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When Did Plate Tectonics Begin? Plate Tectonics Part 4 · PlanetGeo: The Geology Podcast

• 0:02
  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
  unknown: Mr.
• 0:13
  SPEAKER_01: Bullheis.
• 0:14
  SPEAKER_01: Well, hey, Dr.
• 0:14
  SPEAKER_01: Remick.
• 0:15
  SPEAKER_01: How are you doing today?
• 0:16
  SPEAKER_01: I'm well.
• 0:16
  SPEAKER_01: How are you?
• 0:17
  SPEAKER_01: That's twice in a row now you've called me Mr.
• 0:19
  SPEAKER_01: Bullheis.
• 0:20
  SPEAKER_01: That needs to stop.
• 0:21
  SPEAKER_01: Um that's no, I'm on Christmas break right now.
• 0:25
  SPEAKER_01: You can you do not want to be called that at all.
• 0:27
  SPEAKER_01: Going back to 05, baby.
• 0:29
  SPEAKER_01: Back to 05.
• 0:32
  SPEAKER_01: Hey, I I gotta I want to show you something.
• 0:34
  SPEAKER_01: I'm gonna unzip my hoodie here a second.
• 0:36
  SPEAKER_01: I want you to look at this.
• 0:37
  SPEAKER_00: Oh, do you have a shirt on there?
• 0:39
  SPEAKER_00: Okay, you do.
• 0:39
  SPEAKER_00: Hold on.
• 0:40
  SPEAKER_00: Oh, uh Tyrannosaurus riding a mountain bike.
• 0:43
  SPEAKER_00: What is going on with that?
• 0:44
  SPEAKER_00: Yeah.
• 0:45
  SPEAKER_00: That's pretty funny.
• 0:46
  SPEAKER_01: Sweet shirt, isn't it?
• 0:47
  SPEAKER_00: Yeah, we're what's it uh what's the brand we're repping here?
• 0:50
  SPEAKER_01: I don't know.
• 0:51
  SPEAKER_01: I just saw the shirt and um I'm like, I have to have that.
• 0:54
  SPEAKER_01: I love t-shirts.
• 0:55
  SPEAKER_01: And you know, anything geology related that's funny, I gotta have it.
• 0:59
  SPEAKER_00: So we were here in uh over the holidays, and my nieces were here in town, and we went to this state park nearby, which has this amazing light festival.
• 1:08
  SPEAKER_00: Like I'd never been to it before, but it's probably about three-quarters of a mile through the woods where it's just completely lit up, like lights everywhere.
• 1:16
  SPEAKER_00: It was unbelievable.
• 1:17
  SPEAKER_00: And uh, so we're walking along, you know, my nieces are having a great time, they're little and they're kind of you know running around looking at things, and then there's a couple dinosaurs and a volcano, and they're like, Look, Uncle Jesse, a dinosaur.
• 1:28
  SPEAKER_00: You must like that.
• 1:30
  SPEAKER_00: There's a Tyrannosaurus and a volcano, like you know, they have in the little section there.
• 1:34
  SPEAKER_00: So anyway, we had to take a photo in front of it.
• 1:36
  SPEAKER_00: It's great.
• 1:37
  SPEAKER_00: They know you, yeah, totally.
• 1:38
  SPEAKER_01: They know who you are.
• 1:39
  SPEAKER_01: Do you got a photo of it?
• 1:40
  SPEAKER_01: Oh, yeah, yeah, yeah.
• 1:41
  SPEAKER_00: For sure, for sure.
• 1:42
  SPEAKER_01: Good deal.
• 1:42
  SPEAKER_01: All right.
• 1:43
  SPEAKER_01: Well, hey, how are you doing today?
• 1:44
  SPEAKER_00: I'm good, man.
• 1:45
  SPEAKER_00: We're back.
• 1:46
  SPEAKER_00: We're back with new stuff.
• 1:47
  SPEAKER_00: And uh this is uh this is part four in our plate tectonics question series, which has been a fun and uh winding journey, I think.
• 1:55
  SPEAKER_00: So the origin of this, just to go back to what mid-2022, you know, Q3 2022, was that you know, you had this series of questions that you gave your class to kind of test them on, you know, they were bitching and moaning about how they thought they knew plate tectonics.
• 2:11
  SPEAKER_00: So you gave them these kind of some rather cursory but some pretty deep questions about uh plate tectonics and and sort of how it is written in the rock record, I guess, or how it affects the rock record.
• 2:22
  SPEAKER_00: Like how does plate tectonics generate basalt?
• 2:25
  SPEAKER_00: And then we so we went through several of these and we went through sedimentary rocks, we covered sub igneous rocks, we talked about planetary plate tectonics in the last one in this series in episode three of this section.
• 2:37
  SPEAKER_00: And so today we're not done, but we're we're maybe halfway through-ish, around about there.
• 2:43
  SPEAKER_01: Yeah, this episode here is a I'm gonna just be flatter honest with you.
• 2:47
  SPEAKER_01: It's a bunch of questions that I really want to ask you.
• 2:50
  SPEAKER_01: This is higher level stuff.
• 2:51
  SPEAKER_01: I I think today is gonna drive home the point, at least for me, that I think there's more about plate tectonics that we don't know than we actually do know.
• 3:01
  SPEAKER_01: And so when I have a bunch of 15-year-olds telling me, oh, I got this, I know this, you know, that's a that's also a little irritating for me too, because I'm like, well, wait a second.
• 3:10
  SPEAKER_01: I mean, you know, there's a there's a lot here we don't know.
• 3:12
  SPEAKER_01: And I think the biggest thing in this, Jesse, is uh whenever somebody first learns about plate tectonics and you learn about the mechanism of plate tectonics, right?
• 3:23
  SPEAKER_01: Everything out there all says convection.
• 3:27
  SPEAKER_01: Convection within the mantle, convection within the asthenosphere drives these lithospheric plates about.
• 3:33
  SPEAKER_01: And and you know, it's just like there's something about this that rubs me the wrong way.
• 3:37
  SPEAKER_01: It's I just don't think that this is as easy and complete as what it's made to be.
• 3:42
  SPEAKER_01: I there's something more going on.
• 3:45
  SPEAKER_01: It's not it can't be just convection.
• 3:47
  SPEAKER_01: There's a lot going on with that.
• 3:49
  SPEAKER_00: That's right, I think.
• 3:50
  SPEAKER_00: Uh it's not just convection.
• 3:52
  SPEAKER_00: That's that's for sure, that's for sure right.
• 3:54
  SPEAKER_00: Um, it's not as simple as as that.
• 3:56
  SPEAKER_00: And and you know, that's in many ways the textbook answer, which is is good enough for students in your class and and good enough for students in my class, the the intro level class I teach.
• 4:06
  SPEAKER_00: But in the higher level classes, you got to dive deeper and you got to really understand that there's a lot of nuance to this.
• 4:11
  SPEAKER_00: And this is a really complicated one.
• 4:13
  SPEAKER_00: So the theme of this episode, I think, is when did plate tectonics start on earth?
• 4:18
  SPEAKER_00: And let's give away the ghost here immediately that we don't know the answer to that.
• 4:24
  SPEAKER_00: And that is a huge part of my research.
• 4:26
  SPEAKER_00: So it's a good thing we don't know because it's a fair amount of job security for me.
• 4:30
  SPEAKER_00: Is that a lot of what I do is is try and understand when did plate tectonics start on earth, because we don't really have that pinned down well at all.
• 4:37
  SPEAKER_01: That's right.
• 4:38
  SPEAKER_01: So we're gonna start with that, and then I think the next question is well, why is that answering that question important?
• 4:44
  SPEAKER_01: Why is you know answering when did plate tectonics begin, or what do we think?
• 4:48
  SPEAKER_01: What is our best idea about this?
• 4:50
  SPEAKER_01: And and who cares?
• 4:51
  SPEAKER_01: So what?
• 4:51
  SPEAKER_01: That's where we're gonna start.
• 4:52
  SPEAKER_01: And then what we're really gonna do after that is I'm gonna ask a bunch of questions that kind of get into nuances and a little bit of the research world of what's going on today.
• 5:05
  SPEAKER_01: And so we're gonna introduce some things that maybe most of our listeners have never heard of before, things like sagduction and squishy lid tectonics and and things along this line.
• 5:16
  SPEAKER_01: And totally I'm do my best to keep you on track.
• 5:19
  SPEAKER_00: Okay, because you're gonna have your work cut out for you.
• 5:21
  SPEAKER_01: You and I were talking about this.
• 5:23
  SPEAKER_01: We were talking about this before we started, and you know, you're like, well, I mean, this could be really, really big, Chris.
• 5:28
  SPEAKER_01: And and uh I'm like, okay, yeah, it could be, but we don't need to do that.
• 5:31
  SPEAKER_01: And so I'm gonna I'm gonna keep you on track with this.
• 5:34
  SPEAKER_01: Okay, good.
