igneous
Vesicular basalt (probable)
Basalt, mainly plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to slightly glassy, with small vesicles (gas bubbles); typically Mohs 5–6, no obvious cleavage, and specific gravity about 2.8–3.0. The image is too limited for a definitive identification; weathering may account for the brown surface.
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Physical properties
Dark gray-brown, fine-grained, dull to slightly glassy, with small vesicles (gas bubbles); typically Mohs 5–6, no obvious cleavage, and specific gravity about 2.8–3.0. The image is too limited for a definitive identification; weathering may account for the brown surface.
Formation & geological history
An extrusive volcanic rock formed when basaltic lava cooled rapidly at or near Earth’s surface. It may be Precambrian or younger; the specimen’s age cannot be determined visually. Basalt is geologically plausible around northeastern Minnesota, including glacially transported material from regional volcanic terranes.
Uses & applications
Commonly used as crushed aggregate, road stone, riprap, and building stone; attractive polished pieces are occasionally collected, but this small weathered fragment has little commercial value.
Geological facts
Vesicles form when dissolved gases escape from cooling lava; later, they can become filled with minerals such as calcite, quartz, or zeolites.
Field identification & locations
Look for a dense, dark fine-grained interior and magnetic response from iron-bearing minerals. A streak test, hardness test, magnet test, and fresh broken surface would distinguish it from vesicular slag or weathered scoria; common regional alternatives include basaltic volcanic rock and gabbro fragments.
