igneous
Andesite
Andesite (Intermediate Extrusive Igneous Rock)
Hardness: 6-7 on Mohs scale; Color: typically light to dark gray (also greenish or brownish); Luster: Subvitreous to dull; Crystal structure: Aphanitic (fine-grained) to porphyritic (larger crystals like plagioclase in a fine matrix); Specific gravity: 2.4 - 2.8
- Hardness
- 6-7 on Mohs scale
- Color
- typically light to dark gray (also greenish or brownish)
- Luster
- Subvitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically light to dark gray (also greenish or brownish); Luster: Subvitreous to dull; Crystal structure: Aphanitic (fine-grained) to porphyritic (larger crystals like plagioclase in a fine matrix); Specific gravity: 2.4 - 2.8
Formation & geological history
Formed through the rapid cooling of intermediate lava at the surface. It is primarily associated with subduction zones at convergent plate boundaries. The specimen in the image is likely from the Quaternary period (approx. 7,700 years ago) related to the eruption of Mount Mazama.
Uses & applications
Used primarily in the construction industry as crushed stone for road base, concrete aggregate, and railroad ballast. Historically and in modern landscape architecture, it is used for masonry, wall stone, and decorative pavers.
Geological facts
Andesite is the second most common volcanic rock in the Earth's crust after basalt. It is named after the Andes Mountains, where it is found in abundance. The photo features Crater Lake, Oregon, which was formed by the collapse of an andesitic-dacitic volcano.
Field identification & locations
Identify in the field by its characteristic gray color, fine-grained texture, and lack of visible quartz compared to rhyolite. It often contains 'phenocrysts' (small visible white crystals of plagioclase). Found in volcanic arcs globally (Cascades, Andes, Japan).
