igneous
Porphyritic Basalt
Porphyritic Basalt
Hardness: 6-7 (Mohs scale), Color: Dark grey or greenish-grey matrix with lighter tan or white phenocrysts, Luster: Dull to sub-vitreous, Crystal structure: Fine-grained matrix with larger visible crystals (porphyritic texture)
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark grey or greenish-grey matrix with lighter tan or white phenocrysts, Luster: Dull to sub-vitreous, Crystal structure: Fine-grained matrix with larger visible crystals (porphyritic texture)
Formation & geological history
Formed from the rapid cooling of lava containing large pre-existing crystals. These 'phenocrysts' grew slowly underground before the remaining magma was erupted and cooled quickly on the surface.
Uses & applications
Used primarily in construction as crushed stone for road base or railroad ballast. Distinctive specimens are often collected as 'river rocks' for landscaping or decoration.
Geological facts
The 'porphyritic' texture records a two-stage cooling history of the magma. Basalt is the most common volcanic rock in the Earth's crust and makes up most of the ocean floor.
Field identification & locations
Identified by the distinct 'spotted' appearance of larger mineral crystals (usually plagioclase or olivine) embedded in a very fine-grained, dark background. Commonly found in volcanic regions and transported by rivers.
