Igneous
Basalt
Vesicular Basalt (Mafic volcanic rock)
Hardness: 5-6 Mohs; Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.8 - 3.0. Note the 'vesicles' (small holes) and amygdales (mineral-filled holes) visible on the surface.
- Hardness
- 5-6 Mohs
- Color
- Dark grey to black, weathering to brown
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 Mohs; Color: Dark grey to black, weathering to brown; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.8 - 3.0. Note the 'vesicles' (small holes) and amygdales (mineral-filled holes) visible on the surface.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. The pits seen in the image are caused by gas bubbles being trapped as the lava solidified. Basalt ranges from recent volcanic activity to billions of years old.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Dense varieties are used for floor tiles and monument stone. It is also used in the manufacture of rock wool insulation.
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. Famous basaltic formations include the Giant's Causeway in Northern Ireland.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and fine-grained texture. It often exhibits columnar jointing in large outcrops. Common in volcanic regions such as the Pacific Northwest of the US, Iceland, and Hawaii.
