PlanetGeo: The Geology Podcast
PlanetGeo: The Geology Podcast
Evolution of Eruption - Intro to the Long Valley Caldera
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In this introductory episode, we plunge into the depths of the Long Valley Caldera's explosive history! 🌋 Join us as we unravel the volcanic secrets behind one of the biggest eruptions of the last 2 million years. From the initial mantle melting to the mind-blowing eruption 767,000 years ago, discover how this geological marvel evolved. Dive into the intricate interplay of tectonic forces, mantle upwellings, and crustal melting that shaped this volcanic wonder. Plus, catch a glimpse of the differences between Long Valley and Yellowstone, and learn why the Long Valley Caldera is often referred to as the 'geological fruit fly' of eruptive studies. Whether you're a geology novice or a rock-solid expert, this episode is sure to be a groundbreaking exploration!
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Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you. Hi, Chris. Hi, Jesse.
SPEAKER_00I never know how to come in after that. That's such a weird intro. Yeah, you say it in a weird way, though. It's like it's awkward. Do I say it in a weird way?
SPEAKER_02Um here's the uh here's the thing, Chris. This is our intro to Long Valley. And and let me just preface this by saying we have several episodes coming out on the Long Valley Caldera. This episode, when this episode comes out, there'll be uh a new book, a new audiobook, new visual podcast series on our Camp Geo Mobile app. First link in your show notes. There'll be a Long Valley one there. You can go there and get access to this plus all of the other uh episodes here in this seven, currently seven part series on the Long Valley Caldera Geology. But Chris, this has long been a listener request, many different listeners request to do an episode on the Long Valley Caldera, and we decided to tackle it like six months ago, if not more, I would say probably.
SPEAKER_00And yeah, this was uh like I don't know if I would have done it if you know if if I knew all that was involved. I don't know that I look here's there's a backstory for me that I've got to do that. Yeah, yeah.
SPEAKER_02What was the Chris Bullhoy's evolution here to the to this uh what ended up being this series?
SPEAKER_00Yeah, the evolution for me is that for my Earth science and geoscience classes, I do whatever I can sometimes to pull like um standardized test materials and incorporate them into my own tests, right? Because they're they're really not content driven, it's more about interpreting the text and and reading data and graphs and stuff. And so there was one that pertained, and it was it was long and it was complicated, but it pertained to the Long Valley Caldera. This goes back 20 plus years, yes.
SPEAKER_02The Long Valley Caldera had a had a question on just what was it, a math standardized test or just the like the high school It was the Michigan science like standardized test for um I think juniors.
SPEAKER_00Yeah. Juniors.
SPEAKER_02Okay, right, interesting.
SPEAKER_00And and it was it was uh complicated and it was difficult. They needed a little bit of background information on tectonics and volcanism and and so on, but it was more just about understanding the complexity of the Long Valley Caldera, right? So that's how it all started for me. And then we like you said, we got these, you know, listener questions about Long Valley, and and so it just put a bug in my ear about this. I'm like, all right, let's do it. I had to convince you, I had to pitch you on it, and I've been there twice, and you know, it was a little bit out of our way to go here, but I wanted to go here because I was intrigued by this test thing that I put on on my stuff. I wanted to see it. That's how I've kind of like come to this.
SPEAKER_02Yeah, well, what was the that's the backstory? What was the evolution, you know, as we started going into this journey of learning the Long Valley Caldera? Because for me, you're right. I I was kind of like, oh man, that I don't know, like it's an interesting story, but is it really a uh generally applicable one? And then once we started getting to it, for me, it was like, oh my goodness, this is super interesting, this is super detailed. This is there's a really, really rich story here, a really rich story, too rich for us to to cover in any one episode. And then when it once it once it goes past that like one episode category of thing, we kind of struggle with how to deal with it, how to think about it. Like, how deep do you go, how deep do you not go, what do you filter out, what don't you filter out? Um Yeah, that's right.
SPEAKER_00We're curating all kinds of stuff.
