 All right, here's our first try at a recorded video. So today we're gonna do a crash course in geological time and I'm gonna explain as we go along what I mean by that. But all of the ideas that we go over in this lecture are at least touched at, well at least the majority of the ideas are touched at in your reading. So I really strongly recommend you read your reads. You should be reading your readings every week, but this week is really important. All of the ideas we're gonna go through today are gonna be applied in class, or rather in lab. So make sure that you have the reading pre-read and also make sure that you watch this video, the entirety of it, take notes, et cetera, cause you're gonna need to apply these during lab this week. All right, so all that said, let's talk about geological time. This is a geological time scale. This is, believe it or not, the combined effort of 150 years of geologists and chemists and physicists smashing their heads and their hands and their hammers against rocks and resting this information from the world. This did not come easily. So what you will see here is a bunch of noise the first time you look at it, but the more you look at it, the more you'll realize is a very clear arrangement of information. And there's some important stuff within here. The first you're gonna see that there are a lot of different words here. These all represent units of time. So we have the senozoic up here, the mesozoic up here, paleozoic up here, et cetera. Let me get a better little poker, where's the little poker here? Here he is, laser pointer. There we go, you can see it a little bit better. Pre-cambrian, et cetera. Over here, vertically, we have different units, the protozoic. Jumping in here, you can see there are things in these much smaller units, but don't get fooled. These are actually very large units of time, the cryogenian, the ediacrian, the cambrian, or division, Devonian, et cetera. These are all terms that we're talking about. Throughout this course, we are really gonna be principally interested in this whole area over here. All of this stuff here represents almost nine-tenths of Earth's history, but in terms of oil formation, it really isn't important for reasons we'll cover later on in class. So graphically, you can look at it like this. This is the largest units of time, what we call eons. And the eons, again, you don't really need to memorize these things, but just be aware that when we talk about the Archean, we talk about the Hedian, we're talking about really large units of time. I do want you to know this word here, eons. Just understand that that is a division that we put time into. So those are eons. Eons further subdivide into eras and eras further subdivide into periods. So just think about months, which have weeks in them, which have days in them, which have hours in them, which have minutes in them, et cetera, et cetera, et cetera. So pretty much we're looking at, these are our days up right here, these are our hours right here, and these guys here are our minutes or our seconds. Actually, these would be our minutes. In fact, these further subdivide even smaller units, things like stages, but we are not gonna be talking about those in class. We can be aware that they exist. So here's just another graphical representation of these. This one is directly from your textbook. Again, please do go read your textbook. And you can see to scale here, here's the Hadean, and then all these little tiny slices over here, these are vast units of geological time. Each one of these is 50 million years, 100 million years. These are really large units, which gives you an idea that what you're looking at with these eons here are vast, vast, vast units of time. In this case, this is a billion and a half years right there, 600 million years right here. This is two billion years in the protozoic. So everything that you have really experienced in terms of interacting with rocks, vast majority of them, every fossil you have ever looked at or touched, all of that stuff hangs out down here in the Phanerozoic. And this is, the whole story of oil is really over here in terms of economically useful accumulations anyways. And that's because oil is made of organisms and this is where all the real organisms start showing up, the things that are capable of existing in abundance and size that we can actually really make things out of. All right, so that said, let's move on and talk about, not just about links of time, but how we can think about time. So here is your scale over here again and we're gonna come back to each one of these components. But beyond that, we can ask the question, how do we know any of these things? How do we get these numerical dates over here? How do we get these orderings of dates over here? And that's really what I'm gonna be interested in today. It's talking not so much about the time system itself in terms of what the divisions are. I don't care that you memorize that the Jurassic existed between year X and year Y. We can go down here. What would Jurassic 201 up through to 145? This is not the most recent time scale. What year is this? It'll say down here somewhere. This looks like this is 2012, maybe I put it on here. They're reproduced every year and these numbers move around because these numbers are our best estimates based on the current data and the data is being changed every year. It's being refined. And so these actual hard numbers will move around a lot. The relative ordering of these names, you'll see in a second, these never move around. So let me say get subdivided up, but they never move around. And that is an important distinction, this relative versus these actual numbers over here. This is gonna be really what the rest of the talk is gonna be about. It's about these kind of orderings and these kind of orderings. But I'm not really concerned that you know the names that then hear the numbers. In fact, I'm not concerned at all. I really just want you to understand how the system exists, how the system functions, and more importantly, where the data for this entire time system came from. All right, so that brings us to this question. How do we know any of this stuff that I've been telling you right now? How do you know any of it? Well, because you watched the video. Now, how do we know the information to put in the video? You might not be surprised since this is a geology class that we know it from rocks. So if we look right here, there actually already is temporal information in this slide right now. I can tell things from it. You might be able to infer things just by looking at it. By the end of this lecture, you'll be able to look here and see a lot of time information. And it's, in fact, that time information, just superficial appraisal of geologic structures, geologic layers of rocks that we get really the most important first bit of information of a time. But there's a lot more important analytical things that need to be done to pull out those real age dates. So that brings us to this clarifying point right here. Now, when we talk about time, we can mean two different things. We can talk about a relative ordering of things. For example, I can say that my first class of the day happens before the second class. That's obvious. But that doesn't tell you what time. You might have calculus in the morning and you have chemistry in the afternoon. Or you could just say, hey, what do you have first? What do you have? Well, I've got calculus and then after that I've got chemistry. That didn't tell me where you're gonna be. It tells me what order you were taking things, but that doesn't contain a lot of crucial information in it, which is what day are you taking it at? What time does each start? How long is each one? So this discussing things simply as a sequence of events, the order of occurrence, that something occurs before some other thing. And in this case, we're looking at a timeline. This occurred in the morning, this occurred in the afternoon, but this guy here occurs on a Friday and this one occurs on a Monday. I mean, so you can read a schedule in terms of a relative sequence, both left to right and up and down. I mean, that's just how a schedule works. You guys can read the schedule. The other important thing and the more important thing really is this part over here, which is the actual time. I mean, Monday, Tuesday, Wednesday, in terms of days or 10, 10, 30, 11, 11, 30, et cetera. So that actual hours or days in your schedule, that's the portion of time that we call absolute time. Over here, this relative ordering events, that is just relative. One happened before the other. It's a sequence, but a sequence that's not locked into any specific enumerated hour or day system. So time geologically can be thought of in exactly the same way. When we go back and we look at a sequence like this, we can say that the Cretaceous happens after the Jurassic, which happens after the Triassic and all of these things happen before the Paleogene. That's how this particular charts to be read. It goes up like this and then it goes through the long wait, now it goes this direction and then it does this. It's just going from the bottom to the top and all these actually, now I look at it. So that's one way we can do it. The other way is we can look at these actual numbers right over here. So we can look at the absolute time. And both are useful ways of thinking about things. This first thing, this ordering, that's gonna be what the first part of this lecture is gonna be talking about. How do we figure out the relative order of things? And then later on, we're gonna apply some numerical