igneous
Weathered basalt or fine-grained mafic rock
Basalt, likely altered/vesicle-poor tholeiitic basalt; exact identification requires testing
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly submetallic surface with rusty-brown weathering along fractures. Typical basalt hardness is about 6 Mohs, specific gravity 2.8–3.0, with interlocking microscopic plagioclase and pyroxene; no visible cleavage or large crystals.
Identified More igneous →
Explore Weathered basalt or fine-grained mafic rock in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray to black, fine-grained, dull to weakly submetallic surface with rusty-brown weathering along fractures. Typical basalt hardness is about 6 Mohs, specific gravity 2.8–3.0, with interlocking microscopic plagioclase and pyroxene; no visible cleavage or large crystals.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, dikes, and oceanic crust. It may range from recent volcanic material to Precambrian rock, but age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and occasionally building stone or landscaping rock; attractive pieces may be collected but are generally low value.
Geological facts
The reddish-brown lines are likely iron-oxide staining along joints or alteration seams, while the polygonal cracking reflects weathering and natural fractures.
Field identification & locations
Common worldwide, especially in volcanic provinces and oceanic terrains. Field confirmation includes its heavy feel, dark fine texture, lack of fizz with dilute acid, and gray-black streak; a fresh broken surface and magnet test can help distinguish basalt from slag or iron-rich metamorphic rocks.
