Rock of the week - basaltic pillow lavas, Fife

Added: 2026-05-20

[00:00:00]you get different ignous rocks like different compositions of ignous rocks that form at different plate tectonic settings right so yeah like there's a lot to consider with ignous rocks and it gets really complicated but just to put it in basic terms we have intrusive ignous rocks ones that intrude into the crust right so imagine you had a magma chamber here magma this is a crust right and these are layers of sediments okay in between it um over time that magma is molten It's quite hot, right? It'll crystallize very slowly over time, producing like, you know, something like a granite or gabro, something that's coarse grain, but it's intruded. It's intrusive, right? It's intruding into the crust. It wants to ascend and reach the surface. And when it does, it'll erupt as a volcano and spews out, you know, ash and lava, ash and lava, blah blah blah. Oh, over geological time. And it forms like a volcano, right? That's a volcano there. This is known as extrusive. So when it reaches this can spell when it reaches the surface extrusive, right? These are very fine grained rocks. So it's going to be fine grained ground mass which is some of this stuff is extrusive, right? So when you look at this stuff like the ground mass is quite like fine grain, right? It crystallizes straight away or just about straight away like um this stuff in here that forms your intrusive ignous rocks crystallizes very slowly over geological time. Okay. Like so it forms bigger crystals. The crystals are allowed to grow. Now the other types of intrusive ignous rocks is sometimes you get magma that seeps along you know in between two beds of of sedimentary rocks or whatever was there and crystallizes out as a sill. So if it's horizontal and it's concordant with like the the layers in there, it's it's it crystallizes out and is known as a sill. Now, if it's vertical or just a bit vertical or like, you know, like cuts across like different sedimentary rocks or or whatever is there before, this would be known as a dyke. Okay.

[00:02:01]Now, I put a post up recently about dikes and obviously in America dyes known as something else. And I didn't really think about that. It's just the way that in in in Scotland, right, it wasn't trying to offend anyone. It's just the way in Scotland that was spell You'll know that, won't you, Chris? They spelled like d i k e.

[00:02:15]>> Yeah. D i k e. Yeah.

[00:02:16]>> Yeah. Well, anyway, I was like, "Oh, no.

[00:02:18]I didn't mean it that way." Like so that's a dyke. This is a cell and they again crystallize a little bit slower you know throughout geological time. This stuff that erupts at the surface you know lava flow upon lava flow that will you know crystallize out quite quickly or if it's ash ash kind of gets pushed out like or pyassic flows you know they'll all accumulate and it's a mixture of shite and crap all together. I score it again. I'm not going to lie, that's going to happen quite a lot, but we'll Yeah. So, that's your kind of two types. Now, these are separated. Oh, no. I forgot my we rubber hang, but I'll just use my finger. So, these are separated out into different compositions depending on like what plate tectonic setting you're sitting at. But, we're just going to keep it basic for now. And before we move on to talk about metamorphic rocks, we're going to talk about the free plate boundaries. Right? Does anyone know like what I'm talking about when it comes to plate boundaries? in plate tectonics. So like basically the crust, the earth's crust is made up of these rigid plates that move about it's known as the lithosphere. You then have the mantle underneath us and people like tend to fight the mantles also like you know a liquid but it's not actually. It's a solid that moves very slowly over geological time, right? Like I'm taught in something that's like a py or like a clay and sometimes it like melts in certain locations and when it melts it will usually partially melt a wee bit like and form basaltic magmas like the stuff that we're sitting on here is basaltic kind of in composition. Um but basically you get new crust created at what's known as divergent plate boundaries. Right? So this is where two plates are moving away from each other and we've had this happen and it's affected Scotland like a few times over geological time. So this is oceanic crust. This is the sea. This is a fish in the sea. It's just floating about, right? And what you have underneath is like within the mantle, you have like kind of like, you know, what's known as like mantle confection.

[00:04:17]Like that's pushing these two plates away from each other. And as it erupts obviously here it will form usually pillow lavas and stuff but it forms basaltic um in composition magmas which is really it's mafic it's basic it's rich in magnesium and iron which is where the mafic comes from. Do you guys pronounce it as mafic?

