igneous
Porphyritic Rhyolite
Extrusive igneous rock with felsic composition
Hardness of 6 on Mohs scale, reddish-brown to pinkish groundmass with visible lighter phenocrysts (often quartz or feldspar), dull to sub-vitreous luster, aphanitic-porphyritic texture, specific gravity around 2.35-2.55.
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Physical properties
Hardness of 6 on Mohs scale, reddish-brown to pinkish groundmass with visible lighter phenocrysts (often quartz or feldspar), dull to sub-vitreous luster, aphanitic-porphyritic texture, specific gravity around 2.35-2.55.
Formation & geological history
Formed by the rapid cooling of silica-rich lava at or near the Earth's surface, often originating from volcanic domes or explosive eruptions. Geological age ranges from Precambrian to recent.
Uses & applications
Used locally as decorative crushed stone, road aggregate, and sometimes in landscaping or lapidary work.
Geological facts
Porphyritic texture indicates a two-stage cooling history: slow cooling deep underground allowing large crystals (phenocrysts) to form, followed by rapid eruption and cooling at the surface forming the fine-grained groundmass.
Field identification & locations
Identify in the field by its fine-grained pinkish-red background peppered with distinct, smaller light-colored crystals. Commonly found in volcanic regions worldwide.
