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Earth's Leaky Core and Michigan Geology - Your Questions Part 1 · PlanetGeo: The Geology Podcast

• 0:03
  SPEAKER_01: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:14
  SPEAKER_01: Oh, Christopher.
• 0:18
  SPEAKER_00: Hi.
• 0:19
  SPEAKER_00: What was that?
• 0:20
  SPEAKER_00: Hi.
• 0:20
  SPEAKER_00: How you doing, Jesse?
• 0:21
  SPEAKER_01: Hi, good, good.
• 0:22
  SPEAKER_01: How are how are you?
• 0:24
  SPEAKER_01: Doing all right.
• 0:25
  SPEAKER_01: What's uh what's happening?
• 0:28
  SPEAKER_00: Uh let's see.
• 0:29
  SPEAKER_00: Not a lot.
• 0:30
  SPEAKER_00: Just you know, we're at the beginning of summer and just gearing up for a busy, like out of Michigan lots of excursions, lots of things.
• 0:40
  SPEAKER_01: Lots of excursions.
• 0:41
  SPEAKER_01: There you go.
• 0:42
  SPEAKER_01: Yeah, I'm uh well, I'm kind of doing the same a little bit.
• 0:45
  SPEAKER_01: I'm leaving for field camp, teaching field camp in a few weeks.
• 0:48
  SPEAKER_01: So, well, no, next week.
• 0:50
  SPEAKER_01: So getting ready for that.
• 0:52
  SPEAKER_01: Just writing papers, keeping the research enterprise going.
• 0:55
  SPEAKER_01: It's fun.
• 0:56
  SPEAKER_01: Okay.
• 0:56
  SPEAKER_01: Nice summer.
• 0:57
  SPEAKER_00: There we go.
• 0:57
  SPEAKER_01: So far, so good.
• 0:58
  SPEAKER_01: Good deal.
• 0:59
  SPEAKER_01: Hey, we've been talking about listener questions for a while or asking for listener questions, and we have gotten, I don't know, many.
• 1:06
  SPEAKER_01: I don't know how many, but a lot.
• 1:07
  SPEAKER_01: Yeah, we have a lot.
• 1:08
  SPEAKER_01: And we filtered through a few.
• 1:10
  SPEAKER_01: Well, I wouldn't say we filtered.
• 1:12
  SPEAKER_01: We took a few that we thought were quite interesting, and we're gonna address them today.
• 1:17
  SPEAKER_01: Let's do it.
• 1:18
  SPEAKER_01: And so what are we starting with?
• 1:21
  SPEAKER_00: We are gonna start with a question from a listener.
• 1:25
  SPEAKER_00: Her name is Kathy, and she says, I know you guys have talked about hot spots before, but I don't recall if you mentioned this.
• 1:34
  SPEAKER_00: And there was a link to a paper.
• 1:37
  SPEAKER_00: It says high resolution imaging reveals puzzling features deep in the earth's interior.
• 1:43
  SPEAKER_00: And we're talking about the mantle core boundary here.
• 1:46
  SPEAKER_00: That's how deep.
• 1:47
  SPEAKER_00: So, you know, we're we're at like 2,900 kilometers down below the surface of the earth.
• 1:52
  SPEAKER_00: That's uh we're gonna start with that question from Kathy.
• 1:55
  SPEAKER_01: Yeah, I think we should start out by describing the Earth's layering first, right?
• 1:59
  SPEAKER_01: I mean, just in general, so that everybody's on the same page.
• 2:01
  SPEAKER_00: Okay, yeah.
• 2:02
  SPEAKER_01: So you have this great analogy.
• 2:03
  SPEAKER_01: Is it a bowling ball that you use to describe the crust?
• 2:06
  SPEAKER_01: Is that what my or is it a basketball?
• 2:08
  SPEAKER_00: Yeah, usually, yeah, usually a bowling ball.
• 2:11
  SPEAKER_00: Um, if you take a bowling ball and you wrap it in like tissue paper or like that wax paper, the thickness of that tissue paper over the ball is about the scale of the earth's crust to the rest of the earth.
• 2:26
  SPEAKER_00: So it's you know, I'm not saying it's not an important layer because it's extremely important, but it it you know puts it in perspective in terms of how thin this layer actually is.
• 2:37
  SPEAKER_01: Yeah, the crust is not very much.
• 2:38
  SPEAKER_01: And then as you go below the crust, we end up in the mantle.
• 2:42
  SPEAKER_01: And the mantle is the hot plastic kind of material.
• 2:46
  SPEAKER_01: It's still solid, but it is hot, plasticky.
• 2:49
  SPEAKER_01: There's several interesting layers in the mantle that we don't need to talk about right now.
• 2:53
  SPEAKER_01: But as you get down towards the base of the mantle, towards the mantle core boundary, there are two parts of the core.
• 2:58
  SPEAKER_01: There's a liquid outer core and an inner solid core.
• 3:01
  SPEAKER_01: And so this core mantle boundary, well, you said 2,900 kilometers deep, that's the layer that we're focusing on, or that the researchers that this article highlights were focused on.
• 3:12
  SPEAKER_00: That's right.
• 3:13
  SPEAKER_00: And what we just went through with the basic three layers of the earth, the crust, the mantle, and the core, those are compositional layers, those are layers that are distinguished based on their chemical composition.
• 3:25
  SPEAKER_00: And it basically just gets more mafic the deeper you go until you get to the core where it's almost all iron.
• 3:33
  SPEAKER_00: There's iron and then some other components like nickel and cobalt and things like that.
• 3:37
  SPEAKER_00: But there is also another way that we layer the earth, and that's the lithosphere and asthenosphere, and that's based on rock rheology or the behavior of the rock, where the lithosphere spans a big compositional difference because it takes the crust and the upper part of the mantle, but it behaves like peanut brittle.
• 3:59
  SPEAKER_00: It's really cold and crunchy.
• 4:01
  SPEAKER_00: Whereas then the asthenosphere is that really kind of soft, plasticky, hot rock that's like very close to its melting point.
• 4:11
  SPEAKER_00: And so it behaves very differently.
• 4:13
  SPEAKER_01: That is exactly right.
• 4:14
  SPEAKER_01: And Chris, the way that we know the layers of the earth is largely due to seismic waves.
• 4:19
  SPEAKER_01: And we've talked about this before, but there's sort of two ways that seismic waves travel through the crust or that we use them as they travel through the crust.
• 4:26
  SPEAKER_01: So seismic waves, an earthquake happened somewhere, or back in the 60s, you know, a nuclear weapon was detonated somewhere.
