Episode 1 - Assembling Wisconsin

Read on for a transcript of episode 1 of Under Our Feet. I've tried to add images, definitions, or links where it might be helpful.

Many thanks to Carol Grelck for her assistance in editing this transcript! She also added some annotations and definitions, which you'll see in brackets throughout.

Rudy: Before we get started, I want to take a moment to say welcome. This is the first ever episode of Under Our Feet. I've been thinking about this podcast for a few years now and I'm thrilled that this is our first episode. We're going to hear from Marcia Bjornerud, the Walter Schober Professor of Environmental Studies and professor of Geosciences at Lawrence University in Appleton, Wisconsin, and author of two books, Reading the Rocks: The Autobiography of Earth, and Timefulness: How Thinking Like a Geologist Can Help Save The World. I've admired her writing and teaching for a while, and I thoroughly enjoyed talking with her and learning about the earliest history of Wisconsin's geology, and I hope you enjoy learning about it, too.

If you're really liking what you hear, you can support the show on Patreon. There's a link at UOFpod.org. If you're able to give some support, which I deeply appreciate, you can get some cool benefits starting at just $1 a month, like shout outs on the show, stickers, T shirts, and even personalized mini pods that I'll publish on the show's feed with a topic of your choice.

Now, let's get on to the show.

In the mid 1880s, right around 1885, a mining boom changed a remote landscape forever. Fortunes made up of millions of dollars were made and then lost almost overnight. Immigrants from Finland, Sweden, Italy, French Canada, and Austria moved in, hoping to strike it rich in the New World. If you wrack your memory for mining booms of this magnitude, you might be thinking that I'm talking about gold in California or maybe copper in Butte, Montana, but nothing quite so shiny. This was iron and they were digging up the Northwoods: the remote stretches of Minnesota, Wisconsin, and the Upper Peninsula of Michigan that border Lake Superior. In Wisconsin, this was centered in the Gogebic Range, which is called the Penokee Range by some and it's in the far north of the state. It's between the towns of Mellen and Ironwood/Hurley. So to visualize this, if you imagine Wisconsin as being shaped like a mitten, with the thumb being the Door Peninsula, sticking out into Lake Michigan, the Penokee Range is sort of at the tip of the middle finger right up there in the very far north of the state. In the Gogebic, about 325 million tons of iron were extracted in the 90-year period from 1877 to 1967, which is when the last iron mine in the area closed. At the peak of extraction, there were 40 individual underground mines running. And underground mines are really dangerous and expensive. So it shows just how desirable Wisconsin's iron ore was. What drove this boom? Why is some of the richest iron ore in the world found in Wisconsin in the Lake Superior region? And what are the lasting consequences today of this mining? Let's find out.

Map of the land-surface topography of Wisconsin. Highlighted in red are the twin ridges of the Penokee Mountains. Image courtesy of the Wisconsin Geological and Natural History Survey

Welcome to Under Our Feet, the podcast where we go deep into the earth and deep into time to seek out the geologic events and forces that shape the world around us. I'm Rudy Molinek. And this is season one, the Geology of Wisconsin.

Before we get back to our story about iron in the Gogebic/Penokee Range, I want to take a minute to talk about why we're doing a podcast about the geology of Wisconsin. It might seem an unlikely candidate given the spectacular rock vistas you might see in places like Colorado, California, or Utah. But the stories held in the rocks of this Midwestern Great Lake state are just as dramatic, and maybe even more surprising, than the stories out west. More importantly, these geologic stories explain a lot about how people live today in Wisconsin. It turns out that the rocks you find in Wisconsin span about 2.8 billion years of Earth's history. Let's put that in perspective. So if all of Earth history, which is about four and a half billion years was laid out on a football field, and Aaron Rodgers was starting a Green Bay Packer drive at his own goal line, it would take about a 62 yard pass to get from now back to the oldest rocks in Wisconsin's geologic record. That would get The Pack just about in field goal range. For comparison, the Grand Canyon covers about 1.8 billion years. If the Packers were playing the Arizona Cardinals in this game, and the Cardinals started the next drive on their own goal line, then a play spanning the portion of Earth's history in the Grand Canyon would only move them about 40 yards, which isn't even to midfield. By might count that's 3-0 Packers. But it's not just about the amount of time. It's about the variety and the drama of stories we can read in the rocks of Wisconsin. This season we'll hear about towering mountains, rift valleys and volcanoes, vast oceans and tropical reefs, and massive ice sheets grinding into the bedrock. And all that and just this one unassuming Midwestern state.

Alright, let's get back to the Northwoods and our tale of an iron mining boom. To learn more about this earliest chapter of Wisconsin's geologic history, I talked to a Wisconsin geologist who knows a lot more about it than I do.

