14 DAY TRIAL // If pebbles could talk, they might tell tales of tortuous journeys as they were transformed from jagged rock shards to smooth stones. Physicists at the University of Pennsylvania and the Institute de Geologie in France are hoping to deduce the history of a pebble's travels through streams, under glaciers, or along seashores by developing models of pebble erosion.
D. J. Durian and his colleagues evaluated possible models by experimentally simulating erosion with artificial pebbles made of five millimeter thick tiles of clay that were molded in various geometrical shapes including squares, triangles, and other polygons.
The researchers tracked changes in the tile shapes as they tumbled on a tilted, rotating pan. The tiles' sharp corners rapidly wore away, but the physicists were surprised to find that the tiles ultimately approached shapes that were not the perfect circles predicted by simple models based on the proposition that points are gradually polished down on pebbles.
Instead they tended to form more intricate shapes that were almost, but not quite, circles. In addition, the final shapes are the same regardless of the artificial pebble's starting shape.
More involved models that take into account the fracture of protruding points predicted final tile shapes more accurately. Although the research is in its early stages, theories of pebble erosion could soon provide clues to geological history encoded in various pebble shapes. The study will appear in Physical Review Letters.