Where is Porphyritic Andesite found?
Porphyritic Andesite
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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, where it is abundant in the volcanic ranges.
Cascades Range, North America
Volcanoes like Mount St. Helens, Mount Rainier, and Mount Hood are composed largely of andesitic lavas and pyroclastic rocks.
Japan
Numerous active and dormant volcanoes in Japan are characterized by andesitic eruptions.
Indonesia
Part of the 'Ring of Fire,' Indonesia has many volcanoes that produce andesitic lavas.
New Zealand
Volcanic regions, particularly in the North Island, feature significant occurrences of andesite.
Collecting tips
- Look for Volcanic Terrains. Porphyritic andesite is found in areas of past or present volcanic activity, especially stratovolcanoes and volcanic arcs.
- Examine Outcrops for Texture. The key identifying feature is the porphyritic texture. Look for larger, distinct crystals embedded in a finer matrix.
- Check for Mineral Assemblage. Identify common phenocrysts like plagioclase (often white to gray, tabular), hornblende (black, prismatic), or pyroxene (dark green to black, blocky).
- Consider Color and Hardness. Andesite is typically intermediate in color (medium to dark gray) and relatively hard due to its silicate mineral content.
Quick Check
- Color:
- Medium to dark gray, greenish-gray, or reddish-brown.
- Luster:
- Dull to earthy groundmass, vitreous phenocrysts.
- Streak:
- White to light gray (due to high silicate content).
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