igneous
Basalt (Greenstone)
Mafic volcanic rock (Basalt)
Hardness: 5-6 Mohs. Color: dark gray, black, or greenish-tinted due to chlorite (greenstone). Luster: dull to earthy. Structure: fine-grained, massive with occasional columnar jointing or pillow structures. Specific gravity: 2.8–3.0.
- Hardness
- 5-6 Mohs
- Luster
- dull to earthy
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Physical properties
Hardness: 5-6 Mohs. Color: dark gray, black, or greenish-tinted due to chlorite (greenstone). Luster: dull to earthy. Structure: fine-grained, massive with occasional columnar jointing or pillow structures. Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. These specific coastal formations often represent ancient oceanic crust or subduction-related volcanism from the Mesozoic or Cenozoic eras.
Uses & applications
Used heavily in construction as aggregate for roads, railroad ballast, and concrete. Also used for high-quality paving stones and as a material for rock wool insulation.
Geological facts
Basalt is the most common rock in the Earth's crust and makes up most of the ocean floor. When subjected to low-grade metamorphism, it often turns into 'greenstone' due to the formation of green minerals like chlorite and epidote.
Field identification & locations
Identify in the field by its dark color, heavy weight, and fine-grained texture. Coastal exposures often show salt-weathering and blocky fracturing patterns. Common in volcanic regions and along plate boundaries.