dates. And in fact, that's how the history of geology worked. We've got the ordering of things first because the technology to do that is very simple. Again, you can actually get that basic information simply by looking at this picture here. Later on to actually get those absolute dates required some fairly complicated chemistry, some fairly complicated analytical equipment. And we didn't even start cracking that problem until the 1900s and straight through to the 20th century. And we're still working on it. My colleague, Deanne, for example, further refines the numerical date ages for igneous events in Nova Scotia every single year. It's one of her big research projects right now is refining our understanding of when what we already knew was a relative sequence of events, the intrusion of large plutons or blobs of magma into the crust and Cape Breton. We knew relatively when did they happen, but the actual exact date, X number of millions of years ago, that problem is still being worked out. She works on it all the time and sometimes he's summer student, so chat with it. All right, let's go forwards. So now we got to a bishop over here with this fancy big cross and this fancy hat over here looking very serious. Is this serious? Man, have we got a big shark over here. So what does a guy and a shark have to do with time? This is the basis really of geological time is the a few set of really powerful observations and deductions that were made by this guy here. So this is a guy who goes by the Latinized name Steno whose actual name is Niles Stenson. No one calls him that. Everyone just calls him Steno. That was his choice of a name later on, fancy Latinized name. He was a Danish scientist in the very early days of science. We're talking with the 1600s here. Science doesn't really exist. They're philosophers, natural philosophers who are applying the principles of philosophy to the natural world. So he hung out with a group of intellectuals in Italy. So one year this ginormous shark washes up. I think it actually got caught, but they pulled it up on the shore of Italy and it's the shark like no one's ever seen. It's ginormous shark. So it gets sent up to local royalty so that the court can all come and gawk at this amazing monster. And Niles Stenson, who is a anatomist, he gets put in charge of doing the public dissection. And so it's like a movie, right? Everyone's hanging out. They're watching, whoing and alling at this ginormous rotten carcass of a shark. So he starts looking at the teeth of this thing and he makes an interesting observation, which is the teeth of this look just like these teeth-like things that you can find in rocks all around Italy, in particular Malta, right? They're all along the Mediterranean. And we know of course today that that's because they are literal shark's teeth. We're talking about fossils here that are trapped in the rocks. But the time this was not obvious because just like I was showing you all these cool minerals in class that have really neat shapes, some of them are cubes, et cetera, odd but perfect geometry, the earth just seems to throw things out that have sometimes perfect mathematical shapes, crystals, and sometimes have apparently anatomical shapes look really similar to things on the surface of the earth that you might associate with biology, but they're in solid rock. So no one really knew how these things, maybe they'd always been in there, no one really knew. So he started thinking about how you might go about putting a real tooth in the ground. And so he writes this quick little piece at the end of his chapter of a description of these teeth, of the actual shark. Quick little note where he starts arguing that shark's teeth in fossils or fossil shark's teeth are actually shark's teeth. The things in the rocks were originally outside and they got into the rocks when the rocks were in a liquid form or when the rocks were sediment. That's at some thinking about just in general, and how do rocks come to be? How do the layers of relative rocks come to be? How do the structures we see in rocks come to be? And he writes what was supposed to just be an introduction to what was supposed to be a much larger book on the subject. That's all we ever got out of him because as an individual he was intellectually torn between theology and science through his whole life. And after writing the short introduction he moved on to theology and became more heavily involved in the church. And he spent the rest of his life working on problems of faith rather than on problems of science. So perfectly good thing to do with your life but we didn't get the book out of him that we were expecting to get. But the little introduction changed science. So there are a simple set of principles that come out of Steno that are often referred to as the principles of cryptography sometimes just as Steno's principles. Not every single one of these. There's a couple that I at least want to add it to the end that's not directly traceable to Steno but intellectually even these latter ones are but all the first ones are absolutely straight out of his book and he used them to interpret the geology of Southern Italy. You can use them to interpret the geology anywhere. Super straightforward and I'm going to explain it to you in the form of baked goods, principally pancakes. Ready for it? All right, here comes some pancake geology. So here are the things we're going to be talking about. Superposition, original horizontality, lateral continuity, cross cutting, inclusion. So you remember those terms. We're going to come back to them. Fancy words really mean very simple but powerful ideas. Superposition, original horizontality, lateral continuity, cross cutting and inclusion. And by the way, this is a picture from Steno's books. There's the shark's teeth, the fossil ones. Here's the actual shark. This is what, by the way, a shark that's been rotting in the Mediterranean sun for a few days looks like, not a happy thing. Okay, here are your pancakes. So someone offers you this delicious stack of pancakes. Which one of these pancakes do you want to eat? The obvious, well, this looks like a personal survey but imagine that we are dividing these up amongst everybody. The obvious answer is you want to eat this pancake right here, this one on top. Well, apart from the fact that that pancake comes with blueberries and comes with butter, ignore those. Why do you want this pancake? Why do you want this? Why does everybody want that pancake? You know you want that pancake because that is the freshest of the pancakes. This pancake right here is a brand new pancake. On the other hand, this pancake is a sad old pancake. That is a cold pancake at the bottom of the stack. So the further you go up this stack right here, the younger the pancakes are going to get until this one here is the last pancake to form. It is the youngest pancake. Now, when was that pancake made? I don't know. Was it five minutes ago? Was it an hour ago? Was it 30 seconds ago? I don't know. But I do know that this pancake was made after this pancake was made. Because otherwise, it would be underneath because this is a stack which was laid down sequentially. First, second, third, fourth, fifth, sixth pancake. That's the first idea. That is the concept of superposition. Superposition is just a fancy way of saying that things that are oldest are on the bottom of a stack. Things that are youngest are on the top of a stack. And the age of the stack goes from oldest to youngest. That's superposition. Super simple in pancakes in everything you know this. Going back to our layers of rocks over here, I said there was time information. What's the time information? Down here, we have the, these down here are the oldest rocks. And these ones up here are the youngest rocks. Heard in the horrible writing. Those are the youngest rocks. And the time goes like that. There you go, straightforward. That's the first idea. Next up, we have original horizontality. Well, what does that mean? Look at how these pancakes are laid down. But not just in the stack. Think about how they were in the pan. When you pour the batter on, what happens? When you pour the batter on, it's gonna flow. It's gonna flow from left to right until it encounters a barrier. But it's gonna make a flat layer. And so if you find pancakes like these ones here, which are at some angle, you know they had to be removed and turned up to that angle. They were originally flat. They're no longer flat because they have had their angle transformed. And here is a picture of that in more extreme form. You see these rolls here, delicious looking crepes. Those actually look super good. I wanna eat them. You know that these things were originally made flat and then someone came and rolled them in afterwards. So now think about when we were talking about plate tectonics. The very first slide I showed you in plate tectonics was a picture of a giant fold in the Rocky Mountains. So we can deduce then that those rocks were originally horizontal and that something happened. In this case, we know it was a tectonic collision happened that actually bent those rocks and put them into their current position. So that bending was a secondary event to their being laid down. That is the principle of original horizontality. So now we got two more, lateral continuity and cross cutting. These both just follow from the same idea. Lateral continuity, if you look at our pancake stack, just says that pancakes get laid down in sheets, continuous sheets. And if you see it, anything rocks due to, if you see a break in them like this, you know that they used