So You Think You Know Plate Tectonics? Part 6 - Interior of the Earth
Dr. Jesse Reimink: [00:00:00]
Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
Chris Bolhuis: Yay, you. You did it, Jesse. That was nice. I changed things up on you and instead of back counting 3, 2, 1, I went 1, 2, 3. And you didn't miss a
beat. You clapped in on cue
Dr. Jesse Reimink: didn't even look confused or anything like that. Did I?
Chris Bolhuis: Hey, you did a little, you, you looked a little
Dr. Jesse Reimink: did I? Okay.
Chris Bolhuis: Little bit. [00:00:30] Little bit.
Dr. Jesse Reimink: and I cocked my head at you. Like sort of like, what's he doing over there?
Chris Bolhuis: I, yeah, you did actually. And I didn't know if the, your screen froze up again or whether you were just confused. But
Dr. Jesse Reimink: Uh, yeah, generally the latter, but sometimes I blame it on the, the screen freezing up on me, so, you know, that's why I was stuck with, you know, my tongue hanging outta my mouth in a weird, goofy pose for a while, staring off into space. Yeah. Yeah, that's right. I'm, well, I'm used to you just ignoring me and being very polite and looking at one of your three massive monitors that you have in your recording set [00:01:00] up and just staring off into space while you're paying attention to something different.
Chris Bolhuis: I'm not, I'm, I'm looking at what I have going on. So just because I'm not looking straight at you, cuz you're the one right in front of me. I'm looking at what we have going on over here. I'm making sure that our recording is going well. Hey, this is
Dr. Jesse Reimink: So, Let me just one thing, Chris. Um, I've got a little bit of a bone to pick with you so we were reading, We had a New Yorker article that sort of went through, I forget what the title was, [00:01:30] but something about the standards of modern etiquette, like text messaging, etiquette, dating, app etiquette, all these things that don't really apply, but there's one about text message communication, and there's a bit, I was noticing this the last couple days, you and I have. We're not that far apart in age, but there is a bit of a generational divide when it comes to communication, I
Chris Bolhuis: really? I, I'm really curious to hear what you're gonna say.
Dr. Jesse Reimink: So you, and, and I'll be curious to hear, what listeners think about this because my [00:02:00] generation, we are very text. Oriented, I think, and we perhaps over-communicate or we, we know why communication is being initiated you, so you tend to just call me. and you know, it could be in the middle of the day whenever you have your, you know, half an hour, free hour, whatever. You just call me. And if I don't answer, that's it, . So if I can't get to the call, I'm left wondering what did Chris want for like two hours, maybe five hours. And uh, what I tend to do is if I call somebody and I. [00:02:30] Get them and I either leave a message rarely or I just send a text, Hey, I'm checking in. Hey, I got some thoughts on a script we wrote, or Hey, I got some thoughts on episode. So you know, I would appreciate a little text message every once in a while. And actually
Chris Bolhuis: a, response to that. Can I, can I
answer
Dr. Jesse Reimink: I figured you
Chris Bolhuis: I have some thoughts on this. Yeah, yeah, yeah. I called you today because, I felt like I needed to talk to you about it, and I didn't wanna do this long, rambly text, you know, so I, I don't have time for that. So I thought this would be just a one or two [00:03:00] minute conversation, right? You didn't answer. Look, here's the thing, my day is not like your day at all. You have nothing going. Okay. I I have to, I have to plan out when I'm gonna pee during the day. It's a, it's a dilemma. I'm serious. And so I'm like, oh, okay. I got 30 seconds. I'm gonna quick call. I'm, I, it's too long to text and, and you don't pick up. I'm like, all right, screw it. And then I'm, I'm onto something else. You know? It's so
Dr. Jesse Reimink: Don't, don't, [00:03:30] don't play. Don't play the busy, uh, busy day card on me here, . I know, I knew you were gonna do that. And it's a cheap excuse and I can see right through it, Chris, just a short little text. Just say, Hey, uh, call me back with your chance. This thing, you know, that takes you five seconds to send. Maybe, maybe with your big thumbs it takes you a little bit longer cuz you gotta like delete, delete, delete, delete, and then rewrite it. . But you know, just a little
Chris Bolhuis: I, yeah, I do a fair amount of that, but,
Dr. Jesse Reimink: Anyways. I was, uh, it was
Chris Bolhuis: interesting. Uh, how long is, how long have you been harboring this?[00:04:00]
Dr. Jesse Reimink: uh, about, uh, what is it, four hours probably . No, I was actually thinking about it.
Chris Bolhuis: this thought just occurred to you today, or have you
Dr. Jesse Reimink: Oh no, actually, I mean, I thought, I thought of it because we were talking about this New York article where it like sort of went through modern etiquette and you know, it was mostly for my generation where we grew up with cell phones, or at least we got cell phones when we were in
Chris Bolhuis: my gosh. All right. Whatever. So listen you, I, I don't like the way you text actually, and you text me and you, you are three word texts [00:04:30] and I'll get 10 of them in a row. It's so.
