Pseudotachylite

alt=|thumb|322x322px|Purple and green pseudotachylyte veins in outcrop (Sierra Nevada Mountains, California) Pseudotachylyte (sometimes written as pseudotachylite) is an extremely fine-grained to glassy, dark, cohesive rock occurring as veins that form through frictional melting and subsequent quenching during earthquakes, large-scale landslides, and impacts events. Chemical composition of pseudotachylyte generally reflects the local bulk chemistry, though may skew to slightly more mafic compositions due to the preferential incorporation of hydrous and ferro-magnesian minerals (mica and amphibole respectively) into the melt phase.

Pseudotachylyte was first documented by Shand in the Vredefort impact structure and was named due to its close resemblance to tachylyte, a basaltic glass. Though pseudotachylyte rocks are reported to have a glassy appearance, they are extremely susceptible to alteration and are thus rarely found to be entirely composed of glass. Typically, they are completely devitrified into a very fine-grained material with quench textures such as chilled margins, radial and concentric clusters of microcrystalites (spherulites) or as radial overgrowths of microcrystalites on clasts, as well as skeletal and spinifex microcrystalites.