• 4:32
  SPEAKER_01: The energy from that event, either an earthquake or a nuclear weapons blast, would travel through the earth.
• 4:38
  SPEAKER_01: And it travels through the earth and it kind of curves as it's on its way through.
• 4:42
  SPEAKER_01: And so you kind of picture these, they kind of, if you're looking at a map of the earth, a cross-section of the earth, they look like they curve through the earth.
• 4:49
  SPEAKER_01: And those waves, when they hit different boundaries, and these can be compositional or temperature boundaries, any sort of structure in the earth, they will either slow down, speed up.
• 5:01
  SPEAKER_01: That's one thing they can do.
• 5:03
  SPEAKER_01: The second one is they can change their form.
• 5:07
  SPEAKER_01: So they can go from a P wave to an S wave or a compressional wave to a shear wave, they can convert, so they can kind of convert when they hit this boundary, or they can refract, they can kind of get deflected.
• 5:18
  SPEAKER_01: And all those things we can sort of detect if we have a seismic station on the other side of the planet and see where waves are bending, where waves are slowing down, where they're changing from different waves and back again, or where they're reflecting off of boundaries.
• 5:34
  SPEAKER_01: So it gets really complicated, this seismology.
• 5:36
  SPEAKER_00: Yeah, I think we should define though, real quick, and we've done this before, but what is a P wave, what's an S wave?
• 5:42
  SPEAKER_00: You just alluded to those in there, how they can maybe sometimes change as they go from one medium to another.
• 5:47
  SPEAKER_00: The P waves, they're called primary waves.
• 5:50
  SPEAKER_00: Primary waves are compression waves.
• 5:52
  SPEAKER_00: In other words, what they do is they compress and expand the material that they travel through.
• 5:58
  SPEAKER_00: This is a sound wave.
• 6:01
  SPEAKER_00: Okay, now it doesn't have to be in the human audible range in order for it to be a sound wave.
• 6:05
  SPEAKER_00: That's how a sound wave propagates, that's how it moves.
• 6:08
  SPEAKER_00: And so they can travel through sound, can travel through gases, obviously.
• 6:13
  SPEAKER_00: You can hear me, you know.
• 6:15
  SPEAKER_00: That's a good thing.
• 6:16
  SPEAKER_00: Um, they can travel through liquids, they can travel through solids and gases, they can travel through anything, and they are the fastest kind of seismic wave.
• 6:26
  SPEAKER_00: Okay.
• 6:26
  SPEAKER_00: Then you have the S waves, these are called the secondary waves.
• 6:30
  SPEAKER_00: They're the second fastest of the seismic waves.
• 6:33
  SPEAKER_00: They travel roughly at like the half the speed of a primary wave, but they don't compress and expand what they travel through.
• 6:41
  SPEAKER_00: They're actually they're called a transverse wave or a shearing wave.
• 6:44
  SPEAKER_00: In other words, the wave is traveling in one direction, but the particles that are being disturbed are traveling perpendicular to that direction.
• 6:55
  SPEAKER_00: We call it shearing or tearing.
• 6:58
  SPEAKER_00: And because of this, they can only propagate through solids.
• 7:02
  SPEAKER_00: They can't move through gases and they can't move through liquids because liquids and gases don't have shear strength.
• 7:08
  SPEAKER_01: Yeah.
• 7:09
  SPEAKER_01: So what this study focuses on is looking at how these waves travel through the earth, how they get diffracted, how they get reflected, how they get converted from one to the other.
• 7:19
  SPEAKER_01: That gets really complicated really quickly, and it gets more complicated the deeper in the earth you go.
• 7:23
  SPEAKER_01: And so the farther these seismic waves have to travel through the earth, the more opportunities there are for slowing down, speeding up, reflecting, refracting, all that stuff, converting.
• 7:33
  SPEAKER_01: So getting down to 2,900 kilometers deep is really difficult.
• 7:38
  SPEAKER_01: But the feature they're looking at, there's two features in the mantle that are useful.
• 7:42
  SPEAKER_01: One is called a LLSVP, a large, low, shear wave velocity province.
• 7:46
  SPEAKER_01: Very sort of uninteresting acronym there.
• 7:49
  SPEAKER_01: The other one is a ULVZ or ultra-low velocity zone.
• 7:53
  SPEAKER_01: And these are two things that think of them like big mountains in the mantle, where the LLSVPs are these really high, sort of big, beastie, they're like 7% of the mantle, these huge things that kind of sit under, there's one under Africa, there's one under the Pacific.
• 8:09
  SPEAKER_01: The ultra low velocity zones are kind of thin, small things right at the base of the mantle, right at the core mantle boundary.
• 8:18
  SPEAKER_01: And these are really intriguing because nobody knows really what these things are, but they're ultra-low velocity zones, so they slow down the shear waves.
• 8:28
  SPEAKER_01: They really, really slow down the seismic waves traveling through them, and so therefore they're called ultra-low velocity zones.
• 8:34
  SPEAKER_01: And there's a few things that could slow down these waves.
• 8:38
  SPEAKER_01: One of them is temperature.
• 8:40
  SPEAKER_01: Some of them are compositional.
• 8:41
  SPEAKER_01: If there's a lot of iron in them, that might slow down the waves, and that's what some people have proposed that this is actually iron from the core leaking into the mantle in a way.
• 8:50
  SPEAKER_01: So this would be like a really iron-rich part of the mantle at the boundary.
• 8:54
  SPEAKER_01: But these things are tiny, Chris.
• 8:57
  SPEAKER_01: I mean, this it's really amazing that we can even image these things.
• 9:00
  SPEAKER_01: They're like 20 kilometers high and about a thousand kilometers wide, 900 kilometers wide.
• 9:07
  SPEAKER_00: And by the way, let's put a number on this.
• 9:08
  SPEAKER_00: They're talking about slowing these seismic waves down by as much as 40%.
• 9:13
  SPEAKER_00: I mean, that's if for such a small area, that's an abrupt, that's like speeding up to a red light and just slamming on the brakes.
• 9:21
  SPEAKER_00: It's really a dramatic drop in speed.
• 9:23
  SPEAKER_01: I mean, it's just amazing.
• 9:24
  SPEAKER_01: This is not a large thing.
• 9:26
  SPEAKER_01: It's slowing down a lot, and it's so deep, it's incredibly difficult to image.
• 9:31
  SPEAKER_01: And so, what this paper looked at, what they did is they used high performance computing and basically modeled the waveforms going through here to try and isolate exactly what was going on.
• 9:40
  SPEAKER_01: And so it gets really complicated really quickly there.
• 9:43
  SPEAKER_01: But basically, they could resolve these things better.
