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So You Think You Know Plate Tectonics? Part 5 - Magnetism · PlanetGeo: The Geology Podcast

0:01
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: How are you doing, Dr.
0:14
SPEAKER_00: Ramink?
0:15
SPEAKER_00: Oh, I'm good, Chris.
0:16
SPEAKER_00: How are you?
0:16
SPEAKER_00: Hey, you know, I was just thinking about this.
0:18
SPEAKER_00: We haven't done uh introductions in a long time here.
0:20
SPEAKER_00: You are Chris Bullheis.
0:21
SPEAKER_00: Let me just dive right in.
0:22
SPEAKER_00: Yeah.
0:23
SPEAKER_00: And national award-winning.
0:25
SPEAKER_00: I'm gonna go, man.
0:25
SPEAKER_00: We're just all fired up.
0:26
SPEAKER_00: I had too much coffee this morning.
0:28
SPEAKER_00: You are Chris Bullheis, national award-winning earth science teacher, high school teacher.
0:33
SPEAKER_00: You taught me geology.
0:34
SPEAKER_00: You taught me ninth grade earth science.
0:36
SPEAKER_00: Let's see here.
0:37
SPEAKER_00: Geology.
0:37
SPEAKER_00: I was a junior, I think, when I took your geology class.
0:40
SPEAKER_00: Between my junior and senior year of high school, I went on your summer science institute field geology course, really.
0:47
SPEAKER_00: And then I also did a what were we?
0:49
SPEAKER_00: What was it called?
0:51
SPEAKER_01: It was an independent study.
0:52
SPEAKER_01: You actually did a cool project for me, but it just took you longer, way longer than it should have taken.
0:57
SPEAKER_00: I had senioritis that kicked in really early.
1:00
SPEAKER_01: Two things happened when you were my independent study, your senior year.
1:04
SPEAKER_01: One is you are the reason why I flew out to Mount St.
1:07
SPEAKER_01: Helens in 2004 when it got active again, because you called me like all talk and no show or something like this.
1:15
SPEAKER_01: Yeah, you're a good one.
1:16
SPEAKER_00: I'm a good motivator.
1:17
SPEAKER_00: I'm a good motivator.
1:19
SPEAKER_01: And then the other thing that happened because you were in my independent study is you were able to TA my field trip in the spring up to northern Michigan and the upper peninsula to the box area.
1:31
SPEAKER_01: That's right.
1:31
SPEAKER_01: Yes, yes, yes.
1:32
SPEAKER_01: That's right.
1:35
SPEAKER_00: Yeah, yeah.
1:35
SPEAKER_00: The the class that um I was originally on as well.
1:38
SPEAKER_00: That was really fun.
1:39
SPEAKER_00: I forgot about that.
1:40
SPEAKER_00: Yeah, yeah.
1:40
SPEAKER_00: I mean, that's the best.
1:41
SPEAKER_00: Go on a trip with no responsibilities.
1:43
SPEAKER_00: Really, you don't have to do anything.
1:44
SPEAKER_00: You just get to hang out and go camping and look at cool rocks.
1:47
SPEAKER_00: I mean, so fun.
1:48
SPEAKER_01: All right.
1:49
SPEAKER_01: Well, that sums it up for me.
1:51
SPEAKER_01: So you are Dr.
1:51
SPEAKER_01: Jesse Rymanck.
1:53
SPEAKER_01: Um, you went to Hope College right here in West Southwest Michigan and got your undergrad degree in geoscience, and then you went on to the University of Alberta in Canada to get your PhD in geoscience, and you're now a professor at one of the best institutions for geoscience education in the U.S.
2:11
SPEAKER_01: at Penn State University.
2:13
SPEAKER_00: So, Chris, this is I don't know, what is this?
2:15
SPEAKER_00: Part five, part four, part six, part five.
2:19
SPEAKER_00: Okay, part five in part something.
2:22
SPEAKER_00: Part X in our series that we've been kind of chipping away at over the last couple months on plate tectonics questions.
2:30
SPEAKER_00: Really, sort of the title, So You Think You Know Plate Tectonics, right?
2:35
SPEAKER_00: And the origin of this, really quickly, is that you at one point asked your class or said to your class that you were gonna start in on plate tectonics, and they moaned and groaned and said, We already know all this stuff, and that got you all fired up.
2:46
SPEAKER_00: And you you give me a phone call and you're like, This is ridiculous.
2:50
SPEAKER_00: We gotta like, what do you think about this?
2:52
SPEAKER_00: It got you all fired up, and that led to this list of questions that was sort of describing or leading questions that sort of describe how plate tectonics is the grand unifying theory of Earth and the Earth system.
3:04
SPEAKER_01: That's right.
3:04
SPEAKER_01: A couple of things.
3:05
SPEAKER_01: My blood pressure goes up when I just think about that day at school.
3:09
SPEAKER_01: Um, I just kind of creepy.
3:13
SPEAKER_01: That's what I think of.
3:14
SPEAKER_01: And the other thing I that I'm amazed at is when I wrote this, like kind of, hey Jesse, let's do this on Planet Geo.
3:21
SPEAKER_01: I thought this was going to be an episode.
3:23
SPEAKER_01: And it we're on now episode five or six with a couple of more coming, yet, like how dumb was that?
3:30
SPEAKER_01: That I thought, yeah, let's just rip this in one episode, let's answer these questions.
3:34
SPEAKER_01: Now it's much bigger than that.
3:36
SPEAKER_00: Yeah, that is a good representation because it this is a huge topic.
3:39
SPEAKER_00: It is the grand unifying theory of how our planet operates.
3:43
SPEAKER_00: And plate tectonics feeds back into many parts of the Earth system, and also other parts of the Earth system feed back into plate tectonics.
3:51
SPEAKER_00: So, you know, think of the earth system as uh, you know, it's it's a living, breathing organism in some ways, where you turn one dial over here and a whole bunch of stuff cascades off and a bunch of changes happen.
4:01
SPEAKER_00: And we're seeing this with climate change.
4:03
SPEAKER_00: We'll come back to that in one of the later episodes in this series.
