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Transcript

The Basalt Before the Blast - The Leadup to The Long Valley Caldera · PlanetGeo: The Geology Podcast

• 0:00
  SPEAKER_02: The takeaway from this is that Long Valley Caldera Parallels is such a good model, Jesse, to talk about a broader geologic concept, which is how does magma traverse the crust?
• 0:16
  SPEAKER_02: What happens along the way?
• 0:21
  SPEAKER_01: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:30
  SPEAKER_02: They were awesome.
• 0:31
  SPEAKER_02: They were fire.
• 0:32
  SPEAKER_01: Of course you did.
• 0:34
  SPEAKER_01: Of course you did, old man Bull Eyes.
• 0:36
  SPEAKER_01: That's right.
• 0:37
  SPEAKER_01: Um well the the this is uh this episode.
• 0:41
  SPEAKER_01: I don't know how to segue into talking about the Falling Valley Caldera from that.
• 0:46
  SPEAKER_01: But this is hot.
• 0:47
  SPEAKER_01: This is spicy geology.
• 0:48
  SPEAKER_01: How about that?
• 0:49
  SPEAKER_01: It is.
• 0:49
  SPEAKER_01: This is fire.
• 0:50
  SPEAKER_02: This is fire.
• 0:51
  SPEAKER_02: Um this is fire in more ways than one.
• 0:54
  SPEAKER_02: I'm never gonna say that ever again.
• 0:55
  SPEAKER_02: Okay, I promise.
• 0:56
  SPEAKER_01: Um put that one in your bill fold and store it away for a while.
• 0:59
  SPEAKER_02: We're talking about the we're beginning this lead up to a cataclysmic biblical kind of eruption, but really what this is about is magmatic differentiation, right?
• 1:11
  SPEAKER_02: Would you say, would you agree?
• 1:12
  SPEAKER_02: Like that's the that's what we're discussing today.
• 1:15
  SPEAKER_01: That's what we're discussing.
• 1:16
  SPEAKER_01: Yeah, and where we came from last episode, we were talking about why here?
• 1:19
  SPEAKER_01: Like, what's the broader tectonics of the Long Valley Caldera?
• 1:22
  SPEAKER_01: We need to have this extension.
• 1:24
  SPEAKER_01: We're gonna talk about why we need that extension today.
• 1:26
  SPEAKER_01: That's our kind of leadoff topic.
• 1:28
  SPEAKER_01: Why do we have extension?
• 1:29
  SPEAKER_01: Why is Long Valley Caldera here today, in this location?
• 1:32
  SPEAKER_01: We talked about tectonics here, Nevada block on the west, this big block.
• 1:35
  SPEAKER_01: We had basin and range to the far east in between.
• 1:38
  SPEAKER_01: There's some movement associated with the San Andreas Fault in here, and we have we end up having an extensional environment.
• 1:46
  SPEAKER_01: There's a jog in the system, and we get extension in the Long Valley Caldera.
• 1:50
  SPEAKER_01: The jog is really important, Chris, and I'll turn to the first image here.
• 1:54
  SPEAKER_01: We discussed last time, last episode, we discussed how we get melting on Earth, and we discussed three ways adding volatiles, adding heat, that's pretty obvious, and then decompression, which is maybe the least obvious but the most important process.
• 2:10
  SPEAKER_01: And here, this image, I love this GIF.
• 2:12
  SPEAKER_01: I draw something like this every year in my intro-level class when I'm talking about ways to melt the mantle.
• 2:18
  SPEAKER_01: It's a good one, right?
• 2:19
  SPEAKER_01: We have on here, we've mapped on here, this is temperature-pressure space, where temperature is going up to the right, pressures going up as we go down, so it's kind of like a uh looking down into the earth.
• 2:30
  SPEAKER_01: And we have three different fields.
• 2:32
  SPEAKER_01: We have one field where on the far left, low temperature side, where the whole mantle is solid.
• 2:38
  SPEAKER_01: All the rock is solid.
• 2:40
  SPEAKER_01: Then as we can increase in temperature, we get to a field where there's liquids plus solids.
• 2:46
  SPEAKER_01: This is where melting happens or partial melting happens.
• 2:49
  SPEAKER_01: If you keep adding heat and move this thing all the way to the right, eventually the entire rock will melt.
• 2:54
  SPEAKER_01: That rarely happens on Earth.
• 2:56
  SPEAKER_02: But we have an all-the-partial melting that is generally the case.
