igneous
Vesicular basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase, pyroxene, and olivine
AI-generated identification · may be incorrectDark gray-black, fine-grained, porous with abundant vesicles, and generally dull to weakly vitreous; hardness about 5–6 Mohs, no prominent cleavage, and specific gravity roughly 2.7–3.0.
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Physical properties
Dark gray-black, fine-grained, porous with abundant vesicles, and generally dull to weakly vitreous; hardness about 5–6 Mohs, no prominent cleavage, and specific gravity roughly 2.7–3.0.
Formation & geological history
Formed when low-silica basaltic lava cooled rapidly at or near Earth’s surface, with gas bubbles producing the vesicles. It may be Cenozoic to Precambrian depending on its volcanic setting; age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, roadstone, riprap, concrete additive, and sometimes dimension stone or landscaping rock. Attractive vesicular pieces are collected but usually have modest value.
Geological facts
The rounded cavities are frozen gas bubbles, and the specimen may be called scoria if it is exceptionally vesicle-rich and lightweight. Its dark color reflects abundant iron- and magnesium-bearing minerals.
Field identification & locations
Identify it by its black color, fine texture, and bubble holes; basalt is common in volcanic regions, oceanic crust, Hawaii, Iceland, and continental flood-basalt provinces. The image may also show weathering or surface coatings, so a fresh break is most diagnostic.
