igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6 on Mohs scale; Color: Dark grey to black, sometimes with a brownish-red weathered surface; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None, usually breaks with a conchoidal or irregular fracture; Specific gravity: 2.8–3.0.
- Hardness
- 6 on Mohs scale
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black, sometimes with a brownish-red weathered surface; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None, usually breaks with a conchoidal or irregular fracture; 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 of a terrestrial planet or a moon. It is the most common volcanic rock on Earth, making up most of the ocean floor and large continental provinces.
Uses & applications
Extensively used in construction, specifically as an aggregate in concrete and asphalt, road base, and railroad ballast. Fine-grained varieties are also used for high-tensile basalt fiber and in the manufacturing of stone wool insulation.
Geological facts
Basalt is the primary rock type on the moon's lunar maria and also covers much of the surface of Mars and Venus. It often forms striking hexagonal columns during cooling, such as those seen at the Giant's Causeway in Ireland.
Field identification & locations
Identify in the field by its dark color, fine-grained texture (crystals too small to see with the naked eye), and high density. It is commonly found in volcanic regions, oceanic island arcs (like Hawaii), and mid-ocean ridges.