4:06
SPEAKER_00: But today we're going to talk about magnetism.
4:08
SPEAKER_00: So, Chris, I'll be interested to know where you know your brain was when you're writing these questions, because where my mind goes with magnetism and especially ancient magnetism locked up in ancient rocks is really the discovery of plate tectonics.
4:21
SPEAKER_00: That's kind of how how it sort of relates to me.
4:24
SPEAKER_00: So why don't you sort of lead us in?
4:26
SPEAKER_00: You wrote these questions.
4:27
SPEAKER_00: I don't know where we go in this episode.
4:29
SPEAKER_00: What what are the sort of broad overarching questions here?
4:31
SPEAKER_01: Yeah, for sure.
4:32
SPEAKER_01: Mine does too.
4:33
SPEAKER_01: Mine goes to paleomagnetism and the smoking gun of plate tectonics.
4:38
SPEAKER_01: I mean, this discovery was this like kind of irrefutable evidence that our earth is making new crustal material and it it becomes older as it goes away from these spreading centers.
4:49
SPEAKER_01: I mean, this was the nail in the coffin for plate tectonics.
4:53
SPEAKER_01: This this really like set the stage for it, right?
4:57
SPEAKER_00: And so that's where my mind goes.
4:59
SPEAKER_00: Let me interrupt there, Chris.
5:00
SPEAKER_00: It's the nail in the coffin that sort of said plate tectonics is the operating system.
5:05
SPEAKER_00: You know, it was like the it wasn't the death of plate tectonics, it was the birth of plate tectonics as it was.
5:12
SPEAKER_01: But I just want to clarify that's a good point.
5:14
SPEAKER_01: Today what we're gonna do, though, is we are gonna talk about how paleomagnetism on the ocean floor relates to plate tectonics.
5:20
SPEAKER_01: And I think this will be a review for maybe a lot of our listeners, but for a lot of our listeners, no, it won't be.
5:25
SPEAKER_01: This will be new stuff that they maybe haven't learned before.
5:28
SPEAKER_01: And then also, why is our magnetic field important?
5:32
SPEAKER_01: And how is Earth's magnetic field generated?
5:35
SPEAKER_01: What do we think about this?
5:37
SPEAKER_01: What do we know about this?
5:38
SPEAKER_01: And and that's really the that's the stage for today's episode.
5:43
SPEAKER_00: So on that note, Chris, this is a podcast.
5:46
SPEAKER_00: We don't have some images, and I think this is one topic where images are really, really useful.
5:50
SPEAKER_00: And so if you've having a hard time visualizing what we're discussing today, go to our Camp Geo.
5:55
SPEAKER_00: The link is the first link in the show notes.
5:57
SPEAKER_00: Go to Camp Geo.
5:58
SPEAKER_00: There we have images, and we have a couple episodes sort of dedicated to magnetism in our plate tectonics chapter there that really show some of the images.
6:06
SPEAKER_00: So we'll kind of talk about things that that are visually represented there.
6:09
SPEAKER_00: So if you're having trouble visualizing, go there.
6:11
SPEAKER_00: But Chris, I think for me, what would be interesting in this episode is to hear first how your students answered this first question.
6:18
SPEAKER_00: So you said it that how does paleomagnetism on the ocean floor relate to plate tectonics?
6:24
SPEAKER_00: And so I'm gonna ask you how do your students answered, but let me just define paleomagnetism.
6:29
SPEAKER_00: Is that okay to do really quickly here?
6:30
SPEAKER_00: Like, okay, so the paleo is just really quickly, you I don't know if that's I don't know if that's possible or not.
6:38
SPEAKER_00: When rocks form, either sedimentary or igneous rocks typically, they can contain a record, they can lock in Earth's magnetic field at the time they formed.
6:47
SPEAKER_00: And so what paleomagnetism is is going back to old rocks and looking at the magnetic signatures preserved there, and those magnetic signatures were the record of the magnetic field at the time the rock formed.
6:59
SPEAKER_00: So a 200 million-year-old basalt will record the Earth's magnetic field 200 million years ago.
7:04
SPEAKER_01: In other words, when you say that record the magnetic field, you are talking about recording where the the magnetic north and south pole are of the earth.
7:12
SPEAKER_01: That's what you're referring to.
7:14
SPEAKER_00: It records sort of many features about magnetism, including the strength of the magnetic field, direction, north and south, reversals, so a lot of details.
7:22
SPEAKER_00: But what we're gonna focus on is was north up or was north down on the map?
7:26
SPEAKER_00: Magnetic north up or magnetic north down.
7:28
SPEAKER_01: Right on.
7:29
SPEAKER_01: So you asked me how my kids answered this question.
7:32
SPEAKER_01: Um, I don't really they didn't keep in mind, I was talking to freshmen.
7:38
SPEAKER_01: Okay, so what they know about plate tectonics, they knew maybe some of the basics.
7:43
SPEAKER_01: They know the three basic boundaries and and so on.
7:46
SPEAKER_01: They know about continent and Alfred Wagner a little bit.
7:50
SPEAKER_01: They know about his evidence for continent and so on.
7:53
SPEAKER_01: That's about where that stopped.
7:55
SPEAKER_01: I mean, they they really didn't know how paleomagnetism relates at all to plate tectonics because they really had no understanding of what paleomagnetism is.
8:05
SPEAKER_01: It's a big word, you know, but it's really kind of simple if you just break it down.
8:10
SPEAKER_01: Paleo means ancient and magnetism, you have to know about magnetism to you know to understand that a little bit.
8:16
SPEAKER_00: But so when you did you hand out like sheets of paper here and and you get a bunch of blank answers back on this question, or or was it just blank stairs?
8:24
SPEAKER_01: That's absolutely what happened.
8:25
SPEAKER_01: Yeah, that's absolutely like all right, Mr.
8:27
SPEAKER_01: Boyce.
8:28
SPEAKER_01: You know, they're looking at me now, they got you know, some of them are grinning at me because they can tell I'm upset.
