igneous
Weathered basalt or mafic volcanic rock
Basalt; primarily plagioclase feldspar and pyroxene, possibly olivine
AI-generated identification · may be incorrectDark gray-brown, fine-grained to subtly vesicular, with dull to earthy luster and irregular fracture; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. The surface appears heavily weathered, with pale alteration minerals or encrustation.
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Physical properties
Dark gray-brown, fine-grained to subtly vesicular, with dull to earthy luster and irregular fracture; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. The surface appears heavily weathered, with pale alteration minerals or encrustation.
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface; basalt ranges from Archean to modern age and commonly forms volcanic flows and oceanic crust.
Uses & applications
Used as crushed aggregate, roadstone, railway ballast, riprap, and building stone; attractive polished pieces may be sold as inexpensive decorative specimens.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up much of the ocean floor. Weathering commonly produces brown iron-oxide staining and clay-rich coatings.
Field identification & locations
Identify by its dark color, heavy weight, fine texture, and lack of obvious large crystals; a streak is usually gray, and it generally does not fizz with dilute hydrochloric acid unless coated with carbonate. Common worldwide, especially volcanic regions and oceanic islands.
