igneous
Porphyritic Basalt
Basalt with Plagioclase phenocrysts
Hardness: 6-7 (Mohs scale), Color: Dark grey to dark reddish-brown with lighter grey/green spots, Luster: Dull/Earthy, Texture: Porphyritic (fine-grained matrix with visible crystals)
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark grey to dark reddish-brown with lighter grey/green spots, Luster: Dull/Earthy, Texture: Porphyritic (fine-grained matrix with visible crystals)
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The larger spots (phenocrysts) formed slowly underground before the eruption.
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, railroad ballast, and sometimes as decorative garden stones or landscaping rock.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Porphyritic varieties tell a story of two-stage cooling: slow then fast.
Field identification & locations
Identified by its heavy weight (high density), dark color, and characteristic 'spotted' appearance from larger crystals set in a fine-grained background. Often found in volcanic regions or riverbeds.
