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Rocks and Rules - Putting Things in Order · PlanetGeo: The Geology Podcast

• 0:03
  SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:14
  SPEAKER_00: Okay, freaking finally.
• 0:16
  SPEAKER_01: How you doing?
• 0:18
  SPEAKER_01: How you doing, Dr.
• 0:20
  SPEAKER_01: Jesse Rymink?
• 0:21
  SPEAKER_01: How are you?
• 0:22
  SPEAKER_00: I'm great, Chris.
• 0:23
  SPEAKER_00: I mean, now that we finally hit record here, 45 freaking minutes later, you've been down the rabbit hole.
• 0:28
  SPEAKER_00: You've been messing around with your microphone.
• 0:30
  SPEAKER_00: We've been arguing about the title.
• 0:32
  SPEAKER_00: It's all very much.
• 0:33
  SPEAKER_01: Oh my gosh, coming up with the title for this episode.
• 0:35
  SPEAKER_01: Yeah, this was a this was a chore.
• 0:37
  SPEAKER_01: Everybody needs to know this.
• 0:39
  SPEAKER_01: We came up with a whole page full of possibilities.
• 0:42
  SPEAKER_01: Three-quarters of the page were Jesse's horrible ideas.
• 0:46
  SPEAKER_00: Um we rarely agree on a title, let's put it that way.
• 0:51
  SPEAKER_00: We came to some consensus here, though.
• 0:53
  SPEAKER_00: Uh so you know what?
• 0:55
  SPEAKER_01: The last two episodes though, we've actually had some agreement.
• 0:57
  SPEAKER_00: Um It's true.
• 0:58
  SPEAKER_00: It's true.
• 0:59
  SPEAKER_00: We we're on the same page, Chris.
• 1:00
  SPEAKER_00: We're operating on the same wavelength for a while here.
• 1:04
  SPEAKER_00: Yeah, not quite, but yeah, no, that's right.
• 1:06
  SPEAKER_00: So, okay, as the title implies, that we agreed upon.
• 1:10
  SPEAKER_00: The title that we both agree upon.
• 1:11
  SPEAKER_00: We're putting things in order today.
• 1:13
  SPEAKER_00: And this is uh this, I don't know, this is uh how long do you spend you're not even gonna say the title?
• 1:19
  SPEAKER_01: Jesse, it's called Rocks and Rules, putting things in order.
• 1:22
  SPEAKER_01: That's our title.
• 1:23
  SPEAKER_00: There we go.
• 1:24
  SPEAKER_00: So, how long do you spend lecturing in your class about the stuff we're gonna cover today?
• 1:32
  SPEAKER_00: Just out of curiosity.
• 1:34
  SPEAKER_01: Not a long time, maybe 15 to 20 minutes with the things that we're gonna do today.
• 1:39
  SPEAKER_01: So not even a full lecture.
• 1:41
  SPEAKER_01: No, I don't think so.
• 1:42
  SPEAKER_01: Uh what I would do is I would go through the things we're gonna do today.
• 1:45
  SPEAKER_01: I don't know though, you know, that's hard to say because you know how I am.
• 1:48
  SPEAKER_01: I get a little off topic because I try to make this as visual as I can.
• 1:52
  SPEAKER_01: Yeah, I do I make it as visual as I can, so I'm showing pictures of the principles that we're gonna talk about, and then when I show a picture of it's a place I've been, I probably took the picture, and so I've got things to say about that too.
• 2:05
  SPEAKER_01: Sure.
• 2:05
  SPEAKER_01: You know, so maybe a little bit longer.
• 2:07
  SPEAKER_00: And that's kind of what we're gonna do today.
• 2:09
  SPEAKER_00: These are the basic, you know, five slash six basic principles of a part of geology, I would say like half of geology.
• 2:20
  SPEAKER_00: And these are principles that I think on the surface are very simple.
• 2:25
  SPEAKER_00: They are actually are really simple ideas, really simple concepts, but for me at least, they're the kind of thing that you don't come about them naturally.
• 2:34
  SPEAKER_00: Like it's not natural to just think, oh, that makes sense.
• 2:36
  SPEAKER_00: You don't come up with this on your own, but once it's pointed out to you, it's very obvious and it makes complete sense, right?
• 2:41
  SPEAKER_00: I mean, these all these five or six principles are fairly straightforward, but it's hard to come up with on your own, I think.
• 2:49
  SPEAKER_01: Yeah, I think you're right.
• 2:50
  SPEAKER_01: But once you get a hold of it and you think through these processes, then then it becomes like putting together a puzzle almost.
• 2:57
  SPEAKER_01: You're using these to like just sequence things out, and that's what we're doing today.
• 3:02
  SPEAKER_01: This is about sequencing rocks and events in their proper order using basic, basic principles.
• 3:09
  SPEAKER_00: That's exactly right.
• 3:10
  SPEAKER_00: And we're gonna cover five principles here: superposition, horizontality, cross-cutting relations, inclusions, and correlation slash continuity.
• 3:19
  SPEAKER_00: And these five principles were derived back in the 1800s, really early 1800s, well before we had any of the modern chemical or chronological tools that we have today, like my lab did not exist, being able to date rocks did not really exist.
• 3:36
  SPEAKER_00: So these principles are all based on simple observations in the relationships of rocks to one another, and most of them focus on sedimentary rocks and the sedimentary rock record.
• 3:45
  SPEAKER_01: When we do this kind of like sequencing, we're not concerned with putting a number on things, as in this happened so long ago.
• 3:54
  SPEAKER_01: We're just putting things in their proper order.
