What are these strange rocks?

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0:58thinking, is it worth it? Is it worth it? I'm Alex Laffridge and this is listener Liana. And you pull up and you're like, oh wow, a beautiful white sand beach, blue sky, teal ocean, just a beautiful beach. And you walk for about maybe a kilometer and you end up behind a kind of like a hill. She's describing a journey to a place that she found remarkably curious. And as soon as you walk past this hill, you look out and it's just this beautiful change of scenery. It's all these dark gray rocks with waves just crashing on them. And in the center, there's like this like mountain of rocks. Each rock is like an individual rock, maybe like, say like 20 centimeters wide.

1:52And then very long, like two meters long. Liana is Canadian but grew up in Indonesia on the island of Lombok. If geography isn't your strong suit, Lombok sits in one of the 17 ,000 islands between the Pacific and Indian oceans that make up the Indonesian archipelago. On this island, Liana found something special a few years back. They're piled up on each other, almost like spikes but with like a perfect hexagon on top and then cylinders down they're like big columns all lumped together parallel to each other and build up like almost like a mountain of these columns but it's individual rocks that you can even wiggle apart almost like wiggling a loose tooth where like you could almost pull it out and it's these perfectly hexagonal rocks that made her writing to crowd science the whole time while i was there i was thinking how were these rocks formed why did this happen what are the rocks made out of how old are these rocks so I guess my question is how did these amazing rocks form what's your hypothesis why do you think these rocks exist the island that this is on it's a volcanic island so I assume it has something to do with the fact that maybe lava exploded there and it cooled really quickly or it cooled really slowly or something with mixing with the salt or we also have a lot of earthquakes too so I don't even know maybe like some earthquake smushed it together and it created this little like mound of cool rocks I'm not sure but I think it definitely has to do with volcanoes or earthquakes because that's like our whole life is volcanoes and earthquakes on the island I just like the idea that you know you've put crowd science into a box 50 50 volcanoes earthquakes one of the two i'll not accept anything else no i'll accept whatever the super smart geologists have to say but that's my guess all right so the challenge is on liana's baffled by the hexagonal rocks that she found and wants to know where they came from could it be the work of volcanoes maybe a particularly artistic earthquake i'm excited to get underway and so the first step is to find someone who knows those Indonesian islands quite well.

4:22Okay, my name is Dr. Alphen Drudiawan, and I'm a geologist and also a lecturer of geology at the Institute of Technology Bandung, Indonesia. Indonesia has very interesting geology. We have more than 17 ,000 islands that straddle across from east to west, and those islands have all different geological history. The central part is actually a part where we call a triple junctions. It's an area where three or more plates are joined together to create a geological phenomena. Okay, let's go back to basics. The Earth's surface isn't one solid shell. It's made up of tectonic plates, these enormous slabs of rock, which are floating on top of a hot liquid layer underneath, like gigantic jigsaw pieces on top of a hot bowl of custard.

5:15These tectonic plates are in constant motion, albeit very slowly. They grind against one another, crash together or pull apart. And these different motions are what leads to things like mountain ranges, earthquakes and volcanoes. The central part of Indonesia is basically formed by three major plates that are crushing towards another. The plates are moving and converging towards another, and the heavier plates are going down because of its heavier, and then it's going down underneath the other plates, and that creates the activity that we know on the surface, like volcanoes and earthquakes and things related to those plates activities.

5:57Our listener was on a walk around Lombok Island with her friends and they were fascinated by the rocks that they saw. She described them as sort of these very long columns that were almost perfect hexagons and they wobbled a little bit like a wobbly tooth. Can you shed any light on this? Yeah, it's actually quite common in the volcanic islands. In Indonesian, we call them Kakarkolom. I think most Indonesian listeners will also know about it. And in terms geologically, we call it columnar basalts or columnar andesite. It depends on the rock. So it's basically a columnar structure. It's a joint, basically a fracture that creates it on the surface of a cooled lava rocks that create such nice features of hexagons.

