igneous
Basalt (likely vesicular basalt)
Basalt; predominantly plagioclase, clinopyroxene, and olivine, with possible volcanic glass
AI-generated identification · may be incorrectDark gray to black, fine-grained and dense, with a dull to subtly vitreous luster and irregular fracture; Mohs hardness about 5–6, typically no visible cleavage, and specific gravity around 2.8–3.0. Small cavities suggest vesicles, though the surface may also be weathered or artificially broken.
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Physical properties
Dark gray to black, fine-grained and dense, with a dull to subtly vitreous luster and irregular fracture; Mohs hardness about 5–6, typically no visible cleavage, and specific gravity around 2.8–3.0. Small cavities suggest vesicles, though the surface may also be weathered or artificially broken.
Formation & geological history
An extrusive mafic igneous rock formed when low-silica lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions; its exact geological age cannot be determined from the image. It forms in lava flows, volcanic islands, mid-ocean ridges, and flood-basalt provinces.
Uses & applications
Used widely as crushed aggregate, road base, railway ballast, riprap, concrete additive, and dimension stone; attractive polished pieces may be used decoratively. Ordinary hand specimens have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its black, fine-grained appearance, heavy feel, and occasional gas cavities; a streak test is usually pale gray, and a magnet may show weak attraction. Common locations include Hawaii, Iceland, India’s Deccan Traps, and oceanic volcanic regions.