• 3:56
  SPEAKER_01: This happened first, second, third, and so on to get us to the end of the page.
• 4:02
  SPEAKER_01: This is the last event that took place.
• 4:04
  SPEAKER_00: Yeah, and this is what's called relative dating because we're just putting things in relative order.
• 4:09
  SPEAKER_00: It's, you know, I got in my car, I drove to the gas station, I got a flat tire in that order.
• 4:14
  SPEAKER_00: It's not, oh, I got gas at noon on Wednesday last week.
• 4:18
  SPEAKER_00: So these are relatively simple rules, but they're really valuable.
• 4:21
  SPEAKER_00: They're valuable to practicing geoscientists out in the field.
• 4:23
  SPEAKER_00: They're also valuable to just know if you like going outside and you see rocks, you're on a hike.
• 4:28
  SPEAKER_00: It's valuable to know these things and be able to recognize them in the field because I find it adds a you know a decent amount of appreciation when I go hiking with non-geologists, they're always asking about the rocks, and invariably we end up talking about one of these five principles at some point on even like a one-mile hike through a little park in the city.
• 4:46
  SPEAKER_00: They're everywhere, so it's important to know these things.
• 4:48
  SPEAKER_00: So, Chris, superposition.
• 4:50
  SPEAKER_00: Do you start there in class?
• 4:52
  SPEAKER_01: I do.
• 4:52
  SPEAKER_01: I I think it's a good place to start because it's probably the easiest of all these things we're gonna talk about today.
• 4:58
  SPEAKER_01: It's called the principle of superposition, or do you call it the law of superposition, or is it?
• 5:02
  SPEAKER_00: I call them all principles, but I don't whatever.
• 5:05
  SPEAKER_01: Okay.
• 5:06
  SPEAKER_01: So yeah, we're gonna start with the principle of superposition, which all this says is that in a vertical stack of sedimentary rocks, the oldest layer is on the bottom and the youngest one is on top.
• 5:18
  SPEAKER_01: And everything else follows in that proper sequence.
• 5:21
  SPEAKER_01: It's kind of like you know, taking loose leaf paper and making a stack of the paper on the table.
• 5:26
  SPEAKER_01: You take the first one off and you put it down on the table.
• 5:29
  SPEAKER_01: And then you keep putting papers on top, on top, on top.
• 5:32
  SPEAKER_01: And then the last one that you put down is the one that's on the very top of the stack.
• 5:37
  SPEAKER_01: That's the principle of superposition because sedimentary rocks are laid down this way.
• 5:42
  SPEAKER_00: So, Chris, these principles, at least my experience with these principles, is most easily represented in mostly the southwest US or the western United States, where there's a load of sedimentary rocks everywhere.
• 5:53
  SPEAKER_00: There's you can see this in Michigan too, but as we talked about in our listener question episode about Michigan Basin, you know, there's not a lot of rocks exposed.
• 6:00
  SPEAKER_00: But where are some good places that you have seen superposition, or where it's like dead obvious?
• 6:06
  SPEAKER_00: You know, this is a relatively simple principle, but where's it obvious?
• 6:09
  SPEAKER_00: Can we paint a picture?
• 6:10
  SPEAKER_01: Yeah, in one of our earlier episodes, we talked about the Badlands, Badlands National Park in South Dakota.
• 6:15
  SPEAKER_01: It's a great place to see the principle of superposition because you can see these vertical layers are stacked right up on top of each other, and you have all these paleosols, these ancient soil layers that are colored red.
• 6:28
  SPEAKER_01: And so it's very easy to discern bottom layers from the top layers, and it just works.
• 6:33
  SPEAKER_01: The oldest layers are on the bottom and the youngest layers are on top.
• 6:36
  SPEAKER_00: Yeah, that's that's a very clear one.
• 6:38
  SPEAKER_00: I mean, that was the place I first saw this principle for the most part.
• 6:42
  SPEAKER_00: And you can use this principle in a lot of different settings too.
• 6:45
  SPEAKER_00: If you're digging down into the soil around your house, or if you're in a stream valley and you see one of those cut banks from a stream, you know, where the stream is cutting into the edge, uh, it's turning like to the left, and the right side of the stream bank will be cut through.
• 7:00
  SPEAKER_00: You can see different soil layers in there.
• 7:02
  SPEAKER_00: The oldest is usually on the bottom there.
• 7:04
  SPEAKER_00: The oldest is on the bottom, and those get stacked up over time.
• 7:07
  SPEAKER_01: Yeah, and you said too that these principles largely apply to sedimentary rocks.
• 7:12
  SPEAKER_01: This can apply to igneous rocks too, if you have lava flows, lava flow on top of flow, on top of flow, like you get in the Columbia River flood basalts or some of the lava flows in Hawaii.
• 7:23
  SPEAKER_01: That works also.
• 7:25
  SPEAKER_01: Superposition, oldest on bottom, youngest on top.
• 7:28
  SPEAKER_01: Pretty basic.
• 7:29
  SPEAKER_00: Yeah, very basic.
• 7:30
  SPEAKER_00: And you need to be a little bit careful with this at times because there are certain tectonic regimes that can actually overturn a package of sedimentary rocks, they can flip it upside down.
• 7:40
  SPEAKER_00: You imagine a big fold.
• 7:41
  SPEAKER_00: If you take that sheet of paper, that pile of paper you were talking about, you smash it together, the middle of the paper will go up, the stack of paper will kind of bend up, and then it'll get a bit heavy and it'll kind of flop over on the side.