Marcia Bjornerud: I'm Marcia Bjornerud. I'm a professor of Geosciences and Environmental Studies at Lawrence University in Appleton, Wisconsin. I grew up in Wisconsin and Minnesota, but then lived elsewhere in the world, including Norway and the UK, but came back. And part of the appeal is that Wisconsin is a very geo-diverse state. People might not really realize this, but we have a great range of rock types that represent every modern tectonic setting that's on the globe today. We have very different environments represented, and the age of rocks in Wisconsin spans more time than is represented by the rocks in the Grand Canyon.

Rudy: What this means is that in the ancient rocks of Wisconsin, we see evidence of subduction zones, where one tectonic plate dives underneath another one. We see mountain building events, where continents collide into each other like in the Himalaya. We see volcanic arcs like the Aleutian Islands of Alaska, and continent splitting riffs like what's currently going on in East Africa. Of course, none of that's actually happening today, but we see evidence of where it happened in the past. Today, we're going to focus on one of those mountain building events where tectonic plates collide to thrust new peaks high above the landscape.

Marcia Bjornerud: Yeah, so to situate us in space and time, I think first, it is good to know it's a very ancient mountain belt. And so, it formed about 1.8 billion years ago. And in geology, we make a distinction between mountains and what is called an orogen, which means a place where mountains topographically were, but they may not be topographic mountains anymore, but they bear all the signs of having been mountains.

Rudy: And that's an important distinction here, geologically speaking. An orogen, which is spelled o-r-o-g-e-n, rather than origin, like the beginning of something, is an event or period of time when mountains formed. In Wisconsin, we don't have any true mountains left. But we do have evidence of several orogens. At one point, there would have been mountains in these places, but time, water, and ice have eroded them down to their roots, leaving behind just a geologic fingerprint that tells us an orogen once occurred.

Marcia Bjornerud: And so that's really what the Penokees are. They're not a towering mountain range, but they clearly, at one time were. And, all the rocks then record the kinds of tectonic stories that we see in modern mountains.

Rudy: And at this point, I have to cut in and admit that I was a little bit confused on this during our conversation. Colloquially, the Penokees, also known as the Gogebic Range, refers specifically to the 80-mile-long set of ridges that span from Mellon to Ironwood/Hurley on the northern Wisconsin border with the Upper Peninsula of Michigan. It's an area that Backpacker magazine called "the Alps of Wisconsin." But when a geologist like Marcia talks about an orogen like the Penokees, she means the entire extent of these ancient mountains, which it turns out extended much further than the modern Penokee/Gogebic Hills.

Marcia Bjornerud: So this ancient mountain belt extended from East-Central Minnesota, across the northern two thirds of Wisconsin, most of the Upper Peninsula of Michigan, at least certainly the western part. And then over into Ontario, as far as Sudbury area, maybe a little east of Sudbury, we still see the signs that there's deformation of this age. So probably an extent that we can really document something like 500 miles long, maybe 80 to 90 miles wide, and stretching northeast/southwest over that area.

Rudy: So now that we have an idea of the spatial situation of the Penokean orogeny, let's take a minute to get ourselves situated in time. This is a specialty of Marcia's. If you're curious in learning more about how she thinks about time, which I highly recommend doing. Check out her book, Timefulness.

Marcia Bjornerud: Like most mountain belts, it took time to form them, we can talk about the different steps. So we see the first hint around 1.89 billion years ago and the last stages around 1.83. So that's about comparable with modern time of mountain building, about 50 or 60 million years.

Rudy: 60 million years. To put that in perspective. It's been just over 60 million years since an asteroid hit the Earth and caused the extinction of the dinosaurs. So that's a really long time. And in that 60 million year interval from 1.89 to 1.83 billion years ago, collisions kept occurring between tectonic plates driving up the Penokee Mountains and the collisions that formed the Penokees are part of an interval of geologic time that was really important to shaping the world as it is today, with just a few massively large continents. Back then, almost 2 billion years ago....

Marcia Bjornerud: So you would have been looking at a world that did have continental crust, so emergent areas surrounded by ocean, but kind of small, maybe Greenland size areas, no large continents, we think. The time that the Penokees formed was a time of amalgamation of some of these really ancient shield areas. [A shield is a large area of exposed Precambrian-age crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas]. So there's the Penokee Range; there's another ancient mountain belt or orogen called the Trans-Hudson, that are all forming close to this time, and those amalgamated together probably by modern style tectonic processes into what we now call in North America, the Canadian Shield. But those smaller pieces hadn't been brought together yet at this time so the globe would have had these sort of blobs of continental crust, surrounded by ocean.

Rudy: But that scattershot arrangement of small islands scattered around the globe wasn't all that was different about the planet when the Penokees were forming.