[00:04:39]>> We do.

[00:04:40]>> Yeah. So in Scotland I mean everyone had their own pronunciations you know depending on what country you're in but I say mafic right. Um so yeah this would prod mafic like the saltic magmas and lavas right so usually when you have the creation of oceanic crust like think about it this way the north atlantic ocean started splitting apart when you had the split up and break up of pangia that's actually affected like this side of Scotland we can see the vcanism related like it tried to split down here but it failed and split elsewhere right when you see the vcanism related to like the opening of the Atlantic Ocean 60 odd million years ago recorded in the rock record here like and here and here like you know sky I maul aren rum a right so when you have the breakup and the continents start ripping apart you're going to get oceanic crust form and oceanic crust is quite thin right like so it's like 5 to 10 km or so like um compared to continental crust but what happens when oceanic crust meets continental crust sometimes like it will subduct underneath it so our set plate boundary which I'll just draw below here is known as a convergent plate boundary where you have the two plates And what tends to happen is because like the oceanic crust is like a lot thinner than the continental crust and the continental crust is silica rich. It's a lot thicker. This oceanic crust is usually going to subduct underneath it.

[00:06:00]>> Sidebar, >> right?

[00:06:02]>> It's also denser.

[00:06:03]>> Yeah. Denser as well. Yeah. It's made up of those mafy like minerals like magnesium and iron. So it's it's basically going to go underneath like this continental crust, right? And as it does so sometimes it can partially melt also the bits of the mantle and stuff but it also partially melts like the crust in here because the friction of this plate going down and as it's partially melting it will form these blobs of magma which we see quite a lot of this stuff related in in Scotland and here in the highlands in the northern highlands we have these random red blobs. These are granite right granitic plutons that were probably once upon a time 20 km within the crust. those bad boys in there formed like by this subduction plate, right? Sometimes the subducting plate can be carrying a continent with it and also over time this continent kind of gets pulled like and it moves closer and eventually it can smash into you know that's probably a really bad diagram like of it. But what happens in here is you get like a lot of like metamorphism. you get the rocks that were originally horizontal end up getting folded and buckled like they are put under like these stresses like and high temperatures and pressures can affect rocks differently and they can they kind of like go through ductile deformation and and all sorts like so they end up like all folded and foliated like and faulted as well if it's closer to the surface so the rocks here and the central highlands terrain and the northern highlands terrain of Scotland are all pretty much like they formed in what's known as an aerogyny like what's known as a convergent play boundary because England was once upon a time separate from Scotland and Scotland actually had independence like for maybe a we bit of time 450 million years ago and what's happened is you've had this convergent plate boundary you've had frost faults form as well but you've had abalonia which was England come up like there's also been a subduction zone under here England's came up and collided like with Scotland you've had another continent come in from the side here Baltica which caused a lot of the metamorphism up here and like also like the frost fault to form um that's found in the highlands where you have older rocks crust on top of young ones. This is like came in and smashed into the side here which makes up Norway and stuff like and it's caused a lot of the metamorphism and stuff in here. So, and before we move on to metamorphic rocks or third kind of plate boundaries known as a strike slip plate boundary or transform plate boundary and it's when I always really struggle to like draw this so give me give me a minute to like try draw it. Okay. And I don't want to show anyone until I do it because it's going to look funny until I actually Okay. How do we do this again? Like that. I've not done that.

[00:08:42]Right. How How can I not draw? Okay.

[00:08:45]This is the block of crust. Yeah, that's it. Okay. Right. And that's another block of crust. Okay. I've got it. It's fine. It's all good. So, a transform plate boundary is where two plates slide past each other. Oh no, I've done that wrong.

[00:09:05]That's what I get for trying to hide it.

[00:09:07]So this block slid that way and that block slid this way. So think about the San Andreas fault line today. That is a strike slip transform fault line, right?

[00:09:16]Plate boundary. That's your third type of plate boundary is transform. And once upon a time we had one of them in Scotland as well. We've just about had everything in Scotland. We're so lucky where we are in Scotland to come across so much geology. Even when you look at this map, there's so much going on.

