igneous
Weathered mafic rock, likely basalt
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine- to medium-grained, massive, and dull to weakly vitreous; typically Mohs 5–6, with no obvious cleavage and specific gravity about 2.8–3.0. The rounded shape and brown patches indicate surface weathering, possibly with iron-oxide staining.
Identified More igneous →
Explore Weathered mafic rock, likely basalt in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray-brown, fine- to medium-grained, massive, and dull to weakly vitreous; typically Mohs 5–6, with no obvious cleavage and specific gravity about 2.8–3.0. The rounded shape and brown patches indicate surface weathering, possibly with iron-oxide staining.
Formation & geological history
Basalt forms when low-silica magma erupts and cools rapidly at or near Earth’s surface, commonly in lava flows and oceanic crust. Most examples range from recent to several billion years old; this specimen’s age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces may be collected, but this appears to be a common field specimen.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and relatively low quartz content.
Field identification & locations
Identify it by its dark color, high density, fine-grained texture, and resistance to scratching with a steel knife; acid usually produces no reaction. Common worldwide in volcanic regions, especially oceanic islands and continental lava provinces.
