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Shock metamorphism orimpact metamorphism describes the effects of shock-wave related deformation and heating duringimpact events.
The formation of similar features duringexplosive volcanism is generally discounted due to the lack ofmetamorphic effects unequivocally associated with explosions and the difficulty in reaching sufficient pressures during such an event.[1]
Planar fractures are parallel sets of multiple planar cracks or cleavages inquartz grains; they develop at the lowestpressures characteristic ofshock waves (~5–8 GPa) and a common feature of quartz grains found associated with impact structures. Although the occurrence of planar fractures is relatively common in other deformed rocks, the development of intense, widespread, and closely spaced planar fractures is considered diagnostic of shock metamorphism.[2]
Planar deformation features, or PDFs, are optically recognizable microscopic features ingrains ofsilicate minerals (usuallyquartz orfeldspar), consisting of very narrow planes ofglassy material arranged in parallel sets that have distinct orientations with respect to the grain'scrystal structure. PDFs are only produced by extreme shock compressions on the scale of meteor impacts. They are not found involcanic environments.
This form oftwinning in quartz is relatively common but the occurrence of close-spaced Brazil twins parallel to thebasal plane, (0001), has only been reported from impact structures. Experimental formation of basal-orientated Brazil twins in quartz requires high stresses (about 8GPa) and high strain rates, and it seems probable that such features in natural quartz can also be regarded as unique impact indicators.[2]
The very high pressures associated with impacts can lead to the formation of high-pressurepolymorphs of various minerals. Quartz may occur as either of its two high-pressure forms,coesite andstishovite. Coesite occasionally occurs associated witheclogites formed during very high pressure regional metamorphism but was first discovered in a meteorite crater in 1960.[3] Stishovite, however, is only known from impact structures.
Reidite, the high-pressurescheelite-structure polymorph ofzircon, is known only from impact structures.
Two of the high-pressurepolymorphs oftitanium dioxide, one with abaddeleyite-like form and the other with aα-PbO2 structure, have been found associated with theNördlinger Ries impact structure.[4][5]
Diamond, the high-pressureallotrope ofcarbon, has been found associated with many impact structures, and bothfullerenes andcarbynes have been reported.[6]
Shatter cones have a distinctively conical shape that radiates from the top of the cones repeating cone-on-cone, at various scales in the same sample. They are only known to form in rocks beneathmeteoriteimpact craters orunderground nuclear explosions. They are evidence that the rock has been subjected to a shock with pressures in the range of 2-30GPa.[7][8][9]
The effects described above have been found singly, or more often in combination, associated with every impact structure that has been identified on Earth. The search for such effects therefore forms the basis for identifying possible candidate impact structures, particularly to distinguish them from volcanic features.