Marcia Bjornerud: Of course, there was no life on land, the whole biosphere was microbial at this time. There probably was some kind of microbial life on land, but not abundant. So the continents would not have been green, certainly. But life was abundant in the ocean. Microbial life was diverse and abundant in the oceans even though it was all single-celled. On the shores of these oceans, there would have been huge colonies of microbes called stromatolites that are well preserved in the fossil record. They were probably not just one species, but whole ecosystems with photosynthetic cyanobacteria at the top. And then a whole host of fungi and bacteria, maybe not fungi, but sort of different types of bacteria and archaea, [organisms whose cells lack a defined nucleus] living off the waste products of each other in complex biotic communities that built up sort of cabbage-like mounds. There are few places on Earth still today that have these kinds of colonies, the most famous is in northwestern Australia. But these are very well and abundantly preserved in the fossil record at this time. And in fact, in many rocks in Wisconsin, we have beautiful, fossilized stromatolites. So the world was teeming with life. It just wasn't macroscopic life.

Rudy: So that's the lithosphere which is rocks, and the biosphere, which is life. But when we're thinking about how the world 2 billion years ago was different than today, there's one more sphere that we need to consider.

Marcia Bjornerud: The atmosphere at this point would not have been very hospitable to us oxygen-needing organisms. There was some oxygen, but probably a fraction of a percent compared with 21% today.

Rudy: That means that the air and the oceans would have had way less oxygen than modern life would need to survive. And that'll be important for you to remember a little bit later on when we circle back to how the iron got into these rocks.

So Marcia has situated us in space and time, it's about 1.89 billion years ago, and the world is dotted with these little mini-continents. The land is devoid of life. But the oceans are teeming with single celled organisms that are sometimes living in these complex communities. There's not much oxygen. And this is the stage upon which the epic story of the creation of the Penokee Mountains is set. I'm at the edge of my seat ready for the curtain to lift and the orogeny to begin.

Marcia Bjornerud: I hope people appreciate, it's difficult to reconstruct something that happened in 1.8 or more billion years ago, we're partly limited by what rocks have not been eroded away. And then, what rocks are actually accessible because other sediments in many cases have buried the rocks. And so what we know has been pieced together over time by many different workers starting in the late 1800s. And in the last 20 years, really, I think there's been a huge revolution in understanding the Penokees and other ancient mountain belts because of better dating methods.

Rudy: And this is a key point in the science of Geology, generally. We have a hard time figuring out precisely how the tectonic plates and continents are moving today, and we have GPS and real time monitoring. Go back a few billion years ago, and all geologists have to go on, especially in a place like Wisconsin, are a few rock outcroppings strewn throughout the landscape. It's like trying to figure out what the image is on a 1000 piece puzzle when you only have five pieces. Key to this geologic detective work is dating: figuring out exactly how old certain rocks are to help determine how the puzzle pieces fit together. A common dating method, besides dinner and a movie, is to precisely measure the amount of certain radioactive elements and look at their ratios. The degree to which these elements have decayed radioactively tells us how old the rocks are. These elements are very much the clocks in our rocks. So geologists use these clocks along with the types of rocks and their arrangement in these scattered outcrops to piece together the stories of our planet's past, like this one that Marcia is telling us about - the Penokees.

Marcia Bjornerud: So the broad outline would be that about 1.9 billion years ago, what is now the Upper Peninsula of Michigan and the area of Wisconsin along the shores of Lake Superior would have been the edge of a great sea and would have been the edge of what we call the Superior Craton one of these ancient continental areas. [ A craton is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle.].

So it would have been a shoreline environment. And, it was attached to an oceanic plate that was probably subducting somewhere offshore. So, a good analog to this would be modern day Indonesia, where people I'm sure remember the terrible 2004 tsunami that happened on a subducting margin, between the ocean - the Indian Ocean and the archipelago, that is Indonesia.

So, very similar to that there would have been a chain of volcanoes, probably multiple groups of volcanoes somewhere offshore, modern coordinate south of the Lake Superior region, (of) course everything was oriented differently. And so we would have had: the shoreline, intermediate intervening ocean, a subduction zone and then related to that subduction, just like in Indonesia today, a great chain of very explosive volcanoes. And some of those rocks from that volcanic chain still exist today. Some are volcanic actual ash deposits and things like that. Others are probably the magma chambers that fed those volcanoes. Those are exposed, for example, in the Stevens Point area. [ Rib Mountain quartzite] So, we have a pretty good handle on what kind of magmas were coming out of those volcanoes and we can date them.