• 3:00
  SPEAKER_01: Exactly.
• 3:00
  SPEAKER_01: That's right.
• 3:01
  SPEAKER_02: And but Jesse, one thing that you kind of glossed over, and I want to make sure that we hit that.
• 3:05
  SPEAKER_02: That green line with the arrows on it is this is generalized geothermal gradient, which is the rate at which temperature increases with depth.
• 3:13
  SPEAKER_01: If we just drill down underneath our feet, the mantle is all solid underneath of our feet in most places, and and so we would be going increasing pressure as we drill down, and we'd be increasing temperature, but it would not be temperature would not be increasing dramatically fast.
• 3:28
  SPEAKER_01: And that's the that's the key to this geothermal gradient.
• 3:31
  SPEAKER_01: How is heat getting out of the crust?
• 3:33
  SPEAKER_01: It's all solid beneath our feet in most places.
• 3:35
  SPEAKER_01: Not the case in Long Valley, though.
• 3:37
  SPEAKER_02: So so Jesse, one thing though that is subtle with this diagram is that you can get this thing going on partial melting by ascension of you know, soft, plasticky kind of mantle material.
• 3:51
  SPEAKER_02: That's right, but you can also do this by stretching the crust, this tensional force, which is gonna flatten out the geothermal gradient.
• 4:00
  SPEAKER_01: And so it's you know, think about what happens here, Chris.
• 4:03
  SPEAKER_01: That's a great I love that analogy of flattening out the crust.
• 4:06
  SPEAKER_01: Okay, if we take the crust, we talked about basin and range, for instance.
• 4:09
  SPEAKER_01: You take a thick block of crust, you know, 40 kilometers, 35 kilometers thick, and you stretch it out.
• 4:14
  SPEAKER_01: You had used an analogy of taffy.
• 4:16
  SPEAKER_01: Pull that taffy, stretch it out.
• 4:17
  SPEAKER_01: Okay, it gets thinner, the top gets thinner, the bottom gets thinner.
• 4:21
  SPEAKER_01: And in the middle, you know, it's getting stretched out here.
• 4:24
  SPEAKER_01: The mantle wells up to replace that.
• 4:26
  SPEAKER_01: And the mantle is hot, so you're taking hot stuff from deep and bringing it up shallower.
• 4:31
  SPEAKER_01: And just because the structure of this line called the solid is where the melting happens, if you bring something straight up, the same amount of heat, and you bring it straight up, you're gonna melt it.
• 4:40
  SPEAKER_01: You're gonna cross into a field where melting happens.
• 4:42
  SPEAKER_01: And this happens at mid-ocean ridge settings, which we talked about before.
• 4:46
  SPEAKER_01: But this is the process.
• 4:48
  SPEAKER_01: This is an image that the really schematic, you know, intro-level image that shows us how this melting happens.
• 4:53
  SPEAKER_01: And that's what's going on in this in this area.
• 4:55
  SPEAKER_01: I would say not just Long Valley Caldera, but in this general region where there's trans tension, there's a lot of little volcanoes, little basaltic volcanoes coming up and popping off and bringing lava up to the surface.
• 5:07
  SPEAKER_01: So, Chris, when does this happen?
• 5:09
  SPEAKER_01: When do we start to have this melting going on?
• 5:13
  SPEAKER_02: Let's transition now to talk a little bit about just Long Valley Caldera itself.
• 5:18
  SPEAKER_02: Then we're gonna take a step back and talk more about just the the geologic principles that are at play.
• 5:24
  SPEAKER_02: So this began 3.9-ish million years ago.
• 5:28
  SPEAKER_02: This is when magma began to accumulate beneath what is now Long Valley Long Valley.
• 5:33
  SPEAKER_01: Chris, let me just interject real quick and say that we know there's older volcanoes.
• 5:37
  SPEAKER_01: There's a lot, there's older volcanism in this area.
• 5:40
  SPEAKER_01: That's like maybe not tightly related to the Long Valley Caldera story.
• 5:45
  SPEAKER_01: By by what we mean by in this region, we're talking about Long Valley Caldera.
• 5:48
  SPEAKER_01: You know, there is older magmatism in this region.
• 5:51
  SPEAKER_01: We we know that, but we're kind of skipping, glossing over that for the purposes of staying out of the weeds in in details here.
• 5:57
  SPEAKER_01: So, anyway, sorry, we that's why we've zoomed in on age in this diagram here.
• 6:00
  SPEAKER_02: And if you look at the image here on the slide, and Jesse, you have it pulled up right there.
