igneous
Basalt
Basalt; fine-grained mafic volcanic rock, typically dominated by plagioclase and clinopyroxene with possible olivine
AI-generated identification · may be incorrectDark gray to black, generally aphanitic, with dull to subvitreous luster; Mohs hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears dense and massive, though the image cannot confirm mineral grains or vesicles.
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Physical properties
Dark gray to black, generally aphanitic, with dull to subvitreous luster; Mohs hardness about 6, no obvious cleavage, and specific gravity roughly 2.8–3.0. The specimen appears dense and massive, though the image cannot confirm mineral grains or vesicles.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces. It may range from modern to Precambrian in age; age requires locality or laboratory dating.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, riprap, dimension stone, and occasionally basalt fiber. Unusual polished pieces are collected, but ordinary hand specimens have modest value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify by its dark, heavy, fine-grained appearance and lack of visible layering; a streak test is usually gray, while magnetism may be weak to moderate. Common worldwide, especially volcanic regions and oceanic islands; exact identification can require a fresh surface, hand lens, and thin-section analysis.
