igneous
Andesite River Cobble
Andesite (Intermediate Volcanic Rock)
Hardness: 6-7 (Mohs); Color: Grey, greenish-grey, or brownish-grey texture; Luster: Dull/Earthy; Crystal structure: Aphanitic to porphyritic; Specific gravity: 2.4-2.8.
- Hardness
- 6-7 (Mohs)
- Color
- Grey, greenish-grey, or brownish-grey texture
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 (Mohs); Color: Grey, greenish-grey, or brownish-grey texture; Luster: Dull/Earthy; Crystal structure: Aphanitic to porphyritic; Specific gravity: 2.4-2.8.
Formation & geological history
Formed from the rapid cooling of intermediate lava at the surface. This specific specimen was found in the Solomon Islands/Bougainville region, which is geologically dominated by the Pacific Ring of Fire's subduction zones, leading to extensive Cenozoic volcanic activity.
Uses & applications
Used primarily as crushed stone for road construction, high-quality concrete aggregate, and historical use in stone tools or grinding stones.
Geological facts
The name 'Andesite' comes from the Andes Mountains, but it is the primary volcanic rock type found in the island arcs of the Southwest Pacific. Its rounded shape is the result of mechanical weathering and transport in high-energy river systems.
Field identification & locations
Identify by its fine-grained texture, hardness (cannot be scratched by a steel blade), and moderate weight. Its rounded 'potato' shape is characteristic of river-worn volcanic cobbles found in mountainous island terrains.
