TOWARDS UNDERSTANDING THE MECHANISMS AND QUANTIFYING THE RATES OF CRACK GROWTH IN ROCKS

Cracks govern physical and chemical degradation of natural and artificial materials. In particular, sub-critical crack (SCC) growth in rocks is reckoned to be a significant controlling factor in landscape evolution (Eppes et al. 2018). In order to fully appreciate the role of SCC in geomorphology, we need to understand its causal mechanisms on relevant spatial scales, e.g., intrinsic vs. extrinsic stresses at the crack tip, and understand crack evolution over relevant geological time scales.

To address these issues, we are currently testing the hypothesis that SCC growth is influenced by the presence of atomic scale metastable states at the crack tip. Such metastable states are created by exposure of a rock to ambient ionising radiation (alpha, beta, gamma and cosmic rays) over geological time scales and this exposure results in trapping of electrons or holes at the crystallographic defects. The metastable states, e.g. in feldspar or quartz, can exist for millions of years at about 20 ⁰C; however, exposure to visible light photons at the crack tip can destroy the metastable states in seconds to days depending on the available light flux. We reckon that the destruction of such metastable states at the crack tip could potentially release the stored lattice energy in the form of phonons (Blaise & Le Gressus 2018) and thus lead to crack tip propagation. This idea is being tested by inter-comparing the sub-critical crack tip velocities in gamma irradiated and non-irradiated ceramics and rock materials.

Some of us are also developing a method for constraining the timing of crack propagation by applying the principle of luminescence surface exposure dating (Sohbati et al. 2012); specifically, we map the aforementioned metastable states in feldspar mineral using infra-red stimulated luminescence or photoluminescence (IRSL and IRPL, respectively) (Prasad et al. 2017). We find that a luminescence bleaching front develops parallel to the crack surface due to daylight exposure, and the positon of this front changes with time. With appropriate calibration, it is possible to translate the distance between the bleaching font and the crack wall in terms of the timing of crack opening.

Here we will present some preliminary results on both these aspects on some ideal/well constrained materials.

Acknowledgments

We thank Villum foundation for supporting this research through project no. 36321 (CRACK).

Reference list

Eppes MC, Hancock GS, Chen X, Arey J, Dewers T, Huettenmoser J, Kiessling S, Moser F, Tannu N, Weiserbs B, Whitten J. 2018. Rates of subcritical cracking and long-term rock erosion. Geology 46, 951-4.

Blaise G, Le Gressus C. 2018. Electron-trapping and energy localization in insulating materials. Technological impact of space charge electron-beam characterization. AIP Advances. 28, 095228.

Sohbati R, Murray AS, Chapot MS, Jain M, Pederson J. 2012. Optically stimulated luminescence (OSL) as a chronometer for surface exposure dating. Journal of Geophysical Research: Solid Earth. 117, B09202.

Prasad AK, Poolton NR, Kook M, Jain M. 2017. Optical dating in a new light: a direct, non-destructive probe of trapped electrons. Scientific Reports 26,1-5.