to extend across and that's been removed out. So you know that this pancake bed here used to extend between here. This was one sheet. So I can say that this one is the same as that rock layer right there. And then pancake layer right there. So if for example, I mean right here I can see that it's one sheet. But let's say I walked along to the Grand Canyon and in the Grand Canyon, I have one layer of rocks on one side, one layer of rocks on one side. And on the other side, I have, so I've got a stack of rocks like this. Here they are and there's my layers. And on the other side, I've got a stack of rocks like this. And I'm interested in knowing what is the relationship between this rock layer over here and this one right here. So lateral continuity just tells me that these things used to connect across. That is one continuous layer. That's all lateral continuity is. Cross cutting relationships tells you that if anything cuts through something else like the knife cut through this stack of pancakes, by definition something must have existed for it to cut through. Otherwise it couldn't cut through it. That's it. So superposition, right? Oldest on the bottom, youngest on the top with the age going younger as you move up the stack. Lateral continuity, these layers used to connect across like this. Cross cutting, this slice of the knife happened after the pancakes existed. Original horizontality, these guys here used to be horizontal and something happened to bend them up like this. Or in the case of our crepes over here, right? Something happened, something compressed them, whatever to make them bend like this, right? But they were originally horizontal layers. That's it. That's pretty much all of Steno's principles that you need with actually that's a lot. There's a couple of very minor additions. One that's actually Steno's and another one we've added on later on. But on this point I got to move away from, well, actually no, I can do these ones with pancakes as well. If you want to think about, if you're not a pancake person in this sense and you want to think about muffins, chocolate chip cookies, you can do that too, it doesn't matter. You choose your bake good, okay. So you notice that this one has blueberries on it. Is this a blueberry pancake? This is not a blueberry pancake. To be a blueberry pancake, the blueberries need to be, maybe I should make these blueberries a different color. Can I do that? I think I can make these blueberries a different color. Let's make them be blue. All right, now I'll make some blueberries in there and some blueberries. So let's say I wanted to take this blueberry and I wanted to put it inside the pancake. Can I do that? Not without destroying the pancake. So if I want the blueberry pancake, the blueberry inside the pancake, that had to have happened when the pancake was still a batter. That's the principle of inclusion. It says if you have a solid object included inside of another solid object. And for Steno, this was a shark's tooth. Shark's tooth looks something like that. And if there was a shark's tooth inside a rock, the rock had to have been a liquid when it went in, or in the case of the sediment, really a mud is a liquid, but often a slurry of a bunch of sediment of some kind. But it had to be non-consolidated, not a solid as a collective whole. So the same is true with batter. So here is your, you wanna make a blueberry pancake, your blueberries or your fossils or your pebbles if you're talking about a rock that can conglomerate. Those things were solids before the surrounding material was solid. That is the principle of inclusion. Now I'm gonna make a caveat here that we're talking about things like sediments and fossils. And we already said in class that you can get igneous material and shoot it inside of something else. In that case, it's actually not the principle of inclusion we're gonna use. It's the principle of cross-cutting relationships. Which remember just said, if something cuts through something else, it has to be younger than whatever it cuts through. So if you've got a bunch of magma, right? And if you've got a bunch of magma and it intrudes up, a big blob of magma comes up inside and lodges itself inside, then obviously it is cutting into everything that it is going through. And it therefore must be younger than the things it intrudes into. So that's an exception to the case of the principle of inclusion. Which just says if you find something like a pebble or a fossil, you know that thing is older than the stuff around it. In the case of an intrusion, by definition it came in afterwards and intruded it. And so again, we're talking about liquid rock. It would lava underground or magma is the proper word for that. So here is your analogy for an intrusion. So here's my jelly donut. You know, we make a donut, right? Then what do we do? We get a big thing and we squirt our jelly inside of it. So we know that the jelly went in after, in this case. It intruded this donut after the donut existed. But notice that it did something to it. Not only to add jelly to it, obviously. But if I wanted to know, if I wanted additional information that this came in afterwards, there actually is additional information in here. And that's actually in this area right here around. This area just here on the outside. So there's my intrusion of jelly. But notice that it's also soaked into the material around. Kind of done a bad job. Let's go over here. This area right here. And it's soaked actually into the dough. So the same thing actually happens when you have igneous material and it goes squirting inside of rock. Except in this case, it's not obviously jam soaking in. Rather, it's the actual heat, right? The heat from the rock itself, from the magma itself. That heat will actually alter the rocks or metamorphose the rocks. You guys remember back, we used a term for this that is contact metamorphism. So this here is often called a metamorphic halo, right? It is just kind of an area of metamorphism that surrounds the igneous intrusion. At the same time though, remember that when you have two things, when you've got, say, your volcano, here's my volcano, and material comes out of the volcano on the surface, it is gonna cool very slowly. Or very rapidly, sorry. On the other hand, the body of magma underground, which is coming up towards the surface, remember it is gonna cool slowly. It is gonna cool slowly. And as a result, the crystals are gonna be larger. All right, so they're gonna be larger. They're gonna be these big, visible crystals. You can see these are supposed to be crystals. This is really hard with the mouse pad, so these are my crystals. Whereas the ones on the surface are just gonna be little dots, right? They're gonna be much smaller. So the same thing's true here. This is an intrusion of magma coming inside. Where it is touching the cold rock, where it is directly in contact, right along the edge. Now on the magma side, not on the original, on the rock that's intruded side, on the magma side, right around the edge where it's in contact, it's gonna cool faster than it does on the inside. Which means this portion right here is actually gonna have smaller crystals than the inside does. And so that's another one. So this is what's called a chill margin. It's literally the margin of the intrusion, right? And it's been chilled, and so it's smaller crystals. So either by looking at this metamorphic halo, this is on the rock that's been intruded, or on the intrusion itself, on that gradation of sizes of crystals from small on the outside to larger inside. We can figure out which one of these things happen after, or first. Now, this jelly donut is not a question, obviously, but imagine that you are walking along and you come across, here you are, you're a little guy here, there you are, and you are, and you only have one arm. Let's give you another arm. I guess that's a hat. It wasn't intentionally a hat, now it's a hat. You're walking along and you find this. And this here is all granite. And this right next to it is some kind of other rock. Now it's possible that this granite came first, millions of years earlier, and then it eroded away, and this was an open space, and then some new rocks got deposited. You don't know the relationship between these. If you just walk along in the woods, you happen to find a bunch of granite sitting next to it. Or it could be this rock existed and the granite came intruding up into it. You don't know what the relationship is, what we'd call the contact. This is the contact where they are literally touching each other. We don't know what that contact is about, what story it's telling us. So what we would look at then is we would look at the contact itself. We would ask on this side here, right, do you see evidence of metamorphism from the heat coming out of this side? And on this side, do you see evidence of a chill margin? If you do not, then that is telling you that this already existed and this stuff got deposited afterwards, right? If you do, then you know that this material intruded in to this material and baked it and was cooled in contact with it. So this guy here is the principle of metamorphic halos it's sometimes called, sometimes it's metamorphic margins, different names for this. And this is the flip side of that are chill margins. Remember on the edge where the crystals are smaller. So that's all you need. Those are the principles of Sotagraphy, these basic principles, you can apply them. Let's say we look at this right here. This is Five Islands Provincial Park. If you have not been here, you should go here, it's amazing. So you look at this