• 7:52
  SPEAKER_00: Now, the bottom side of that, the rocks are upside down.
• 7:55
  SPEAKER_00: That sequence of paper is upside down now.
• 7:58
  SPEAKER_00: And we can see this.
• 7:59
  SPEAKER_00: So actually, when you're working in areas that have rocks that are really deformed and folded and faulted, you always need to be able to say which way's up because that will tell you which one is the oldest, and you know, which ways up points in the younger direction.
• 8:11
  SPEAKER_00: So determining that is important.
• 8:13
  SPEAKER_01: And sometimes faulting can do the same thing.
• 8:16
  SPEAKER_01: Faulting can displace rocks and put younger rocks on top of older rocks, but you can't use this alone.
• 8:23
  SPEAKER_01: You have to use other things in conjunction with superposition to put the whole area in order.
• 8:29
  SPEAKER_01: So let's move on.
• 8:30
  SPEAKER_01: Let's go to number two, Jesse.
• 8:32
  SPEAKER_01: What do we got?
• 8:33
  SPEAKER_00: Uh, this is the principle of horizontality or the principle of original horizontality.
• 8:37
  SPEAKER_00: And this just states that sedimentary rocks were deposited horizontally.
• 8:41
  SPEAKER_00: You don't deposit sediments on their sides, basically, is all it says.
• 8:46
  SPEAKER_00: Like sediments are laid down in a flat manner.
• 8:49
  SPEAKER_00: So they're stacked up vertically and they're layered just like your paper sheet.
• 8:53
  SPEAKER_00: You take your paper sheets and you're stacking them on a flat table, they're laid down flat.
• 8:58
  SPEAKER_00: You're not stacking them on some ramp where the top sheets would slide off, right?
• 9:01
  SPEAKER_00: Sediments are laid down flat, and the same goes for basalt flows as well.
• 9:06
  SPEAKER_00: So these things are originally flat, and if you see them tilted, that tilting came later.
• 9:12
  SPEAKER_00: And so I see this every week for the last year and a half.
• 9:16
  SPEAKER_00: I've been driving back and forth between State College, Pennsylvania, and eastern Pennsylvania, and there's this road on Highway 322 in Pennsylvania.
• 9:23
  SPEAKER_00: There's an exit called Arch Rock Road exit, and it's called Arch Rock Road because there's a big road cut, it's like a hundred feet high road cut, and there are stacked sedimentary rocks in there.
• 9:33
  SPEAKER_00: So you can see little tiny like coal seams and shales and limestones, very finely layered, and there's this huge arch in it.
• 9:41
  SPEAKER_00: And so these sediments are bent in a big arch.
• 9:43
  SPEAKER_00: You could actually see two folds in the road cut.
• 9:46
  SPEAKER_00: It's really pretty, really beautiful.
• 9:48
  SPEAKER_00: But this original horizontality principle tells me those things were actually flat, and the folding came afterwards.
• 9:55
  SPEAKER_00: They were folded after they were deposited because they had to be deposited in flat layers.
• 10:00
  SPEAKER_01: So original horizontality, this principle is not about placing rocks in their proper order.
• 10:06
  SPEAKER_01: You use superposition for that.
• 10:08
  SPEAKER_01: This is about putting an event in its proper order.
• 10:11
  SPEAKER_01: It was deposition first and then folding, and the folding or tilting is a tectonic event.
• 10:19
  SPEAKER_00: We talked about this in the answer to our listener questions uh a couple weeks ago when we were talking about these rainbow hills in Peru, talked about this beautiful banded sediment, uh really brightly colored banded sediment, but it's tilted and it's tilted so that the sediment layers dip down into the mountainside.
• 10:35
  SPEAKER_00: That's not original.
• 10:36
  SPEAKER_00: The sediment was deposited, it was turned into a rock, and then it was tilted on edge like that.
• 10:41
  SPEAKER_00: So it puts these tectonic events in order.
• 10:43
  SPEAKER_00: So, Chris, that brings us again.
• 10:45
  SPEAKER_00: We always have to use these things together because usually we will be able to leverage a couple of these laws or principles against one another or together.
• 10:53
  SPEAKER_00: So this brings us to cross-cutting relations, which uh to me is perhaps the most valuable out there, at least in my you know, functional field work days, is like this is the one I use probably the most.
• 11:05
  SPEAKER_01: Yeah, I agree.
• 11:06
  SPEAKER_01: So cross-cutting relations can mean a lot of things.
• 11:10
  SPEAKER_01: It can mean if you have a sequence of rocks, a vertical sequence of rocks that has a fault cutting across it, the rocks had to be there first, and then the fault cut across it.
• 11:20
  SPEAKER_01: So cross-cutting relations says that if there is an intrusion, like an igneous intrusion, a fault that happened, the rocks had to be there first.
• 11:31
  SPEAKER_01: Then the intrusion or the faulting took place.
• 11:34
  SPEAKER_01: So it's again putting an event in its proper place, in its proper order.
• 11:39
  SPEAKER_01: So, Jesse, where have you seen cross-cutting relations?
• 11:42
  SPEAKER_01: Where's the first time you saw it?
• 11:43
  SPEAKER_00: Well, it's near and dear to my heart because you taught me cross-cutting relations on this beach.
• 11:49
  SPEAKER_00: I think it was like a beach, if I'm remembering correctly, up in the upper peninsula, Michigan.
• 11:54
  SPEAKER_00: And I just I don't know how long this exercise is supposed to be.
• 11:57
  SPEAKER_00: Like, how long is it typically for you?
