Earth’s Leaky Core and Michigan Geology - Listener Questions Part 1

Jesse Reimink: Welcome to planet geo the podcast where we talk about our amazing planet, how it works and why it matters to you.

Oh, Christopher Hi.

Chris Bolhuis: know what was that? Hi, how you doing Jesse?

Jesse Reimink: Hi. Good, good. How, how are you?

Chris Bolhuis: Doing all right.

Jesse Reimink: What's uh,

Chris Bolhuis: Oh, let's see. Not a lot, 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 going, how about you?

Jesse Reimink: There you go. Yeah. I'm uh, well, I'm kind of doing the same a little bit. I'm leaving for field camp teaching field camp in a few weeks so, well, no next week. So getting ready for that, just writing papers, keeping the research enterprise going. It's fun. Nice summer.

Chris Bolhuis: There you go. There we go. Good.

Jesse Reimink: 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, I don't know how many but a

Chris Bolhuis: Yeah. We have a lot

Jesse Reimink: and we've filtered through a few. Well, I wouldn't say we filtered, we took a few that we thought were quite interesting and we're gonna address them today.

Chris Bolhuis: Let's do it.

Jesse Reimink: And so what are we starting with?

Chris Bolhuis: We are gonna start with a question from a listener. Her name is Kathy, and she says, I know you guys have talked about hotspots before, but I don't recall if you mentioned this and there was a link to a paper. It says high resolution imaging reveals puzzling features deep in the Earth's interior. We're talking about the mantle core boundary here. That's how deep, so, you know, we're, we're at like 2,900 kilometers down below the surface of the earth. That's, we're gonna start with that question from Kathy.

Jesse Reimink: Yeah. I think we should start out by describing the Earth's layering first. Right. I mean, just in general, so that everybody's on the same page. So you have this great analogy. Is it a bowling ball that you use to describe the crust? Is that what, or is that a

Chris Bolhuis: Uh, yeah, usually, yeah, usually a bowling ball. 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. So it's, you know, I'm not saying it's not an important layer cause it's extremely important, but it, it, you know, puts it in perspective in terms of how thin this layer actually

Jesse Reimink: Yeah, the crust is not very much. And then as you go below the crust, we end up in the mantle and the mantle is the. Plastic kind of material it's still solid, but it is hot plasticy. There's several interesting layers in the mantle that we don't need to talk about right now. But as you get down towards the base of the mantle towards the mantle core boundary, there are two parts of the core. There's a liquid outer core and an inner solid core. And so this core mantle boundary, you said 2,900 kilometers deep, that's the layer that we're focusing on, or that the researchers that this article highlights we're focused.

Chris Bolhuis: That's right. 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. And it basically just gets more. mafic the deeper you go until you get to the core where it's almost all iron. There's iron and then some other components like nickel and cobalt and things like that. , but there is also another way that we layer the earth and that's the lithosphere and asthenosphere, and that's based on rock reality or the behavior of the rock where the lithosphere spans a big compositional difference, cuz it takes the crust in the upper part of the mantle, but it behaves. Peanut Burle it's really it's cold and crunchy. Whereas then the asthenosphere is that really kind of soft plasticy, hot rock. That's like very close to its melting point. And so it behaves very differently. So cool.

Jesse Reimink: That is exactly right. Chris, the way that we know the layers of the earth is largely due to seismic waves 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. So seismic waves, an earthquake happened somewhere, or back in the sixties, you know, a nuclear weapon was detonated somewhere. The energy from that event, either an earthquake or a nuclear weapons blast would travel through the earth and it travels through the earth and it kind of curves as its on its way through. And so you kind of picture these, they. If you're looking at a map of the earth, a cross section earth, they, they look like they curve through the earth and those waves when they hit different boundaries and these could be compositional or temperature, boundaries, any sort of structure in the earth, they will either slow down, speed up. That's one thing they can do. The second one is they can change. Their form. So they can go from a P wave to an S wave or a compressional wave to a sheer 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 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, of boundaries. So it gets really complicated this seismology,