8:32
SPEAKER_01: I'm not really mad at them, I'm just like, you know, I'm excited about what we're about to dive into, and I wanted them to be excited about it too.
8:38
SPEAKER_01: And when they don't get that way, then I get upset.
8:41
SPEAKER_01: So um, you know, you were very upsetting to me when you were a student too.
8:46
SPEAKER_01: So yeah, I sat in the back like a stone, but uh you were yeah, you did not have personality back in those days.
8:52
SPEAKER_01: So yeah, I didn't get a lot.
8:54
SPEAKER_01: Um well let's let's get into this, Jesse.
8:57
SPEAKER_01: Then paleomagnetism on the ocean floor.
9:00
SPEAKER_01: Can I go ahead and just set the stage and have you jump in?
9:03
SPEAKER_01: Okay, why is this important?
9:04
SPEAKER_01: How does it relate to plate tectonics?
9:06
SPEAKER_01: So every ocean on our planet has a mid-ocean ridge somewhere near the middle of it.
9:11
SPEAKER_01: Okay.
9:12
SPEAKER_01: And so what happens then is this mafic magma, rich in iron and magnesium, uh, pours out onto the ocean floor and it begins to cool.
9:20
SPEAKER_01: Then as magma, or in this case now lava, as the lava begins to cool off, these early formed minerals that tend to be, you know, rich in iron and magnesium begin to crystallize.
9:32
SPEAKER_01: And so think of them.
9:33
SPEAKER_01: I this is the way I describe it, and you tell me if this is like not a good idea or not.
9:37
SPEAKER_01: But I describe these early formed minerals as kind of being long and skinny, like pencil-shaped, because they then it's it's easier to think about magnets then if you think about them being long and skinny and separating out a north and a south pole.
9:50
SPEAKER_01: Okay, so they begin to cool and they begin to crystallize and they begin to grow.
9:55
SPEAKER_01: If these long and skinny minerals, uh pencil-shaped minerals are cooling, they get below the curie point, below 530 degrees Celsius-ish, and they're in a soup of magma.
10:06
SPEAKER_01: So what happens then?
10:08
SPEAKER_01: Why is that an um such an important thing when we're talking about paleomagnetism?
10:12
SPEAKER_00: Yeah, so uh it's kind of this is in the keep me out of the weeds here, Chris, because you know, but really what's important here is that those minerals are now locked in place, they're not going to move around.
10:24
SPEAKER_00: Or more importantly, their magnetic, their internal magnetic fields are not going to realign or re-orient.
10:29
SPEAKER_00: And so below the cure point.
10:30
SPEAKER_01: Hold on, you gotta back up.
10:31
SPEAKER_01: When they get below the cure point, then the minerals are gonna align because they become magnetized, you know.
10:37
SPEAKER_01: They and so start with that.
10:39
SPEAKER_00: Okay, yeah, Chris.
10:40
SPEAKER_00: So the main thing here is that, you know, the you're describing sort of the minerals aligning within the within the magma, like you're imagining minerals that are little blades are flowing with the magma and they're kind of orienting themselves, but the magnetic fields are kind of randomly uh pointing in every which way.
10:57
SPEAKER_00: At the curie point, their magnetic fields do not change anymore.
11:01
SPEAKER_00: And that curie temperature is usually below what we call the solidest.
11:04
SPEAKER_00: So the rock is solid, the minerals are already like locked in their orientation, and the magnetic fields are also below the curie point, become locked in their orientation.
11:15
SPEAKER_00: So, what this means is that a rock becomes solid, and at in this case, you're talking about basalt, so it's something like you know, a thousand degrees centigrade, nine hundred degrees centigrade, they become solid.
11:28
SPEAKER_00: Then below that temperature, the magnetic fields can kind of realign.
11:31
SPEAKER_00: If the Earth's magnetic field shifts around, the minerals' internal magnetic fields will realign to that, but below the curie point, it's locked in.
11:39
SPEAKER_00: And so as soon as it cools down below the curie point, which sort of varies different depending on which minerals, but think of it in that temperature range that you described.
11:48
SPEAKER_00: As soon as you get below that temperature, it's locked in now.
11:51
SPEAKER_00: And so that means you can take that rock and we can move it around by plate tectonics.
11:55
SPEAKER_00: We can shift that rock around for 200 million years, let's say, can move around, and it's still going to maintain a record of the magnetic field back in time.
12:04
SPEAKER_00: So it will point in a direction of the magnetic north pole.
12:09
SPEAKER_00: The magnetic field within that rock is locked in place.
12:13
SPEAKER_01: So what you're saying is at look, every magnetic field has a north and a south pole.
12:17
SPEAKER_01: So the south pole of the minerals that are in that basaltic lava flow are pointing to the magnetic north pole of Earth at the time it cooled below the Curie point.
12:30
SPEAKER_00: Exactly.
12:31
SPEAKER_00: Exactly.
12:31
SPEAKER_00: And, you know, there's details here at the equator, the magnetic fields kind of align with the the surface of the earth.
12:37
SPEAKER_00: When they're near the poles, they kind of point down into the earth.
12:40
SPEAKER_00: So we can kind of get two measurements.
12:42
SPEAKER_00: We can get is north pole up or is north pole down, magnetic north pole up or down.
12:47
SPEAKER_00: And we can also get was this rock near the equator or was it up near the poles somewhere?
12:52
SPEAKER_00: We can kind of get latitude out of this too, out of this paleo magnetic measurement.
12:56
SPEAKER_00: And so I think there's two main ways that this is related to plate tectonics or related to the discovery of plate tectonics, right?
13:06
SPEAKER_00: The first, Chris, is seafloor spreading.
13:08
SPEAKER_00: And that's kind of really one of the major ones.
13:11
SPEAKER_00: And there's an interesting story here.
13:12
SPEAKER_00: So I did my postdoc at the Carnegie Institute for Science.
13:16
SPEAKER_00: I was there for about four years, which is a nonprofit research institute that has a long history of research in Earth Sciences.
13:23
SPEAKER_00: And it started out, the department that I was in was called the Department of Terrestrial Magnetism, which DTM, baby.
