igneous
Dacite
Dacite (intermediate volcanic rock)
Hardness: 6-7 (Mohs); Color: Gray to light tan with visible white and black crystals; Luster: Dull/earthy matrix with glassy phenocrysts; Structure: Porphyritic (fine-grained groundmass with larger crystals); Specific Gravity: 2.4-2.5
- Hardness
- 6-7 (Mohs)
- Color
- Gray to light tan with visible white and black crystals
- Luster
- Dull/earthy matrix with glassy phenocrysts
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Physical properties
Hardness: 6-7 (Mohs); Color: Gray to light tan with visible white and black crystals; Luster: Dull/earthy matrix with glassy phenocrysts; Structure: Porphyritic (fine-grained groundmass with larger crystals); Specific Gravity: 2.4-2.5
Formation & geological history
Formed by the rapid cooling of silica-rich lava at the Earth's surface, typically at subduction zones above continental plates. It represents an intermediate composition between andesite and rhyolite.
Uses & applications
Used primarily in construction as crushed stone for road base, aggregate, and fill. Occasionally used in stone carvings or as a source of high-silica volcanic material.
Geological facts
Dacite takes its name from Dacia, a province of the Roman Empire (now Romania) where it was first identified. The 1980 eruption of Mount St. Helens was characterized by the explosion of dacite magma.
Field identification & locations
Identified by its light gray color and the presence of visible rectangular white plagioclase crystals and dark needle-like hornblende or biotite flakes. Commonly found in the Cascade Range of the USA and the Andes Mountains.