So, the subduction continued as the sea floor was being consumed as that oceanic plate sank back into the mantle. There were probably great earthquakes happening all the time, just like there are in Indonesia and other settings like that today, tsunamis. And then at about 1.89 billion years ago, that ocean plate was completely consumed. And the volcanic chain, we call it an island ark collided with the southern margin of the Superior Craton, and that was the first sort of tectonic event, the first collisional event that started building these Penokee Mountains. And so that complex of island arcs is what's called the Wisconsin Magmatic Terranes [ A terrane is a crust fragment formed on a tectonic plate and accreted or "sutured" to crust lying on another plate] and it makes up probably a third at least of the basement rock in Wisconsin. Quite, when you get about, you know, 50 miles south of Lake Superior from maybe Park Rapids south, that's all these volcanic rocks, sea floor too kind of mashed in their oceanic sea floor. And people will perhaps remember: the Crandon mine proposed in the late 90s, the Flambeau mine near Ladies Smith that did exist and is now closed, and also the very controversial Back 40 Project, a gold prospect right on the Menominee River on the border with the U.P. that's potentially going to happen. All of those metal ore deposits are related to the Wisconsin Magmatic Terranes. So, they have a lingering effect on us today. Then, a new subduction zone is thought to have begun that pulled another even older continent into contact with the Wisconsin Magmatic Terranes on the south and this is called the Marshfield Terrane, named after the city of Marshfield, uses some very old rocks, some of the oldest in the state. And that very ancient continental mass collided with the Wisconsin Magmatic Terranes, again, presumably through the process of subduction and that sea floor being consumed at about 1.83 billion years. And that's sutured together what is now Wisconsin with other three very different parts of the state in some sense: the Superior Craton, that messy middle area with all the volcanoes, and then this really old chunk of crust called the Marshfield Terrane. And, that final collision is probably what caused most of the typography and the deformation that formed the Gogebic Range and other parts of the Penokees.

I think, again, the best analog would be if we were trying to think of the cinematically how these mountains are forming over time. They didn't just spring up. It, you know, it happened over 10s of millions of years. It would be imagining modern day Southeast Asia, with the subduction zone off the coast of Indonesia, and then Australia slowly encroaching to the north. Eventually, this is probably going to happen. People have done projections of future plate tectonics, think that probably there's going to be a new supercontinent formed and in that part of the world, mainland Asia, and Australia collide, and crunched in between them will be the archipelagos of Indonesia. And that's very much like the geography that probably would have existed in the Penokees.

Rudy: Okay, so I started the story with an anecdote about iron mining. And it may seem like we've gotten way off course here talking about all these tectonic events. But the whole time that these tectonic events were underway, there's a parallel story happening underwater. So between all these continents, remember, there are oceans. And to understand this story, it helps to zoom out and appreciate the scale of what Marcia has been talking about.

This orogen is taking place along a shoreline that's at least 500 miles long. That's like going from Savannah, Georgia, all the way down to Miami, or for anyone on the west coast from San Francisco to San Diego. And it takes place over 60 million years, which seems short if we're talking about events almost 2000 million years ago, but it's still a vast, incomprehensible amount of time on its own. During this interval along the shoreline of collision between the continents that are amalgamating, there are ocean basins full of sedimentary rocks being deposited. These rocks on the ocean floor are then caught up in the collision and piled up onto the shoreline and added to the mountains. And it's in these shallow sea basins where the sedimentary rocks are forming where the story of iron takes place. And it's here that you need to reach back about 10 minutes into your memory, when Marcia told us about how little oxygen there was in the atmosphere and in the oceans.

Marcia Bjornerud: So one thing I find fascinating about this moment in geologic time, the early Proterozoic is that it was a time not only of transition in the tectonic regime, but also a revolution in the atmosphere. So prior to about 2.4 billion years ago, we're quite confident that the atmosphere of the Earth had very little if any free oxygen (O2) as just oxygen like we enjoy. But beginning at about that time 2.4 billion years ago, photosynthesis had been going on long enough for some free oxygen to start accumulating in the atmosphere. And that changed all the geochemical rules on earth.

Rudy: Because it turns out oxygen is super important to the formation of minerals. When there's free oxygen around it likes to bond with other elements like iron, and form oxide minerals. When there's not much oxygen, those iron atoms can just hang out floating around in the oceans. So, prior to about 2.4 billion years ago …

Marcia Bjornerud: …we don't see oxide minerals forming at the surface because there just wasn't oxygen to be bonding to elements. But after that time, especially in shallow marine settings, where there was lots of photosynthesis going on, we start to see these very characteristic rocks called banded iron formation. And what we think was happening was in a pre-oxygen world, iron, which was spewed out at sea floor vents could actually stay in solution in seawater in the absence of oxygen. But as soon as there's any oxygen, that oxygen will bond with the iron in the water and rust out essentially. And so, as soon as you started having free oxygen in the atmosphere, the ocean and the atmosphere are a well mixed system - at least in the shallow ocean. And so, we start seeing evidence of the iron that's coming mainly from these black smoker volcanoes being precipitated out in the shallow marine environments.

Rudy: So to break it down so far, for millions and millions of years, iron from underwater volcanoes and sea floor vents was being spewed into the oceans. And it mostly just floated there until in shallow seas like the ones between the continents that are colliding in the Penokee Orogeny. Enough of these little single celled organisms were taking in carbon dioxide and pumping out oxygen with photosynthesis, that the iron floating around rusts and settles to the bottom, eventually forming a rock called banded iron formation.