6:48So how old would they be? Would they be the same age as the islands? Yes, some of the lava, some of the volcanic products there were dated to be around 7.5 to 5 million years ago. So it's very young. So, Liana, you guessed correctly. Those hexagonal rocks, technically known as columnar basalts, are the result of fractures made when the magma spewed out from volcanoes starts to cool. And they're only a few million years old, which is pretty young, geologically speaking. And these aren't unique to Indonesia. There's another, because I love the screensaver that's on the computers, and I'm always trying to find out where those pictures are coming from.

7:38And one of them I found out is the Giant Coastway. It's an island that was formed in Paliocene age, which is 50, 60 million years old. And it's still there. 50 to 60 million years old. They've been around for a really long time. Much older than Lombok Islands. And it's funny that he mentioned the Giant's Causeway because listener Leanna isn't the only one with a columnar curiosity this week. Listener Sarah in England was just as baffled by some geological symmetry after a visit to Northern Ireland. And she wasn't convinced that they were natural at all. The rocks at the Giant's Causeway in Ireland, they come up straight from the ground like columns but the edges at the top and all the way down the sides are straight and at the top it's hexagonal and they have corners and I don't know anything else in nature which has got such exact straight edges and which has got corners because usually in nature everything's rounded.

8:42I understand Sarah's suspicions. Now when you see a stack of rocks in a perfect hexagonal column, it's hard not to think that this was done by somebody who just got a bit bored of making sandcastles. But even seeing them up close and personal didn't do too much to dissuade her. I went there about two years ago and I've just been curious because at first I didn't believe they were natural, I thought they looked man-made. I originally thought, no, that's cement, that's been done in a mould. And then I just realised it was just natural and it was strange, but it could be so symmetrical. All of them are the same size, all of them the same shape.

9:19So how can that naturally happen?

9:26It's probably around by a ten minute walk. Yeah. Well, you should have a lovely day. It's misty. And it's not the kind of mist that you'd see in an artistic film, revealing wide panoramic views of an ancient castle. It's the kind of mist that makes you question if your jacket is actually waterproof. And I'm descending downhill into the rain with a geologist that I met 10 minutes ago. Sorry, but we can see nothing. Really? Yeah, we should be able to see some cliffs in the causeway, but we can just see mist. I'm with Dr. Kirsten Lemon, Science Program Manager for the Geological Survey of Northern Ireland, which is part of the British Geological Survey.

10:05The Giant's Causeway is tucked away on Northern Ireland's north coast, very close to Scotland, and is a World Heritage Site. Apparently, I'm standing right next to it.

10:20I can hear the swells of the North Atlantic, restlessly throwing spray onto any land it can reach. But I can only see about 5 metres in front of me. The mist has swallowed everything, which is a shame, because apparently the view across the water is stunning. Scotland over that way. I mean, nobody can see Scotland, but obviously not today. Really? Yeah, it was very close. Scotland's maximum about 30 miles away. Swimble, very swimble. Well, maybe you can, but... I don't have a 30-mile swimming badge, so I'm not going to brave the North Atlantic. But finally, the mist is slowly beginning to part, and I can just about make out the edges of the rocks.

11:00I can see for the very first time why we came here. Okay, just around the corner. Oh, we're on the coast and I'm seeing lots of vertical slabs. They're quite low down. There's a big bunch of rocks all sitting together. These columns go far up, you know, behind grassland. You're surrounded by water. You know, you've got these waves coming inland and just breaking softly against the rocks. As I look further out, you can see small islands of rocks. But looking further, the horizon is completely empty. It looks like I'm looking over the edge of the world. This is one element of the Giant's Causeway.