13:30
SPEAKER_00: DTM.
13:31
SPEAKER_00: It no longer is named that, but it was DTM, Department of Terrestrial Magnetism.
13:36
SPEAKER_00: And this name goes back to like 1905, when magnetism was kind of first starting to sort of make its way on the scene.
13:44
SPEAKER_00: People could measure the modern magnetic field and see variations in the magnetic field.
13:48
SPEAKER_00: So, like, oh, we need to do research on this.
13:49
SPEAKER_00: Andrew Carnegie gave a bunch of money at the time to just fund this type of research, right?
13:54
SPEAKER_00: So, born the Department of Terrestrial Magnetism.
13:57
SPEAKER_00: For the next like 30 years, from 19, you know, oh five through to the 40s and early 50s, Department of Terrestrial Magnetism applied.
14:07
SPEAKER_00: There was people working on Earth's magnetic field there.
14:09
SPEAKER_00: Then in the early 50s, so the story goes, the director at the time walked into the one person who was doing rock magnetism, who was looking at rock magnetism, and was like, What is the point?
14:20
SPEAKER_00: What are you doing here?
14:21
SPEAKER_00: What's the future look like?
14:22
SPEAKER_00: And apparently the guy gave the wrong answer because the director at the time said, No, not interesting.
14:28
SPEAKER_00: We're moving out of magnetism.
14:30
SPEAKER_00: We're not going to be studying magnetism anymore.
14:32
SPEAKER_00: And this was right at the very cusp of plate tectonic discovery.
14:36
SPEAKER_00: So the scientific community had not recognized the value of studying ancient magnetic signatures in rocks well enough to really know that this was leading into this monumental discovery of plate tectonics.
14:48
SPEAKER_00: So then from then on, you know, the sort of department of terrestrial magnetism, which should have been on the leading front of discovering plate tectonics, right?
14:55
SPEAKER_00: Given the story we're about to tell, they sort of missed out and nobody worked on rock magnetism in the Department of Terrestriomagnetism for a long time.
15:02
SPEAKER_00: So it's kind of one of these funny funny stories of like, you know, uh they sort of missed out because they weren't in the right field at the right time, you know.
15:09
SPEAKER_00: Yeah, it's a cool story.
15:10
SPEAKER_01: It's a good it's it's awesome history.
15:12
SPEAKER_01: I remember when I when Jenny and I came to visit you in DC, you took us through DTM, and and that was that was awesome for me, you know, like from a historical standpoint.
15:21
SPEAKER_01: I loved it.
15:21
SPEAKER_01: So, Jesse, we got to talk though about what happens.
15:25
SPEAKER_00: Let me just say we're about to tell the story of the Discovery Plate Tectonics here, right?
15:30
SPEAKER_00: Like that that's why I sort of told the story about DTM is we're about to tell this story, and that Chris is exactly right.
15:35
SPEAKER_00: So, you know, looking at Earth's modern magnetic field, you can see it moves around.
15:40
SPEAKER_00: Like the North Pole is moving around, it's it's not stagnant, it's moving around, and we can watch it move around on year or decade timescales.
15:48
SPEAKER_00: And every once in a while, North Pole and South Pole flip.
15:50
SPEAKER_00: The magnetic north pole and south pole flip directions and then they flip back and they flip back.
15:55
SPEAKER_00: And so it's doing this over and over on the several hundred thousand years to millions of years time scale.
16:01
SPEAKER_00: The flips happen at that rate, basically.
16:03
SPEAKER_00: Right.
16:03
SPEAKER_01: So and there's a pattern to this.
16:05
SPEAKER_01: I mean, the magnetic field will weaken, it'll weaken, it'll weaken, and then eventually it shuts down for a period of time before it starts up again in a reversed polarity situation, and then it flips back to normal and then it weakens again and flips back to reverse, and it just keeps doing this throughout time.
16:23
SPEAKER_01: So, what happens to the rocks then, Jesse, at the mid-ocean ridges?
16:29
SPEAKER_01: What happens then?
16:30
SPEAKER_01: What's preserved?
16:31
SPEAKER_01: What can we see?
16:32
SPEAKER_00: If you imagine the mid-ocean ridge, which we've talked about before, but this is where new oceanic crust is formed.
16:39
SPEAKER_00: New basalt, the rock you mentioned, high in iron magnesium, that rock is being formed and it's being formed along a ridge system.
16:45
SPEAKER_00: So it's sort of like you know, they're spreading apart along a line on the map.
16:49
SPEAKER_00: So we're forming new crust and moving off away from the ridge at the same rate.
16:54
SPEAKER_00: Now, let's imagine a rock where the magnetic north pole is pointing in the true north pole direction.
17:02
SPEAKER_00: So, Chris, I I think this is very confusing to me, and I always get lost in this space because the textbook is wrong in one part of at least the textbook I use is has an image that's reversed.
17:14
SPEAKER_00: But this nomenclature gets confusing because we're talking about Earth's magnetic field reversing.
17:19
SPEAKER_00: So, like North Pole becomes South Pole, the magnetic north becomes the mag, you know, points down.
17:24
SPEAKER_01: We're not talking about Earth flipping around.
17:26
SPEAKER_01: Okay, we're talking about just the shutdown of the magnetic field and then a reversal of the magnetic field.
17:32
SPEAKER_00: That's exactly right.
17:33
SPEAKER_00: So as far as magnets are concerned, you have a north pole and a south pole, those little bar magnets that have an N and an S on them, right?
17:39
SPEAKER_00: You've people have maybe played with those or seen those things.
17:42
SPEAKER_00: Right now, the magnetic south, the S on that bar is pointing up, is pointing to the north.
17:48
SPEAKER_00: But as geologists, you know, we don't really care about the physics of magnetism so much.
17:53
SPEAKER_00: So we call that normal.
17:54
SPEAKER_00: Like that is a normal period because it's what we have today.
17:57
SPEAKER_00: And then as we go back in time when it's flipped, when magnetic north is is true north, then that's reversed.
18:04
SPEAKER_00: So it gets really confusing in here, and the textbook is wrong.