SPEAKER_02Yeah, and then you have a really old, you know, a lot of uh information that's like on old USGS websites that's not really up to date with the current literature discussion. So you're kind of going through all of this stuff, and um it's a really it was a really interesting and thought-provoking exercise that I loved and I think is giving me a really um frankly, a new insight into the the science of the Long Valley Galdera and the Bishop Tuff and all this stuff. I've got a new kind of perspective on it uh from what I had before after reading so much about it and putting this together.
SPEAKER_00100% agree with that. This was definitely an evolution. We Jesse, do you remember back how long we were on three episodes? You know, we thought this is gonna be a three, it turned into seven. Yeah, yeah, you know, and we just kept going. We just kept digging, and it got comp it got more complicated. The more I understood and the more I learned about Long Valley, the more questions I had about it as we went along. And it was that part was frustrating and exciting all at the same time. It's very weird kind of thing with this, but I think back to like I wonder maybe this is just because this is new to me, right? You know, this geology of the tectonic setting of this is new, and maybe Yellowstone was that way for me. I don't remember that, but it had to be because Yellowstone's complicated in its own right, and um I don't know.
SPEAKER_02So maybe, maybe, Chris, let's let's just give it like a high a couple high-level sort of statistics here about why Long Valley, why it's so interesting, what this uh series kind of covers and goes through and how we're thinking about it, how we're gonna cover it. Basically, this was a very large, what we call a large silicic eruption, very, very, very large eruption.
SPEAKER_00Hold on, I'm gonna put a number to that, Jesse. I'm gonna put a number to that. Because if you take the last two million years, there have been four cataclysmic eruptions on our planet, and this ranks as number three out of the four eruptions. So you had Yellowstone one, that was the biggest. You had Yellowstone three, that was number two on the list. Long Valley Caldera is number three on the list, and then you have Yellowstone Two that comes in as number four. So right there, that puts Long Valley, it's important. You just right there, that's it. That's all you need to know.
SPEAKER_02Absolutely. You can see why this is a hot topic. And Chris, this happened 767,000 years ago. This eruption we're talking about, it took place over a six-day long interval with, as you would imagine, huge volumes of ash fall, 70 kilometers down Owens Valley, all the way out into the Pacific, all the way to Nebraska. This was this was a big event, big geological event. But there's also we kind of bookmark this discussion, end this discussion with uh with speaking to Andy Calvert, a research scientist who's worked in this area for the U.S. Geological Survey and the California Volcano Observatory. And just this is an exceptional place for geology, and there's a bunch of different reasons for that. And we get into all of them in this topic. But Chris, I mean, what are some of the highlights? I I know I have my own highlights of why this is an exceptional story to tell, apart from the fact that it's like one big erupt, one you know, massive eruption. Like what stood out to you as you know, interesting. I have an answer.
SPEAKER_00I know exactly what I want to say is that uh as we got into this, we I think quickly re maybe not quickly, but we realized, we began to realize that the Long Valley Caldera has so many basic geologic concepts and principles embedded in it, in its story. That's why we wanted to tell the story. Is that it's uh it gives context to so many important uh topics in a intro or beyond intro, you know, a beyond an intro course in in geology, getting into let's say you're gonna major in geoscience, you're gonna get into all of this stuff that's wrapped up into the Long Valley Caldera.
SPEAKER_02There's so much. It's we get in the weeds. We get in the weeds here for sure. And you know, just a couple of these, right? A couple of the the classic uh stories that kind of all coalesce in the Long Valley Caldera story. How does the mantle melt? How do we do this type of melting? Well, this is a kind of a unique tectonic setting. There's some really interesting tectonics going on here. It's at the edge of the basin and range province, and now there is large silic silica-rich, large silicic volcanoes in the basin and range, kind of all over the basin and range, but they're focused on the edges a little bit. And it's in between this basin and range extensional environment and the big Sierra Nevada block in California, this big block of 100, 120 to 80 million year an arc complex, like it, you know, granites, all the transcrustal arc system over there, and that's behaving tectonically like one big block. And the Long Valley Caldera sits kind of in a weird bend, and that has something to do with why why here? Why do the volcanoes, why did this volcano form here, and why did we get a little extension and mantle melting there? Chris, a little anecdote on that I thought was really interesting was that the San Andreas Fault, which we all I think a lot of people know is this kind of right lateral stretch slip fault, very famous fault out in in Southern California, kind of going up the coast of California. That's not the only place that this sense of motion or this this um this stress is accommodated. On the other side, on the east side of the Sierra Nevadas, inboard, like towards the continent, there's an additional shear zone that is taking up like 25% of the movement along in this region of the tectonic stresses in this region. Only 75% are accommodated on the San Andreas. The other 25 are inboard. So the Sierra Nevada blocks kind of caught up, and you can imagine why the geology is really complicated and spectacular. So that was one thing that stood out to me as like a really interesting storyline in why Long Valley Caldera is super interesting, apart from just the eruption, but it's kind of the lead up to the eruption or the initial stages here.