11:45So this is known as the Grand Causeway. So it is the largest exposure of the basalt columns. But we will see them all along the coast as we go on. Now this is the best place to see them. This is where all the tourists come and basically any time you come here there will always be somebody clambering over the cones. Are you allowed to clamber over this? You are, yeah. Okay, clambering up here every single step. One, two, three. Clambering is a nice way to describe my very inelegant attempts to get purchase on these strange hexagonal rocks. It's like trying to climb stacks of dinner plates. with each plate about 30 centimeters tall.

12:26I'm having to carefully place my feet to make sure I don't slip. It's like I'm a baby giraffe on ice. Complaining aside, the view from the top is well worth it. Okay, so right now, underneath us, there are all of these hexagonal shaped rocks. And that's me not as any sort of specialist. So you, as someone who knows these things, what am I seeing? What we are looking at is essentially a formation made up of hexagonal columns of basalt, which in itself is not unique. We do find formations like this on pretty much every continent on Earth. In fact, there are hexagonal columns on both sides of the ocean here, one on the Irish side and one on the Scottish side.

13:16And before we had science to help us understand how these strange rocks came to be, people came up with a story to try and explain it. So the Giant's Causeway, as you can probably guess, gets its name from its association with a giant. And that giant is the local hero known as Finn McCool. So the story goes is that Finn McCool wanted to prove how mighty a giant he was. So he had decided to build a causeway across the sea to Scotland to challenge the famous Ben and Donner, who was a very well-known Scottish giant. so Finn started to build his causeway little did he know that the Scottish giant Ben and Donner was doing the same thing and he was coming to challenge him so Finn got halfway across the sea and he took one look at Ben and Donner and thought my word isn't he big and he's a lot bigger than I am so Finn ran back home to his wife said oh my goodness what have I done I've challenged this Scottish giant and now he's chasing me across the sea on the causeway so his wife being the very clever woman that she was decided I know what we'll do so they actually put Finn into a crib and they dressed him up like a baby and whenever the Scottish giant came in and said, where's Finn?

14:27His wife said, I don't know where he is would you like to meet his small son? So of course because the small son was actually giant sized Ben and Donner said, oh my word if that's the size of the son, what size is the giant? And he ran all the way back to Scotland and he tore up the causeway as he went so that's why we have a little bit of the causeway here in Northern Ireland and there's a very similar formation on the other side of the sea in Scotland. OK, now I love that story. You're going to tell me the geology underpinning it. So the rock itself is a rock called basalt. Now basalt is a very common rock, so it's not unique to have it here, but what is unique is the shape that it's taking at this particular location, because here the basalt is formed into hexagonal vertical columns that protrude out into the sea.

15:12And the basalt columns themselves, we think some of them are up to around about 100 metres in height. The reason why it's in this shape is because it's cooled down really slowly and it's cooled down really evenly. So whenever this area was forming, around about 56 million years ago, the lava that ended up turning into basalt actually flowed into a lake. So it flowed into a depression. And as that lava pooled, it basically stayed there and I was able to cool down really slowly and really evenly. So as it slowed, as it cooled, it contracted. And as it contracted, stress has built up. And the easiest way to relieve those stresses is for that lava to crack.

15:53And it always cracks at an angle of 120 degrees. That's when you get the hexagons forming. It's the easiest way in nature for stress to be relieved. It's for cracks to form at 120 degrees. The cracks generally form perpendicular to the cooling surface. and the cooling surface is the top of that pool of lava. So if you imagine that's the top, and the right angle of that would be going downwards, so all of our cracks go downwards. So that's why they are vertical. So these columns are caused by magma coming up from underground into a depression in the landscape, which means it pools into a lake. It's a big volume of lava, which means that it takes a long time to cool.

16:33And that's what causes the hexagons. As the magma slowly cools, it contracts. cracks, cracking at 120 degree angles, which is the internal angle at every corner of a hexagon. But both Liana and Sarah also wanted to know, why hexagons? Why not a rhombus, a pentagon or circle? If it just needs to crack up to relieve stress, surely any shape would do. To get to the bottom of this, I think we're going to have to switch our geologists for a geometrist. That's coming up. How has America shaped the world? I'm Asma Khaled, host of the Global Story podcast from the BBC. As the United States marks its 250-year anniversary, we've been exploring the surprising and often hidden ways the U.S.