Chris Bolhuis: 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. If you just alluded to those in there, how they can maybe sometimes change as they go from one medium to another, the P waves they're called primary waves. Primary waves are compression waves. In other words, what they do is they compress and expand the material that they travel through. This is a sound wave. Okay. Now it doesn't have to be in the human audible range in order for it to be a sound wave. That's how a sound wave propagates. That's how it moves. And so they can travel through sound can travel through gases. Obviously you can hear me, you know, That's a good thing. 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. Okay, then you have the swaves. These are called the secondary waves. they're the second fastest of the seismic waves that travel roughly at like the half the speed of a primary wave. , but they don't compress and expand what they travel through. They're actually they're called a transverse wave or a shearing wave. In other words, the wave is traveling in one direction. but the particles that are being disturbed are traveling perpendicular to that direction. We call it shearing or tearing. And because of this, they can only propagate through solids. They can't move through gases and they can't move through liquids because liquids and gases don't have sheer strength.

Jesse Reimink: yeah. So what this study focuses on is looking at how these waves travel through the earth, how they. Deed, how they get reflected, how they get converted from one to the other that gets really complicated really quickly, and it gets more complicated, the deeper in the earth you go. 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. getting down to 2,900 kilometers. Deep is really difficult. , but the feature they're looking at, there's two features in the mantle that are useful. One is called a L L SVP, large, low sheer wave velocity province, very sort of uninteresting acronym there. The other one is a U L VZ or ultra low velocity zone. and these are two things that think of them like big mountains in the mantle where the L L SVPs are these really high sort of big. Beasty. 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. The ultra low velocity zones are kind of thin small things, right at the base of the mantle, right at the core mantle boundary. And these are really intriguing cuz nobody knows really what these things are. but they're ultra low velocity zones. So they slow down. The sheer waves. They really, really slow down the seismic waves traveling through them. And so therefore they're called ultra low velocity zones and there's a few things that could slow down these waves. One of them is. Temperature. Some of them are compositional. 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. So this would be like a really iron rich part of the mantle at , the boundary. But these things are tiny, Chris. I mean, this it's really amazing that we can even image these things. They're like 20 kilometers high and about a thousand kilometers wide, 900 kilometers wide.

Chris Bolhuis: and by the way, let's put a number on this. They're talking about slowing these seismic waves down by as much as 40%. I mean, that's for such a small area, that's a abrupt, that's like speeding up to a red light and just slamming on the brakes. It's really a dramatic drop in.

Jesse Reimink: I mean, it's just, just amazing. This is not a large. It's slowing down a lot and it's so deep, it's incredibly difficult to image. And so what this paper looked at, , what they did is they used high performance computing. And basically model the wave forms going through here to try and isolate exactly what was going on. And so it gets really complicated really quickly there, but basically they could resolve these things better. We've known these ULV Zs have existed for a while. 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 'em. So we're seeing. 10 kilometer, five kilometer layering, thousands of kilometers down deep in the mantle. It's really amazing feet of, of

Chris Bolhuis: hold on before we get into, like, what might be the cause of it and where are they? Right. I mean, they're, their locations are kind of cool. 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. Does the idea of a numerical model? Does that diminish what their findings are to you at all? Do you know what I'm saying? Like, I, 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, you know, I don't know. Is that diminished

Jesse Reimink: I think, um, no, not really for this. This is the way seismology is done because it gets so complicated so quickly. 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. 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. That's, 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. I mean, we do this in I took geochemistry a bit. Hey, the earth is super complicated and it's really big. And it's, there's a lot that could happen to seismic waves. They're going through the mental. 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.