• 12:00
  SPEAKER_01: It depends.
• 12:01
  SPEAKER_01: It depends upon the engagement of the class.
• 12:04
  SPEAKER_01: You know, I mean, you can spend an hour there.
• 12:07
  SPEAKER_01: I think we probably spent three or four hours there because it's extremely complicated.
• 12:11
  SPEAKER_00: As far as I remember, it was like kind of a cold, dreary, rainy day, and it is kind of that slight drizzle overcast, kind of cold.
• 12:19
  SPEAKER_00: But man, was I interested in this outcrop because it's basically this huge broad swath of mostly basaltic rocks, if I remember correctly, or at least Mafic rocks, with some intermediate and felsic rocks.
• 12:31
  SPEAKER_00: So you have this dark, like small peninsula that goes out into the Lake Superior, and then there's all these cross-cutting veins going across it.
• 12:40
  SPEAKER_00: Veins slash dikes.
• 12:41
  SPEAKER_00: They're you know, maybe 10 centimeters wide at most, and then down to millimeters wide, and they're all different colors and they all cross-cut each other.
• 12:49
  SPEAKER_00: And so you basically say, Okay, all right, kids, work out what came first, put these things in order.
• 12:54
  SPEAKER_00: There's however many generations of veins and dikes going through here, put them in order.
• 12:59
  SPEAKER_00: And that was the exercise.
• 13:00
  SPEAKER_00: And man, it's just like putting a puzzle together because obviously the dark rock is the one that is being cut, so it had to be there first.
• 13:08
  SPEAKER_00: Like the background dark rock was there first, and then you're going through and you're saying, okay, which one?
• 13:13
  SPEAKER_00: There's a white vein here, a small white vein, and that cuts a bigger gray vein.
• 13:20
  SPEAKER_00: So the white vein has to be younger than the gray vein, but then the gray vein has to be younger than the black rock that is the most of the outcrop.
• 13:28
  SPEAKER_00: And then you kind of have to work through this because not every vein cuts every other generation of vein, so you gotta like put them in relative order.
• 13:34
  SPEAKER_00: Oh, it's a great puzzle.
• 13:35
  SPEAKER_00: That that is just a great exercise.
• 13:38
  SPEAKER_01: Because you said it, I mean, you have to identify all of the different like chemical compositions of the veins that are cutting across the black rock, and then work out the the sequence of what veins are older and which one is the last one, and then put everything else in its place in between.
• 13:55
  SPEAKER_01: So it's really complicated if you dig down like the way you did, you know, if you dig into it and and really analyze all right, what's this vein here?
• 14:03
  SPEAKER_01: What's it made of?
• 14:04
  SPEAKER_01: What's this vein made of, and so on.
• 14:06
  SPEAKER_01: So it's really cool.
• 14:07
  SPEAKER_01: Cross-cutting relations, like you said, is really, really a valuable tool in the field.
• 14:11
  SPEAKER_00: Yeah, it's extremely valuable.
• 14:13
  SPEAKER_00: And I use this all the time in field work, you know.
• 14:15
  SPEAKER_00: It is the one we lean on the most because we're dealing with not necessarily sedimentary rocks, we're dealing with like very deformed, ancient 3 billion, 4 billion year old rocks that have seen a lot of alteration and deformation.
• 14:28
  SPEAKER_00: Usually when I'm going out sampling, it's this really complicated outcrop, like you taught me, uh the principal on it back when I was in high school.
• 14:35
  SPEAKER_00: Very complicated outcrop, and I'm interested in sampling the oldest one.
• 14:39
  SPEAKER_00: I can't bring a sample of every rock back, so I want the oldest one or the oldest couple.
• 14:43
  SPEAKER_00: So you have to work through this process to decipher which one's older, which one has the potential to be the oldest rock in this given package of rocks.
• 14:50
  SPEAKER_01: That's a cool application.
• 14:51
  SPEAKER_01: Real quick, continuing to cross-cutting relations, Paradise Valley, which uh earlier in the summer was just devastated by these floods that started in Yellowstone.
• 15:01
  SPEAKER_01: Paradise Valley runs from Gardner, Montana up through Livingston, Montana.
• 15:06
  SPEAKER_01: And there are just these gorgeous basaltic dikes that cut across sedimentary rocks, and it's like it's a great example of a lot of geology because most of the students haven't seen sedimentary rocks that are that folded before.
• 15:23
  SPEAKER_01: I mean, these things are vertically up, I mean, they're straight up and down, and they're cut across by a basaltic dike.
• 15:30
  SPEAKER_01: So you have superposition, original horizontality, cross-cutting all going on in a field of view that you can all see at the same time.
• 15:38
  SPEAKER_01: Pretty cool stuff.
• 15:40
  SPEAKER_01: And then also the Grand Tetons.
• 15:41
  SPEAKER_01: We've talked about the Tetons earlier in an episode.
• 15:43
  SPEAKER_01: Uh, we did the geology of the Tetons and that dye-base dike that cuts across Mount Moran, and then there are all kinds of other, you know, dye-based dykes that are smaller that cut across other peaks in the Tetons, cross-cutting relations.
• 15:58
  SPEAKER_01: That dye base dike is younger than the peaks that it cuts across.
• 16:03
  SPEAKER_01: So yeah, it's just a it's a fun one to apply.
• 16:06
  SPEAKER_01: It's it's really cool when you first see this one in the field.
• 16:09
  SPEAKER_01: You remember that.
• 16:10
  SPEAKER_00: That's a great point, Chris.
