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Office Hours - Pollution and Slab Pull · PlanetGeo: The Geology Podcast

• 0:03
  SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:13
  SPEAKER_00: Get your radio voice, Chris?
• 0:15
  SPEAKER_00: I got it.
• 0:16
  SPEAKER_00: How's the it sounds good?
• 0:18
  SPEAKER_00: Chris Bullhouse from the basement of the Bullhouse Household.
• 0:22
  SPEAKER_00: How um how has your recording voice recovered or has it been damaged with all of your outdoor work and yelling back and forth between you and Jenny?
• 0:32
  SPEAKER_00: You seem to be doing this weekend?
• 0:34
  SPEAKER_01: Yeah, well, we've cut down probably a hundred trees in my backyard.
• 0:38
  SPEAKER_01: You're not going to recognize my backyard when you move over.
• 0:40
  SPEAKER_00: You know what?
• 0:40
  SPEAKER_00: That kind of hurts my heart a little bit.
• 0:42
  SPEAKER_00: We I don't like cutting down trees.
• 0:43
  SPEAKER_01: Well, these are all like scrub trees.
• 0:45
  SPEAKER_01: They're really crappy and they're they're just they're getting choked with grapevine, you know.
• 0:49
  SPEAKER_01: So we're just cleaning it out.
• 0:50
  SPEAKER_01: We're leaving all the evergreens in there.
• 0:52
  SPEAKER_01: So all the pine trees and all the maple trees are staying, but everything else is going.
• 0:57
  SPEAKER_01: So it's a lot of wood and then uh lots of cutting.
• 1:02
  SPEAKER_01: And so I rented a uh a tractor with it that has a front end loader on it, and then a backhoe on the back, and the seat swivels back and forth.
• 1:09
  SPEAKER_01: And I'm gonna tell you if I had a do-over, that might be a career choice because I had so much fun driving that thing around and then yeah, yelling for Jenny, you know, like Jenny, go do this, and she's yelling back at me, and neither one of us can hear each other.
• 1:24
  SPEAKER_00: See you in your swivel chair.
• 1:26
  SPEAKER_00: Do you have your little BB gun there or your shotgun to shoot at the ground dogs or whatever?
• 1:30
  SPEAKER_00: You're always hassling the ground dogs on your property.
• 1:33
  SPEAKER_01: They're my nemesis.
• 1:34
  SPEAKER_01: I was driving the thing and I went past my wood pile and a little chipmunk went skirting across me, and it just instantly made me mad.
• 1:41
  SPEAKER_01: Yeah, you know, because chipmunks they do so much damage, and I'm like, oh, that one got away from me.
• 1:47
  SPEAKER_00: You are living out in it, man.
• 1:48
  SPEAKER_00: It is.
• 1:49
  SPEAKER_00: I wish I could have been there.
• 1:50
  SPEAKER_00: I would have loved to just post up on your beautiful deck there, watching you with a beer in my hand, watching you and shouting some directions and encouraging.
• 1:59
  SPEAKER_01: You would have needed multiple beers.
• 2:01
  SPEAKER_01: You would have needed multiple beers.
• 2:02
  SPEAKER_00: This is an all-day affair.
• 2:04
  SPEAKER_00: Uh hey, well, Chris, by the time this episode launches, yeah, that's right.
• 2:09
  SPEAKER_00: We will be together.
• 2:11
  SPEAKER_00: There is a session in your honor at the Geological Society of America's North uh what is it, North section?
• 2:21
  SPEAKER_00: Yeah, the the section meeting, the Regional Geological Survey of America meeting.
• 2:24
  SPEAKER_00: We're having a session in your honor, which will be quite fun.
• 2:27
  SPEAKER_00: I'm giving a talk, a bunch of other alumni are giving a talk.
• 2:30
  SPEAKER_00: So we'll be together.
• 2:31
  SPEAKER_00: I'm flying to Michigan.
• 2:32
  SPEAKER_00: It's gonna be great.
• 2:33
  SPEAKER_00: This is uh it'll be a good time.
• 2:34
  SPEAKER_00: Yeah, I'm looking forward to it.
• 2:35
  SPEAKER_01: It's a huge honor.
• 2:36
  SPEAKER_01: You and Dr.
• 2:37
  SPEAKER_01: Maddox from Grand Valley got together and behind my back and put this together.