• 9:46
  SPEAKER_01: We've known these ULVZs have existed for a while.
• 9:49
  SPEAKER_01: They could get a little bit better picture, well, maybe a lot better picture of what they actually looked like, and they proposed this layered approach to them.
• 9:57
  SPEAKER_01: So we're seeing 10 kilometer, 5 kilometer layering, thousands of kilometers down deep in the mantle.
• 10:03
  SPEAKER_01: It's really amazing feat of that.
• 10:05
  SPEAKER_00: Hold on, before we get into like what might be the cause of it and where are they, right?
• 10:10
  SPEAKER_00: I mean, they're the their locations are kind of cool.
• 10:13
  SPEAKER_00: Before we get into that other stuff, like the cause of these things and and where they are, I want to ask you, because you're a researcher.
• 10:23
  SPEAKER_00: Does the idea of a numerical model does that diminish what their findings are to you at all?
• 10:34
  SPEAKER_00: Do you know what I'm saying?
• 10:36
  SPEAKER_00: Like I'm I'm talking about somebody that's listening to this and they're like, wait a minute, we're using numerical computer modeling to determine what this is.
• 10:45
  SPEAKER_00: Uh, you know, I don't know, is that diminish it at all?
• 10:48
  SPEAKER_01: I think um no, not really for this.
• 10:50
  SPEAKER_01: This is the way seismology is done because it gets so complicated so quickly.
• 10:55
  SPEAKER_01: And the way to think about this is they're using a numerical model to basically add precision or uncertainty to what we're seeing.
• 11:06
  SPEAKER_01: So you basically like run a whole bunch of computer simulations and say what reproduces the data the best, and then which ones are reasonable within there.
• 11:14
  SPEAKER_01: That's uh you know, in a sort of simplistic way, what a numerical model would be is forward modeling your data, which I that doesn't really concern me too much.
• 11:23
  SPEAKER_01: I mean, we do this in isotope geochemistry a bit.
• 11:26
  SPEAKER_01: Hey, the Earth is super complicated and it's really big, and it's there's a lot that could happen to seismic waves as they're going through the mantle, so it's really the only way to do it, to get at these, you know, really small scale features down at the base of the mantle.
• 11:40
  SPEAKER_00: Okay, all right.
• 11:40
  SPEAKER_00: And then I I just want to go back and also just reiterate that by studying seismic waves, this is how we were able to hone in on where the liquid outer core is, that boundary is, which is super important, right?
• 11:56
  SPEAKER_00: Because the liquid outer core, you have this it's this molten iron that's circulating that generates our magnetic field, which obviously is very important for protecting us from cosmic and solar radiation and so on.
• 12:09
  SPEAKER_00: But it's because of studying us these seismic waves that we know exactly where it is, the size of it, the location of it, and so on, because S waves cannot travel through the liquid outer core boundary, whereas P waves can.
• 12:25
  SPEAKER_00: They change speed abruptly, but they can still propagate their way through.
• 12:29
  SPEAKER_01: So yeah, that's right.
• 12:30
  SPEAKER_01: So I think we just maybe wrap this up by saying that it's still unclear what these ULVZs or ultra-low velocity zones are, or even what the larger LLSVP things are as well.
• 12:42
  SPEAKER_01: It's not really clear what they are.
• 12:44
  SPEAKER_01: They could be high temperature regions, really high temperature regions.
• 12:48
  SPEAKER_01: They could be regions where there's a bit of iron, like I said before, leaking from the core into the mantle there, that there's some chemical communication between the iron and the core.
• 12:58
  SPEAKER_01: Some people have suggested they're remnants of ancient crust, you know, as a subducting oceanic crust goes down, it could potentially pile up at the base of the mantle.
• 13:09
  SPEAKER_01: They can get piled up down at the base of the mantle and sort of be this ancient subducting slab graveyard, some people call it, um, down at the base of the mantle.
• 13:18
  SPEAKER_01: So it's a little bit unclear.
• 13:20
  SPEAKER_01: Well, we don't know.
• 13:21
  SPEAKER_01: We we just don't know what these things are.
• 13:22
  SPEAKER_00: But but there is some relationship or maybe correlation is a better word, with these ultra-low velocity zones and hot spots.
• 13:33
  SPEAKER_00: Is that correct?
• 13:34
  SPEAKER_00: I mean, we've found these below famous hotspots.
• 13:37
  SPEAKER_01: Yes, the ULVZs are the ultra-low ones, which they're studying here, are at the base of the larger ones, the LL SVPs, these bigger things, and there's a correlation with the bigger things with mantle plumes.
• 13:49
  SPEAKER_01: So they're kind of all three linked there in some way.
• 13:52
  SPEAKER_01: These small-scale mountains at the base of the mantle, the really big, massive pieces of the mantle, the the LLSVPs.
• 14:00
  SPEAKER_01: So people who have studied these mantle plumes, the basalts coming out of the mantle plumes, have suggested that there's little bits of core in the mantle plume basalt.
• 14:08
  SPEAKER_01: So that would imply that you know this core mantle interaction, iron is leaking into the mantle in the ULVZ, and that's getting transferred all the way up to the surface through this volcanic plumbing network that is that transverses, that travels through the entire mantle, which would be kind of incredible.
• 14:26
  SPEAKER_00: Yeah, I feel like this is just at the beginning stages of like really learning something something new.
• 14:33
  SPEAKER_01: Yeah, totally.
• 14:33
  SPEAKER_01: And this is definitely different than the old school textbook style of explaining the mantle, you know, the three-layered mantle and all that.
• 14:41
  SPEAKER_01: That that's kind of gone away, and this has replaced replaced it for understanding the mantle.
• 14:46
  SPEAKER_01: So very cool, Kathy.
• 14:48
  SPEAKER_01: Great article, great suggestion, very cool subject.
• 14:51
  SPEAKER_01: The second question, Chris, and we're gonna have two parts to these because we got so many good questions.
• 14:56
  SPEAKER_01: So we're breaking these apart.
• 14:57
  SPEAKER_01: We've got to mention this at the beginning.
• 14:58
  SPEAKER_01: We're breaking these apart into two episodes.
• 15:00
  SPEAKER_01: So stay tuned.
• 15:01
  SPEAKER_01: In a couple weeks, we'll have part two coming out of more listener questions.
• 15:05
  SPEAKER_01: But the second one we're gonna cover today, Chris, is from Matt.
• 15:08
  SPEAKER_01: And Matt says, I would wonder if you would consider spending some time talking about the evolution of Michigan.
• 15:14
  SPEAKER_01: And so I don't know if Matt is aware of the can of worms and the long lecture he's about to unleash on himself by this because you and I are both from Michigan.