17:21has shaped the modern world. And today on the show, we answer your questions about this moment and what to expect in the years to come. From the BBC, it's the United States at 250. Listen to The Global Story on bbc.com or wherever you get your podcasts.

17:40Next.

17:46You're listening to CrowdScience from the BBC World Service, the show that tries to crack your science mysteries. I'm Alex Laffbridge and I'm trying to answer a question from listeners Liana and Sarah, both perplexed by hexagonal rocks on opposite sides of the globe. We've already found out why these rocks form, and know that they form hexagons. But what is it about this shape that makes it so appealing for cooling magma? So I'm Jean-Marc Schlacker. I'm a professor of mathematics at the University of Luxembourg. And I've been studying for now many decades questions related to geometry. Close your eyes right now.

18:27Picture a hexagon. If you're anything like me, the first thing that comes to mind is a honeycomb, those little shapes that bees build, every single one sitting flush against the other, no gaps whatsoever. There's a reason why it looks the way it does, and it's sort of related to why the colonel basalts in Lombok and Northern Ireland look the way they do. It's all down to geometry. There are probably two explanations to this phenomenon that we see hexagons often in nature. One is related to a very deep and important mathematical fact, which is the following. So suppose you take a plane, an infinite plane, and you want to cut it into small regions of equal area, let's say area one, what mathematicians call tiling.

19:17Jean-Marc isn't talking about paper planes or tiling your roof. In mathematics, a plane is a flat 2D surface. Think like a piece of paper that stretches out infinitely in every direction, with zero curves or edges. Tiling means covering that entire surface with shapes, fitted edge to edge, so there's zero paper showing underneath. No gaps and no overlapping. So, what shapes can you use? So if you want a tile plane with only one shape, one tile, there are not many possibilities. You can do it by triangles, you can do it by squares, or you could do it by hexagons. And I think those are the only possibilities with only one type of tiles.

20:03So triangles, squares, hexagons, you can pack them together or flush without any gaps or overlaps. And you can't do that with a circle. So why aren't the rocks at Giant's Causeway or Lombok Island triangular or square shaped? The hexagonal tile is the best of those three possibilities because the ratio of the perimeter of a hexagon, so the length of its boundary to its area, is smaller than for a triangle or a square of the same area. So it's more efficient to use hexagons than to use squares of triangles. Actually, you know, the worst is triangles, then you can use squares, but hexagons are actually better.

20:45Because the more round the shape is, the more area you have for a given unit of length. Right, okay. So, hexagon is great because it is the closest you can get to being a circle while still having all of those borders. That's exactly the point, you got it. I believe the same phenomenon is happening when you look at those hexagonal columns that we see in nature. It's the same optimization phenomenon that leads to honeycombs used by bees and to those hexagonal columns that appear in a natural setting. But back at the Giant's Causeway with Kirsten, I'm noticing while clambering over them that these rocks aren't all perfect hexagons.

21:29Turns out, while maths is perfect, nature rarely is. I've been a little bit disingenuous in calling them all hexagons, because nature, as we all know, is not always black and white. There will be little pockets where it's not quite the same, it's maybe a bit hotter, it's maybe a bit cooler, maybe the chemical composition's a bit different. So actually, whilst most of our columns are six-sided, we have got four-sided, we've got five-sided, we have got seven-sided, and there's even a couple of eight-sided. They may be uneven, but they're still very cool. And they're the same on the opposite sides of the world.

22:06So the key thing here is that these are all volcanic rocks. So whilst we were volcanically active here in Northern Ireland 56 million years ago, Indonesia at the current day is still volcanically active. They still have lava being produced. And that's why you get features that are similar to this in many places around the world. Either they've had volcanic activity in their geological past, or they have volcanic activity right now and that's what's happening in Indonesia. But that raises another question. If it's just a matter of volcanic activity and there are volcanoes all over the world, why don't all volcanoes produce these hexagonal rock columns?

