igneous
Weathered mafic rock, likely basalt
Basalt; primarily plagioclase feldspar, clinopyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to subtly glinting, with rounded weathering surfaces; typical hardness is about 5–6 Mohs, with no visible cleavage and a dense crystalline texture. The metallic-looking specks may be weathered iron oxides or exposed pyroxene, though a photo cannot confirm the identification.
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Physical properties
Dark gray-brown, fine-grained, dull to subtly glinting, with rounded weathering surfaces; typical hardness is about 5–6 Mohs, with no visible cleavage and a dense crystalline texture. The metallic-looking specks may be weathered iron oxides or exposed pyroxene, though a photo cannot confirm the identification.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age may range from recent eruptions to Precambrian rocks, depending on location.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and building stone; attractive polished pieces may be collected but are generally low-value.
Geological facts
Basalt is the most common volcanic rock and forms much of the ocean floor. Vesicles, if present, may later fill with minerals such as zeolite, calcite, or quartz.
Field identification & locations
Look for a heavy, dark, fine-grained rock that does not fizz with dilute acid and may attract a magnet weakly if magnetite-rich; a streak test and fresh broken surface help distinguish it from weathered sedimentary rocks. Common worldwide, especially volcanic regions and river gravels.