• 5:34
  SPEAKER_01: Um, but that's really the theme of of today, then is we're gonna, I'm just gonna ask you a bunch of questions, and it's really this is gonna be a conversation back and forth that really started with the topic for today.
• 5:47
  SPEAKER_01: When did plate tectonics begin?
• 5:49
  SPEAKER_01: And why is that an important thing?
• 5:51
  SPEAKER_01: So let's can we start?
• 5:52
  SPEAKER_01: Yeah, let's start.
• 5:53
  SPEAKER_01: Let's go for it.
• 5:54
  SPEAKER_01: Well, then I'm let's go with that then.
• 5:56
  SPEAKER_01: The question is when did plate tectonics begin?
• 5:59
  SPEAKER_01: And then why is that important?
• 6:01
  SPEAKER_01: Let's start with that.
• 6:02
  SPEAKER_00: Well, the short answer is that we don't know when plate tectonics began on Earth.
• 6:07
  SPEAKER_00: So I'll kind of give you my view of the landscape of the research landscape.
• 6:11
  SPEAKER_00: Like what do most researchers, knowledgeable, thoughtful people think about this?
• 6:17
  SPEAKER_00: Most people agree.
• 6:18
  SPEAKER_00: I would say there's a consensus view that plate tectonics began at least by about 2.5 billion years.
• 6:24
  SPEAKER_00: So plate tectonics has been operating for the last two and a half billion years of Earth history.
• 6:30
  SPEAKER_00: Now, you'll see scientific papers written to the contrary.
• 6:33
  SPEAKER_00: That would say plate tectonics only began 500 million years ago or only began 700 million years ago.
• 6:38
  SPEAKER_00: Those are fringe ideas and in my opinion, not super well founded.
• 6:43
  SPEAKER_00: Um, consensus view is that plate tectonics has been operating at least for the last two to two and a half billion years on Earth.
• 6:50
  SPEAKER_00: Before that, it's anybody's guess.
• 6:53
  SPEAKER_00: I have my own opinions, but if you talk to 10 people who study this thing, you'll get 10 different opinions about when plate tectonics started.
• 7:00
  SPEAKER_01: But there has to be a ballpark, right?
• 7:02
  SPEAKER_01: There has to be a range in this that's more acceptable.
• 7:06
  SPEAKER_00: I think most people would really agree that around between 2.7 and 3 billion years ago was probably a good time.
• 7:15
  SPEAKER_00: They would sort of if you had if you force somebody to pick a number, they'd probably pick something in between there.
• 7:20
  SPEAKER_00: I mean, maybe half the community would pick something in that window and and half would be earlier than that.
• 7:25
  SPEAKER_01: Okay.
• 7:26
  SPEAKER_01: So here's where my mind goes with this.
• 7:28
  SPEAKER_01: Listening to you talk about, you know, the nuances and so on about when plate tectonics began.
• 7:33
  SPEAKER_01: My mind goes to this.
• 7:35
  SPEAKER_01: We talked about this in some of our earlier episodes, that the great diversity of rocks that we have on our planet, they exist because of plate tectonics, right?
• 7:44
  SPEAKER_01: And so if we can then date, you know, radiometric date these rocks, these diverse set of rocks, then doesn't that give us an idea?
• 7:54
  SPEAKER_01: Like, is that a how do you do this?
• 7:56
  SPEAKER_00: Is that is this where you start?
• 7:58
  SPEAKER_00: That's exactly right, Chris.
• 7:59
  SPEAKER_00: That's exactly the the thinking that goes into this.
• 8:01
  SPEAKER_00: Is you you gotta really think about well, what what are the key things?
• 8:06
  SPEAKER_00: If I'm gonna go to some other planet with my spaceship and I'm gonna go down and land on the surface and say, does plate tectonics operate here?
• 8:13
  SPEAKER_00: You know, I don't have any other evidence apart from the rock record.
• 8:17
  SPEAKER_00: You know, what do you need?
• 8:18
  SPEAKER_00: What do you need to see to say yes, plate tectonics is operating?
• 8:22
  SPEAKER_00: And on the modern Earth, it's easy.
• 8:24
  SPEAKER_00: We can watch GPS movement, we can see the ground uh you know uplifting, we can see volcanoes popping off, we can see seduction zone trenches.
• 8:31
  SPEAKER_00: It's dead easy on a modern planet.
• 8:33
  SPEAKER_00: Going back in time where we only have the rock record, it becomes really, really hard.
• 8:37
  SPEAKER_00: Paleomagnetism is a really, really powerful one, and we'll kind of get into that next episode.
• 8:43
  SPEAKER_00: But if you can look at a paleomagnetic record and see that a single area in a sequence of rocks of a single location had been moving at a rate of centimeters per year, it's a good indication of maybe that's a plate tectonic-like movement that's sort of similar.
• 8:58
  SPEAKER_00: But as you go further back in time, paleomagnetism becomes less reliable or non-existent in the really old rock record.
• 9:06
  SPEAKER_00: So it's really a a sample, there's a sampling bias here that the further back in time we go, the less rocks we have.
• 9:12
  SPEAKER_00: So the less information we have, and and you know, we can't play this game of do we see the diversity of rocks that we see on the modern earth that tells us plate tectonics is exists.
• 9:22
  SPEAKER_00: Okay, uh, but why not though?
• 9:24
  SPEAKER_00: If we if we see So um Let me ask you, let me turn the question around real quick.
• 9:29
  SPEAKER_00: What is the single rock that is most diagnostic of plate tectonics to you?
• 9:34
  SPEAKER_01: Um okay, let me think this this requires some thought.
• 9:37
  SPEAKER_01: Like the rock that most signifies or or like screams this is produced because of plate tectonics, I would have to say granite.
• 9:48
  SPEAKER_00: So that's definitely one of the one of the ones people use.
• 9:52
  SPEAKER_00: And and when we start to see real true granites in the rock record is around 2.7 billion years ago.
• 9:59
  SPEAKER_00: So that's why most people kind of pin that number down in there.
• 10:02
  SPEAKER_01: Hold on, but but you kind of disagreed with that.
• 10:05
  SPEAKER_01: Were you thinking more along the lines of gneiss and these really intensely metamorphosed rocks?
• 10:10
  SPEAKER_01: Is that what you're where your thought went?
• 10:12
  SPEAKER_01: Because I I the reason why I said granite is because granite is produced by distillation, distillation happens because of plate tectonics, you know.
• 10:20
  SPEAKER_01: So that's why I said granite, but uh, I was torn between do I say granite or do I say the Acosta gneiss?
• 10:25
  SPEAKER_01: I mean that's your that's your rock.
• 10:27
  SPEAKER_01: Sometimes we just call you Acosta.
• 10:29
  SPEAKER_01: Um Right.
• 10:30
  SPEAKER_00: So that's that's one um I don't disagree with you at all that granite is definitely one of the ones, and that's what people use.
• 10:36
  SPEAKER_00: You know, researchers have used that historically for a long time to say, oh, plate tectonics has been operating because there's these certain types of granites that arise on the scene 2.7 billion years ago.
• 10:45
  SPEAKER_00: Another one that people point to is blue shists, blue shists and eclogites, because blue shists are these really high pressure, low temperature ones that are indicative of subduction zone metamorphism.
• 10:55
  SPEAKER_01: And so Can I interject a second?
• 10:57
  SPEAKER_01: What what you're saying with this is that these rocks, eclogite and you said blue schist, right?
• 11:04
  SPEAKER_01: Eclogite and blue shist, they only happen in the temperature pressure recipe that can only happen in a subduction zone.
• 11:12
  SPEAKER_01: It can't happen anywhere else.
• 11:13
  SPEAKER_01: And so the minerals that you find in this rock are indicative of those temperatures and pressures which tell you of subduction.
• 11:20
  SPEAKER_01: So okay.
• 11:21
  SPEAKER_00: Exactly right.
• 11:22
  SPEAKER_00: And there are that makes sense.
• 11:23
  SPEAKER_00: There are researchers, you know, very well respected researchers who, you know, want that to be the distinguishing feature that say if there is blue schist in the rock record, if we find an old blue schist, then it means that there was a subduction zone system in that location at that time.
• 11:38
  SPEAKER_00: So let's say there's a 2.7 billion year old blue shit somewhere, and then that means subduction was happening there 2.7 billion years ago.
• 11:45
  SPEAKER_00: Therefore, that's plate tectonics if you see enough of those around, if you see a bunch of 2.7 billion year old blue schists.
• 11:52
  SPEAKER_00: Now, you have to question, is blue shist a stable you know, rock?
• 11:57
  SPEAKER_00: Is it going to last through multiple plate tectonic cycles, through multiple collisional erogenes?
• 12:02
  SPEAKER_00: Is it going to be overprinted ever?
• 12:04
  SPEAKER_00: And blue shist is a a pretty easily overprinted rock.
• 12:07
  SPEAKER_00: So it's not necessarily the best indicator.
• 12:10
  SPEAKER_00: Okay, hold on.
• 12:11
  SPEAKER_00: Does that make sense what I'm getting at?