• 2:42
  SPEAKER_01: Bella's actually coming, she's gonna drive down from Northern.
• 2:45
  SPEAKER_01: She's gonna make a seven-hour drive so she can attend the session too.
• 2:49
  SPEAKER_01: Which, like, here's I gotta I have to ask you this.
• 2:52
  SPEAKER_01: My parents found out about it and now they want to go.
• 2:55
  SPEAKER_01: Is that like, do you think they should or were they just bored out of their mind?
• 2:59
  SPEAKER_00: No, I think they should totally go.
• 3:00
  SPEAKER_00: That sounds great.
• 3:01
  SPEAKER_00: I was just talking to our little buddy Andrew, who I graduated with, who's also giving a talk.
• 3:05
  SPEAKER_00: And um, and we're you know, we're talking about what our talks should look like because this is a bit of a different thing.
• 3:10
  SPEAKER_00: You know, this happens.
• 3:11
  SPEAKER_00: Let me just back up and explain this.
• 3:12
  SPEAKER_00: This happens in academia quite frequently when somebody very high profile, high-profile research professor, or somebody, you know, with with a with a great deal of um success in their career retires.
• 3:26
  SPEAKER_00: Usually there'll be a session in their honor.
• 3:28
  SPEAKER_00: And so my postdoc mentor retired and there was a session in his honor, and basically a lot of people that he mentored would give talks in the session, and it's a really cool thing to sit there and watch.
• 3:37
  SPEAKER_00: You get a feel for how much stuff these people are doing, the mentees are doing, and it's really because of the one person, right?
• 3:47
  SPEAKER_00: So you kind of get this view of like, wow, there's a lot of stuff going on that's because of in this case Rick Carlson, inspired a whole bunch of really cool science.
• 3:54
  SPEAKER_00: And so our goal was kind of to have a similar thing.
• 3:57
  SPEAKER_00: So, all that to say, so are you are you guys pushing me out?
• 4:00
  SPEAKER_01: No, I think you guys uh is this a a farewell to Chris Bullheis?
• 4:06
  SPEAKER_00: Yeah, you can't carry banded iron around anymore, so it might be time, Chris, to to settle in to you know, the easy life.
• 4:12
  SPEAKER_00: No, I'm kidding.
• 4:13
  SPEAKER_00: Uh, but yeah, I think your parents would would get a lot out of it, probably.
• 4:17
  SPEAKER_01: Okay.
• 4:17
  SPEAKER_01: All right, because then and Jenny's parents want to go to so nice.
• 4:22
  SPEAKER_00: Yeah, then they totally should.
• 4:23
  SPEAKER_01: Um you've been ripping on me a lot, by the way, for the whole banded iron thing.
• 4:27
  SPEAKER_01: You even texted me yesterday about you think that's pretty funny, don't you?
• 4:31
  SPEAKER_01: That I do don't care my own banded iron.
• 4:33
  SPEAKER_00: Um get ready because I'm gonna make a lot of hay out of that one.
• 4:36
  SPEAKER_00: That it's gonna be a running joke for uh the next year at least, I think.
• 4:40
  SPEAKER_01: So just just buckle up.
• 4:41
  SPEAKER_01: I stopped by my parents like uh yesterday after I dropped off the tractor that I rented, and they made a comment to your Kermit voice.
• 4:49
  SPEAKER_01: So um my mom thought that was pretty funny.
• 4:52
  SPEAKER_01: So yeah.
• 4:53
  SPEAKER_00: You did.
• 4:53
  SPEAKER_00: Okay, so that's actually a great lead-in, Chris.
• 4:55
  SPEAKER_00: We're what we're doing today.
• 4:57
  SPEAKER_00: We have gotten, I just want to say this and and and thank everyone who has contributed questions and very nice complimentary emails about our podcast.
• 5:06
  SPEAKER_00: So if you're listening to this and you did that for us, that was great.
• 5:09
  SPEAKER_00: We really, really appreciate that.
• 5:11
  SPEAKER_00: But because of that, we've gotten a lot of questions and a lot of really good episode suggestions that we're gonna start to kind of chip away at, I think, and it's far too many for one episode.
• 5:22
  SPEAKER_00: So, what we're doing today, Chris, you and I, we've picked one question for each other that we're gonna ask.
