igneous
Basalt (Vesicular)
Extrusive mafic igneous rock; primarily (Ca, Na)(Mg, Fe, Al)(Si, Al)2O6 with (Ca, Na)(Al, Si)4O8
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None/conchoidal fracture; Specific gravity: 2.8 - 3.0. Note: This specimen appears vesicular/porous.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None/conchoidal fracture; Specific gravity: 2.8 - 3.0. Note: This specimen appears vesicular/porous.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Vesicular basalt specifically forms when gas bubbles are trapped in the cooling lava. It can range from Cenozoic to Precambrian in age.
Uses & applications
Used extensively in construction as road base, concrete aggregate, and railroad ballast. Larger pieces are used in landscaping and as 'lava rocks' for gas grills due to heat retention.
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, indicating similar volcanic history on other planets.
Field identification & locations
Identify in the field by its dark color, heavy weight (high density), and fine-grained texture. It often shows 'holes' (vesicles) or a rusty exterior from iron oxidation. Common in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.