SPEAKER_00That's a really interesting point that you brought up, though. That bend that you have in the San Andreas fault. The San Andreas is a transform fault. And any geoscience beginning student knows what this is. It's where you have two plates that are sliding past each other in opposite directions or at different speeds, but the fault isn't straight and there's a bend in it. And the bend in the San Andreas there is a transpressional kind of fault, which is not something that you get into in a into like an intro-level geoscience class, but it's basically it's a transform fault overall, but it creates a convergent boundary there. The motion of the plates relative to each other and the direction of the bend creates this kind of convergence right there. Well, uh in Long Valley, you have a bend, but it bends the other way, which instead of compression creates trans tension. So it's a transform tensional fault all at the same time, and that's hugely important the geology of Long Valley Caldera. And uh I just I thought that was I don't it's really cool, you know. You like you said, it's it's much more inboard, it's further to the east than the San Andreas is. Same kind of thing, just the bend is different, which creates then compression versus tension.
SPEAKER_02Yeah, it kind of speaks to the um we often think in geology of at least our our kind of you know sort of intra-level, uh dare I say, ignorant understanding, is we think of these things as like singular points in time or singular points in space, where a subduction zone system is a subduction zone system and it's just diving down and there's two plates interacting. But the stress fields involved are propagated throughout the entire continent here. Like, you know, there's a lot of action happening away from that one point where the subduction zone system's happening. This is a these tectonic events affect regions, not just little parts of regions. And and I think that's an important like point here. And this comes to sort of the the later the other aspect that really stood out to me, Chris, was the the story of the eruptive sequence here, the sort of lead up to the eruption, and then the eruption itself, and the the sort of progressive nature of that, I guess. The fact that there's mantle melting that is pumping heat into the crust, and that heat is doing melting of the crust, which changes the magma composition, then all that stuff erupts in a big eruption, and actually you can see this progression in the eruptive cycle, and you can Andy Um Calvert alluded to this unzippering of the caldera. You can kind of they can kind of track in really creative way where the eruption was happening along the caldera. That's a cool story. The fact that this thing was unzipping over a six-day eruption, they can track based on looking at the rocks included in the eruptive tough layers where the the focus, where the central eruption point was coming from in this caldera complex. Just an amazing story. That evolution is just such a beautiful. It's not, I wouldn't say it is textbook, I would say like second year or third year textbook. It's not intro level stuff, but it's yeah, it's next level.
SPEAKER_00It's a textbook, but it's it's next level. Yeah, for sure. Exactly. Um Jesse, how many episodes have we had in Planet Geo that deal with Bowen's reaction series, fractional crystallization, the evolution of magma, all of that is a huge part of the Long Valley story. To me, I think it's a great way to learn because it's it's it's not a concept that's out there on its own. This is these are these are geologic principles that are tied to the third largest eruption in modern history.
