igneous
Vesicular basalt
Basalt, predominantly plagioclase + clinopyroxene ± olivine, with vesicles
AI-generated identification · may be incorrectDark gray-green, fine-grained, dull to slightly glassy; abundant rounded gas cavities (vesicles). Hardness about 6 on Mohs scale, no obvious cleavage, typically dense except where vesicular; individual crystals are mostly microscopic.
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Physical properties
Dark gray-green, fine-grained, dull to slightly glassy; abundant rounded gas cavities (vesicles). Hardness about 6 on Mohs scale, no obvious cleavage, typically dense except where vesicular; individual crystals are mostly microscopic.
Formation & geological history
Extrusive volcanic rock formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping expanding gas bubbles; age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and building stone; vesicular varieties may be used decoratively. This specimen has modest collector value.
Geological facts
Vesicles distinguish it from most dense basalt; cavities may later fill with zeolites, calcite, or silica, producing amygdaloidal basalt. The green tint can reflect olivine alteration or weathering.
Field identification & locations
Identify by its fine dark matrix, high hardness, and abundant cavities; it is commonly found in lava flows, volcanic fields, and basaltic river or glacial gravels. A streak test is usually gray, and it is generally nonmagnetic to weakly magnetic.
