We can subdivide the effects of cratering into those related to high pressure, those resulting from high temperature, and those resulting from high strain rate. In reality all three conditions are met almost simultaneously during impact. We have seen that the projectiles can explode in the atmosphere before they even reach their targets. The energy is sufficient to locally produce temperatures of 10,000^oC and pressures of 100 GPa (1GPa = 10 Kbar which is approximately 10,000 atmospheres) in an event which is essentially complete in minutes.

The following is a combined phase diagram for both carbon and SiO₂. We have already noted that carbon is a fairly common component of most chondritic and some achondritic meteorites while SiO₂ is a common component of terrestrial crustal rocks, usually in the form of quartz. At high pressures quartz is replaced by its higher density polymorph, coesite. At even higher pressures coesite is replaced by stishovite. Both coesite and stishovite would require the pressures equivalent to those in the Earth's mantle in order to form. Then they have to get to the earth's surface. Igneous processes with the accompanying high temperatures would cause both minerals to revert to quartz on a very short timescale, thus their presence at what were once labeled "cryptoexplosion structures" is indisputable evidence for meteorite impact. Likewise graphite changes to diamond at approximately the pressures of the quartz-coesite transition. Diamond is found associated with terrestrial ignous rocks (kimberlites) but the diamond-graphite transition is much slower than that of coesite-quartz. Diamonds have been found associated with some terrestrial impact craters and, as we have seen, are a common component of some meteorites.

The first ever conference on the shock metamorphic effects on natural materials was held as recently as 1966 and presented in a volume edited by Bevan French and Nicholas Short. It outlines most of the information known at the time of shock effects at terrestrial meteorite craters, shock produced by atomic bomb blasts, and known shock effects in meteorites. A lot of experimental work has been carried out since that time but the basic principles remain largely unchanged.

Most minerals have a well-defined crystal structure such that when they deform by "regular" processes, that deformation is along pre-determined and well known crystallographic directions, no matter what the orientation of the crystal. However, since in most natural environments (excluding foliated rocks) the crystals are haphazardly arranged with respect to the path of the shock wave, the effect is to produce internal deformation of the crystals in whatever direction the shock went, regardless of the crystal orientation. In the extreme case, there is no crystal structure left and the mineral is essentially glass. Such a scenario is seen in the mineral maskelynite which is the shock metamorphic form of feldspar. The rectangular outline of the feldspar is retained but the crystal is isotropic. The following image pair from Becker shows the elongate white feldspar in the upper image in ordinary light and as a black isotropic centered crystal under crossed polars.

Less extreme are planar deformation lamellae seen in the following two images of pyroxenes. The coarser bands in the left image can obviously be seen to have a different crystal orientation to the major part of the crystal. The image on the right shows a domain structure in addition to the deformation lamellae. (images from Zeiss)

The next image shows spectacular deformation lamellae in a lunar plagioclase breccia.

At extreme temperature and pressure conditions the rock will melt. The next pair of images show a regular lunar mare basalt on the left and a mare basalt that has been melted on the right. Note the unaltered rock has well-formed crystal faces, especially the black ilmenite crystals, whereas in the right image we have wispy, blurry glass. Quartz transforms to a glass known as lechatelierite while zircon transforms to baddeleyite.

At low pressure and temperature conditions the passage of the shock wave may have only caused kinking of elongate crystals.

At the outcrop scale, a feature frequently found at terrestrial impact sites are shatter cones as seen in the next image of a shatter cone from the Sierra Madera structure in Texas. These typically form in fine grained, homogenous rock (in this case limestone). The apex of the cone points toward the location of the impact and usually terminates at a bedding plane, so their size is partly determined by bed thickness.

The next image shows B. Ray Hawke standing next to giant shatter cones at the Sudbury impact structure in Ontario. Ironically, Sudbury was long regarded as volcanic and has a large layered igneous intrusion. It is also the world largest producer of nickel.

The following image, also from Sudbury, shows a breccia enclosed by impact melt.

The next image shows a dike, but it is no dike in the normal sense; the material which passed through it went downwards rather than upwards. Impact melt was pushed down through cracks, joints and faults by the force of the blast.

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