Dacite: A Fine-grained, Light-Colored Volcanic Rock

Dacite is a fine-grained, light-colored volcanic or extrusive igneous rock. Its composition is felsic and mainly has plagioclase feldspar, quartz, lesser alkali feldspar, minor hornblende, biotite, and other mafic minerals. Plagioclase is more alkali feldspar in this rock.

Guido Stache, an Austrian geologist, proposed the term dacite in 1863. It referred to calc-alkaline quartz-bearing rock with oligoclase phenocrysts from Dacia (former Roman Empire province, now in Romania and Moldovia). This name was to separate this rock from rhyolite, another calc-alkaline rock with mainly orthoclase phenocrysts.

However, today, dacite refers to a felsic, acidic rock with 63-69% silica. Such a rock must have more plagioclase feldspar than alkali feldspar. Quartz makes 20-60% of QAPF by volume of the rock, and it has minor mafic content (hornblende, biotite, and sometimes pyroxenes).

Let us learn more about dacite, including what it is, its texture, colors, and chemical and mineral composition. We will also discuss how it forms, where it is found, its uses, and more.

Quick properties and characteristics

• Rock name: Dacite
• Rock type: Igneous
• Origin: Extrusive or volcanic
• Color: White, gray, pink, brownish, blue-gray, yellow, or reddish
• Texture: Fine-grained or aphanitic to porphyritic
• Chemical composition: Felsic
• Silica content: 63-69 wt. %
• Plutonic or coarse-grained equivalent: Granodiorite
• Eruption temperature: ≈900-1000°C at 1 atm
• Density: 2.3-2.7 g/cm³ (non-vesiculated), lower for vesicular types
• Mohs hardness scale: 6 to 7
• Tectonic settings: Convergent boundaries (mainly active continental margin and island arcs) and other tectonic settings where sub-alkali basalt can fractionate

What does dunite look like?

Color, texture, and fabric are essential in identifying dacite rocks. They are usually massive, fine-grained, often porphyritic, and sometimes vesiculated off-white, gray, pink, and blue-gray rocks.

However, other colors exist. Also, they may show flow banding, and others, like dacitic pumice, are highly vesiculated.

Let us talk more about color, texture, and flow banding characteristics.

1. Colors

According to Romaine (2020), dacite colors are off-white, shades of gray, pink, blue-gray, and less often black, yellow, pale brown, or red. However, it may have deeper browns or reds.

These colors of dacitic rocks are influenced by their minerals. For instance, felsic or light-colored minerals like feldspars or quartz will give this rock lighter colors. These minerals are often colorless, whitish, pink, yellowish, or grayish.

On the other hand, dark colors (green, brown, or black) come from mafic minerals like hornblende, pyroxenes biotite, and some inclusions.

Apart from minerals, refraction from crystallites, microlites, and other small inclusions may cause various colors.

Lastly, dacite is leucocratic with a typical color index of M < 20 due to the abundance of felsic minerals compared to mafic.

2. Textures

Dacite has a fine-grained or aphanitic texture, often porphyritic. A few specimens may show serrated or rapakivi textures.

Porphyritic dacite will have mostly plagioclase and quartz (or tridymite) phenocrysts in aphanitic groundmass. However, a few phenocrysts of hornblende, biotite, augite (clinopyroxene), enstatite (orthopyroxene), Fe-Ti oxide, or apatite may occur too.

These phenocrysts will appear as large speckles of various colors set in a groundmass of minute grains of feldspars, mafic minerals, interstitial quartz or tridymite, opaques, and some glass.

Usually, plagioclase phenocrysts are blocky. However, they can be zoned or show a sieved texture, while quartz may be rounded and corroded.

Sometimes, alkali feldspar phenocrysts may be mantled by plagioclase, creating a rapakivi texture. This texture is also common in granites. Others may show glomeroporphyritic texture, where phenocrysts cluster into clots or aggregates.

Also, some porphyritic dacite may show seriated texture characterized by gradual variation of grains, suggesting crystallization continued as magma rose.

Lastly, terms like cryptodacite indicate glassy groundmass silica phase, and phanerodacite means that the excess silica phase is crystalline quartz, not glass.

3. Flow banding

The cross-section of a young dacite lava dome may show flow banding or foliation with layers showing differences in vesicularity or crystallinity, not necessarily chemical composition, notes Gill (2010). It refers to magma mingling at the subvolcanic level, with the Aliso lava dome, AZ, USA, a perfect example.  

Some streaks or layers are visible to the unaided eye, and others on a less than mm scale. They represent chemical and physical magma variations (heterogeneity) sheared, folded, or attenuated by lamina flow during extrusion. Phenocrysts will also show flow foliation.