• 12:12
  SPEAKER_01: What do you mean when you say overprinted?
• 12:14
  SPEAKER_01: Um, because what you're what you're saying, I think, is that these minerals that are in there may have existed in a rock before.
• 12:21
  SPEAKER_01: That's exactly right.
• 12:22
  SPEAKER_01: And and so if you date the eclogite or blue schist, then you're not dating when that rock formed.
• 12:29
  SPEAKER_01: You're dating when the minerals that are in that rock formed.
• 12:32
  SPEAKER_00: Exactly.
• 12:32
  SPEAKER_00: So there's two parts to that.
• 12:34
  SPEAKER_00: You're dating when the minerals formed and when the rock was metamorphosed to that particular pressure and temperature window that tells you it's subduction, that creates a blue schist.
• 12:43
  SPEAKER_00: So we can date blue schist metamorphism.
• 12:46
  SPEAKER_00: But the question is let's say a blue schist formed 3.2 billion years ago, but then it didn't survive till present because that rock might have been eroded, it might have been broken down and it into its constituent parts and broken apart, or it could have been caught in a collisional orogeny where the blue schist was heated up to higher temperature conditions and the blue shist metamorphic signature has been overprinted now.
• 13:11
  SPEAKER_00: Now it's an amphibolite or a granulite or a migmatite or some gneiss, you know, some other higher temperature rock.
• 13:16
  SPEAKER_01: Yeah, but doesn't it still give you a minimum age though of that?
• 13:21
  SPEAKER_00: It would give you a younger age, but it wouldn't give you it wouldn't be a blue schist anymore.
• 13:26
  SPEAKER_00: You could take a blue schist and turn it into a migmatite just by putting, you know, increasing the temperature.
• 13:31
  SPEAKER_00: So you would lose that that key.
• 13:34
  SPEAKER_01: I get that.
• 13:34
  SPEAKER_01: I get that that has value though.
• 13:36
  SPEAKER_01: If you have a minimum age on it, or a younger age, I should say, if it got overprinted, it turned into a different mineral.
• 13:44
  SPEAKER_01: Plate tectonics had to have happened at least this long ago, at least 2.7 billion years ago, right?
• 13:51
  SPEAKER_01: I mean, that's what I'm saying.
• 13:53
  SPEAKER_01: There it has value at least in saying it has to be older than this.
• 13:57
  SPEAKER_00: That's exactly right.
• 13:58
  SPEAKER_00: So, you know, this kind of comes into the the key, the battle we're fighting, the uphill battle we're fighting when we're trying to answer the question of when did plate tectonics start is we're dealing with very few rock samples.
• 14:10
  SPEAKER_00: The further back in time we go, the less rocks there are.
• 14:13
  SPEAKER_00: If you want to find a rock that's 3.8 billion years old, it's less than one part per million of the continental land mass is that old.
• 14:21
  SPEAKER_00: So it's a tiny fraction of the surface of the earth that is that old.
• 14:25
  SPEAKER_00: And so it's really these are rare rocks, and we deal with a survivorship bias, a preservation bias when we deal with you know small numbers of samples.
• 14:34
  SPEAKER_01: All right.
• 14:35
  SPEAKER_01: I don't know what you meant when you said a preservation bias on that.
• 14:38
  SPEAKER_01: Like I want to come back to that in a second.
• 14:40
  SPEAKER_01: But uh first though, all right, you have found rocks that are 4.03 billion years old.
• 14:48
  SPEAKER_01: Is that right?
• 14:48
  SPEAKER_01: The Acosta Nice in the Northwest Territory.
• 14:50
  SPEAKER_00: So I didn't find them, but I I sort of teased out some details, but they had been discovered before me.
• 14:56
  SPEAKER_00: But yeah.
• 14:56
  SPEAKER_01: All right, and that is intensely metamorphosed uh rock.
• 15:01
  SPEAKER_01: I mean, it's it's nice, it's a beautiful rock, it's gorgeous, it's this toothpaste, squished rock, it's awesome, right?
• 15:08
  SPEAKER_01: Um so uh help me with this because if that rock is 4.03 billion years old, then that rock is had to have formed because of plate tectonics.
• 15:20
  SPEAKER_01: You said you know, nice is indicative of tectonics.
• 15:24
  SPEAKER_01: So doesn't that mean that that's when it uh started or or at least minimum?
• 15:31
  SPEAKER_00: Nice is not a very diagnostic one.
• 15:34
  SPEAKER_00: Nice can be produced in other tectonic settings or other geodynamic systems like non-plate tectonic plants.
• 15:40
  SPEAKER_00: Like what?
• 15:41
  SPEAKER_00: There's probably nice deep down in Mars.
• 15:44
  SPEAKER_00: You know, it just all it requires is some heat and pressure to get a gice.
• 15:47
  SPEAKER_00: And the deep down.
• 15:48
  SPEAKER_01: But it requires differential pressure though.
• 15:50
  SPEAKER_01: This doesn't it doesn't require lithostatic equally applied pressure, it requires this you know, differential kind of the pressure you get if you put something in a vice.
• 16:01
  SPEAKER_01: That's tectonics.
• 16:03
  SPEAKER_00: It's very rare that it that you don't have any differential pressure in a in a real situation, like a real planet situation.
• 16:11
  SPEAKER_00: So in Mars, if you have the the lid is stagnant, it doesn't mean that the lid is not internally deforming, like you can have little bits of deformation, it's just not breaking across the lithosphere.
• 16:22
  SPEAKER_00: So you you will get gneiss and magmatite in these really high temperature rocks down in the deep crust of Venus, let's say, for instance, even if there's no plate tectonics.
• 16:32
  SPEAKER_01: Okay, so Jesse, let's go back to what you said a second, just real quick, okay?
• 16:36
  SPEAKER_01: Um, because I want to move on to the next question.
• 16:39
  SPEAKER_01: What did you mean when you said preservation bias?
• 16:41
  SPEAKER_01: What is that?
• 16:43
  SPEAKER_00: Let's take an example, I guess.
• 16:45
  SPEAKER_00: Let's say, well, what types of rocks are the easiest to erode, do you think?
• 16:51
  SPEAKER_00: Like, I mean, which which types of rocks are eroding the most right now?
• 16:55
  SPEAKER_00: Sedimentary.
• 16:56
  SPEAKER_00: Sedimentary rocks, right?
• 16:57
  SPEAKER_00: Yes.
• 16:58
  SPEAKER_00: Sedimentary rocks.
• 16:59
  SPEAKER_00: And but uh maybe pick a tectonic setting.
• 17:01
  SPEAKER_00: Like what tectonic setting has the fastest erosion rates?
• 17:06
  SPEAKER_00: The ones that have the fastest uplift rates.
• 17:09
  SPEAKER_00: For sure.
• 17:09
  SPEAKER_00: For sure, right?
• 17:10
  SPEAKER_00: And you know, so any sort of active tectonic regime, mountain chains, uh, you know, especially in the mid-latitude.
• 17:17
  SPEAKER_01: I'm feeling the pressure, by the way, when you're asking me these questions.
• 17:19
  SPEAKER_01: You know, sorry, sorry.
• 17:21
  SPEAKER_01: Damn, I better get these right.
• 17:22
  SPEAKER_01: Putting you on point.
• 17:26
  SPEAKER_00: No, that's exactly right.
• 17:28
  SPEAKER_00: So, you know, like if you you know, the the rock type that exists the least, the shortest amount of time on earth is like ash, you know, volcanic ash or volcanic tephra.
• 17:39
  SPEAKER_00: That stuff is gone tomorrow, basically, on a geologic time scale.
• 17:43
  SPEAKER_00: So you really knock off the surface stuff really easily.
• 17:46
  SPEAKER_00: So if we go back in time, there's very few ashes preserved back in time.
• 17:51
  SPEAKER_00: I mean, they are there, but proportionally there's much less.
• 17:53
  SPEAKER_00: So, what is preserved as we go back in time, it's actually the deep crust.
• 17:57
  SPEAKER_00: Most of the rocks that are really old are gneisses, they're migmatites, they're amphibolites, they're these really high grade rocks.
• 18:04
  SPEAKER_00: Why?
• 18:04
  SPEAKER_00: Because the top of the continent has been eroded off over time, and the middle part has been uplifted up.
• 18:10
  SPEAKER_00: In the middle, the middle parts, the gneisses and the amphibolites, we don't have as much information.
• 18:15
  SPEAKER_00: Like it's not as much useful information in there.
• 18:17
  SPEAKER_00: It's not as diagnostic.
• 18:19
  SPEAKER_00: Again, also because these rocks have been like the Acosta Nys, four billion-year-old rocks, those things have been metamorphosed probably eight times in their life, maybe twelve.
• 18:28
  SPEAKER_00: Just wow, overprinted, overprinted, overprinted.
• 18:31
  SPEAKER_00: And the highest grade one, the most the one that defines their mineralogy today happened 1.9 billion years ago.
• 18:38
  SPEAKER_00: So the rock itself is a metamorphic rock that was made 1.9 billion years ago, but the elements there in some of the minerals, zircon being one, preserve their primary magmatic chemistry, right?