22:44So different plate tectonic settings produces different types of volcanoes. Whenever I talk about plate tectonic settings, there's three different types I can really talk about. Number one is whenever you've got plates pulling apart, that's what's happened here. We also have plates sliding past each other that really only leads to things like earthquakes and then the other one is whenever we've got plates going towards each other. So whenever we've got them going towards each other what will generally happen is you'll get one plate going beneath or they'll simply be pushed together and they'll form mountains and they tend to produce very different types of volcanoes.

23:21They are the volcanoes in people's heads they think of the cone volcanoes, the classic volcano here it's a very different plate tectonic setting so we get very different types of rock formations. So what would a volcano here have looked like? So if I'm going to be honest with you a volcano here would have been really unimpressive it would have been a fissure eruption so it essentially would have been a crack in the earth up through which lava bubbled and it would have flowed out across the surface so we're talking about volcanic activities as opposed to classic volcanoes. Like a letterbox? Pretty much.

23:53Ah, another piece to the puzzle. So does that mean that the rocks themselves are exactly the same? So Indonesia is not exactly the same. They do have the same type of rocks in that they are basalt, but basalt has got many different types of variations depending on the slight differences in composition. And the difference between here and Indonesia is also the volcanic setting. So here we've got volcanoes that formed from seafloor spreading, from tectonic plates moving apart, but in Indonesia we have got a process called subduction. Whenever one plate is going by one another, they produce very different types of basalt.

24:34Now to the non-geologist basalt is basalt, but to geologists they are very, very different depending on where you are. So the basalt here for example has got a lot less of a mineral called olivine and the one in Indonesia has got a lot more olivine. The one in Indonesia has also got a lot more water in it and because it's got a lot more water in it and it cools down a bit faster it tends to not be as perfectly hexagonal as it is here. So they do look regular to a degree but they're a bit more jaggedy and it's got a slightly different chemical composition. so whilst it is similar to here it's not exactly the same.

25:13The chemistry might differ a bit of olivine here a little bit more water there but at a fundamental level it's about volcanoes spewing out slowly cooling magma that contracts cracks and releases stress in the form of these hexagonal shapes. If it's more about the process than it is the location where else can I spot these? The most Dove's example is Iceland. They have got really good examples of basalt columns. The Devil's Postpile in USA, it's not at the sea, but it's got very, very similar rock types. We do have similar formations in places like Korea and in Hong Kong, where the rock types are different, but the way they formed is the same.

25:56It's still lava that has cooled really, really slowly. So there are loads of places. I think in every single continent on Earth, we have examples of this. Right, so these are still being made around the world? Absolutely are. So anywhere where you hear there may be some kind of volcanic activity, you may get features like this. So listeners Liana and Sarah, although both of you were confused by the geometry of strange rocks on the opposite sides of the globe, they're connected by one simple origin, volcanoes. When magma pulls, cools, cracks and contracts, It makes hexagons, because the angle of hexagons are the most efficient way to relieve stress.

26:40Whether that's 56 million years ago in Northern Ireland, or just a few million years ago on a beach in Indonesia, the same rules apply. So the next time you see a hexagon in the natural world, know that it's nature being efficient. Thank you for your cracking questions, Liana and Sarah. while I go hunting for hexagons, over to you for the credits. You've been listening to CrowdScience from the BBC World Service. The question was for me, Leanna, in Canada. And also for me, Sarah, in England. The presenter was Alex Lathbridge and the producer was Hannah Fisher. If you've got a science question you'd like the team to answer, you can email it to crowdscience at bbc.co.uk or send a message or voice note on WhatsApp.

27:28The number is plus 44 8000 314 773. That's plus 44 8000 314 773. Thanks for listening.

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