Dacite composition

What is the chemical and mineral composition of dacite rock?

Dacite chemical composition

Dacite is an acidic, felsic rock with 63-69% feldspar and ow in alkali oxides (Na₂O and K₂O). Its composition is intermediate between andesite and rhyolite.

Data from Le Maitre (1976) puts the average weight percentage chemical composition of dacite from 651 samples to be SiO₂: 65.98%, TiO₂: 0.59%. Al₂O₃: 15.15%, Fe₂O₃: 2.47%, FeO: 2.33%, MnO: 0.09%, MgO: 1.81%, CaO: 4.38%, Na₂O: 3.85%, K₂O: 2.20% and P₂O₅: 0.15%

Mineral composition

Dacite mineral composition is mainly plagioclase feldspar, quartz (or tridymite), lesser alkali feldspar, and smaller amounts of hornblende and biotite. Sometimes it may have pyroxenes (augite, enstatite, or pigeonite), iron-rich olivine, or glass.

Typical accessory minerals in this rock include apatite, Fe-Ti oxides (magnetite and ilmenite), titanite (sphene), zircon, garnet (rarely), etc. Usually, garnet indicates anatexis alkali feldspar.

Plagioclase is mostly sodic, usually oligoclase, andesine to labradorite, and is more than the alkali feldspar. This rock may also have sanidine, an alkali feldspar, whose relative abundance increases in dacitic rocks transitioning to rhyolite.

Quartz and tridymite occur mostly as interstitial crystallites in holocrystalline dacites or as phenocrysts. Also, some samples may have tridymite veins.

Based on the QAPF (Quartz, Alkali feldspar, plagioclase, and feldspathoids) mineral by volume, dacite is defined as a volcanic rock in which quartz makes up 20-60% feldspathoids by volume and plagioclase at least 65% of the total feldspars.

Volcanic rocks like dacite, andesite, rhyolite, or basalt have an aphanitic texture with too tiny mineral crystals to distinguish. Thus, using the IUGS recommended mineral composition is hard. Instead, geologists use Total Alkali Silica (TAS) based on silica (SiO₂) and total alkali (Na₂O + K₂O).

In the TAS diagram, dacite is a volcanic rock with up to 8% total alkalis and > 63% silica.

Lastly, dacitic rock alteration will form various secondary minerals. For instance, this rock may have chlorate and uralite from hornblende, biotite, and pyroxene alteration. Also, epidote and sericite may replace feldspars. Those with olivine may have iddingsite or serpentine, while cavities may contain cristobalite (vesicle walls) and epidote.

Classification of dacite rocks

Dacite falls into subalkaline and low, medium, or high potassium (K). Subalkaline dacitic rocks are further divided into calc-alkaline and tholeiitic, which are similar but will vary in mineralogical composition, evident in intermediate rocks.

The tholeiitic series will produce iron-enriched tholeiitic basalt, ferrobasalt, andesite, dacite, and rhyolite. These rocks are common in mid-ocean ridges and island arcs, especially at early stages.

On the other hand, calc-alkaline rocks will produce basalt, andesite, dacite, and rhyolite, and their intrusive equivalent, i.e., gabbro, diorite, and granite. They are common in volcanic arcs (continental and island).

Lastly, note that alkaline magmas will evolve into trachytes and phonolites, not dacitic rocks.

Varieties and related rocks

Rocks closely related to dacite are rhyodacite, trachydacite, adakite, and sanidine dacite.

Let us look at each briefly.

1. Rhyodacite 

Rhyodacite is a fine-grained, silica-rich intermediate rock between dacite and rhyolite not represented on the QAPF diagram. This volcanic or extrusive igneous rock mainly has quartz, plagioclase, alkali feldspar, and minor amounts of mafic mineral.

2. Trachydacite

Trachydacite is an intermediate to felsic, extrusive igneous rock chemically defined to have more than 20% normative quartz. It occupies the same position as trachyte on the TAS diagram but has more than 20% normative quartz.

However, QAPF classification doesn’t recognize it since alkali feldspar-rich rocks with > 20% quartz are rhyolites.

3. Sanidine dacite

It has large sanidine phenocrysts and a transitional composition between rhyolite and dacite, i.e., close to dellenite, notes Haldar & Tisľjar (2014).

4. Adakite

Adakite is a collective term for andesite, dacite, and some sodic rhyolite extrusive rock with unusual composition named after Adak Island, Alaska, USA. However, they also occur in Mindanao (Philippines) and the Trans-Mexican Volcanic belt.

