igneous
Vesicular basalt (weathered volcanic rock)
Basalt, a fine-grained mafic igneous rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-green, fine-grained, dull to weakly vitreous; irregular vesicles and weathering cavities are visible, some lined with secondary minerals. Typical hardness is about 6 on Mohs scale, with no prominent cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark gray-green, fine-grained, dull to weakly vitreous; irregular vesicles and weathering cavities are visible, some lined with secondary minerals. Typical hardness is about 6 on Mohs scale, with no prominent cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows or underwater pillow lavas. Basalt ranges from Precambrian to modern age; the specimen’s age cannot be determined from the image.
Uses & applications
Used extensively as crushed aggregate, road metal, railroad ballast, riprap, and building stone; attractive polished pieces may be sold as decorative or educational specimens.
Geological facts
Vesicles formed from gas bubbles trapped during lava cooling; later groundwater can enlarge them or deposit zeolite, calcite, or quartz. The greenish weathering may reflect chlorite, epidote, or altered olivine.
Field identification & locations
Identify by its heavy weight, dark fine-grained matrix, weak magnetism sometimes caused by magnetite, and rounded gas cavities; it commonly occurs in volcanic regions, oceanic crust, and river or beach gravels.
