Spatial and temporal analysis of crust deterioration under particle impact

Recent work by Rice et al. (Earth Surf. Process. Landforms 21 (1996) 279; Earth Surf. Process. Landforms 22(9) (1997) 859; Earth Surf. Process. Landforms 26 (2001) 721), and McKenna Neuman and Maxwell (Catena 38(2) (1999) 151; Earth Surf. Process. Landforms 27 (2002) 891) has contributed much to the measurement and understanding of crust strength in relation to the impact force of particles striking a crusted surface. However, the spatial and temporal aspects of crust destruction under impact remain largely unknown. The present paper expands the analysis of data obtained from an earlier wind tunnel simulation of crust destruction (McKenna Neuman and Maxwell, 2002). A detailed spatial and temporal analysis is presented for over 4000 sites of crust loss, as preserved in photographic images that have not been previously considered. The results suggest that the fractal dimension of the digitized deflation features falls between 1.09 and 1.24, is consistent for any given crusted surface, and may be positively correlated with independent measurements of crust strength. Small erosion pits (<10 mm2) can appear at any time, even after an hour or more of impact. This observation reinforces our earlier suggestion (McKenna Neuman and Maxwell, 2002) that the duration of impact is significant and that a fatigue-like mechanism is likely at work. While the orientation of any given deflation feature can vary through 180°, analysis of a large sample (n=4100) shows that on average and regardless of size, they are aligned with the air stream. The results from this study present a number of possible simplifications for those interested in modeling these aeolian systems in future.