• 15:25
  SPEAKER_01: You lead field trips in Michigan in your class up to the northern peninsula.
• 15:29
  SPEAKER_01: I mean, you must you must cover a lot of Michigan geology in your class.
• 15:32
  SPEAKER_01: I frankly don't remember how much you cover from you know 2005.
• 15:36
  SPEAKER_00: Hey, thanks for that.
• 15:37
  SPEAKER_00: I appreciate that.
• 15:38
  SPEAKER_00: Like that really pisses me off.
• 15:39
  SPEAKER_01: I went to college in Michigan.
• 15:41
  SPEAKER_01: We did, you know, I we learned a lot about Michigan geology, and uh there's there's a lot to tell.
• 15:48
  SPEAKER_01: And you know, both you and I can be a little bit long-winded at times.
• 15:51
  SPEAKER_01: So that's right.
• 15:52
  SPEAKER_01: Get ready, Matt.
• 15:55
  SPEAKER_00: Wow, this is so big.
• 15:57
  SPEAKER_00: Okay, we could do a lot just on this topic.
• 16:01
  SPEAKER_00: We could do several episodes just on this.
• 16:02
  SPEAKER_00: So we simplified things.
• 16:04
  SPEAKER_00: We're gonna slow down on some other things that I think like people should know about Michigan geology, you know.
• 16:10
  SPEAKER_00: Like we're just gonna be general and then we'll get specific and then we'll go back to general and and so on.
• 16:16
  SPEAKER_01: So, where are we gonna where do we start with, Chris?
• 16:18
  SPEAKER_01: I mean, like the the general I don't know.
• 16:21
  SPEAKER_00: Um well, you have to start with the Michigan basin.
• 16:25
  SPEAKER_00: Okay, so we live in what is called the Michigan basin.
• 16:29
  SPEAKER_00: And I think of when I think of the Michigan basin, I think of just an like what's an analogy.
• 16:36
  SPEAKER_00: Do you do our listeners know what a sincline is?
• 16:39
  SPEAKER_01: Oh, we might need to define that.
• 16:41
  SPEAKER_01: So a syncline is a type of fold.
• 16:44
  SPEAKER_01: So when rocks rocks fold typically, sometimes they break, but and there'd be little breaks along the folds, but at large scales you can fold rocks.
• 16:52
  SPEAKER_00: And a syncline is just one of bending, not like folding clothes, it's like bending.
• 16:57
  SPEAKER_01: Yeah, that's right.
• 16:58
  SPEAKER_01: It's a bend in a rock.
• 16:59
  SPEAKER_01: And so a syncline is when you bend the sides up, you make a U shape out of the rock.
• 17:05
  SPEAKER_01: So you take a flat line group of rocks and you bend the edges up to make a U-shaped.
• 17:09
  SPEAKER_01: The opposite is an anticline where you bend it down and it makes an A kind of shape.
• 17:13
  SPEAKER_01: So a syncline is just the U-shaped version.
• 17:16
  SPEAKER_00: So the reason why I asked that is because a basin is like a circular syncline.
• 17:22
  SPEAKER_00: So instead of just having you know two sides bent up, it's it's bent up all the way around it.
• 17:27
  SPEAKER_00: And uh think of what this looks like.
• 17:30
  SPEAKER_00: If you take these rocks and you bend them this way, like a circular syncline, and then you put sedimentary rocks inside of that, they resemble like nested uh measuring cups.
• 17:42
  SPEAKER_00: Is that that's pretty good?
• 17:43
  SPEAKER_00: You know, like because you can see the edges of all of the measuring cups, but the one that's in the center is the one that's on top.
• 17:54
  SPEAKER_00: So that's the youngest one, right?
• 17:56
  SPEAKER_00: And then they get progressively older as you go down in the nesting cups, and that's how sedimentary rocks work, right?
• 18:03
  SPEAKER_00: Oldest is on bottom and youngest is on top.
• 18:05
  SPEAKER_01: So the opposite of a basin is a dome, and a dome is a little bit easier.
• 18:09
  SPEAKER_01: You just kind of you know bend all the rocks up around a central point, and you have the youngest one still on top, but you get these domes and basins, which it's a three-dimensional folding.
• 18:19
  SPEAKER_01: You know, you're pushing in one spot up and it bends up around it, or you're pushing in one spot down and it bends, so sinks down around it as well.
• 18:26
  SPEAKER_01: And so, sorry, Chris, you were describing what this looks like when you erode a basin.
• 18:30
  SPEAKER_00: Yeah, so when you take these sedimentary rocks that are inside of the basin and you erode it, you you get a bullseye kind of pattern, then where the same rocks show up in these rings.
• 18:43
  SPEAKER_00: And the ring that is the youngest ring, the youngest sedimentary rock that's exposed, is then going to be this smaller circle, like the small measuring cup, the smallest measuring cup that's nested in there.
• 18:56
  SPEAKER_00: And then you get progressively older as you go out toward the margins and even beyond the state of Michigan.
• 19:03
  SPEAKER_00: We call it the Michigan Basin, but it goes beyond actually Michigan.
• 19:07
  SPEAKER_00: That's what we can see with the Michigan Basin.
• 19:09
  SPEAKER_00: And this, like when if you ever look at a geologic map where it shows the rock layers exposed at the surface for the state of Michigan, it looks like a bullseye pattern.
• 19:19
  SPEAKER_00: It looks like you're ready to start throwing darts.
• 19:21
  SPEAKER_00: Okay.
• 19:22
  SPEAKER_00: But Jesse, what's below that sedimentary cover?
• 19:25
  SPEAKER_01: Well, uh, below sedimentary rocks, usually we call these basement rocks, which are high-grade metamorphosed rocks, some igneous rocks.
• 19:34
  SPEAKER_01: They're the deeper roots to the crust.
• 19:36
  SPEAKER_01: So they're they're things that exist in the crust.
• 19:38
  SPEAKER_01: They were exposed at sea level at some point in time, and then the sediments were laid down on top of them.
• 19:43
  SPEAKER_01: So we call that basement because it's, you know, like the basement to your house, it's below the stuff on top, and the sediments are the stuff on top.
• 19:50
  SPEAKER_01: And so we have a lot of these Precambrian, older than about 650 million-year-old basement rocks that are exposed, especially in the western part of what we call the UP, which most people don't know what that is.
• 20:01
  SPEAKER_01: That's the upper peninsula of Michigan.
• 20:03
  SPEAKER_01: So if you look at a map of Michigan, it has two parts.
• 20:05
  SPEAKER_01: The lower peninsula is the main part, the glove, and then the upper peninsula is this the place where the lower hand and the upper hand.