Chris Bolhuis: Okay. All right. And then 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? Because the liquid outer core you have this it's. Molten, iron that's circulating that generates our magnetic field, which obviously is very important for, protecting us from cosmic and solar radiation, so on, but it's because of studying of the 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. Whereas P waves, can they change speed abruptly, but they can still propagate their way

Jesse Reimink: Yeah, that's right. So I think we just maybe wrap this up by saying that still unclear what these LV Zs or ultra velocity zones are, or even what the larger L L SVP things are as well. It's not really clear what they are. They could be. High temperature regions, really high temperature regions. 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. Some people have suggested their remnants of ancient crust. You know, as a subducting ocean crust goes down, it could potentially pile up at the base of the mantle. 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. So it's a little bit unclear. Well, we don't know. We, we just don't know what these

Chris Bolhuis: but, there is some relationship or maybe correlation's a better word with these. Ultra low velocity zones and hotspots, is that correct? I mean, , we've found these below famous hotspots. Mm-hmm

Jesse Reimink: Yes, the LV Zs are the ultra low ones, which they're studying here are at the base of the larger ones, the L L SVPs, these bigger things. And there's a correlation with the bigger things with mantle plumes. So they're kind of all three linked there in some way. , these small scale mountains at the base of the mantle, they're really big, massive pieces of the mantle, the, the L L SVPs. so people who have studied these mantle plumes the basalts coming outta the mantle plumes have suggested that there's little bits of core in the mantle plume basalt. So that would imply. This core mantle interaction, iron is leaking into the mantle in the ULVZs. 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

Chris Bolhuis: Yeah. I feel like this is just at the beginning stages of, like really learning something, something new

Jesse Reimink: Yeah, totally. And this is definitely different than , the old school textbook style of explaining the mantle, you know, the, three layered mantle and all that, that that's kind of gone away and this has replaced, replaced it for understanding the mantle. So very cool, Kathy, great article. Great suggestion. Very cool subject. the second question, Chris, and we're gonna have two parts to these cuz we got so many good questions so we're breaking these apart. We've got to mention this at the beginning. We're breaking these apart into two episodes. So stay tuned in a couple weeks. We'll have part two coming out of more listener questions, but the second one we're gonna cover today, Chris is from Matt and Matt says, I would wonder if you would consider spending some time talking about the evolution of Michigan. 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, you lead field trips in Michigan, in your class, up to the Northern peninsula. I mean, you must, you must cover a lot of Michigan geology in your class. I frankly don't remember how much you cover from, you know, 2005,

Chris Bolhuis: Hey, thanks for that. I appreciate that. Like that. really pisses me off. You don't remember

Jesse Reimink: We did, you know, I. We learned a lot about Michigan geology and, uh, there there's a lot to, to tell and you know, both you and I can be a little bit long winded at times. So

Chris Bolhuis: that's right. That's right. I while this is so big. Okay. We could do a lot just on this topic. We could do several episodes just on this. So we simplified things. We're gonna slow down on some other things that I think like people should know about Michigan geology, you know? 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.

Jesse Reimink: So where are we gonna, where do we start

Chris Bolhuis: Yeah. Um, well, you have to start with the Michigan basin. Okay. So we live in what is called the Michigan basin. And I think of when I think of the Michigan basin, I think of just an, like, what's an analogy. Do you do our listeners know what a syncline is

Jesse Reimink: Ooh, we might need to define that. So a sink line is a type of fold. 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 and a syncline

Chris Bolhuis: it's just the bending. Not like folding clothes. It's like bending.

Jesse Reimink: that's right. It's a bend in a rock. And so a syncline is. When you bend the sides up, you make a U shape out of the rock. So you take a flat line group of rocks and you bend the edges up to make a U-shaped. The opposite is, anti incline where you bend it down and it makes an, a kind of shape. So a syncline is just the, the U-shaped version.

