Rhyolites are extrusive igneous rocks with very high silica content. It’s usually gray or pink, with tiny grains that are difficult to observe without magnification or a 10x-powered loop. Rhyolite is composed of Plagioclase, Quartz, and Sandine, with minor amounts of Biotite and Hornblende.
If you’ve dug for rocks and minerals then you’ve come across pockets in host rock or some type of pay gravel. More than likely you came across a rhyolite vug created by trapped gases. They often contain Opal, crystals, or other minerals.
Many Rhyolites form from Granitic Magma that’s partially cooled in the subsurface. When these magma eruptions occur, a rock with two different grain sizes can form. The large crystals formed beneath the surface are called Phenocrysts, and the tiny crystals formed at the surface are called Groundmass.
As with most rocks, there are various types of Rhyolite. Let’s break them down for you.
We use the Porphyritic or Rhyolite Porphyry terms to indicate that the Rhyolite rocks have larger crystals in a fine-grained to glassy matrix. Keep in mind that rhyolites are volcanic or extrusive igneous rocks that might have fine-grained to glassy texture, for example, non-porphyry or larger crystals in a fine grain matrix, including Porphyritic Rocks.
The term Porphyry only tells us more about the texture. It has absolutely nothing to do with the actual size of crystals (larger and finer-grained matrix), chemical composition (mineralogy), or chemistry.
Most Rhyolite rocks are porphyritic, which indicates that crystallization began before extrusion. Crystallization may sometimes have started while the magma was buried below the surface. In such situations, the stones may consist principally of well-developed, large single crystals (Phenocrysts) at the time of extrusion.
The amount of microcrystalline matrix or groundmass in the final product may then be so small it can escape detection without the use of a microscope; such Nevadites (rocks) are easily mistaken for Granite. However, in most Rhyolites, the period of such crystallization is relatively short, and the stones consist primarily of microcrystalline or partial glassy matrix containing few Phenosrysts. The matrix can sometimes be granophytic or micro pegmatitic. Glassy Rhyolites include Perlite, Obsidian, Pitchstone, and Pumice.
Rhyolites are felsic extrusive rocks. Because of the high silica content, Rhyolite lava is incredibly viscous and flows relatively slowly, similar to toothpaste being squeezed out of a tube, and naturally tends to pile up and form lava domes.
When Rhyolite magma is gas-rich, it can erupt explosively, which forms a frothy solidified magma we know as Pumice ( a very lightweight, light-colored, vesicular form of Rhyolite) along with Ignimbrite and/or ash deposits. In certain situations, incredibly porous Rhyolite lava flows can develop. The extreme porosity of these flows allows degassing and subsequent collapse of the flow, which forms Obsidian.
Rhyolite is the volcanic equivalent of Granite.
How To Identify Rhyolite
There are various tests one can do to identify rock specimens.
Rhyolite is widespread and so relatively recognizable that one might think that identifying the stone is a trivial matter. While this is often the case, it can lead us to misidentify other rock types as Rhyolite.
To identify Rhyolite, we must first ensure its color is relatively light, usually light gray or light pink. Then, check the crystals within the mass of the rock cannot be seen with your naked eye. Some larger crystals can be identified as Feldspar or Quartz. Rhyolite is dense, without air pockets or bubbles.
A stone should meet all of these requirements to be identified or considered a Rhyolite:
- Igneous: The rocks must be formed from cooling lava with interlocking crystal grains.
- Fine-grained: Porphyritic or aphanitic texture with crystals invisible to our naked eye should be present.
- Massive: There shouldn’t be any layering or internal structures.
- Felsic Mineralogy: Must have a high Feldspar or Quartz content.
Suppose your specimen meets all of the above criteria. In that case, it is incredibly likely a Rhyolite or at least a closely related specimen.
How is Rhyolite Formed
Rhyolite is created by violent volcanic eruptions. During the eruptions, the silica-rich magma is so incredibly viscous that it doesn’t flow in a river of lava. Instead, the volcanoes are more likely to explosively eject the materials.
While Granite forms when magma crystallizes beneath the surface, Rhyolite forms when lava or ejected magma crystallizes. In some cases, magma partially solidified into Granite can be ejected from a volcano, becoming Rhyolite.
The volcanic eruptions that produce Rhyolite have occurred throughout geological history. There have only been three Rhyolite eruptions to occur since the beginning of the 20th century: the St. Andrew Strait Volcano in Papua New Guinea between 1953 and 1957, the Novarupta Volcano in Alaska in 1912, and Chaiten in Chile in 2008. Other active volcanoes that are capable of producing Rhyolite include those in Iceland, Tambora in Indonesia, and Yellowstone in the United States.