igneous
Vesicular Basalt
Extrusive mafic igneous rock
Hardness: 6 (Mohs scale); Color: Dark gray, greenish-gray, to black; Luster: Dull to sub-earthy; Texture: Fine-grained (aphanitic) to vesicular; Specific gravity: 2.8-3.0; Contains visible gas cavities (vesicles).
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray, greenish-gray, to black
- Luster
- Dull to sub-earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray, greenish-gray, to black; Luster: Dull to sub-earthy; Texture: Fine-grained (aphanitic) to vesicular; Specific gravity: 2.8-3.0; Contains visible gas cavities (vesicles).
Formation & geological history
Formed from the rapid cooling of mafic lava rich in dissolved gases at or near the Earth's surface. As lava erupts, pressure decreases, allowing gas bubbles to expand and solidify into vesicles before cooling completely.
Uses & applications
Used widely as crushed stone for road base, aggregate in concrete, railway ballast, shoreline protection (riprap), and occasionally carved or polished for decorative landscaping.
Geological facts
Vesicular basalt makes up a massive portion of the oceanic crust and volcanic islands like Hawaii. When the vesicles later fill with secondary minerals like quartz, zeolites, or calcite, it is called amygdaloidal basalt.
Field identification & locations
Identified by its fine-grained dark matrix interspersed with distinctive open gas pockets (vesicles) and rounded, water-worn beach pebble shape. Common in volcanic terrains, beaches near volcanic flows, and ancient lava fields.
