igneous
Vesicular basalt
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray, fine-grained, dull to weakly glassy, with irregular vesicles and weathered tan mineral patches; hardness about 5–6 Mohs, no obvious cleavage, typically dense except where vesicular. The visible rounded cavities and massive texture are characteristic of basaltic lava.
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Physical properties
Dark gray, fine-grained, dull to weakly glassy, with irregular vesicles and weathered tan mineral patches; hardness about 5–6 Mohs, no obvious cleavage, typically dense except where vesicular. The visible rounded cavities and massive texture are characteristic of basaltic lava.
Formation & geological history
Formed when low-silica magma rapidly cooled at or near Earth’s surface, likely as an ancient lava flow; individual flows may range from Precambrian to very recent in age. Vesicles formed from gas bubbles trapped during eruption, later partly filled or weathered.
Uses & applications
Widely used as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished basalt is also used decoratively. Ordinary field specimens generally have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its dark color, fine grain, heavy feel, and vesicles; a streak test is usually gray, and a magnet may show weak attraction if magnetite is present. Common near volcanic fields, oceanic islands, and flood-basalt provinces.
