igneous
Weathered iron-rich basalt or volcanic breccia
Mafic volcanic rock, likely basalt; composition dominated by plagioclase and pyroxene with iron oxides
AI-generated identification · may be incorrectDark brown to gray, fine-grained and strongly weathered, with rusty iron-oxide staining and irregular fractures; hardness about 5–6 Mohs, dull to subtly earthy luster, no obvious cleavage, and typical specific gravity around 2.8–3.2.
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Physical properties
Dark brown to gray, fine-grained and strongly weathered, with rusty iron-oxide staining and irregular fractures; hardness about 5–6 Mohs, dull to subtly earthy luster, no obvious cleavage, and typical specific gravity around 2.8–3.2.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, probably during the Cenozoic or older; oxidation and groundwater altered the original minerals to brown iron-rich coatings. The image cannot establish an exact age or distinguish basalt from breccia confidently.
Uses & applications
Commonly used as aggregate, road base, riprap, and building stone; attractive dense pieces may be collected as volcanic specimens, though this example appears ordinary and heavily weathered.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and pillow lavas.
Field identification & locations
Identify it by its dark fine-grained matrix, high density, magnetic response if magnetite-rich, and rusty weathering; common worldwide, especially in volcanic regions. A fresh broken surface and thin-section or chemical test would improve identification.
