Polarised light micrograph of rhyolite from the Auer formation of the Etschtaler Volcanic group, Bolzano, Italy. Rhyolites are volcanic rocks that have large, well-formed individual crystals in a very fine-grained matrix. They arise when magma in the interior of the Earth cools down slowly. Few, but large, crystals form in the subsurface. If there is then a rapid ascent of the magma to the surface, the remaining liquid magma cools down very quickly and crystallises. This creates numerous, microscopic crystals, which are known as the base mass, or matrix. The faster the cooling, the finer-grained the base mass. Rhyolites consist mainly of quartz and feldspar (sanidine and plagioclase). Magnification: x30 when printed at 15 centimetres wide.