igneous
Fine-grained dark igneous rock, likely basalt
Basalt, composed chiefly of plagioclase feldspar and pyroxene, with possible olivine or magnetite
AI-generated identification · may be incorrectDark gray, fine-grained, dull to subtly vitreous surface; typically hardness 5–6, massive texture, no obvious cleavage, and specific gravity about 2.8–3.0. The uniformly tiny crystals indicate rapid cooling, though the image cannot confirm the exact variety.
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Physical properties
Dark gray, fine-grained, dull to subtly vitreous surface; typically hardness 5–6, massive texture, no obvious cleavage, and specific gravity about 2.8–3.0. The uniformly tiny crystals indicate rapid cooling, though the image cannot confirm the exact variety.
Formation & geological history
Formed when mafic lava cooled quickly at or near Earth’s surface, commonly during volcanic eruptions; basalt ranges from ancient Precambrian flows to modern volcanic deposits. Common sources include oceanic crust, lava plateaus, and volcanic islands.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, concrete aggregate, and building stone; attractive dense pieces may be polished for decorative objects. Ordinary hand specimens have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark color, fine interlocking grains, great density, and lack of visible layering or large crystals; a streak, magnet, and fresh broken surface can help distinguish it from dark sedimentary rocks. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic volcanic regions.
