igneous
Andesite
Andesite (Intermediate Extrusive Volcanic Rock)
Hardness: 6-7 (Mohs scale); Color: light to dark grey, sometimes reddish-brown; Luster: dull/earthy; Crystal Structure: Aphanitic (fine-grained) often with visible phenocrysts of plagioclase or hornblende; Specific gravity: 2.4 - 2.8
- Hardness
- 6-7 (Mohs scale)
- Color
- light to dark grey, sometimes reddish-brown
- Luster
- dull/earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: light to dark grey, sometimes reddish-brown; Luster: dull/earthy; Crystal Structure: Aphanitic (fine-grained) often with visible phenocrysts of plagioclase or hornblende; Specific gravity: 2.4 - 2.8
Formation & geological history
Formed through the rapid cooling of intermediate-composition lava at or near the surface, typically at volcanic arches above subduction zones. Can range from Cenozoic to ancient Precambrian ages depending on the tectonic history of the region.
Uses & applications
Primarily used in the construction industry as crushed stone for road base, concrete aggregate, and railroad ballast. Occasionally used for landscaping, pavers, or as a dimension stone for architectural cladding.
Geological facts
Andesite is named after the Andes Mountains in South America, where it is found in great abundance. It represents the chemical intermediate between basalt and dacite/rhyolite. Some of the world's most famous volcanoes, like Mount St. Helens, are primarily composed of andesite.
Field identification & locations
Identify in the field by its medium-grey color (lighter than basalt), fine-grained texture, and lack of visible quartz crystals. Common in volcanic regions of Western North America, the Andes, Japan, and Indonesia. Look for 'porphyritic' textures where larger crystals are embedded in a fine-grey matrix.