18:06
SPEAKER_00: Like we had to look this up because I always get confused about which one's which, because it deals with the physics of magnetism.
18:11
SPEAKER_00: For us, it doesn't really matter.
18:12
SPEAKER_00: We treat it as normal, like it is today, and back in time, it's reversed sometimes, it goes back the other direction.
18:18
SPEAKER_00: Okay.
18:19
SPEAKER_00: So let's say you're dragging a magnetometer, a thing that could measure the magnetic field across a series of rocks, and these rocks have an internal magnetic field as well.
18:30
SPEAKER_00: They recorded an ancient magnetic field, right?
18:33
SPEAKER_01: Any rocks where the magnetic field at hold on, because at the time those rocks formed and they cooled below the curry point, the magnetism was preserved inside those rocks at that time.
18:48
SPEAKER_01: And so any change that happened after that in the magnetic field on Earth would not affect those rocks.
18:56
SPEAKER_01: It would not affect the magnetism in those rocks.
19:00
SPEAKER_01: It would only affect the magnetism in newly forming rocks.
19:05
SPEAKER_00: So let's play this out.
19:06
SPEAKER_00: Let's say we have a magnetometer, we can measure the strength of the magnetic field and we drag it across a rock.
19:10
SPEAKER_00: You know, right now it's measuring Earth's magnetic field pointing to the north, the north arrow's pointing up towards the south magnetic pole, which is the real north pole, north on Earth, and the rock is also pointing that direction.
19:21
SPEAKER_00: So we kind of get this strength, you know, we they're both pointing the same direction.
19:24
SPEAKER_00: So our magnetometer registers like a strong signal.
19:27
SPEAKER_00: When we get over a sequence of rocks where that is flipped, where the polarity is flipped is reversed.
19:33
SPEAKER_00: Now Earth's modern magnetic field and the rock's magnetic field are fighting against each other, which results in a weaker signal.
19:40
SPEAKER_00: And this is how these early magnetic measurements were made is just looking at the strength of the signal, and you can see when the polarity is flipped, when the rock is pointing the other direction, those signals they cancel out a little bit, which means you measure a decreased magnetic field strength.
19:56
SPEAKER_00: And that tells us that the rock is pointing in the opposite direction from Earth's modern magnetic field, or when the rock formed, it was in a reversed state.
20:04
SPEAKER_00: And this was really monumental because if you go to a mid-ocean ridge and you start making these measurements and you go in either direction away from the ridge, you get a striping pattern where you have normal, then reverse, then normal, then reverse, then normal, then reverse.
20:20
SPEAKER_00: And it's symmetrical, it looks the same on either side of that ridge.
20:25
SPEAKER_00: It's a barcode.
20:26
SPEAKER_01: And we often represent that in geology as literal stripes on the ocean floor.
20:33
SPEAKER_01: We represent it usually with black stripes representing, let's say, normal polarity, white stripes representing reverse polarity, and they flip-flop back and forth.
20:42
SPEAKER_01: And the thing is that you use the analogy of driving a boat with a magnetometer away from a mid-ocean ridge, and it shows us those stripes, right?
20:52
SPEAKER_01: Well, as you go away from a mid-ocean ridge, the age of the ocean floor also increases.
21:00
SPEAKER_01: So I always just say this you know, ocean floor that is farther away from a mid-ocean ridge is older and colder because day one for that rock, the age zero is when it came out onto the ocean floor.
21:13
SPEAKER_01: Then it cools and the ocean floor conveys it away from that spreading center, from that mid-ocean ridge.
21:20
SPEAKER_01: So the further away you go from mid-ocean ridge, the older that plate gets.
21:23
SPEAKER_01: And so what we're looking at then is paleomagnetism on the ocean floor are those magnetic stripes.
21:31
SPEAKER_00: So, Chris, that's a great lead-in to the sort of the barcoding thing, right?
21:36
SPEAKER_00: Like, let's say that this spreading center is spreading at a rate of, I don't know, four centimeters per year, let's say.
21:41
SPEAKER_00: Uh, and and and that is a constant spreading rate for that ridge.
21:45
SPEAKER_00: Now, these magnetic field reversals don't occur at the same time.
21:49
SPEAKER_00: There's no real consistency to them.
21:51
SPEAKER_00: They sort of occur maybe 10,000 years apart, maybe 150,000 years apart.
21:55
SPEAKER_00: They vary, and so sometimes you'll be in a long period of reversal, then you'll Be in a short normal period and then a long reversal period again.
22:03
SPEAKER_00: And this variation, now that is represented as time, which means the width of the sea floor, it is a barcode.
22:10
SPEAKER_00: So it's just like a barcode scanner that you'd see on your grocery items, right?
22:14
SPEAKER_00: Like it narrow bands and wide bands in the sequence of narrow, wide, on off, white and black is the sequence of the record of the magnetic field.
22:24
SPEAKER_00: And the reason that it's really important is because they match on either side of the spreading center, which means that this is a symmetrical movement.
22:32
SPEAKER_00: The rocks are forming and moving away, and as they're moving away, they're recording Earth's ancient magnetic field, and it matches on either side of the ridge.
22:40
SPEAKER_00: It has to be plates moving horizontally away from each other, which is plate tectonics.
22:45
SPEAKER_01: Plate tectonics is the movement of tectonic plates, right?
22:50
SPEAKER_01: And the only way to get this barcode pattern on the ocean floor is to have rock created and moved aside.
22:58
SPEAKER_01: As that happens, new rock is created and pushed aside.
23:02
SPEAKER_01: And that results then in paleomagnetism preserved on the ocean floor.
23:07
SPEAKER_01: It is the smoking gun of plate tectonics.
23:10
SPEAKER_01: There's no other way to get this pattern preserved.
23:14
SPEAKER_01: And the thing is, Jesse, that I think is awesome, is every ocean shows the exact same pattern.
23:22
SPEAKER_01: Okay, now newer oceans, younger oceans, aren't gonna go as far back in time because they're not old enough for that.
