Shapes of Mud Cracks
February 06, 2008
When mud dries out, it cracks and forms polygonal structures. Most of them have six sides but owing to the distribution of subsurface stresses, they can have as few as three or as many as nine. It's often difficult to count the sides and in many cases the polygons have concave boundaries on part of their peripheries. Curiously, the major cracks that define adjacent polygons tend to meet at right angles.
This photograph shows a 3 foot (1 m) square section of a dry seasonal pond near the Salton Sea in southern California. In addition to the obvious cellular structures, each polygon is itself broken into a family of smaller tiles. For some reason, there are two size regimes: about 8 inches (about 20 cm) and a little over an inch (about 3 cm) diameter cells.
Mud cracks have been well studied in the laboratory as well as in theory. Their regularity is related to the formation of columnar basalt as it cools, shrinks and cracks from the molten state. Yet, much remains unknown about such processes and research is still underway. On an artistic level, mud cracks are appealing for their geometrical regularity. Potters go out of their way to produce similar patterns in their glazes. To “craze” a pot is to develop a fine network of cracks in the glaze or surface.
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