• 6:04
  SPEAKER_02: This this is a great schematic just showing not all of the eruptions, but some of the eruptions that occurred within a window of time.
• 6:12
  SPEAKER_02: This was not one event that happened.
• 6:16
  SPEAKER_02: So from 3.9 million years ago when it began to accumulate to now, we've had over 110 eruptions that have accumulated 820 cubic kilometers of material that ejected that was ejected out as lava flows and ash and and and everything in between.
• 6:34
  SPEAKER_02: So that's a lot of stuff.
• 6:36
  SPEAKER_02: This is a big system from 2.2 million years ago to 230,000 years ago, and the big eruption fits right in the middle of that time window.
• 6:47
  SPEAKER_02: It was mostly this really sticky, thick, viscous, high silica material, rhyolite or rhyolytic uh material that was ejected out.
• 6:59
  SPEAKER_02: And that's very similar to like a Yellowstone kind of composition.
• 7:03
  SPEAKER_02: Yeah, subtle differences, but very similar.
• 7:05
  SPEAKER_01: And that's gonna be our our our we're gonna sort of talk about that next episode, really.
• 7:09
  SPEAKER_01: But this episode's focusing on this early phase, right?
• 7:12
  SPEAKER_01: You said 3.9 to 2.2.
• 7:13
  SPEAKER_01: That's all that's shown on this this plot right here, is that first phase.
• 7:17
  SPEAKER_01: And these are small eruptions, there's a lot of them, but they're all relatively small.
• 7:23
  SPEAKER_01: Most of these, there's you know, hundreds of eruptions in this 3.9 interval tilt today.
• 7:28
  SPEAKER_01: A lot of them are really quite small, like 0.1 cubic kilometers, which is small, not by human standards, but by volcano standards.
• 7:37
  SPEAKER_01: This is a relatively small amount of time.
• 7:39
  SPEAKER_02: And even even by Long Valley standards, these are small, you know, in terms because of this the culmination of the big eruption, right?
• 7:47
  SPEAKER_02: Um, so from 3.9 to 2.2 million years ago, that time frame of like 1.7 million years, what came out was mostly basalt to and acidic to day site um eruptions.
• 8:02
  SPEAKER_02: In other words, there was no rhyolite.
• 8:04
  SPEAKER_02: Um, and and so Jesse, real quick, I think a question for you.
• 8:09
  SPEAKER_02: When you talk about the rocks in your in your intro class, you 250 big lecture hall, you know, class, you talk about basalt, obviously.
• 8:17
  SPEAKER_02: I'm sure you talk about andesite.
• 8:19
  SPEAKER_02: Do you talk about day site?
• 8:20
  SPEAKER_01: Um, no, not at that level.
• 8:23
  SPEAKER_01: No, I I you you you kind of have to just actually, Chris, maybe this is a good time to to get a little bit more specific.
• 8:29
  SPEAKER_01: This is we're moving into you know, second or third year of an undergrad geology degree type of content here, talking about this.
• 8:36
  SPEAKER_01: We're using these terms.
• 8:38
  SPEAKER_01: These are our specific the words basalt, andesite, rhylite, we kind of use those, you and I and and and a lot of geologists use those in a very general sense to mean mafic to felsic to inter mafic to intermediate to felsic.
• 8:54
  SPEAKER_01: They actually have more specific meaning to geologists and geochemists, because we can subdivide basalt by other types of composition, and this is the way that we do this as geochemists and petrologists.
• 9:07
  SPEAKER_01: I'm plotting here um a very typical diagram, which is called a total alkalize versus silica, TAS diagram, and all it's plotting is the amount of sodium and potassium in a rock versus the amount of silica in a rock.
• 9:22
  SPEAKER_01: And there's a couple different pathways that these things can take, but we have basalt, basaltic andesite, andesite, dacyte.
• 9:29
  SPEAKER_01: Those kind of have low sodium, low potassium, relatively low.
• 9:33
  SPEAKER_01: And so as you increase silica content, the alkalis, potassium, and sodium stay low.
• 9:39
  SPEAKER_01: You can have melt or magma that has more sodium and potassium in it while still having low silica, basalt to trache basalt, tracheandesite, trache dacyte, which has higher silica, excuse me, higher sodium and potassium.
• 9:54
  SPEAKER_01: Yeah, you mean you don't you don't need to use these on a day-to-day basis unless you're really getting specific about the tectonic setting here.
