igneous
Vesicular basalt
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, and matte to weakly vitreous, with abundant rounded gas vesicles; hardness about 5–6 Mohs, nonfoliated, and lacking cleavage as a whole. The specimen appears weathered and may contain lighter alteration minerals in some cavities.
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Physical properties
Typically dark gray to black, fine-grained, and matte to weakly vitreous, with abundant rounded gas vesicles; hardness about 5–6 Mohs, nonfoliated, and lacking cleavage as a whole. The specimen appears weathered and may contain lighter alteration minerals in some cavities.
Formation & geological history
Formed when low-silica basaltic lava cooled rapidly at or near Earth’s surface, trapping bubbles of volcanic gas. Basalt ranges from Precambrian to modern age and commonly forms lava flows, volcanic islands, and oceanic crust.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive vesicular pieces are collected or used decoratively. It is generally low-value unless unusually shaped or from a notable locality.
Geological facts
Vesicles distinguish it from many massive igneous rocks; if vesicles are filled with minerals such as zeolite, calcite, or quartz, the rock is called amygdaloidal basalt. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark fine-grained matrix, irregular volcanic texture, and gas holes; it is common in Hawaii, Iceland, the Deccan Traps, western North America, and oceanic volcanic regions. Field confirmation may require checking that it is not lightweight scoria or artificial slag.
