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Devils Tower National Monument · 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.
• 0:13
  SPEAKER_01: That was the weakest clap I know.
• 0:16
  SPEAKER_01: I missed I missed the There was no way you got a spike out of that.
• 0:22
  SPEAKER_01: So what kind of Jesse do I have tonight?
• 0:24
  SPEAKER_01: Like what's what's up?
• 0:25
  SPEAKER_01: You have fun Jesse.
• 0:27
  SPEAKER_01: Do I though?
• 0:28
  SPEAKER_00: Yeah.
• 0:28
  SPEAKER_01: This is fun Jesse.
• 0:30
  SPEAKER_01: All right.
• 0:30
  SPEAKER_01: That was not an enthusiastic clap in.
• 0:33
  SPEAKER_00: No, that was you're right.
• 0:34
  SPEAKER_00: That was not.
• 0:34
  SPEAKER_00: That was that was shockingly bad performance by me there on the leader.
• 0:38
  SPEAKER_00: It really was.
• 0:39
  SPEAKER_00: I really need to step it up for the rest of this year.
• 0:42
  SPEAKER_00: You do.
• 0:43
  SPEAKER_01: Let's go.
• 0:44
  SPEAKER_01: How are you doing, Dr.
• 0:44
  SPEAKER_01: Remake?
• 0:45
  SPEAKER_01: Oh, I'm doing really well, Chris.
• 0:47
  SPEAKER_01: How are you?
• 0:48
  SPEAKER_01: We I'm doing great.
• 0:49
  SPEAKER_01: Um, I'm out for summer now.
• 0:51
  SPEAKER_01: Actually, yeah, when this comes out, I'll probably be out west, won't I?
• 0:55
  SPEAKER_00: You will.
• 0:55
  SPEAKER_00: You will be out on the trip that I fell in love with geology on while you were teaching me the basics of geology.
• 1:03
  SPEAKER_00: I mean, this trip, and and this is in the theme of what we're doing here.
• 1:08
  SPEAKER_00: We're gonna talk about Devil's Tower today.
• 1:10
  SPEAKER_00: We talked about Badlands National Park last week.
• 1:14
  SPEAKER_00: And the reason is because you start drive from Michigan, you head out west to the western United States, you stop in the Badlands.
• 1:20
  SPEAKER_00: It's kind of the first real interesting geology, and then you hit Devil's Tower, which gets a little bit more serious because let's face it, there's igneous rocks there, and those are pretty exciting.
• 1:30
  SPEAKER_01: So yeah, that's right.
• 1:31
  SPEAKER_01: But don't forget about the Black Hills, too.
• 1:33
  SPEAKER_01: So when you go to the Badlands, you spend a little bit of time there, but then go to the Black Hills.
• 1:38
  SPEAKER_00: That's right.
• 1:39
  SPEAKER_00: Absolutely.
• 1:40
  SPEAKER_01: And go back and listen to our episode on that too.
• 1:42
  SPEAKER_00: Because you're out on that trip right now and you're showing 26 high school soon-to-be seniors, you're teaching about geology.
• 1:49
  SPEAKER_00: You might be, you know, carrying on a little bit too long for some of them.
• 1:54
  SPEAKER_00: But but but you're providing great information nonetheless.
• 1:59
  SPEAKER_01: Okay, you have no right to say that.
• 2:01
  SPEAKER_01: When you joined us out west, okay, you I gave you a task.
• 2:05
  SPEAKER_01: I said, All right, Jesse, talk about what is a magma chamber.
• 2:09
  SPEAKER_01: It's one of your favorite things to talk about.
• 2:11
  SPEAKER_01: That was the problem.
• 2:13
  SPEAKER_00: We found this favorite things to talk about.
• 2:15
  SPEAKER_01: We've we found this gorgeous place in Yellowstone.
• 2:19
  SPEAKER_00: Let me interject there a minute, Chris.
• 2:21
  SPEAKER_00: This was like three years ago.
• 2:22
  SPEAKER_00: I don't even think I'd started at Penn State at that point.
• 2:24
  SPEAKER_01: Was that a long time to you?
• 2:26
  SPEAKER_01: No, no, no.
• 2:26
  SPEAKER_00: But I was like, I had done my PhD.
• 2:28
  SPEAKER_00: I'd spent three years as a postdoc, four years as a postdoc.
• 2:31
  SPEAKER_00: So this was not like in high school, this was recently.
• 2:34
  SPEAKER_01: Oh, yeah, yeah.
• 2:34
  SPEAKER_01: That's what you're saying.
• 2:35
  SPEAKER_01: Yeah, it was recent, and we were all excited.
• 2:38
  SPEAKER_01: You spent a week with us, actually, and we had a riot.
• 2:41
  SPEAKER_01: However, though, I gave you a job, I said, Hey, put a lesson together on what is a magma chamber.
• 2:46
  SPEAKER_01: So we found this great spot in the middle of the woods in Yellstone National Park.
• 2:49
  SPEAKER_01: It was amazing, and you managed to put to sleep 26 high school kids.
• 2:53
  SPEAKER_01: Like that was impressive.
• 2:55
  SPEAKER_00: They had a really nice nap, though.
• 2:57
  SPEAKER_00: It was really comfy moss that they were laying on, some nice logs they're leaned up against.
• 3:02
  SPEAKER_00: Straight out, straight to sleep.
• 3:04
  SPEAKER_01: You have to remember your audience, Jesse.
• 3:07
  SPEAKER_01: That's true.
• 3:08
  SPEAKER_00: That's true.
• 3:08
  SPEAKER_00: I did I did have a comment though.
• 3:10
  SPEAKER_00: One of your students, I had I think I had like an REI backpack and I'd brought along like an REI like hiking chair or something.