• 10:00
  SPEAKER_02: So I I just want to think of I think of something with this, Jesse.
• 10:04
  SPEAKER_02: You know, it's a quote that your professor, well your supervising professor Tom Chaco said to you is that you really can't identify a rock in the field.
• 10:11
  SPEAKER_02: You can't do that, you need to get it back in the lab before you can identify it.
• 10:14
  SPEAKER_02: This graphic here shows that these little subtleties that separate out, they tease out these variations of, like you said, basalt.
• 10:23
  SPEAKER_02: You know, what I would pick up in the field and say, oh, this is this is an awesome looking basalt, or you go to Devil's Tower and you pick it up, and you you're like, oh, this is a porphytic andesite, it's a textbook-looking porphyric andesite.
• 10:34
  SPEAKER_02: Actually, it's a phonolite, you know, and it's got the That's right.
• 10:37
  SPEAKER_02: Exactly.
• 10:37
  SPEAKER_01: You know, phonolite on there, you can see it on this chart.
• 10:39
  SPEAKER_01: It's got screaming high sodium and potassium, but it has the silica content of basaltic andesider andesite.
• 10:45
  SPEAKER_01: So this is why I love geochemistry, is it tells us so much.
• 10:49
  SPEAKER_01: When we get into the details of the geochemistry here, the point that I wanted to that I think I want to make, and you just made an excellent one, Chris, on that, that the devil's in the details a little bit when it comes to this.
• 11:00
  SPEAKER_01: We can get to rhyolite.
• 11:02
  SPEAKER_01: There's multiple ways on this chart to get to rhyolite.
• 11:05
  SPEAKER_01: You can get to it by having low alkali content, going basalt, basaltic andesite, andesite, daycyte, then a little bit of rhyolite, but you got to get really high, really far up on the right side of this diagram to get to rhyolite.
• 11:17
  SPEAKER_01: You can also get there by going basalt, basaltic andesite, tracheandocyte, hold on, and then rhyolite.
• 11:23
  SPEAKER_02: We need to explain what you mean when you say that.
• 11:25
  SPEAKER_02: When you say going from basalt to basaltic andesite to andesite, what you're talking about is the evolution of the magma.
• 11:34
  SPEAKER_02: Right.
• 11:34
  SPEAKER_02: You know, assimilation and this kind of mixing that can happen, or you can get there by distilling the rocks, right?
• 11:42
  SPEAKER_02: Mart melting them in bits and pieces, and you gradually then shift the dial from mafic to intermediate to felsic that way.
• 11:51
  SPEAKER_02: And so I just want sorry to interrupt, I but I wanted to make sure that our our viewers like kind of know what you're referring to.
• 11:59
  SPEAKER_01: I know that's a really great point, Chris.
• 12:00
  SPEAKER_01: And the way that we'll we'll talk about in the next little bit here, we'll talk about how that actually happens, how we go from basalt to day site, basically.
• 12:09
  SPEAKER_01: The main point I think I wanted to make here is that the last image here is plotting a field of samples.
• 12:15
  SPEAKER_01: And those samples are from a bunch of papers, you know, a lot of Wes Hildreth work has been done on Long Valley Caldera, but all these these this orange field at the end is the field for the actual geochemistry of the samples that we're talking about in this window, 3.9 to 2.2 million years ago.
• 12:30
  SPEAKER_01: So they actually are not basalt, basaltic andesite andesite.
• 12:35
  SPEAKER_01: They they're up in this trachey field, which has higher sodium, higher potassium.
• 12:39
  SPEAKER_01: So a lot of the literature, like the National Park Service literature or you know, intro, you know, blogs you'll read about the Long Valley Caldera, you'll hear basalt, andesite, day site.
• 12:49
  SPEAKER_01: We used that term before.
• 12:50
  SPEAKER_01: It's a little bit of a simplification.
• 12:52
  SPEAKER_01: Instead of saying we don't need to say trache day site, you know what I mean?
• 12:56
  SPEAKER_02: Like we say day site, I have a okay.
• 12:58
  SPEAKER_02: It actually burning question.
• 13:00
  SPEAKER_01: Yeah, okay.
• 13:01
  SPEAKER_02: So, all right, I this is off topic, um, but question for you.
• 13:06
  SPEAKER_02: Most geoscience programs at university are shifting away from the track that you and I took.
• 13:14
  SPEAKER_02: We took mineralogy, we took petrology, and you wouldn't get into any of this until you got in a lab in petrology.
