igneous
Basalt (with weathered crust)
Basalt
Hardness 6 on the Mohs scale. Color ranges from dark gray to black when fresh, weathering to brownish-tan hues. Luster is dull to sub-vitreous. Fine-grained (aphanitic) microcrystalline structure consisting mainly of plagioclase feldspar and pyroxene.
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Physical properties
Hardness 6 on the Mohs scale. Color ranges from dark gray to black when fresh, weathering to brownish-tan hues. Luster is dull to sub-vitreous. Fine-grained (aphanitic) microcrystalline structure consisting mainly of plagioclase feldspar and pyroxene.
Formation & geological history
Formed from the rapid cooling of low-viscosity basaltic lava at or near the Earth's surface. Extrusive igneous rock commonly produced at mid-ocean ridges, hotspot volcanoes, and continental flood basalt provinces.
Uses & applications
Extensive use in construction as crushed stone, concrete aggregate, asphalt pavement, railway ballast, and dimension stone. Cast basalt is also used for wear-resistant industrial tiles.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up almost all oceanic crust. It is also abundant on the Moon (forming the dark lunar maria) and Mars.
Field identification & locations
Identify by its dark fresh interior, fine-grained texture, high density, and brownish weathered outer rind. Widely found worldwide in volcanic terrains such as Hawaii, the Pacific Northwest (Columbia River Basalts), and Iceland.