Dr. Jesse Reimink: No, you like that, you like the buzz in your pocket. , you like the phone buzzing. I do that on purpose.
Chris Bolhuis: it. I, I, I cannot stand it. So, um,
Dr. Jesse Reimink: well, we have, uh, we have some work to do in our, electronic communication, protocols. I would say probably
Chris Bolhuis: But I am actually offended by this whole discussion because you have notoriously been the person that. You won't respond. You are hard to get ahold of. [00:05:00] And that was one of the conditions of me doing this podcast with you, is like, I need access to you and I need it When I called and when I text, like I, I need to know what's going on. And so now for you to be criticizing me about this is very hurtful.
Dr. Jesse Reimink: How the worm has
Chris Bolhuis: I'm a
Dr. Jesse Reimink: and going, let me take my coat off here. I'm getting all, I'm getting all worked up about this now. Uh, no, I think, uh, you're totally right. I, that was one of your conditions about the podcast is, Jesse, you gotta answer when I call. You gotta respond to my [00:05:30] text messages. And I think I've been pretty dang good about that actually.
Chris Bolhuis: you actually have, you've been good.
Dr. Jesse Reimink: Okay. Well Today is part six. So you think, you know, plate tectonics and I think we have this really ironed out now, Chris, this is gonna be a seven part series . Like we're, we're at a stage, we know the length , we're not gonna go 10, we're gonna cut it off at seven and we might pick it back up later on down the road. But with the current set of questions, we've got seven. So what's [00:06:00] part six, Chris?
Chris Bolhuis: internal dynamics and plate tectonics. So what we're gonna talk about today is the interior structure of the earth. , you know, we're gonna talk about the chemical properties, the physical properties of the, like, the layering of the earth and how that's done. Um, how do we know what the interior structure is? Because this is stuff that most of the time we never get to lay eyes on and we never will. What is the hottest layer of the earth and why is the hottest layer a solid? And then we're gonna talk a little bit [00:06:30] about the liquid outer core and why it's a liquid. And then we'll wrap. By talking about the mechanisms for plate tectonics. So I think this is pretty basic stuff that I think a lot of our listeners are gonna be, it might be a refresher or it might just be straight up review, for things that, they already know. However, I said that with the last episode on magnetism, and I'm listening back to the episode with my wife and, and Jenny's like, Chris, [00:07:00] I have no idea what the heck you're talking about. So you know what you said about most people. Like, I don't know if I don't fit into the category of most people, but I do not know
Dr. Jesse Reimink: Yeah. Right. It, it's, it's a fair point, Jenny. It's I think this is a, a common question though too, like, especially the parts about how do we know the interior of the earth? So I think we should probably start there. And the first question that, and again, the structure of this is you basically put together a bunch of questions to ask your class that sort of [00:07:30] tested their knowledge about plate tectonics because it is the operating paradigm or the paradigm by which we understand how the earth works. So, The first one here is what is the interior structure of the earth? The chemical and physical behavior is, is kind of what we're talking about here. And Chris, why don't you just run us down on the basic layering of the earth. And we have two categories, right? Chemical and physical. So those are two different categories of layering.
Chris Bolhuis: Um, which one do you wanna do first? Do you wanna do chemical
Dr. Jesse Reimink: I prefer [00:08:00] chemical. I'm a ju. Yeah. I like chemistry. So
Chris Bolhuis: That's, that was a dumb question. I . Yeah. I shouldn't have. Yeah. Okay. All right. Okay. the chemical layering of the earth basically goes crust, mantle, and. . those are the three basic layers that are distinguished based upon their stark chemical compositional differences. Okay, so outside in crust mantle core, now with the crust, there are two [00:08:30] kinds of crust. We have oceanic crust and continental crust. And this layer, when you compare it to. , the overall size of the earth, the crust is really thin skinned. I mean, it's, it's almost like a, it is, it's like a, a bowling ball with wax paper over it. That's about the proportion of the crust. very, very thin. so let's talk about the two kinds of crust then Oceanic versus continental [00:09:00] Jesse. So what's the difference between. What are the main differences between them?
Dr. Jesse Reimink: Yeah, this kind of goes back to the first, uh, episode in this, so you think, you know, plate tectonic series, we were talking about igneous rock types. You know, the rock basalt and the rock Granite. Really oceanic crust is thin. It's about seven kilometers thick. and it is mostly basalt. It is composed mostly of basalt.