23:30
SPEAKER_01: But they will show every mid-ocean ridge where the rock is being created now shows the same polarity.
23:38
SPEAKER_01: Every single one, every corner of the world.
23:41
SPEAKER_00: That's awesome.
23:42
SPEAKER_00: And if you look at this map, if you you know you can Google a map of uh the the sort of ancient you know seafloor stripes or something like that, uh, Google paleomagnetism on seafloor, they don't look the same.
23:51
SPEAKER_00: Like the barcodes don't look like they match up.
23:53
SPEAKER_00: That's because the plates are spreading at different rates.
23:55
SPEAKER_00: And we've talked about this a little bit before, but the East Pacific rise is spreading much faster than the mid-Atlantic, and uh that's why the plates don't look the same.
24:04
SPEAKER_00: So you kind of have to squish and squeeze these barcodes to make them match up, but they do align.
24:08
SPEAKER_00: If you sort of normalize for the speed of spreading, they they align for sure.
24:13
SPEAKER_00: And they provide a really interesting way to match up rock record back in time because sediments preserve magnetic field as paleomagnetic signature as well.
24:22
SPEAKER_00: So we can tank ancient sediments, and the oldest oceanic crust is like 220, 230 million years old.
24:28
SPEAKER_00: So that's how far back in time our seafloor magnetic record goes, but we can barcode that and match it with sediments preserved on the continents that go back further in time, and so we can barcode further back into the past by using this sort of alignment of magnetic field reversals.
24:45
SPEAKER_01: Yeah, that's a good point.
24:47
SPEAKER_01: So I want I I wish that you know, this is where podcasts are very difficult because this is such a visual thing that we're talking about here.
24:54
SPEAKER_01: So one of the things that I I'm gonna describe something, Jesse, here.
24:58
SPEAKER_01: What I do to show this and what this looks like is I take two desks and I shove them together.
25:03
SPEAKER_01: And then I take a long, like 11 by 8 piece of paper and I fold it in half.
25:09
SPEAKER_01: And uh I put it, that folded paper, I put it in between my two desks that are shoved together.
25:16
SPEAKER_01: So just a a little bit of that paper sticking out.
25:19
SPEAKER_01: And that little bit of paper sticking out in between the desks, that represents the mid-ocean ridge.
25:24
SPEAKER_01: So I I reach in with my thumbs and I pull those papers up and out from the mid-ocean ridge.
25:30
SPEAKER_01: And I stop because you know, I maybe let's say I pull that paper out an inch on each side.
25:36
SPEAKER_01: Okay, and I I I stop right there and I take a marker and I draw an arrow pointing up, let's say, to represent, okay, this is the magnetism preserved in the rocks.
25:45
SPEAKER_01: And they're the same, they're pointing in the same direction.
25:48
SPEAKER_01: And then I pull it further out and I draw an arrow pointing the other way because that stuff now that represents a magnetic reversal.
25:55
SPEAKER_01: And so by the time you do this, you just draw a bunch of arrows that are pointing in opposite directions.
25:59
SPEAKER_01: And by the time the whole paper is pulled out from in between the desks, it's a mirror image on each side where the mid-ocean ridge was, where arrows are flip-flopping back and forth.
26:08
SPEAKER_01: And that's what every ocean looks like, where the paleomagnetism is preserved.
26:13
SPEAKER_01: I don't I don't know if that works because it's visual.
26:15
SPEAKER_01: It definitely does, Chris.
26:16
SPEAKER_00: That's actually a really good one.
26:17
SPEAKER_00: I'm sitting here thinking, wow, I wish I could use that in a classroom, a big lecture hall of 250 people, but I don't think people would be able to see the uh the front of the classroom if I did that.
26:25
SPEAKER_00: Because that's a great record it.
26:28
SPEAKER_01: You know what I mean?
26:28
SPEAKER_01: Like on your iPad or something like that, and and put it up on a big screen, something like that.
26:32
SPEAKER_00: Yeah, that's a that's a good analogy.
26:33
SPEAKER_00: I like that.
26:34
SPEAKER_00: That works.
26:35
SPEAKER_00: Um, so I think this is sort of getting to the point of why Earth's magnetic field is important or why our magnetic field is important.
26:42
SPEAKER_00: We're leading in that direction, right?
26:44
SPEAKER_00: So we've sort of shown this is definitely really important for the discovery of plate tectonics, and and we can there's a lot more to paleomagnetism that we're not gonna be able to do.
26:52
SPEAKER_01: So it's important because of that.
26:53
SPEAKER_00: It's definitely important for because of that.
26:55
SPEAKER_00: I mean, it's also really important biologically.
26:58
SPEAKER_00: There are people who argue that you can't have life on a planet when we're looking at all these exoplanets that the astronomers are discovering.
27:07
SPEAKER_00: One of the big arguments is when you talk about habitability, or can those exoplanets have any life on them?
27:13
SPEAKER_00: One of the parameters is having a magnetic field because it protects us from all of this cosmic radiation and solar wind and all of this radiation coming from both the sun and from uh you know, sort of outside of our solar system.
27:25
SPEAKER_00: So lots of background radiation which would be damaging to life and DNA and RNA uh is is sort of blocked and deflected by the magnetic field.
27:34
SPEAKER_01: However, this is hotly debated though, Jesse, because you know, we've had many, many reversals over time.
27:39
SPEAKER_01: And when the magnetic field shuts down, it takes maybe I don't know, this is also a hotly debated area, and I'm sure you know a lot more about this than I do, but it can take upwards of 20,000 years for a reversal to complete.
27:52
SPEAKER_01: So that's a long time for Earth to go without a uh much of a magnetic field at all to speak of.
28:00
SPEAKER_01: And so we're subjected to this cosmic radiation, which in our atmosphere is our only protection from this, which is substantial, okay.
28:08
SPEAKER_01: And it's it's debated, I think, a lot because, well, there are no mass extinctions that are related to magnetic reversals.
28:16
SPEAKER_00: It just that is a that's a really good point to point out here is that magnetic reversals don't correlate with mass extinction events.