• 13:21
  SPEAKER_02: You're looking at rocks under thin section like crazy.
• 13:24
  SPEAKER_02: You be, you know, that's all you do.
• 13:27
  SPEAKER_02: Um they don't have those classes anymore very often.
• 13:31
  SPEAKER_02: It's more like a rocks and minerals or earth materials classes.
• 13:36
  SPEAKER_02: So when do they get exposed to this kind of stuff?
• 13:40
  SPEAKER_01: That's a great question, Chris.
• 13:41
  SPEAKER_01: Um, I mean, we don't.
• 13:42
  SPEAKER_01: At Penn State, we have a 200 level.
• 13:46
  SPEAKER_01: The only required class in this like rocks and minerals kind of thing is called earth materials, and it's basically that it's mineralogy and petrology, it's mineralogy one, mineralogy two, igneous petrology, and metamorphic petrology all grouped into one class.
• 14:00
  SPEAKER_01: So you don't get into this.
• 14:02
  SPEAKER_01: I mean, maybe they'll see a TAS diagram.
• 14:05
  SPEAKER_02: Like imagining that class.
• 14:06
  SPEAKER_01: Um, so it you because you you just end up skimming the surface on all this stuff.
• 14:11
  SPEAKER_01: So you don't get into it.
• 14:12
  SPEAKER_02: Is this now a graduate level thing?
• 14:14
  SPEAKER_01: Then independently get into this if it pertains to your studies.
• 14:18
  SPEAKER_01: We have an optional advanced petrology class, which is uh metamorphic and India's petrology combined into one.
• 14:25
  SPEAKER_01: It's advanced, quote unquote, advanced.
• 14:28
  SPEAKER_01: We have other classes, you know.
• 14:29
  SPEAKER_01: I teach like a discussion class that's like continental distillation that we might touch on this type of stuff.
• 14:35
  SPEAKER_01: But yeah, it ends up being as part of their research, if they want to.
• 14:39
  SPEAKER_01: It you know, it's part of this conversation of broadening the geoscience degree, broadening from geology to geoscience, probably.
• 14:48
  SPEAKER_01: And you know, in Canada, where the geologists are certified professional geologists who go work in mineral exploration, they have to take igneous petrology because it's part of the certification process, and they need to actually go out and be able to identify a basalt and say, Oh no, is this a phonolite or is this an andesite?
• 15:06
  SPEAKER_01: Like what do I think about that in the field?
• 15:09
  SPEAKER_01: So Canada, they still have igneous petrology, metamorphosology, mineralogy one, mineralogy two.
• 15:14
  SPEAKER_01: Um, it's different to me why they would.
• 15:17
  SPEAKER_02: Yeah.
• 15:18
  SPEAKER_02: Um, so I have mixed feelings about that.
• 15:21
  SPEAKER_02: Um as long as as long as it's replaced with You and me, Chris, and a lot of other people have mixed feelings about this.
• 15:29
  SPEAKER_02: Yeah, it's it's a as long, but as long as Earth systems is a part of the replacement of that, I'm all for it.
• 15:38
  SPEAKER_02: This bigger umbrella, the um more of a global context, I guess.
• 15:44
  SPEAKER_02: You know, this is very niche, what we're talking about here.
• 15:47
  SPEAKER_02: But he was very niche.
• 15:49
  SPEAKER_02: What is it?
• 15:49
  SPEAKER_01: You have to kind of ask, what does it get replaced with?
• 15:51
  SPEAKER_01: You know, if you take igneous petrology, which you'd where you'd learn this stuff and you replace it with something, what are you replacing it with?
• 15:56
  SPEAKER_01: And you know, for us it's replaced by a variety of global ocean chemistry cycling.
• 16:01
  SPEAKER_01: Good.
• 16:02
  SPEAKER_01: That's important, right?
• 16:03
  SPEAKER_01: Like that's really relevant.
• 16:04
  SPEAKER_01: Earth science policy classes, maybe.
• 16:07
  SPEAKER_01: Um Data Analytics in the geosciences.
• 16:10
  SPEAKER_02: What about climate science too?
• 16:12
  SPEAKER_01: What about climate science, you know, earth system science?
• 16:15
  SPEAKER_01: So it's I I think it's broadening, not watering down.
• 16:20
  SPEAKER_01: And maybe there's there's a a lever between, you know, that's kind of a seesaw, but I think we're where our students are getting depth just just in different fields.
• 16:29
  SPEAKER_01: And a lot of what you learned about.
