Effects high pressure radiometric dating
Shock pressures are estimated at several hundred kilobars based upon the presence of the high-pressure polymorphs of silica.
In contrast volcanic activity produces pressures of just a few kilobars at the most.
For instance the 1956 eruption of Bezymianny was calculated at three kilobars based upon erratics thrown up to 30 km (Milton et al. The Charlevoix cratering event has shattered Ordovician rocks, which may have been deposited both before and after the event (Rondot 1971).
Mistastin Lake, Labrador, shows a K-Ar age of 202 m.y. for an area outside of the shock zone (Taylor and Dence 1969).
The most obvious criterion is the presence of shatter cones, conical fractures with diverging striations along the length of the cone.
Orientations of the cone apices were originally in the direction of the impact object and thus served as indicators of the amount of tilt in the beds subsequent to impact, some beds even overturning (Manton 1965; Howard and Offield 1968).
The tremendous pressures (425-500 kilobars with coesite) and the high temperatures (1000C with coesite and 400 with stishovite formation) result presumably in a resetting of the potassium-argon and the fission-track radiometric clocks.
Examples of the resetting of the clocks are numerous. Impactites and pseudotachylites are unique rock types that are good indices of impact.