• 18:51
  SPEAKER_00: So there's this long tortured history here, which means it's hard to really tell did that rock form in a plate tectonic scenario or not four billion years ago?
• 19:01
  SPEAKER_00: That's where it gets contentious.
• 19:04
  SPEAKER_01: All right.
• 19:04
  SPEAKER_01: If subduction happens in only one place on a planet, does that equal plate tectonics on that planet?
• 19:13
  SPEAKER_01: So if you have this planet, this outside of our solar system or even inside of our solar system, and it has subduction that we know has happened in one spot, does that mean oh, that planet is tectonically active?
• 19:25
  SPEAKER_00: Well, I think there's a difference between tectonically active and plate tectonics as the main operating force.
• 19:32
  SPEAKER_00: Like you could so I gosh.
• 19:35
  SPEAKER_00: Oh my gosh.
• 19:36
  SPEAKER_00: I know this is in the weeds.
• 19:37
  SPEAKER_01: You're but hey, this is where your job gets hard.
• 19:41
  SPEAKER_01: I feel like a little kid that just got scolded.
• 19:44
  SPEAKER_01: Well, Chris, there is a difference between tectonically active and plate tectonics.
• 19:49
  SPEAKER_01: Oh my gosh.
• 19:50
  SPEAKER_01: Okay, school me, Jesse.
• 19:52
  SPEAKER_00: Nobody said that your job keeping me out of the weeds was going to be easy here.
• 19:56
  SPEAKER_00: I mean, come on.
• 19:56
  SPEAKER_00: Holy cow.
• 19:57
  SPEAKER_00: All right.
• 19:58
  SPEAKER_01: Nobody sorry to interrupt, but I could not avoid that.
• 20:01
  SPEAKER_01: Like, I couldn't resist.
• 20:02
  SPEAKER_00: Let's go.
• 20:03
  SPEAKER_00: But this is kind of where it gets fun, I think, is like it this is where geology is very artistic because we can dream up many different scenarios of this.
• 20:13
  SPEAKER_00: So, you know, it plate tectonics is the earth is broken up into however many, 30 different tectonic plates, and they all interact different plate boundaries, right?
• 20:23
  SPEAKER_00: So, does one subduction zone system, like let's say the earth was a stagnant lid planet and it had one subduction zone system, would that subduction zone system like start plate tectonics across the planet?
• 20:33
  SPEAKER_00: Would the planet then break up apart into a bunch of different plates and sort of kickstart plate tectonics?
• 20:38
  SPEAKER_00: Or does it mean, oh, you know, some random subduction zone system just kind of started randomly and then it shut off and then it wasn't actually a plate tectonic planet?
• 20:47
  SPEAKER_00: So my opinion is that subduction on one place does not equal plate tectonics.
• 20:53
  SPEAKER_00: Um, subduction on three or four different places would maybe be a better indicator of a global system of plate tectonics.
• 21:00
  SPEAKER_01: Okay.
• 21:00
  SPEAKER_01: And you're saying this, I think, correct me if I'm wrong, but you're saying no, that does not mean it it has plate tectonics, because there are these other niche settings that can cause subduction that are not plate tectonic, you know, this forceful driving around of these lithospheric plates.
• 21:21
  SPEAKER_01: That's what plate tectonics is, and that subduction doesn't always mean that.
• 21:27
  SPEAKER_00: That's exactly right.
• 21:28
  SPEAKER_00: That's exactly right.
• 21:29
  SPEAKER_00: And if you're listening to this, just let me say one thing, because that's a great point, Chris, and that brings us nicely into the next series of discussion points, I think.
• 21:36
  SPEAKER_00: But let me interject one thing here is that I think a lot of our listeners do read science news or see science news headlines, especially geoscience news headlines.
• 21:44
  SPEAKER_00: And I would caution to always be very careful when reading headlines that say plate tectonics started X time on Earth.
• 21:54
  SPEAKER_00: So and so pointed it out in this paper and showed it definitively started at this time because a lot of that stuff.
• 22:00
  SPEAKER_00: And I've I have papers out there that do make this claim too.
• 22:04
  SPEAKER_00: But be very, very cautious in reading those headlines and thinking, wait, now we know plate tectonics started 3.7 billion years ago or 2.5 or 4.3 billion years ago.
• 22:15
  SPEAKER_00: That's a really attention-grabbing news headline that is used for a lot of different papers that do exactly what we're talking about.
• 22:21
  SPEAKER_00: They identify a subduction zone, potential subduction zone signature in some suite of rocks in one location at one point in time, and then say, oh, must be global tectonics.
• 22:30
  SPEAKER_01: Okay.
• 22:30
  SPEAKER_01: So question for you then.
• 22:32
  SPEAKER_01: You said be cautious, but uh so if they're going to read an article like this with that very attention-grabbing headline that's published not in the original journal.
• 22:41
  SPEAKER_01: This is somebody that is synthesizing the research and putting out a paper in a pop science kind of setting for everybody to read and absorb.
• 22:51
  SPEAKER_01: Um, whose fault is that?
• 22:53
  SPEAKER_01: Is that the research paper that is responsible for that attention-grabbing headline, or is that the person that writes for science today or something like that?
• 23:05
  SPEAKER_01: Or is that too touchy of a topic?
• 23:07
  SPEAKER_00: That's a really tough question.
• 23:08
  SPEAKER_00: No, no, I that no, I it's a great, it's a really, really great question.
• 23:13
  SPEAKER_00: My opinion, and this is just, you know, one guy's opinion.
• 23:16
  SPEAKER_00: My opinion is that it I'm not sure if it's anybody's fault.
• 23:19
  SPEAKER_00: I'm not sure if it's a problem, because the reader, you know, you you need to read things carefully.
• 23:23
  SPEAKER_00: And and so the part of the ownership is on the reader.
• 23:27
  SPEAKER_00: Um, so part of it's like not a problem that there's a bunch of you know news headlines that say this.
• 23:32
  SPEAKER_00: You just have to read cautiously.
• 23:34
  SPEAKER_00: But there are researchers, there are science journalists, many of them are great.
• 23:39
  SPEAKER_00: And and there are some that that sort of really put the splashy, catchy title out there.
• 23:44
  SPEAKER_00: And you know, it's it behooves all of us.
• 23:46
  SPEAKER_00: And and I'm I'm not um I'm not blameless here because I do this too.
• 23:51
  SPEAKER_00: Like it's good for me if I publish a paper that has us that gets some public, you know, interest, right?
• 23:56
  SPEAKER_00: That that's a very good thing.
• 23:58
  SPEAKER_00: And uh, you know, as long as it doesn't.
• 24:00
  SPEAKER_01: But we all know that you are not very good at coming up with splashy, catchy, awesome titles.
• 24:05
  SPEAKER_01: So terrible at it.
• 24:06
  SPEAKER_00: And everybody I've ever published a paper with will back that up and say, Yeah, you know, you are bad at that, Jesse.
• 24:13
  SPEAKER_00: So um okay, fair enough.
• 24:16
  SPEAKER_01: Um, I want to move on, Jesse, to the next set of questions that deal with these kind of niche studies that you've alluded to uh actually several times.
• 24:26
  SPEAKER_01: You've already talked about stagnant lid tectonics and so are there variations on plate tectonics?
• 24:35
  SPEAKER_01: Things like sagduction and squishy lid tectonics and episodic tectonics, stagnant lid, like I think Mars is a stagnant lid tectonic system, right?
• 24:48
  SPEAKER_01: I mean, or for sure.
• 24:51
  SPEAKER_01: Let's go through some of them.
• 24:52
  SPEAKER_01: We don't have to talk about all these little nuances, but like I think just to bring home the point that there's a lot that we don't know about plate tectonics yet.
• 25:00
  SPEAKER_00: That's exactly right.
• 25:01
  SPEAKER_01: Let's start with sagduction.
• 25:03
  SPEAKER_01: I want to hear what you say about this.
• 25:04
  SPEAKER_00: Let me just list those again, Chris, in a in a slightly different order because there's I'm gonna put them as a gradient.
• 25:09
  SPEAKER_00: There's plate tectonics, which we know and love on Earth, which is that system of global plates, the lithosphere, the crust and upper mantle are broken apart into lithospheric plates, they move around, interact with each other.
• 25:19
  SPEAKER_00: That's plate tectonics.
• 25:20
  SPEAKER_00: That's one end member, as far as we know, of how planets can operate.
• 25:25
  SPEAKER_00: Basically, how do you get heat out of the planet?
• 25:27
  SPEAKER_00: How is heat escaping the planet?
• 25:29
  SPEAKER_00: It's plate tectonics.
• 25:30
  SPEAKER_00: The other end of the spectrum is stagnant lid tectonics.
• 25:33
  SPEAKER_00: There is a single plate, there is one tectonic plate.
• 25:36
  SPEAKER_00: It is not broken, it's a solid thing, and heat is escaping through there in a different way.