• 16:31
  SPEAKER_02: I think that's great.
• 16:32
  SPEAKER_02: I I love that.
• 16:33
  SPEAKER_02: I think that that's necessary to for the survival of geoscience.
• 16:37
  SPEAKER_02: I think that that's the direction that we have to move, and I think it's an appropriate direction.
• 16:42
  SPEAKER_02: I love it.
• 16:42
  SPEAKER_01: So I mean, it still hurts that we don't that that it still hurts me a little bit that you can graduate with a Penn State geoscience degree not being able to identify and a site in the field.
• 16:52
  SPEAKER_01: Uh, you know, there's some part of me that struggles with that kind of thing.
• 16:54
  SPEAKER_02: I know we need to let go of that.
• 16:56
  SPEAKER_02: I I do I when I first um learned of this shift, you know, years ago, I was upset.
• 17:02
  SPEAKER_02: I'm like, wait a minute, you you're gonna have people graduated from my alma mater that have taken mineralogy and petrology.
• 17:07
  SPEAKER_02: Um, and those were the hard classes, those were the ones that you had to survive that.
• 17:13
  SPEAKER_02: There's a there was a point in pride, like, yes, I got through it, you know, and I not only did I get through it, but I nailed it.
• 17:19
  SPEAKER_01: You nailed it.
• 17:20
  SPEAKER_01: Um but then is it is inspired by that too.
• 17:24
  SPEAKER_01: Yeah.
• 17:24
  SPEAKER_01: Well, I mean it's a big well, okay.
• 17:26
  SPEAKER_01: Let's let's not get too distracted by that conversation.
• 17:29
  SPEAKER_01: That's a long one.
• 17:30
  SPEAKER_01: All right.
• 17:31
  SPEAKER_02: I I want to throw a stat at you.
• 17:32
  SPEAKER_02: Second, Jesse.
• 17:33
  SPEAKER_02: Okay.
• 17:34
  SPEAKER_02: Hit me.
• 17:35
  SPEAKER_02: You ready?
• 17:36
  SPEAKER_02: So we were saying that from 3.9 to 2.2 million years ago that most of the eruptions were basalt and andesite and day site, and then that that derailed us the discussion about day site.
• 17:47
  SPEAKER_02: No rhyolite, in other words.
• 17:49
  SPEAKER_02: Um, but uh crazy stat with these early flows that you had, which were mostly mafic basalt um and some andesite in there, they have been offset.
• 17:59
  SPEAKER_02: So these are like 3.9 million years ago when these happened.
• 18:03
  SPEAKER_02: They've been offset by a thousand meters, a kilometer by the Sierra Nevada fault.
• 18:10
  SPEAKER_02: Amazing.
• 18:11
  SPEAKER_02: Goes back to episode two in this, where those normal faults are.
• 18:14
  SPEAKER_02: In other words, high up on the Sierra Nevada are these basaltic lava flows, that normal fault has cross-cut that lava flow, and it's a thousand meters below in the valley floor.
• 18:27
  SPEAKER_02: In in the to me, what is seemingly not an incredible amount of time.
• 18:33
  SPEAKER_02: That's I I don't know.
• 18:35
  SPEAKER_02: I think that when I when I came across that, I like that that has to be a part of this discussion.
• 18:39
  SPEAKER_02: That's a lot of offset in the last you know, three and a half-ish million years ago.
• 18:45
  SPEAKER_01: So super interesting.
• 18:47
  SPEAKER_02: Yeah, so as you alluded to, Jesse, most of these eruptions were small, but we're talking about Long Valley Caldera, you know, and and this massive explosion, the you know, cataclysmic kind of thing.
• 18:59
  SPEAKER_02: Yeah, not basalt.
• 19:01
  SPEAKER_02: So not basalt transition, you know.
• 19:03
  SPEAKER_02: What gives?
• 19:04
  SPEAKER_02: You know, let's let's move in that direction.
• 19:06
  SPEAKER_01: I wanna we've we've kind of covered all of the important aspects of this the lead up, I guess, at this point.
• 19:14
  SPEAKER_01: But there's a great phrase, Chris, that comes from Wes Hildreth, one of the 2023 papers about this.
• 19:19
  SPEAKER_01: That I think it's this might I'm not sure.
• 19:21
  SPEAKER_01: You let me know if this is a useful uh learning exercise or not.
• 19:24
  SPEAKER_01: Here's the phrase mantle derived basaltic power supply, transcrustal self-refining, melt mush fractionation column.
