igneous
Weathered volcanic rock, likely andesite or basalt
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase ± pyroxene, amphibole, and iron oxides
AI-generated identification · may be incorrectGray-brown, rough and strongly weathered, with a dull earthy luster and no obvious crystal faces; likely Mohs hardness about 5–6, massive texture, and density roughly 2.6–2.9 g/cm³. The image is insufficient to distinguish basalt from andesite confidently.
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Physical properties
Gray-brown, rough and strongly weathered, with a dull earthy luster and no obvious crystal faces; likely Mohs hardness about 5–6, massive texture, and density roughly 2.6–2.9 g/cm³. The image is insufficient to distinguish basalt from andesite confidently.
Formation & geological history
Formed when lava cooled at or near Earth’s surface, generally during the Cenozoic or older volcanic episodes; surface oxidation and weathering produced the brown coating and softened texture.
Uses & applications
Common volcanic rock is used as crushed aggregate, road base, riprap, and occasionally building stone; ordinary hand specimens have little commercial or jewelry value.
Geological facts
Basalt is typically darker and richer in pyroxene, while andesite is commonly medium gray and more silica-rich with visible plagioclase. A fresh broken surface would provide a much more reliable identification.
Field identification & locations
Look for a fine-grained, non-layered texture, hardness sufficient to scratch glass, and possible weak magnetism; common worldwide in volcanic terrains, especially subduction zones and oceanic crust. Acid reaction is absent unless coated with carbonate.