Chris Bolhuis: And what does Basalt look
Dr. Jesse Reimink: Basalt is a dark, fine grain colored rock. it basically looks like a black, maybe greenish, maybe slightly brownish rock, fine grain. Can't [00:09:30] really see many minerals in it, if any at all. Uh, it's what you think of when you have a lava flow. If you are picturing a lava flow in your head, it's probably a basaltic lava flow. The continental crust is significantly thicker. It's about 35, sometimes up to 70 kilometers thick. And it is mostly an intermediate rock called Endocyte, or that's the average composition, but we have a lot of Granite in there. We do have some basalt in the continental crust, but on average it's intermediate, what we'd call endocyte. and so it kind of floats higher [00:10:00] and sinks a little bit lower cause it's thicker.
Chris Bolhuis: Okay. That's a good point, Jesse, that I want you to expound upon just a little bit that you said that it's andesitic because this is something new For people that have taken one or two Geology classes, it's typically said that continental crust is fsic or granitic. The opposite of basalt. Basalt is black and fine grained. Granite is very light colored, rich in quartz and felds bar, and it's coarse grained, but you just kind of [00:10:30] contradicted it a little bit by saying that the average composition of the crust is anesthetic. So explain that a second. I think you just kind of glossed over it. Go
Dr. Jesse Reimink: Yeah, no, that's, that's a good point. So the continental crust, this 35 on average kilometer stick piece of Continental crust. There are a bunch of different rocks in the continental crust. We can look at this on 80 geologic map. You look and you see all this complexity. You see there's granites over there. There's a bunch of sediments over there. There's some limestone cliffs over there. There's a huge basalt flow over there, like it's an amalgam. There's a [00:11:00] whole bunch of stuff. In the continental crust, if we look really, really at, at sort of the volumes, , the volume breaks down into two parts. The upper part of the crust is granitic, mostly on average. That's sediments. That's granites. They're really high in silica, aluminum, potassium, these types of elements. So that's kind of a granitic composition. The upper part of the continental crust, the lower part, There's some debate about what the composition of the lower crust is cuz we don't have a ton of samples of it. But on average it's more meic, it's more like [00:11:30] basalt than it is like Granite probably. And so if you average those two things, you get intermediate . If you average meic and Fsic, you get intermediate and you get something on the order of an acidic.
Chris Bolhuis: that makes sense. so we talked about the crust then this really, really thin skin. But when we're talking about the chemical compositional layers, The crust can be fsic, it can be intermediate, and it can be mapic. That is the chemical composition of the crust. Okay? [00:12:00] The layer right below that is the mantle, and there's a boundary that separates 'em. It's called the, the moho. We, we usually just abbreviate it and call it the moho. do you formalize that in your class, Jessie, that you teach?
Dr. Jesse Reimink: I just call it the moho, but like I said, like we've belabored before. I think you can get into a lot more detail than, than I can in my, uh, one semester class. But yeah, I just called it the mojo. The cross mantle boundary done.
Chris Bolhuis: And that's all that it is. It's not a layer, it's just a boundary. This was discovered, you know, [00:12:30] roughly a little over a hundred years ago because when seismic waves traveled through the crust and they got to this boundary, there was an abrupt change in the velocity of the p n s waves. And so that. Change in velocity was a result of a very sharp, compositional difference. The, the minerals were different. Okay. And that marks the beginning of the, mantle and the mantle. How it's different from the crust. The mantle is ultra [00:13:00] meic, so it's mapic on steroids. you know, we talked about oceanic crust being meic basaltic. This is ultra meic. So, Jesse, you're the geochemist in the house. So what is the ultra meic? What does that mean?
Dr. Jesse Reimink: Yeah, ultra Mavic is rich in iron and magnesium. Silica is a lot lower. Something on the order of 45% of the rock will be silica, whereas in a Granite it could be up to 70. 75% of the rock can be silica. [00:13:30] Soic, you also hear this. Termed peridotite or peridotitic is the rock composition of a piece of the mantle will be a Rock Tite. So you'll hear tic, you'll also hear pyrolytic sometimes to describe a mantle that has
Chris Bolhuis: Oh, I haven't heard that word in a long time. Wow.
Dr. Jesse Reimink: of creeping in a bit. I've seen it in a bunch of these, uh, you know, science news publications recently, pyrolytic Mantle and, uh, so it's kind of creeping back in a little bit. So those are all three things that broadly mean, the [00:14:00] mantle.