• 19:36
  SPEAKER_01: Okay.
• 19:37
  SPEAKER_01: That's a fun phrase.
• 19:39
  SPEAKER_01: I mean it's a lot.
• 19:41
  SPEAKER_01: Mantle derived basaltic.
• 19:43
  SPEAKER_01: Let me say it again real quick.
• 19:44
  SPEAKER_01: Mantle derived basaltic power supply, transcrustal self-refining, melt mush fractionation column.
• 19:50
  SPEAKER_01: And I think it might be useful to just work through these words, and they all have meaning, and it and it'll it'll kind of make the story tight, I think.
• 19:58
  SPEAKER_01: But what do you think?
• 19:59
  SPEAKER_02: I'm not sure Joyce understands this.
• 20:02
  SPEAKER_02: Uh my mom would uh let's work through it for Joyce.
• 20:09
  SPEAKER_01: Yeah, she's gonna struggle with this.
• 20:10
  SPEAKER_01: This is a mouthful.
• 20:11
  SPEAKER_01: It is, it is a mouthful, but all those words, it's uh that's like the whole story is kind of tied up in in that sentence right there.
• 20:18
  SPEAKER_01: So I broke this apart into a couple different important phrases.
• 20:21
  SPEAKER_01: Let's work on mantle-derived basaltic, first of all.
• 20:24
  SPEAKER_01: Mantle-derived basalt.
• 20:26
  SPEAKER_01: That's kind of what we've been talking about this whole time, right, Chris?
• 20:29
  SPEAKER_01: Like transtension, mantle upwelling.
• 20:33
  SPEAKER_01: Exactly.
• 20:34
  SPEAKER_01: The tensional environment, mantle upwells, it starts to melt.
• 20:38
  SPEAKER_01: So the extension leads to melting.
• 20:40
  SPEAKER_01: We've got basaltic, broadly basaltic, trache basaltic melt in the system.
• 20:46
  SPEAKER_01: All right.
• 20:46
  SPEAKER_01: Is that part of the?
• 20:47
  SPEAKER_01: Well, why?
• 20:47
  SPEAKER_02: Hold on, hold on.
• 20:48
  SPEAKER_02: No, we got no, we gotta we're not done teasing that out because you're talking about mantle, which is ultramafic, but that's not what came out.
• 20:54
  SPEAKER_02: It was basalt and this kind of trache basalt, you know, so or tracheandocyte that came out.
• 20:59
  SPEAKER_02: So, well, why?
• 21:01
  SPEAKER_02: How do you get that out of the ultramafic upwelling of the mantle?
• 21:06
  SPEAKER_02: And I always go back to this.
• 21:07
  SPEAKER_02: Yeah, my Bowen's reaction series.
• 21:10
  SPEAKER_02: Bowen's reaction series.
• 21:11
  SPEAKER_01: The why, the why and why MCA.
• 21:14
  SPEAKER_01: Yeah.
• 21:14
  SPEAKER_02: You know, it's it's melting temperatures of certain yeah.
• 21:18
  SPEAKER_01: Let me interrupt here.
• 21:19
  SPEAKER_01: You you always say the the earth does not melt, earth partially melts, and that's so key for differentiation and distillation.
• 21:25
  SPEAKER_01: The earth is distilling itself by partial melting, it does not totally melt, it partially melts, and so that's what you're talking about here.
• 21:33
  SPEAKER_01: The the why bones reactions why.
• 21:35
  SPEAKER_01: Go for it.
• 21:36
  SPEAKER_02: So you take these ultramafic minerals that make up that the peritotite and the rock that's coming up, and you're gonna only partially melt it, and you're gonna melt the only the lower temperature minerals then.
• 21:46
  SPEAKER_02: And so the net effect of that is you take ultra mafic and you partially melt it, you generate mafic.
• 21:52
  SPEAKER_02: If you j if you partially melt mafic, you generate intermediate magma and then and so on down to felsic.
• 21:58
  SPEAKER_02: And so it's like this perfect.
• 22:00
  SPEAKER_02: It's in stages, right?
• 22:01
  SPEAKER_02: Yeah.
• 22:02
  SPEAKER_02: In stages.
• 22:03
  SPEAKER_01: And we're we're just focusing on the first part.
• 22:04
  SPEAKER_01: The mantle's melting to produce this basalt.
• 22:06
  SPEAKER_01: There's probably something else, you know, melting the lower crust in there too, or melting some of the acenospheric uh lithospheric mantle, which is mantle still.
