What does an insect eye, Saturn’s North pole, beeswax, and a rock formation called columnar basalts all have in common? They are all hexagonal in shape. Why is this the case?
Hexagons are the most efficient way to fill a space with the least amount of material
The science of bubbles and bubble sculptures is very interesting and I do encourage you to go check out some other blogs that talk about bubble geometry in more detail (Fig. 1), however lets now talk about hexagonal basalts.
Columnar Basalts form by cracking when igneous rock rapidly cools after being emplaced. They constitute some of the most unique and awe-inspiring geological features we see on Earth. They are so regular and geometrically organised they look hand cut or man made. Some stories in mythology hypothesize they were built by giants (check our previous post about the Giant’s Causeway myth), or by the Great Spirit (wait for the next post about the Devils Tower myth). Once you analyse the physics behind them though, it becomes easier to understand why columnar basalt organise into such perfect geometrical shapes.
Imagine a big pool of lava (molten basaltic rock) that has settled and isn’t being topped up by more lava. Just like hot stew, the lava will cool faster at the edges and on the top, the solidified lava takes up less space than the molten lava because it contracts as it cools. The cooler contracted rock pulls on the still molten lava in the middle of the pool to the edge evenly. This generates cracks in the lava as the whole surface cools, much like dried mud (or dried starch slurry! Fig. 2a).
If the lava is well mixed and smooth it will crack evenly and in predictable and repeated patterns; the mechanics of this are shown in Figure 3. during the cooling, the produced tensile stress accumulates elastically and initially generates randomly distributed micro-fractures while the cooler parts of the lava are pulling on other parts. Such micro-fractures subsequently tend to develop into a more organized set of polygonal fractures (Lamur et al., 2018); the most efficient regular pattern to relieve the most tension on the rock is if it cracks into regular hexagonal shapes.
At first the hexagonal shapes only exist on the surface of the lava but the hexagonal surface cracks propagate downwards into the lava as the deeper parts of the lava, initially kept molten by insulation, eventually cool too. Much the same as splitting wood, the surface cracks are weak points which propagate deep into the rock. This is how meter tall columnar basalts form.
Sometimes the presence of impurities in the lava will affect the way it cools down, causing some columns to develop 5 (pentagon) or 7 (heptagon) sides but the pattern “nature favours” (i.e. the pattern that is most efficient) is hexagons. The same pattern can be replicated at home with a starch slurry, like as Hofmann et al., (2015) did in a lab. Hofmann et al. (2015) looked inside of the dried starch using tomography, and measured the angles that form between the polygons as the slurry dries. Nature wants the 120 degree angle of a hexagon and 120 degrees is the most frequently occurring angle (Fig. 2b); but of course the model isn’t perfect and local effects will reduce or increase the angle – making a pentagon, heptagon or another irregular shape. In the case of the slurry, this may be because the slurry dried unevenly (i.e., if there was a temperature difference or a breeze) or if the slurry wasn’t mixed well enough and parts were stickier or runnier than others, or also if there was some contaminant in the slurry like leftover sugar. All the same can be applied to lava. Many columnar basalts do have imperfections, most likely because of one or more of the reasons mentioned above.
How far down do the columns go?
Columns (or colonnade) can become few meters to several tens of meters long (Fig. 4), however they can only continue to form to the base of the lava flow that was cooling.
Subsequent lava flows, or parts of the lava flow above or below the columnar basalt formation can produce more fractured lava formations called entablature, which are formed by curved and irregular columns (Fig. 5). Different structures and geometries in lava generally occur because of the different chemical composition of different lava flows or the eventual interaction with water, which cause it to cool faster and asymmetrically. Although columnar jointing is typical of basaltic rocks, the same joint pattern can be also found on other types of volcanic rocks such as Phonolites (Zavada et al., 2015) and ignimbrites (Giordano and Cas, 2021).
Written by Hannah Davies and edited by Filippo Carboni and Samuele Papeschi