Marcia Bjornerud: This went on for quite a while. The oldest iron formations of this kind are, as I said 2.4 billion. We don't have those in the Lake Superior region. Ours are some of the younger ones like 1.85 million. But all of the great iron ranges of the Lake Superior region from Minnesota, the Mesabi Range, the Cuyuna Range, the Gogebic Range in Wisconsin, the Ironwood area at Ironwood Hurley area, the Negaunee in the Upper Peninsula of Michigan are all the same age, probably deposited in a shallow marine setting, where again, photosynthetic oxygen was combining with iron in the water, and precipitating out vast quantities of this iron oxide.

Rudy:So it's been well over a billion years since the last major iron formation on Earth was deposited. And this isn't the only way that iron forms into rocks, but it's the most important one. And it's really important when you look around you and notice how much of our world is built with iron as a raw material.

Marcia Bjornerud: And most of the steel that's ever been produced in the world comes from these banded iron formations, which are really an extinct rock type, because they're not forming today, because we have way too much oxygen there. As soon as iron is exhaled by seafloor volcanoes, it bonds with oxygen, and it's precipitated out immediately. So we don't have the ocean chemistry to form these things anymore. So that's a huge story. And the banded iron formations are important economically.

Rudy: And these iron formations can also give us clues about the climate of the Earth all those 1000s of millions of years ago.

Marcia Bjornerud: That’s fascinating from a biogeochemical standpoint. They have textures that look very much like modern day limestones. So, they have physical structures at the sort of hand specimen scale that tell us these are being deposited in warm, shallow marine environments with agitated water. Some have stromatolites suggesting that there were organisms that were part of the oxidation process, and they themselves were actually perhaps processing the iron and precipitating it as these minerals.

Rudy: And, importantly to Wisconsin, iron formations are really strong rocks that resist erosion.

Marcia Bjornerud: Yeah, so that's really kind of the reason there's any topographic expression these days of the Gogebic Range is that it’s a really resistant iron formation that's up there.

Rudy: So, if you visit the Gogebic range up near Ironwood, and Hurley, those hills are there because of the changes in the atmosphere 1.8 billion years ago. But we're not quite done with the geologic story of the Penokees. The capstone so to speak, still remains. And in this rock layer is a story of a single day, really just a few hours that occurred 1.85 billion years ago.

Marcia Bjornerud: Yeah, so the Sudbury Ontario meteorite impact site is well known. It's the town of Sudbury (which) is inside a giant crater that's now an oval shape. But, it's presumed to have been originally circular and the diameter would have been something like 250 kilometers so quickly a 130 miles or something - sizable craters. And the city of Sudbury is entirely in this crater, and it was recognized probably starting in the late 1950s as a meteorite impact crater. Prior to that time, people didn't like to think that Earth had really been struck by impact or by meteorites, but especially as understanding of the Moon's history grew, we realized, oh my gosh, yes, there are some impact craters. So this is a crater that's larger than the one in the Yucatan that's implicated with the dinosaur extinction event. The Sudbury crater is the second largest known on Earth. The first largest is in South Africa. And there's a nickel mine at Sudbury that is directly related to the melting of rocks at this site. So, since the 50s, Sudbury has been known as a very large impact crater that was dated at about 1.85 billion years. Economically hugely important and environmentally hugely destructive the nickel that's mined there is nickel sulfide and it's also the poster child for acid rain damage. Anyway, so Sudbury had been known for a long time, but nobody in the geological community had thought of looking for the ejecta layer - the stuff that was thrown out of the crater at the time it happened.

Rudy: Yeah, ejecta layers are crazy. So just imagine a giant rock that's hurdling into the earth when it hits an incredible amount of really hot rock and debris would explode out of the impact site. And then wherever that debris settles back down is the ejecta layer, which can spread hundreds and hundreds of miles away from where the actual impact happened.

Marcia Bjornerud: That the actual crater and all the damage it caused had been well explored by Ontario geologists. But no one had thought - is there any remnant of the stuff that was thrown out? And the reason that people hadn't really thought about this is because these are old rocks. You know, it's kind of unlikely it seems it would be one day 1.85 billion years ago you would be looking for that particular layer. But a high school teacher, a biology teacher in Thunder Bay, Ontario named Bill Addison, starting in the early 1990s. started thinking, you know, I know that the rocks in the Thunder Bay, Ontario area on the west side of Lake Superior more than 300 miles away from Sudbury are the same age. They are banded iron formation. They're part of the Gunflint Iron Formation. They're the same age broadly as the age of the impact. Maybe we could find the ejecta layer and he got people from the local university Laurentian or Lakehead University in Thunder Bay, who had the capacity to date rocks using zircon dating uranium lead zircon dating, and they started combing through the Gunflint Formation looking for volcanic zircons.

