igneous
Basalt (Vesicular)
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 Mohs scale; Color: Dark grey to black, weathering to reddish-brown; Luster: Dull/Sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8-3.0; Contains vesicles (gas bubbles).
- Hardness
- 6-7 Mohs scale
- Color
- Dark grey to black, weathering to reddish-brown
- Luster
- Dull/Sub-metallic
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Physical properties
Hardness: 6-7 Mohs scale; Color: Dark grey to black, weathering to reddish-brown; Luster: Dull/Sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8-3.0; Contains vesicles (gas bubbles).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. Vesicular basalt specifically forms when gas bubbles are trapped in the cooling lava. Found in volcanic regions and oceanic crusts throughout geological history.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Larger pieces are used as landscaping rocks or 'lava rocks' for grills and garden decoration.
Geological facts
Basalt is the most common rock type on Earth's crust and also covers large areas of the Moon, Mars, and Venus. The dark 'seas' on the Moon (Maria) are actually ancient basaltic lava plains.
Field identification & locations
Identify in the field by its high density, dark color, and characteristic small holes (vesicles). It is commonly found in basalt flows, near volcanic vents, or as glacial till in certain regions. Collectors look for pieces with interesting gas bubble patterns or secondary mineral fillings (amygdules).
