Columnar joints are a common, yet spectacular pattern that can be easily seen in volcanic areas.  The names associated with these types of features, such as the Giant's Causeway , the Devil's Postpile, the Devil's Tower, or Samson's Ribs, reflect the impression that such near-perfect order could not be caused by nature, but only by supernatural agents.  This is far from true.  The beautiful hexagonal pillars, common to all these places, were formed by the gradual cooling, and cracking, of lavas, the products of volcanoes.  Anyone can set up a simple experiment to demonstrate how these remarkable landscapes form.

Columnar joints can be made in any kitchen.  You will need to mix:

250 ml (1 cup) corn starch (aka corn flour)

150 ml water

15 ml (1 tbs.) bleach

Actually, the proportions don't matter too much, and potato starch can be used in place of corn starch. The resulting liquid should be runny, but you should still be able to feel some resistance when stirring.  Handle the bleach with care, as it is caustic, and even small spills will quickly cause clothes to fade, and skin to become dry and irritated.

Pour the mixture into a clean dish, at least 10 cm or so wide, so that you have a layer 2 to 4 cm thick.  If it will not pour readily, you will need to mix in more water.  If you can, use a glass container, so you can see what's going on inside.  Put your creation in a warm, but not hot (<50°C) place , where it can dry, such as 20 cm underneath an incandescent lamp. 

It will take a couple days to fully dry your starch-cake.  Before the columns start, the surface of the starch-cake will dry into little flakes of starch. The columns will then start, and grow downwards. When they reach the base of the container, you may be able to see a network of polygons through the underside of the dish, and the starch-cake will be done.  If you can’t see through your container, leave the experiment at least a day after the point when you can see that the surface has fully dried.

To remove the starch-cake, cover the container with a hard surface, such as a cutting board.  Carefully turn it upside down, and the cake should slide out onto the board.  If it does not, tap on the base of the container, or shake it a little.  The colonnade is very fragile, so handle it with care.  The best columnar joints are near the middle of the starch-cake.  You can try pick off columns from one side, to expose these.  Alternatively, hold the whole sample together, by gently squeezing it from all sides with both hands, and pull apart two halves of it, to expose the centre.

You should also be able to see a beautiful polygonal pattern of cracks on the exposed base of the starch-cake.  Each of these polygons is the end of a single column, which will stretch into the sample.  On average, these columns are hexagonal, but there will be many pentagons and heptagons, and a few other shapes. 

You may notice that your columns curve, slightly, towards the edges of your container.  The columns will always grow perpendicularly to the drying front.  Due to the container walls, this front will tend to be slightly convex, causing the columns to curve.  Since lavas will cool from all sides, they will tend to have more complex patterns than a starch-cake. Typically, basalt colonnades come in pairs, one cooling up, and one cooling down.  In the field, curving columns can be used to show exactly how a lava flow cooled.