[00:09:30]You're just like, "What the," you know, like, but basically, you know how the fault line that runs from like Inesse and cuts all the way through down into Fort William, that's known as the Great Glen fault line. Once upon a time that slipped and you have the rocks that are part of the Al Radiant Super Group, these metal Murphy rocks in here, they were once upon a time down here somewhere and that's slid like that and this has kind of came in that way.

[00:09:54]Right. So the fault line obviously it's no longer active like not much is happening in Scotland now but once upon a time was active during the Caledonian arroyny. This is the geological map of Scotland right and there's a lot going on and we are situated up in five here.

[00:10:09]So we are like just like in here right king horns there. So this area that we're in in F and not in F in Scotland is known as in geology terms the Midland Valley terrain. So Scotland's separated in different terrains that each have their own individual geological history, but the terrain that we're in just now is mainly made up of like carboniferous rocks here and around F and Carboniferous rocks in around Edinburgh and Glasgow, right? It's probably why some of the main cities are where they are because obviously the industrial revolution and that carboniferous rocks, it's named carbon, right? Like you get all your coal measure formations and stuff within these rocks. On you go.

[00:10:51]>> I'm going to ask you age of the carbon period.

[00:10:55]>> 359 to 299 million years ago. I think >> 359 to 299 million years ago.

[00:11:07]>> I'm not going to lie, I don't know every period. So, I'm going to get questioned at the end on another one. I >> Oh my god, there's a bug on me.

[00:11:15]>> The only reason I know that is because I've done most of these rocks in here.

[00:11:19]Carbonifice I've done like like you know I've had to like memorize that number.

[00:11:24]Um no yeah I got that just a bit right.

[00:11:26]I like it. I love it when I'm a geologist they get to question me on things and then tell me when I'm wrong.

[00:11:33]>> I believe me I've forgotten most of my period so I'm all jumbled up myself at this point.

[00:11:38]>> Okay. I'll be testing you later on. Hi.

[00:11:40]So carbon period is named after carbon.

[00:11:43]Scotland was situated the equator back in then right like so the environment was a lot different today and here in F we had a shallow marine environment was happening at the beginning we're going to go through it like in geological time and because as we walk just along this little bit of the coastline we come across like the oldest rocks here and then you kind of walk and you're coming across younger ones but they are roughly around the same age like but as as you know Chris has just asked me Carbonus period is 300 well 299 to 359 million years ago Right. So that was a pretty long time ago and we were pretty much at the equator and environment it was humid. You had the deposition of a lot of sediment because in this Midland Valley the reason it's called the Midland Valley there was extension of the crust going on. So this is actually a failed drift valley. So the crust has started to rift apart creating space for sediment. As you create space you have these faults that form. Usually normal fault lines would form because as you rip things apart, you start forming these normal fault lines. And this is going to be really funny because see every time I try draw this like you'll probably be better at drawing it than me, Chris. Like um but basically as you start like rifting apart everything, these bits of blocks are going to like slip down and form this kind of what's it called again?

[00:13:02]Grabins or something.

[00:13:04]>> Can you remember the exact familiar with that? We just call them normal faults or >> Yeah. So these are like normal fault lines that form and as it's spreading apart obviously lava's coming up it's erupting as obviously it doesn't even got this right like it's just it's try to explain it. So you're creating loads of normal faults where like the football goes down and the hanging wall kind of goes up. No wait that's not right is it?

[00:13:25]Oh I can't even remember. And um because it creates space you obviously have the accumulation of sediment in here and and that's what's happened here in the Midland Valley. you're creating space and you're getting sedimentary rocks been accumulated but also you're getting magma seeping up through the crust cuz it's pulling apart and as that magma comes to the crust and it rerups as lava it's going to you know like spirit lava and we can see the remnants of this lava like in the record so the light pink stuff here near King Horn is lava and basically mafic which is magnesium iron rich magma associated with pulling apart of the crust extension of the crust like that's lava that's came out and and a mixture of ash and magma and stuff So, when we're looking at this area that we're in now, does anyone want to guess what kind of rock we're on?

