shock spallation - a typical impact process in the chiemgau meteorite crater strewn field

8/2/2019 Shock spallation - a typical impact process in the Chiemgau meteorite crater strewn field

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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. Its 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 fieldwe 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 Tttensee 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.


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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 cobbles 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 ofthe 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.


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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


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Azuara http://pubs.giss.nasa.gov/abs/er01000b.html Rubielos de la Crida 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 http://pubs.giss.nasa.gov/abs/er01000b.html) where the full

article can be downloaded), and an extended report may be read here:

http://www.impact-structures.com/impact-spain/shock-deformation-in-triassic-buntsandstein-conglomerates-spain/.

Fig. 6. Limestone cobble from the ejecta (Bunte Breccia) of the Ries impact structure(Nrdlinger 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 Crida impact

structures showing very typical shock-induced open spallation fractures. For some of thefissures the rough mirror symmetry of surface and fracture geometry becomes again evident.

shock spallation - a typical impact process in the chiemgau meteorite crater strewn field

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