Rhyolite is an extrusive igneous rock with a high silica concentration. The rock is usually pink or gray. The grains on the rock surface are so small that it is difficult to observe them without a hand lens.
A typical rhyolite rock specimen has numerous extremely small vugs (cavities in rock lined with mineral crystals) with some evidence of flow structure.
The name rhyolite was first used in Geology in 1860 by a German geologist, Ferdinand von Richthofen. It is derived from the Greek word rhýax, meaning a stream of lava.
The rock is mostly composed of quartz and feldspar. Most deposits have a silica content of more than 68%. The quartz content may be as low as 10%, but it is mostly 25-30%. The rock also contains feldspar from 50% to 70%. The potassium feldspar in the rock is almost double the amount of plagioclase feldspar.
Dark minerals like phenocrysts and biotite are present in trace amounts. The trace accessory minerals may include muscovite, pyroxenes, amphiboles, and oxides.
Rhyolite’s composition is similar to granite but has much smaller grains.
Rhyolite Rock Formation
Rhyolite is formed after granitic magma eruptions. Granitic magma eruptions also result in pumice, obsidian, or tuff. All of these rocks have similar compositions but different cooling conditions.
There are two types of eruptions; effusive or explosive. Effusive eruptions produce rhyolite or obsidian when lava cools immediately. Explosive eruptions produce tuff or pumice.
Granitic magma eruptions don’t frequently happen as other eruptions. Only 3 eruptions have been reported since the 1900s; Novarupta Volcano in Alaska, Chaiten Volcano in Chile, and St. Andrew Strait Volcano in Papua New Guinea.
A typical granitic magma is rich in silica and gas. The silica starts to connect into complex molecules, giving magma a high viscosity. It makes magma sluggish. A high gas percentage and viscosity cause the magma to erupt explosively. The viscosity is so high that the gas-only escapes by blasting the magma from the vent. The most explosive volcanic eruptions in earth’s history include Yellowstone in Wyoming, Valles in New Mexico, and Long Valley in California.
Rhyolite lava is sluggish. It exudes slowly from a volcano and pile-up around the vent, producing a mound-shaped structure known as a lava dome. The lava domes can grow up to several hundred meters.
Lava domes can become dangerous. The brittle dome can become highly fractured and unstable as additional magma extrudes. The ground can also change slope as the volcano inflates and contracts, triggering a dome collapse. The collapse suddenly lowers the pressure on the extruding magma, resulting in an explosion.
Rhyolite and Gemstones
Rhyolite provides a wide variety of gems. The gems are formed when granitic magma cools quickly, and gas pockets are trapped inside the lava. The trapped gas cannot escape when the magma cools quickly, creating vugs.
The lava cools afterward, and hydrothermal gasses or groundwater precipitate in the vugs. It forms high-quality topaz, red beryl, agate, jasper, and opal deposits. This valuable property makes rhyolite one of the most sought-after rocks.
Where is Rhyolite Found?
Rhyolite is commonly found along convergent plate boundaries, where a slab of oceanic lithosphere is subducted into the Earth’s mantle below the oceanic or continental lithosphere. Rhyolite is more commonly found in overriding continental lithosphere.
Rhyolite deposits are found in European countries such as France, Germany, Canada, and the USA. The Taupo Volcanic Zone in New Zealand has large concentrations of young rhyolite volcanoes. The Gondwana Rain forests of the Australian World Heritage area contain rhyolite-restricted flora along the Great Dividing Range.
The biggest rhyolite use is to extract valuable minerals from the stone. It was used in ancient times to manufacture stone tools such as scrappers, blades, and projectile points.
It is not used in the construction or manufacturing due to its highly fractured lookout. Its properties make it unfit to be used as an aggregate.