• 3:16
  SPEAKER_00: And one of your students was like, um, Mr.
• 3:19
  SPEAKER_00: Rimeek, are you sponsored by REI or something?
• 3:25
  SPEAKER_00: Remember that what?
• 3:26
  SPEAKER_00: And she's like, Well, I well, I don't know.
• 3:28
  SPEAKER_00: You're kind of cool and you have a lot of REI stuff.
• 3:31
  SPEAKER_01: You had the you had your guide pants on.
• 3:32
  SPEAKER_01: Remember the the Paul Reddit.
• 3:34
  SPEAKER_01: That's exactly right.
• 3:35
  SPEAKER_00: Yeah, I was like, uh so REI, if you're listening, you know, we're looking for sponsors here.
• 3:39
  SPEAKER_00: So that's great.
• 3:41
  SPEAKER_00: Now maybe we could be uh anyway.
• 3:43
  SPEAKER_00: Uh yes, so you so let's get back to it here.
• 3:46
  SPEAKER_00: You are on this trip right now.
• 3:48
  SPEAKER_00: You are probably not at Devil's Tower, but you've been there recently or you're headed there soon.
• 3:52
  SPEAKER_00: Lead us in, Chris.
• 3:53
  SPEAKER_00: Like give us the high-level summary of this half an hour episode here on Devil's Tower.
• 3:57
  SPEAKER_01: Well, first of all, push pause right now.
• 3:59
  SPEAKER_01: If you don't know what Devil's Tower looks like, then you should you need to Google image this.
• 4:03
  SPEAKER_01: If you've never been there, you've never seen it, like look at this thing.
• 4:06
  SPEAKER_01: It is it is so amazing.
• 4:08
  SPEAKER_01: So do that first and then come back.
• 4:11
  SPEAKER_01: But there are two broad types of rocks that are exposed at Devil's Tower.
• 4:14
  SPEAKER_01: We're gonna talk about each of them.
• 4:15
  SPEAKER_01: We're gonna talk about their relative ages and how we know that.
• 4:18
  SPEAKER_01: Then we're gonna talk about the theories that go into what we think about how this really weird uh structure came to be from a geoscience standpoint.
• 4:30
  SPEAKER_01: It's not quite settled.
• 4:32
  SPEAKER_01: We know some things, everybody agrees on some things, but you know, it's not settled yet.
• 4:36
  SPEAKER_01: So that's really what we're gonna do today is uh just kind of go through those things.
• 4:41
  SPEAKER_01: What we know, what we think we know, and and what's open for debate yet.
• 4:45
  SPEAKER_00: Yeah, so where do we start with this, Chris?
• 4:47
  SPEAKER_00: I mean, in geology, we always start, you know, the oldest first, the bottom first.
• 4:50
  SPEAKER_00: So what predated Devil's Tower itself?
• 4:54
  SPEAKER_01: Yeah, sure.
• 4:55
  SPEAKER_01: There are two types of rocks exposed at Devil's Tower.
• 4:58
  SPEAKER_01: There's sedimentary and there's igneous, and uh, just strikingly different from each other.
• 5:03
  SPEAKER_01: The sedimentary rocks are actually gorgeous.
• 5:05
  SPEAKER_01: I mean, they're beautiful, beautifully colored oranges and reds and browns and some whites thrown in there too.
• 5:13
  SPEAKER_01: And then you have this kind of gray, bland looking igneous rock, and I say that it's bland looking until you pick up a chunk of it and hold it in hand.
• 5:21
  SPEAKER_01: Specimens really actually it's an awesome rock.
• 5:24
  SPEAKER_01: This is different though from stuff that we've talked about before.
• 5:28
  SPEAKER_01: The oldest rocks at Devil's Tower are actually the sedimentary rocks, and that's different from a lot of the other places that we've discussed.
• 5:37
  SPEAKER_01: Usually though those old basement rocks are igneous, and the younger ones are sedimentary, so not the case.
• 5:45
  SPEAKER_01: So, how would we possibly determine that the sedimentary rocks are the oldest, they're older than the tower itself?
• 5:54
  SPEAKER_01: How would we do that?
• 5:54
  SPEAKER_01: How does how do we know that?
• 5:56
  SPEAKER_00: This is one of the most basic laws of geology.
• 6:00
  SPEAKER_00: And you know, we I teach this in the intro-level class that I teach at Penn State now, and you taught me this when I was a student taking the college-level class that you taught in high school.
• 6:10
  SPEAKER_00: But there's a lot of laws in geology or rules in geology, first principles that are really simple once you hear it, but until you hear it and think about, you wouldn't really think about.
• 6:21
  SPEAKER_00: I mean, uh, at least that's the way I kind of feel about them, is they're they're dead obvious once they're pointed out to you.
• 6:25
  SPEAKER_00: And this one is one of them.
• 6:27
  SPEAKER_00: This is the law of cross-cutting relationships.
• 6:29
  SPEAKER_00: And what it is is if something cross-cuts another thing, it is younger.
• 6:34
  SPEAKER_00: And the way this works is picture a sequence of sedimentary rocks, oldest on the bottom, youngest on top, they're deposited horizontally in layers.
• 6:43
  SPEAKER_00: And then you inject a magma through that, the magma will cross-cut the layering.
• 6:49
  SPEAKER_00: And that means that the magma is younger than the sediments around it, because it cuts across the layers.
• 6:54
  SPEAKER_00: So the sedimentary layers, if you like found one of those sedimentary layers, Chris, like let's say there's an oxidized red rock sitting out right on top of a reduced green rock, you find that boundary between red and green, and you trace it back, that boundary will end, it'll die out when it hits the igneous rock.
• 7:11
  SPEAKER_00: And that means that the igneous rock cross-cuts the sediments, therefore the sediments are older.