• 16:11
  SPEAKER_00: You can see this at a variety of scales in the field, like a big dike cutting across an entire mountain in the Tetons, and then also these tiny little veins that we were mapping when I was in high school in your exercise.
• 16:24
  SPEAKER_00: You can see this cross-cutting relations at a wide variety of scales.
• 16:27
  SPEAKER_00: That same variety of scales thing can be applied to principle number four, which is the principle of inclusions.
• 16:33
  SPEAKER_00: And this, much like cross-cutting relations, tells us of two events which one came first.
• 16:39
  SPEAKER_00: And the principle of inclusions states that if a rock is included in another rock, it is older.
• 16:47
  SPEAKER_00: And the way I always describe this, and Chris, you usually have better analogies than I do, so you know, give a better one if mine's not that good.
• 16:55
  SPEAKER_00: The way I always describe it is used in baking.
• 16:58
  SPEAKER_00: Like if you're gonna make a chocolate chip cookie, you had to have chocolate chips.
• 17:02
  SPEAKER_00: The chocolate chips had to be made before the cookie.
• 17:05
  SPEAKER_00: And that same thing kind of applies to rocks.
• 17:08
  SPEAKER_00: Like if you have well, in my lab, we do a lot of dating of certain minerals called zircons, and we find some of these in sedimentary rocks, but the zircons are in the sediments, and therefore the zircons were formed before the sediments, if that makes sense.
• 17:21
  SPEAKER_00: So it's kind of the chocolate chips and the cookie there away.
• 17:24
  SPEAKER_00: Is that a No?
• 17:25
  SPEAKER_01: I actually I like the chocolate chip analogy.
• 17:27
  SPEAKER_01: It works.
• 17:28
  SPEAKER_01: The chocolate chips are the inclusions, so they have to be older than the cookie because you can't have the cookie and then just you know, smash in a bunch of chocolate chips, it doesn't work that way.
• 17:37
  SPEAKER_01: Uh, that scenario would never work in geology.
• 17:40
  SPEAKER_01: So we see this a lot with igneous rocks, these intrusions, okay, that break off some of the rock that they're intruding.
• 17:50
  SPEAKER_01: The magma doesn't have enough heat to melt the rock that it broke off, so it just becomes included in it.
• 17:56
  SPEAKER_01: So then that different rock is the inclusion.
• 18:00
  SPEAKER_01: It's older than the magma that caused the whole thing in the first place.
• 18:06
  SPEAKER_00: This one I think students find a little bit more complicated to interpret at times because you know it's not as obvious.
• 18:13
  SPEAKER_00: Like I've seen this in the field school I used to teach when I was a PhD student at University of Alberta.
• 18:17
  SPEAKER_00: We go out to British Columbia.
• 18:18
  SPEAKER_00: This is beautiful granite, beautiful white granite.
• 18:21
  SPEAKER_00: It's uh formed by melting of sediments, so it's this lower temperature granite, and it has these big pieces of black amphibolite in it, what we call rafts of amphibolite in it.
• 18:32
  SPEAKER_00: And it does not look obvious what's going on because this amphibolite's like layered in there and the granite's not layered.
• 18:38
  SPEAKER_00: So I found students, you know, usually can kind of get a little bit confused there when you're actually looking at the rock because the structures can be misleading.
• 18:46
  SPEAKER_00: You see this layered thing inside of this other rock.
• 18:48
  SPEAKER_00: It can get a little bit sort of confusing in that way, but you have to sort of step back, look at it, be like, okay, what is included in what?
• 18:56
  SPEAKER_00: Which one surrounds the other one?
• 18:58
  SPEAKER_00: The one that surrounds the other one is almost always younger.
• 19:00
  SPEAKER_00: And so the granite is younger, the amphibolite was older and was around before the granite formed.
• 19:06
  SPEAKER_00: I always just found this kind of got confusing.
• 19:08
  SPEAKER_00: Do you find this as well, Chris, or or not really?
• 19:11
  SPEAKER_01: I do.
• 19:11
  SPEAKER_01: I do.
• 19:12
  SPEAKER_01: So here's an example that we come across in the field that you can predict.
• 19:16
  SPEAKER_01: Okay, hey, we're gonna see the law of inclusions here, and it's in Glacier National Park.
• 19:21
  SPEAKER_01: There's a great rock that I absolutely love.
• 19:24
  SPEAKER_01: It's called a mud chip breccia.
• 19:26
  SPEAKER_01: The chips of mud are bright red and they're embedded in what is now a quartzite, but it used to be just a quartzose sandstone.
• 19:34
  SPEAKER_01: And so the bright red chips of mud, those are the inclusions.
• 19:39
  SPEAKER_01: They're older than the sandstone, which is now quartzite, right?
• 19:44
  SPEAKER_01: And so you can see that grade.
• 19:45
  SPEAKER_01: And the way this happens is when mud dries out, it cracks and it kind of peels up like potato chips, it curls up like potato chips.
• 19:56
  SPEAKER_01: Well, then imagine a storm comes ripping down out of the mountains across these chips that have curled up, they rip them up, and then it comes to rest when the water slows down but lays down this quartz sandstone, it has these bright red chips of mud in it.
• 20:12
  SPEAKER_01: And so those are the inclusions.
• 20:15
  SPEAKER_01: Those are older than the rock that they're included in.
• 20:19
  SPEAKER_01: So it's just a great example, but you're right.
• 20:22
  SPEAKER_01: This one can be harder for students to like recognize right off the bat.
• 20:28
  SPEAKER_00: And you know, this is you can see this everywhere.
