igneous
Dacite
Dacite (intermediate volcanic rock high in silica)
Hardness: 6-7 Mohs; Color: Light gray to bluish-gray; Luster: Dull or earthy; Crystal structure: Porphyritic with a fine-grained (aphanitic) groundmass; Cleavage: None (conchoidal fracture)
- Hardness
- 6-7 Mohs
- Color
- Light gray to bluish-gray
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Light gray to bluish-gray; Luster: Dull or earthy; Crystal structure: Porphyritic with a fine-grained (aphanitic) groundmass; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed by the rapid cooling of silica-rich lava from subduction zone volcanoes. It is common in continental volcanic arcs and is typically associated with explosive eruptions.
Uses & applications
Primarily used in construction as crushed stone for road base or aggregate. Occasional use in historical architectural facades or as a filler material.
Geological facts
Dacite is the intermediate rock between andesite and rhyolite. The famous 1980 eruption of Mount St. Helens was characterized by dacite lava and ash.
Field identification & locations
Identify by its light gray color, rough texture, and visible small black crystals (phenocrysts) of hornblende or biotite against a pale, uniform background. Often found in proximity to andesite flows.
