impactmelt

Impact melt refers to rock that forms when target rocks are melted by the energy released during a hypervelocity impact, such as a meteorite or comet striking a planetary surface. The melt product can appear as glassy sheets or pods, as vesicular glass, or as part of impact melt breccias in which melt glass surrounds or cements together shattered clasts.

Formation and textures: Extremely high temperatures and pressures during impact can liquefy rock; rapid cooling then preserves glassy textures or quenched glass. Melt rocks are found in crater floors, central uplifts, and ejecta blankets; tektites and splash glasses are common forms produced by rapid ejection and cooling in the atmosphere or airless environments.

Identification: In hand specimen and thin section, impact melts are typically glassy with flow features; they can host fragments of unmelted material in a glassy matrix. Shock metamorphism signs such as planar deformation features in quartz and occasionally high-pressure minerals may accompany melts, though not always.

Occurrences: Well-known terrestrial impact melts are found at the Sudbury Basin, Chicxulub crater, and Ries crater; Moon samples from Apollo missions include impact melt rocks; many other craters worldwide preserve melt sheets and suevite deposits containing melt glass.

Significance: Radiometric dating of impact melts provides ages for the corresponding impact events. In some basins, the melt phase is linked to ore formation, as in the Sudbury Complex, where an impact melt sheet underlies nickel-copper-PGE ore deposits.