igneous
Amygdaloidal basalt (vesicular basalt)
Basalt, commonly containing plagioclase, pyroxene, iron oxides, and secondary zeolite/chlorite/calcite fillings
AI-generated identification · may be incorrectDark gray, fine-grained volcanic rock with rounded vesicles and pale green mineral-filled amygdules; typically dull to slightly waxy, hardness about 5–6, with no obvious cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark gray, fine-grained volcanic rock with rounded vesicles and pale green mineral-filled amygdules; typically dull to slightly waxy, hardness about 5–6, with no obvious cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Formed when fluid basaltic lava cooled rapidly at or near Earth’s surface, trapping gas bubbles later filled by groundwater minerals. Its exact age cannot be determined from the image; such rocks range from Precambrian to modern, commonly occurring in lava flows and oceanic volcanic terranes.
Uses & applications
Used as crushed aggregate, road stone, riprap, and building stone; attractive polished pieces are collected as decorative lapidary material.
Geological facts
The pale green fillings are likely zeolites, chlorite, epidote, or prehnite, while the dark spots are original vesicles or basaltic matrix. This texture is called amygdaloidal basalt.
Field identification & locations
Identify it by its dark aphanitic groundmass, abundant rounded cavities, and contrasting pale secondary fillings; a knife usually fails to scratch fresh basalt. Common in volcanic regions such as the Columbia River Basalt, Iceland, India, and ocean-island lava fields.