• 25:42
  SPEAKER_00: And then in between there, there's a huge gradient.
• 25:44
  SPEAKER_00: So I would say that you kind of go from um stagnant lid being the one end member, I'd put sagduction next, then I'd put squishy lid tectonics, then episodic tectonics, then plate tectonics in kind of that order, going from one end of a spectrum to the other.
• 26:00
  SPEAKER_00: Gotcha.
• 26:01
  SPEAKER_00: Is that a helpful framing?
• 26:02
  SPEAKER_01: And then yeah, well, one end member is stagnant lid.
• 26:05
  SPEAKER_01: So it makes sense to start with the end member.
• 26:07
  SPEAKER_01: So we'll go from stagnant lid tectonics.
• 26:09
  SPEAKER_01: What is this?
• 26:10
  SPEAKER_01: And give us an example of where this happens and and kind of why.
• 26:14
  SPEAKER_00: So stagnant lid tectonics is where there's only one tectonic plate.
• 26:19
  SPEAKER_00: It's the entire planet is covered by one plate, it's not broken anywhere.
• 26:23
  SPEAKER_00: And you have to ask yourself, how does heat get out of the planet?
• 26:26
  SPEAKER_00: Because there's this heat that is in the core, there's radioactive decay going on in the planet, it needs to escape somehow.
• 26:32
  SPEAKER_00: So, how does that heat get out?
• 26:33
  SPEAKER_00: And it usually gets out by mantle plume volcanism.
• 26:36
  SPEAKER_00: Olympus mons on Mars is heat escaping.
• 26:39
  SPEAKER_00: That's volcanism escaping.
• 26:41
  SPEAKER_00: It's taking heat from the interior of the planet, dumping it out onto the surface and into the atmosphere.
• 26:45
  SPEAKER_00: And that's a stagnant lid.
• 26:47
  SPEAKER_01: Olympus Mons, by the way, is the solar system's largest known volcano.
• 26:52
  SPEAKER_01: It's on Mars.
• 26:53
  SPEAKER_01: Um, this thing dwarfs anything that we have on an Earth scale.
• 26:58
  SPEAKER_00: That's right.
• 26:59
  SPEAKER_00: So it's a great way to frame stagnant lid tectonics because it's not moving, the plate's not moving.
• 27:04
  SPEAKER_00: Um, the next one on that list I would have put sag duction.
• 27:08
  SPEAKER_00: So this is kind of a play on subduction.
• 27:10
  SPEAKER_00: So subduction is one plate dives down beneath another plate.
• 27:15
  SPEAKER_00: Sag duction would be one plate kind of sags down and then goes into the mantle.
• 27:21
  SPEAKER_00: So the way you could envision this happening is if you took, let's say, Olympus Mons, which is this giant, giant volcano, right?
• 27:29
  SPEAKER_00: And has an equally thick lithospheric root beneath it, most likely.
• 27:33
  SPEAKER_00: A volcano doesn't just stack up vertically, it's like an iceberg.
• 27:36
  SPEAKER_00: It builds up and it sinks down as well.
• 27:38
  SPEAKER_00: And so the crust right next to Olympus Mons is not nearly as thick as Olympus Mons.
• 27:45
  SPEAKER_00: So at some point, Olympus Mons might build up so thick that the root drops off.
• 27:50
  SPEAKER_00: And that could create some like buoyancy differences that could allow that crust to kind of sag down underneath its own weight and it could suck little bits under it.
• 28:00
  SPEAKER_00: So it's kind of like a suction happening at the surface where you have heavy stuff dripping off and light stuff kind of rebounding up.
• 28:09
  SPEAKER_01: Okay.
• 28:10
  SPEAKER_01: So from my limited outside of your world experience, okay, um, because I certainly do not live in this world like you do.
• 28:20
  SPEAKER_01: Um I envision maybe this being a possibility in let's say a continental margin, sediment and material being shed off the continents into the ocean basin, and it just the sediment accumulates and then it it sinks down due to isostic adjustment or isostasy, and which allows for more deposition to happen and then it sinks due to the weight and more deposition and so on.
• 28:44
  SPEAKER_01: And eventually this weight can get to the point where it causes that margin, the continental margin area there to get so heavy that it starts to sink and forms this kind of subduction zone that was created due to kind of weight sagging.
• 29:03
  SPEAKER_00: That's exactly right, Chris.
• 29:04
  SPEAKER_00: That is definitely one of the scenarios that people would say this is a potential sagduction scenario.
• 29:11
  SPEAKER_00: The other one is if you have a sort of a mini continental block and you erupt a ton of basalt on top of that.
• 29:18
  SPEAKER_00: The basalt is much more dense than the sort of intermediate andesite rock that makes up the continent.
• 29:23
  SPEAKER_00: And so you have this density inversion where that basalt wants to kind of slough off the edges of the continent, kind of wants to slide off.
• 29:30
  SPEAKER_00: And if you build up enough of that, you could create this density, sort of what we call density inversion, where the top is heavier and more dense than the bottom, and it wants to invert.
• 29:39
  SPEAKER_00: It wants to slide off the top and kind of slough off the margins.
• 29:43
  SPEAKER_00: And so that would mean the continent would kind of be buoyed up through the basaltic pile on top of it.
• 29:49
  SPEAKER_00: And uh, and this is one model that people would use to explain some of the Archean rocks that we have on the earth.
• 29:55
  SPEAKER_00: Three three billion dollars.
• 29:56
  SPEAKER_01: Archean means very, very, very old, yes.
• 29:58
  SPEAKER_01: Some of the oldest rocks we have.
• 29:59
  SPEAKER_01: So, okay, um, real quick then, back to uh a question that I asked earlier.
• 30:04
  SPEAKER_01: Sagduction, which would be a subduction zone due to one of these mechanisms that we just described, would be an example of well, subduction occurring in one place that does not prove plate tectonics exists on that planet.
• 30:19
  SPEAKER_00: It's close to subduction, it's not quite subduction because it's not like a continuous uh conveyor belt.
• 30:26
  SPEAKER_00: Whereas modern subduction, you know, the oceanic crust has this slab pull mechanism that pulls more slab down and it creates more oceanic crust.
• 30:32
  SPEAKER_00: Sagduction does not have that mechanism, it's like one pile of basalt sloughing off the edge and then it's done.
• 30:38
  SPEAKER_00: So it's kind of a an episodic breaking of the lid or an episodic movement, which kind of leads us nicely into the next one, which would be episodic or squishy lid tectonics.
• 30:48
  SPEAKER_00: Those are kind of similar things, but it's basically the way to think about that is just as the name implies, you don't have this continuous subduction zone system.
• 30:58
  SPEAKER_00: These come out of a lot of geodynamical models, so big computer simulations of the physics of a planet, where they see subduction will happen, but it'll happen for a brief amount of time, a couple million years, and then it'll stop.
• 31:12
  SPEAKER_00: The idea being that in the modern subduction system, the oceanic plate dives down, it gets really dense, and that begins to be an anchor.
• 31:20
  SPEAKER_00: That anchor pulls down more of the plate, it's called slab pull, pulls down more of the plate, and the plate is strong enough, it's like your anchor rope is strong enough that it holds onto the anchor, and the anchor can hold that thing down in the mantle.
• 31:34
  SPEAKER_00: It kind of pulls the rest of the stuff down into the mantle.
• 31:37
  SPEAKER_00: Back in the early Earth, the mantle was probably a couple hundred degrees hotter than it is today, and so that anchor rope was weaker.
• 31:44
  SPEAKER_00: And so as soon as that anchor forms and starts to pull down, the slab will kind of break off because it's weaker.
• 31:51
  SPEAKER_00: Stretches it like taffy things and then it breaks and then it shuts off subduction for a little while until the forces build up enough to start subduction again.
• 32:00
  SPEAKER_01: Uh, hence hence the term episode.
• 32:02
  SPEAKER_01: That's the term episodic.
• 32:03
  SPEAKER_00: And so that's that's usually people would say that that does happen, or it probably has happened.
• 32:08
  SPEAKER_00: We just don't really know how to identify it in the rock record.
• 32:11
  SPEAKER_00: It comes out of a lot of computer models, so a lot of people see results that are kind of episodic tectonic settings.
• 32:17
  SPEAKER_00: So, yeah.
• 32:18
  SPEAKER_01: Okay, interesting.
• 32:20
  SPEAKER_01: Let's go to the let next question then.
• 32:22
  SPEAKER_01: Let's leave these variations on plate tectonics and go into this, which I don't know if you've already answered, but what key signature do you need to see that convinces you that okay, this planet has plate tectonics?
• 32:38
  SPEAKER_00: Yeah, that it's a really tough one.
• 32:40
  SPEAKER_00: I so I deal a lot in geochemical signatures, which are one step removed from the rocks.
• 32:50
  SPEAKER_00: They're in the rocks, but usually I'm dealing with formerly igneous rocks that have been metamorphosed multiple times, like the Acostanis complex samples.
• 32:58
  SPEAKER_00: And so, you know, I'm looking at relatively armwavy geochemical signatures.