• 5:28
  SPEAKER_00: And so I'm gonna ask you a question from a listener, and you don't know what I'm gonna ask you, and then you're gonna return the favor, and we're kind of gonna use this as just a way to sort of chip away at the many, many great questions.
• 5:40
  SPEAKER_00: So these are coming in no particular order.
• 5:41
  SPEAKER_00: And Chris, are you ready for your question?
• 5:44
  SPEAKER_00: I think so.
• 5:44
  SPEAKER_00: Hit me up.
• 5:45
  SPEAKER_00: Okay, this is an easy one, and I'm gonna get to ask you two because of this one right now.
• 5:49
  SPEAKER_00: So, first of all, your mother, your mother Joyce.
• 5:53
  SPEAKER_00: Oh, Joyce, she sent an email and it was very complimentary, but it was also um suggesting perhaps that the the nomenclature in the words are often above her head.
• 6:04
  SPEAKER_00: And she alluded to the fact that she had trouble keeping the the names of her four boys straight in the household.
• 6:10
  SPEAKER_00: So geology words are difficult.
• 6:12
  SPEAKER_00: This is true.
• 6:13
  SPEAKER_00: And so Joyce is ready for a new word.
• 6:15
  SPEAKER_00: She's got Feldspar down and she's ready for a new word.
• 6:19
  SPEAKER_00: So, what's the new word you think that your mother Joyce should learn?
• 6:25
  SPEAKER_00: The next bit of terminology.
• 6:27
  SPEAKER_01: Okay, well, first of all, I'm taken off guard because I had no idea that my mom sent you an email.
• 6:32
  SPEAKER_01: So it wasn't just to me, it was through our planet email.
• 6:35
  SPEAKER_01: That one got past me, so I didn't see that.
• 6:38
  SPEAKER_01: I thought maybe she was hitting you up on the sly button or something.
• 6:41
  SPEAKER_01: No, no, no.
• 6:42
  SPEAKER_01: This is uh up front.
• 6:43
  SPEAKER_01: You're just not paying for it.
• 6:45
  SPEAKER_01: Should my mom Joyce add to her repertoire?
• 6:49
  SPEAKER_01: That's the question, right?
• 6:50
  SPEAKER_00: Yeah, that's right.
• 6:50
  SPEAKER_00: She's got Feldspar down.
• 6:52
  SPEAKER_00: Okay.
• 6:52
  SPEAKER_00: Andy Feldspar.
• 6:54
  unknown: Okay.
• 6:54
  SPEAKER_01: She's got Feldspar.
• 6:56
  SPEAKER_01: All right, that's the basic of the basic mom, just so you know.
• 6:59
  SPEAKER_01: Um let's see.
• 7:02
  SPEAKER_01: This is the so there are so many that jump to mind.
• 7:04
  SPEAKER_01: Do you have one that you think she should know?
• 7:06
  SPEAKER_01: Well, I'm thinking, I'm still.
• 7:08
  SPEAKER_00: This is directed for you, man.
• 7:09
  SPEAKER_00: This is I she didn't say Jesse, I want one from you.
• 7:11
  SPEAKER_00: She said, I'm ready for a new word, Chris.
• 7:14
  SPEAKER_01: Okay.
• 7:15
  SPEAKER_01: Uh, let's let me think about this a second.
• 7:18
  SPEAKER_01: Um, I guess my mind right away drifts to the basics of geology.
• 7:23
  SPEAKER_01: The unifying thing is plate tectonics.
• 7:25
  SPEAKER_01: So it probably has to do something with plate tectonics.
• 7:29
  SPEAKER_01: So, mom, here you go.
• 7:31
  SPEAKER_01: The word that you need to know is paleomagnetism.
• 7:36
  SPEAKER_01: Ooh, good one.
• 7:36
  SPEAKER_01: Okay.
• 7:37
  SPEAKER_01: Yeah.
• 7:37
  SPEAKER_01: Now she should be define it for us a minute.
• 7:40
  SPEAKER_01: Okay.
• 7:40
  SPEAKER_01: All right, mom, here you go.
• 7:42
  SPEAKER_01: Paleomagnetism.
• 7:43
  SPEAKER_01: Paleo is ancient, and magnetism is a reference to the magnetic record that is preserved in the basaltic or mafic rocks on the ocean floor, usually, not always, but usually.
• 7:55
  SPEAKER_01: And this gives us a record then of where the magnetic north and south pole was on the planet at the time that those rocks solidified.
