Rhyolite is an extrusive igneous rock with high silica content due to primarily being comprised of felsic minerals, such as quartz and plagioclase, but can occur in a number of shades. In some instances, they can have trace amounts of mafic minerals, like biotite, amphibole or pyroxene. Its composition is equivalent to that of granite but the two types of rocks differ from their formational processes and textures. Both are sourced from magmas of very similar composition, however granite crystallizes intrusively resulting in phaneritic and equigranular grains, whereas rhyolite cools rapidly extrusively during volcanic eruptions, typically resulting in an aphanitic texture. However, rhyolite can take up several forms depending on the cooling rate. For example, this particular sample crystallized gradually enough before surface exposure to form large phenocrysts but also has a high density of vugs, indicating gas content. Vugs are pockets that form from the entrapment of gases in the magma and are released upon bursting at the Earth's surface. The amount of gas content in a rhyolitic magma can be very influential on its cooling rate as the greater amount of gas, the more explosive the eruption and the faster the cooling rate. These can result in different classifications of rocks based on textural characterisitics, such as glassy obsidian or highly vesicular pumice rhyolites.