ME Seminar: Rock sculpture by windblown sand: the origin of regular patterns and singularities on eroded surfaces

Dr. Peter Varkonyi, Budapest University of Technology, visiting Caltech ME

Rocks abraded by wind-blown sand (called ventifacts) exhibit a wide variety of morphological characters, including sharp edges, polished flat facets and regular surface patterns. Ventifacts are important indicators  of present and past wind conditions, which occur in deserts, coastal areas, periglacial environments, and also on Mars. Experimental and field data on the connection between wind characteristics and ventifact morphology has been accumulating for over a hundred years. At the same time, it is still not clearly understood what mechanism causes polishing  on some rocks, but coarsening and pattern formation in others. In this talk, I introduce a multi-scale approach to abrasion, which combines partial differential equations and trajectory tracking of individual grains bouncing on the rock surface. I show how abrasion by direct impacts may lead either to very smooth or to very coarse surface textures depending on the initial surface geometry, and some material properties.  The main mechanism driving pattern formation is the intensified abrasion  of concave surface regions due to multiple impacts from a single sand grain. I also demonstrate why erosion creates sharp edges on rock surfaces instead of rounding them off