• 8:08
  SPEAKER_01: So very close to the time that they erupted out.
• 8:11
  SPEAKER_01: And then this really was the definitive proof that plate tectonics has been happening on Earth for a very, very long time.
• 8:22
  SPEAKER_01: Because it shows reversals, and the only way to get that is to have rock created at a spreading center and then pushed aside, and then new rock is created.
• 8:33
  SPEAKER_01: And so as Earth's magnetic field shuts down and reverses as time goes on, and it just keeps doing this again and again and again, that record is preserved on the ocean floor, and the only way to have that is to have plate movement.
• 8:46
  SPEAKER_00: Gotcha.
• 8:46
  SPEAKER_00: Okay, that's a good one.
• 8:47
  SPEAKER_00: Paleomagnetism.
• 8:49
  SPEAKER_00: That's a good one.
• 8:50
  SPEAKER_00: Okay, so that that was a bit of a cheap one.
• 8:52
  SPEAKER_00: So I got another one for that.
• 8:53
  SPEAKER_00: A little bit more detailed.
• 8:54
  SPEAKER_00: Okay.
• 8:55
  SPEAKER_00: Yep.
• 8:56
  SPEAKER_00: Okay.
• 8:56
  SPEAKER_00: So this question comes from Candace.
• 8:58
  SPEAKER_00: And Candace, I'm gonna kind of summarize this a little bit, but Candace asks about human chemical change in plate tectonic cycles.
• 9:09
  SPEAKER_00: And really what she's after is she says, okay, well, the theme here is oceanic crust goes down into the mantle in a subduction zone.
• 9:15
  SPEAKER_00: The water gets squeezed off that plate and it melts the mantle, right?
• 9:19
  SPEAKER_00: And that's how we have arc volcanism.
• 9:22
  SPEAKER_00: Really, her question, there's a few nuances to it, but basically it's saying, okay, there's a lot of trace stuff that humans have done to affect the ocean.
• 9:30
  SPEAKER_00: We have a lot of chemicals that are higher concentration in water than in the background before humans really human progress was substantial.
• 9:38
  SPEAKER_00: And the question is, how will that, if at all, affect magmas?
• 9:43
  SPEAKER_00: So if we look two billion years in the future, will we see a record of human chemical disturbance in the arc volcanic record?
• 9:52
  SPEAKER_00: And would that change how plate tectonics behaves is another kind of side question.
• 9:56
  SPEAKER_00: So that there's a lot of there's a lot of stuff in here, but what's your knee-jerk reaction to that, Chris?
• 10:00
  SPEAKER_01: Okay.
• 10:01
  SPEAKER_01: I'm interested to uh hear your reaction to this question as well.
• 10:05
  SPEAKER_01: But here's here's my knee-jerk reaction is in terms of how we're affecting humans, the oceans, the oceans are getting more carbon dioxide because of increased greenhouse gases, particularly CO2, being put into the atmosphere, therefore the oceans soak those up.
• 10:23
  SPEAKER_01: And so I guess that if we were gonna see a change in you know several hundred million years in in what comes out, if there was anything noticeable in terms of a difference, I uh it would have to do with carbon dioxide, I would think.
• 10:40
  SPEAKER_00: What do you think?
• 10:42
  SPEAKER_00: The theme of your answer I totally agree with, and I that's where my mind went as well, is that when we're talking about you know, trace concentrations of lead or even radioactive materials from nuclear waste, which is something Candace asked about specifically, they're not in a high enough concentration to really affect how melting happens.
• 11:00
  SPEAKER_00: Like they're not changing how the mantle melts.
• 11:02
  SPEAKER_00: And so we don't I I I wouldn't think that humans are affecting the plate tectonics system that much.
• 11:10
  SPEAKER_00: Like we just we're not big enough, we don't have that much change.
• 11:14
  SPEAKER_00: It's the the the the masses involved and the the rates of movement are too big.
• 11:18
  SPEAKER_00: So in that regard, I would say no, we probably don't.
• 11:21
  SPEAKER_00: But your point is a good one.
• 11:23
  SPEAKER_00: We're definitely perturbing the carbon cycle, and so there could be some sort of unintended consequences about c more carbon maybe going down in the slab or something like that.
• 11:32
  SPEAKER_00: But again, there's not much CO2 in magma.