and you ask yourself, okay, what in this stack? Which is the youngest rock in the stack? Well, we know that they are getting younger as they go up. I'm gonna get rid of my, I'm gonna want my red pen back with my red pen color. Let's get my red pen back, right? So this one up here is the youngest and these guys down here are the oldest. But I can also see that they are no longer horizontal. They have been rotated up just slightly. And so I know that these things were originally horizontal. So I get both the time information, but I can also tell that sometime after the deposition of this last layer, because all of them, remember just that all of them have been moved. That wasn't that one, they're supposed to go that direction. All of them have been shifted up like this. And it's sometime after the deposition of every rock in this package, they were all bent up slightly. So that gives you a temporal sequence, right? If I ask when did that, when did that tectonic event happen that caused this whole thing to bend upwards? Well, I can date this and that doesn't give me an actual time, but it gives me a relative sequence. It was sometime after the deposition of that rock. So however old that rock is, this tectonic event has to be younger than that. It gives me a bracket, an age bracket for it. So there's my youngest, et cetera. This one here has got a bunch of different information. So here's this rock, I don't know what kind of rock this is, but you can see there's been intruded and it's been intruded twice. So here is an intrusion, right? There's one right here and there's another one right here and this is the same one over here and over here. That's the same intrusion. So this is lava that is forced, or rather magma that's forced its way along a crack. So we know that this guy here has to be, and this is the same, all this rock here is the same, right? This is the oldest thing. Why? Because this cuts through it and this cuts through it. So principle of cross-cutting relationships, right? These guys here have to by definition be younger than the things they cut through, just like our knife cutting through the pancakes was younger. Okay, which one of these is the youngest? Well, notice that this one actually cuts through this one. How do I know that? Because here you can see it goes, but then it gets moved over here and moved over here. So these have actually been displaced as well after, whereas this one has been unaffected by that. So I know that it went one, then these ones were deposited two, and then this thing was deposited three, and I actually put your numbers in there for you to help to see them. So there's one, two, three. I've got a sequence of events right there, a little bit more complicated than what we've seen previously. All right, what is next up? You guys remember this slide? I showed you the slide. This one is from Ireland. So this right here, remember, is an intrusion of basalt. And then over the side here, this is all limestone. And I use this to show you about differential weathering. And this is more resistant rock, and this rock here, weather is much more rapid. And that's why this is sticking up above, right? Both of these at some point in the past were way underground, but erosion has taken away all this material, but it's taken this material away easier. So if you actually go up and look at this, this tells you two things. This tells you that, well, first, that this weather is slower. But the other thing it's telling you is that this one has to be younger than this one. Why is that? Because it cuts through it. This is a cross-cutting relationship. For what have you said? No, no, Jason, that was there like a wall and this material got deposited around it. Is that possible? Yeah, it absolutely is possible. That happens for sure. So if you wanted to double check, we could go and look, and this photograph over here is actually a photograph directly down on top of this guy right here. So this is looking down like this. So what would be the prediction? If this was liquid rock and it intruded into it, I should expect to see the area on the edge here and the area on the edge here. I should expect to see metamorphism, right? So this should be metamorphosed, right? Contact metamorphism. And right along the edge here, and right along the edge here, I should expect to see finer crystals on the inside. You can't really see it in this photograph, but I promise you can see both of those things in reality when you get there. This area here has a chill margin. And right over here, you can see contact metamorphism. In fact, the cool thing about this locality, that contact metamorphism, there's an interaction between the carbonates here and the hot material here and the salicyc minerals that are in here, fluids associated with it that actually produce new minerals right in here. And in this case, at this locality, there's a mineral that occurs right on that contact that occurs only there as far as we know in the entire world. So this is the type locality for a particular mineral. It's a really rare mineral that no one cares about unless you're a mineralogist, but it's pretty cool to go to the only spot in the world where the sequence of geologic events and the chemistry available was perfect to produce that particular mineral. I mean, it probably exists elsewhere in the world, but we've never seen it. This is the only place we know it exists. So a lot of cool spots. If you want to go, this is up on the, not Northern Ireland, but the Northern part of Ireland. It's a place called Enos Crone, something like that. It actually has about four different names because it's really a name in Irish and then it's been translated into English. So you see about four different spellings of it. It's something like Enos Crone. All right, so this is a bit of a blurry picture, but we can start deciphering it. This guy here is a layer of coal. This is some shale on top of the coal. This is sandstone. This is sandstone. So what you should see is that this layer is not flat here. And what you can see inside of it is that we actually have a fault that runs right there and another fault that runs right there. So this fault cuts through this sandstone. So it must be younger than the sandstone. What about this shale or this coal? Well, this coal is sitting on top of, it's on top of the sandstone and therefore it must be younger than the sandstone. But how do the faults relate to the coal? Well, notice that up along the top of the coal, along the top of the coal and along the top of the sandstone here, notice they are not displaced like this sandstone here. This sandstone has got this major displacement along the fault. So the fault does not continue through the coal, which suggests that this area was deposited, was faulted and then the coal got put on top of it. Just filling in these kind of indentations and then the shale on top of that and then this on top of that. So this should go one, then the faults are two, then this is three, then this is four and this guy here is five is the order that it should go in. And there you go. This is the interpretation and this image is also from your textbook. You can see there. So there's your faults, two little faults. There's your sandstone, your coal. This is some kind of coley shale, some carbon rich shale up top and then you've got your sandstone there. And that's the sequence of events. All right, so let's go back to Steno. So Steno with his principles has given us the ability to date things, but he's also given us the ability to correlate between things. And we saw that with the principle of lateral continuity. So if I come walking along and I see this rock layer right here and I wanna know how does that rock layer refer to this one right here? The principle of lateral continuity says that they can connect across like this. And so that gives us the ability to start doing what we call stratigraphic correlation or stratigraphy. So correlating between two different rock horizons over distance. In an ideal world, we wanna correlate between a little rock outcrop we see in one spot and another one five kilometers away. Not just where you can visually look across. This becomes problematic pretty quickly. So lithology just refers to the kind of rock. So this is a sandstone. This is a limestone. You could do this. The problem, and you can see it already in this picture is that the types of rocks repeat themselves over time, right? You've had sandstones being deposited on earth for at least four billion years. At least we've got sandstones that are over four billion years old. So if you see this sandstone and some kilometers away this sandstone, do they really connect or does this one connect down to this one here? Don't know. I mean, that's an open question. So on a really local level, these help, they help us interpret the orientation and the history of one particular outcrop. But in the task of correlating over distance or building a time series that describes all of earth history, taking all the little outcrops and putting it into one long timeline, this doesn't really help us. The other problem, which is actually even more important is that the types of rocks themselves change laterally when you're talking about sediments. And you know this if you go to the beach. Let's say you go to Dominion Beach. If you haven't been to Dominion Beach, well, it's too late this year, but next summer, go there. It's very nice, it's very close. So when you go to Dominion Beach, you will see that if you walk out towards the ocean, you are in sand. If you walk back up towards the beach, you are in gravel. So you can move