• 7:16
  SPEAKER_01: That's right.
• 7:16
  SPEAKER_01: And we've also discovered what we call this intrusive breccia, where the the sedimentary rocks have been brecciated or kind of busted up by this intrusion.
• 7:28
  SPEAKER_01: And we call it a breccia because breccia in geology means it's kind of angular, you know, there's no transport with it.
• 7:34
  SPEAKER_01: So they're just kind of like this welded together, busted up sedimentary rock, and that happened because of the intrusion.
• 7:43
  SPEAKER_01: So, like you said, that means that the sedimentary rocks are older than the igneous intrusion that that did that.
• 7:48
  SPEAKER_01: Um, and again, like we said, they're beautifully colored.
• 7:51
  SPEAKER_01: They're oxidized reds and browns and yellows, and there's some white in there too.
• 7:55
  SPEAKER_01: They're beautifully colored.
• 7:56
  SPEAKER_01: They were laid down.
• 7:58
  SPEAKER_01: This is a theme from the Badlands, the Black Hills.
• 8:01
  SPEAKER_01: We're in the same vicinity by a shallow sea that existed during this time, which was the Triassic and the Jurassic.
• 8:09
  SPEAKER_01: So this is not the same sea, the Western interior sea that deposited the pure shale that we see in the uh Badlands.
• 8:17
  SPEAKER_01: The Western Interior Sea is actually younger than this interior seaway, but there were lots of seas that came and went as time went on in this part of the country.
• 8:28
  SPEAKER_01: You also get in within this, and did you allude to it, Jesse, a green sedimentary layer?
• 8:34
  SPEAKER_01: Why would we get reds and greens?
• 8:36
  SPEAKER_01: What does that mean?
• 8:37
  SPEAKER_00: Well, it's basically reduced iron and oxidized iron, and the reds are the oxidized iron.
• 8:42
  SPEAKER_00: Think of rust.
• 8:42
  SPEAKER_00: It's just iron that is rusted.
• 8:44
  SPEAKER_00: Iron is bonding with oxygen to make these iron oxide minerals that are often red, usually it's hematite, and the green is iron in the reduced state, which tends to make green minerals.
• 8:55
  SPEAKER_00: And so that's really the only difference there.
• 8:57
  SPEAKER_00: So if you see this red-green layering, it's an oxidized versus a reduced environment.
• 9:02
  SPEAKER_00: It's just the amount of available oxygen, free oxygen in the system.
• 9:07
  SPEAKER_01: Yeah, think back to your high school chemistry class and redox reactions, and that's kind of what we have going on in a practical.
• 9:15
  SPEAKER_00: Yeah, it's quite simple to just think of it, leave it at that.
• 9:19
  SPEAKER_01: So the next thing that we have to talk about then are the igneous rocks.
• 9:22
  SPEAKER_01: Okay, and the igneous rock is actually kind of a rare rock.
• 9:26
  SPEAKER_01: It's what's called a phonolite porphyry.
• 9:28
  SPEAKER_00: And let me interrupt you, Chris, because the igneous rock is what Devil's Tower is.
• 9:32
  SPEAKER_00: So the tower is this igneous rock.
• 9:34
  SPEAKER_00: That's what we're speaking about now.
• 9:36
  SPEAKER_01: That's right.
• 9:36
  SPEAKER_01: Yeah.
• 9:37
  SPEAKER_01: And it's a it's kind of a rare, like very niche kind of rock.
• 9:40
  SPEAKER_01: It's called a phonolite porphyry.
• 9:42
  SPEAKER_01: To me, uh, I don't know, Jesse.
• 9:44
  SPEAKER_01: For me, I have introductory level students.
• 9:47
  SPEAKER_01: Some of them have had very minimal earth science or geoscience education.
• 9:51
  SPEAKER_01: I call it a phonolite porphyry, but I say this is very similar to an andesite in terms of chemical composition.
• 9:58
  SPEAKER_01: In other words, what I'm saying is it's intermediate.
• 10:00
  SPEAKER_01: It's not felsic, it's not mafic.
• 10:03
  SPEAKER_01: The magma was kind of sticky that did this.
• 10:06
  SPEAKER_01: Um, but all right, what, Jesse, is a phonolite porphyry?
• 10:12
  SPEAKER_01: And feel free, go ahead, get doctory on us, get professory, let's go.
• 10:16
  SPEAKER_00: Maybe let's focus on the texture that you can see when you pick it up, first of all, which is the porphyritic texture.
• 10:21
  SPEAKER_00: So, what is that, Chris?
• 10:22
  SPEAKER_00: I mean, you you said andesite porphyry is kind of similar.
• 10:24
  SPEAKER_00: That porphyry term is applied to a bunch of different rocks.
• 10:27
  SPEAKER_00: What does that mean?
• 10:28
  SPEAKER_01: Yeah, so porphyritic means two distinct grain sizes in an igneous rock.
• 10:33
  SPEAKER_01: And the rule with igneous rock, uh, and there are exceptions, but the general rule is the slower it cooled, the larger the crystals get.
• 10:43
  SPEAKER_01: So if you have a a rock, an igneous rock that has two distinct grain sizes, it means that it had two distinct rates of cooling.
• 10:51
  SPEAKER_01: It had slow cooling, probably deep down, and then it moved up toward the surface.
• 10:57
  SPEAKER_01: Sometimes it comes out onto the surface where it cools much faster.
• 11:01
  SPEAKER_01: And so you're gonna get then, you know, big crystals that are gonna be entombed in this fine grain matrix.
• 11:06
  SPEAKER_01: And that's what a porphyritic texture means.
• 11:09
  SPEAKER_01: Anything that's porphyritic, two distinct grain sizes from at least two rates of cooling.