Chris Bolhuis: So the reason why I ask that is because a basin is like a circular sin line. So instead of just having, you know, two sides bent up, it's, it's bent up all the way around it. And I think of what this looks like. 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 measuring cups is. May, 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. So that's the youngest one. Right. And then they get progressively older as you go down in the nesting cups. And that's how sedimentary rocks work. Right. Oldest is on bottom and youngest is on top.

Jesse Reimink: So the opposite. Basin is a dome and a dome is a little bit easier. 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, 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 sinks down around it as well. And so sorry, Chris, you were describing what this looks like when you erode a basin.

Chris Bolhuis: Yeah. So when you take these sedimentary rocks that are inside of the basin and you rewrote it, you, you get a bullseye kind of pattern then where the same rocks show up in these rings and the ring that is, the youngest ring, the youngest sedimentary rock that's exposed is then gonna be this smaller circle. Like the small measuring cup, the smallest measuring cup that's nested in there. , and then you get progressively older as you go out toward the margins. And even beyond the state of Michigan, we call it the Michigan basin, but it goes beyond actually Michigan. That's what we can see with the Michigan basin 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. It looks like you're ready to start throwing dart. Okay. But Jesse what's below that sedimentary.

Jesse Reimink: Well below sedimentary rocks. Usually we call these basement rocks, which are high grade metamorphose rocks, some igneous rocks, they're the deeper roots to the, cross. So they're, they're things that exist in the cross. They were exposed at sea level at some point in time, and then the sediments were laid down on top of them. So we call that basement because it's. Like the basement to your house. It's below the stuff on top and the sediments of the stuff on top. And so we have a lot of these pre-Cambrian 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. That's the upper peninsula of Michigan. So if you look at a map of Michigan, it has two parts to lower peninsula is the main part, the glove. And then the upper peninsula is the, the place where

Chris Bolhuis: the, the lower hand and the upper hand that's right. That's right. Oh, so

Jesse Reimink: is, 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 magnetic veins cutting through them. Dare I say, they're more interesting than sedentary rocks, at least to me. Uh, but there.

Chris Bolhuis: interesting.

Jesse Reimink: Yeah. And they're exposed in the Western part of the upper peninsula and then into Minnesota and Wisconsin. And this is where there's historically been a lot of mining, iron, silver, copper, um, there's huge amounts of, or deposits up there that were mined in the, you know, early industrial days and continue to be still a little bit mine.

Chris Bolhuis: That's right. That's right. So and a lot of the, we call these or deposits where you have a rock that has enough of a metal in it to make it profitable to mine. It. That's called an or, and the, the up is just, it's known for copper. You know, there's 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 our informations form, but they're super important. And this is related to this mid continental rift that it took place during pre cam brain time. Well, the bottom line is, is that you have this continental rifting, that's taking place, the, the continent's being kind of pulled and torn apart. 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 rock, it's gonna dissolve these metals and then. It's gonna concentrate them in certain areas when the chemical conditions or the physical conditions changes. And that's what was going on in the up, we are being torn apart in this mid continental rift and the conditions were just right to concentrate really valuable metals.

Jesse Reimink: Right. Exactly. 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. This failed rift. You know, the continent tried to split apart. It didn't quite work out that gives us the mid continent rift because it's still in the middle of a continent and it failed. It was about 1.1 billion years old. Then later on, we get this Michigan basin formation and. So on top of the basement rocks, we have the start of all of these sediments and these are 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. So there's a huge length of time where sediment was being nearly continuously deposited in Michigan in the michigan.

