary bodies, including the Moon (forming the dark Lunar Maria) and Mars. It can form unique columnar jointing patterns when cooling slowly in thick flows.
Field identification & locations
Identify by its characteristic dark color, heavy weight (density), and fine-grained texture with no visible crystals to the naked eye. Commonly found in volcanic regions and ocean basins.
