igneous
Basalt
Mafic extrusive igneous rock
Hardness: 6 on Mohs scale; Color: Dark grey to black, heavy weathering can turn it reddish-brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often contains micro-crystals of plagioclase feldspar and pyroxene; Cleavage: None (conchoidal fracture).
- Hardness
- 6 on Mohs scale
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black, heavy weathering can turn it reddish-brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often contains micro-crystals of plagioclase feldspar and pyroxene; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (basaltic lava) exposed at or very near the Earth's surface. Most common in oceanic crust and volcanic regions.
Uses & applications
Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Can also be polished for architectural stones or used in the manufacturing of rock wool insulation.
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, Mars, and Venus. Giant columns of basalt (columnar jointing) form famous landmarks like the Giant's Causeway.
Field identification & locations
Identify in the field by its dark color, high density (heavy for its size), and lack of visible large crystals to the naked eye. It often shows vesicules (small holes from gas bubbles) or a weathered crust. Found in volcanic fields or as cobbles in riverbeds downstream from volcanic areas.
