Rhyolite is a silica-rich igneous rock found throughout the world. The rock received its name from German geologist Ferdinand von Richthofen (better known as the Red Baron, a World War I flying ace). The word rhyolite comes from the Greek word rhýax (a stream of lava) with the suffix "-ite" given to rocks. Rhyolite is similar in composition and appearance to granite, but it forms through a different process.
Key Takeaways: Rhyolite Rock Facts
- Rhyolite is an extrusive, silica-rich igneous rock.
- Rhyolite has a similar composition and appearance to granite. However, rhyolite forms as a result of a violent volcanic eruption, while granite forms when magma solidifies beneath the Earth's surface.
- Rhyolite is found all over the planet, but it is uncommon on islands located far from large land masses.
- Rhyolite takes many different forms depending on the rate at which the lava cools. Obsidian and pumice are two very different types of rhyolite.
How Rhyolite Forms
Rhyolite is produced by violent volcanic eruptions. During these eruptions, the silica-rich magma is so viscous that it does not flow in a river of lava. Instead, the volcano is more likely to explosively eject material.
While granite forms when magma crystallizes beneath the surface (intrusive), rhyolite forms when lava or ejected magma crystallizes (extrusive). In some cases, magma partially solidified into granite may be ejected from a volcano, becoming rhyolite.
The eruptions that produce rhyolite have occurred throughout geologic history and all over the world. Given the devastating nature of such eruptions, it is fortunate that they have been rare in recent history. Only three rhyolite eruptions have occurred since the beginning of the 20th century: the St. Andrew Strait volcano in Papua New Guinea (1953-1957), the Novarupta volcano in Alaska (1912), and Chaitén in Chile (2008). Other active volcanoes capable of producing rhyolite include those found in Iceland, Yellowstone in the United States, and Tambora in Indonesia.
Rhyolite is felsic, which means it contains a significant amount of silicon dioxide or silica. Usually, rhyolite contains greater than 69% SiO2. The source material tends to be low in iron and magnesium.
The rock's structure depends on the cooling rate when it formed. If the cooling process was slow, the rock may consist mostly of large, single crystals called phenocrysts, or it may be composed of a microcrystalline or even glass matrix. Phenocrysts typically include quartz, biotite, hornblende, pyroxene, feldspar, or amphibole. On the other hand, a quick cooling process produces glassy rhyolites, which include pumice, perlite, obsidian, and pitchstone. Explosive eruptions may produce tuff, tephra, and ignimbrites.
Although granite and rhyolite are chemically similar, granite often contains the mineral muscovite. Muscovite is rarely found in rhyolite. Rhyolite may contain much more of the element potassium than sodium, but this imbalance is uncommon in granite.
Rhyolite occurs in a rainbow of pale colors. It can have any texture, ranging from a smooth glass to a fine-grained rock (aphanitic) to a material containing obvious crystals (porphyritic). The hardness and toughness of the rock is also variable, depending on its composition and the rate of cooling that produced it. Typically, the rock's hardness is around 6 on the Mohs scale.
Starting about 11,500 years ago, North Americans quarried rhyolite in what is now eastern Pennsylvania. The rock was used to make arrowheads and spear points. While rhyolite may be knapped to a sharp point, it is not an ideal material for weapons because its composition is variable and it readily fractures. In the modern era, the rock is sometimes used in construction.
Gems commonly occur in rhyolite. The minerals form when lava cools so quickly that gas becomes trapped, forming pockets called vugs. Water and gases make their way into the vugs. Over time, gem-quality minerals form. These include opal, jasper, agate, topaz, and the extremely rare gem red beryl ("red emerald").Fire opal precipitates in rhyolite vugs. Coldmoon_photo / Getty Images
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