Relatively abundant around the world, rhyolites have been formed in all geologic ages. Most of the rocks are porphyritic in texture, consisting of a fine-grained base through which sizable crystals, called phenocrysts, are dispersed. The two different sizes of crystals in these rocks is an indication that the rhyolite cooled in two separate stages. Crystallization that begins when the magma is still buried deep within the Earth produces the phenocrysts, while the groundmass they are lodged in does not form until after an eruption brings the magma to the Earth’s surface.