• 20:13
  SPEAKER_00: Yeah, that's right.
• 20:14
  SPEAKER_00: That's right.
• 20:14
  SPEAKER_00: So in the so beautiful though.
• 20:16
  SPEAKER_01: Oh my gosh, it is that that is where I uh cut my teeth in field geology on your field camp or your field trip during your geology class, where we go up and look at some of these basement rocks, these high-grade igneous, metamorphic rocks, lots of magmatic veins cutting through them.
• 20:33
  SPEAKER_01: Uh dare I say they're more interesting than sedimentary rocks, at least to me.
• 20:37
  SPEAKER_01: Um, they're super interesting.
• 20:39
  SPEAKER_01: Yeah, and they're exposed in the western part of the upper peninsula and then into Minnesota and Wisconsin.
• 20:44
  SPEAKER_01: And this is where there's historically been a lot of mining iron, silver, copper.
• 20:49
  SPEAKER_01: Um there's huge amounts of ore deposits up there that were mined in the you know early industrial days and continue to be still a little bit mined.
• 20:58
  SPEAKER_00: That's right, that's right.
• 20:59
  SPEAKER_00: So, and a lot of the we call these ore deposits, where you have a rock that has enough of a metal in it to make it profitable to mine it.
• 21:07
  SPEAKER_00: That's called an ore.
• 21:09
  SPEAKER_00: And the the UP is just it's known for copper.
• 21:13
  SPEAKER_00: You know, there's uh the whole western part of the of the UP um is often referred to as copper country, and then you got just tons and tons of iron because there's a lot of what's called banded iron formation up there, and that could be a whole episode just in and of itself, Jesse, with the ideas behind how banded iron formations form, but they're super important, and this is related to this mid-continental rift that it took place during Precambrian time.
• 21:42
  SPEAKER_00: Well, the bottom line is that you have this continental rifting that's taking place, the the continents being kind of pulled and torn apart.
• 21:50
  SPEAKER_00: You have a potent heat source and a lot of fractured rock, and so when you take hot, salty water that's circulating through this.
• 22:00
  SPEAKER_00: Rock, it's going to dissolve these metals and then it's going to concentrate them in certain areas when the chemical conditions or the physical conditions changes.
• 22:09
  SPEAKER_00: And that's what was going on in the UP.
• 22:12
  SPEAKER_00: We were being torn apart in this mid-continental rift, and the conditions were just right to concentrate really valuable metals.
• 22:22
  SPEAKER_01: Right, exactly.
• 22:23
  SPEAKER_01: And and this is all sort of metals flowing through rock, or uh, and this is all fluids flowing through rock, and so it's occurring at depth.
• 22:33
  SPEAKER_01: So this failed rift, you know, the continent tried to split apart, it didn't quite work out.
• 22:39
  SPEAKER_01: That gives us the mid-continent rift because it's still in the middle of a continent and it failed.
• 22:43
  SPEAKER_01: It was about 1.1 billion years old.
• 22:46
  SPEAKER_01: Then later on, we get this Michigan basin formation.
• 22:50
  SPEAKER_01: And so on top of the basement rocks, we have the start of all of these sediments.
• 22:56
  SPEAKER_01: And these are uh significantly younger, they're mostly post-Cambrian, so younger than about 540 million years old, and they go all the way through to about 280 million years old.
• 23:07
  SPEAKER_01: So there's a huge length of time where sediment was being nearly continuously deposited in Michigan in the Michigan basin.
• 23:16
  SPEAKER_00: Yeah, and it's interesting too that we were in Michigan, we were at the bottom of a sea.
• 23:23
  SPEAKER_00: And that's something that I think is important to note because we're so far away from that right now.
• 23:28
  SPEAKER_00: But the rock record shows us that this is where we were, the kinds of rocks we had.
• 23:36
  SPEAKER_00: We have sandstones, we have shales, we have limestones, we have extensive coral reefs, we have evaporites, tons of evaporites like gypsum and rock salt.
• 23:49
  SPEAKER_00: Okay.
• 23:49
  SPEAKER_00: I mean, you and I are both from the Grand Rapids area, which is southwest Michigan.
• 23:54
  SPEAKER_00: And crisscrossing underneath the city of Grand Rapids are gypsum mines.
• 24:01
  SPEAKER_00: And gypsum forms in a restricted salt water basin that gets super warm and lots of evaporation is taking place, and it begins to lay down just these massive accumulations of the rock gypsum.
• 24:16
  SPEAKER_00: So all of this points to this kind of like warm, salty sea.
• 24:22
  SPEAKER_01: And that's a great point, Chris.
• 24:23
  SPEAKER_01: This is a shallow, warm, salty inland sea.
• 24:27
  SPEAKER_01: So it's still a sea that's in the ocean that has intruded into the continental center, the interior part of the continent, and is depositing all this stuff.
• 24:36
  SPEAKER_01: And these are really bioactive areas as well.
• 24:40
  SPEAKER_01: There are loads of fossils in Michigan and loads of you know sea life fossils.
• 24:46
  SPEAKER_01: You get all sorts of stuff.
• 24:47
  SPEAKER_01: Some of the most famous are the Potoski stones, at least in Michigan, that's the state rock or the state fossil.
• 24:52
  SPEAKER_00: And what is the Potoski stone?
• 24:54
  SPEAKER_00: What kind of rock is it?
• 24:56
  SPEAKER_01: It's a fossilized coral.
• 24:58
  SPEAKER_01: So it's uh an ancient coral, it's actually a colonial coral.
• 25:02
  SPEAKER_00: Looks like this honeycomb.
• 25:03
  SPEAKER_01: Yeah, yeah, it's beautiful.
• 25:04
  SPEAKER_01: Black and sort of a dark brown and light gray honeycomb structure.
• 25:09
  SPEAKER_01: So really beautiful.
• 25:10
  SPEAKER_01: And all of these rocks were deposited in this basin environment.
• 25:15
  SPEAKER_01: And so the rocks get thicker in the center of the basin, really, really thick, up to 15,000 feet thick in the center of the basin, where on the margins the rocks are a little bit thinner because this was this sort of basin feature that uh, you know, the same rock units you can correlate all across them, they just get thinner on the edges because the sea wasn't quite as thick there.
• 25:34
  SPEAKER_00: Hey, I want to go back to one thing because I absolutely love this about the state of Michigan.
• 25:39
  SPEAKER_00: Do you love pasties?
• 25:42
  SPEAKER_01: You know, I haven't had a pasty since like 04 when I went on.
• 25:46
  SPEAKER_01: That was my trip, wasn't it?
