igneous
Dark fine-grained rock, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectTypically black to dark gray, fine-grained, and matte to faintly vitreous, with interlocking crystals too small to see; hardness about 6, no prominent cleavage as a rock, and specific gravity around 2.8–3.0.
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Physical properties
Typically black to dark gray, fine-grained, and matte to faintly vitreous, with interlocking crystals too small to see; hardness about 6, no prominent cleavage as a rock, and specific gravity around 2.8–3.0.
Formation & geological history
An extrusive igneous rock formed when low-silica basaltic lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and hotspots; specimens may range from recent to Precambrian in age.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, riprap, building stone, and occasionally dimension stone or lapidary material.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later be filled by zeolite, calcite, or quartz.
Field identification & locations
Identify it by its uniform dark color, fine texture, heavy feel, and possible tiny feldspar crystals or gas holes; distinguish it from slag by its natural texture and lack of metallic bubbles. Common worldwide, especially volcanic regions and oceanic islands.