literally 10 meters and the type of rock changes from a sand, right? The type of sediment from a sand to a much coarser material. This is a shale here, ignore that, but a much coarser material that's got big pieces in it. So something that we would call a conglomerate if it became a rock, it's a gravel right now. So if you kept going out deeper in the water, eventually that sand would transform into mud. Transform a lot all into rock and imagine you were walking along the rocks that resulted and you would be on shale at one point, a mudstone, then you'd be on sandstone, then you'd be on a conglomerate made of gravel. So if you were trying to correlate over distance, because the environments themselves change as you move along, and because of sediments, and we'll talk about this in detail later on, the environments directly determine the types of rocks being deposited, then the lithology or the resulting rocks are going to change over distance. They're gonna change laterally with the environments. So all of this is super problematic and really limits the ability of litho-stratigraphy, stratigraphy-bilithology to help us correlate over anything beyond really close distances. So the answer to that comes up actually pretty early in geology in the 1800s and it's absolutely key to everything we do beyond. This is actually what my PhD was on, that is this application I'm gonna explain to you in a second. So this guy here, William Smith, just like Steano, he's definitely worth reading about. Multiple books have been written by both these guys. In particular, a book called The Map that Changed the World about this guy. So he was at a canal surveyor. He was an engineer who helped build canals. At the time, the Industrial Revolution, this is the late 1700s, early 1800s, the Industrial Revolution was just taking off in Britain. And there'd been this explosion of a need, a demand for coal. And there are no trains. The only way that you can bring coal into London, where all the factories are, from the coal fields, many of which are up in Northern England, is by horse and buggy. Now coal obviously weighs a lot, that's massively inefficient. The easiest way would be to float it down. But these things are inland, so you can't just take them out to the ocean and take them around. Instead, they literally built a network of water highways. These large canals, which they'd flood, they could put a barge on them, fill it with coal, have horses on either side of the land, they just dragged it along. And boom, you suddenly have functionally a train network, where before trains existed, a water train network more or less. So he's out there supervising the digging of these canals. And as he's doing it, he's getting down in the rocks and seeing the rocks exposed below the dirt that covers them up in most parts of England, all over England. And he starts to notice something. What he notices is that the fossils that occur within them, occur with a particular and predictable order. So in some strata, in some layer of rock, you always see some fossil A, and it always occurs before some fossil B, and fossil B always occurs before some fossil C, which then occurs before some fossil D, all the time, everywhere he's looking. So if you find a stack of rocks somewhere that goes A, B, and you find another stack of rocks, the lowest layer of that rocks contains fossil B, and the upper one contains fossil C, you can, in sense, take that first stack of rocks, take that second stack of rocks, and combine them together to make one single imaginary stack of rocks like this. The other thing is that if you see B, and that's the last one exposed on the surface, you can make a prediction that if you were to drill down, you should hit something of age A down here. It's gonna contain that fossil. The other thing he noticed is that sometimes these fossils would move outside of lithology. So that is, you might find this fossil species, A, occurring in both a limestone and a shale, and that allowed you then to correlate the time these things had been deposited, even if the lithology was not the same. So to get across this problem that this thing might change laterally into some different kind of rock, just like a Dominion Beach that I was explaining. So this idea, this idea we call biostratigraphy. So stratigraphy is the correlation of rocks over distance based on some characteristic. Lithostratigraphy uses the characteristics of the rock themselves. Biostratigraphy, as is implied by the biopart, uses fossils, uses biology. So this guy here, these are all trilobites. They were kind of like marine cockroaches that lived in the ocean. These things are extinct. They've been dead for about 250 million years. So in this case, this species over here, those two species, they occur at that point in time. That's when they first evolve. This species first evolves at this point in time, and these guys have been around at this point in time. So I could say I will define the beginning of the range of this thing. When this thing first evolves, that guy, I will define that as a point of time. And so if I have that thing, I know that I am at this. And I could even say, since it goes extinct here, that if I have this fossil, I know I am in that interval of time right there. So we call these things biozones. There's a bunch of different kinds of biozones. It could be the total range of things. It could be the combined, the overlapping range. It could be from the beginning of one thing's range to the beginning of another. There's a bunch of different ways of defining a biozone. And we'll talk about this, actually, not this lab, but the coming lab. We're going to talk about biostratigraphy. And you're going to have some practice. You're going to play with some fossils. This is to give you an idea about it. So right after this discovery, and this is his observation, start coming out in the early 1800s, and very soon after, suddenly we had the power to start putting rock layers. Not just saying, this is a sandstone. This is a shale. But this sandstone came before this other sandstone. And people start studying all of England, mostly England, but other areas as well, it spreads outside. And they start dividing up the earth into these really broad temporal categories that I saw over here. All these names right here, Cambrian, Ordovician, Silurian. These are these guys right over here, the periods. So we start naming all the periods throughout the 1800s. And in fact, with the exception of two new periods that have come along just recently that aren't included on here, these ones that are way back over here, all of the periods we use, as you can see, were all named really rapidly. Within about 40 years of the advent of this kind of new theoretical approach. All right, so that is how we put the broad categories of time, how we erected them first on, based on fossils, and how we order them, right? It's in a relative sequence of oldest through the youngest. So that's relative dating. Everything we've talked about to this point is relative dating, which just says we're putting a sequence of things in order from oldest to youngest, that's it. The time scales I showed you though had actual dates in terms of millions of years. So how do we do those dates in terms of millions of years? That is absolute dating. So what is absolute dating? Putting terms of millions of years on them. Okay, that's easy. How do we do it? A little bit harder, but actually pretty neat and pretty easy to understand when you go through it. So the most commonly applied principle is something called radiometric dating. So radiometric dating dates things based on the decay of radioactive isotopes and the rate at which they transform into stable, non-radioactive isotopes. So I'll take you through this. Pretty simple idea, a little bit complicated to think about at first, but pretty simple the more you think about it. So the first thing is remember, rocks are made of minerals. They're collections of minerals. This is a mineral called zircon, these teeny little guys. This is what Deanne, my colleague, she does all of her dating work on these little critters right here, on these little zircons. So these things naturally include in their crystalline structure, small amounts of radioactive material, isotopes of elements. And isotope remember is just an element that has more neutrons. And so some of them are stable, some of them are unstable. So if they're unstable, they are radioactive, which means they will break down, they will spontaneously decompose and transform into something else, usually in the process. So these will have small amounts, just naturally occurring of radioactive material within them, usually. Once they crystallize, once these things form from a magma, they then become functionally closed systems so that any of the new radiogenic material, radiogenic material, that is material which is produced as a result of decay, any radiogenic material is going to be trapped within them and it can't get out. So it's stuck inside here. So I can then look at how much of that material has been produced inside. And if I know the rate at which that material is produced, I can get an age, right? A date for those things. It's gonna tell me when did these things form first as crystals. So I'll take you through that again from just a couple of different angles and then I'll talk about some problems with this in a second. I mean, all things that make sense theoretically are usually a lot more complicated in terms of applications, right? So here's an example. This is from Green Cove, which you guys, again, it's around Ingenis, you should definitely