Chris Bolhuis: so one thing, Jesse, When my students see Tite, I mean I've, you know, they've heard me pronounce it several times, but when they read it, the most common thing that they see, they call it perdot , like, no, no, that's a gemstone. Um, this is peridotite. You got a couple syllables, you gotta slip
Dr. Jesse Reimink: I know, just another win for Geology naming, being so complicated and frustrating, for a student to learn. Um, and then Chris to, just, to [00:14:30] take it, we've got the mantle, which is a huge amount of the earth,
Chris Bolhuis: Yeah, about 2,900 kilometers. Yeah. I mean, it's, it's a massive
Dr. Jesse Reimink: Yeah, massive part. And then we hit the core, which is the iron nickel alloy part, and that's just iron nickel ally. There are some, what we call quote unquote light elements in there, something silica or lighter. We don't really know the composition of the core very well. We don't have many samples. Well, we don't have any samples of our core. We have. Iron meteorites that we [00:15:00] can kind of say are maybe analogous to the core, but we don't know exactly what the core composition is. Uh, but it's iron and nickel mainly. So that's the chemistry structure. Are we good
Chris Bolhuis: So again, the crust is fsic intermediate or meic. The mantle is ultra meic and the core is iron nickel. So those are the stark chemical differences that separate each one of these layers out. So that leads us into the other way that geologists [00:15:30] layer the earth or or think about layering of the earth, and that's based upon physical or mechanical behaviors of the rock.
Dr. Jesse Reimink: And, and this really brings us to plate tectonics a little bit because the first layer we would hit outside of the atmosphere in the ocean is going to be a tectonic plate. It is the lithosphere, what we call the lithosphere, and that is if you bend it, it will break part of the earth. It's the cold and crunchy part, and it has the crust. Is all of the crust and [00:16:00] a part of the mantle, the upper part of the mantle. And so the lithosphere is crust plus the top of the mantle. And just like crust, we have two different categories of lithosphere. We have oceanic, lithosphere, and continental lithosphere. Chris, what, uh, broadly, what are the differences here?
Chris Bolhuis: Uh, well, it's just about what sits on top of the mantle. If you have oceanic lithosphere, we're talking about oceanic crust sitting on top of the mantle. Continental lithosphere is continental crust sitting on top.
Dr. Jesse Reimink: Yeah. And the oceanic lithosphere, just to put [00:16:30] some numbers on this, is, again, this is a pretty thin layer for the most part, but the, the, we said oceanic crust is about seven kilometers thick. On average, the oceanic lithosphere is about 35 kilometers thick, plus or minus a few kilometers. The oceanic lithosphere, the upper part of the mantle is the residual. Of the melting to form basalt. So you take ultra meic mantle, that tic or pyrolytic oric mantle, you partially melt it and you get basalt. The stuff that's left behind the stuff that's not partially melted, [00:17:00] that stuff is what we call depleted. And that forms the lithosphere. So that forms the upper mantle, the base of the lithosphere. So then those two things go together and form a tectonic plate, so that's Oceanic Lithosphere. Continental Lithosphere can be anywhere from like a hundred, 150 kilometers thick to 350 kilometers thick. It's, it can get very, very thick in some places and it's a mess. So we don't really understand how it forms necessarily. So
Chris Bolhuis: That's right. Because we're
Dr. Jesse Reimink: definition,
Chris Bolhuis: snapshots down to that stuff. It's, it's deep. It's deep. Yeah. Yep. [00:17:30] All right. So Jesse, what's below it? What's below the lithosphere?
Dr. Jesse Reimink: Then we go into the asthenosphere, uh, which. Convecting mantle. This is the mantle that will flow.
Chris Bolhuis: So Jesse, I think like the best way to describe the essere is this rock that is hot, it's soft, it's, it's plastic like in, it can then flow in the form of convection. And that's really, really important in terms of, you know, driving plate tectonics one of the main mechanisms that we'll talk about at the end of the episode. [00:18:00] So that's what's below the lithosphere.
Dr. Jesse Reimink: Exactly, and, and this is because it's hot. I mean, it, it gets hot, it gets weak, it can flow, it's like plastic and you know, depending on the, era of the textbook that you might have in front of you or you know, what you find on the internet, there are different terms. People have historically broken up the mantle, the asthenosphere, into an upper mantle and a lower mantle because there is a big, seismic velocity shift. There is something happening at about 660 kilometers deep that there's some change. Most people [00:18:30] these days believe that material goes across that boundary. So the whole mantle is kind of convecting across that boundary. That boundary is a mineral logical change rather than like a, a barrier of any kind. So this kind of begs the question how we know this, and I think the answer's fairly short , right? Like how do we know the interior structure? There's basically one way
Chris Bolhuis: yeah. The, yeah. It's based on the study of seismic waves. This is geophysics at [00:19:00] its best. there are two kinds of waves that travel through the body of the earth, P waves and s waves, and the way that they refract and reflect when they encounter new compositional layers, when they encounter new physical. Properties. They do certain things and, you know, for instance, the best thing I think to explain how this works is talking about the liquid outer core. How do we know that the liquid outer core is liquid S waves? they're shearing waves, and so they can't [00:19:30] travel through a liquid because liquids don't have sheer strength. you can't tear a liquid. And so these waves, they abruptly halt when they encounter the liquid at a quarter. So studying the way these waves come back, we've determined like the size and the shape and the nature of the liquid outer core.