• 11:14
  SPEAKER_00: Yes, and so that brings us to the phonolite part, and this is P H O N O L I T E, like a phonograph.
• 11:22
  SPEAKER_00: And what this tells us, this is a chemical designation.
• 11:25
  SPEAKER_00: It means it has intermediate amounts of silica, so something like 60, 65% silica, which is kind of normal for this type of felsic or intermediate kind of composition rock, but it's has a screaming high concentration of what are called alkali metals.
• 11:40
  SPEAKER_00: That's sodium and potassium.
• 11:42
  SPEAKER_00: Really, really, really high amounts.
• 11:44
  SPEAKER_00: And the interesting thing about this rock, and this is a rare kind of suite of rocks, but we call these rocks alkali rocks because they're rich in the alkali elements, sodium and potassium.
• 11:54
  SPEAKER_00: What's interesting about these rocks is although they have a decent amount of silica in them, they have such a high concentration of the alkali metals that feldspar cannot compensate for them.
• 12:03
  SPEAKER_00: There's too much sodium and potassium to fit into feldspar because there's not enough silica.
• 12:09
  SPEAKER_00: And I don't have a great analogy for this, Chris.
• 12:11
  SPEAKER_01: You might ridicule me about what does it form then?
• 12:13
  SPEAKER_01: If it what do you mean?
• 12:15
  SPEAKER_01: What do you get then?
• 12:16
  SPEAKER_00: So it's basically mean what it means is there's no free quartz.
• 12:19
  SPEAKER_00: There's no free silica around, so you don't get quartz ever in these rocks.
• 12:23
  SPEAKER_00: You do get feldspars, so you do get feldspar, but there's more sodium and potassium floating around.
• 12:29
  SPEAKER_00: And so you get minerals that are called feldspethoids, a really, you know, original name here.
• 12:35
  SPEAKER_01: I'm gonna interject, Jesse.
• 12:36
  SPEAKER_01: You're first of all, you're a funny guy.
• 12:38
  SPEAKER_01: Um, you took a long time to get to really what makes a phonolite a phonolite, is there's no quartz.
• 12:46
  SPEAKER_01: Okay, like that's really it, which is really weird.
• 12:50
  SPEAKER_01: Um, you know, you pick this up and you'd expect to see some quartz in it, and you certainly do not.
• 12:56
  SPEAKER_01: There's no quartz in it.
• 12:58
  SPEAKER_00: All right, give us an analogy.
• 13:00
  SPEAKER_00: You do get weird minerals like uh you get leucite, you get analceme, you get a lot of weird minerals, which are these what are called feldspethoids.
• 13:08
  SPEAKER_00: And these are minerals that have less silica in them than feltspar.
• 13:13
  SPEAKER_00: They do have silica, but they have less silica and they just accommodate more sodium and potassium.
• 13:18
  SPEAKER_00: And, you know, I always like to think of mineral making like recipes, and you need the right amount of constituent parts to make a good chocolate chip cookie.
• 13:28
  SPEAKER_00: And so if you're after a good chocolate chip cookie, you want to have a lot of chocolate chips in there, make it nice and gooey and doughy, but you're starting to make a chocolate chip cookie and oh shit, I don't only have half a bag of chocolate chips, right?
• 13:39
  SPEAKER_00: Think of the chocolate chips being the silica, tetrahedrons.
• 13:42
  SPEAKER_00: We've talked about this before, as silica with four oxygen around it.
• 13:45
  SPEAKER_00: You use all your silica you can because you don't have any left over, or you don't have that much, you dump all of it in and you end up with a chocolate chip cookie that doesn't quite have enough chocolate chips in it.
• 13:55
  SPEAKER_00: It's still kind of a chocolate chip cookie, and you don't have any left over, right?
• 13:58
  SPEAKER_00: You know, you always want to grab a little handful and taste some chocolate chips just while you're making the cookie, but you don't have enough.
• 14:04
  SPEAKER_00: So there's no free chocolate chips.
• 14:05
  SPEAKER_00: It's the same thing in these rocks, there's no free silica to make quartz.
• 14:09
  SPEAKER_00: And so you get a very different mineralogical sequence there at the later stages.
• 14:13
  SPEAKER_01: Good good job.
• 14:15
  SPEAKER_01: Good job.
• 14:15
  SPEAKER_01: Thanks.
• 14:17
  SPEAKER_01: That was a good analogy.
• 14:18
  SPEAKER_01: I get it now.
• 14:19
  SPEAKER_01: I get it.
• 14:19
  SPEAKER_01: So phonolites are weird.
• 14:21
  SPEAKER_01: And let me just wrap this up.
• 14:22
  SPEAKER_00: I'll just wrap this up because they're weird rocks.
• 14:24
  SPEAKER_00: So, like, you know, they form in weirder environments.
• 14:27
  SPEAKER_00: And these rocks are typically thought to be formed by melting of the lithosphere beneath continents.
• 14:34
  SPEAKER_00: So the mantle that exists beneath continents, it's usually thought to be formed by melting that material.
• 14:40
  SPEAKER_00: And you get alkali-rich basalt, and then that sort of differentiates up towards phonolite.
• 14:47
  SPEAKER_00: So the alkali-rich basalt will differentiate and then end up as a phonolite.
• 14:51
  SPEAKER_00: And that's what we see in Devil's Tower is the phonolite.
• 14:54
  SPEAKER_01: That's right.
• 14:54
  SPEAKER_01: And this is where geology gets in the weeds a little bit because you know, a phonolite on like just physical appearance looks just like the rocks that you get from Mount St.
• 15:03
  SPEAKER_01: Helens, Mount Rainier, the Three Sisters in Oregon, Crater Lake, or the Andes Mountains in South America.
