igneous
Porphyritic Basalt or Porphyritic Andesite
Porphyritic volcanic rock
Hardness on Mohs scale: 6-7. Color: Purplish-brown or reddish-brown groundmass with distinct white or light-colored plagioclase feldspar phenocrysts. Luster: Dull to sub-vitreous. Crystal structure: Aphanitic (fine-grained) groundmass with larger crystalline phenocrysts. Specific gravity: 2.7-3.0.
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness on Mohs scale: 6-7. Color: Purplish-brown or reddish-brown groundmass with distinct white or light-colored plagioclase feldspar phenocrysts. Luster: Dull to sub-vitreous. Crystal structure: Aphanitic (fine-grained) groundmass with larger crystalline phenocrysts. Specific gravity: 2.7-3.0.
Formation & geological history
Formed through a two-stage cooling process. Magma initially cools slowly deep underground, allowing larger crystals (phenocrysts) to form, followed by a rapid eruption and cooling at or near the Earth's surface, creating the fine-grained groundmass.
Uses & applications
Used primarily as crushed stone for construction, road aggregate, and sometimes in landscaping and decorative gravel.
Geological facts
The distinctive spotted appearance is known as a porphyritic texture. The contrasting white laths of feldspar stand out sharply against the darker iron- and magnesium-rich volcanic matrix.
Field identification & locations
Easily identified in the field by its contrasting large white mineral crystals embedded in a dark, fine-grained reddish or brown volcanic base. Commonly found in volcanic regions, ancient lava flows, and glacial drift.