28:23
SPEAKER_00: So, you know, this time period when the magnetic field is down does not relate to more sort of genetic variation or anything like that.
28:31
SPEAKER_00: There's no sort of biological impact on the the planet because the magnetic field is down for a little while, for actually a long while, uh, which is kind of interesting.
28:41
SPEAKER_00: So I think the the best arguments I've heard about this is that the magnetic field protects our atmosphere.
28:46
SPEAKER_00: So the the Earth can kind of hold on to its atmosphere more because otherwise the solar wind would would sort of blast away the atmosphere, uh the upper parts of the atmosphere.
28:54
SPEAKER_00: But the magnetic field protects the atmosphere, which then protects us from the sort of radiation.
28:59
SPEAKER_00: Um yes, you're right.
29:00
SPEAKER_00: It's hardly debated here.
29:02
SPEAKER_00: So maybe should we just wrap up, Chris, with just a brief sort of overview of how the magnetic field's created?
29:09
SPEAKER_01: Yeah, I think we have to.
29:11
SPEAKER_01: Um again, what we know about the magnetic field and how it's generated, obviously we can't, you know, directly observe this.
29:18
SPEAKER_01: Um, and so this is all done with modeling, you know, and and what we're able to do in labs actually, this looks to me to be really, really interesting.
29:28
SPEAKER_01: Creating these dynamos in lab and how circulating metallic fluids can generate an electric field, which in turn generate a magnetic field.
29:37
SPEAKER_01: It's you know, this electromagnetic dynamo.
29:40
SPEAKER_01: So what's going on in our liquid outer core, Jesse?
29:45
SPEAKER_01: And why is that the important layer in the discussion here?
29:49
SPEAKER_00: So the core is liquid, well, the outer core is liquid iron, the inner core is solid iron or solid iron nickel alloy, but the outer core is liquid.
29:59
SPEAKER_00: And so the important thing here is that when you get liquid metal spinning around, it generates a magnetic field.
30:06
SPEAKER_00: Now, this is really complicated.
30:08
SPEAKER_00: People much smarter than me, as you said, model this and do huge supercomputer simulations trying to understand you know what is forming the core.
30:16
SPEAKER_00: But the way I think about this, the way I visualize this, is let's you got to visualize Earth as a hollow sphere.
30:23
SPEAKER_00: So Earth's crust, lithosphere, asthenosphere, the mantle, crust and mantle is solid, and that's like a shell, right?
30:30
SPEAKER_00: And then inside of it is liquid.
30:32
SPEAKER_00: That's let's let's make it really, really like dumb simple.
30:36
SPEAKER_00: Like if you take a uh, I don't know, a water balloon, right?
30:39
SPEAKER_01: And you start to let me let me do this.
30:41
SPEAKER_01: I got you, Jesse, here.
30:42
SPEAKER_01: So like let's imagine a peanut MM, okay?
30:45
SPEAKER_01: But we gotta make a peanut M better.
30:48
SPEAKER_01: So we have the the peanut is the crust, the the the mantle is the chocolate, but let's put some caramel below the chocolate in between the the chocolate and the peanut.
30:56
SPEAKER_01: Let's put some like caramel in there, some like warm, fluid, movable caramel.
31:01
SPEAKER_00: Well, that's an I'm not sure that you're making the MM taste better with caramel inside of it.
31:05
SPEAKER_00: Oh, I think that's interesting.
31:06
SPEAKER_00: I think definitely.
31:07
SPEAKER_01: And so the that caramel layer is the important layer because that in in terms of the earth that would represent our liquid outer core, which is a metal.
31:15
SPEAKER_01: And when you circulate, when you have a circulating metallic liquid, it generates electricity.
31:22
SPEAKER_01: Okay, so we have circulation in the liquid outer core for like right, Jesse, two main reasons.
31:27
SPEAKER_01: Right.
31:28
SPEAKER_01: We have uneven heating, right, which generates convection currents within the liquid outer core, but we also have earth spinning on its axis, right?
31:38
SPEAKER_01: So you have this solid, you know, crust mantle is solid, the core is solid.
31:44
SPEAKER_01: But the liquid outer core, if you take this this thing and and rotate it rapidly, that liquid part is not gonna quite keep up with the solid part of the earth, right?
31:55
SPEAKER_00: That's right.
31:56
SPEAKER_00: So I think it's that's hard to kind of visualize when you think of the scale of the planet, right?
32:01
SPEAKER_00: We always think the mantle's super big, but actually the liquid outer core is is really a thick layer too.
32:06
SPEAKER_00: And so when you have this hollow thing spinning around, it's caramel inside of your MM, it doesn't spin like the outside does.
32:13
SPEAKER_00: Like it, you know, that that turbulence effect.
32:15
SPEAKER_00: Like think of turbulence, there's turbulence in there, it's not quite the same.
32:18
SPEAKER_00: But then you nailed it, heat has to escape because the outer core is liquid, it's crystallizing as it cools down to form the solid inner core.
32:27
SPEAKER_00: In that crystallization, what's called the latent heat of crystallization, it gives off heat.
32:31
SPEAKER_00: And so there's heat being generated at the base of the liquid outer core that's trying to escape.
32:36
SPEAKER_00: And so we have these convection cells, think of the uh circular convection cells like you see in a lava lamp, except take that lava lamp and spin it like a top, and you're gonna get vortexes, vortices.
32:46
SPEAKER_00: You're gonna get these vortices, yep, that's right.
32:48
SPEAKER_00: That kind of go north and south, and they're kind of close to the inner core, the solid inner core, and then they head up towards the poles and they kind of run vertically.
32:56
SPEAKER_00: But vortices that are like slinkies, they look like slinkies where the material is moving up and down in a vortex, and that's what generates the magnetic field.
33:04
SPEAKER_00: So you're exactly right.
33:05
SPEAKER_00: It's the spinning like a top, except it's a liquid, so it has these weird currents inside of it, and then it's the heat escaping.
33:12
SPEAKER_00: And so those two things generate these vortices.
33:15
SPEAKER_01: And it doesn't have to be just iron, it can really be any liquid metallic substance.
