Where is Porphyritic Rhyolite found?
Porphyritic Rhyolite
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Open the appPorphyritic Rhyolite occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Porphyritic Rhyolite
Yellowstone National Park, USA
Extensive rhyolitic lava flows and domes, often porphyritic, are characteristic of the Yellowstone Caldera complex.
Nevada, USA
Numerous rhyolitic volcanic fields and caldera complexes, such as the Black Rock Desert, contain porphyritic rhyolite.
New Zealand
The Taupo Volcanic Zone is a major rhyolitic volcanic province with abundant porphyritic rhyolite occurrences.
Japan
Associated with subduction-related volcanism along the Pacific Ring of Fire, porphyritic rhyolites are found in various volcanic regions.
Andes Mountains, South America
Widespread felsic volcanism in the Andes produces significant amounts of porphyritic rhyolite.
Collecting tips
- Look for Volcanic Terrains. Porphyritic rhyolite is an extrusive igneous rock, so search in areas with past or present volcanic activity, such as ancient lava flows, volcanic domes, and caldera rims.
- Identify Phenocrysts. The key to identifying porphyritic rhyolite is the presence of visible, larger crystals (phenocrysts) of quartz and/or feldspar embedded in a fine-grained groundmass. Quartz phenocrysts often appear as glassy, bipyramidal crystals.
- Observe Color and Texture. Note the light color (pink, gray, reddish) and the characteristic porphyritic texture. Flow banding, where layers of different textures or colors are visible, can also be present.
- Check for Hardness. Rhyolite is a hard rock due to its high quartz content. It will scratch glass and steel.
Quick Check
- Color:
- Light (pink, gray, reddish, yellowish)
- Luster:
- Dull to vitreous (groundmass), vitreous to pearly (phenocrysts)
- Streak:
- White
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