SPEAKER_02I think Chris, you're you're exactly right. And and just to touch on that a little bit more, you know, we often refer to okay, when when you were when you and I are teaching, I and I'm curious how you teach this. Uh I teach about differentiation, meaning crustal differentiation, usually magmatic differentiation. But I I kind of break that apart into a bunch of different processes. Fractional crystallization is one way to go from mafic to felsic. Differentiation means going from mafic to felsic. I say fractional crystallization, that's one way. Okay, good. Assimilation, that's another way. Okay, good. Um, you know, uh Magma mixing. Magma mixing, thank you. Another way. So we kind of treat these as discrete things. When actually, when we refer to these in the real world, we'd say AFC processes, meaning assimilation, fractional crystallization, or people have come up with the term mash, melting, assimilation, storage, homogenization. To kind of link all these together and say, actually, this all happens all at once. And in this Long Valley Culter is it is a really, again, a kind of type locality for that process, the linking of these, because mag you know, mafic magmas intrude into the crust, they intrude at 1200 degrees centigrade. Well, guess what? The continental crust is pretty felsic and therefore it melts at 1200 degrees centigrade. So, you know, that mafic stuff brings a lot of heating power in, which drives both cooling of the magma and fractional crystallization, and then melting of the stuff around it, which changes the magma composition. It changes the composition of this magma chamber. So all these processes are really complicated and intertwined, and this is a really beautiful way to see that because we have a nice record through time of the eruptive sequences, which we get into in in later later episodes here into more detail in maybe perhaps gruesome detail, but interesting detail nonetheless.
SPEAKER_00Absolutely. I I think that when I was younger, uh let's say a a young geoscience student, or even like progressed, you know, pretty far through the through the program, I didn't have an understanding of like how a volcano or a volcanic system can evolve over time. I knew about fractionation, I knew about magma mixing and assimilation, but you're right, I didn't I didn't understand how they all kind of like played together in the same system at the same time. And and so going to a place like St. Helens and seeing lava, old lava flows that are very different chemically, it didn't make sense to me. I'm like, wait a minute, this came from the same volcano, right?
SPEAKER_02That uh that was a and actually, Chris, that's a great point because I mean we've been you know, we went to Mount St. Helens, I don't know, uh many years ago now, but even just look even without knowing anything about the chemistry of the rocks, you just look at the rocks. You can say, Oh, that's kind of reddish, that one's pretty dark gray, that one's really actually quite black. I wasn't expecting that here because I have, you know, because I was thinking andesite, and then that one's you know, salt and pepper andesite, and this one's really white. What the heck? And they're all in one little gully, and you're like, okay, these all came from a very restricted area right here, but it's totally different. What gives? I agree. To the to the even if you have uh uh, like you said, a pretty good geoscience education, geology training, that can be a little shocking or a little confusing. Um and and understanding how complicated these systems are is really important. Um and to see it live in real in real life in a place like Long Valley makes a lot more sense of it.
SPEAKER_00That's right. And that they are all snowflakes, you know. They they they don't have to fit in a box, they have their own individuality, and I don't know, it it takes a while, I think, to come to grips with that. And I think it did for you and I, as deep as our backgrounds are, this was still not an easy story for us to parse out. No, that's right.
SPEAKER_02And I think in part, Chris, for me, I'll speak for me, and I'd be curious to get your actions to this. It was not easy to filter because there's been so much work on the Long Valley Caldera, and in part because we have the geological fruit fly of rock layers here, the eruptive unit.
SPEAKER_00Hey, yeah, that yeah. Explain can you explain a second, Jesse, what to some people? I mean, I think a lot of people are familiar with, but why is a fruit fly important to the scientific community? Love it. I'm clapping right now. That's awesome.
SPEAKER_02Yeah, and then reaffirmed by I don't think I'm trying to remember that I'll I'm gonna have I'm gonna take heat for this, but I don't remember if we actually did anything with the fruit fly in AP biology. Um, do you know if they do if the AP used to do experiments with fruit flies or anything like that? I I don't I don't remember off. I have no idea. I remember in college, uh in college biology, we we did some stuff with the fruit fly. But basically the fruit fly is really important because it's been studied a lot, so people know a lot about the fruit fly. So if you want to you know test something, you go to this template. It's kind of like a type, it's like a type locality. In geology, it's a type locality. In biology, you know, you get this fruit fly thing, right? You get this this type organism that people have studied a lot, and therefore there's this feedback loop. It becomes more valuable because there's so much known about already. And the Bishop Tuff, which is the rock that was formed during the Long Valley Caldera eruption, the big eruption, is one of those famous rocks that is studied, has been teased apart by so many people over the years that it has become really valuable and a really good proving ground for new geochemical technique development. And so that stood out to me as a really it's hard to filter down which stories to tell with the bishop tuff because there's so many.