[00:14:14]>> Lava.

[00:14:15]>> So, this is like the salt composition.

[00:14:20]And you'll notice like from a distance and even some of the stuff that you're sitting on and like some of this stuff here like it's got like a kind of like bubbly shape to it like and up there and stuff. So we're actually like geologists can come down and like just about every rock will tell you a story. Um so when I'm looking at this rock the first thing I notice is it's quite fine grain. Well the color probably the color it is slightly green kind of like it's probably been hydrothermally altered a we bit like um but it's still dark in color dark gray like and the first thing I notic is like the not just the color but that it's fine grain. So that suggest to me it came out like and it crystallized straight away because it's fine grained like it's crystallized straight away. It's got these bubbles in it like visicles they're called like or vesicles in America.

[00:15:08]So it's physicular. But then the next thing I notice about it is some of these bubbles have been filled up with minerals which usually can form a um geodess, you know, like so there's been hydrothermal fluids that have ran through this roof at one point in time precipitating out these minerals. Now some of them don't look that pretty down here, but when you get further along, you'll see what I mean. And then you'll notice as well the veins are cutting in between. So there's a vein here. It's cutting between some of like the you know the cracks the joints that naturally formed. So to me when I look at this and I see the pillow shape like of like the lava is telling me a story that once upon a time this lava erupted into a shallow marine environment where there was like water and you had like the the because when lava erupts into water it tends to form pillows and when later on Google what a pillow lava eruption looks like right because then you'll kind of see what I mean it like pushes like the lava comes out obviously but because it's hitting cold water it crystize is just a bit straight away.

[00:16:12]But the way it forms and does this is it pushes it out and forms these random pillows. So geologists refer to it as pillow lavas, but I wouldn't put your head on it to sleep on it, right?

[00:16:21]Because it's not like that. Yeah. So this is lava to me. It's basaltic and composition dark. It's obviously been hydrothermally altered. We've got hydrothermal veins running through it.

[00:16:31]We've got amigdals like that filled the cavities. It's physicular as well. So there's a lot going on. And then obviously we have this line. It's probably a fault line of some sort of weak point. and the rock and then the stuff that's exposed at the surface here is a little bit stained like at the surface like this is kind of red. So you need to think about it this way but salty because iron has got iron in it.

[00:16:51]So sometimes when you find rocks that are exposed at the crust they tend to you know rust away like a car would rust away. Okay. So in this case this stuff's been rusting away. So these are pillow lavas right? Does anyone get is everyone get that pillow like they're not the best examples of pillow lavas. This is actively happening in Hawaii right now, >> right?

[00:17:14]>> Yeah. Yeah, you're right. Yeah. So, you're getting the ripping up like as well like Iceland and stuff.

[00:17:20]>> Yeah.

[00:17:21]>> You got the volcanism.

[00:17:22]>> There are videos you can see >> even right now. Has anyone seen in the news recently the recent volcanic eruption in Iceland?

[00:17:28]>> That's like the middle oceanic ridge which is where the crust is splitting apart. This is like, you know, this is 330 million years older than that stuff, right? But like it's still similar. And when we look at this stuff up here, we notice it's got like some it just looks quite blocky and it's actually a little bit sheered a wee bit like it's probably been affected by the fault line a we bit but it's like breiated and stuff and sometimes we can look at rocks and see well that doesn't look like it's you know lava. It's got like a combination of different things. It was maybe a little bit of a volcanic classic flow like where it it kind of had like some ash in there too as well as like pieces of you know like they were a little bit explosive but most of the time basaltic magma when it reaches across the surface it's not explosive. It's unless it hits you know shallow shallow water like or ice like that eruption back in 2010 like where the the planes weren't flying because the volcano in Iceland. I don't know if anyone can remember that but that was a bit of a disaster.

[00:18:25]And we have these lovely veins on the outside of our pillow. So this is a big massive, you know, cross-section of a pillow. Once upon a time, it's erupted and, you know, like into a pillow shape has kind of been eroded away in that.