• 33:01
  SPEAKER_00: And I have written papers that suggest that plate tectonics started around 3.7 billion years ago.
• 33:08
  SPEAKER_00: You know, I wouldn't stick my career on that result, but I sort of what I would like to see is not just one location, but multiple.
• 33:14
  SPEAKER_00: So multiple locations around the world that have some similar signature, the where the simplest answer is plate tectonics.
• 33:23
  SPEAKER_00: Maybe not the only answer, but the simplest answer is that signature is a plate tectonic signature.
• 33:28
  SPEAKER_00: So that's what I've used previously in in my research.
• 33:32
  SPEAKER_00: And um, you know, I'm I'm happy with it being the simplest, if not only, answer.
• 33:37
  SPEAKER_01: Um, so I don't know if I'm really satisfied with your answer, and here's why.
• 33:41
  SPEAKER_01: Maybe some of our listeners do understand what you mean when you say I'm looking for this chemical signature, but I I would say that most don't, because I don't really feel like I do either.
• 33:52
  SPEAKER_00: I don't that's totally fair.
• 33:53
  SPEAKER_00: I I don't feel like so I think like one thing that you could look at is in modern subduction zones.
• 34:01
  SPEAKER_01: Hold on.
• 34:01
  SPEAKER_01: I got like, do you have a way to explain this?
• 34:04
  SPEAKER_01: Because like I have to throw this out there sometimes because um my mom, you know, I'm trying to catch her not listening to this podcast because that would hurt me.
• 34:14
  SPEAKER_01: You know, I'm I'm her top and if she's really so this is gonna be like can you please explain a chemical signature so that Joyce.
• 34:22
  SPEAKER_01: So this is for Joyce.
• 34:23
  SPEAKER_00: So Joyce, here's what happens.
• 34:25
  SPEAKER_00: There we go.
• 34:26
  SPEAKER_00: Um there are certain elements in it in modern subduction zone systems, the volcanic arc, the volcanoes above a subduction zone system, Mount Fiji, the Cascades in the U in the northern U.S.
• 34:38
  SPEAKER_00: and in British Columbia, the Andes Mountains, those are produced by water leaving the slab and going into the mantle.
• 34:44
  SPEAKER_00: So there's water involved in this process.
• 34:47
  SPEAKER_00: And that's not common on all those other sagduction, stagnant lid, squishy lead tectonics.
• 34:54
  SPEAKER_00: Water is not necessarily involved in those systems there.
• 34:57
  SPEAKER_00: So water is key here on modern plate tectonics.
• 35:00
  SPEAKER_00: So there are particular elements that water will move them more than they'll move other elements.
• 35:06
  SPEAKER_00: So you could look for ones that are quote unquote water mobile or fluid mobile, things like uranium are fluid mobile, things like barium are fluid mobile, and you compare those water mobile ones to ones that are not water mobile that stay behind when water leaves.
• 35:21
  SPEAKER_00: And so if you see a granite or an andesite that is enriched in uranium and barium, that might be a subduction zone signature.
• 35:32
  SPEAKER_01: So then if they're mobile, if these elements, these chemical signatures are concentrated, that concentration happens because they were water mobile.
• 35:43
  SPEAKER_01: And so the water is what concentrates in certain areas.
• 35:46
  SPEAKER_01: And if you go back to some of our previous episodes on plate tectonics, we talk a lot about the importance of water in plate tectonics.
• 35:56
  SPEAKER_01: For sure.
• 35:57
  SPEAKER_00: And you know, if you're sitting there listening, Joyce, or anybody else, and and you're thinking, well, that's a really arm wavy argument, it is completely, but it's all we have.
• 36:07
  SPEAKER_00: Because as I described, the rocks have been metamorphosed multiple times, all their magnetic signatures are gone.
• 36:12
  SPEAKER_00: We don't know where they initially formed on the surface, down in the crust, you know, what latitude.
• 36:17
  SPEAKER_00: There's so much we don't know.
• 36:18
  SPEAKER_00: We only have geochemistry for many of these old rock locations.
• 36:22
  SPEAKER_00: And so that's why the many of the pieces of evidence though you'll read those Science Daily or those science uh news articles about when plate tectonics started, a lot of them are speaking about geochemical signatures because that's all we have.
• 36:33
  SPEAKER_00: That's all the information we have.
• 36:36
  SPEAKER_01: Okay.
• 36:36
  SPEAKER_01: Which are usually these geochemical signatures, are usually going to be elements that concentrate in water.
• 36:43
  SPEAKER_00: Many many times.
• 36:44
  SPEAKER_00: Many times there'll be that, or there'll be different isotopic uh systems that get really complicated and nuanced, and you know, we'd have to have charts to really explain this well.
• 36:53
  SPEAKER_00: Uh so it is beyond the scope of uh an hour podcast.
• 36:57
  SPEAKER_01: That leads me into our final question for today.
• 37:00
  SPEAKER_01: I think.
• 37:00
  SPEAKER_01: I think I might come up with another one.
• 37:03
  SPEAKER_01: But do planets evolve into and out of plate tectonic states?
• 37:11
  SPEAKER_01: Because I I'm struck with this thought.
• 37:14
  SPEAKER_01: Why is Earth so unique in that it has plate tectonics?
• 37:20
  SPEAKER_01: And most systems that we are aware of, most other planetary systems do not.
• 37:25
  SPEAKER_00: That, Chris, my friend, is the question.
• 37:28
  SPEAKER_00: Like, why is Earth unique?
• 37:30
  SPEAKER_00: What is the key thing that makes it unique?
• 37:34
  SPEAKER_00: That we don't we don't know that.
• 37:36
  SPEAKER_00: You know, we talked last episode about water, you know, we could talk about magnetism, we could talk about an atmosphere and oceans, we can talk about many different things that make Earth unique, but time is maybe one of them because the answer to this question, do planets evolve into or out of plate tectonic states, the answer is probably yes.
• 37:55
  SPEAKER_00: That Venus used to be in a different tectonic state than it is today.
• 37:59
  SPEAKER_00: Mars used to be in a tectonic state that is different today.
• 38:02
  SPEAKER_00: Maybe.
• 38:03
  SPEAKER_00: Earth, maybe, was in a different tectonic state.
• 38:06
  SPEAKER_00: It didn't, it wasn't born and had plate tectonics right away.
• 38:09
  SPEAKER_00: It had some magma ocean phase, then it went into maybe some stagnant lid or episodic lid, um, and then went into plate tectonics.
• 38:18
  SPEAKER_00: And in the future, Earth will probably cool down such that it becomes a stagnant lid planet.
• 38:22
  SPEAKER_00: Like the end state of a planet is cooling down such that there's no heat to actually break the lithosphere anymore.
• 38:29
  SPEAKER_00: And so um or when the lithosphere maybe gets too thick, right?
• 38:33
  SPEAKER_00: Exactly.
• 38:34
  SPEAKER_00: Because you've lost so much heat that the planet is cooling down and getting, you know, the lithosphere is getting thicker and thicker when that it can't be broken.
• 38:41
  SPEAKER_01: Because again, the lithosphere, we're talking about this crust, the the the you know, the part that the the skin of the earth, right?
• 38:48
  SPEAKER_01: And then this crunchy, brittle mantle below it.
• 38:52
  SPEAKER_01: And so we're when we talk about lithosphere, these are the tectonic plates that we're discussing that are moving and shifting about and so on.
• 38:59
  SPEAKER_01: And um, it's it's all about the rock behavior.
• 39:02
  SPEAKER_01: The lithosphere is cold, crunchy, relatively cold.
• 39:06
  SPEAKER_01: I, you know, I'm I'm this is in relative terms, but it's cold and crunchy, it's brittle.
• 39:11
  SPEAKER_01: Okay, so that's what we're talking about.
• 39:12
  SPEAKER_01: So that over time, as Earth continues to cool off, that crunchy outer layer of the earth is going to get thicker and thicker and thicker.
• 39:19
  SPEAKER_01: And eventually, you probably get to a point where it's evolved out of the ability to do this.
• 39:24
  SPEAKER_00: That that's exactly right.
• 39:25
  SPEAKER_00: Like as a planet cools down, it kind of evolves towards a stagnant lid regime, and but it could start in a stagnant lid regime as well.
• 39:33
  SPEAKER_00: So, yes, you can evolve in and out.
• 39:35
  SPEAKER_00: And and there are pretty good models or people who model planetary heat loss and they model the the geodynamics of the system that say that plate tectonics is really this kind of Goldilocks environment where it's gotta be not too hot, not too cold, and then you get plate tectonics is happy for a little while in the sort of middle age of a planet, which um is a really interesting thing.
• 39:54
  SPEAKER_01: So and then the size of the planet probably becomes really important.
• 39:57
  SPEAKER_00: Absolutely.
• 39:58
  SPEAKER_00: A lot of things.
• 40:00
  SPEAKER_01: Mars is too small.
• 40:01
  SPEAKER_01: Yep, and it's already cooled off.