go here. And you've seen these. This was number 36 in your sample kit. These little pink guys here. They're the things that gives the pink color to rhyolites, the pink color to many granites. This is a granite you're looking at here. So this is potassium feldspar. This is a K right there for potassium. This is potassium feldspar. And so potassium feldspar, as you might guess, has potassium in it. Some of the potassium, I had a small amount of that potassium, is potassium 40. And potassium 40 is an unstable form of potassium, which will transform over time to argon 40. To argon 40. And it's gonna do that at some given rate. Now argon is a gas, and it's gonna be trapped within that lattice. This is gonna get, you're already gonna see a problem here potentially trapped. What if some gets out, you might say to Jason. Well, that's an issue. We'll talk about it in a second. But assuming none gets out, I can look at the amount of this stuff relative to the amount of stuff it transforms into. And I assume, since this was a gas, that any that was originally present before this between the crystal would have just gone away and it's not gonna be there. Whereas anything that was produced after it became a crystal is gonna be trapped within it. And so I can just measure, what is the relative abundance of this to this? And if I know the rate at which that transformation takes place, I know the rate, that's a T. This is the worst, look at the rat. I know the rate at which the transformation takes place, then I get a time from it. So here is your zircon again. In the zircons, we are not using potassium and argon. In zircons, we are principally gonna be using uranium and it's transforming to a form of lead. So here's some terminology for you. The original isotope, that radioactive thing, we're gonna call that the parent. And the radiogenic isotope, the thing it transforms into, that is the daughter. And so we're gonna be looking at the ratio of parents to daughter right there. All right, so let's look at this graphically. Graphically actually in a second. This is showing some of the instrumentation. So this actually, this device here is a device that shoots beams of ions at, shoots beams called a shrimp, shoots beams of ions at minerals and actually ablates the mineral, blows off some of the material. And then we run that material through a mass spec and the mass spec will figure out the relative weights of the different things inside. And that will allow us to figure out the relative abundance of different materials. That's how we figure out, how we actually do the work. It's done in a laboratory somewhere. Unlike the tests that we can do in lab where we're scratching things, putting acid on them, this requires a bunch of fairly sophisticated equipment. And some of these analyses are quite expensive. Some of them are pretty cheap, kind of like a grand per sample, but you want more than one sample. Some of them, depending on how precise you want the date to be and the particular things you're dating can get really pricey. All right, so what about rates? So when we talk about rates, we're gonna talk about rates in terms of half-lives. What is a half-life? A half-life is the amount of time it takes for half of the original or parent material to transform into the daughter material. In this case, this is showing uranium and lead. So after some amount of time, x, that is one half-life, half of the material, half of this material has transformed into lead. After another half-life, right? After one more half-life, then you get half of the remaining uranium is going to transform after. So after one half-life, the ratio is going to be one to one, right? Then we are going to take half of the remaining stuff and make that be there again. So after one more half-life, the ratio is going to be one to three, right? And so on and so on and so on and so on. If you click this link, it's gonna take you, actually this is a video, you can't click that link, but if you Google Evolution 101, Radiometric Dating, something like that, it's where I got this from. It's from a really neat interpretive thing. It's mostly about evolutionary theory, about biology, but because we use these to give dates to biological systems as well, to evolution, fossil record, then that information is included in that introduction. So you can check that out. So we're gonna have that ratio changing along and you can look at it. Here it is graphically displayed. So here is uranium lead as a system again. This thing has a half-life of four and a half billion years. So after four and a half billion years, 4.5 billion years, that's the age of the earth, right? Roughly the age of the earth. So after one earth age, right up where we are today, you're gonna have half of it left. So if I get something and I look at the ratio and half of it is uranium 238 and half of it is lead 206, which is what it becomes, I know that for that to happen, four and a half billion years, four and a half giga-anims, four and a half billion years had to go. If I go in the ratio is three to one, then I know nine billion years had to go and so on and so on and so on, right? Each one of these represents another half-life and in each point, half of my original material. Now here's a problem you're gonna see. At some point, I'm gonna have none of the thing left and at that point, the clock stops working. 22 billion years is well more than the entire age of the universe. The age of the universe is something like 13 billion years. So somewhere around here is the age of the universe itself. But you can see that if you had something that had a shorter half-life, you can run into that problem pretty quick, where you have none of the material left or so little left that you're not within the range that people can actually detect it. So here's the caveats. The first one is this whole thing assumes you have a closed system. That is that in your mineral, right, as the new things were appearing within it, here's the new things appearing within it, this might be the argon or it might be the lead, that none of that escaped out. Because since we're looking at the ratio, if some of the daughter material escaped, we are going to get, we are going to get an age which is younger than the actual age of that material. The second thing we're assuming is that none of the daughter was present there originally. So we started off with just a ratio of one to zero. If we started off with some of that lead in the system already, then it's gonna look artificially older. This is gonna make it look younger, this is gonna make it look older right here. That's C, that's supposed to be O. Can't get an O there. Okay, we'll make an O right here. Make this one go away. Holder. The second thing is that this is really as a result, this is an extension really of number one, is that this actually isn't getting us necessarily the point at which the thing was created. It's gonna get to the point at which it went beyond its curry point, right? When things could escape away from it. So if you have a mineral which was heated sufficiently, the entire thing will reset back to age zero. Even though the thing obviously has an age, it will restret back to age zero. So you gotta be really aware of what the heating history, was it heated enough to allow some of that material to escape or maybe all of the material to escape and it'll reset back to zero. So you've gotta be very careful and we're gonna look for evidence of things like heating in the mineral beforehand. Although as you're gonna see in a moment, the fact that some of these things reset easily is actually useful as a technology, particularly in petroleum. And finally, you've gotta choose the right isotope. Because these things are decaying at some rate and because there's a limitation on how accurate, how accurately we can count up the ratio of these things, if you have too little of the material, so the half-life is too long for the thing you're trying to date, you won't have enough of the data produced and you can't date it. If on the other hand, it's been going too long, we pick something with too short a half-life for the particular thing we're trying to date, we're trying to date something that's a million years old and we pick something that has a half-life of 50 years, none of that material is gonna be left anymore and so you can't date it. The final major caveat is with a couple of exceptions, this dating technique does not work on sedimentary rocks, which remember are very common on the surface of the earth, not common overall, but it's what you encounter most on a day-to-day basis. So this doesn't work there at all. That's a pretty big limitation. All right, here is a table just showing some of the half-lifes of some really commonly used isotope systems. So this right here is the parent as the daughter, parent, daughter, parent, daughter, parent, daughter, et cetera. This is the half-life. So this thing has a half-life of 1.3 billion, but this thing has a half-life of 47 billion. So each one of these is gonna term in the useful age and this is the minimum amount of time, the time at which sufficient has decayed that we can use it and this is the maximum amount of time and the reason these are 4.57 actually here is that this is just taking you back to the origin of the earth. But if you were trying to date something which was older than our solar system itself, then you could use rubidium strontium. Let's say that some kind of meteorite came in from a totally different solar system. It came in from a different galaxy or something. You wanna date that or in the future like Star Trek we're flying around, we