Dacite

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Light gray to tan with darker phenocrysts; Luster: Sub-vitreous to dull; Crystal structure: Aphanitic/Porphyritic (fine-grained matrix with larger crystals); Specific gravity: 2.4-2.5.

Formation & Geological History

Formed through the rapid cooling of silica-rich lava at or near the Earth's surface, typically at convergent plate boundaries (subduction zones). It is associated with explosive volcanic activity.

Uses & Applications

Primarily used as crushed stone for road construction, fill material, and occasionally as a building stone or for landscape decoration.

Geological Facts

Dacite is intermediate in composition between andesite and rhyolite. Mount St. Helens is famously composed largely of dacite domes and ash flows.

Field Identification & Locations

Identify in the field by its light color and the presence of visible crystals (phenocrysts) of plagioclase, quartz, or hornblende in a very fine-grained background. It often breaks with a rough or sub-conchoidal fracture.

Identified on: 4/25/2026

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