Rudy: Zircons - they're a mineral that's incredibly hardy. They can resist alteration from chemical and physical processes and remain stable, kind of like a time capsule for billions of years. Also, critically, they contain bits of those radioactive elements we use for geologic dating that I talked about earlier. So, in terms of clocks in rocks, zircons are kind of like a Rolex timepiece.

Marcia Bjornerud: So these would have been zircon is probably coming from the Wisconsin Magmatic Terranes. In big eruptions, tiny little crystals that were (like) clock's locking in the age of the eruptions. And little by little, they started narrowing down where in the iron formation they should be looking for this ejector layer. And as they started doing this, they found some astonishing outcrops right in the city of Thunder Bay, that now in retrospect, people are wondering what did we think these were before because they are crazy looking rocks. In the Gunflint Iron Formation that are stromatolites fossils. And in rocks that now we recognize as part of the ejecta layer the stromatolites, huge like meter sized chunks are just upside down ripped up either from a tsunami related to the impact or maybe just an air blast ripping up part of the coast at the time and tearing up these poor stromatolites. And then, on top of that are very characteristic pellets, about marble sized pellets that are interpreted as hailstones essentially of huge amounts of pulverized rock that the ejecta just blasted into the atmosphere. And maybe with raindrops, there would be amalgamation of these particles into these pellets. That would have been raining down for quite a long time. So, one of the really diagnostic features as people then subsequently looked around the Lake Superior region were these. They call them Accretionary Lapilli. Lapilli is an Italian word meaning pebbles, that's really diagnostic of the impact layer. And there are other telltale signs like quartz grains that have shock features, strange crystal scale features and blebs of glass tektites they’re called [small, round, dark glassy objects, composed of silicates, formed by the rapid cooling of meteorite fragments that hit the earth]. And also some very strange, very holey or porous looking black rock that looks a bit like the basalt if people know what like Hawaiian basalt looks like. But this is interbedded with these other rock types. It's interpreted as vapor rock - vapor that boiled as it cooled. So that's what you got to get your head around - that it was actually in the gaseous state, superheated and then as it fell out of the atmosphere, it boiled and became a liquid and then solidified into a glass. And pieces of this very vuggy porous material are interlayered with the pellets and the shocked quartz. And so this is clearly the ejector layer and it is one day 1.85 billion years ago. And it all started because of a high school teacher just saying, you know, we should look for this layer and now it's been found not only in the Thunder Bay area, but up on the Gunflint Trail in Minnesota, in the Marquette area in Michigan, and sadly, it does not seem to crop out as a surface in Wisconsin, but it was intercepted by drill cores that the company called G-Tac (Gogebic Taconite, LLC.) that had intended to mine the Gogebic range did in their exploratory drilling. So, it's in the subsurface in Wisconsin. And it's really become this really remarkable datum. We know that was that day, that bad day 1.85 billion years ago.

Rudy: Wow. So this is amazing. To be able to look back in time, millions and billions of years and say, “ This happened on a single day”. As geologists we don't get that very often. We will often be talking about something that happens over 1000s of years. And that is pretty quick. So a single day is just out of this world incredible. And to use a cliche, which is actually appropriate here it's literally like finding a needle in the haystack of geologic time.

Marcia Bjornerud: And not only that, people who have worked on this have kind of worked out hour by hour, what probably would have happened based on you know, knowledge of how, how quickly the seismic waves could have passed, how quickly that the air blast would have happened, how quickly a tsunami would have happened. So, you can really kind of reconstruct that day in real time.

Rudy: And so after that impact, things calmed down for the Penokees. Today, this formerly violent collisional zone is near the stable center of the North American continent, far from any dramatic tectonics. Millions upon millions of years wore down those high mountains, leaving behind just a few ridges held up by the strong iron formations. And this is what's now known as the Gogebic Range, or the Penokee Hills, or even as I said, Backpacker Magazine called it the Alps of Wisconsin.

And so this brings us back to where we started - the iron mines, the large quantities of economically viable iron that have been mined from the Banded Iron Formations in the Penokee Orogeny over the past 150 years, or what turn this from a geological point of interest into a culturally and economically vital region and into part of our human story.

In this banded iron formation, there are two distinct grades of iron ore. The first….

Marcia Bjornerud:…. it's really just the unaltered Banded Iron Formation, the iron content is in the range of 15% maybe 20.

Rudy: And the second grade of iron ore in the Banded Iron Formation is that's which really kicked off the early mining booms and powered the economic engine of the North Woods for decades. This ore is…

Marcia Bjornerud: ….typically called Direct Shipping ores, or natural ores which were mined in the early days is even in the 1800s. In the, say, Ironwood Hurley area where there were tremendously deep mines. Those were natural ores, and they had something like 50 to 55% iron. And the reason they were so rich is not related just to the deposition of the rocks, it was subsequent hydrothermal alteration which means water getting into the rocks, and leaching the silica. So, these banded iron formations are not all iron, they're alternating layers of essentially, quartz, Jasper, which is read iron stained quartz, and mainly hematite and magnetite. Those would be the iron bearing minerals, as well as an iron carbonate called siderite. So the silicate silica part, the quartz part is not valuable at all. But in some places hydrothermal alteration had leached a lot of the silica away leaving a very enriched iron remnant.

