igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark brown to black, locally rusty from oxidation, with a dull to faintly waxy luster and rounded weathering pits. Typically hardness 5–6 Mohs, dense, nonfoliated, and massive; individual crystals are mostly too small to see.
Identified More igneous →
Explore Weathered basalt cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark brown to black, locally rusty from oxidation, with a dull to faintly waxy luster and rounded weathering pits. Typically hardness 5–6 Mohs, dense, nonfoliated, and massive; individual crystals are mostly too small to see.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. The rounded shape and surface pitting indicate prolonged stream, beach, or soil weathering; its exact age cannot be determined from the image.
Uses & applications
Commonly used as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished pieces may be collected or used decoratively.
Geological facts
Iron-bearing minerals commonly weather to reddish-brown oxides, producing the chocolate-brown surface. Vesicles or weathering cavities may resemble holes, but this specimen appears heavily altered and rounded rather than freshly vesicular.
Field identification & locations
Identify it by its high density, dark fine-grained interior, lack of layering, and weak response or no response to acid; a fresh broken surface should be gray-black. Basalt is widespread worldwide, especially in Hawaii, Iceland, India’s Deccan Traps, and oceanic islands.
