Look around before a rain, and find yourself a depression in the dirt that's sure to collect water. Inspect its surface. After a long dry spell, chances are it looks like any other piece of ground. Now go back after the rain. Have a mud puddle? Good! In our desert climate, you'll only have to wait a day or two until the water has evaporated—and the magic has happened.
The depression's dry floor is now covered by a layer of sediment
divided by deep cracks into more or less matching polygons. What happened? No
magic—just physics. The agitation of the rainwater sorted the bits of dirt by size,
leaving a layer of small clay particles on top. Clay absorbs water and, in doing so,
swells. But as water evaporates from the fine material, it shrinks. Whereas before the
rain the surface was a mixture of larger and smaller pieces, now only the small clay
particles are on top. As the moist clay dries, it takes up less space and the layer has
no choice but to crack apart as stresses become too great.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.