• 25:48
  SPEAKER_01: Yeah, on the field trip where you stop at the one.
• 25:50
  SPEAKER_00: Because I thought we stop at that place.
• 25:51
  SPEAKER_00: It's my favorite pasty place.
• 25:53
  SPEAKER_00: All right.
• 25:54
  SPEAKER_00: If you're listening to this, this is a it's a it's an upper peninsula thing, and it actually goes back to the mining days.
• 26:00
  SPEAKER_00: Um, because you know, you have these miners that moved from Europe actually, the tin mines of Cornwall, England.
• 26:07
  SPEAKER_00: These people brought with them their tradition of this meat potato pie that still stands today.
• 26:13
  SPEAKER_00: And so all over the place in the UP, you'll see signs of people advertising pasties.
• 26:19
  SPEAKER_00: It's spelled like a pasty, and they right away know if you're not from there, they look at you and they're kind of like not in a nice way.
• 26:27
  SPEAKER_00: If you order a pasty instead of a pasty, don't make that mistake.
• 26:30
  SPEAKER_01: No, that's right.
• 26:31
  SPEAKER_01: But yes, that I mean, I remember enjoying them.
• 26:33
  SPEAKER_01: I don't uh I haven't had one in a you know decades, but uh it would be good to go back and check it out.
• 26:38
  SPEAKER_01: I'm actually I got a grant funded to work on some rocks in the upper peninsula of Michigan as well.
• 26:43
  SPEAKER_01: So there's some really old part of the basement has some really old rocks, 2.5 billion year old rocks on the western side.
• 26:49
  SPEAKER_01: So we're gonna go up there.
• 26:50
  SPEAKER_01: Maybe I'll have opacity next summer.
• 26:52
  SPEAKER_00: You better.
• 26:53
  SPEAKER_00: You have to.
• 26:53
  SPEAKER_00: Lido's that's the place to go, man.
• 26:55
  SPEAKER_00: I'm telling you right now.
• 26:56
  SPEAKER_00: All right, so anyway, hey, listen, the angle at which these rocks are dipping toward the center of the Michigan basin is about 60 feet per mile.
• 27:07
  SPEAKER_00: So this isn't super steep, right?
• 27:10
  SPEAKER_00: We're talking about these rocks that are gonna drop down toward the center of the basin, just 60 feet every mile that you go.
• 27:18
  SPEAKER_00: Okay.
• 27:19
  SPEAKER_00: I'm gonna throw this to you a second.
• 27:21
  SPEAKER_00: What do you what are we thinking about the formation of the basin itself?
• 27:27
  SPEAKER_00: What caused this subsidence?
• 27:29
  SPEAKER_01: So during the time that this basin was around, we had several of these types of basins.
• 27:34
  SPEAKER_01: There's an Illinois basin, Iowa basin, the Appalachian Basin in western Pennsylvania, which I kind of live in.
• 27:39
  SPEAKER_01: These were all depositing sediments.
• 27:41
  SPEAKER_01: And so, you know, one model for this is that basically it's it's related to the fact that there was this older 1.1 billion-year-old mid-continent rift that failed.
• 27:51
  SPEAKER_01: And when that failed breaking apart of the continent happened, it kind of destroyed the roots to the continent a bit.
• 27:58
  SPEAKER_01: It kind of broke apart the deep lithosphere that used to exist underneath of that area.
• 28:03
  SPEAKER_01: And so when the Appalachian mountains began to form on the eastern seaboard of what is now North America, then it kind of caused this sort of reactivation of that and this sort of down warping of the interior of the continent.
• 28:16
  SPEAKER_01: Because you have this big mountain range kind of being smashed together on the outside, and it could reactivate the rift environment, this failed rift environment on the interior part and cause it to subside down.
• 28:26
  SPEAKER_01: And then you have, when it subsides down, you have water flows in there, then you have this inland sea.
• 28:31
  SPEAKER_01: And there's several that intruded into this region Appalachian Basin, the Michigan Basin, and the Illinois Basin are three of the um more well-known ones in this area.
• 28:41
  SPEAKER_00: Okay.
• 28:42
  SPEAKER_00: So there's the Michigan Basin and the sedimentary rocks that cover the Precambrian basement rocks and so on.
• 28:49
  SPEAKER_00: And then you know what?
• 28:50
  SPEAKER_00: We gotta we can't just make it as simple as that, because then it all has to get covered with a veneer of glacially deposited material.
• 28:58
  SPEAKER_01: Right.
• 28:58
  SPEAKER_01: And so we're done.
• 29:00
  SPEAKER_01: So we're done talking about the rocks themselves, and now we're talking about you know, the glacial stuff that's on top of it, which creates some really interesting geological features as well and landforms that you see in Michigan all over the place.
• 29:12
  SPEAKER_00: So it's what makes Michigan one of look, I love this state.
• 29:18
  SPEAKER_00: We live in an amazing state, and uh most of that is because of the glacial veneer that was left over uh at the end of the last ice age.
• 29:27
  SPEAKER_01: So yeah, all the lakes up north.
• 29:29
  SPEAKER_01: If you live in Michigan, you know, you're familiar with going up north for the in the summer to lakes and stuff.
• 29:34
  SPEAKER_01: All that's due to glacial stuff.
• 29:35
  SPEAKER_01: All that is due to glacial formations.
• 29:38
  SPEAKER_01: We don't have that in Pennsylvania.
• 29:39
  SPEAKER_01: We don't have the glaciers that came through, we just have the rocks, we don't have nearly as many lakes in Pennsylvania.
• 29:45
  SPEAKER_00: It's important to note that there are very few places in the state of Michigan where we can see the sedimentary rocks that we just got done talking about.
• 29:52
  SPEAKER_00: We get to see what the rocks are, and we get to see the angle that they're dipping or tilted toward the center of the basin through boreholes, through wells, exploration wells, and things like this.
• 30:05
  SPEAKER_00: All of that has been compiled to give us a picture of what's below because we have you know anywhere between three and five hundred feet of glacially deposited material that covers up all those rocks we just got done talking about.
• 30:19
  SPEAKER_01: That's right.
• 30:19
  SPEAKER_00: Okay, so Jesse, let's move into then what did the glaciers do to sh like they really reshaped the Michigan landscape?
• 30:28
  SPEAKER_00: How?
• 30:28
  SPEAKER_01: Yeah, and we got to move forward in time now.
• 30:30
  SPEAKER_01: We're thinking relatively recently, geologically speaking, right?
• 30:34
  SPEAKER_01: We got to jump forward to the last several hundreds of thousands of years in the last glaciers that came through.
