igneous
Weathered volcanic rock, likely andesite or basaltic-andesite
Fine-grained plagioclase–pyroxene volcanic rock; possible altered andesite (exact identification requires testing)
AI-generated identification · may be incorrectGray to brown, dull and strongly weathered, with a fine-grained groundmass and pale angular mineral or altered-rock patches. Approximate hardness 5–6; cleavage is absent at the rock scale, and specific gravity is typically 2.6–2.8.
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Physical properties
Gray to brown, dull and strongly weathered, with a fine-grained groundmass and pale angular mineral or altered-rock patches. Approximate hardness 5–6; cleavage is absent at the rock scale, and specific gravity is typically 2.6–2.8.
Formation & geological history
Formed when lava cooled at or near Earth’s surface, likely during a Cenozoic or older volcanic eruption; the rusty colors and friable texture reflect surface oxidation and alteration. The image alone cannot establish its geological age.
Uses & applications
Commonly used as crushed aggregate, road base, riprap, and occasionally building stone; this weathered specimen has little jewelry value.
Geological facts
Andesite commonly contains abundant plagioclase with pyroxene or amphibole, while basaltic-andesite is compositionally intermediate between basalt and andesite. Weathering can obscure the original texture and make identification uncertain.
Field identification & locations
Look for a hard, dense, dark gray fine-grained rock with scattered white plagioclase crystals; a streak test, fresh broken surface, hand lens, and hardness or acid testing help distinguish it from tuff, shale, or concrete. Common in volcanic arcs and lava fields worldwide.
