Where is Porphyritic Andesite found?
Andesite (porphyritic texture)
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Open the appPorphyritic Andesite 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 Andesite
Andes Mountains, South America
The type locality for andesite, found extensively along the entire length of the mountain range, associated with active and extinct volcanoes.
Pacific Ring of Fire
Numerous occurrences in volcanic arcs around the Pacific Ocean, including Japan, Indonesia, the Philippines, Alaska, and the Cascade Range (USA).
Aegean Volcanic Arc, Greece
Present in volcanic islands such as Santorini and Milos.
Caribbean Volcanic Arc
Found in islands like Montserrat, Martinique, and St. Vincent.
Collecting tips
- Look for Volcanic Terrains. Porphyritic andesite is an extrusive igneous rock, so it will be found in areas of past or present volcanic activity, particularly stratovolcanoes and lava flows.
- Examine Outcrops for Texture. Look for rocks with a distinct two-stage texture: larger, well-formed crystals (phenocrysts) embedded in a much finer-grained matrix. Plagioclase phenocrysts are often white or light-colored and tabular.
- Check for Associated Minerals. The presence of plagioclase, pyroxene, and hornblende phenocrysts is characteristic. Quartz phenocrysts are rare or absent in true andesite, distinguishing it from dacite.
- Consider Color and Density. Andesite is typically intermediate in color (mesocratic to leucocratic) and density, falling between basalt (darker, denser) and rhyolite (lighter, less dense).
Quick Check
- Color:
- Gray to dark gray, reddish-brown, or black.
- Luster:
- Dull to earthy groundmass, vitreous to pearly phenocrysts.
- Streak:
- White to light gray (not typically tested for rocks).
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