Chris Bolhuis: Yeah. And it's interesting too, that we were in Michigan. We were at the bottom of a sea and that's something that I think is important , to note because we're so far away from that right now. But the rock record. Shows us that this is where we were the kinds of rocks we had. we have sandstones, we have shales, we have limestones, we have extensive coral reefs. We have evaporates tons of evaporates, like gypsum and rock salt. Okay. I mean, you and I are both from the grand rapids area, which is Southwest, Michigan. And crisscrossing underneath the city of grand rapids are gypsum mines. in gypsum forms in a restricted salt water basin, that gets super warm and lots of evaporations taking place. And it begins to lay down just these massive accumulations of the rock gypsum. So all of this points to this kind of like warm salty,

Jesse Reimink: and that's a great point, Chris. This is a, a shallow, warm salty inland sea. So it's still a sea that's in, the ocean has intruded into the continental center, the interior part of the continent and is depositing all this stuff. And these are really bioactive areas as well. There are loads of fossils in Michigan. Loads of, you know, sea life fossils, you get all sorts of stuff. Some of the most famous are the Petoskey stones, at least in Michigan, that's the state rock or the state fossil

Chris Bolhuis: what is the Petoskey stone? What, what kind of rock is it?

Jesse Reimink: It's a fossilized coral. So it's a, an ancient coral is actually a colonial coral.

Chris Bolhuis: looks like this honeycomb.

Jesse Reimink: yeah, it's beautiful black and sort of a dark brown and light gray honeycomb structure. So really beautiful. And all of these rocks were deposited in this basin environment. 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 basement. We're 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 could correlate all across them. They just get thinner on the edges. Cuz the sea wasn't quite as thick there

Chris Bolhuis: Hey, I want to go back to one thing because I absolutely love this about the state of Michigan. Do you love pasties?

Jesse Reimink: you know, I haven't had a past east since like, oh four when I went on year. Yeah. On the field trip where you stop at the

Chris Bolhuis: Because I, we stop at that place. It's my favorite Passy place. All right. 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, um, because you know, you have these, minors that move from Europe, actually. the tin mines of Cornwall, England, these people brought with them, their tradition of this meat, potato pie that still stands today. And so all over the place in the up, you'll see signs of people advertising, pastes. 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. if you order a pasty instead of a pasty, don't make that mistake.

Jesse Reimink: no, that's right. But yes, they, I mean, I remember enjoying them. I don't, uh, I haven't had one in, you know, decades, but, uh, it would be good to go back and check it out. I'm actually, I got a grant funded to work on some rocks in the upper peninsula of Michigan as well. So there's some really old part of the basement as some really old rocks, 2.5 billion year old rocks on the Western side. So we're gonna go up there. Maybe I'll have a pass next summer or in two summers

Chris Bolhuis: you have to Lidos. That's the place to go, man. I'm telling you right now. 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. So this isn't super steep. Right. We're talking about these rocks that are gonna drop down toward the center of the basin, just mm, 60 feet, every mile that you go. Okay. I'm gonna throw this to you a second. What do you, what are we thinking about the formation of the basin itself? What caused this subsidence?

Jesse Reimink: During the time that this basin was around, we had several of these types of basins. There's an Illinois basin, Iowa basin, the Appalachian basin in Western Pennsylvania, which I kind of live in. , these are all depositing sediments. 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. And when that failed breaking apart of the continent happened, it kind of destroyed the roots to the continent, a. It kind of broke apart. The, the deep lithosphere that used to exist underneath of that area. 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, cuz 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. And then you. When it subsides down, you have water flows in there. Then you have this inland sea, 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.

Chris Bolhuis: Okay. So there's the Michigan basin and the sedimentary rocks that cover the pre-Cambrian basement rocks and so on. And then, you know what we gotta, we can't just make it as simple as that. Because then it all has to get covered with the Vene of glacially deposited material.

Jesse Reimink: Right. And so we're done 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 land forms that you see in Michigan all the place. So.

Chris Bolhuis: what makes Michigan one of. It look, I love this state. We live in an amazing state and most of that is because of the glacial veneer that was left over, uh, at the end of the last ice age.

Jesse Reimink: Yeah, all the lakes up north, if you live in Michigan, you know, you're familiar with going up north for the, in the summer to lakes and stuff, all that's due to glacial stuff, all that is due to glacial formations. We don't have that in Pennsylvania. We don't have the glaciers that came through. We just have the rocks. We don't have nearly as many lakes in Pennsylvania.

