Sometimes in nature, you  stumble across something that just seems too perfect to have  been made without prior planning.

Like those carrots that grow  to look like little people, or the fact that the Earth is just the right  distance from the sun to not freeze or fry. Or the fact that, occasionally,  lava can cool into monumental hexagonal structures that look like  temples and walkways for titans.

But, impressive as the basaltic columns  like the ones at Giant’s Causeway in northern Ireland are, they aren’t  really ancient engineering projects. Instead, they’re the product of some  particularly elegant math and physics. And by understanding them, scientists can tap into our planet’s geological history  and its engineering potential. [♪ INTRO] The Giant’s Causeway is one of the most impressive natural structures on the planet.

The UNESCO World Heritage  site is made up of more than 40,000 hexagonal basalt columns that descend down into the sea across roughly  3 kilometers of coastline. For a long time, people struggled to  believe that such an impressive landscape could be natural, so they’ve  come up with some pretty fantastical tales to explain  how it came to look that way. The most famous story is about an  Irish giant called Finn McCool, who built the causeway to get to  Scotland to fight a rival giant.

And another version of the myth  describes how Finn battled storms and built the causeway to reach  a Scottish maiden who he loved. However, while both of these  make for good fireside tales, geological evidence can provide an  even more dramatic origin story. Sixty million years ago, the ancient  continent of Laurasia started to break apart into what would eventually  become North America and Europe, as the North Atlantic ocean grew between them.

This tectonic rifting was caused  by cataclysmic volcanic eruptions, which saw lava pouring out over the landscape that now makes up northern  Ireland and western Scotland. It was a similar situation to  what we see in Iceland today, where runny basaltic lava formed  rivers and pooled into lakes. The first influx of lava came  from at least six eruptions, but then there was a pause in volcanic activity.

The lava solidified and then  weathered into a landscape with low hills and wide valleys  that became covered in forests. Eventually the volcanoes started up again, and this time the lava filled up the valleys, burning away trees and creating  a 90 meter-deep lava lake. That’d be deep enough to swallow pretty much the whole of the Statue of  Liberty, pedestal and all.

Over time, the lava lake cooled  slowly and evenly into basaltic rock. And at some point in the process of  cooling, the basalt became sculpted into tens of thousands of hexagonal columns, which have been further exposed and  shaped by relentless coastal weathering. The regular hexagonal pattern of columns  do seem to be too perfect to be natural, but their shapes are the result of some  fairly straightforward math and physics.

Most substances take up more space when  they’re liquid than they do when solid, so when something cools and solidifies, it becomes smaller, contracting  in three dimensions. If the volume of liquid starts out contained, like in a lava lake contained  between valley walls, then it puts tension across the  whole volume as it contracts. It’s still trying to fill that volume, but it  has a lot less stuff to do the actual filling.

The first bit to cool is the top surface  because it’s exposed to the cold air. And it releases tension by cracking, just like the mud cracks that form on a dry lake bed. At first, those stress-relieving  microfractures all have random orientations.

But before long, these fractures  self-organise into regular polygons. And that’s where nature gets to show  off its favorite shape - the hexagon. In lots of different structural  systems, hexagons turn out to be the strongest and most efficient  way of packing things together.

Their trick lies not in the shapes  themselves, but in the joints between them. The sides of all the adjacent hexagons  meet at an angle of 120 degrees, which distributes stress equally  across an area without shearing. So, if mechanical strength is what you’re after, then hexagons really are the bestagons.

Once the surface has started to crack hexagonally, then the lower insulated layers follow suit. Just like when you’re splitting wood, the  split that starts at the top propagates downward as the material breaks  following the path of least resistance. Continue that down through 90  meters of cooling lava lake, and you’re left with eerily  perfect basalt columns.

And sure, if you look hard enough you’re  bound to find a bunch of not-hexagons among them too, caused by small-scale  irregularities in the lava or the landscape, but on the whole the hexagons and their  120 degree angles are going to dominate. However, even though math  can explain the geometry, it doesn’t tell us about how this  process actually happens in real time. We needed experiments.

One of the biggest questions we had  about these hexagonal clusters was whether the fracturing happened  at the moment of solidification, or when the solid basalt was hot,  cold, or somewhere in between. So in 2018 scientists from the UK  did some experimental research, heating up columns of basalt until  they were at their melting point and watching them solidify again, in the  hopes of catching the fractures in action. This was the first time something  like this had been done by people rather than the planet, and  it allowed them to pin down the precise temperature of column formation.

They found that the hexagonal fractures  form not long after the lava solidifies into rock, between about  890 and 840 degrees Celsius. That’s around 100 degrees lower  than the solidification temperature, and much warmer than what  most scientists had assumed. Figuring out the conditions that create  these columns is useful outside of the Giant’s Causeway, because these  columns are found all over the world.

There’s colonnades in Iceland, Portugal,  Romania, Mexico, Greece, Vietnam, and America, among others,  so knowing how they form gives us insights into the  geology of each of these places. And these columns aren’t  even limited to our planet. They’ve been spotted in satellite  images of a crater wall on Mars, revealing some of the red  planet’s fiery tectonic past.

The columns aren’t identical  everywhere we find them, likely due to varying cooling  rates and chemical differences. And researchers from China have shown  that some fancy numerical modeling can help to simulate just how  these columns are likely to form. That can be important if you’re  planning to build around your basalt, since it’s useful to predict just  how fractured the bedrock is.

Not only that, but engineers can  also apply the physics of lava fracturing to our quest for  efficient, clean, geothermal energy. In 2009, geothermal engineers were drilling  deep into hot volcanic rock in Iceland, when all of their cooling, lubricating  fluid was suddenly and unexpectedly lost, draining away into the rock. But the 2018 experimental results  revealed that hexagonal cracking can open up solid rock when it’s still hot, increasing its permeability to heat  and fluids up to a billionfold.

This helps to explain the engineers’ fluid loss but also offers engineers  an opportunity in the future. Since geothermal energy relies on the  release of heat from deep in the Earth, these hot, fractured rocks could be targeted for more efficient geothermal boreholes. Of course, nobody’s going to be  drilling into the Giant’s Causeway for geothermal power any time soon.

But this natural wonder, along with its  hexagonal fellows across the solar system, is helping us to better understand our  planet’s history, as well as our own future. So while it may not be the result of some  giants’ love affair, it’s still pretty special. [♪ OUTRO]