• 11:34
  SPEAKER_00: So it like 100 parts per million is not that much.
• 11:36
  SPEAKER_00: So if you add 120 parts per million, it doesn't really change the system that much.
• 11:40
  SPEAKER_01: It is the second most abundant gas typically that's in magma, though.
• 11:44
  SPEAKER_01: You know, anywhere from 10 to 15 percent of the dissolved gas in magma is carbon dioxide.
• 11:49
  SPEAKER_01: Obviously, water is the most abundant one, but yeah, I forgot about that part of the question because when you read that to me about the nuclear waste, I automatically dismissed that.
• 11:58
  SPEAKER_01: And and so when I answered it, I didn't address that is because I was thinking, okay, because really, this is on the spot.
• 12:05
  SPEAKER_01: I had no idea what you were gonna ask me.
• 12:07
  SPEAKER_01: So it is it's a little bit of pressure to be honest.
• 12:11
  SPEAKER_00: But that's okay, it's fun.
• 12:12
  SPEAKER_00: Uh no way.
• 12:12
  SPEAKER_00: So well, you did a good job, Chris.
• 12:14
  SPEAKER_00: I I think we're on the same page about this question, and it's a really insightful question, actually.
• 12:18
  SPEAKER_00: Like, you know, we we talk a lot about how humans are perturbing the carbon cycle, and this is a huge, like big, big cycle that operates on relatively short timescales.
• 12:28
  SPEAKER_00: And so it's sort of an obvious and interesting question of okay, can humans affect billion-year timescale processes on Earth?
• 12:35
  SPEAKER_00: And and I think at the moment it does not look that way, but um, so it's a good question.
• 12:41
  SPEAKER_00: Okay, okay.
• 12:43
  SPEAKER_01: All right, I have one for you, Jesse.
• 12:45
  SPEAKER_01: Uh, I think this is right up your alley, actually.
• 12:47
  SPEAKER_01: I didn't I want to make sure you get your geology correct.
• 12:50
  SPEAKER_01: So I'm I'm giving you this is this is a softball right here.
• 12:53
  SPEAKER_01: Is this gonna be in the weeds?
• 12:54
  SPEAKER_00: Is my answer am I gonna have to stay out of the weeds?
• 12:56
  SPEAKER_01: Probably.
• 12:56
  SPEAKER_01: You're gonna get all doctor and everybody, but all right.
• 12:59
  SPEAKER_01: This question is from Ava.
• 13:01
  SPEAKER_01: She is a master's student at Georgia Institute of Technology, and she's currently doing research on aerospace engineering.
• 13:07
  SPEAKER_01: And so this question goes back to our episode on So You Think You Know Plate Tectonics part six, the interior of the earth.
• 13:15
  SPEAKER_01: And we had a little bit of a discussion about the mechanisms for plate tectonics, and I pointed out that slab pull is becoming you know a very primary driving force for plate tectonics, which is a little bit of a shift.
• 13:29
  SPEAKER_01: And I made the comment kind of off the cuff that that doesn't answer how slab pull began.
• 13:36
  SPEAKER_01: There there seems to be a gap in it.
• 13:37
  SPEAKER_01: I can see it once it starts, how it would become a major driving mechanism, but our how does it start?
• 13:43
  SPEAKER_01: And and so she says her background is that she's taken this class where she's learning about planet formation, and it seems intuitive to her that as the body cooled, the outer shell or the crust would form cracks, right?
• 13:56
  SPEAKER_01: And because they're they're shrinking in volume.
• 14:00
  SPEAKER_01: And she says, Would this not be a viable explanation for some kind of beginning to the process of slab pull?
• 14:07
  SPEAKER_00: Ah okay.
• 14:09
  SPEAKER_00: Hmm.
• 14:11
  SPEAKER_00: Great question, first of all.
• 14:12
  SPEAKER_00: So, okay, we're imagining a magma ocean planet where the whole thing's molten and then the skin is kind of cooling down and starts to crack, right?
• 14:19
  SPEAKER_00: That's kind of the visual we have in our head, I think.
• 14:21
  SPEAKER_00: Um can that start plate tectonics?
• 14:25
  SPEAKER_00: Um it probably can start plate tectonics, but what we're talking about specifically is a slab pull, which is basalt turning to eclogite at depth.
• 14:36
  SPEAKER_00: And so that it has to get deep down.