wanna date some eruption on some planet and the planet might be 15 billion years old. Then you could date that using this, but it means that any of these have a maximum useful range which is sufficient to date any earth object. On the other hand, something like carbon nitrogen over here, the radiocarbon, this thing has a maximum effective usefulness of 60,000 years. The other caveat for radiocarbon dating is that it is going to only be useful for organic materials. This does not work if you are trying to date any kind of rocks themselves. So we use these to date organic materials, not fossils, but actual organic material, bits of wood and stuff in very young sediment. So this is for archeology really. It's also for kind of glacial systems. You wanna understand that, but this is a maximum working age of only 60,000 years. So you gotta pick the right pair. If you picked, say, rubidium strontium, or even if you picked uranium lead and you try to date, say, Mount St. Helens, you're gonna get a completely erroneous date from it because it has a minimum useful age of a million years and Mount St. Helens erupted in 1985, something like that, 1980s. So you can't use that. So you gotta be really careful. You have to have some already hypothesis of how old you think something is and then you're gonna use this to try to narrow in on that. And you're not just gonna use this. You're gonna use a number of other things to double check it. All right, so here is the geologic time scale. I told you this represents 150 years of a converging group of scientists, geologists, geophysicists, chemists, all of these guys working on this together. And the reason for that is because of the limitations of all of these dating. So for example, me as a biocertographer, I'm going to tell you the date potentially in terms of relative time, in terms of relative time of some rock. So I could tell you how old this rock is in the sense of it is younger or older than or rather older or younger than some sequence of rocks. But I cannot tell you how old this thing is in a million years. On the other hand, a geocrinologist, someone who does radiometric dating, they can't tell you how old this thing is at all because it is a sedimentary rock. So you can't see anything at all here. All right, so because of that, we need to work together in teams. And we're gonna be applying a lot of these things together into a converging understanding of Earth's history. So here's one way we can do that, very simple way. So in this case, you've got a series of fossiliferous horizons. So these things cannot be dated using radiometric dating, but they can have a relative order between them. Each one of these represents a volcanic ash layer. And volcanic ash is, remember, an igneous rock. So think about the tough we were looking at in lab. But the advantage of a volcanic ash layer is that it distributes over really broad areas. When a volcano erupts, that material is going way up and it's gonna blanket really large areas on either side. And so we can get these thin layers and they can fall in the ocean, they can fall on land, they can fall everywhere. And so anywhere they fall, if you're lucky enough to get a little air of ash, you can date that. So if I wanna know how old in absolute years is this clam? I know this clam is younger than that trilobite down there because it's on top of it. I wanna know how old is it in actual years? If I have an ash here, and this is 495, and I have an ash here, which is 510 down below, I know this clam has to be older than 495, so it's at least 495, and it cannot be older than 510 because this one's 510. And I mean, I could make an assumption this is probably closer to 495 than 510 because it seems to be closer to this. That's actually a dangerous assumption to make because this all could have been eroded. There might have been 10 meters of material on top that was eroded and this got put in afterwards. We'll talk about unconformities in the lab later on. So that's a bit of an erroneous assumption, but you can make that if you wanted to. So if I keep dating this, let's say I keep dating this in 10 different places in the world, and I find that this clam is always a roughly 500 million years. Every time I get, every time I am lucky enough to find layers of ash above or below it to give it constraints on its age, I find it's always roughly 500 million years. If I do that, then I can start using that thing as a proxy, even when I do not have ash. So I can say based on the fact that I have consistently seen that this thing where I have ash available is always 500 million years. In the future, I can just start saying that fossil itself represents 500 million years by inference from past experience. So we do this, and this is how we build these timescales together, right? It's these convergent lines of evidence using fossils and absolute estimates together. And there's a lot of different tools that are used in here in addition that are quite complicated that we're not gonna get into this class. You can come and talk to me afterwards if you want. But this is the idea, is these convergent lines of evidence creating an increasing resolution of our understanding. All right, here's just a couple of extra dating methods that we use. So one of them surprisingly are trees. So you know, or you probably know, that trees grow every single year and that they leave behind rings that represent one year. So each one of these rings represents one year. That's one year, right, from year to there. That's one year from here to here. This is one year from here to here, et cetera, et cetera, et cetera. So we can date these rings. But you can also see that the rings are not of equivalent thickness as you move through them. And the reason for that is a tree will grow more in a good year than a bad year. So let's say it's a year where it's a drought. There is not enough water. It's gonna grow really slowly. And a drought doesn't happen just in three square meters. A drought happens on a region, which means all the trees in that region are gonna have a really short year. Maybe that year they only grow this much. They only grow this much here. On the other hand, if the conditions are really good, they might grow a ton in that year. And not just this tree, all the trees on average will grow a ton. And so as a result, what we'd expect is we could build a diagram that showed units of lots of growth punctuated by units of short growth. These are maybe two drought years or three drought years. And then we had a really good year. In fact, we had two really good years and then we had another drought year and then we had another really good year. And so that sequence of long, short, short, short, long, long, short, long. I can go and I can find another tree and that tree is younger than this one. And that tree just has long, short, long. And I can connect these together and say these guys at that point in time, these trees were existing at the same time. And so I can create a database that covers all the trees through time in one particular region. And then I can just get my tree, I take a sample of this guy right here, I just compare that over to this giant database I have made and say, oh, this one then was growing at this point in this giant sequence of events I've made. So we've actually done that going back quite a ways. In the Europe and in the US, we have particular databases that go back seven, 14, those sorts of thousands of years. And they are importantly regionally specific because we're talking about regional climate events, right? And they are also species specific. So they might be just pine tree, something like that. Now where you might wanna use this is in something like in glaciology sometimes, but in particularly like let's say in the West Coast around Vancouver, you get tsunamis, massive tidal waves that come in as a result of seismic activity, earthquakes. So when a tsunami comes in, it will bring saltwater inland, which inundates a forest, it can kill the forest. So if I wanna know when did that happen? I wanna know exactly when did those trees die? And that's an aspect where I could use this to try to figure out, because if they died 500 years ago, none of the other methodologies I have are really gonna provide sufficient information. The other thing is this is organic material. So I can't actually use a lot of the same things I would use, I can't use, stratigraphy, et cetera. All right, here's another one. This is actually, this is a colleague of mine named John Goss. He was actually on my PhD supervisory committee. Super nice guy. If any of you guys end up taking your engineering degree down to Dow or you end up in Halifax and you interact with this guy, really nice guy, really smart guy. So he's interested in surface process. He wanna know when did this landscape like this with all these curves and stuff, when did that happen? In particular, this happened because a mass of glacier used to cover all of this stuff during the ice age. It wasn't known when did the glaciers arrive? When did they leave? So you're interested not in how old the rocks are, but rather when were the rocks on the surface? And this is another absolute method and it uses exactly the same idea as radiometric dating but in a slightly different way. So these things are constantly getting bombarded by energy from outer space in the form of particles flying out at neutrons and other things. So these are all zipping in and they are hitting and penetrating just the surface area of a rock. And as they do that, they smash into the atoms and they can actually knock some of the protons out and transform