• 40:03
  SPEAKER_00: The starting chemistry of the planet matters, how much radioactive decay is going on, lots of different things go into this.
• 40:09
  SPEAKER_00: Um, and so Chris, let me just end it and keep me out of the weeds.
• 40:13
  SPEAKER_00: I'm gonna try and keep this 20 seconds here, but I just I I I'm feeling a little self-conscious that I've given my opinion here a lot.
• 40:19
  SPEAKER_00: And and my opinion is that plate tectonics, some sort of mobile lid system, meaning plate tectonics, the lithosphere is breaking.
• 40:26
  SPEAKER_00: It's either squishy lid or episodic tectonics, started about 3.7 billion years ago, and then we got to a modern like plate tectonic regime around 2.7 or 3 billion years ago.
• 40:39
  SPEAKER_00: But I just want to say that you will talk to very good researchers who disagree vehemently with me on that one.
• 40:46
  SPEAKER_00: And so there's a lot of people with a lot of different opinions on that.
• 40:50
  SPEAKER_00: Some people would pin plate tectonics at 4.3 billion years old, they think plate tectonics was going on then.
• 40:56
  SPEAKER_00: Um, and some people say it didn't happen until 600 million years ago.
• 40:59
  SPEAKER_01: So you know what, Jesse, I think that's okay.
• 41:02
  SPEAKER_01: I appreciate you doing that, putting out your little disclaimer, okay.
• 41:05
  SPEAKER_01: Um, but I I don't think that it's absolutely necessary to do that because this is our podcast.
• 41:11
  SPEAKER_01: We get to have our thoughts.
• 41:12
  SPEAKER_01: Now, these are educated thoughts.
• 41:13
  SPEAKER_01: This is an educated discussion that we're having.
• 41:16
  SPEAKER_01: And you said that right from the get-go, that this is what we think.
• 41:20
  SPEAKER_01: This is you know where my research has led me.
• 41:22
  SPEAKER_01: Yeah, uh, yeah, it's it's fun.
• 41:24
  SPEAKER_01: Okay, relax, okay.
• 41:26
  SPEAKER_01: Well, there we go.
• 41:27
  SPEAKER_01: You're you're gun shy, I think, from all these conferences where people sit back and get yelled at, get yelled at for some idea.
• 41:34
  SPEAKER_00: Yeah, for sure.
• 41:34
  SPEAKER_00: For sure.
• 41:35
  SPEAKER_01: Oh, which which again is really entertaining to watch.
• 41:39
  SPEAKER_01: Super entertaining.
• 41:40
  SPEAKER_01: I've never seen this happen to you, but uh man.
• 41:42
  SPEAKER_00: No, it's uh it's it's very entertaining to watch happen.
• 41:45
  SPEAKER_00: I um on that note, you know this paper that I was struggling to get published.
• 41:50
  SPEAKER_00: Oh yes.
• 41:51
  SPEAKER_00: So it is it probably by the time this airs, it will have been accepted.
• 41:55
  SPEAKER_00: I just got back over the holidays, I got back news that I had two reviews and they were very nice, minor comments, and the editor was happy to accept it.
• 42:03
  SPEAKER_00: Um, you know, if I made the changes.
• 42:05
  SPEAKER_00: So that one will hopefully be coming out.
• 42:08
  SPEAKER_00: Yeah, it's kind of fun to finally see something through.
• 42:10
  SPEAKER_00: I mean, this thing's been two years in the making through reviews and negative reviews, hard criticisms, valid criticisms, invalid criticisms.
• 42:18
  SPEAKER_01: So I want to ask you about that.
• 42:19
  SPEAKER_01: Okay, can I ask you a little bit about this?
• 42:21
  SPEAKER_01: Um, because we've been, you know, we've hit this time to time on our episodes because I I want updates on this.
• 42:27
  SPEAKER_01: How much did you change due to the responses that you got, the feedback that you got?
• 42:33
  SPEAKER_01: Was there then all right back to the drawing boards?
• 42:35
  SPEAKER_01: I got I gotta look at some things again and so on.
• 42:37
  SPEAKER_01: Like what it what was the process?
• 42:39
  SPEAKER_00: So the process goes, you really what I do is I first take I first look at the read the reviews, which I always and I think everybody does this.
• 42:50
  SPEAKER_00: I get pretty defensive.
• 42:51
  SPEAKER_00: It feels like personal attacks.
• 42:52
  SPEAKER_00: Some of them are, many of them are not.
• 42:55
  SPEAKER_00: Um, and so I just put it away for a week, right?
• 42:58
  SPEAKER_00: Like read them, put it away for a week and calm down.
• 43:00
  SPEAKER_01: I think the personal attacks are understandable as well because the reviewer is probably doing research in a related area that may contradict what your conclusions are, and so that reader took your findings to be personal, you know, so it's just kind of this this yeah, it goes back and forth like that.
• 43:19
  SPEAKER_00: So, you know, I always put it away for a couple weeks and then you know, revisit it, and you can kind of hell of calm down by that point, and and you can kind of say, All right, what are reasonable critiques here?
• 43:30
  SPEAKER_00: And in this case, in this paper, um there were a couple reasonable critiques, and there was a lot of of sort of not understanding.
• 43:39
  SPEAKER_00: And the way to read this is if people didn't quite get it, it's my fault for not explaining it well enough, and my fault for not like giving enough of a backstory or giving the right introduction to why we did this or how this fits in.
• 43:53
  SPEAKER_00: So I kind of have to have to say, you know, okay, they they clearly don't understand this, or they're they're giving.
• 44:00
  SPEAKER_00: Yeah, they don't agree with this point, but maybe they're not understanding that point I'm trying to make, so I need to explain it clearer.
• 44:05
  SPEAKER_00: That's kind of the way it goes.
• 44:07
  SPEAKER_00: But I would say the end product here is significantly improved from the starting product, and that makes it worth it, right?
• 44:15
  SPEAKER_00: That makes the stress of this review process and the how long it takes.
• 44:19
  SPEAKER_00: It makes it worth it when the end paper is a lot better.
• 44:22
  SPEAKER_01: Okay.
• 44:23
  SPEAKER_01: Interesting.
• 44:24
  SPEAKER_01: Very interesting.
• 44:24
  SPEAKER_01: Um, could can we have the title?
• 44:27
  SPEAKER_00: The title is Yes, you may, and it's going to be published in Earth and Planetary Science Letters, uh, which is a nice journal.
• 44:34
  SPEAKER_00: It's like a disciplinary journal, so it's it's very much like solid earth science.
• 44:38
  SPEAKER_00: Um it the title is going to be Quantifying the Effect of Late Bombardment on Terrestrial Zircons.
• 44:46
  SPEAKER_00: Oh, wow.
• 44:47
  SPEAKER_00: Okay.
• 44:48
  SPEAKER_00: Which is um late heavy bombardment to meteor.
• 44:52
  SPEAKER_01: Yeah, this is late bombardment to the early bombardment era, correct?
• 44:56
  SPEAKER_00: Yes, that's right.
• 44:57
  SPEAKER_00: We we basically what we did is we we calculated the probability that any point on the surface of the earth would have experienced enough pressure from a meteorite impact, what we call shock pressure.
• 45:10
  SPEAKER_00: A meteorite impact hits and it sends a shockwave through a planet, and that can shock mineral grains.
• 45:14
  SPEAKER_00: In zircons, it can really shock mineral grains, and it's very diagnostic.
• 45:17
  SPEAKER_00: So we calculated the probabilities that zircons would be shocked given different impact scenarios, basically, like um in the timing of different impact scenarios.
• 45:27
  SPEAKER_00: So it's an interesting, I think it's interesting.
• 45:29
  SPEAKER_00: Some people don't think it's interesting, but it might get published.
• 45:32
  SPEAKER_00: So so there we go.
• 45:34
  SPEAKER_01: Okay, well, very cool, very cool.
• 45:36
  SPEAKER_01: So uh, all right, what's next for you?
• 45:38
  SPEAKER_01: Uh are you already working on your next publication?
• 45:40
  SPEAKER_01: Uh is this is your next thing gonna be a spin-off from this?
• 45:44
  SPEAKER_00: No, this is uh a little bit of a one and done uh paper.
• 45:48
  SPEAKER_00: We've got some collaborators who are maybe gonna help take the lead on the next paper on this, but I've got probably six papers that are at various stages.
• 45:56
  SPEAKER_00: I've got some students who are writing papers, which is exciting for me.
• 45:58
  SPEAKER_00: A PhD student and a master's student who just finished.
• 46:01
  SPEAKER_00: Um, and they're both writing papers at the moment, so I'm helping them.
• 46:04
  SPEAKER_00: Um they'll be first author and I'll be a co-author, but we're we're sort of getting those papers written and hopefully submitted.
• 46:10
  SPEAKER_00: So that's kind of the main thing.
• 46:12
  SPEAKER_01: How how many publications have you you've been at Penn State now for a couple years, right?
• 46:18
  SPEAKER_01: Two and a half years.