Chris Bolhuis: It's important to note that there are very few places in state of Michigan where we can see the sedimentary rocks that we just got done talking about. 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 bore holes, through Wells exploration, Wells and things like this. All of. Has been compiled to give us a picture of what's below because we have, you know, anywhere between three and 500 feet of glacially deposited material that covers up all those rocks. We just got done

Jesse Reimink: That's right.

Chris Bolhuis: Okay.

So Jesse, let's move into, then what did the glaciers do to sh like they really reshaped the Michigan landscape? How what'd they

Jesse Reimink: and we gotta move forward in time. Now we're thinking relatively recently, geologically speaking, right? We gotta jump forward to the last several hundreds of thousands of years in the last glaciers that came through. 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, uh, uh, in this part of north America. But the last one was about 15,000, started to recede at least about 15,000, 12,000 years ago. Glaciers sculpted the landscape. First of all, glaciers are an amazing erosive force, so they can just come through. They just scrape off everything for the most part. So they scrape everything mostly flat, except in Michigan, they didn't, they took advantage of weak rock layers, and this formed the great lakes. So the great lakes kind of outlined Michigan really nicely, right? Like Michigan, like here on, they kind of outlined this half of an oval or maybe even more than half of an oval around the Lower peninsula of Michigan. And that's because that's where the weak rocks were exposed. And the glaciers took advantage of the wheat rocks and carved out these deeper valleys, which are now the great lakes they're now filled in with water.

Chris Bolhuis: That's right, Jesse. So what you're saying, I think is that look, water takes the path of least resistance, right? Rivers follow the path of least resistance. So do glaciers. So. the glaciers that reshaped and sculpted the state of Michigan, they came outta Canada. The zone of accumulation for this was in the, the Hudson bay area where the ice was over three miles thick. and that ice then began to ooze out in every direction. We're just straight south of it. So our glaciers came out of the north when they encountered us, when they encountered where Michigan currently sits, they encountered soft rock and they began to then excavate that deeper and exploit that more than the harder rock. And When you melt the ice and fill it with water, you have the leftovers are the great lakes. That's what you have. So if you were in Michigan, near the end of the last ice age and you wanted to, well, let's, we're the glaciers, you know, what did it look like outside? Well, you were guaranteed to see glaciers, if you went to one of the great lake. That's where they spent most of their time. Now, granted then things would get colder 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.

Jesse Reimink: Yeah, that's right. And these glaciers also formed all sorts of features across north America in the us and Canada, but they formed beaches in dunes, at least on the west side of Michigan, which is where we're from Chris. They formed these beautiful. Sandy beaches and sand dunes and so the sand dunes, at least in Michigan or on the west side and on the east side, there's not much sand dunes. And that's because the prevailing wind direction is from the west, the Southwest. So it blows the sand off the land on the east, and it blows it onto the land on the west side. So that's why we have these beautiful Sandy beaches and sand dunes that make Michigan really spectacular in the summertime. Uh, on the west side of the.

Chris Bolhuis: I would stack our coastlines, our beaches up with anywhere that I've ever been. I mean, they are absolutely spectacular. And then if, if you go

Jesse Reimink: in the wintertime, less

Chris Bolhuis: I don't know. I like it during the winter too, but that's just me. But if you go to the Wisconsin side, the Chicago side, right then they're on the we have to get our directions clear with this because. They're on the Western side of lake, Michigan. You know what I mean? And we're so we keep switching back and forth. Does that make sense?

Jesse Reimink: we're on the, we're talking about the, land on the west side of Michigan

Chris Bolhuis: Okay.

Jesse Reimink: but on the east side of the lake. Yep.

