MEMIN Sub-Project: Microscale shock processes in sandstone

Impacts of cosmic projectiles on Earth produce characteristic shock wave effects in the rock, which can be used to verify the impact and to determine the pressure within the impacted rock. The mineral quartz, which is commonly found in the Earth’s uppermost crust, exhibits particularly typical effects.

In this sub-project, ‘Microscale shock wave processes in sandstone’, the shock wave metamorphism of sandstone was investigated through explosive experiments carried out at the Fraunhofer Ernst-Mach Institute. The experimental setup replicates on a small scale what happens when an asteroid impacts the Earth’s uppermost crust. The experimental setup—more precisely, the ‘shock wave recovery assembly’—provided reference material for classifying naturally shock-deformed rocks.

In non-porous, quartz-rich rocks such as granite, so-called planar deformation elements form during impacts at pressures of approximately 10 GPa or higher. These are amorphous defects that form regularly within the crystal structure of the quartz grains. At higher shock wave intensities – and thus pressures – so-called diaplectic glasses (i.e. material amorphised in the solid state, which, unlike glass solidified from a melt, was not liquid) form from quartz from around 35 GPa onwards, and in some cases also coesite, a high-pressure modification of quartz. Under even higher pressures, from around 55–60 GPa, the quartz melts and glass solidifies from the melt.

However, in porous, quartz-rich rocks, such as sandstone, the typical planar deformation features are rarely observed, particularly in the case of small impacts (crater diameters less than 2 km). It was therefore difficult to reliably identify small impact craters, such as those found in the Sahara, on the basis of such shock wave effects. The analysis of sandstone samples from a total of 20 shock wave recovery experiments provided important data for classifying the traces in the rock. As part of the project, thin sections of the deformed sample material, approximately 25 micrometres thick, were prepared and structurally characterised using light and electron microscopes as well as a Raman spectrometer.

The analysis largely confirmed the researchers’ expectations. As suggested by knowledge of traces in natural craters, the typical planar deformation effects were scarcely observed in the experiments either. The shock wave pressure is distributed very unevenly within the rock, which means that even at comparatively low pressures, porous quartz rocks partially melt. The laboratory results help to identify impact craters on Earth and to estimate the pressure experienced by their rocks.

It was particularly interesting that the high-pressure mineral stishovite was sporadically discovered even in sandstone subjected to low shockwave pressure. Transmission electron microscopy played a key role in this discovery and has contributed significantly to our understanding of the formation of this high-pressure mineral in impact rocks.

Collaborating partners

Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut, EMI
Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena
Institut für Geo- und Umweltnaturwissenschaften, Albert-Ludwigs-Universität Freiburg

Publications and conference papers

• Ebert M., Kowitz A., Schmitt R.T., Reimold W.U., Mansfeld U., Langenhorst F. (2018). Localised shock-induced melting of sandstone at low impact pressures (<17.5 GPa): An experimental study. Meteoritics and Planetary Science 53(8), 1633–1643. doi:10.1111/maps.12948
• Mansfeld U., Langenhorst F., Ebert M., Kowitz A., Schmitt R.T. (2017). Microscopic evidence of stishovite generated in low-pressure shock experiments on porous sandstone: constraints on its genesis. Meteoritics and Planetary Science 52(7), 1449–1464. doi:10.1111/maps.12867
• Kowitz A., Güldemeister N., Schmitt R.T., Reimold W.U., Wünnemann K., Holzwarth A. (2016). Revision of existing shock classifications for quartzose rocks using low shock pressure recovery experiments (2.5–20 GPa) and meso-scale numerical modelling. Meteoritics and Planetary Science 51(10), 1741–1761. doi:10.1111/maps.12712
• Buhl E., Kowitz A., Elbeshausen D., Sommer F., Dresen G., Poelchau M.H., Reimold W.U., Schmitt R.T., Kenkmann T. (2013). Particle size distribution and strain rate attenuation in hypervelocity impact and shock recovery experiments. Journal of Structural Geology 56, 20–33. doi:10.1016/j.jsg.2013.08.007
• Kowitz A., Schmitt R.T., Reimold W.U., Hornemann U. (2013). The first MEMIN shock recovery experiments at low shock pressure (5–12.5 GPa) with dry, porous sandstone. Meteoritics and Planetary Science 48, 99–114. doi:10.1111/maps.12030
• Kowitz A., Güldemeister N., Reimold W.U., Schmitt R.T., Wünnemann K. (2013). Diaplectic quartz glass and SiO₂ melt experimentally generated at only 5 GPa shock pressure in porous sandstone: Laboratory observations and meso-scale numerical modelling. Earth and Planetary Science 384, 17–26. doi:10.1016/j.epsl.2013.09.021