Where is Porphyritic Basalt found?
Igneous rock, extrusive, porphyritic texture
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Open the appPorphyritic Basalt 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 Basalt
Hawaii, USA
Active shield volcanoes (e.g., Kilauea, Mauna Loa) frequently erupt porphyritic basalts with olivine phenocrysts.
Columbia River Basalt Group, USA
Extensive flood basalts in the Pacific Northwest, often exhibiting porphyritic textures with plagioclase phenocrysts.
Deccan Traps, India
One of the largest volcanic provinces on Earth, containing vast quantities of porphyritic basalts.
Iceland
Mid-ocean ridge volcanism produces abundant basalts, including porphyritic varieties with plagioclase and olivine.
Oceanic Islands (e.g., Galapagos, Canary Islands)
Volcanic activity in these settings commonly produces porphyritic basalts.
Collecting tips
- Look for Volcanic Regions. Porphyritic basalts are found in areas of past or present volcanic activity, such as lava flows, volcanic plateaus, and eroded volcanic structures.
- Examine Outcrops. Look for dark, fine-grained rocks with distinct, larger crystals embedded within them. Freshly broken surfaces provide the best view of texture.
- Check for Vesicles. While not always present, some porphyritic basalts may contain vesicles (gas bubbles), indicating rapid cooling and gas escape during eruption.
- Consider Associated Rocks. Porphyritic basalts are often found alongside other volcanic rocks like non-porphyritic basalt, scoria, and volcanic ash.
Quick Check
- Color:
- Dark gray to black
- Luster:
- Dull to sub-vitreous (groundmass), vitreous to pearly (phenocrysts)
- Streak:
- White to light gray (due to fine powder of silicates)
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