Silicate glasses can be formed by several geological processes, all of which involve melting of rock and rapid cooling to prevent crystallization. The most common and widespread natural glasses are volcanic glasses produced when magma is extruded onto the Earth's surface. The type of volcanic glass is determined by the composition of the magma and type of eruption, which in turn are related to the geological setting. During high velocity impacts of large extraterrestrial bodies, melted rock can be ejected into the atmosphere, and sometimes into space, where it cools to form large glass bodies centimetres in size or larger, called tektites, and smaller glass bodies, called impact spherules. Tektites and spherules can be thrown over large areas of the Earth's surface. Impact spherules can also form by condensation from an impact plume of vaporized surface rocks and impactor. Lightening strikes generate high enough temperatures to melt and vaporize rock. When they hit sand, they can produce tube-like structures called fulgurites. Burning of organic material like coal or oil underground can melt rock and produce combustion-metamorphic glasses. Glasses called frictionites can be produced by frictional heating at the base of massive landslides. Frication glasses can also be generated during faulting.