• 30:40
  SPEAKER_01: We've had many glacial events in the last two and a half million years that have come through and put large glaciers on Earth in this part of North America.
• 30:48
  SPEAKER_01: But the last one was about 15,000, started to recede at least about 15,000, 12,000 years ago.
• 30:54
  SPEAKER_01: Glaciers sculpted the landscape.
• 30:56
  SPEAKER_01: First of all, glaciers are an amazing erosive force.
• 30:58
  SPEAKER_01: So they can just come through and they just scrape off everything for the most part.
• 31:03
  SPEAKER_01: So they scrape everything mostly flat, except in Michigan they didn't.
• 31:08
  SPEAKER_01: They took advantage of weak rock layers, and this formed the Great Lakes.
• 31:13
  SPEAKER_01: So the Great Lakes kind of outline Michigan really nicely, right?
• 31:16
  SPEAKER_01: Lake Michigan, like Huron.
• 31:18
  SPEAKER_01: They kind of outline this half of an oval, or maybe even more than half of an oval around the lower peninsula of Michigan.
• 31:25
  SPEAKER_01: And that's because that's where the weak rocks were exposed.
• 31:28
  SPEAKER_01: And the glaciers took advantage of the weak rocks and carved out these deeper valleys, which are now the Great Lakes.
• 31:34
  SPEAKER_01: They're now filled in with water.
• 31:36
  SPEAKER_00: That's right, Jesse.
• 31:37
  SPEAKER_00: So what you're saying, I think, is that look, water takes the path of least resistance, right?
• 31:42
  SPEAKER_00: Rivers follow the path of least resistance, so do glaciers.
• 31:46
  SPEAKER_00: So the glaciers that reshaped and sculpted the state of Michigan, they came out of Canada.
• 31:51
  SPEAKER_00: The zone of accumulation for this was in the Hudson Bay area where the ice was over three miles thick.
• 31:58
  SPEAKER_00: And that ice then began to ooze out in every direction.
• 32:01
  SPEAKER_00: We're just straight south of it.
• 32:02
  SPEAKER_00: So our glaciers came out of the north.
• 32:04
  SPEAKER_00: When they encountered us, when they encountered where Michigan currently sits, they encountered soft rock.
• 32:13
  SPEAKER_00: And they began to then excavate that deeper and exploit that more than the harder rock.
• 32:19
  SPEAKER_00: And when you melt the ice and fill it with water, you have the leftovers are the Great Lakes.
• 32:26
  SPEAKER_00: That's what you have.
• 32:27
  SPEAKER_00: So if you were in Michigan near the end of the last ice age and you wanted to, well, let's it were the glaciers, you know, what did it look like outside?
• 32:37
  SPEAKER_00: Well, you were guaranteed to see glaciers if you went to one of the Great Lakes.
• 32:42
  SPEAKER_00: That's where they spent most of their time.
• 32:45
  SPEAKER_00: Now, granted, then things would get colder and the glaciers would spill out and cover the land area also, but they spent the majority of their time in what are now known as the Great Lakes.
• 32:55
  SPEAKER_01: Yeah, that's right.
• 32:56
  SPEAKER_01: And these glaciers also formed all sorts of features across North America in the US and Canada, but they formed beaches and dunes, at least on the west side of Michigan, which is where we're from, Chris.
• 33:06
  SPEAKER_01: They formed these beautiful sandy beaches and sand dunes.
• 33:11
  SPEAKER_01: And so the sand dunes, at least in Michigan, are on the west side, and on the east side, there's not much sand dunes.
• 33:16
  SPEAKER_01: And that's because the prevailing wind direction is from the west and the southwest.
• 33:21
  SPEAKER_01: So it blows the sand off the land on the east and it blows it onto the land on the west side.
• 33:25
  SPEAKER_01: So that's why we have these beautiful sandy beaches and sand dunes that make Michigan really spectacular in the summertime on the west side of the state.
• 33:34
  SPEAKER_00: I would stack our coastlines, our beaches up with anywhere that I've ever been.
• 33:39
  SPEAKER_00: I mean, they are absolutely spectacular.
• 33:42
  SPEAKER_00: In the summer, in the summer, they're quite nice.
• 33:44
  SPEAKER_00: In the wintertime, less so.
• 33:46
  SPEAKER_00: No, I like it during the winter too, but that's just me.
• 33:49
  SPEAKER_00: But if you go to the Wisconsin side, the Chicago side, right?
• 33:54
  SPEAKER_00: Then they're on the we have to get our directions clear with this because they're on the western side of Lake Michigan.
• 34:04
  SPEAKER_00: You know what I mean?
• 34:04
  SPEAKER_00: And we're so we keep switching back and forth.
• 34:08
  SPEAKER_00: Does that make sense?
• 34:09
  SPEAKER_01: Yeah, we're on the we're talking about the land on the west side of Michigan.
• 34:12
  SPEAKER_01: Yeah.
• 34:13
  SPEAKER_01: And on the east side of the lake.
• 34:15
  SPEAKER_01: Yeah.
• 34:16
  SPEAKER_00: So I go to Chicago a lot, which is on the other side of the lake.
• 34:21
  SPEAKER_00: So the wind comes across Chicago, and it any sand that's deposited on the beaches on that side of the lake, then the sand gets blown into the lake.
• 34:31
  SPEAKER_00: And so they don't have the beaches that we have.
• 34:34
  SPEAKER_00: They don't have any dunes to speak of.
• 34:36
  SPEAKER_00: In fact, in Chicago, they actually engage in beach nourishment every year where they come in with dump truck loads of sand and just dump it, dump it in big piles and rework it with bulldozers because you know, give it a year and it's the sand is going to be blown back into the lake.
• 34:54
  SPEAKER_00: So they have to do beach nourishment on that side of the lake, whereas we don't.
• 35:00
  SPEAKER_00: Ours are natural and they're yeah, they're awesome.
• 35:03
  SPEAKER_01: So the other major features that we see about glaciers.
• 35:06
  SPEAKER_01: So we have the sand dunes that are formed there.
• 35:07
  SPEAKER_01: We have the scouring of the Great Lakes.
• 35:09
  SPEAKER_01: We also have inland lakes that are called kettles, and most uh most of them are kettles in Michigan, at least, the inland lakes.
• 35:16
  SPEAKER_01: We have these rolling hills, moraines that are where sediments being dumped off, drumlins, which are the glaciers kind of scouring the land into these sort of canoe-shaped features, eskers, erratics, gravel pits, all of these are glacial features that were either formed when the glacier was there, when the glacier was scouring over the land, or formed as the glacier was retreating.
