igneous
Basalt
Mafic Igneous Rock (primarily Plagioclase and Pyroxene)
Hardness: 6 (Mohs), Color: Dark grey to black, Luster: Dull/Sub-metallic, 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: Dark grey to black, Luster: Dull/Sub-metallic, 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 very near the Earth's surface. Found in oceanic crust and volcanic regions; ages vary from modern to billions of years old.
Uses & applications
Used extensively in construction as aggregate for roads, railroad ballast, and concrete. It is also used as a thermal insulator (rock wool) and for high-durability floor tiles.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Large-scale cooling can result in 'Columnar Basalt' formations like the Giant's Causeway.
Field identification & locations
Identify in the field by its high density, dark color, and lack of visible crystals. It will not scratch easily with steel. Common in volcanic zones and coastal regions.
