igneous
Weathered porphyritic volcanic rock, likely rhyolite or andesite
Porphyritic rhyolite–andesite; mainly quartz/feldspar phenocrysts in a fine iron-stained groundmass
AI-generated identification · may be incorrectHardness about 6–7 Mohs; reddish-brown to purplish, with pale gray-white crystals and dull to slightly vitreous luster. The visible porphyritic texture indicates larger crystals embedded in a fine-grained matrix; cleavage is absent, and specific gravity is roughly 2.5–2.7.
Identified More igneous →
Explore Weathered porphyritic volcanic rock, likely rhyolite or andesite in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness about 6–7 Mohs; reddish-brown to purplish, with pale gray-white crystals and dull to slightly vitreous luster. The visible porphyritic texture indicates larger crystals embedded in a fine-grained matrix; cleavage is absent, and specific gravity is roughly 2.5–2.7.
Formation & geological history
Formed when silica-rich to intermediate lava cooled relatively quickly at or near Earth’s surface, preserving earlier-formed feldspar or quartz crystals. Its precise age cannot be determined from the photograph and depends on the local volcanic terrain.
Uses & applications
Used locally as landscaping stone, riprap, aggregate, and occasionally decorative facing stone; generally has little jewelry value.
Geological facts
The reddish coloration likely reflects oxidation of iron-bearing minerals during weathering. A fresh broken surface, streak, and acid or hardness tests would help distinguish rhyolite from andesite or altered volcanic rock.
Field identification & locations
Common in volcanic belts worldwide, including the western United States, Mexico, Japan, and New Zealand. Look for phenocrysts, lack of foliation, and a fine volcanic matrix; a hand lens and fresh fracture are useful.