• 15:11
  SPEAKER_01: It's very similar looking to a porphyritic andesite.
• 15:15
  SPEAKER_00: Yeah, these weird felds with the minerals are hard to identify.
• 15:18
  SPEAKER_00: You know, you gotta have a pretty practiced eye, and usually you need to bring them back to the lab to actually distinguish them.
• 15:23
  SPEAKER_00: They do have a greasy appearance.
• 15:25
  SPEAKER_00: They're a little bit greasier than quartz, which is what I always kind of think of if you come across this rock.
• 15:30
  SPEAKER_00: It looks greasy even when you break a fresh surface.
• 15:33
  SPEAKER_01: Yep, they do.
• 15:34
  SPEAKER_01: They do, they have this kind of like weird sheen to them.
• 15:37
  SPEAKER_01: Yep, for sure.
• 15:39
  SPEAKER_01: All right, so let's move on, Jesse.
• 15:40
  SPEAKER_01: We spent a lot of time talking about what a phonolite porphyr is, but that's important, okay?
• 15:44
  SPEAKER_01: But here's the deal.
• 15:45
  SPEAKER_01: Let's get into how the tower actually formed.
• 15:49
  SPEAKER_01: And this is this is shocking to me, but there's still debate on how this incredible igneous feature formed.
• 16:01
  SPEAKER_01: I've lived through some cycles with this.
• 16:04
  SPEAKER_01: When I was a kid, it was a volcanic neck.
• 16:06
  SPEAKER_01: And then, you know, when I began teaching and taking students there, ah no, it's not a volcanic neck.
• 16:12
  SPEAKER_01: Now it's like a shallow, intrusive plug.
• 16:15
  SPEAKER_01: And now we've gone to other ideas, and nobody really knows what the hell is going on with this thing.
• 16:21
  SPEAKER_01: I so when I I think probably when I took you there, young Jesse, to Devil's Tower, we were walking around the base, and they have these interpretive signs, you know, and they have the diagrams and they're in color.
• 16:32
  SPEAKER_01: They're pretty well done, they look good.
• 16:34
  SPEAKER_01: But they were showing this as a volcanic neck.
• 16:38
  SPEAKER_01: And so we looked at the sign, and at the time, everyone had the consensus was this is not a volcanic neck, it's a shallow intrusive plug.
• 16:45
  SPEAKER_01: And so there was a ranger, an interpretive ranger that was had a group of people, and he looked at us and he's like, All right, who's in charge here?
• 16:52
  SPEAKER_01: And I'm like, Well, I am, and he's begrudgingly, uh, I guess I am.
• 16:57
  SPEAKER_00: What did I do wrong?
• 16:59
  SPEAKER_01: He puffs out his chest and he's like, So, volcanic neck or shallow intrusive plug.
• 17:05
  SPEAKER_01: I said, shallow intrusive plug, we think.
• 17:09
  SPEAKER_01: And and he said, Okay, good enough, carry on.
• 17:12
  SPEAKER_01: And he walked away.
• 17:14
  SPEAKER_01: But but you're I wanted to say, hold on, but your signs are telling everybody else that it's not, that it's something else.
• 17:22
  SPEAKER_01: You know, like, ah, fix it.
• 17:24
  SPEAKER_00: I mean, Chris, it must be interesting to have been born before geology was invented in your case, right?
• 17:29
  SPEAKER_00: So let's get into the difference there a little bit because it at first pass it sounds kind of like what's the difference between a volcanic neck and a volcanic plug?
• 17:35
  SPEAKER_00: Think of volcanoes like a plumbing system.
• 17:37
  SPEAKER_00: Think of the pipes in your basement that are feeding your plumbing system in your house.
• 17:41
  SPEAKER_00: The neck would be any pipe that's leading up to a faucet.
• 17:45
  SPEAKER_00: You go down your basement, you see this pipe, that's like in the neck of a volcano.
• 17:49
  SPEAKER_00: It's you know, down in the roots that's feeding the stuff up to the surface, right?
• 17:54
  SPEAKER_00: Because what's a volcanic plug?
• 17:55
  SPEAKER_00: Like, can you kind of contrast that with a volcanic neck?
• 17:59
  SPEAKER_01: Sure.
• 17:59
  SPEAKER_01: If you just imagine the plumbing network leading to the surface uh beneath a volcano, it's very intricate.
• 18:05
  SPEAKER_01: You know, we think we know what it looks like, but you know, it's hard to see and model and so on.
• 18:09
  SPEAKER_01: But as as you take thick toothpaste like magma that's oozing up through this plumbing network, uh and it gets close to the surface, you know, into the plumbing system leading to the volcano, but it cools and it just gets stuck, and it cools and hardens there without becoming volcanic.
• 18:29
  SPEAKER_01: Whereas a volcanic neck, it actually was volcanic at one time.
• 18:34
  SPEAKER_01: That's the that's the difference.
• 18:35
  SPEAKER_00: And then, you know, there's this we're relatively recent paper, I suppose, in 2015 that they proposed that actually this is actually really in the shallow parts.
• 18:44
  SPEAKER_00: It's actually really close to the surface, like half a kilometer from the surface.
• 18:47
  SPEAKER_00: It's more like sort of an eruptive equivalent from the magma chamber, right?
• 18:51
  SPEAKER_00: This is what's called a diatreme.
• 18:53
  SPEAKER_00: And uh basically it sort of entails eruption and then stuff falling back into that crater a little bit, but also stuff oozing up from the surface and cooling right near the surface, almost in contact with the atmosphere.
• 19:08
  SPEAKER_00: And so that's another sort of option or thing that's proposed for the formation of Devil's Tower igneous rocks.
