igneous
Basalt (or Gabbro/Diabase)
Mafic Igneous Rock (plagioclase, pyroxene, olivine)
Hardness 6 on Mohs scale; dark grey to brownish-black color; fine-grained to microcrystalline texture; dull to sub-metallic lustre on weathered surfaces; specific gravity ~2.8-3.0.
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Physical properties
Hardness 6 on Mohs scale; dark grey to brownish-black color; fine-grained to microcrystalline texture; dull to sub-metallic lustre on weathered surfaces; specific gravity ~2.8-3.0.
Formation & geological history
Formed through the rapid cooling of mafic lava rich in magnesium and iron near or at the Earth's surface, or shallow sub-surface dikes/sills. Common throughout geologic history from Archean to recent volcanic activity.
Uses & applications
Widely used as crushed stone for road base, concrete aggregate, railway ballast, dimension stone, and cast basalt wear-resistant tiles.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up almost all of the oceanic floor. It is also extremely common on the Moon (lunar maria) and Mars.
Field identification & locations
Identify by its dark color, high density compared to felsic rocks like granite, fine grain size, and lack of visible quartz crystals. Found in volcanic terrains and oceanic basins worldwide.