Dr. Jesse Reimink: And just to double click on that, we have some great images that we have for our Camp Geo, conversational textbook that is online. That's the first link in your show notes in the earthquakes chapter. Chris, we had some [00:20:00] great schematics and gifts made, uh, that are, that are really, really cool, um, and are uploaded, uh, they should be uploaded soon, if not already. So check that out if you want to see this visually really well. So, that's exactly right Chris. I mean that's a good example of this. You talked originally about the moho, where seismic wave velocities change. Seismic waves also bounce off of stuff, so they bounce off of boundaries. They also change, they convert energy between p and s wave and between different waves. And so you get all these conversions. It gets super complicated. But the complexity's [00:20:30] really important cuz it allows us to really get a good view of the interior of the.
Chris Bolhuis: you would be proud of me. Because we were talking about magma chambers, and I asked the question in a very Jesse Reimink way, I said, what? Is a magma chamber
Dr. Jesse Reimink: you, did you have your pinky in the air too? And
Chris Bolhuis: I , I had, I was, I was really pretentious about it. And, and then we talked about how, all of the models that you [00:21:00] see in books and online and so on have these idealized magma chambers. And I'm like, well, no. And I use the analogy of a. Fill in the face with marbles and how a magma chamber is mostly solid and there's a little bit of liquid in there. It's all hot, but it's mostly solid stuff. and so anyway, we talked about that because that makes me think of it when you talk, I mean, if, if magma chambers were idealized like that, they would be very easy to see the footprint of what's below this, all this vulcanism. Right? But it's very [00:21:30] difficult to do that because the magma chamber. Mostly solid. And so you don't see this, know, stark contrast between the P and the S waves.
Dr. Jesse Reimink: Yeah, no,
Chris Bolhuis: A thought
Dr. Jesse Reimink: exactly right. Nicely done. I am indeed proud of you, Chris. Well done. Well, well done
Chris Bolhuis: Ah gee. Thanks. Ah,
Dr. Jesse Reimink: Um, so I think we can move on from that one cuz that's a, a fairly straightforward answer and we have covered that in previous episodes as well. Why is the hottest layer of the earth a solid? This is the next question we have in our [00:22:00] list here. Why is the hottest layer of the earth a solid, which this is a, a great question, I think, because if you sit down and think about it, it's not very intuitive that that is the case. but if you re, if you think deeply about it, as we talk about it, I think it becomes intuitive that this is the case.
Chris Bolhuis: I think so. This is something that, you know, a, a couple of my students sitting there when I ask this question, they don't have a lot of exposure. I'm talking to my upper level Geology students. Some of 'em are gonna, are gonna know [00:22:30] the hottest layer of the earth is the solid inner core. And then of course, that begs the next question, well then why is the liquid outer core, why is it liquid? If that's not the hottest layer? And a couple of them will be able to think that. And they'll come to the right conclusion. They begin to talk about pressure, and that's really what it is. you have this geothermal gradient, this steady increase with temperature, with depth. So the hottest layer of the earth is the deepest layer of the earth, and why is that solid [00:23:00] then? If it's hottest and it's simple. It really, it is. I mean, it's not intuitive, but like you said, I think it becomes more intuitive if you think it through pressure squeezes it into a solid. In other words, pressure becomes more of an important factor than temperature at that depth and the liquid outer core, the recipe of temperature and pressure is just right for it to be a liquid.
Dr. Jesse Reimink: And I think, again, I don't want to belabor the point, but if you can go back [00:23:30] to our Camp Geo chapter on igneous rocks, we again have some cool examples, cool schematics of how melting happens. And I think the way I explain this in class is I, I think about it from the reverse, not why is it a solid, but why is it not melted? And If you think about how melting happens, it makes more intuitive sense. So if you start with something that is solid, it's under high pressure, it's being squished, and you can think of this like squeezing out a sponge. If you squish that, the water comes out of it. And if you squish a rock, all the stuff [00:24:00] that wants to be water is going to leave it. And the magma, the liquid part is gonna rise to the surface cuz it's less, dense. , if you think of it that way. It makes a little bit of sense now, think about how we would melt this stuff. Well, if it's solid, it's sitting there, it's solid at the same pressure. If you heat it up, usually things will melt, so you can add heat to it and it'll melt. It'll just sort of increase the temperature. Inducing some melting. The opposite thing can happen, and this is what's going on in the lower part of the liquid outer [00:24:30] core. So the bottom part of the liquid outer core, that's gradually cooling down. As it cools down, it starts to crystallize at the same pressure. It's starting to crystallize. So the inner core is growing as the outer core is crystallizing. Metal . Basically metal crystals are crystallizing right at the base because that's the highest pressure part of the liquid outer corn. And the same thing happens in the mantle too. if stuff upwells, it decompresses, it can melt. Now, kind of just think about the earth. We've got rock down [00:25:00] below than liquid ocean on top of it, then gas above it. That's because of the pressure differences and those things are happy at those pressure differences
Chris Bolhuis: If you take really, really hot rock and do nothing else to it, but lower the pressure on it, that rock. Become liquid, it can melt. And that's a, it's a very important thing that happens in generating ocean floor, this rising and convecting. Asthenosphere, well, it's that rock we, we just said. It's right near its melting point. As it rises up toward the [00:25:30] surface towards lower pressure, it partially melts and that's what comes out then on the ocean floor. So it's a, this pressure relief melting. Really, I think, important in understanding why the liquid water core is liquid,
Dr. Jesse Reimink: And I think that it, it is quite an intuitive thing. because if you just ask people what's more dense, a solid or a liquid, they'll usually say a solid is more dense. The vast majority of people, I mean, unless you're thinking about ice in water, but heavy things or solid things, sink, , that's [00:26:00] intuitive. And so, you know, of course the solid stuff is gonna be at the bottom and that is the case with the mantle mostly. Uh, and especially with the core, which leads us very nicely into the next question. Why is the liquid outer core liquid, which is kind of the inverse of why is the hottest layer of the earth, the solid right?
