Where is Porphyritic Basalt found?
Igneous Rock, Volcanic
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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 porphyritic basalts, often with plagioclase or olivine phenocrysts.
Oceanic Islands and Mid-Ocean Ridges Worldwide
Commonly found in active volcanic regions associated with divergent plate boundaries and mantle plumes.
Collecting tips
- Look for Volcanic Terrains. Porphyritic basalts are found in areas of past or present volcanic activity, such as lava flows, volcanic cones, and dikes.
- Examine Outcrops for Texture. The key identifying feature is the porphyritic texture. Look for larger, distinct crystals embedded in a fine-grained matrix.
- Check for Dark Color and Density. Basalts are typically dark and relatively dense due to their mafic mineral content.
- Observe Mineral Phenocrysts. Identify the phenocrysts: white/gray tabular crystals (plagioclase), green granular crystals (olivine), or black prismatic crystals (pyroxene).
Quick Check
- Color:
- Dark gray to black
- Luster:
- Dull to earthy (groundmass), vitreous (phenocrysts)
- Streak:
- White to grayish-white (due to fine-grained nature, not diagnostic of individual minerals)
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