igneous
Andesite
Andesite (Intermediate Extrusive Volcanic Rock)
Hardness: 6-7 (Mohs scale); Color: Gray to grayish-black; Luster: Dull or sub-vitreous; Crystal structure: Aphanitic (fine-grained) to porphyritic (larger crystals in fine matrix); Specific gravity: 2.4 - 2.8.
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray to grayish-black
- Luster
- Dull or sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray to grayish-black; Luster: Dull or sub-vitreous; Crystal structure: Aphanitic (fine-grained) to porphyritic (larger crystals in fine matrix); Specific gravity: 2.4 - 2.8.
Formation & geological history
Formed by the rapid cooling of intermediate lava at the Earth's surface, typically at subduction zones (convergent plate boundaries). These rocks can date from the Archean to the Holocene.
Uses & applications
Used primarily in the construction industry as crushed stone for road base, concrete aggregate, and railroad ballast. Occasionally used for decorative masonry or tiles.
Geological facts
Andesite is the second most common volcanic rock in Earth's crust after basalt. It is named after the Andes Mountains where it is found in abundance. The 1980 eruption of Mount St. Helens produced significant amounts of andesitic material.
Field identification & locations
Identify in the field by its medium gray color and the presence of small, visible needle-like crystals (plagioclase or hornblende) within a fine-grained matrix. It is commonly found in volcanic arcs like the Cascades or the Andes. Collectors should look for porphyritic specimens with well-defined phenocrysts.
