Shock metamorphism

[1] Planar fractures are parallel sets of multiple planar cracks or cleavages in quartz grains; they develop at the lowest pressures characteristic of shock waves (~5–8 GPa) and a common feature of quartz grains found associated with impact structures.

[2] Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals (usually quartz or feldspar), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure.

Coesite occasionally occurs associated with eclogites formed during very high pressure regional metamorphism but was first discovered in a meteorite crater in 1960.

[4][5] Diamond, the high-pressure allotrope of carbon, has been found associated with many impact structures, and both fullerenes and carbynes have been reported.

The search for such effects therefore forms the basis for identifying possible candidate impact structures, particularly to distinguish them from volcanic features.

Shocked quartz with two sets of ‘decorated’ planar deformation features in impact melt rock from the Suvasvesi South impact structure, Finland (thin section photomicrograph , plane polarized light).
Shatter cones developed in fine grained dolomite from the Wells Creek crater , USA.