igneous
Vesicular Basalt (River Cobble)
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 5-6 Mohs. Color: Dark grey to blackish-green or brownish when weathered. Luster: Dull/earthy. Structure: Fine-grained/aphanitic with vesicles. Cleavage: None. Specific Gravity: 2.8–3.0.
- Hardness
- 5-6 Mohs
- Color
- Dark grey to blackish-green or brownish when weathered
- Luster
- Dull/earthy
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Physical properties
Hardness: 5-6 Mohs. Color: Dark grey to blackish-green or brownish when weathered. Luster: Dull/earthy. Structure: Fine-grained/aphanitic with vesicles. Cleavage: None. Specific Gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The small holes, called vesicles, were formed by gas bubbles trapped in the cooling lava. Often found in volcanic regions and transported via rivers.
Uses & applications
Primarily used in construction as crushed aggregate, for road base, or in landscaping. In its rounded state, it is common in riverbed decor or as garden stone.
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 and Mars.
Field identification & locations
Identify in the field by its dark color, heavy weight (relative to size), and the presence of small pockmarks or holes (vesicles). Commonly found in stream beds near volcanic provinces.
