igneous
Weathered mafic rock, likely basalt
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-green, fine-grained and dull to weakly granular, with irregular fracture and no visible cleavage; hardness about 5–6 Mohs, specific gravity roughly 2.8–3.1. The small specimen appears strongly weathered, so exact identification from the image is uncertain.
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Physical properties
Dark gray-green, fine-grained and dull to weakly granular, with irregular fracture and no visible cleavage; hardness about 5–6 Mohs, specific gravity roughly 2.8–3.1. The small specimen appears strongly weathered, so exact identification from the image is uncertain.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. It may range from recent to Precambrian in age.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive varieties are used for polished architectural surfaces and modest specimen collecting.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Weathering can produce reddish-brown iron oxides and clay minerals, obscuring its original texture.
Field identification & locations
Look for a dense dark color, fine texture, high weight, and lack of obvious quartz; a streak, fresh broken surface, and magnet test can help, though this fragment may also be weathered gabbro or another mafic rock. Common worldwide, especially volcanic regions and oceanic crust.
