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Spallation

Many of the features observed in the Buntsandstein quartzite cobbles can be explained by spallation.

In fracture mechanics, spallation is a well-known process typically combining compressive and tensile stress (not to be confused with nuclear spallation). Spallation occurs when a dynamic compressive pulse impinges on a free surface, where it is reflected as a rarefaction pulse. The associated tensile stress may lead to internal spallation fractures and to complete spalls detaching from the free surface. The image below shows strong spallation fracturing in an experimentally shock-loaded Armcor iron.

Photograph by courtesy of Michael Hiltl.

In impact cratering, spallation is well known to occur in the near-surface zone of the target (Melosh 1989), has been described for lunar micrometeorite craters (Hörz et al. 1971), and has been studied in the laboratory by experimental impacts in, e.g., gabbro targets (Lange et al. 1984, Polanskey & Ahrens 1990).

The fractures in the deformed Buntsandstein cobbles have much in common with the spallation fractures experimentally produced in shock experiments, as is shown in the images below.

Sets of closely spaced open spallation fractures in a quartzite cobble which, otherwise, is macroscopically untouched. A tectonic origin can be excluded. Note that the master fracture at the right mirrors the cobble surface (arrows). For geometrical reasons, this is expected as a result from the reflection of a dynamic pulse at the free surface. In the case of the Buntsandstein cobbles, there was not a free surface in the strict sense but a distinct contrast of shock-wave impedance between the very dense quartzite cobbles and the sandy, porous matrix. 

Small-spaced spallation fractures in a quartzite pebble. Note the lens-shaped spall nearly completely detached from the pebble! 

Pock-marked quartzite cobble (left) showing small circular spallation fractures (arrow). Different from the irregular grazes, the spallation is defined by circular brittle fracturing and an annular halo of micro-fracturing (see close-up; right). Note that the center of the structure is largely undisturbed. This is also observed in experimental impact spallation cratering (see below).