• 19:16
  SPEAKER_01: Yeah, so it's just interesting that we don't have this nailed down yet because it seems to me that there are far more complicated things in geoscience, you know, where rather than this big, you know, monolith sticking out of the ground like this, I'm so surprised that there's still debate on this.
• 19:36
  SPEAKER_01: Anyway, so that's where there's disagreement on exactly how the magma got him placed and exactly where it got him placed in terms of depth.
• 19:46
  SPEAKER_01: You know, was it two kilometers deep or was it a 500 meters deep, that kind of thing.
• 19:51
  SPEAKER_01: There's some debate on this, right?
• 19:53
  SPEAKER_01: But where there's not debate really is that the tower was covered with these sedimentary rocks, and then it was exposed by erosion.
• 20:03
  SPEAKER_01: And the river that's there right now is it's called the Bel Foosh River.
• 20:07
  SPEAKER_01: Um, and it that the Foosh part of it is is spelled F-O-U-R-C-H-E.
• 20:12
  SPEAKER_01: So it's probably the most mispronounced word that people are reading the interpretive signs.
• 20:19
  SPEAKER_01: It's really humorous to see this, but um anyway, uh the Belfouche River, as it just meandered back and forth across its floodplain, uh, just began to strip away these soft, kind of weak and wimpy sedimentary rocks and eroded that sediment downstream and just gradually from the top down, exposing the tower.
• 20:44
  SPEAKER_00: Right.
• 20:45
  SPEAKER_00: And that is what we call differential weather and erosion, where this basaltic igneous rock, which is a really solid rock, that is a hard piece of material, preferentially resists weathering and erosion so that it now stands at a high elevation relative to the sediments, even though sediments used to be on top of it.
• 21:02
  SPEAKER_00: The sediments are relatively easy to erode compared to the phonolite.
• 21:06
  SPEAKER_01: Yeah, right.
• 21:07
  SPEAKER_01: And for me, and I think for some of my students too, it's one of the coolest things to sit there and contemplate, imagine it just being covered with sedimentary rocks, relatively flat-line sedimentary rocks, and just then gradually being stripped down and exposing the tower from the top down.
• 21:24
  SPEAKER_01: It's just kind of a cool thing to imagine, but it doesn't really take a real vivid imagination to see this because if you stand on the south side of the tower and you look south, you can see the Belfouche River in the distance there, and you can see terraces where the Balfouche River has like got reinvigorated and cut down and left the old floodplain kind of high and dry.
• 21:47
  SPEAKER_01: You can see it's it's kind of like step like look to the floodplain.
• 21:51
  SPEAKER_01: It's very cool.
• 21:52
  SPEAKER_01: Those are called stream terraces, and and so you can see this kind of incremental incision of the River on the on its own floodplain.
• 22:02
  SPEAKER_01: It's awesome.
• 22:03
  SPEAKER_00: So, Chris, the other, you know, really obvious and very cool feature of Devil's Tower has to do with the jointing in it.
• 22:11
  SPEAKER_00: And you have climbed.
• 22:12
  SPEAKER_00: If I remember, you've climbed Devil's Tower, rock climbed it.
• 22:15
  SPEAKER_00: Is that right?
• 22:15
  SPEAKER_01: That's right.
• 22:16
  SPEAKER_01: Yeah, we did.
• 22:17
  SPEAKER_01: Um, Jenny and I did it, and then we came back.
• 22:20
  SPEAKER_01: Well, our kids were there, but they didn't climb.
• 22:21
  SPEAKER_01: And then we took our kids up two years later when they were uh eight and ten, I think.
• 22:29
  SPEAKER_01: Which is such a long time ago now.
• 22:32
  SPEAKER_01: The kids are gone.
• 22:34
  SPEAKER_01: I was thinking, wait a minute, they were little when we did this.
• 22:36
  SPEAKER_01: Okay, I have a feeling that you're gonna get very professory on us with this, but we do need to talk about columnar joints because you and I we're huge fans of columnar joints.
• 22:46
  SPEAKER_01: We we love them.
• 22:47
  SPEAKER_01: Um if we ever have chances to collect columnar joining, and yeah, it's one of the more spectacular things you can see.
• 22:53
  SPEAKER_00: And you can see this in many different parts of the world.
• 22:55
  SPEAKER_00: The Columbia River Gorge, we have been to the Columbia River Gorge together.
• 22:58
  SPEAKER_00: We've seen these unbelievable columnar joints and the basalts there.
• 23:02
  SPEAKER_00: The famous Giants Causeway is one where people you can walk on top of the Columnar joints.
• 23:07
  SPEAKER_00: So it's these big hexagons.
• 23:09
  SPEAKER_00: The rock has broken into hexagons, and it's called the Giants Causeway because it looks like a Giants Causeway, right?
• 23:14
  SPEAKER_01: Um Which is interesting from the top of the tower, you can't see the columns.
• 23:20
  SPEAKER_01: There's actually soil up there, and it's it's grassy, and there's some cacti up there, and so on.
• 23:26
  SPEAKER_01: It's not like Giant's Causeway where you can literally walk uh like steps on the corner joints.
• 23:32
  SPEAKER_00: But it must define the climbing here, you know.
• 23:34
  SPEAKER_00: You must climb basically up these cracks in the in the columnar joints.
• 23:47
  SPEAKER_01: Something to catch, you know, and you just keep doing it with your feet and your hands.
• 23:50
  SPEAKER_00: Horrible.
• 23:51
  SPEAKER_00: Um so these columnar joints, this is uh one of the really beautiful features in nature, I think, that forms naturally.
• 23:59
  SPEAKER_00: It's relatively simple to figure out what's going on here.
• 24:02
  SPEAKER_00: It's the rock cooling and contracting, and when the rock contracts, it's losing volume and it needs to compensate for that space, so it breaks into joints.