Adakites are high in strontium and low in heavy rare earth metals (HREE) and yttrium (Y). Also, they have high strontium to yttrium (Sr/Y) ratio > 40 against < 40 in other calc-alkaline rocks.

Common phenocrysts in these rocks are plagioclase, mica, amphibole, and rarely orthopyroxene with no clinopyroxenes. Their composition is similar to tonalites, trondhjemites, and granodiorites that made young Archean continental crust.

Lastly, adakite forms at subducted young oceanic crusts (≤ 25 Ma) and in non-subduction zones from the 1) partial melting metamorphosed basalt or 2) fractionation of arc magma

How is dacite formed?

Dacite rocks form from the quick cooling of silica-rich, low alkali metal oxides (Na₂O and K₂O) magma, mainly on the Earth’s surface. However, some form dikes, sills, or other intrusions close to the surface.

The fast cooling doesn’t allow large crystals to form. Thus, they have small mineral grains that are invisible to the naked eye.

However, rapid quenching of dacite and rhyolite will form obsidian glass. If it is high in volatile gases, you will have dacitic pumice.

Dacitic magma originates from the fractionation of mantle-derived magma and the partial melting of older continental rocks.

However, larger volumes of andesitic, rhyolitic, or dacitic magma originate from anatexis of subcrustal rocks. Fractionation will require huge volumes of basaltic magma.

Also, magma mixing (andesite and rhyolite) and contamination by subcrustal rocks play a role in dacitic magma generation.

Dacite lava eruption

Dacite magma may erupt effusively or explosively.

Magmas low in volatiles will be effusive, forming domes like Chaos Crags Lassen Peak (California USA) and UNzen volcano dome (Japan), plugs, domes, and less often pillow lavas (on land).

Dacitic blocky lavas don’t flow fast or far like less viscous pahoehoe, and aa lava flows except for flows Opal Cone British Colombia, Canada that flowed 15km, a remarkably far distance.

On the other hand, volatile-rich (4-8%) dacitic magma will erupt explosively. Typical eruptions are Vulcanian, Peléan, and Plinian styles. These eruptions result in pyroclastic flows, surges, lahars, or eject pyroclasts or tephra (ash, lapilli, bombs or magma blobs) blanket covering extensive places, traceable 100s to 1000s of kilometers.

For instance, in 1883, Krakatau heard 4800 km away near Mauritius in the Indian Ocean with clouds spread around the globe. Also, the Fish Canyon Tuff represents the largest explosive eruption, with 5000 km³ of dacitic volcanic ash flow or ignimbrite deposits.

Some of the eruptions associated with dacite will form stratovolcanoes. Examples include Pinatubo (1991, Philippines), St. Mt. Hellen (Washington, USA, 1980-1986), Mt. Ruapaehu (1995-1996, New Zealand), Montagne Pelée (1902-1903), Mount Unzen and (Japan, 1991-1995). More examples are Ollagüe (Chile-Bolivia), Nevado Ojos del Salado (Argentina-Chile), and Mount Haruna (Japan).

Other mountains with dacitic lava eruptions include Mt Shasta (California, USA) and Myojinsho (Japan, 1952-1953), a submarine volcano.

Lastly, phreatomagmatic eruptions (if they interact with water) may occur. This kind will be explosive.

Where is dacite found?

Dacite occurs mainly in volcanic arcs, i.e., active continental and mature island arcs above the subduction zone. These volcanic arcs are mostly along the Ring of Fire around the Pacific Ocean, including in related orogenic belts.

However, it can occur in other tectonic settings where alkali basalt magma fractionation happens. Such places may include continental rifts, large igneous provinces (LPIs), bark arc basins, mid-ocean ridges, etc.

Depending on the tectonic setting, dacite will often occur with other rocks like rhyolite, rhyodacite, felsite, obsidian, andesite, trachyte, basalt, quartz latite, granophyre, pitchstone, comendite, ignimbrite, etc. It may be a major constituent or minor.

Some of the areas with dacite and related rocks include:

1. Active volcanic margins (continental and island arcs)

Dacite is most common in continental margins, especially those with thick crusts and long-lasting subduction zones. As magma rises through thick continental crust, it becomes chemically contaminated, becoming more silicic and having sialic elements. Also, it gives time for magma to evolve.

Some active continental margins with this rock include the Cascade Range (Canada and USA) and the west coast of South America (Andes). For instance, dacite is prominent in the central Andes, with a thick crust of dacitic ignimbrite sheets covering over 50,000 km².

