Dacite
Dacite is a fine-grained, extrusive igneous rock of intermediate to felsic composition, chemically positioned between andesite and rhyolite with roughly 63 to 68 percent silica. It usually shows a porphyritic texture, with visible crystals of plagioclase, quartz, and hornblende or biotite set in a dense, glassy to microcrystalline groundmass. Dacite magma is typically thick and gas-rich, making it prone to explosive eruption.
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Dacite properties at a glance
| Type | Igneous Rocks |
|---|---|
| Mohs hardness | 6–7 |
| Luster | Dull to vitreous |
| Fracture | Conchoidal to uneven |
| Specific gravity | 2.6–2.7 |
| Transparency | Opaque |
| Common colors | Gray, Light gray, Tan, Pale brown |
| Rarity | common |
Geology & background
Dacite forms above subduction zones, where partial melting of the mantle wedge and subsequent fractional crystallization or crustal melting produce a magma richer in silica than basalt or andesite but not as evolved as rhyolite. Because dacitic magma is viscous, it often erupts explosively or piles up as thick lava domes rather than flowing far, and it is closely associated with major stratovolcanoes such as Mount St. Helens, Mount Pinatubo, and the volcanic chain of the Andes. In the field, dacite is usually gray to light tan and noticeably lighter in color than basalt, with visible phenocrysts giving it a speckled look. Its explosive eruptive history makes dacite domes and pyroclastic deposits important for understanding volcanic hazard, and collectors often gather samples tied to specific historic eruptions, such as the 1980 Mount St. Helens dome.
How to identify Dacite
Dacite is typically light gray to tan and porphyritic, with visible plagioclase, quartz, or dark hornblende and biotite crystals scattered through a fine-grained to glassy groundmass. Overall hardness is close to that of its feldspar phenocrysts, around 6, so testing is best done on individual grains. It lacks the dark color and abundant iron-magnesium minerals of andesite and basalt. Dacite is easily confused with rhyolite and andesite, and distinguishing them often requires checking mineral content or silica percentage in a lab.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Dacite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Dacite worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Construction or aggregate when suitable
- Landscape interpretation
- Education
Interesting facts
- The 1980 eruption of Mount St. Helens produced a dacite lava dome that kept regrowing inside the crater for years afterward.
- Dacite sits chemically between andesite and rhyolite, making it one of the more silica-rich rocks erupted at subduction-zone volcanoes.
- Because dacitic magma is thick and gas-charged, it fuels some of the most explosive eruptions on Earth, including Mount Pinatubo in 1991.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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