igneous
Dark basalt (likely porphyritic basalt)
Basalt, composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, with a dull to slightly vitreous luster and scattered pale feldspar/zeolite crystals or veins. Hardness about 5–6 Mohs; no prominent cleavage; dense, typically about 2.8–3.0 specific gravity.
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Physical properties
Fine-grained, dark gray to black, with a dull to slightly vitreous luster and scattered pale feldspar/zeolite crystals or veins. Hardness about 5–6 Mohs; no prominent cleavage; dense, typically about 2.8–3.0 specific gravity.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. Individual pieces may range from very young volcanic material to Precambrian age.
Uses & applications
Widely used as crushed aggregate, road base, railway ballast, riprap, asphalt aggregate, and building stone; attractive polished varieties are used decoratively. Ordinary hand specimens generally have little monetary value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Pale patches may be plagioclase phenocrysts, weathering products, or secondary minerals filling vesicles and fractures.
Field identification & locations
Identify it by its dark, dense, fine-grained texture, lack of visible quartz, and possible magnetic response from iron-bearing minerals; a streak test is usually gray. Common localities include Iceland, Hawaii, India’s Deccan Traps, oceanic islands, and volcanic regions worldwide.
