igneous
Dark mafic rock, likely basalt
Basalt; primarily plagioclase feldspar and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectTypically fine-grained, dark gray to black, dull to faintly glassy, with hardness about 5–6 and specific gravity 2.8–3.0; cleavage is not visible in the whole rock. The image is insufficient to confirm texture or exact mineralogy.
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Physical properties
Typically fine-grained, dark gray to black, dull to faintly glassy, with hardness about 5–6 and specific gravity 2.8–3.0; cleavage is not visible in the whole rock. The image is insufficient to confirm texture or exact mineralogy.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, especially in volcanic fields, mid-ocean ridges, and oceanic crust. Specimen age depends on locality and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, dimension stone, and occasionally as a decorative material.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, olivine crystals, or a fine-grained groundmass can help distinguish varieties.
Field identification & locations
Identify it by its heavy, dark, compact appearance and lack of obvious large crystals; a streak, hardness, magnetism, and fresh broken surface would improve identification. Common worldwide, including Hawaii, Iceland, India, and oceanic crust.