SPEAKER_00Yeah, because Jesse, how many, how many papers? You don't have to put a number to it, but you can just describe it. How many papers have been written about the bishop the Bishop Tuff?
SPEAKER_02I mean, way more than the Acostanized complex, which is arguably already arguably one of the most famous rocks in the world. Speaking from myself, yeah, it's uh one of the oldest rocks in the world has had maybe a fourth or a fifth of the papers written about it as the Long Valley Caldera, or as the the Bishop Tuff from the Long Valley Caldera. So that just gives you an idea. It's become this really important rock unit because it's been studied by a ton of other people. There's loads of data about all the minerals in this rock, loads of geochemical data. So if you want to develop some new thing, you go, you can you can go and look at the Bishop Tuff. It's a good spot. And then Chris, there's an also this really interesting this story of of what happens at the end of a big magma chamber. And I wouldn't say that this is like a a prelude to what might happen to Yellowstone necessarily, because there's debate about whether this magma chamber's still alive or not, if we could put it that way. But the end stages of a big eruptive center, this is an important data point to to look at and to study, and the geophysicists have gone after a lot of this stuff. But there's some really interesting. Yeah, where does the magma chamber go to die? Yeah, yeah, right. It's really interesting because and Chris, I I'm curious on your thoughts on this because you've you've driven along the Snake River Plain, you've you've seen at least uh at some level the the extinct Yellowstone hotspot locations, the I don't know, what are there, six, eight different eruptive some number of eruptive centers. I don't think we have uh an under st we know there's none of them are active, not even like remotely close to act as active as the Long Valley Caldera. Do you know this offhand or no?
SPEAKER_00Uh I mean you know, I guess like uh that's hard. Craters of the moon is pretty recent activity anyway. Yeah. You know, in the last 1500 years or so. You know, but but Long Valley Caldera has recent activity just a few hundred years ago. You know, so in Mono Lake and in some of those islands in Mono Lake.
SPEAKER_02Uh yeah, and some really interesting geology outside of the outside of the the caldera, yeah, north of the caldera, potentially, other uh as Andy Calvert alluded to at the end here, um a potentially uh interesting spot to pay attention to. Absolutely. To kind of track the value of volcanology research here, like understanding risks and hazards in a place like that.
SPEAKER_00Yeah. I like our approach to this, and uh just a reminder to everyone is that we were pretty deliberate about tying Long Valley back to basic geologic concepts and using it to really use this as a case study to teach uh practical geology. Exactly.
SPEAKER_02Practical geology. And Chris, maybe just by way of uh of uh tying up this episode, why don't we just go go through the story briefly in bullet point form here, real quick, of the story. And it starts out with mantle melting in this really interesting setting, this really, I would say non-intro level setting, right? This is not a mid-ocean ridge, this is not necessarily plate margin, but it is uh adjacent to this basin range extension, and we get basically an extensional environment, a transtensional environment along a bend in the fault zone, and we get mantle melting. And that mantle melting produces basalt, and that kind of starts this process, starts the storyline here. Um it's decompression melting.
SPEAKER_00Decompression melting, very simple, very simplified nutshell, yes. Yep.
SPEAKER_02That's exactly right. So when the mantle wells up, it decompresses and it produces basaltic melt. That basaltic melt then makes its way into the crust, and we have clear evidence for this because eruptions started in this area, what, four million years ago, three and a half million years ago, basalt started coming out of the crust and erupting onto the surface. And so we could tell this is mantle melts, it's coming out to the surface. Good. We know that there's heat being distributed through the crust, and that evolved then, all that heat got dumped into the crust, which started to melt the crust, and that ended up forming a relatively large magma chamber down at depth. And eventually we got to a point where that magma chamber was ready to go, and 767,000 years ago, over a six-day period, we had the Bishop Tuff forming eruption. And uh we go into some detail about this that there's the unzippering of this caldera complex, where over six days the eruption moved over in different locations and created this caldera complex. And then, Chris, after we have the geological fruit father, the bishop tuff formed.