• 14:38
  SPEAKER_00: And then once that starts.
• 14:39
  SPEAKER_01: Can you hold on a second?
• 14:40
  SPEAKER_01: Sorry, I don't want to interrupt your train of thought, but I think a lot of people have heard us talk about basalt a lot.
• 14:46
  SPEAKER_01: But what about eclogite again?
• 14:48
  SPEAKER_01: Can you please explain that a second?
• 14:50
  SPEAKER_00: Sure.
• 14:50
  SPEAKER_00: Eclogite is the Christmas tree rock.
• 14:52
  SPEAKER_00: It's a stunningly beautiful rock, but it's garnet and sodium-rich pyroxene.
• 14:56
  SPEAKER_00: And so uh what we call oomphhocyte.
• 14:59
  SPEAKER_00: Here's another word for just to look up oomphocyte.
• 15:02
  SPEAKER_00: But um, it's basically basalt that's been metamorphosed to high pressures and low temperatures.
• 15:08
  SPEAKER_00: So it only really forms in subduction zone settings for the most part.
• 15:12
  SPEAKER_00: And once that rock forms, it's super, super dense.
• 15:14
  SPEAKER_00: It's more dense than the mantle around it.
• 15:18
  SPEAKER_00: And because it's kind of a cool rock, it bec it's very, very dense.
• 15:22
  SPEAKER_00: And so that's the anchor that pulls the slab down.
• 15:25
  SPEAKER_00: That's the anchor that does the pulling and slab pull.
• 15:29
  SPEAKER_00: So I guess let me back up and say that there's a lot of debate about how plate tectonics might have started.
• 15:34
  SPEAKER_00: And there are theories out there that range from a meteorite impact that kind of hits the earth and punches through the lithospheric lid, and then you could potentially have that dense stuff start to kind of drag down through the hole that the meteorite punched through the lithosphere.
• 15:51
  SPEAKER_00: There's other ways to do it that are not really dependent upon slab pull, but slab pull is like the modern mechanism and has been operating for the last like two and a half billion years.
• 16:02
  SPEAKER_00: So it's a great question.
• 16:04
  SPEAKER_00: I don't have a clear answer.
• 16:05
  SPEAKER_00: I will answer the last part that just cooling the outer part of the earth is probably not the thing that did it.
• 16:11
  SPEAKER_00: So plate tectonics doesn't seem to turn on right away after a planet cools down from a magma ocean phase.
• 16:18
  SPEAKER_00: The planet needs to cool more than that to actually have the slab pull mechanism start to be an important uh driver.
• 16:25
  SPEAKER_01: Okay.
• 16:26
  SPEAKER_01: Interesting take.
• 16:26
  SPEAKER_01: My take on this too is that there's something else to consider with this question because plate tectonics it turns on and it turns off in certain areas.
• 16:38
  SPEAKER_01: And so just cooling and contracting isn't enough to talk about plate tectonics starting up again in a different area, you know what I mean?
• 16:48
  SPEAKER_01: And and so like I it doesn't explain everything.
• 16:52
  SPEAKER_01: It's just kind of where my mind went with it.
• 16:54
  SPEAKER_01: You know, we talk about the these supercontinents in the past, like Rhodinia and Pangaea, where they they seem to come together at somewhat regular intervals, and so you're talking about how you get spreading that stops and then it comes back together and then it spreads again.
• 17:12
  SPEAKER_01: That has nothing to do with initial contraction of the continent.
• 17:16
  SPEAKER_00: That's a really good point, Chris.
• 17:17
  SPEAKER_00: And that that makes me think of something else that I think helps helps clarify some of this is that what do you need to form eclogite?
• 17:26
  SPEAKER_00: You need a basalt that's cool and deep.
• 17:29
  SPEAKER_00: And so if you have a mid-ocean ridge, imagine like a mid-ocean ridge system that's forming new basalt really in the middle of the ocean.
• 17:36
  SPEAKER_00: But if the ocean's really small and compact in that plate is subducting like a hundred miles away, that plate is still really hot when it hits the subduction zone and it starts going down the trench, like it hasn't cooled off very much.
• 17:50
  SPEAKER_00: And so that slab will be less likely to form eclogite and actually form this slab pole mechanism.
• 17:57
  SPEAKER_00: If you have a plate that is forming at a mid-ocean ridge, like the Pacific Ocean, and it's thousands of miles away from the subduction zone, it cools off.