Rudy: So in some areas, we have this hydrothermally enriched ore - really high grade stuff. This is rich enough that miners can just take the rock out of the ground and ship it directly. The cost of shipping is a huge part of mining costs, since volume and weight of material is so great. So, to have an iron ore that doesn't need to be refined further, before it makes economic sense to put it on a train or a boat to send it to market is a really valuable ore.

Marcia Bjornerud: So most of those really high grade ores were mined out pretty early on, and they were high enough grade that underground mining was profitable. Underground mines are super expensive and dangerous. So even in the bad-old-days when workers were not protected, you only did that if you had a really rich product.

Rudy: And these rich ores began to be mined out in the post World War II era when military buildup and a subsequent rise in consumerism drove huge demand for steel. So, we're in the situation where demand is up, but the historically rich supply is dwindling. And this is the moment when a change in mining begins to change the landscape of the Northwoods forever.

We switch from mining that hydrothermally altered and enriched or to unaltered iron formation. And because this new ore is only 15 to 20% iron, it needs to be processed before it can be shipped profitably. And because underground mining is so expensive and dangerous, and we're already incurring the extra cost of that processing, the lower grade ore needs to be mined in these massive open pits where they strip the earth on top. And it's just it's literally a big hole in the ground. And this new style of mining is known as taconite mining.

Marcia Bjornerud: So what they do is they take all the rock both the silica or quartz rich part and the iron oxide or iron carbonate part they grind it, they magnetically separate the iron part, and then they reconstitute it in these little pellets that I'm sure people have seen with clay and bake that and that's shipped as these little marble like things. And that has a high iron content like 60% iron, but they've had to mine a ton of rock, crushed and separate it, you're ending up with like half of the rock is just waste. And that's really all that's left.

Rudy: Whenever it occurs, mining leaves a legacy that long outlives the miner. This waste is one part of that legacy, the physical part and it results in a permanently altered landscape.

Marcia Bjornerud: In the Hurley area there are huge waste piles from the underground mining there that are vegetated now. I mean, this was 70 years ago that they closed. But there's a lot of rock taken out even in the underground mining days. And in the Minnesota, I mean in Hibbing the so called Hull Rust Mine is vast - 1000s of acres. It's huge. Some of the mines up in the Mesabi Range have been allowed to fill up with water. So now there are manmade lakes. But it's a devastated landscape. If you look at it, say in Google Maps, terrain view or land satellite view, you can see from space, especially up in the Minnesota iron ranges, the scale of these open pit mines. It’s amazing.

Rudy: Another part of the legacy of mining is its lasting effects on people, communities and culture. Marcia, beyond her academic expertise in Geology, knows this well.

Marcia Bjornerud: My own Father worked in in the open pit mines. As a young man, it was very hard work. But yeah, there was a lot of wealth that came out of those mines.

Rudy: There was a lot of wealth, but an unfortunate aspect of mining is its boom and bust nature. The iron boom in the Northwoods was a long one by mining standards from the 1880s to a peak in the 1950s. Then taconite mining stayed big until the late 1970s. But booms don't last. Iron production fell off a cliff in the 1980s. And now mostly only comes from a few mines in the Mesabi range in Minnesota. So if you drive through the upper peninsula of Michigan, along Lake Superior through Ashland, Wisconsin, and into Minnesota, at Duluth, you see the evidence of this past boom. There's huge infrastructure and much of it’s now sitting there unused and rusting - just like the iron in the oceans when it was first exposed to oxygen. It's also evident in the communities. These mines didn't just make wealth for the companies, they made jobs, they made livelihoods and these supported families. As is so often the story in many mining areas, many of those jobs are now gone. And that's to the detriment of the communities. Many areas of the Northwoods though, are now once again beginning to thrive. They're relying not on extraction, but on tourism which is built upon the natural beauty of the remote landscape. I mean, part of that too, is the hills of the Penokee Range because that Banded Iron Formation, makes it pretty scenic. It's a great place to go hiking these days. But this raises a sticky problem. There's still iron up there and when the economics are right, companies are going to want to start mining it again. But, there's a growing awareness of the environmental costs of mining. One particular moment in Lake Superior history helped raise that awareness.

Marcia Bjornerud: Another chapter that I think is fading from memory, is that some of the Mesabi Range ores are right up against these younger midcontinent rift age intrusions, called the Duluth Complex.

Rudy: And just a note, you probably don't know what the Midcontinent Rift is yet and that's totally okay. Because it turns out, that's the topic of Episode Three. So stay tuned.

