igneous
Weathered basalt (possibly vesicular basalt)
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine, with iron-oxide alteration
AI-generated identification · may be incorrectDark gray to black, fine-grained, with rusty brown weathering and possible vesicles or mineral-filled cavities; typically dull to slightly glassy. Hardness about 5–6 Mohs, no prominent cleavage, and specific gravity around 2.8–3.1.
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Physical properties
Dark gray to black, fine-grained, with rusty brown weathering and possible vesicles or mineral-filled cavities; typically dull to slightly glassy. Hardness about 5–6 Mohs, no prominent cleavage, and specific gravity around 2.8–3.1.
Formation & geological history
An extrusive volcanic rock formed when mafic lava cooled rapidly at or near Earth’s surface, generally during Cenozoic or older volcanic activity. Brown coloration reflects oxidation and weathering of iron-bearing minerals.
Uses & applications
Used as aggregate, roadstone, riprap, ballast, and building stone; attractive polished pieces are collected but usually have modest gem value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. Vesicles may later become lined or filled with zeolite, calcite, or quartz.
Field identification & locations
Identify it by its heavy, dark fine-grained texture, iron-rich weathering, and lack of visible large crystals; a streak, magnetism, and fresh broken surface help confirm it. Common worldwide in volcanic regions, including oceanic islands, Iceland, Hawaii, and continental lava fields.
