igneous
Weathered vesicular basalt (probable)
Basalt; primarily plagioclase, clinopyroxene, and iron-rich olivine, with secondary weathering minerals
AI-generated identification · may be incorrectDark gray to brown, fine-grained and dull, with an irregular porous surface and pale weathering crust. Hardness is approximately 5–6 Mohs; cleavage is absent at the rock scale and specific gravity is typically 2.8–3.0.
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Physical properties
Dark gray to brown, fine-grained and dull, with an irregular porous surface and pale weathering crust. Hardness is approximately 5–6 Mohs; cleavage is absent at the rock scale and specific gravity is typically 2.8–3.0.
Formation & geological history
An extrusive mafic igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. Its tan coating likely reflects oxidation and chemical weathering.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally as building or landscaping stone. This small weathered fragment has little jewelry or commercial value.
Geological facts
Basalt commonly contains microscopic crystals because of rapid cooling; visible pores indicate trapped volcanic gas bubbles (vesicles). The photograph does not show enough detail for a definitive identification, and it could also be a weathered mafic rock such as andesite.
Field identification & locations
Common worldwide in volcanic regions, including Hawaii, Iceland, India’s Deccan Traps, and oceanic crust. Confirm by checking a fresh dark interior, lack of fizz with dilute acid, and magnetic response; a hand lens may reveal tiny feldspar crystals.
