igneous
Likely weathered basalt
Basalt; fine-grained mafic rock composed mainly of plagioclase, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectDark gray-green, dull to weakly earthy luster, massive and fine-grained; typically Mohs 5–6, with no visible cleavage and density about 2.8–3.0 g/cm³. The specimen appears weathered and may contain small vesicles or altered mineral patches.
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Physical properties
Dark gray-green, dull to weakly earthy luster, massive and fine-grained; typically Mohs 5–6, with no visible cleavage and density about 2.8–3.0 g/cm³. The specimen appears weathered and may contain small vesicles or altered mineral patches.
Formation & geological history
Basalt forms when low-silica magma erupts and rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands. Its exact age cannot be determined from the photograph; basalt ranges from Archean to modern.
Uses & applications
Crushed basalt is used for road aggregate, railroad ballast, concrete, and construction stone; attractive varieties are used as landscaping material and occasionally polished decorative stone. This small weathered piece has little commercial value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. Iron- and magnesium-rich minerals commonly weather to greenish or brownish alteration products.
Field identification & locations
Look for a dark, heavy, fine-grained surface that does not react with dilute acid; a streak test is usually gray. Common localities include volcanic regions, Hawaii, Iceland, the Deccan Traps, and oceanic crust; exact identification may require a fresh broken surface or thin section.
