igneous
Porphyritic Basalt
Porphyritic Basalt with Plagioclase Phenocrysts
Hardness of 6 on the Mohs scale for feldspar phenocrysts, dark grey to black groundmass with contrasting white or light grey rounded phenocrysts, dull to sub-aphanitic luster, aphanitic-porphyritic texture, specific gravity around 2.8-3.0.
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Physical properties
Hardness of 6 on the Mohs scale for feldspar phenocrysts, dark grey to black groundmass with contrasting white or light grey rounded phenocrysts, dull to sub-aphanitic luster, aphanitic-porphyritic texture, specific gravity around 2.8-3.0.
Formation & geological history
Formed via a two-stage cooling process where early-formed minerals (phenocrysts) grew slowly in a magma chamber before the remaining magma erupted and cooled rapidly at or near the Earth's surface.
Uses & applications
Used primarily in construction as crushed aggregate, roadstone, and building stone. Unique spotted specimens are also collected by rockhounds and lapidary enthusiasts.
Geological facts
The distinctive spotted appearance has led to common regional nicknames such as 'leopard rock' or 'porcupine rock' depending on the shape and distribution of the crystals.
Field identification & locations
Easily identified in the field by its contrasting large, light-colored crystals embedded in a dark, fine-grained volcanic matrix. Commonly found in ancient and modern volcanic provinces worldwide.
