igneous
Porphyritic Basalt
Aphanitic-porphyritic mafic volcanic rock
Hardness: 6 (Mohs scale), Color: Dark grey to black with light-colored phenocrysts (plagioclase), Luster: Dull to sub-vitreous, Crystal structure: Fine-grained matrix with larger visible crystals, Specific gravity: 2.8 - 3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black with light-colored phenocrysts (plagioclase), Luster: Dull to sub-vitreous, Crystal structure: Fine-grained matrix with larger visible crystals, Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The larger crystals (phenocrysts) formed at depth before the lava erupted and cooled quickly on the surface. Commonly associated with oceanic crust and volcanic hotspots.
Uses & applications
Primarily used as a construction aggregate, for road base, concrete, and railroad ballast. Exceptionally durable specimens are used in stone masonry and occasionally for ornamental garden stones.
Geological facts
Basalt is the most common volcanic rock on Earth and also makes up most of the surface of the Moon and Mars. The porphyritic texture indicates a two-stage cooling history.
Field identification & locations
Identify by looking for a dense, dark, heavy rock with small needle-like or rectangular white crystals embedded in a dark grey background. Found in volcanic regions like Hawaii, the Pacific Northwest, and Iceland.