• 35:40
  SPEAKER_01: And all of the sort of what are called postglacial or periglacial regions where you have the glacier front is right there, and there's a lot of water coming off the glacier and dumping sediment right at the edge of the glacial front.
• 35:52
  SPEAKER_01: So all of these features are basically all, for the most part, glacial features, the the surface stuff, the lakes, the rivers, the streams, the hills, almost all glacial.
• 36:03
  SPEAKER_00: Yeah.
• 36:04
  SPEAKER_00: Michigan would look completely different if it weren't for glaciers.
• 36:08
  SPEAKER_00: I mean, we would be much flatter.
• 36:10
  SPEAKER_00: We wouldn't have all kinds of oh, you know, we have over 10,000 lakes in the state of Michigan.
• 36:15
  SPEAKER_00: And most of those are kettles, like you said, and that's where chunks of ice get broken and buried in the own glacial sediment.
• 36:22
  SPEAKER_00: And then if you take this chunk of ice and then it slowly melts, it leaves this depression behind that then fills with water.
• 36:29
  SPEAKER_00: And so we have these awesome lakes.
• 36:33
  SPEAKER_00: They're not huge, but they're the size and the shape of a chunk of ice that was broken, buried, and subsequently then melted out.
• 36:40
  SPEAKER_00: I don't know.
• 36:41
  SPEAKER_00: We have a lot of really cool stuff.
• 36:43
  SPEAKER_00: We don't have mountains in Michigan, but damn it, we have water.
• 36:46
  SPEAKER_00: Lots of water, lots of water.
• 36:48
  SPEAKER_01: So the the last thing we should touch on here, Chris, is what's happening now because the glaciers retreated, and this is isostatic rebound.
• 36:55
  SPEAKER_01: The amount of glaciers, the amount of glacial weight of the ice that was in the region, and especially up to the north part where you said the glaciers were the thickest, that actually pushes the crust down.
• 37:06
  SPEAKER_01: It actually weighs down the crust, the lithosphere.
• 37:09
  SPEAKER_01: So it pushes the lithosphere down into the mantle a little bit.
• 37:12
  SPEAKER_01: When that weight is removed, we get what's called isostatic rebound or isostatic adjustment, where the land starts to kind of float back up a little bit.
• 37:20
  SPEAKER_01: It kind of starts to flex back up.
• 37:23
  SPEAKER_01: And what this does is it changes the topography of the entire region and it changes how water flows around.
• 37:30
  SPEAKER_01: So the Great Lakes have migrated to the south a bit because of this flexure upward in the north.
• 37:36
  SPEAKER_01: The Hudson Bay, the center near the Hudson's Bay, is moving up the fastest because that's where the most ice weight was.
• 37:42
  SPEAKER_01: So the Hudson Bay is retreating because of this, it's getting shallower, but also down in the south, the Great Lakes are being shifted around as well.
• 37:50
  SPEAKER_01: And it's changed some of the drainage patterns over time as this isostatic rebound, as the land starts to rise up and rebound from the weight of the ice.
• 37:59
  SPEAKER_00: That's right.
• 38:00
  SPEAKER_00: If you look at the state of Michigan or the Michigan basin, because you know, Michigan is the only state that is entirely within the basin, but Wisconsin part of you know, the eastern part of Wisconsin's in the Michigan basin, Illinois' in it, Minnesota's in it.
• 38:14
  SPEAKER_00: There are, I think, eight states total that are a part of the Michigan basin.
• 38:19
  SPEAKER_00: If you look at that, we are just weirdly isolated from the Mississippi River drainage basin.
• 38:28
  SPEAKER_00: It's it's very strange.
• 38:29
  SPEAKER_00: I mean, the central part of the United States all drains into the Mississippi.
• 38:35
  SPEAKER_00: But Michigan doesn't we actually drain out the northeast, and that's because of isostasy.
• 38:40
  SPEAKER_00: I think the best way to see this and to like envision it is to think of Lake Michigan.
• 38:45
  SPEAKER_00: Because Lake Michigan is this really long, skinny, north-south-shaped lobe, right?
• 38:52
  SPEAKER_00: It's like tongue-shaped, okay?
• 38:54
  SPEAKER_00: Well, like you said, the ice was thickest and heaviest in the northern part of the state of Michigan.
• 39:02
  SPEAKER_00: So if you think of Lake Michigan, the ice was thickest and heaviest in the northern part of it than it was in the southern part of it.
• 39:09
  SPEAKER_00: And so that what that did it caused the whole basin to gently rock back to the north due to just uh isostic adjustment, this gravitational balance.
• 39:21
  SPEAKER_00: Well, that rocking back to the north isolated us from the Mississippi River basin then.
• 39:27
  SPEAKER_00: And a new drainage began out the St.
• 39:32
  SPEAKER_00: Lawrence River to the North uh Atlantic.
• 39:36
  SPEAKER_00: And so we're just weirdly isolated because before the ice age, just like everybody else, we drained into the Mississippi.
• 39:44
  SPEAKER_01: And if you look at a map no longer the case.
• 39:46
  SPEAKER_01: And if you look at a map of the Mississippi River drainage system and you look at the boundary of that, it's like so close to Chicago, it's so close to, you know, it's like 10 miles from the Lake Michigan border.
• 39:56
  SPEAKER_01: And so you think, oh, that lake must drain out in the Mississippi, but it doesn't.
• 40:00
  SPEAKER_01: And so it actually takes this long circuitous route all the way through the St.
• 40:03
  SPEAKER_01: Lawrence Seaway.
• 40:04
  SPEAKER_01: Um, so that's uh okay, well, that's a long, detailed summary of Michigan geology, actually, uh, in response to the question.
• 40:12
  SPEAKER_01: But I think we covered most of the important details there.
• 40:15
  SPEAKER_01: And we're gonna cover more listener questions.
• 40:18
  SPEAKER_01: We got a little carried away with the Michigan and with the hot spots.
• 40:21
  SPEAKER_01: So we'll we have a few more coming up in a later episode in a few weeks.
• 40:24
  SPEAKER_01: So stay tuned to that and keep the questions coming.
• 40:27
  SPEAKER_01: We love them, they're very fun.
• 40:28
  SPEAKER_01: They send us down some rabbit trails to figure out exactly what's going on.
• 40:33
  SPEAKER_01: Sometimes, you know, we get into the weeds with stuff we super like and know a lot about.
• 40:37
  SPEAKER_01: So thanks for those two questions and keep them coming.
• 40:39
  SPEAKER_01: Appreciate it, Matt and Kathy.
• 40:41
  SPEAKER_01: Those were great ones.
• 40:42
  SPEAKER_01: Cheers.
• 40:43
  SPEAKER_01: Cheers.