• 20:30
  SPEAKER_00: We we've been talking about going out in the mountains and stuff.
• 20:32
  SPEAKER_00: You can see this in city buildings a lot.
• 20:34
  SPEAKER_00: You go walk into the city wherever you're living, and you go past the building stones.
• 20:38
  SPEAKER_00: A lot of the granite or even the sandstone that is used to make the building, a lot of those will have inclusions in them.
• 20:43
  SPEAKER_00: And so you can kind of work through this inclusion and cross-cutting relations principles just by looking at buildings in a city.
• 20:49
  SPEAKER_00: It's it's kind of fun.
• 20:50
  SPEAKER_01: Or go out and look at concrete, go look at pavement, okay?
• 20:54
  SPEAKER_01: And the pavement has gravel in it.
• 20:56
  SPEAKER_01: The gravel are those are pieces of rock, right?
• 21:00
  SPEAKER_01: And they're included in the concrete, and they're older than the concrete themselves.
• 21:05
  SPEAKER_01: So yeah.
• 21:06
  SPEAKER_00: Absolutely.
• 21:07
  SPEAKER_00: So, Chris, that brings us to number five correlation or continuity.
• 21:11
  SPEAKER_00: Um, this again, for me, best represented in the Western US, but it's relatively simple.
• 21:17
  SPEAKER_00: Like a sequence of rocks, if a sequence of rocks forms in one side of a valley, you could correlate that across to the other side of the valley.
• 21:24
  SPEAKER_00: Think of the Grand Canyon.
• 21:26
  SPEAKER_00: I mean, the rocks on one side have a certain sequence, the rocks on the other have the same sequence.
• 21:30
  SPEAKER_00: There used to be rocks in between.
• 21:32
  SPEAKER_00: I guess that's kind of the way they didn't form with this valley.
• 21:35
  SPEAKER_00: There used to be rocks in between, they used to be one continuous correlated unit.
• 21:39
  SPEAKER_01: Most sedimentary rocks are laid down laterally continuous.
• 21:43
  SPEAKER_01: In other words, think of it as this really broad blanket that's distributed across a relatively flat area.
• 21:50
  SPEAKER_01: That's lateral continuity.
• 21:52
  SPEAKER_01: So, what happens often in geology is you get this deposition spread out over the same rock layer, spread out over a massive.
• 22:00
  SPEAKER_01: Massive lateral area, right?
• 22:02
  SPEAKER_01: Well, then later on, rivers come through and mess it all up.
• 22:05
  SPEAKER_01: So they dissect it.
• 22:07
  SPEAKER_01: Okay.
• 22:07
  SPEAKER_01: And like what you said, you can stand at the Grand Canyon with a Colorado River that dissected that whole place.
• 22:14
  SPEAKER_01: And you can stand on a very prominent layer of limestone and look way across the valley and see that same layer of limestone.
• 22:22
  SPEAKER_01: And that's the principle of lateral continuity.
• 22:25
  SPEAKER_01: It has the same sequence of rocks above it, the same sequence of rocks below it.
• 22:30
  SPEAKER_01: And so using that, you can start to put things in their proper order because you know what?
• 22:36
  SPEAKER_01: Oftentimes in geology, we can't see the other side of it.
• 22:42
  SPEAKER_01: So it's hey, get in a car, go to this outcrop, chart it all out on a piece of paper, get in your car and drive 15 miles down the road, and there's another outcrop, and you do the same thing, and you're now putting things in their proper sequence in that way.
• 23:00
  SPEAKER_01: And that's stratigraphic correlation.
• 23:02
  SPEAKER_01: That's what we're talking about here.
• 23:03
  SPEAKER_00: Yeah, and this is used again at a variety of scales.
• 23:06
  SPEAKER_00: We've talked about the canyon or driving 15 miles away, and you see the same rock unit.
• 23:10
  SPEAKER_00: Okay, they used to be a continuous sequence.
• 23:13
  SPEAKER_00: You can extend that out to regional and actually global correlation.
• 23:18
  SPEAKER_00: So, for instance, the southwest of the US is a great example of this.
• 23:21
  SPEAKER_00: The Grand Canyon units of rocks, you can correlate similar ones all the way over to Las Vegas and into California, the same general sequence of rocks that was deposited.
• 23:31
  SPEAKER_00: Now they're offset from one another.
• 23:32
  SPEAKER_00: Some are missing in some places, but if you're careful and do very careful work, you can correlate the general timing all the way across an entire region.
• 23:40
  SPEAKER_00: And then when we start to leverage things like index fossils, which we're going to talk about here in a minute, we can start to do this across the world.
• 23:47
  SPEAKER_00: We can link rocks in China with rocks in the US, with rocks in Africa.
• 23:52
  SPEAKER_00: We can link them all up and determine when they formed relative to one another using this really valuable thing called index fossils.
• 23:58
  SPEAKER_00: And Chris, what can we go into that briefly here?
• 24:01
  SPEAKER_01: We can, but first I want to say that was impressive, my young sage.
• 24:05
  SPEAKER_01: Wow.
• 24:06
  SPEAKER_01: Well, well put.
• 24:08
  SPEAKER_01: Oh, thank you.
• 24:08
  SPEAKER_01: Well extremely well put.
• 24:10
  SPEAKER_01: Nice job.
• 24:10
  SPEAKER_01: Yeah.
• 24:11
  SPEAKER_01: Um, okay, index fossils.
• 24:13
  SPEAKER_01: So important, so cool.
• 24:16
  SPEAKER_01: Everybody knows what an index is, right?
