Dacite Identification Guide
Identify Dacite, an intermediate-to-felsic volcanic rock, by its light gray porphyritic groundmass, plagioclase and quartz phenocrysts.
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What Dacite Looks Like
Dacite is a fine-grained extrusive (volcanic) igneous rock intermediate between andesite and rhyolite in silica content. It typically has a light to medium gray (sometimes pinkish or tan) fine groundmass studded with visible phenocrysts (larger crystals): white-to-clear plagioclase feldspar, glassy quartz, and dark biotite, hornblende, or pyroxene. The overall look is a pale, speckled volcanic rock.
- Color: light gray to medium gray, sometimes pink, buff, or pale brown
- Texture: porphyritic and aphanitic (visible crystals in a fine matrix); may be glassy or flow-banded
- Phenocrysts: plagioclase laths, rounded/embayed quartz, dark biotite/hornblende
- Habit: lava flows, domes, and pyroclastic deposits
Step-by-Step Field-ID Checklist
- Assess color. Lighter than basalt/andesite but usually grayer than pinkish rhyolite.
- Find phenocrysts. A hand lens should reveal blocky white feldspar, clear/gray quartz, and a few dark minerals in a fine groundmass.
- Confirm fine groundmass. Most of the rock is too fine-grained to see individual grains without magnification (volcanic).
- Hardness/scratch. Overall hard; quartz and feldspar phenocrysts scratch glass.
- Acid test. No effervescence (silicate rock, not carbonate).
Key Diagnostic Tests
- Mohs hardness: mixed; feldspar ~6, quartz ~7; the rock as a whole resists a knife.
- Streak: none useful (light, gives a pale streak/powder).
- Density: ~2.4-2.6 g/cm3, lighter than dark basalt.
- Acid: inert to dilute HCl.
- Quartz check: presence of clear, often rounded/embayed quartz phenocrysts distinguishes dacite from andesite.
Common Look-Alikes and How to Tell Them Apart
- Andesite: Generally darker gray and lacks free quartz phenocrysts; dacite has visible quartz and is paler. The quartz is the key separator.
- Rhyolite: Lighter/pinker and more silica-rich, often with more quartz and alkali feldspar and frequent flow banding; dacite is grayer with more plagioclase and dark minerals.
- Granodiorite: The coarse-grained (intrusive) chemical equivalent of dacite, with all crystals visible; dacite has a fine groundmass.
- Basalt: Much darker, denser, and quartz-free.
- Trachyte/latite: Feldspar-rich but typically lack the quartz phenocrysts; check for quartz under a lens.
Where It Is Found
Dacite is common at subduction-zone volcanoes and continental arcs. Famous occurrences include Mount St. Helens, Lassen Peak, and many Cascade and Andean volcanoes, plus volcanic domes worldwide where viscous intermediate-felsic lava erupts.
Frequently asked questions
How can you tell if it's real Dacite?
Look for a light-to-medium gray fine-grained volcanic rock with visible phenocrysts of white plagioclase, clear-to-gray quartz (often rounded), and a few dark biotite or hornblende crystals. The presence of quartz phenocrysts in a pale, fine groundmass is the key sign it is dacite rather than andesite.
What does Dacite look like?
It looks like a pale gray (sometimes pink or tan) volcanic rock with a fine groundmass speckled by larger blocky feldspar crystals, glassy quartz, and scattered dark minerals.
Dacite vs andesite: what's the difference?
Dacite has more silica and contains visible free quartz, making it lighter colored, while andesite is generally darker gray and lacks quartz phenocrysts. The quartz is the main field distinction.
Dacite vs rhyolite: how do I tell them apart?
Rhyolite is more silica-rich, usually lighter or pinker with more quartz and alkali feldspar and common flow banding, whereas dacite is grayer with abundant plagioclase and more dark minerals.
Is Dacite an intrusive or extrusive rock?
Dacite is extrusive (volcanic), erupting as lava flows and domes. Its coarse-grained intrusive equivalent is granodiorite.
Dacite identified by the community
Recent Dacite specimens identified with Rock Identifier.
