igneous
Basalt with Scoria/Vesicular Texture
Extrusive Mafic Igneous Rock (primarily Plagioclase Feldspar and Pyroxene)
Hardness: 5-6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) with vesicular (porous) voids; Specific gravity: 2.8–3.0.
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) with vesicular (porous) voids; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface. The holes (vesicles) are created by gas bubbles escaping as the lava solidifies. It can range in age from modern volcanic flows to billions of years old.
Uses & applications
Used extensively in construction as road base, railroad ballast, and aggregate in concrete. Highly vesicular varieties (scoria) are used in landscaping, as gas grill 'lava stones', and as lightweight abrasive materials.
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 in abundance on the Moon and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and the presence of small holes or pits. It is commonly found in volcanic regions such as the Pacific Northwest (USA), Iceland, and Hawaii. For collectors, look for specimens with unique mineralization inside the vesicles (amygdules).