Chris Bolhuis: right? Yeah. I think, I don't know. We think we've kind of answered it. Haven't, haven't we?
Dr. Jesse Reimink: I would agree. Yeah. Yeah. I think we've gotten that. It's, but I think Chris, the, the one important thing and, and actually a fun visual that I, at least a [00:26:30] visual I find quite fun, is thinking about liquid iron metal. Convecting, that's the liquid outer core. That's where a magnetic field, uh, is generated, which we talked about in the last episode in this series. Think about that. Hitting the base of the mantle, which is solid, the mantle solid down there, the lower mantle is solid, and so you've got this liquid convecting underneath of it, and you think about how does that happen? Why is there solid above liquid at the core mantle boundary? [00:27:00] And I, I think that's kind of a fun visual to think about and to think about why that might be the case.
Chris Bolhuis: Um, do you want to answer why that is the case? are you just gonna leave us hanging
Dr. Jesse Reimink: Well, I, yeah, we, we can answer. I mean, it's just that metal is more dense than rock, so, even liquid metal is more dense than solid rock. So solid rock will float on top of liquid metal, of iron nickel alloy. So that's, it's just a density difference even though there's this phase difference that seems inverted to our intuition. So,
Chris Bolhuis: Jesse, that brings us [00:27:30] into the last part of this episode, which are the mechanisms for plate tectonics. So this is an opportunity for you to show us how overeducated you are. Uh,
Dr. Jesse Reimink: Always, always my
Chris Bolhuis: It's the truth. It it is the truth. Like you just light up like a
Dr. Jesse Reimink: It's always what I'm trying to do. I'm just trying to find people out there in the world who
I can show how overeducated I am too.
Chris Bolhuis: Okay, well, let's go then. All right. I don't know we've talked about this actually, and I think we've talked about it on Planet Geo [00:28:00] before. The main mechanism for plate tectonics is always, everything you read talks about how convection is this main mechanism. I just, it just doesn't do it for me. The, you know, there's something missing in this, and I think that I don't know. I'm asking you this. Does the literature. Um, I'm sure that, that you're coming across things that are contradicting this as the main mechanism for plate tectonics at the research level.
Dr. Jesse Reimink: Yeah, absolutely. I [00:28:30] think actually there's, there's more of a consensus around one of the other mechanisms. So let's just list the mechanisms. Can you just list the four mechanisms that we're talking about here, Chris? So, and then we can dive into '
Chris Bolhuis: Convection number one, because that's always the one first listed. Slab push and slab pull, and then mantle, plumes. Those are the four main mechanisms or slab, pull and slab push kind of work together. So maybe three if you combine 'em and lump 'em together in one.
Dr. Jesse Reimink: Yeah. And, and I'll just kind of give you a visual. Let's just [00:29:00] work through visuals of these four really quickly. Convection the mantle. We talked about the asthenosphere mantle, that's convecting. So if this is convecting and upwelling, this stuff, when the convecting part rises to the surface, it hits the base of the lithosphere. It spreads out laterally, it it moves right and left. And that could kind of drag the lithosphere along. So that's one idea. Convection. It's kind of the, the mantle's convecting. It's causing upwelling and it's moving stuff. Think about a boiling pot of water where you see the water moving away from the center of the [00:29:30] pot where it's hottest. And that's, that's kind of convection. If you put some stuff on the surface, it'll spread laterally. Um, slab push or sometimes called ridge push is basically the hot part of the oceanic plate right along the mid-ocean ridge. Oceanic plate is being formed. That stuff's high and it's hot, higher elevation, hence the ridge and it's hot, and that is kind of sloughing off to the sides. It's pushing the plate away from the ridge. That's the ridge push mechanism. or slab. Push slab pole is really the main one where [00:30:00] consensus I think has been built around this being a main driving force is when an oceanic plate abducts, it's diving down into the mantle. At some point it becomes super dense, converts to elo. Fat behaves like an anchor and
Chris Bolhuis: big words here. we go.
