What Is Rhyodacite Rock Composition and How Does It Form?

Rhyodacite is a fine-grained, light-colored volcanic or extrusive rock. This felsic rock has mainly plagioclase, quartz, alkali felspar, and smaller amounts of mafic minerals. The mafic minerals present are biotite, hornblende, and sometimes pyroxenes.

The name rhyodacite combines part of rhyolite and the word dacite, i.e., rhyo + dacite, since it is an intermediate rock between these two.

Let us discuss rhyodacite, including its properties (texture and color), chemical and mineral composition, and how it forms. We will also look at where it is found and some uses.

Quick properties and facts

• Name: Rhyodacite
• Rock type: Igneous
• Origin: Extrusive or volcanic
• Texture: Fine-grained or aphanitic but may be porphyritic
• Color: Light colored, mainly light to medium gray, but may be pinkish, reddish, brownish, or other colors
• Chemical composition: Felsic
• Silica content: ~ 69-72%
• Mohs hardness: 6.
• Extrusive equivalent: Rocks between granodiorite and monzogranite
• Tectonic settings: Mainly in the convergent plate boundaries, especially continental margins and island arc, but can occur in other tectonic settings too

What does rhyodacite look like?

Color and texture are essential in identifying rhyodacite rock in the field. Minerals are too small to see with the naked eye. However, it might not be easy to tell it apart from rhyolite or dacite.

Usually, rhyodacite is mostly light to medium gray, but it may be reddish, brownish, tan, pinkish, purplish, or have a greenish hue, with dark gray ones less common. It somewhat resembles dacite and rhyolite.

On the other hand, the texture of rhyodacite rock is fine-grained. However, most specimens are porphyritic, i.e., they have larger crystals called phenocrysts in a fine-grained to a bit glassy matrix.  

Porphyritic rhyodacite will have mainly plagioclase and quartz phenocrysts in a fine-grained or microcrystalline matrix. Sometimes, it may have a few phenocrysts of biotite, pyroxene, or hornblende, which are darker in color.

Chemical and mineral composition

Chemically, rhyodacite is an acidic or felsic rock with 69-72% silica. It is relatively low in alkali oxides (Na₂O and K₂O) and ferromagnesium elements (iron and magnesium).

The mineral composition of rhyodacite is mainly sodic plagioclase, potassic alkali, quartz, and minor amounts of biotite, hornblende, and sometimes pyroxene.

The common accessory minerals in this rock may include magnetite and apatite.

Also, according to Deer et al. (2013), magmatic epidote minerals can occur in dacite and rhyodacite. Also, TTG (tonalite-trondhjemite-granodiorite) and rhyolite may have epidote.  

Lastly, rhyodacite isn’t represented in the QAPF diagram. However, its composition is 20-60% quartz of QAPF content by volume, with plagioclase feldspar accounting for about 2/3 of the feldspars.

How is rhyodacite formed?

Rhyodacite rock is formed by quick cooling silica-rich magma on or near the Earth’s surface (shallow sills, dikes, and other intrusions). The fast cooling allows the formation of fine-grained crystals.

However, if the magma quenches too quickly, it will form a volcanic glass, i.e., rhyodacitic obsidian.

Lastly, magma originates from fractionating (calc-alkaline or tholeiitic magma series) or partial subcrustal rock melting. However, magma mixing and subcrustal rock contamination play a role, too.

How does it erupt?

Rhyodacite magma is silica-rich and may erupt effusively, forming lava domes, plugs, or slow-moving blocky lava flow morphology if relatively low in volatile magmas. This lava flow has a limited extent and results in massive rocks, some showing flow foliation.

However, more often than, this magma will erupt explosively, i.e., the Peléan Vulcanian or Plinian style if high in volatiles. Explosive eruptions will form pyroclasts or ejecta (volcanic ash, lapilli, bombs, and other debris). Some may result in pyroclastic flows and surges.

These explosive eruptions will result in various pyroclastic deposits, while those that are volatile-rich may form frothy, vesicle-filled rocks, i.e., rhyodacitic pumice.

Where is rhyodacite found?

Dacite rock mainly forms in volcanic arcs on convergent plate boundaries, i.e., continental margins and island arcs. However, smaller amounts occur in mid-ocean ridges, seamounts, and other tectonic settings.

According to Blatt & Tracy (2006), latites, dacite, rhyodacite, and rhyolite rock suites are common in some active continental arcs above subduction zones. Fractionation, secondary rock melting, contamination, and assimilation of magma passing through the thick continental crust result in intermediate and felsic rocks with diverse compositions, including rhyodacite.

Similarly, although island arcs have mainly andesite and some basalt, they may have rhyodacitic and dacitic rocks.

Also, Philpotts & Ague (2022) note that minor amounts of plagiogranite or rhyodacite may occur in mid-ocean ridges. These rocks are common in environments near “transform, propagating rift tips and overlapping spreading centers and propagating rift tips with high Fe-Ti basalts,” according to Perfit & Davidson (2000).

To specific locations, rhyodacite lava flows occur in northwestern Ferry County (Washington, USA) and An Sgùrr Eigg Island (Scotland). Also, Mammoth Mountain (California, USA) has overlapping dacite rhyodacite domes.

Lastly, the island arc rhyodacite includes Point Formation lavas on Seguam Island and Piip seamount (western Aleutian arc). Also, the Ordovician of Wales has pillow lava from rhyolitic to dacitic composition.

What is rhyodacite used for?

Some of the uses of rhyodacite rock include making aggregate for construction, roading, fills, unpaved patios, driveways or walkways, and as a dimension stone building houses or monuments, paving, cladding, etc. However, its relatively high silica content makes it not an ideal stone for concrete making.

Also, you can use it for landscaping, as riprap rock to control erosion, among other uses.

References

• Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company.
• Philpotts, A. R., & Ague, J. J. (2022). Principles of igneous and Metamorphic Petrology (3rd ed.). Cambridge University Press.
• Perfit, M. R. & Davidson, J. P. (2000). Plate tectonics and volcanism. In Sigurdsson, H. (ed.) Encyclopedia of volcanoes. (1st ed. p. 99) San Diego: Academic Press.
• Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (3rd ed.). The Mineralogical Society.