• 18:06
  SPEAKER_00: There's a long time for that plate to cool off and reside under the ocean before it hits the subduction zone, and that forms what we call a cold subducting slab.
• 18:15
  SPEAKER_00: And that is much more likely to form eclogite and form a lot of eclogite, and so the anchor is bigger there.
• 18:20
  SPEAKER_00: And so on an early Earth, an early planet, the plates are all really hot.
• 18:26
  SPEAKER_00: Like the first little crust that's forming, those are still really hot plates, unlikely to have slab pull happen.
• 18:32
  SPEAKER_00: What you said just made me think of that to kind of clarify this.
• 18:36
  SPEAKER_00: So back to the supercontinent cycle.
• 18:37
  SPEAKER_00: If you have a supercontinent, all the continents are in one place on the planet.
• 18:40
  SPEAKER_00: Let's say they're all in the south pole, then there's like maybe a mid-ocean ridge on the north pole.
• 18:45
  SPEAKER_00: The subduction zone's happening around the continent on the south pole, and the plate has to traverse the entire planet halfway around the planet.
• 18:51
  SPEAKER_00: That's a really cold plate by the time it hits the subduction zone.
• 18:55
  SPEAKER_00: And so this is a really important thing for like how much volatiles get down into the mantle and and how the subduction zone actually operates.
• 19:02
  SPEAKER_00: So great question.
• 19:03
  SPEAKER_00: A lot of legs to that topic.
• 19:06
  SPEAKER_01: I think that you're right, because as you're talking, my mind is just just going in a thousand different directions because I'm thinking, all right, well, you know, the severe erogeny that led to the you know, Laramide erogeny began on the order of 120 million years ago.
• 19:21
  SPEAKER_01: In other words, what I'm saying is that's a long time after the cooling and contracting of the crust that would have happened after Thea collided with Earth and formed our moon and all that.
• 19:31
  SPEAKER_01: Um, so it doesn't explain more recent plate tectonics.
• 19:35
  SPEAKER_00: No, that that's absolutely true.
• 19:37
  SPEAKER_00: And and I think I alluded to this, I kind of threw some criticism at the modelers and said that you know we don't actually know how to break the lithosphere.
• 19:44
  SPEAKER_00: Like what it doesn't just happen randomly.
• 19:46
  SPEAKER_00: And so probably what's happening is that it's reactivating old sutures.
• 19:50
  SPEAKER_00: So old breaks in the lithosphere are getting reactivated by later tectonic forces because we don't really know how it breaks.
• 19:58
  SPEAKER_00: It's it's really hard.
• 19:58
  SPEAKER_00: Like the lithosphere is really thick, it's it's really sort of hard to break.
• 20:02
  SPEAKER_00: You have to put a crack that's like hundreds of kilometers deep through the lithosphere to get a subduction zone starting.
• 20:09
  SPEAKER_00: That's a hard thing to do.
• 20:10
  SPEAKER_01: So you know that that brings up an interesting point.
• 20:12
  SPEAKER_01: That that is a very important principle in tectonics, are these cracks that form that get reactivated?
• 20:17
  SPEAKER_01: Yeah, think of of the Tetons.
• 20:19
  SPEAKER_01: You know, you have this the Buck Mountain fault that happened during the Lair Maturogeny, and then much, much, much later, extensional processes take over, but the faulting happened along a pre-existing weakness, the Buck Mountain Fault, which you know, so it is a very important principle, those pre-existing cracks.
• 20:38
  SPEAKER_00: Yeah, those cracks actually don't heal very easily, uh, especially in the crust.
• 20:42
  SPEAKER_00: They can they can heal in the mantle a little bit faster just because it's hotter, but in the crust, you're you're totally right.
• 20:46
  SPEAKER_00: Like it's really, really amazing when you go around and look at faults, big, big, huge, you know, faults like the Teton fault.
• 20:54
  SPEAKER_00: They're often, very, very often reactivated old faults.
• 20:57
  SPEAKER_00: Just and it makes sense, right?
• 20:58
  SPEAKER_00: Like if you start squeezing something, it's gonna break where it's already been broken if it doesn't have time to heal.
• 21:04
  SPEAKER_00: So they're not like human bones, which you know they say get stronger where they break.
