igneous
Basalt
Vesicular basalt; chiefly plagioclase, clinopyroxene, olivine, and volcanic glass
AI-generated identification · may be incorrectDark gray to brown, fine-grained, dull to weakly subvitreous, with abundant small vesicles and a massive structure; hardness about 5–6 Mohs, no obvious cleavage, specific gravity roughly 2.7–3.0.
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Physical properties
Dark gray to brown, fine-grained, dull to weakly subvitreous, with abundant small vesicles and a massive structure; hardness about 5–6 Mohs, no obvious cleavage, specific gravity roughly 2.7–3.0.
Formation & geological history
An extrusive mafic igneous rock formed when low-viscosity lava cooled rapidly at or near Earth’s surface. Its age cannot be determined from the image; basalt forms from Precambrian to modern eruptions.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, riprap, concrete aggregate, and paving stone; attractive polished pieces are collected but generally have modest value.
Geological facts
Vesicles are gas bubbles trapped during cooling, and the brown coloration may reflect weathering or oxidation of iron-bearing minerals.
Field identification & locations
Identify it by its dark fine-grained matrix, high density, lack of visible large crystals, and vesicles; a streak and magnetism test may help, though composition varies. Common worldwide in volcanic regions, oceanic crust, flood-basalt provinces, and lunar-style basalt collections.