SPEAKER_00I mean, what happens in the waning stages of well hold on though, before we get to that, we actually do talk about the hot springs and geysers and and things like this, the hydrothermal activity at the surface, because this is it's very silica-rich material. And then you have obviously you start to think, well, this must be similar to Yellowstone then, because that that's what makes Yellowstone Yellowstone.
SPEAKER_01Right, right.
SPEAKER_00And so we talk about that. You know, we get into the comparisons of all right, what features do they share? What do they not share, and why? And I think that's an important part of the.
SPEAKER_02So what Chris to you stands out in that the comparison or the judge deposition, maybe with Yellowstone. Geysers, is that we said?
SPEAKER_00Geysers, lack of geysers, yep. Lack of geysers in terms of hydrothermal activity at the surface. And that was something that, you know, as you're digging into this, you get you're thinking, I'm thinking, you're thinking, what do they share and what do they not share? Yellowstone and Long Valley. And to me, the thing is that they don't share geysers. There, you know, Long Valley does not have any true geysers, and you know, the story behind that I think is pretty cool. It's uh you know, the rock is a little bit different, it's it's more highly fractured, and so you just don't quite have the uh ingredients to create this pressure tight system at the surface for geysers to be uh at work as you do in Yellowstone. So it's just these subtleties, right? And that I it's I find that exciting. I think it's amazing and and yeah.
SPEAKER_02It's a dramatic difference between in the hydrothermal system between Yellowstone and the Long Valley Caldera. And I think that you're you're hitting on a really the exactly interesting point here. And Chris, the other aspect of this was there was some um activity to kind of come back to the you know the the death of a magma chamber, how does a magma chamber die point? There was this was a hot bed of activity seismically and geophysically at a time when Mount St. Helens was very fresh in geologists' minds, right? So it attracted a lot of attention and a lot of rethinking about, for instance, what does an earthquake mean in an area that we know was a volcanically active province? And there's a a decent amount of debate about what's happening now. Is there still a big magma chamber down there? Is there potential for another big eruption? Or is it actually just the waning stages of magma chamber's life? Is it this thing we call second boiling where basically all the water boils off a cooling down and fully crystallizing magma chamber? There is a decent amount of debate between a couple different scientific communities about that. But regardless, it's a really interesting story and a really interesting type locality to again study these processes. So I don't know, Chris. I I uh it's a really cool story. I want to go there um as a fully or at least relative more mature, maybe not fully mature, but a more mature geologist. And we should go stomp around uh along that way. Absolutely we should go uh out there, Andy Calvert. Uh uh, he did invite us out. Yes, I know, he invited us out to his great personal risk. But that would be great. We gotta do that. That'd be so fun. Go check out some rocks over there. And I'd like to see this hydrothermal plant that's out there too. Check out some geothermal uh geothermal energy systems, that'd be cool too. Little uh planet geo field trip. I think that'd be good. We should uh we gotta arrange that, man. We do, we absolutely do. Yeah, one of these days. Well, you gotta retire from teaching so that we can uh we got more time for fish.
SPEAKER_00Yeah, because you'll have all kinds of time on your hands, right?
SPEAKER_02Yeah.
SPEAKER_01We'll be able to throw pack the kids in the tent. I'll retire. I'll be annoying you're like, come on, Jesse, let's go. Yeah, come on, what's going on, man? What are we doing? It's September. Come on, let's go. That sounds good. I like that.
SPEAKER_02All right. Well, Chris, what do you think? Is that a good intro to Long Valley? Have we have we done the the intro, the high-level view justice here? I it's just such a it's such a great story. There's so many weeds and rabbit holes to go down to. And we we do go down some and we will in coming episodes here. What do you think?
SPEAKER_00I hope you enjoy it.
SPEAKER_02It's uh it's fun to it's a fun series. Um, so again, this is uh the first part of our series on Long Valley Caldera. You can find the full visual podcast series in the Camp Geo Mobile app. First link in your show notes, give that a click. If you like Planet Geo, please leave us a rating, review, and a subscribe. Follow if you're on YouTube or Spotify, leave us a comment, send us an email, planetgeocast at gmail.com, and uh, we'd love hearing from you. Cheers.