igneous
Porphyritic Basalt
Basalt (Mafic volcanic rock)
Hardness: 6 (Mohs scale), Color: Dark grey to black with light-colored phenocrysts (spots), Luster: Dull to sub-metallic, Structure: Fine-grained matrix with larger visible crystals (porphyritic texture).
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black with light-colored phenocrysts (spots), Luster: Dull to sub-metallic, Structure: Fine-grained matrix with larger visible crystals (porphyritic texture).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The lighter spots (phenocrysts) represent crystals that grew slowly underground before the lava erupted and cooled quickly.
Uses & applications
Used extensively in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used for decorative landscaping and as floor tiles.
Geological facts
Basalt is the most common rock type on Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.
Field identification & locations
Identify in the field by its dark, dense appearance and small visible crystals against a dark background. Often found in volcanic regions, riverbeds, and coastal areas where it has been rounded by water.