On the other hand, island arcs like Antilles, Tonga–Kermadec, Japan, Aleutian, and South Sandwich Island, Fiji Islands, Sunda, Hellenic arc (Nea Kameni and Palea Kameni), etc., have considerable amounts of dacitic and rhyodacitic rocks.

However, primitive immature island arcs with thin crusts will form only basaltic suites.

2. Other tectonic settings

Small amounts of dacite may form in oceanic mid-ocean ridges like the Galapagos spreading center, Juan de Fuca Ridge, and Heiðarsporður Ridge (Iceland).

Also, this rock occurs in back-arc basins such as in North Island, New Zealand, and Kasuga seamounts in the northern Mariana arc.

Furthermore, dacite may occur in intra-continental settings. For instance, the Great Basin has dacitic ignimbrites. Also, these rocks occur in Central American and Mexican plateaus, notes McBirney (1989).

LPIs like Panjal Traps (Kashmir Valley), Rajmahal-Sylhet Traps, Madagascar LIP, and Warakurna (Australia) may have dacitic rocks, too.

Lastly, this rock can occur in continental rift zones. For instance, some dacite occurs in the Rio Grande Rift at Francisco Peaks (Arizona), Valles Caldera (New Mexico), and East African Rift.

Dacite rock uses

Dacite is a durable, tough, or hard rock with a Mohs hardness of 6-7. Here are the modern and historical uses of dacite

1. Modern uses

Its uses include making aggregate for construction, the roading industry, fills, unpaved driveways, walkways or patios, etc., and as a dimension stone for building houses, monuments cladding, paving, etc. However, its high silica makes gravel or aggregate unsuitable for concrete making.

Also, rhyolite and dacite are associated with ores, which are important economically. Examples include the Iberian pyrite belt (Spain and Portugal) and Cu–Zn–Pb ±Ag deposits in the Kuroko belt in Japan.

In Europe, gold is associated with subvolcanic andesitic to dacitic in the Carpathian Mountain arc. Also, these rocks are associated with Cu-Ag skarn deposits in the Slovakian Ore Mountains, Vihorlat Mountains in Slovakia, and Transylvanian Ore Mountains in Romania, according to Okrusch (2020).

Additionally, dacitic pumice has many uses, including making lightweight aggregate, landscaping and horticulture, beauty and personal care, abrasive cleaning material, etc. See more on pumice uses.

More uses of dacite include decorative landscaping, riprap to control erosion, carving, or making sculptures.  

Historical uses of dacite

Historically, purple porphyritic dacite to trachyandesite, known as the Roman Imperial Porphyry, was highly valuable to Romans and Byzantines for building and sculpturing. It came from Mons Porphyrites at the Rea Sea Mountains of Egypt. Also, these rocks made Sarcophagi for only sovereigns in Egypt.

Roman Imperial Porphyry had white plagioclase feldspar phenocrysts (mainly andesine to oligoclase) and brown augite embedded in a fine-grained gray matrix or groundmass.

According to Ingham (2013), some of the most valuable kinds of this dacitic porphyritic rock, also known as porfido rosso antico (Ancient Red Porphyry), were:

• Lapis Porphyrites with white feldspar in a deep purple groundmass color from hematite,
• Rubet Porphyrites that has pink feldspar from piemontite set in purple groundmass, and
• Lapis Porphyrites Niger with white feldspar set in a black, greenish-tinged groundmass.

That is not all. As Okrusch (2020) notes, Santa Maria church in Cosmedin, Rome, Italy, built in 1294, used yellow marble, basaltic trachyandesite, i.e., Porfido verde antico from Krokees, Peloponnese, Greece), and Porfido rosso antico from Mons Porphyritic.

Porfido rosso antico had plagioclase phenocrysts embedded in a claret-red matrix, while Porfido verde antico had greenish plagioclase set in a dark-colored matrix.

Frequently Asked Questions (FAQs)

References

• Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
• Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
• McBirney, A. R. (1989) Andesite and dacite. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (pp. 18-21). New York: Van Nostrand Reinhold.
• Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.
• Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
• Haldar, S. K., & Tisľjar, J. (2014). Introduction to Minerology and petrology (1st ed.). Elsevier.
• Frost, B. R. (2014). Essentials of igneous and metamorphic petrology. Cambridge University Press.
• Ingham, J. P. (2013). Geomaterials under the microscope. Manson Publishing.
• Romaine, G. (2020). Rocks, gems, and Minerals (3rd ed.). FalconGuides.
• Okrusch, M., & Frimmel, H. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits (1st ed.). Springer.
• Le Maitre, R. W. (1976). The chemical variability of some common igneous rocks. Journal of Petrology, 17(4), 589–598. https://doi.org/10.1093/petrology/17.4.589