• 46:19
  SPEAKER_00: Uh I've been here three and a half years now.
• 46:21
  SPEAKER_01: Three okay, wow.
• 46:22
  SPEAKER_01: You okay, so you've been at Penn and State for three and a half years now.
• 46:25
  SPEAKER_01: How many publications during that time?
• 46:28
  SPEAKER_00: During that time, that's a good question.
• 46:31
  SPEAKER_00: I would say I have had probably on average, let's see, five, six.
• 46:38
  SPEAKER_00: I've probably had twelve publications in that time, something like that.
• 46:42
  SPEAKER_00: And and I I'm hoping this next year will be a big slug of them with student papers coming out and everything like that.
• 46:48
  SPEAKER_00: So all right.
• 46:49
  SPEAKER_01: Interesting.
• 46:49
  SPEAKER_01: I I don't know if this is interesting to our listeners, but it is to me.
• 46:54
  SPEAKER_01: Um obviously because you know, I yeah, I care about you and all that, but I um it's just like this this world that you live in is so different from the world I live in.
• 47:05
  SPEAKER_01: And we talk about that.
• 47:06
  SPEAKER_01: I think that's one of the we come at this podcast from very different angles, and and like I I just getting an insight into that um is helpful for me anyway.
• 47:14
  SPEAKER_01: I hope it is for everybody else.
• 47:15
  SPEAKER_00: But yeah, well, I think it is very interesting and and well, maybe maybe not interesting, but it's important for people to know how the scientific publication process works because I think most people or many people think, oh, if it's published, it must be right.
• 47:29
  SPEAKER_00: It's been through peer review, it's right.
• 47:31
  SPEAKER_00: And there is a lot of published stuff that is wrong.
• 47:34
  SPEAKER_00: Not all of it.
• 47:35
  SPEAKER_00: I uh you can't just write off publications as wrong, but you can't just wholesale accept them either as it's been published in a scientific journal, therefore it's right.
• 47:46
  SPEAKER_00: Um, and that's the benefit.
• 47:47
  SPEAKER_00: That's the process of of this whole scientific endeavor is sort of going back and forth and finding the correct path.
• 47:53
  SPEAKER_00: Eventually we'll come to some consensus on the correct ideas, but it might take a while.
• 47:58
  SPEAKER_01: Okay.
• 47:58
  SPEAKER_01: So one final question before we sign off here, then.
• 48:02
  SPEAKER_01: Do you think that during your career that we'll have an answer about when plate tectonics began on Earth that is accepted and definitive?
• 48:16
  SPEAKER_00: Ooh, that's a good question.
• 48:18
  SPEAKER_00: I struggle with this one because part of I want to say yes.
• 48:22
  SPEAKER_00: I want to say yes in the worst way, because you know, it's something I probably will dedicate many decades to.
• 48:28
  SPEAKER_00: Um so I want to say yes in the worst way, and there's some new techniques that are coming out, there's more and more researchers, there's more and more discoveries, and so my hope would be yes, but I'm cynical only because of the way the earth works that we just have so few rocks that are really old that it's going to be hard to really pin that time down.
• 48:50
  SPEAKER_00: Like, and part of this is philosophical.
• 48:52
  SPEAKER_00: Like, I'll just give me 20 seconds here to just there's a full philosophical difference here.
• 48:58
  SPEAKER_00: Some people would say that when you're approaching the question of was plate tectonics operative on Earth before, you know, 3.5 billion years ago, let's just say that.
• 49:08
  SPEAKER_00: Some people would say plate tectonics happens today, so therefore you have to disprove it.
• 49:13
  SPEAKER_00: What's your null hypothesis?
• 49:15
  SPEAKER_00: You have to prove that it didn't happen 3.5 billion years ago.
• 49:18
  SPEAKER_00: And some people would say that the null hypothesis is that plate tectonics did not happen.
• 49:22
  SPEAKER_00: So you have to prove it happened 3.5 billion years ago.
• 49:25
  SPEAKER_00: So the kind of onus of evidence is like, do you have to prove it or disprove it?
• 49:30
  SPEAKER_00: Is it plate tectonics until proven otherwise, or is it not plate tectonics until proven otherwise?
• 49:35
  SPEAKER_00: So part of it comes down to this really deep philosophical question of what is your your default state when you think about the early earth?
• 49:42
  SPEAKER_00: Which one do you have to prove?
• 49:44
  SPEAKER_01: Damn, Jesse.
• 49:46
  SPEAKER_01: Oh my gosh.
• 49:47
  SPEAKER_01: Uh I I want to have a bourbon with you right now.
• 49:50
  SPEAKER_01: Sit on the front porch and philosophizing.
• 49:55
  SPEAKER_01: Oh man.
• 49:56
  SPEAKER_01: All right.
• 49:57
  SPEAKER_01: So um where do you fall on that?
• 50:00
  SPEAKER_00: I've had very, very dear mentors who I respect so much who fall on either side of this line.
• 50:09
  SPEAKER_00: I I just don't know where I stand on it, really.
• 50:11
  SPEAKER_00: I think if you pin me to the ground and said you have to make a decision, I would default that you have to disprove it.
• 50:19
  SPEAKER_00: But I I wouldn't hold like I'm not strongly in favor of that one.
• 50:24
  SPEAKER_00: So I don't have a very strong opinion.
• 50:25
  SPEAKER_00: Do you have like a knee-jerk reaction to that?
• 50:27
  SPEAKER_00: What you what do you think?
• 50:28
  SPEAKER_00: I think my reaction would be you have to prove it.
• 50:31
  SPEAKER_00: You have to prove it.
• 50:32
  SPEAKER_00: Okay.
• 50:33
  SPEAKER_00: That I think is most people's.
• 50:35
  SPEAKER_00: That I think is most people's.
• 50:36
  SPEAKER_01: The reason is because to fall on the side of having to disprove that plate tectonics happened back then assumes that everything is just always the same.
• 50:47
  SPEAKER_01: You know, it's this kind of this uniformitarianism that geology takes a lot of shit over because we really don't make the assumptions that are tied in with uniformitarianism.
• 50:57
  SPEAKER_01: Um I think that's why.
• 50:59
  SPEAKER_01: Does that make sense?
• 51:00
  SPEAKER_00: I I agree completely with that.
• 51:01
  SPEAKER_00: And the part of me that agrees with with your sentiment there is exactly for the reasons you're outlining.
• 51:07
  SPEAKER_00: Like seeing what the modelers have done where they can they can see sagduction in geodynamical models, they can see episodic tectonics, they can see mantle overturn tectonics, they see all these different varieties of tectonics, not plate tectonics, different varieties of of how planets could operate.
• 51:25
  SPEAKER_00: That kind of makes me think we need to prove plate tectonics because there's uh other options out there.
• 51:32
  SPEAKER_00: So uh yeah, it's a it's a tough one for me.
• 51:34
  SPEAKER_01: Yeah, I I think I find myself like thinking through my thought process, and I can I can see that.
• 51:41
  SPEAKER_01: I can see myself going either way.
• 51:43
  SPEAKER_01: You know, if you if you say you have to prove it, well, it's going on now, so when did it, you know, it's it had to start at some point.
• 51:50
  SPEAKER_01: It's just kind of this circular logic thing, totally.
• 51:53
  SPEAKER_01: I I get it.
• 51:54
  SPEAKER_00: You get into this chicken or the egg kind of thing where you're like, ah, I don't know which one's which.
• 51:58
  SPEAKER_00: I mean it's complicated.
• 51:59
  SPEAKER_00: So uh anyway, well, that's a very philosophical ending to this uh this this little topic here, but uh a fun one.
• 52:06
  SPEAKER_01: Never know where we're gonna go.
• 52:07
  SPEAKER_01: We didn't know this was a thing.
• 52:08
  SPEAKER_01: We did we with this episode, it was really kind of wide open.
• 52:12
  SPEAKER_01: Um, this is more of just a discussion and not as scripted as our normal episodes are.
• 52:18
  SPEAKER_01: So but I liked it.
• 52:19
  SPEAKER_01: It was good, good talk.
• 52:20
  SPEAKER_00: Yeah, yeah, me too, man.
• 52:21
  SPEAKER_00: Hey, with that, you can follow us on all the social medias at Planet Geocast, send us an email.
• 52:26
  SPEAKER_00: If uh hopefully there's something that struck you about this conversation, and uh if so, send us an email, planetgeocast at gmail.com.
• 52:33
  SPEAKER_00: Go to Camp Geo, our conversational textbook for the geosciences.
• 52:36
  SPEAKER_00: It's the first link in the show notes.
• 52:38
  SPEAKER_00: And uh what else?
• 52:40
  SPEAKER_00: Oh, go to our website.
• 52:41
  SPEAKER_00: You could there you can donate to us, you can support us and check stuff out about us and see all of our past episodes.
• 52:46
  SPEAKER_00: That's planetgeocast.com.
• 52:49
  SPEAKER_00: Cheers.
• 52:49
  SPEAKER_00: Cheers.