igneous
Porphyritic Basalt or Porphyritic Andesite
Porphyritic volcanic rock
Hardness: 6-7 on Mohs scale; Color: Dark brown to reddish-brown groundmass with distinct lighter-colored plagioclase or quartz phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic groundmass with distinct phaneritic phenocrysts (porphyritic texture); Specific gravity: 2.7-3.0
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark brown to reddish-brown groundmass with distinct lighter-colored plagioclase or quartz phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic groundmass with distinct phaneritic phenocrysts (porphyritic texture); Specific gravity: 2.7-3.0
Formation & geological history
Formed through a two-stage cooling process: slow cooling deep underground allowing larger crystals (phenocrysts) to form, followed by rapid eruption and cooling at or near the Earth's surface creating the fine-grained groundmass.
Uses & applications
Used in construction, as crushed stone for aggregate, road metal, and occasionally polished for decorative architectural stone.
Geological facts
The distinct spotted or speckled appearance is known as a porphyritic texture, which tells geologists the complex thermal history of the magma before it erupted.
Field identification & locations
Can be identified in the field by its dark, fine-grained background embedded with contrasting, lighter blocky crystals. Commonly found in volcanic regions, ancient lava flows, and glacial drift deposits worldwide.
