|At the Devil's Postpile National Monument in California one can see spectacular columns of basalt lava that formed 100,000 years ago as a thick volcanic flow slowly cooled. The average diameter of the columns is 2 feet and some are as long as 60 feet.|
As the lava cooled, starting at the surface of the flow, it shrank and cracks developed. The pattern of cracks that provides for the most shrinkage for the fewest cracks is hexagonal, and slightly over have of the columns have this shape. There are also columns with 3, 4, 5, and 7 sides, however, according to this USGS site. Once the pattern of cracks was established at the surface, it was propagated downward as the deeper layers of lave slowly cooled and shrank.
Similar patterns of cracks are found in lave flows located in other parts of the world, including the Palisades Sill, which is well exposed along the west site of the Hudson River north of New York City.
When mud dries it also shrinks and tends to crack with a similar pattern.
Rare Arial Photo of Devil's Postpile after a Snow?
Not really. You can make your own columns from corn starch and water. Click on the photo to the left to see the scale of these columns.
Mix about a half cup of corn starch with just enough water to make a uniform mixture. Play with some of this stuff in your hands for a while, as this material, often called oobleck, has some interesting properties! Then put the oobleck into a small bowl; it should be about 2 cm thick. let this mixture dry out thoroughly. This could take a week or more, depending on the humidity. When its thoroughly dry, you will see cracks on the surface and columnar joints will have formed in the oobleck.
I don't know how! The following two methods failed. It may be that the process can't be sped up, but if you discover a way, let me know!
microwave the bowl
place the bowl in a vacuum