• 24:18
  SPEAKER_00: Well, I don't know.
• 24:18
  SPEAKER_00: This generation, the Google index you know, we're in the Google generation.
• 24:22
  SPEAKER_00: Maybe not.
• 24:22
  SPEAKER_00: Maybe not.
• 24:23
  SPEAKER_01: That's true.
• 24:24
  SPEAKER_01: All right.
• 24:24
  SPEAKER_01: Well, all right.
• 24:25
  SPEAKER_01: Most books, especially textbooks, at the end of the book, you know, the last few pages has an index where you can look up keywords and it'll tell you what page to go to and you can read about those.
• 24:35
  SPEAKER_01: So an index is something that we use to look something up, right?
• 24:39
  SPEAKER_01: So an index fossil is the same thing, it's an index to look up the certain ages of rocks.
• 24:46
  SPEAKER_01: And here's what we're talking about there are certain fossils that have existed over geologic time that were really, really widespread.
• 24:56
  SPEAKER_01: In other words, they lived in all corners of the world, right?
• 25:00
  SPEAKER_01: Most of the time we're talking about oceans.
• 25:02
  SPEAKER_01: So they were very prolific, but they lived during a very specific time period.
• 25:07
  SPEAKER_00: Narrow, a very narrow time period, right?
• 25:10
  SPEAKER_01: Like a restricted time window is that the best ones if it's a very narrow time period.
• 25:16
  SPEAKER_01: So if a rock has that fossil, then we know this rock existed during this bracket of time.
• 25:25
  SPEAKER_01: And so that's what index fossils are.
• 25:28
  SPEAKER_01: They're very, very common and they live during a short period of time.
• 25:33
  SPEAKER_01: So it tells us specifically then, hey, this rock is this age.
• 25:38
  SPEAKER_01: So we can correlate it then back to what you said.
• 25:42
  SPEAKER_01: A rock in China is the same age as a rock in the United States because they have the same index fossils in them.
• 25:49
  SPEAKER_00: That's exactly right, Chris.
• 25:51
  SPEAKER_00: Any global event you can correlate across the globe, right?
• 25:54
  SPEAKER_00: And certain species of fossils or certain species of organisms will live for a million years.
• 26:01
  SPEAKER_00: They lived from 251 to 250 million years ago, and they were all over, they were in every ocean around the world, and so they're deposited.
• 26:08
  SPEAKER_00: Those fossils are found in sediments around the world at that time.
• 26:12
  SPEAKER_00: And so if we go and we find a rock in Michigan, or if we find a rock in, you know, Nevada, if we find a rock in China, and it has that fossil in it, say, ah, okay, this was alive between 251 and 250 million years ago.
• 26:25
  SPEAKER_00: It must have formed then.
• 26:26
  SPEAKER_00: This sediment is now that old.
• 26:28
  SPEAKER_00: We have a pin in time for that rock's age, and then we can link that using this correlation to regional rocks or around that particular discovery place.
• 26:38
  SPEAKER_00: So these index fossils are really valuable, and the index fossils are usually a specific species of organism.
• 26:43
  SPEAKER_00: You know, there are classes that we'll go through a little bit, I think, Chris.
• 26:47
  SPEAKER_00: There's a couple classes that are important here too, right?
• 26:50
  SPEAKER_01: Should we just go through like a couple examples?
• 26:52
  SPEAKER_00: Yeah, totally.
• 26:53
  SPEAKER_01: Um, so a couple of really good examples of index fossils are ammonites and trilobites.
• 26:59
  SPEAKER_01: And I we were talking about this before.
• 27:02
  SPEAKER_01: Like, don't you think most people have heard of these before?
• 27:05
  SPEAKER_01: And you said, ah, I don't know.
• 27:06
  SPEAKER_01: Maybe trilobites.
• 27:08
  SPEAKER_01: Trilobites are are they were very, very, very common, and a lot of people have trilobites, and a lot of people have seen them.
• 27:14
  SPEAKER_01: But so let's describe them a second.
• 27:17
  SPEAKER_01: Ammonite, these are the spiral-shaped fossil shells on the side.
• 27:23
  SPEAKER_01: They look like tightly coiled rams' horns, like a bighorn sheep.
• 27:28
  SPEAKER_01: Okay.
• 27:29
  SPEAKER_01: These were index fossils for the Mesozoic.
• 27:31
  SPEAKER_01: In other words, they were common from 245 million years ago to 65 million years ago, and they met an abrupt extinction at that point, and that was related to the KT boundary extinction, the meteor that wiped out the dinosaurs and two.
• 27:46
  SPEAKER_01: It also wiped out the vast majority of marine life, too, which is what Ammonites lived in.
• 27:52
  SPEAKER_01: Um, the other one is trilobites.
• 27:54
  SPEAKER_01: Jesse, what do trilobites look like to you?
• 27:56
  SPEAKER_01: To me, they're horrifying.
• 27:58
  SPEAKER_00: I mean, they are.
• 27:58
  SPEAKER_00: I they're like, oh, I don't know.
• 28:01
  SPEAKER_00: They're like strange, you know, ocean, they're like sea millipedes with antlers or something gross like that.
• 28:10
  SPEAKER_00: You know, they're they're like very strange looking.
• 28:13
  SPEAKER_00: They're kind of gross looking.
• 28:15
  SPEAKER_00: I would I would not want to see one alive.
• 28:17
  SPEAKER_00: People say they look like horseshoe crabs.
• 28:19
  SPEAKER_00: Maybe people have seen these uh horseshoe crab kind of things.