Chris Bolhuis: So, , I go to Chicago a lot, which is on the other side of the lake. So the wind comes across Chicago and any sand that's deposited on the beaches on that side of the lake, then the sand gets blown into the lake. And so they don't have the beaches that we have. They don't have any dunes to speak of. 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 gonna be blown back into the lake. So they have to do beach nourishment on that side of the lake. Whereas we don't, ours are natural and they're. Yeah. They're awesome.

Jesse Reimink: So the other major features that we see about glaciers. So we have the sand dunes that are formed there. We have the scouring of the great lakes. We also have inland lakes that are called kettles in most. Uh, most of them are kettles in Michigan. At least the inland lakes. We have these rolling Hills Marines that are. Where sediments being dumped off drumlin, which are the glaciers kind of scouring the land into these sort of canoe shaped, , features Esthers erratics, gravel pits. All of these are glacier 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 and all of the, , sort of what are called post glacial or per glacial 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. 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.

Chris Bolhuis: Yeah, Michigan would look completely different if it weren't for glaciers. I mean, we would be much flatter. We wouldn't have all kind. Oh, you know, we, we have over 10,000 lakes in the state of Michigan and most of those are kettles, like you said, and that's where chunks of ice get broken and buried in, in the owned glacial sediment. And then. If you take this chunk of ice and then it slowly melts, it leaves this depression behind that then fills with water. And so we have these awesome lakes 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. I don't know. We have a lot of really cool, so we didn't have mountains in Michigan, but dammit, we have water.

Jesse Reimink: Lots of water, lots of water. 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 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, the glaciers were the thickest that actually pushes the Cru down. It actually weighs down the cross, the lithosphere. So it pushes the lithosphere down into the mantle a little bit. 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. It kind of starts to flex. Back up. And what this does is it changes the topography of the entire region and it changes how water flows around. So the, great lakes have migrated to the south a bit because of this Fletcher upward in the north, the Hudson bay, the center near the Hudsons bay is moving up the fastest because that's where the most ice weight was. 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. And it's changed some of the drainage patterns over time as this isostatic rebound, as the land starts to rise up in rebound from the way to the weight of the

Chris Bolhuis: That's right. 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, at Minnesota's in, uh, there are, I think eight states total that are a part of the Michigan basin. If you look at that, We are just weirdly isolated from the Mississippi river drainage. You know, it's, it's very strange. I mean, the central part of the United States all drains into the Mississippi, but Michigan doesn't, we actually drain out the Northeast and that's because of IOY I think the best way to see this into it, like envision it is to think of lake Michigan, cuz lake Michigan, is this really long, skinny north south shaped lobe, right? It's like tongue shaped. Okay. Like you said the ice was thickest and heaviest in the Northern part of the state of Michigan. So if you think of lake Michigan, the ice was thickest and heaviest in the Northern part of it, then it was in the Southern part of it. And so that, what that did is it caused the whole basin to gently rock back to the north due to just ISIC adjustment this gravitational balance. Well, that rocking back to the north. Isolated us from the Mississippi river basin then. And, a new drainage began out the St Lawrence river to the north, uh, Atlantic. And so we're just weirdly isolated because before the ice age, just like everybody else, we drained into the Mississippi

Jesse Reimink: if you look at a map,

Chris Bolhuis: the case.

Jesse Reimink: 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. And so you think, oh, that lake must drain out in the Mississippi, but it doesn't. And so it actually takes this long circuitous route all the way through the St. Lawrence Seaway. Um, so that's, uh, okay, well, that's a long detailed summary of Michigan geology actually in response to the question. I think we covered most of the important details there, and we're gonna cover more listener questions. We got a little carried away with the Michigan and with the hotspot. So what we have a few more coming up at a later episode in a few weeks. So stay tuned to that and keep the questions coming. We love them. They're very fun. They send us down some rabbit trails to, to figure out exactly what's going on. Sometimes, you know, we get into the weeds with stuff we super like and know a lot about. So thanks for those questions and keep 'em coming. Appreciate it, Matt and Kathy, those were great.

Chris Bolhuis: cheer.