Where is Porphyritic Basalt found?
Basalt (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
Extensive basaltic lava flows, many of which are porphyritic, are found on the Hawaiian Islands, particularly on the Big Island (e.g., Kilauea, Mauna Loa).
Columbia River Basalt Group, USA
Vast flood basalts covering parts of Washington, Oregon, and Idaho, often exhibiting porphyritic textures.
Deccan Traps, India
One of the largest volcanic provinces globally, with numerous porphyritic basalt flows.
Iceland
Active volcanic island with abundant basaltic volcanism, including porphyritic varieties.
Mid-ocean ridges
Commonly found in samples dredged from mid-ocean ridges, representing newly formed oceanic crust.
Collecting tips
- Look for volcanic terrains. Porphyritic basalt is an extrusive igneous rock, so it is found in areas of past or present volcanic activity, such as lava flows, volcanic plateaus, and shield volcanoes.
- Examine road cuts and quarries. These artificial exposures often reveal fresh rock surfaces where the porphyritic texture is clearly visible.
- Check for phenocrysts. The key identifying feature is the presence of larger, distinct crystals (phenocrysts) embedded in a fine-grained matrix. Look for white (plagioclase), green (olivine), or black (pyroxene) crystals.
- Consider the color and density. Basalts are typically dark and relatively dense. If the rock is light-colored or unusually light, it's likely not basalt.
Quick Check
- Color:
- Dark gray to black
- Luster:
- Dull to sub-vitreous (groundmass), vitreous to pearly (phenocrysts)
- Streak:
- Grayish-white to colorless (due to silicate minerals)
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