The term spallation is used in various meanings, e.g. in nuclear physics and fracture mechanics. For impact processes, spallation plays an important role (however seldom appreciated appropriately) and is closely related with the propagation of shock waves. To put it simply, the process runs as follows: On impinging on a free surface, the shock compressive wave is reflected as a tensile wave of practically identical energy. And while a compressive pulse is squeezing a rock, a tensile pulse is stretching the material thus enabling the development of tensile fractures and in an extreme case leading to the detachment of a spall or series of spalls. This is favored by the fact that the tensile strength of all materials and, hence, also of rocks is considerably less than the compressive strength. This is why it is often disregarded that the enormous destructions upon meteorite impact are not so much the result of the shock wave pressure as of the pull of the rarefaction waves. Spallation may take place also when a compressive shock pulse impinges on a boundary of material with reduced impedance (= the product of density and sound velocity) where part of its energy is reflected as a rarefaction pulse that may likewise enable tensile fracturing. It is worth remarking, however compatible with shock physics, that the process of spallation can be observed on arbitrary scales, from microscopically small deformations right up to the movement of huge rock complexes.
Fig. 1. A limestone cobble (14 cm long) exhibiting the typical open spallation tensile fractures. The process is nicely documented by the observation that the running fractures have come to standstill midway through the cobble. In case they had continued running, the cobble would have been fractionized to pieces, and nothing of note would have remained. Continue reading “Shock spallation – a typical impact process in the Chiemgau meteorite crater strewn field”
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The air photo (1)* originates from the northern part of the impact strewn field near Perach and shows (arrow) a crater leveled on an acre.
Image 1: Chiemgau impact; leveled crater near Perach
By image processing, the in the original photo only adumbrated structure gains amazingly sharp contours (2) clearly exhibiting four different concentric zones (3):
- a 5 m-diameter central area (black)
- a 12 m-diameter adjoining zone (red)
- an annular Zone not quite 10 m wide and with an outer diameter of c. 30 m (yellow)
- an exterior zone (60 – 70 m diameter) displaying extensions radiating up to 50 m from the center.
The following attribution of the individual zones is suggested:
The interpretation of the central spot orients by the GPR (ground penetrating radar) measurements (Dr. Patzelt, Terrana Geophysik; inhttp://www.rssd.esa.int/SYS/docs/ll_transfers/295499_Roesler_pres.pdf and http://www.rssd.esa.int/SYS/docs/ll_transfers/295499_roesler.pdf) that have been conducted over another crater (our crater no. 004 – also see http://www.chiemgau-impakt.de/mineral.html) located in the northern part of the strewn field. The GPR soundings show prominent reflections from the crater floor possibly related with strong compaction of the underground material. Assumed this compaction also exists within the Perach crater serving as impermeable rock, a corresponding moisture penetration could optically be traceable to the surface.
The adjoining 12 m-diameter zone could represent the material from the leveling of the crater. The annular zone is suggested to reflect remnants of the original rim wall. Taken the middle of this zone to have been the location of the rim crest, a 20 m diameter of the original crater results.
Finally, the exterior zone is suggested to reflect the zone of the crater ejecta radiating up to 50 m from the crater center.
This documentation of a ring structure illustrates that alternate explanations (geologic glacial formations, anthropogenic (archeological) structures, primitive industrial sites) readily and frequently given by opponents of the Chiemgau impact and skeptics of the meteorite origin for the innumerable craters, continue to have a hard time.
In any case, it is planned to investigate the Perach leveled crater by various geophysical measuring systems trying to characterize the optical zoning in more detail physically.
* In earlier texts, the air photo was erroneously labeled an infrared image.