igneous
Vesicular basalt (weathered basaltic cobble)
Basalt; primarily plagioclase, pyroxene, and olivine
AI-generated identification · may be incorrectFine-grained, gray-green to brown, dull luster, with small rounded vesicles and weathered mineral spots; hardness about 6, no obvious cleavage, and specific gravity roughly 2.7–3.0.
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Physical properties
Fine-grained, gray-green to brown, dull luster, with small rounded vesicles and weathered mineral spots; hardness about 6, no obvious cleavage, and specific gravity roughly 2.7–3.0.
Formation & geological history
An extrusive igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, likely during the Cenozoic or older depending on locality. Vesicles formed from gas bubbles trapped in the cooling lava; surface alteration produces the greenish coating.
Uses & applications
Commonly used as crushed aggregate, road stone, riprap, and building stone; attractive polished pieces may be used decoratively. This small weathered cobble has minimal commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and volcanic islands. The holes distinguish vesicular basalt from many dense dark volcanic rocks.
Field identification & locations
Identify it by its dense fine-grained texture, dark basaltic color, scattered gas bubbles, and lack of visible large crystals; a steel scratch test should generally fail or barely mark it. Common worldwide, especially in volcanic terrains and river gravels.