• 28:22
  SPEAKER_00: They look sort of like that a little bit, but they're sort of strange things.
• 28:26
  SPEAKER_00: But they're around from about 540 million years ago to about 245, and much like ammonites, they're really valuable as index fossils because there are very specific species that formed at given times and only existed during given times.
• 28:41
  SPEAKER_00: So they're kind of this evolutionary branch where there's a bunch of different species, and they kind of one species would come out and then it would proliferate around the world, and then it would die off, and another species would take over.
• 28:51
  SPEAKER_01: These organisms were primarily like filter feeders, they would just feed on the bottom of a shallow seafloor and just filter through the sediment, and they would leave tracks and things like that, and then they would live and die and get buried quickly.
• 29:05
  SPEAKER_01: So, because of all that, they're very, very common.
• 29:08
  SPEAKER_01: I I have some trilobites that I break out in class because I'm a fan of them.
• 29:13
  SPEAKER_01: They they're a little horrifying to me, but I also think they're really cool looking too.
• 29:16
  SPEAKER_01: So yeah, yeah.
• 29:18
  SPEAKER_00: I actually don't have any trilobites.
• 29:19
  SPEAKER_00: I need to, I don't have that many fossils, actually.
• 29:21
  SPEAKER_00: I was, you know, yeah, I don't need super turned out by fossils.
• 29:25
  SPEAKER_00: But uh they're very cool, very useful.
• 29:27
  SPEAKER_01: So, Jesse, go ahead.
• 29:28
  SPEAKER_01: Let's wrap this up.
• 29:29
  SPEAKER_00: Okay.
• 29:30
  SPEAKER_00: So we covered the five really important, but very simple, if they're already pointed out to you, very simple principles in geology.
• 29:38
  SPEAKER_00: First, superposition, bottom sediments are oldest or bottom lava falls are oldest, top are the youngest.
• 29:43
  SPEAKER_00: Two, horizontality, the sediments were originally deposited horizontally.
• 29:48
  SPEAKER_00: If they're tilted or if they're folded, that happened afterwards.
• 29:51
  SPEAKER_00: Principle of cross-cutting relationships, where the younger event cross-cuts the older stuff.
• 29:58
  SPEAKER_00: The stuff that is cut is older.
• 30:00
  SPEAKER_00: The fourth one, principles of inclusions.
• 30:02
  SPEAKER_00: The stuff that is included is older than the younger thing that does the inclusion.
• 30:07
  SPEAKER_00: Think about a chocolate chip cookie.
• 30:09
  SPEAKER_00: Chocolate chips are older than the cookie itself.
• 30:11
  SPEAKER_00: And then the last one, correlation and continuity.
• 30:13
  SPEAKER_00: And in there, we talked about index fossils.
• 30:15
  SPEAKER_00: And with those five tools, you can go out into the world, either look at buildings in your city, go out on a hike, look at some rocks, anywhere you find stuff, sediments in a riverbank, you can do geology.
• 30:28
  SPEAKER_00: You could do the basics of geology if you think about it and just ponder it a little bit.
• 30:32
  SPEAKER_00: So I find this very fun exercise.
• 30:35
  SPEAKER_00: Like I said, go hiking with people who are not geologists.
• 30:38
  SPEAKER_00: It's fun to kind of give them that basic idea.
• 30:40
  SPEAKER_00: Like we did most recently a hike with a couple friends.
• 30:43
  SPEAKER_00: Tess and I went with a couple friends in the Grand Canyon.
• 30:45
  SPEAKER_00: And, you know, on the way down for about a half an hour, point out these very simple principles.
• 30:49
  SPEAKER_00: And then the rest of like the four-hour hike, they're having fun testing it and experimenting with these new tools they have.
• 30:55
  SPEAKER_00: They're like, oh wait, I see cross-cutting relationships.
• 30:57
  SPEAKER_00: Okay, this one's older, right, Jesse?
• 30:59
  SPEAKER_00: You're like, yeah, well done.
• 31:00
  SPEAKER_00: It's amazing.
• 31:01
  SPEAKER_00: So it doesn't take a lot to build these tools, and they're very, very valuable, I think.
• 31:05
  SPEAKER_01: And I think for the most part, too, students like working through these exercises, whether you're out in the field or you're whether you're doing it in a lab with a cross section drawn on a piece of paper, and they're using the faults, the inclusions, the horizontality to put things all in order, too.
• 31:21
  SPEAKER_01: So it's you're right.
• 31:22
  SPEAKER_01: It's just good practice, it's good thought process.
• 31:24
  SPEAKER_01: It's like it's logical, is what this is.
• 31:26
  SPEAKER_01: This is logic.
• 31:27
  SPEAKER_01: This is the way geologists think.
• 31:29
  SPEAKER_00: Um if you like puzzles or you like crosswords, you'll like doing this.
• 31:33
  SPEAKER_00: I guarantee it.
• 31:34
  SPEAKER_00: They're very fun.
• 31:35
  SPEAKER_00: So all right, Chris, with that, that's a wrap.
• 31:37
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• 31:38
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• 31:43
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• 31:48
  SPEAKER_00: That would be great.
• 31:49
  SPEAKER_00: If you could stop and do that right now for us, we would love it.
• 31:52
  SPEAKER_01: That's right.
• 31:53
  SPEAKER_01: And we just want you to share the podcast with somebody that you think might love to learn about the earth.
• 31:58
  SPEAKER_00: That's right.
• 31:59
  SPEAKER_00: Take care.
• 32:00
  SPEAKER_00: Cheers.