Dr. Jesse Reimink: and it pulls the plate down the pla. It sort of behaves like an anchor with an anchor rope, and it kind of drags the rest of the anchor chain down off the boat. Pulls it away. The last one, what did you say? Mantle
Chris Bolhuis: Well, hold on. [00:30:30] Let's, I'm not ready to leave this yet. so it's being pulled and pushed in the same direction, you know, and it's all connected. That's why it's called slab often, where, you know
Dr. Jesse Reimink: pull is what you're referring to, those two, right?
Chris Bolhuis: That's right. That's right. And they're being push. And pulled at the subduction zone in the same direction. But I wanna ask you, cuz this is, this doesn't quite make sense either. if slab pull is a main driving mechanism, what [00:31:00] causes the subduction then it's kind of like putting the cart before the horse or the horse before the cart. Like, you know, what do you do? Uh, you know, something had to cause the subduction zone to begin.
Dr. Jesse Reimink: I think this is, uh, this is a really interesting question and a really deep question because you sort of ask the people who model the planet, uh, from a plate tectonics or GEODynamics standpoint, and we don't actually know. It's hard to do with modeling. you don't just start with a lithosphere and asthenosphere and then the lithosphere breaks and subduction [00:31:30] starts. That actually doesn't really happen in these models. So we don't actually know how you break the litho here to start a subduction zone system. There's a whole bunch of idea. Potential ways to do it. It's hard to really model the physics of this accurately enough to do it. there's a bunch of ideas, but you're keying in on a really. Important point is that this is a sort of a chicken or the egg argument once it started, it's easy to keep it going. And actually if you start subduction in one place, it's easy to start plate tectonics globally that way because of the slab pole mechanism. but how do you get that going in the first [00:32:00] place? Is it. Is an open question. You kind of have to have some horizontal movement due to convection or due to uh, this sort of slab push mechanism. a big huge mountain range piles up. Think Olympus Mons on Mars, this huge thing that's sort of sloughing off to the sides and getting really dense and that could potentially pull stuff down. There's other people who. Giant impacts early on in earth history, a big impact punches through the lithosphere and then you start subduction basically slab polk and start on the edges of [00:32:30] that. so there's a whole bunch of different ideas about that, but it's a great question.
Chris Bolhuis: Which leads us into the last part of this then , the other kind of newer mechanism of plate tectonics mantle plumes. So let's talk about mantle plumes a little bit. Um, Jesse, why is so new and, and why is this becoming, I don't know. I kind of get the feel that it's becoming more and more prominent. It's getting traction.
Dr. Jesse Reimink: I think it, it's uh, so why is it becoming Well, [00:33:00] we've started to see that, and this is because of seismic imaging technique development. So really, advanced computer models that can really tease apart the details of these really complicated seismic waves that are penetrating through the earth. And people do what's called tomography, which is imaging the whole Earth. And we've seen that. What are called ? We've had listener questions about this before. They're called l L SVPs, large, low shear wave velocity provinces, like the most boring name in the world, right? [00:33:30] But basically what these things are are huge, huge piles of. Either hot or chemically different stuff at the base of the mantle and there's two piles. There's one kind of under Africa and there's one kind of un kind of under the Pacific, and these things are thought to feed mantle plumes, or maybe they are the mantle plumes and they feed the. Plumbing systems that feed Hawaii or, Yellowstone or these different, ocean Island basalt provinces, , or volcanic island chains. [00:34:00] So these things are like big piles of stuff down in the mantle that are piled up on the core mantle boundary. And they come up, I forget what it is, maybe 1500 kilometers high or a thousand kilometers high. So these things are, are really big and they. 10% of the mantle is these things, these lls SEPs and
Chris Bolhuis: Okay. So they rise up.
Dr. Jesse Reimink: so they've been newly identified. That's the answer to that.
Chris Bolhuis: Okay. So they rise up they will plume out or pool at the base of the lithosphere.
Dr. Jesse Reimink: let me interrupt there [00:34:30] real quick, Chris, and say the LCPs are down there and those are kind of not moving, at least that we see, but these, there's little plumes coming off. So think of like little jets of stuff coming off of them that are then feeding the magma system. And this is what you're talking about. So this magma will kind of rise up, hit the base of the lithosphere and pool,
Chris Bolhuis: Yes. Okay. So how is this a mechanism for plate tectonics? This kind of pooling at the base of the lithosphere?