• 21:08
  SPEAKER_00: It's not like that.
• 21:09
  SPEAKER_00: It it remains weak for a long time.
• 21:11
  SPEAKER_01: So thanks again, Ava.
• 21:13
  SPEAKER_01: That was an awesome question.
• 21:15
  SPEAKER_00: Totally great question.
• 21:16
  SPEAKER_01: Keep them coming.
• 21:17
  SPEAKER_01: It really, really made my mind go a thousand miles an hour there.
• 21:20
  SPEAKER_00: So that's the thing, Chris.
• 21:22
  SPEAKER_00: I think all these questions, we got a lot of questions, and and most, the vast majority of them are not simple and not short answers, really.
• 21:30
  SPEAKER_00: And so uh, we're gonna have a bunch of episodes over the next many months that are based on many of these questions, I think.
• 21:38
  SPEAKER_00: That's our plan at least.
• 21:39
  SPEAKER_00: So stay tuned to more office hours uh episodes and more episodes that are directly based on questions we've gotten because they take a while to explain.
• 21:47
  SPEAKER_01: Hey, I have one thing um way off topic.
• 21:49
  SPEAKER_01: This is gonna seem like it comes out of the blue.
• 21:51
  SPEAKER_01: But I was at a I was at a track meet the other day, and I came across uh our maintenance guy who takes care of the stadium and does an awesome job.
• 22:00
  SPEAKER_01: Well, his son is on the trap team, and he is also an avid listener of Planet Geo, but he always makes comments to me about like, you know, the T-Rexes and you know how many T-Rexes are in Ottawa County bullheis, you know, stuff like that.
• 22:14
  SPEAKER_01: And I I passed by him at this track meet and I shook hands with him and he made a comment to me, and he said, give him hungry water, coach.
• 22:23
  SPEAKER_01: And uh and I shook his hand and I I walked away and I was thinking, you know, Rob hasn't made a he hasn't made a comment about Planet Geo in a long time.
• 22:33
  SPEAKER_01: And I'm like, I wonder if he still listens.
• 22:35
  SPEAKER_01: And then 20 seconds later, I remember, wait a minute, he just made a reference to an old episode about hungry water about dams with hungry water.
• 22:44
  SPEAKER_01: Yeah, remember how the water comes out the other side and it's hungry and it's hungry.
• 22:48
  SPEAKER_01: What's that sediment?
• 22:49
  SPEAKER_00: That's a great one.
• 22:50
  SPEAKER_00: I love that I actually had this just recently.
• 22:52
  SPEAKER_00: A uh colleague of mine was like, Yeah, you know, I we're kind of friends, and he said, you know, I there's a whole bunch of people who come up to me and I I talk about you, and then they say, Oh yeah, the the the guy from the podcast, the Planet Geo podcast.
• 23:04
  SPEAKER_00: The guy from the podcast.
• 23:05
  SPEAKER_00: That's like he's like, dude, he's like, dude, it's really annoying.
• 23:09
  SPEAKER_00: You know, yeah.
• 23:11
  SPEAKER_00: Anyway, yeah, it's fun.
• 23:13
  SPEAKER_00: It's fun.
• 23:14
  SPEAKER_00: So thank you very much.
• 23:15
  SPEAKER_00: We appreciate all the questions, all the nice emails uh that have questions and episode suggestions, keep them coming.
• 23:20
  SPEAKER_00: We are not ignoring them, even if we don't get to replying uh, you know, in a timely manner.
• 23:26
  SPEAKER_00: Uh we're not ignoring them, and we're putting together lists of of these things to do over the summer, really.
• 23:30
  SPEAKER_00: So, hey, if you like Planet Geo, please give us a subscribe, a like, and a review and a rating.
• 23:37
  SPEAKER_00: Those are really powerful for us.
• 23:38
  SPEAKER_00: You can go to our website, planetgeocast.com, and there you can subscribe, you can listen to old episodes, see transcripts, learn a little bit about us, and you can support us as well.
• 23:48
  SPEAKER_00: We always appreciate that, and and we're very appreciative to everybody who has supported us in the past.
• 23:53
  SPEAKER_00: So I think that's a good wrap, Chris.
• 23:56
  SPEAKER_00: Stay tuned for more Office Hours episodes coming your way.
• 23:59
  SPEAKER_00: Cheers.
• 24:00
  SPEAKER_00: Peace.