igneous
Weathered porphyritic volcanic rock
Likely andesite or rhyolite porphyry, composed chiefly of feldspar and quartz with iron-oxide staining
AI-generated identification · may be incorrectGray-brown to reddish, dull earthy luster, fine-grained groundmass with indistinct lighter phenocrysts, massive and non-cleaved; estimated Mohs hardness 6–7 and specific gravity about 2.5–2.7.
Identified More igneous →
Explore Weathered porphyritic volcanic rock in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Gray-brown to reddish, dull earthy luster, fine-grained groundmass with indistinct lighter phenocrysts, massive and non-cleaved; estimated Mohs hardness 6–7 and specific gravity about 2.5–2.7.
Formation & geological history
Formed when silica-rich lava cooled, leaving larger crystals in a finer matrix; likely Cenozoic or older, though age cannot be determined from the photograph. Surface rounding and reddish coloration indicate weathering and transport.
Uses & applications
Used locally as crushed aggregate, riprap, landscaping stone, and occasionally as a cabinet or collecting specimen; generally not gem-quality.
Geological facts
The reddish areas likely reflect oxidation of iron-bearing minerals, while the darker fractured zones may be alteration or weathering rather than a separate rock type.
Field identification & locations
Identify by its hard, dense, volcanic texture, lack of foliation, and possible feldspar crystals; a fresh broken surface and acid test would help distinguish andesite from rhyolite. Common in volcanic belts worldwide, including the Andes, Cascades, Japan, and Mediterranean regions.