them into different elements entirely. So these are radio, these are what we call cosmogenic nucleotides. So these are isotopes which were formed by radiation from space, literally smashing in particles from space, smashing elements and transforming them into new things. Some of those elements are gonna be radioactively unstable, I mean radioactive. And so they're gonna start transforming. So we can do two things. We can look at the amount, the total amount of these cosmogenic nucleotides to tell you how long this thing was on the surface. But if we already get that rock and bury that rock underground after it had already been on surface for a while, these rays can't make it all the ways down. And so as a result, they are now no longer building up new materials, but the things they already had are gonna be decomposing. So now I can go just like with standard radiometric dating. I can look at the ratio of these cosmogenic nucleotides to their daughters and that'll tell you how long I go to this rock and bury. So I can tell how long go did a rock get brought to the surface and then if it eventually got buried, how long it was the burial. And this is super important for figuring things like when were giant ice sheets on the land? Or what was the rate of deposition of sediment in a particular area, which has some application to petroleum? What else have we got? This is a thing called fission track dating. Fission track dating is an extension really of radiometric dating, which just says this. Remember, radiometric dating was the spontaneous breakdown of radioactive isotopes into some more stable isotope. So when that happens, they release energy and they release particles. Those particles will go flying through a crystal and smash their way through the crystal. Just imagine that somebody has a gun and they just start shooting inside a hotel room, right? They're just shooting bullets everywhere. Those bullets are actually gonna make holes in the walls. So if I wanna know how many shots were fired, I can go and I can actually just count up the holes in the wall. And that's what we're doing. So what we do is we take a mineral, we cut it down to a thin section, we etch it with acid, which just take these little scratches and they make them much larger. And then we literally visually count them. We just go through, there's no computer, it's just a human being goes one, two, three and we count them all up. And because these things are a record of each atom that decayed, right? The more of these things there are, the longer this thing has been being exposed to radiation, which is a proxy for how long that material has been decomposing. Now there's a caveat to this that's really strong, which is that these holes, which are damages to the lattice, they heal themselves if you heat this thing even just a little bit. So this form, which is called fission track dating, literally this is the track, right? And it's formed as a result of the fission of some atoms within here, right? This form here is really sensitive to any heating afterwards. But that's actually super useful for petroleum, because let's say I'm interested in not exactly how old a rock was, but I wanna know at what time did a rock get heated to a particular point or cooled down from a particular point? That's gonna allow me to figure out, for example, we're gonna talk about this later on the oil window, the temperature which oil forms is actually quite low in the sense of geologic temperatures. So if I wanna know at what point did this thing start being, say, 100 degrees centigrade? Pretty low, right? That's the kind of temperatures that these things will start collecting data in the form of fission tracks. This thing will reset depending on the particular, each mineral is different. But let's say the mineral appetite, I think the mineral appetite resets at something like 125 C, quite low temperatures, geologically. And so if I wanna know when did this thing cool down below 125 C, right? I could choose appetite and I could start counting up all of these little things. It's not gonna tell you when that mineral of appetite formed, but it will tell you exactly when this thing fell down below a particular temperature. So there's fission track dating. So we can use these to date the actual mineral itself, but they're often used to date heating events that happen afterwards. Sometimes in fact, you get something like zircon, which is quite resistant to heating in terms of things like uranium and lead in it. And we can use the uranium and lead to figure out exactly how old the mineral is. And then you can use fission track dating to figure out what the heating history after the formation of that mineral was like. So these things are sometimes used in concert as well. So here's one final guide to just end off with here. So remember we talked about as a line of evidence for plate tectonics, we talked about the fact that the Earth as it spreads, Earth as it actually spreads open along spreading ridges, that new magma is formed and that new magma is going to take on whatever the signature of the Earth's magnetic field at that point in time is. So just like with the tree ring dating, since the time that the Earth is oriented, the Earth's magnetic field is oriented in some particular thing, normal or reversed, and is almost totally arbitrary, just like the ordering of droughts versus really good growing years, we're gonna get different links of time associated with normal orientation versus reversed orientation, normal, reverse, normal, reverse, et cetera. And we can do the same thing where I take this and I compare it to another spot and I match up this pattern of long, short, short, long, short, et cetera, which is going to be the same in all sections. We're only gonna capture a little bit in some one section. I can put these things together and start making a giant system by comparing this little bit here to this little bit here somewhere else. I can start putting these together in one giant system. I can then just take a little sample from somewhere, go compare it to my giant system and go, oh, that matches up right here. If I've also dated some of these rocks, because this is only giving me relative dating, remember, this is only relative, but if I've also radiometrically dated some of these rocks, and I say this one's about 4.47, if I find a rock somewhere that has the same pattern of long, short, short, medium, medium, medium, short, it matches this pattern, I can go, that's about four million years old. So this is magneto-certigraphy, which is, again, a relative system, but I can provide just like the fossils, I can provide absolute dates to it by also radiometrically dating either these rocks themselves or adjacent igneous rocks, if these ones are sedimentary rocks. And this is a technique I can use both, not just here, but I can also use these in sedimentary rocks, because sedimentary rocks often have little particles of magnetite, and those little particles of magnetite will also record the magnetic signature. So this is actually super useful for dating sedimentary rocks, especially in the absence of fossils. So here is the time scale, again, oh, this, by the way, that thing you see on the side, those are that barcode. This is the magnetic stuff combined with this is the radiometric stuff, and this is the fossil-y stuff over here. All of this stuff is presented kind of together, and this whole thing is a result of convergence of all of these things here. Anyways, the big take-home from this is we're gonna use all of these methods in concert to understand the global history of the earth. So I wanna know how old is the earth? How have the systems changed over time? What is the rate of evolution been? When did continents break up? When did they come together? All of those things, that's big human history kind of stuff, just understanding what is our role in the universe? What is the planet we lived in like, et cetera? But if I also wanna know something, if I wanna know very local problems, I wanna know something like a particular layer of rock, when did it get heated to a particular temperature that's, well, it could be produced? When did it get heated to a certain temperature that would kill off bacteria that might break down any wheels that accumulated in it? Or how do I connect through space one unit of sandstone to another unit of sandstone? How do I correlate? Are they the same age and therefore the same unit? Or are they very different ages and therefore different units, even though they look similar? Those very local and very specific questions are also answered using the same methodology we use to answer these big, broad questions about Earth history. All right, that's the end of your flash introduction to geologic time and stratigraphy. In lab this week, we are gonna be going through and applying all of these methods. So make sure you watch or re-watch this video. If you don't understand anything, make sure you send me an email or make sure you just Google any of the terms that are in here. Read your textbook as well, it goes through all of this stuff. If you want additional readings, additional resources or something you're struggling with or something you're really interested in, ask me and I will tell you. But otherwise, that is the end of the class for today. You remember that next week's class is also gonna be online, it's gonna be a video. I will provide the link to that video on Moodle. If for some reason you can't see the Moodle page, some of you still don't have access, email me directly and I will provide the link to you. But otherwise, I will see you next week.