igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine and iron oxides
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, fine-grained, with dull to slightly vitreous luster; massive and lacking visible cleavage. The specimen shows rounded weathering, rusty iron-oxide staining, and possible alteration or vesicles.
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Physical properties
Typically Mohs 5–6, dark gray to black, fine-grained, with dull to slightly vitreous luster; massive and lacking visible cleavage. The specimen shows rounded weathering, rusty iron-oxide staining, and possible alteration or vesicles.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, generally in volcanic regions or oceanic crust; its exact age cannot be determined from the photograph. Rounding indicates transport by water, ice, or abrasion.
Uses & applications
Used as aggregate, road base, riprap, railroad ballast, and occasionally as a decorative stone or lapidary material.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the ocean floor. Iron-bearing minerals commonly weather to orange-brown coatings like those visible here.
Field identification & locations
Identify by its dark, dense, fine texture, high relative weight, and resistance to scratching; a steel file may scratch it slightly, while it generally does not fizz in dilute hydrochloric acid. Common worldwide, especially volcanic islands, river gravels, and glacial deposits.