[00:18:39]But then we've had later on, we've had hydrothermal flu running through these rocks. It's precipitated out. I think calsite in this case. I mean, I can scratch it with my fingernail just about, so I'm going to say it's calite.

[00:18:51]And it's just amazing to see like, you know, veins and stuff like that. It just indicates that the geologist you kind of look at it and be like, "How did these get here?" Well, there must have been hydrothermal activity. Like there must have been fluids running through cats and the rocks like joints. Obviously, these joints form naturally on the outside of this pillow like and it it must have had fluids running through it when it's buried a little bit, you know.

[00:19:13]So, there's been lava flow up on lava flow. There's probably been other stuff on top of it, which is maybe that stuff over there. And then you've had fluids running through the rocks and they precipitate out these minerals. So, calite is calcium carbonate. It's like quite a I mean it's only like number three I think or number four on the most scale. So in most cases you can kind of like scratch it with either a copper coin sometimes your fingernail just depends. But it's filled not just like these veins it's filled some of the the ammy bails here. And when we look at these we can sometimes find like that one looks like it might have been potentially a deal. Sometimes we can find the crystals have formed in the inside of these. Like last time I picked up a rock like randomly and I lifted it up like just one that was lying there and it had like a geode in it and I was like oh my god you know like I can't believe you can get that down here. So these rocks these are the ones that will contain like geodess and a stuff and then they get weathered and eroded away and they end up you know in the shingle and stuff. So again, just the saltic lava hydrothermally altered gives it that green kind of color as well.

[00:20:20]Calite veins running through the the joint like that one looks really cool there. There's also one in there for anyone that's wondering. But as we move our way along the beach on you go >> uh looks like it's roughly got a hexagonal shape as well.

[00:20:38]So that also kind of indicates that there was maybe columns and stuff >> from that >> the way that is filled and contracted.

[00:20:48]Yeah.

[00:20:48]>> So when you're saying this is flowing through here, does that mean this to be quite soft at that point?

[00:20:53]>> Oh no, this is still hard. It's just been that what tends to happen is especially when you've got like basalt basalt likes to if it's got the space to do so sometimes it will form these hexagons like these joints and like which are weak points in the rocks. So like they're naturally already there at the formation of like when the lava pools and if there's space for it to like pool and pond cuz this is maybe actually cuz it's funny this I read at this area that these were like pillows and stuff but then when you say that and mention that to me now and you can clearly see the hexagons.

[00:21:29]>> I was standing at cave the other day and that's the size of you know like so it could have been that they contracted formed the columns. I And it'd be interesting to drill a hole down >> and the pools it contracts and then you get the heck there >> and that gives you an avenue for all of Stafa. Have you ever been to Stafa?

[00:21:52]>> Oh, the trans co.

[00:21:54]Yeah. So it's like basalted magi likes to like the way that if sometimes if there's space and stuff for it to do so it will contract and form these naturally like it will have a center like obviously the center was in here and what it does is it fills the space and the most natural way to fill it and each center is is forming these hexagons like it's proper math you see it and you're like how did that form that looks mental you know >> there's a thing recently I a mathematician has just proven you see I seen her did a video on it as well. She's a mathematician. Maybe she was the one that actually worked at Cambridge, but she videos and stuff online as well. So they basically known the most efficient shape.

[00:22:40]>> Yeah, the most efficient shape >> and it was a theory and this I think managed to prove it.

[00:22:47]I guess >> it's about efficiency.

[00:22:50]>> Yeah. Don't ask.

[00:22:52]>> No.

[00:22:55]I mean, it makes sense. If you got sort of a magma, there's no natural faults in it. It's not like they've been subject to any pressure or anything. So, if you got a uniform >> um substance and it starts to cool and it's going to contract, >> it really stands to reason that's the most efficient shape that's going to get.

[00:23:11]>> You're right with beehives and stuff is the way that the bees, they'll put their honey just on like soft. It's not as if they make the the the hexagons themselves. It is just the way that the honey will naturally form and spread out and it's the most efficient way for it to fill the