Marcia Bjornerud: They have minerals in them that are what we would call asbestos. And there was a huge, early 70s environmental case brought by citizens against the mining companies because they had dumped huge amounts of mine waste, at Silver Bay up on the north shore of Lake Superior, into the lake and had created essentially huge submarine fans of mine waste that included particles of asbestos formed minerals. And it was a really notorious early environmental suit that was really, I think, groundbreaking in getting people to wake up and think “Wait a second, what are what are the larger consequences of these huge scale mining operations?”

Rudy: So now, when new mines are proposed, there's not a straightforward answer. There's an undeniable benefit to mining. It can bring prosperity to a community. Though, there's always the risk of the next bust. There's also an undeniable environmental cost. Huge quantities of waste that often contain dangerous materials and water has a way of moving those dangerous materials to places where they pose a risk to plants to animals and to us. The question gets even trickier when the mining is for the types of rare earth elements that are essential to the batteries, solar panels and other technological tools of the critical transition away from fossil fuels. The Penokee Orogeny is not immune to these questions. A mine has been proposed in recent decades in the Penokee Hills, it's called the G-Tac Mine. It's in the lower grade ore that we talked about, so this would be an open pit mine. But unlike the flatline layers that they have in the Minnesotan pits, the strata here is steeply tilted. It's like if you took a flat stack of pancakes, which is what the Mesabi Range looks like in Minnesota, and stood them on end and if one of those pancakes is iron formation, at the surface, you just see a narrow linear strip. To mine more, you'd have to remove all the pancakes above it and that's a lot of wasted pancakes. And that's one reason why Marcia got involved in speaking out against mining in the Penokee Hills.

Marcia Bjornerud: Because of the nature of the structure, the iron formation dips about 60-65 degrees to the north, and it's overlain by this thick black shale. It's full of pyrite. And so, to get at the iron formation, other than just the part, the small part that's exposed naturally, they would have to create huge amounts of this pyritic - black shale as waste rock, which would have been very bad. I mean, just the volumes, first of all, but the fact that is full of finely disseminated sulfide - really bad. And the other thing that's kind of interesting is there is some interest, I always go to the Institute on Lake Superior Geology meetings most years when that happens. Apparently, the US Geological Survey has been looking at these black shales as a strategic reserve, either of phosphorus and or rare earth elements - which is sinister. It would definitely be open pit mining on a colossal scale to harvest rare earths out of these things. So there's organic rich turbidites, essentially what they are. [the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amount of clastic sediment into the deep ocean ]. And you know, it's kind of strange to think about it as their Proterozoic. But, they're black shales because of very, very high organic content. And apparently, people have done assays and there's quite a lot of phosphorus and where there's phosphorus, there's often rare earths. So, this could be the next environmental fight. And ironically, the demand for rare earths is largely coming from green energy demands.

Rudy: And so I wish I could end this podcast by solving the dilemma of mining versus not mining. But I don't have the answers. I don't know the right way to balance the climates need for green technologies, people's needs for good jobs, and our collective need for a healthy environment. Perhaps you though listener, having just learned more about the geologic and human history of the spectacular, the Penokean Orogeny, which is the first chapter Wisconsin's history, maybe you might be able to help find that balance. Just remember, like Marsha said:

Marcia Bjornerud: These, I always tell students that the Proterozoic still casts a shadow over us. I mean, it's still very much with us. We’re in its Thrall in some sense.

Rudy: Yeah, we're still living in the shadow of the Proterozoic, which is the time period when the Penokean Orogeny occurred. And this gets to the whole point of this podcast - that the past isn't over. It continues to live with us today. It's important to learn about the past and not just human history, but the deep earth history, which informs human history, so that we can plan for the future and understand the present. And we've only just begun to explore the deep past of Wisconsin. So, if you're interested in continuing along this journey, join us for the next episode, where we'll jump forward about half a billion years in time and visit the home of the Ringling Brothers Circus - that's Baraboo, Wisconsin.

Thanks for listening. I've enjoyed making this and I hope you've enjoyed listening. If you did, it really helps if you leave a rating or review that helps other people find us. You can also send it to your friends, anyone you think might be interested. They don't have to be in Wisconsin. For more information about the show, you can check out our website, uofpod.org. That's the letters U, O, F. and then pod.org. Also, there, you can find my contact info, and you can let me know if I got anything wrong. Or if there's anything about Wisconsin Geology that you're curious about and want me to cover. I'd really appreciate that feedback. It'd be fun. And before you go, I want to first of all thank Marcia Bjornerud for the excellent interview. If you like what you heard, check out her books or her great writing in The New Yorker and elsewhere. Also, many thanks to the American Geophysical Union's Voices for Science Program, which lent me financial support, podcasting expertise and immeasurable encouragement. The music you heard was the song Arizona Moon by the Blue Dot Sessions. And finally, thanks to Jeremy Randolph-Flagg, who you didn't hear but he has been instrumental in helping me conceive and develop the ideas for Under Our Feet podcast.