igneous
Weathered basalt or mafic metavolcanic rock
Basalt, possibly altered to greenstone (chlorite–epidote–actinolite assemblage)
AI-generated identification · may be incorrectDark gray-green, fine-grained and strongly weathered, with dull earthy luster and irregular fracture; apparent green patches may be chlorite, epidote, or actinolite. Hardness is approximately 5–6, with no obvious cleavage; exact identification is limited by the small image and weathering.
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Physical properties
Dark gray-green, fine-grained and strongly weathered, with dull earthy luster and irregular fracture; apparent green patches may be chlorite, epidote, or actinolite. Hardness is approximately 5–6, with no obvious cleavage; exact identification is limited by the small image and weathering.
Formation & geological history
Likely volcanic basalt formed from rapidly cooled lava, later altered by low-grade metamorphism or hydrothermal fluids. Such rocks are commonly Precambrian in northern Minnesota, although the specimen’s exact age cannot be determined visually.
Uses & applications
Usually a common field specimen with little commercial value; fresh basalt is used as aggregate, road stone, and riprap. Attractive greenstone pieces may be collected or polished, but this example appears mainly of educational interest.
Geological facts
The reported location near 46.7°N, 92.0°W is geologically compatible with abundant Precambrian mafic volcanic and metamorphic rocks of the Duluth–Lake Superior region.
Field identification & locations
Look for a dense, fine-grained texture, dark green-black color, and a pale green streak or crystals. A knife-scratch test, magnet test, and fresh broken surface would help distinguish basalt/greenstone from serpentinite or epidote-rich rock; avoid acid testing unless carbonate veins are suspected.
