Ever wonder what people mean when they use the word ‘pebble-shaped’? Well, thanks to a combined research by boffins in France and the US, we now have a definition of the term.
Material physicist Doug Durian of the University of Pennsylvania in Philadelphia and his colleagues say that technically, a pebble is a rounded body with a near-gaussian distribution of curvatures.
They state that though no two pebbles are exactly alike, all eventually have the mathematical form, and that once attained, this shape never changes, even if a large pebble erodes over time into a tiny grain, reports Nature magazine.
Durian and his colleagues looked at pebbles of hardened mud formed at Mont St-Michel bay on the coast of northern France, where the tides produce flattish, dried mud fragments that are progressively eroded over many months. As erosion proceeds, the pebbles 'mature' from sharp-edged fragments to rounded forms.
They took 2-dimensional pictures of more than 60 pebbles at various states of erosion, and calculated the distribution of curvatures around the circumference of each pebble. They then plotted this distribution on a graph.
They found that although the exact shapes of mature pebbles varied, all showed the same curvature distribution, called a gaussian curve, once past a certain stage of erosion.
The same thing was noted in pebbles made in the laboratory from clay, moulded initially into various sharp-edged polygons - including triangles, squares and odd-shaped chunks - and then bashed about in a square metal pan to erode them. In this case, the curvature distribution was a little closer to the ideal gaussian form.
The distribution of curvatures depends rather solely on the erosion process itself then the pebble's starting shape, they now state.