Where is Porphyritic Rhyolite found?
Rhyolite with phenocrysts of feldspar and possibly quartz
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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 ignimbrites, often porphyritic, associated with the Yellowstone hotspot.
Sierra Nevada, USA
Volcanic fields with rhyolitic domes and flows, including porphyritic varieties.
Andes Mountains, South America
Common in volcanic arcs, where subduction-related volcanism produces felsic magmas.
New Zealand
Taupo Volcanic Zone features numerous rhyolitic domes and ignimbrites.
Japan
Volcanic islands with active and extinct volcanoes often host rhyolitic rocks.
Collecting tips
- Look for Volcanic Terrains. Porphyritic rhyolite is found in areas of past or present volcanic activity, particularly where explosive eruptions or viscous lava flows occurred. Look for volcanic domes, flow fronts, and caldera rims.
- Examine Outcrops for Texture. Carefully inspect rock outcrops for the characteristic porphyritic texture – larger, distinct crystals (phenocrysts) embedded in a fine-grained or glassy matrix. Quartz phenocrysts often appear as glassy, irregular grains, while feldspar phenocrysts are typically opaque and blocky.
- Check for Flow Banding. Many rhyolites exhibit flow banding, which are parallel or contorted layers of different colors or textures, indicating viscous flow. This can be a good indicator of rhyolitic composition.
- Consider Color and Hardness. Rhyolite is typically light-colored and hard. Test its hardness with a steel tool (it should scratch steel) and observe its color, which can range from white to pink, gray, or reddish.
Quick Check
- Color:
- Light (white, gray, pink, tan, red)
- Luster:
- Dull to earthy (groundmass), vitreous to pearly (phenocrysts)
- Streak:
- White
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