Dr. Jesse Reimink: I think it does a lot of stuff. I mean, you can envision this providing a bunch of [00:35:00] heat and kind of eroding the lithosphere above it. The, the mantle plume can kind of destroy lithosphere cause it's just punching a ton of heat into this system, A bunch of magma. It can also focus other. Plate tectonic scenarios. So for instance, Iceland. Iceland has a mantle plume under it and the mid ocean ridge system running through it. And some people would argue that the mantle plume has captured the ridge system. So the ridge system is kind of locked on this mantle plume source, and it's not migrating around like other
Chris Bolhuis: [00:35:30] Okay. I gotta interrupt you here though, because yeah, we see that with Iceland, but you know, we don't see that with. It's well established that we have this mantle plume beneath Hawaii, but we don't see a divergent boundary there. We don't see that with Yellowstone either. We don't see that extension how does this fit into the plate tectonics theory then.
Dr. Jesse Reimink: the mental plumes. A little bit independent in some ways of plate tectonics. Now they can interact with the plate tectonic [00:36:00] surface of the earth. Like in places like Iceland where they can kind of capture a ridge. they also, there's, some thoughts that, mantle plumes, especially big mantle plumes, could potentially break the lithosphere. So could form a triple junction where the, the mental starts to break apart or where the litre starts to break apart. So, for instance, where we're from in Michigan, there's this big mid-continent rift system, which is a failed rift. The continent almost broke apart there. And the idea was maybe there's a plume that hit the base of the lithosphere, tried to break it, but didn't. And so plumes can sometimes [00:36:30] break continents in half by, by doing this.
Chris Bolhuis: Makes sense. All right.
Dr. Jesse Reimink: So I think
Chris Bolhuis: Jesse,
Dr. Jesse Reimink: it. Chris. I, yeah. These, uh, this was a, a fun episode. Fun conversation.
Chris Bolhuis: it was, it was, um, I, I, so I talked to my mom. Joyce. Um, and so she sent me a couple of questions, not really questions today because, you and I talked about this, like which, which kind of try to keep her in mind as we go through this. And I know that today we probably lost her a little bit and, and I think she might be sleeping by now, but [00:37:00] she, she did make two points. Okay? The first one is
Dr. Jesse Reimink: like, you know, Jesse should do this better. Chris, you should do this
Chris Bolhuis: Well, no. No. So no, not really. Not really. She, I think she's gonna come up, she's making lists of questions that she wants to ask us, but, two things today is I, I was over there. I usually try to go over there on Friday afternoons after work and, and, you know, spend an hour, an hour and a half with him and so on. I'm a, I'm a good son. Okay? And so my mom, we were just talking and I dropped a word. [00:37:30] I said perseverate and Like that word shocked her. So she texted me today and said, what are you gonna perseverate on today? And so that was her first. First, first point. Was using that word in a sentence. Okay.
Dr. Jesse Reimink: what were you gonna perseverate on? I think Chris, I thi I, I think we perseverated on the, uh, the chemical structure of the earth. We perseverated on that one today.
Chris Bolhuis: We did. We, we absolutely did. The second point that she made is [00:38:00] she thinks that your voice is very melodic. So it, I, well, I don't know if it's a good thing though, because it does tend to put her asleep
Dr. Jesse Reimink: Well, that's okay. I mean, you know, if there's anybody who wants me to be the voice of their, you know, do some audio recording for advertising, I'm open to it. I can be the voice for, car company or, you know, maybe Rock Hammers, S Wing, maybe S Wing wants me to be the voice of rock hammers for them. I'd be up for that. So I, I sound better on the radio than you do, Chris, it sounds like, [00:38:30] and coming from your mother who,
Chris Bolhuis: know how to take it.
Dr. Jesse Reimink: He was
Chris Bolhuis: I know
Dr. Jesse Reimink: to appreciate your voice more than mine,
Chris Bolhuis: well, like I said, I don't know if it's a good thing cuz you do put her to
Dr. Jesse Reimink: Fair enough. Fair enough.
Chris Bolhuis: but you're boring and, and so I don't, I don't know.
Dr. Jesse Reimink: I'll, I'll take it. I agree with both of those. Probably . All right. Hey, that's a wrap for this episode. You can follow us on all the social medias. We are at Planet Geo Cast. Give us a like and a review and a rating on your podcast platform that really helps us. You can go to our [00:39:00] website, planet geo cast.com. There you can support us, find all our past episodes and look at some transcripts and learn about us. See pictures of Chris as well, There's a lot of pictures of Chris on there. And, uh, last thing, go to our camp Geo conversational textbook. First link in your show notes. There you can learn all the basics with images, where basically we put podcast meets textbook. That's what's going on there. So first link in your show notes. Check that out.
Chris Bolhuis: Right on. Cheers.
Dr. Jesse Reimink: Peace.
[00:39:30]