• 24:14
  SPEAKER_00: Like the joints break, the rocks kind of fall apart or or break apart a little bit to make space because the rock is contracting and cooling.
• 24:22
  SPEAKER_00: And the reason that it breaks into these hexagons is but you gotta think of the joints where they touch.
• 24:29
  SPEAKER_00: And this is just thermodynamically the most energetically efficient way for stuff to form is to have three pieces touching, all with 120 degree angles.
• 24:40
  SPEAKER_00: It's like dividing up a pizza, you just cut it into thirds, you have 360 degrees all the way around, so each one of those is 120 degrees, each slice is 120 degrees.
• 24:50
  SPEAKER_00: That's the most energetically efficient way to do it.
• 24:53
  SPEAKER_00: And you can think through this really in a fairly simple way.
• 24:58
  SPEAKER_00: The less points of contact, the more efficient it is energetically.
• 25:03
  SPEAKER_00: So you want as few points touching as possible.
• 25:05
  SPEAKER_00: If you take a piece of paper and you want to cut it up, you could cut it into a grid.
• 25:10
  SPEAKER_00: You could cut horizontally and then cut vertically.
• 25:12
  SPEAKER_00: You would have squares then all touching each other.
• 25:14
  SPEAKER_00: But where squares touch, there's four squares touching at one point.
• 25:18
  SPEAKER_00: So that's less efficient than having three.
• 25:22
  SPEAKER_00: Now you could think, oh, we could make this two squares touching, probably if we wanted to, which would mean you just have to draw parallel lines to each other and cut along parallel lines.
• 25:31
  SPEAKER_00: But that's inefficient because you need to compensate for space in the other direction.
• 25:35
  SPEAKER_00: And if you kind of find the happy medium between there, it's three.
• 25:38
  SPEAKER_00: And if you combine three together, you make hexagons.
• 25:42
  SPEAKER_00: And so every hexagon is touching two other hexagons at the points.
• 25:46
  SPEAKER_00: And that's just the way it happens.
• 25:49
  SPEAKER_00: It's it's everywhere in physics.
• 25:51
  SPEAKER_00: This happens in a lot of different places.
• 25:52
  SPEAKER_00: What's called a triple junction, having three points touching at once.
• 25:56
  SPEAKER_00: Two doctor e Chris was like is my really in the weeds.
• 26:00
  SPEAKER_01: I love you.
• 26:01
  SPEAKER_01: I you're you're hilarious.
• 26:03
  SPEAKER_01: He just you know, you make me laugh, yeah.
• 26:06
  SPEAKER_01: But I I don't want to laugh in the microphone.
• 26:08
  SPEAKER_01: So I'm just sitting there smiling.
• 26:09
  SPEAKER_01: I'm thinking, he is such a funny little guy over there.
• 26:12
  SPEAKER_01: Except you're not little, you're a giant.
• 26:14
  SPEAKER_01: Um but so basically to do this, it's the most efficient way to crack, right?
• 26:23
  SPEAKER_00: Yes, you know, the other interesting point here, Chris, which pertains to Devil's Tower, and actually we saw this in the Columbia River Gorge, is that those cracks will form perpendicular to the cooling direction.
• 26:33
  SPEAKER_00: So in Devil's Tower, the cracks are vertical, which means that it was cold on top and hot on the bottom.
• 26:40
  SPEAKER_00: And so that's why those cracks are vertical there.
• 26:43
  SPEAKER_00: In Columbia River Gorge, we saw Columnar joints that had big curves to them and kind of swept down into the left, right?
• 26:50
  SPEAKER_01: And that formed You actually see this in Devil's Tower, though.
• 26:53
  SPEAKER_01: At the base the bottom of the tower.
• 26:55
  SPEAKER_01: Yeah, yeah.
• 26:56
  SPEAKER_01: They curve and they become almost horizontal.
• 26:59
  SPEAKER_00: So this tells us that the temperature gradient changed orientation.
• 27:03
  SPEAKER_00: And we saw this in the Clumber River Gorge with you know big sweeping columns that kind of curved to the left, and then another one that intersected them and curved to the right.
• 27:11
  SPEAKER_00: The usual interpretation in the Clumber River Gorge is that lava flowed out and then a river flowed over top of it, which created like a really cold spot, which changed how the contours of the temperature gradients.
• 27:22
  SPEAKER_00: Devil's Tower, this is probably has to do with like a larger magma chamber down below that kind of made it hotter out to the sides than at the surface, potentially.
• 27:32
  SPEAKER_00: This sort of sweeping outwards pattern to it.
• 27:34
  SPEAKER_00: So anyway, it's an interesting, you could kind of like look at how the thing cooled, like actually physically, what parts were hot, what parts were cold based on the shape of these uh cracks.
• 27:44
  SPEAKER_01: Yeah, right.
• 27:45
  SPEAKER_01: The geometry of the columnar joints themselves, uh it's a strong indicator that this was not a volcanic neck, right?
• 27:53
  SPEAKER_01: I mean, that the cooling surface was top down.
• 27:55
  SPEAKER_00: That's right.
• 27:56
  SPEAKER_01: Good job, Jesse.
• 27:57
  SPEAKER_00: You didn't get too like stayed out of the weeds.
• 28:00
  SPEAKER_00: I was poking my head up above the weeds looking around.
• 28:02
  SPEAKER_01: I'm not gonna say you stayed out of the weeds, but you you did all right.
• 28:05
  SPEAKER_00: Well, good.
• 28:06
  SPEAKER_00: So what Chris, like, I don't know, you teach this amazing field class.
• 28:09
  SPEAKER_00: You go there every summer, and you have 26 new, you know, bright-eyed, bushy-tailed high school students.
