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. For a better understanding we add that fractures always begin at a definite point within the material propagating from there with a certain fracture velocity which may change during propagation and may even become zero. Then the fracture stops unless it is again fed with energy and continues running.
As for the Chiemgau impact and shock spallation quite peculiar conditions are met namely particularly in the form of very solid cobbles of Alpine lithology. Apart from the occurrence as components of the strongly cemented Nagelfluh plates, the cobbles are in general found in loose bulk and, hence, predestined for a reaction to the passage of shock waves with resulting spallation. It’s not just the extreme contrast of impedance at the cobbles’ surface, also their frequently nodular shape may boost the effect by internal focusing of the shock and rarefaction waves in part yielding enormous energy densities.
As early as in the beginning of our impact research in the Chiemgau crater strewn field we have reported on these deformations and have shown typical photos of spallation fractures down to microscopic scales. Here, we present new examples from recent investigations near the small town of Obing north of Lake Chiemsee. The reader may forgive our keeping secret about the precise coordinates of the impact sites. Our bad experiences with ransacked smaller craters and with the Tüttensee crater where practically all rocks with impact-typical deformations have been removed by rock hunters or people disliking our impact research are forcing us in order to preserve these peculiar impact features for science and interested scientists.
Fig. 2. A quartzite cobble exhibiting a prominent spallation fracture which like the rock in Fig. 1 has not completely split the cobble. In Fig. 3 we point to features very characteristic of spallation fracturing.
Fig. 3. A close-up of the spallation fracture in Fig. 2 shows some typical behavior: Frequently, the pathway of the spallation fracture proves to be a mirror image of the cobble’s surface curvature, and in the case under discussion we have marked the axis of mirror symmetry by a blue dashed line. This may be understood as a consequence of the reflection of the shock (compressive) wave at the free surface for geometrical reasons leading to a mirrored front of the reflected rarefaction wave.
Fig. 4. Spallation fractures in a gneiss cobble. Here again the open fractures do not split the cobble completely, and here again the geometry of the roughly perpendicularly oriented ruptures mirrors the shape of the angular cobble.
Fig. 5. Open spallation fractures in a garnet amphibolite.
The examples of spallation fractures in quartzite, limestone, gneiss and amphibolite cobbles demonstrate that the process is independent of rock lithology und produces recurrent features.
To obviate objections these deformations have already originated from tectonics in the Alps (regularly claimed by local geologists) and have been transported in the form of cobbles in rapid glacial and post-glacial streams and in the end to have been deposited near Obing north of Lake Chiemsee, we point to the frequently very fragile character of the cobbles. Moreover, any strong pressure having acted on the cobbles can basically be excluded because they would inevitably have been broken and sheared.
For comparison spallation like in cobbles from the Chiemgau crater strewn field is shown in the next figures with examples from the Ries impact crater und the Spanish Azuara http://pubs.giss.nasa.gov/abs/er01000b.html Rubielos de la Cérida impact structures. The latter occurrences have been investigated more intensively including spallation experiments. A related article has been published in the prestigious GEOLOGY journal (see here where the full article can be downloaded), and an extended report may be read here.
Fig. 6. Limestone cobble from the ejecta (Bunte Breccia) of the Ries impact structure (Nördlinger Ries crater) showing spallation fractures that have not split the cobble. After the shock spallation the deformation of the cobble continued – probably in the course of excavation – without dissecting it.
Fig. 7. Quartzite cobbles from the Spanish large Azuara and Rubielos de la Cérida impact structures showing very typical shock-induced open spallation fractures. For some of the fissures the rough mirror symmetry of surface and fracture geometry becomes again evident.