Dacite Rock Type, Composition, Formation, Occurrence & Uses

Dacite Rock
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Dacite is a volcanic igneous rock. It is a fine-grained rock that is normally light in color and is often considered a quartz-bearing variety of andesite.

Rock Composition

Dacite is often considered a fine-grained equivalent of granodiorite. It has a generalized mineral composition between rhyolite and andesite. Dacite has more quartz than andesite and more plagioclase than rhyolite. The plagioclase feldspars are often oligoclase, andesine or labradorite. The other minerals that may be found in some specimens of Dacite are biotite, augite, enstatite, and hornblende. The rock’s color also varies based on its composition. Dacite rocks with more hornblende and biotite are light grey to brown. Dacite specimens with a high concentration of augite or enstatite are dark in color.

Dacite from the Western Carpathians
Dacite from the Western Carpathians

Dacite has 63-69% silica content, one of the highest in igneous rocks.

Dacite Rock Formation

Dacite magma is formed due to the subduction of oceanic crust below a thick felsic continental plate. The oceanic crust is hydrothermally altered with the addition of quartz and sodium. This causes the subducted slab to melt partially and interact with the upper mantle via convection and dehydration reactions. It creates metamorphism in the subducting slab. The dehydration reaction breaks down minerals like talc, serpentine, mica, and amphiboles, generating a more sodic melt.

The magma continues its upward journey, becoming more sodic and silicic. The sodium-rich magma crystallizes plagioclase, quartz, and hornblende once it reaches a cold surface. The dacite rock formation explains the connection between oceanic and continental crust.

AphaniticQAPF diagram denoting dacite
AphaniticQAPF diagram denoting dacite

Where is Dacite Found?

Dacite is a common rock on earth and occurs in oceanic volcanic series such as Iceland and Juan de Fuca Ridge. It is also found in the calc-alkaline and tholeiitic volcanic series of the subduction zones. Examples include Japan, the Philippines, the Aleutians island, the Sundra Arc, Tonga, and South Sandwich islands.

Dacite Uses

It can be used as an aggregate fill in the construction and road-building industries. It is not ideal for concrete aggregate because of its high silica content.

Grey, red, black, altered white/tan, flow-banded pumice dacite
Grey, red, black, altered white/tan, flow-banded pumice dacite

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