• 28:14
  SPEAKER_00: What do you tell them?
• 28:15
  SPEAKER_00: Like, what are the what's the take-home point when you're sitting there looking at Devil's Tower, the spectacular monument?
• 28:21
  SPEAKER_00: Like, I don't know, how how do you convey how interesting this place is?
• 28:26
  SPEAKER_00: Give us the spiel, you know?
• 28:28
  SPEAKER_00: That's what I'm looking for, I guess.
• 28:29
  SPEAKER_01: Like a little bit you don't have to.
• 28:33
  SPEAKER_01: I don't I don't think because it's when we're approaching it and they're they got their noses down, they're probably reading something, whatever, you know, and they happen to look up and you can just feel the energy change on the bus, you know.
• 28:45
  SPEAKER_01: They look at it and they're like, Oh my gosh, I've seen that before.
• 28:48
  SPEAKER_01: I've he's had that in class, like we've seen pictures of that, and what you know how it is, right?
• 28:53
  SPEAKER_01: When you see pictures in a book or pictures on a screen, and then you get to see it for the first time in real life, that's an emotional moment, you know what I mean?
• 29:04
  SPEAKER_00: Especially when it's like 1200 feet tall and it's shockingly big, you way bigger than you thought it was gonna be.
• 29:10
  SPEAKER_01: That's right.
• 29:11
  SPEAKER_01: And it's just it's just stands up and smacks you in the face and says, Here I am.
• 29:16
  SPEAKER_01: So it's pretty cool.
• 29:17
  SPEAKER_01: Uh you know, like I don't know, I had the same reaction.
• 29:20
  SPEAKER_01: It's a such an easy sell.
• 29:21
  SPEAKER_01: But you know what, Jesse?
• 29:22
  SPEAKER_01: That's geology.
• 29:23
  SPEAKER_01: Geology is our planet is an easy sell.
• 29:25
  SPEAKER_00: And it's a good thing to try to take in the timescales of geology, too, because that volcano, that ancient volcano that formed this this igneous rock, this phonolite, that's about 40 million years old.
• 29:35
  SPEAKER_00: And that thing is still sitting there, resistant to erosion.
• 29:39
  SPEAKER_00: It it gets eroded.
• 29:40
  SPEAKER_00: You can see the boulders, you know, falling off.
• 29:42
  SPEAKER_00: These columnar joints are kind of like peeling off the thing.
• 29:45
  SPEAKER_00: And there is erosion going on, but the volcanic part of it is still there and standing pretty tall above the sedimentary rocks.
• 29:52
  SPEAKER_00: We can see cross-cutting relationships, we can see differential weather and erosion, we can see really cool igneous rocks, we can see really cool structural features in igneous rocks that tell us how this ancient thing cooled 40 million years ago.
• 30:04
  SPEAKER_00: Very, very cool.
• 30:05
  SPEAKER_01: One other thing that I think is so cool is kind of goes back to like the you know, the more you know, the more you can think about, and the kind of the more enriched your life becomes, right?
• 30:18
  SPEAKER_01: And when as you're climbing up the talus slope of Devil's Tower, and anybody can do that.
• 30:24
  SPEAKER_01: You there are certain places you can't go beyond without a climbing permit to go up this, and you'll see these like bright white, like really different looking rock on these uh columnar joints that have cascaded off the tower, and they're surrounded by this bright orange lichen, and those things that's that's lightning strike where the rock was hit.
• 30:52
  SPEAKER_01: It is so cool.
• 30:53
  SPEAKER_01: And you just you look at these things and you wonder, well, how many people have walked right on past this?
• 30:58
  SPEAKER_01: And it never occurred to them that that's what happened, but it's kind of cool to look at a this big you know hexagonal columnar joint that's flaked off the tower, and you know, and you're like, wow, lightning hit right there at that spot.
• 31:10
  SPEAKER_01: I mean it's pretty cool.
• 31:11
  SPEAKER_00: Very cool, very cool.
• 31:12
  SPEAKER_00: So yeah, um, well, Chris, I think that's a wrap.
• 31:15
  SPEAKER_00: What do you think?
• 31:17
  SPEAKER_01: I think so.
• 31:18
  SPEAKER_01: Go to Devil's Tower.
• 31:18
  SPEAKER_01: It's awesome.
• 31:19
  SPEAKER_00: Go to Devil's Tower, do it on the way, go to the Black Hills, go to the Badlands, and you know, now you can think of Chris out there lecturing, talking about the Devil's Tower and how cool geology is.
• 31:33
  SPEAKER_00: Well, we hope you return safely, Chris, and we can keep doing Planet Geo.
• 31:37
  SPEAKER_00: Me too.
• 31:38
  SPEAKER_00: Well, that's a wrap.
• 31:39
  SPEAKER_00: Follow us on all the social medias.
• 31:40
  SPEAKER_00: We're at Planet Geocast.
• 31:42
  SPEAKER_00: Send us an email, planetegeocast at gmail.com.
• 31:44
  SPEAKER_00: Visit our website, www.planetgeocast.com.
• 31:49
  SPEAKER_00: And Chris.
• 31:50
  SPEAKER_01: Yeah.
• 31:50
  SPEAKER_01: And share the podcast with somebody that would might like geoscience or geology or our planet.
• 31:56
  SPEAKER_00: Absolutely.
• 31:56
  SPEAKER_00: Or loves to travel to cool places and see cool stuff.
• 31:58
  SPEAKER_01: That's right.
• 31:59
  SPEAKER_00: Absolutely.
• 32:00
  SPEAKER_00: Just know a little bit about it.
• 32:01
  SPEAKER_00: Alright, man.
• 32:02
  SPEAKER_00: That's a wrap.
• 32:03
  SPEAKER_00: We'll see you soon.