igneous
Dark mafic rock, likely basalt
Basalt; chiefly plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray to black, fine- to medium-grained, dull to subtly glassy luster; typically massive, with no obvious cleavage. Mohs hardness about 5–6 and specific gravity approximately 2.8–3.1; the photograph suggests weathered, possibly vesicle-poor basalt, though exact identification requires a fresh surface.
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Physical properties
Dark gray to black, fine- to medium-grained, dull to subtly glassy luster; typically massive, with no obvious cleavage. Mohs hardness about 5–6 and specific gravity approximately 2.8–3.1; the photograph suggests weathered, possibly vesicle-poor basalt, though exact identification requires a fresh surface.
Formation & geological history
Basalt forms when low-silica mantle-derived magma cools rapidly at or near Earth’s surface, commonly in volcanic lava flows, oceanic crust, and flood-basalt provinces. Most examples are geologically young relative to continental rocks, ranging from recent eruptions to Precambrian units.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, concrete aggregate, building stone, and sometimes basalt fiber; attractive polished pieces are occasionally used decoratively.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of Earth’s oceanic crust. It commonly weathers to reddish-brown iron-rich surfaces.
Field identification & locations
Look for a dense dark rock with tiny interlocking crystals; a glassy streak or abundant vesicles supports volcanic basalt. Common locations include Hawaii, Iceland, the ocean floor, India’s Deccan Traps, and volcanic regions worldwide; a fresh broken face and acid test help distinguish it from dark metamorphic rocks.