33:20
SPEAKER_01: So in the case of, let's say, planet Jupiter, Jupiter has this immense magnetic field, but it's created by metallic hydrogen, actually, or or you know, this ionized hydrogen, and it's circulating for the same reasons really that Earth is.
33:35
SPEAKER_01: It's a different substance, but it's a metallic form of hydrogen that is generating then this immense magnetic electromagnetic field.
33:43
SPEAKER_01: So, anyway, a lot of times in lab they will use other metals.
33:47
SPEAKER_01: I think I've seen one simulated using sodium.
33:50
SPEAKER_01: Oh, yeah, okay.
33:51
SPEAKER_01: Um, a reactor.
33:52
SPEAKER_01: So anything that is liquidized metallic form that's circulating is going to generate electromagnetism.
34:01
SPEAKER_00: Yeah.
34:02
SPEAKER_00: You know, that's the big the big dog in Earth is the liquid outer core that generates this magnetic field.
34:07
SPEAKER_00: It's very cool.
34:08
SPEAKER_00: So look up outer core Earth's magnetic field simulation.
34:11
SPEAKER_00: There's some really cool videos on the web done by people at NASA or or research scientists anywhere who publish these videos.
34:18
SPEAKER_00: It's very, very cool, very impressive.
34:20
SPEAKER_01: And they can be very hard to keep up with, though, because it's so complicated.
34:22
SPEAKER_01: I mean, we're really kind of simplifying this down.
34:25
SPEAKER_01: So, Jesse, if that's how the magnetic field is generated, then why do reversals happen?
34:31
SPEAKER_00: Good question.
34:32
SPEAKER_00: I I and it it's not super well understood exactly why.
34:36
SPEAKER_00: It's a sort of a hotly debated thing.
34:37
SPEAKER_00: It does come out of some models.
34:39
SPEAKER_00: You know, it might have to do with you can only convect heat in one direction for so long.
34:43
SPEAKER_00: It might have something to do with sort of the atmosphere and and sort of the feedback between the magnetic fields that that are induced.
34:49
SPEAKER_00: Um, I've never seen a consensus view on this.
34:53
SPEAKER_00: I don't know.
34:53
SPEAKER_00: Have you, Chris?
34:54
SPEAKER_01: First of all, I think we have to say that you know the spinning of the Earth on its axis is a major player in in like determining the way that these currents in the liquid outer core are generated, right?
35:06
SPEAKER_01: And that's why the the magnetic poles are very closely aligned with the geographic poles.
35:13
SPEAKER_00: That's right.
35:14
SPEAKER_01: And so the idea though that I've come across is that you get these competing currents in the liquid outer core, and they they eventually compete to the point where they can't, you know, the up and comer competes with the established current and it cancels it out, and then the up and comer wins, and there's your reversal.
35:31
SPEAKER_01: And then that can only dominate for so long, and then you get another competing current and it cancels it out and it flips again.
35:37
SPEAKER_00: I I think that's the the sort of um you know what happens during it.
35:42
SPEAKER_00: I've never understood really or seen a coherent story about why like you have these competing currents.
35:48
SPEAKER_00: Like why is the current not always running in one direction?
35:50
SPEAKER_00: But they're not regular, but there's some systematics to them.
35:54
SPEAKER_00: You know, I think they don't usually last more than a million years or several hundred thousand years, and they're and they're not too short necessarily either, these reversal periods.
36:03
SPEAKER_00: But if you look at the rock record, there are some instances of very long magnetic fields that that sort of last for a while.
36:11
SPEAKER_00: So uh it's an interesting question.
36:13
SPEAKER_01: And then, of course, you know, you could if you want to learn more about the core and all that, just go back to this movie from like 20 plus years ago called the core.
36:23
SPEAKER_00: Yeah, it's a great diamonds in the earth.
36:27
SPEAKER_00: It's great.
36:28
SPEAKER_00: Oh man, so fun.
36:30
SPEAKER_00: So fun.
36:30
SPEAKER_01: Well, you know, if we ever have to jumpstart the core, we just have to get a nuclear bomb down there and we can just kind of you know jumpstart the circulation again.
36:37
SPEAKER_00: So seems so easy, right?
36:39
SPEAKER_00: Oh man, that's great.
36:40
SPEAKER_00: Yeah, that's right.
36:41
SPEAKER_00: Well, Chris, this is a really uh I think this is a good topic question here, and I'm glad we didn't try to fit all of this into one episode, uh, including all the other stuff we've talked about with plate tectonics so far.
36:51
SPEAKER_00: But magnetism's very, very cool and uh and very important.
36:55
SPEAKER_00: I think you know, it's important for ancient rock record, for understanding ancient rock record, it's important for understanding how we sort of came about plate tectonics as an idea, and also you know, sort of the future of the magnetic field is really important for society because we have a lot of stuff that depends on the magnetic field.
37:11
SPEAKER_01: Yeah, yeah, I agree.
37:12
SPEAKER_01: It's important too to point out, Jesse, that you know, magnetism on the ocean floor proves plate tectonics.
37:19
SPEAKER_01: It has nothing to do with causing plate tectonics.
37:22
SPEAKER_01: These plates are not moving because they're being pulled by Earth's magnetic field or anything like that.
37:27
SPEAKER_01: And that's something that you know sometimes my students will get tripped up on.
37:30
SPEAKER_00: That's a great point, Chris.
37:32
SPEAKER_00: That's a great point.
37:33
SPEAKER_00: All right.
37:34
SPEAKER_00: Hey, I think that's a good one to end on, man.
37:35
SPEAKER_00: Yeah, me too.
37:36
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37:38
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37:39
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37:41
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37:45
SPEAKER_00: Go to our Camp Geo product.
37:47
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37:50
SPEAKER_00: If you want to learn basically what Chris and I teach in our classes every year, you can go there and get a really good intro into geoscience.
37:57
SPEAKER_00: Check that out, let us know what you think, and send us your questions.
38:01
SPEAKER_00: Cheers.
38:01
SPEAKER_00: Have a good week.
38:02
SPEAKER_00: Peace.