igneous
Weathered vesicular basalt (likely amygdaloidal basalt)
Basalt, chiefly plagioclase + pyroxene ± olivine; cavities may contain secondary zeolites, calcite, or chlorite
AI-generated identification · may be incorrectDark volcanic rock heavily altered and weathered to tan, green, and brown; fine-grained groundmass with abundant rounded vesicles/amygdules and dull to earthy luster. Hardness is approximately 5–6, with no obvious cleavage; identification is limited by surface alteration.
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Physical properties
Dark volcanic rock heavily altered and weathered to tan, green, and brown; fine-grained groundmass with abundant rounded vesicles/amygdules and dull to earthy luster. Hardness is approximately 5–6, with no obvious cleavage; identification is limited by surface alteration.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping gas bubbles; later groundwater alteration filled some cavities and weathered the exterior. The precise age and locality cannot be determined from the image.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and occasionally decorative stone; cavity-filled specimens may interest collectors, though this example appears common and strongly weathered.
Geological facts
Vesicles are former gas bubbles, while amygdules are those cavities later filled by minerals. Green coatings or fillings commonly reflect chlorite, epidote, or zeolite-group minerals.
Field identification & locations
Look for a fine-grained volcanic texture, irregular vesicles, high density, and absence of visible quartz grains; a streak, fresh broken surface, magnetism, and acid test would improve identification. Common in basaltic volcanic regions worldwide, including Hawaii, Iceland, the Columbia River Basalt Province, and oceanic crust.
