Thursday, August 03, 2006

Pebbles: They can't escape their shape!

` Here's something I am quickly extracting from the depths of my drafts and slapping together just for you. It may interest a few people:

` This is the gist of what Philip Ball wrote in What shape is a pebble? Scientists head for the beach to find out.

` Doug Durian of the University of Pennsylvania and his colleagues have found the pattern that makes a pebble look like a pebble: No matter what it looks like, after the sharp edges are worn away pebbles always have a near-gaussian distribution of curvatures, even as it is ground down into a speck.
` Curvature is the radius of a circle that matches the countour at a given point. So, a sphere has equal curvature everywhere. Pebble curvatures vary, in a specific way.
` Durian's team examined over sixty hardened mud pebbles that were formed at Mont St-Michel bay on the coast of northern France. These pebbles went from sharp-edged to rounded as they are eroded. They calculated the distribution of curvatures around the circumfrance and plotted it on a graph.
` On this type of graph, the calculation of a circle would be one spike. The pebbles, on the other hand, showed a broad bell curve, and the more 'mature' the pebble, the more gaussian that curve was. Basically, past a certain stage of erosion, all the pebbles showed the same curvature on the graph, no matter their shapes.
` This also happens when pebbles are made in a laboratory by smashing clay polygons around in a square metal pan (in order to erode them). These laboratory pebbles, when their curvatures were plotted, were even closer to the ideal gaussian curve. These pieces of clay were bashed up pretty good; it is possible that a much gentler erosion would produce something more similar to the spike a sphere produces.
` With this information, it may be possible for geologists to figure out if a pebble has been worn away by a river, the wind, or a glacier, just by looking at its shape!

` Hooray for being able to tell where pebbles come from! Who knows how useful that might turn out to be?
` Anyway, it's lunch time. I shall have to eat now.

` Hooray for lunch time!! Without lunch time, I might forget to eat! And hooray for warning colors: Without yellow stripes, I might forget not to eat bees!


Galtron said...

Oh, are you allergic to bees? I find them quite tasty.

S E E Quine said...

` Well, it's just that I prefer grasshoppers....

Anonymous said...

Form is destiny --- of form?


S E E Quine said...

` Preslicedly!