• 22:14
  SPEAKER_01: But mantle derived basalt.
• 22:16
  SPEAKER_02: So that covers mantle derived basalt.
• 22:18
  SPEAKER_01: Perfect.
• 22:19
  SPEAKER_01: Okay.
• 22:19
  SPEAKER_01: Now let's get to power supply.
• 22:21
  SPEAKER_01: This one's interesting because I like this a lot.
• 22:24
  SPEAKER_01: This is mantle-derive basaltic power supply.
• 22:27
  SPEAKER_01: So this is a power supply.
• 22:28
  SPEAKER_01: And you have to think about the temperatures involved here.
• 22:32
  SPEAKER_01: Basalt is liquid at 1300 degrees centigrade, 1200 degrees centigrade, pretty high temperature.
• 22:39
  SPEAKER_01: That's hot.
• 22:41
  SPEAKER_01: It's intruding, and you have to think about what it's doing.
• 22:44
  SPEAKER_01: We're forming basalt down in the mantle.
• 22:47
  SPEAKER_01: That basalt is coming out in these eruptions.
• 22:49
  SPEAKER_01: I'll skip ahead.
• 22:50
  SPEAKER_01: We've seen all these all these eruptions from between 2.2 and 3.9 million years ago.
• 22:54
  SPEAKER_01: It's coming out on the surface.
• 22:56
  SPEAKER_01: So we've got 1200 degree stuff going from the mantle through the crust all the way to the surface.
• 23:02
  SPEAKER_01: What does that mean, power supply here?
• 23:05
  SPEAKER_02: Yeah, well, it's traversing continental crust.
• 23:09
  SPEAKER_02: This basalt is coming from the mantle, it's traversing the crust though.
• 23:13
  SPEAKER_02: And and so it's traversing rocks that are much, much more felsic, which have lower temperatures.
• 23:22
  SPEAKER_02: You know, you said 1300 degrees, basalt is liquid.
• 23:25
  SPEAKER_02: Well, it's usually going to be fully solid at 900 degrees, but felsic material is melting at that temperature.
• 23:34
  SPEAKER_01: Yeah.
• 23:34
  SPEAKER_01: Felsically melting at that temperature.
• 23:36
  SPEAKER_01: It's melting at 700, 680, 700, 750.
• 23:39
  SPEAKER_01: You're starting to melt felsic material.
• 23:41
  SPEAKER_01: So we're bringing hot stuff, punching it through the crust, it's transiting the crust, taking heat really from the mantle through this partial melting process and bringing it up to the surface and bringing it through the entire crustal column.
• 23:54
  SPEAKER_01: Okay.
• 23:54
  SPEAKER_01: So Jesse, what do you refer?
• 23:56
  SPEAKER_02: How do you refer to this in your classes?
• 23:58
  SPEAKER_02: Because I talk about this as uh I use the word assimilation for this.
• 24:02
  SPEAKER_01: Yeah.
• 24:02
  SPEAKER_01: Yeah.
• 24:03
  SPEAKER_01: I think it's both assimilation and partial melting.
• 24:06
  SPEAKER_01: I I refer to it as as you know, kind of both.
• 24:08
  SPEAKER_01: Um they go together, though, right?
• 24:10
  SPEAKER_02: Because partial melting, you're melting the felsic part of the rocks.
• 24:15
  SPEAKER_02: The lowest temperature minerals are going to melt the fastest and the easiest, but then they have to blend into the magma, right?
• 24:22
  SPEAKER_02: So you're taking this basaltic magma and you're blending in now really, really felsic quartzes and felt bars and things like that minerals, and you're changing the composition of the melt that way.
• 24:33
  SPEAKER_02: So they almost always go together.
• 24:35
  SPEAKER_01: And this brings up, so Chris, let's let's move ahead in this phrase because you're touching on the next couple things here.
• 24:40
  SPEAKER_01: So mantle-derived basalt, we talked about that power supply, this stuff's hot.
• 24:43
  SPEAKER_01: These rocks, basaltic magma's hot.
• 24:45
  SPEAKER_01: The next clause is transcrustal, and that's pretty simple.
• 24:48
  SPEAKER_01: It just means crossing the crust.
• 24:51
  SPEAKER_01: And well, why do we use transcrustal?
• 24:53
  SPEAKER_01: Well, we know it's basaltic magma forming down in the mantle, and then it's we're seeing it come to the surface.
• 24:58
  SPEAKER_01: So it's clearly transiting the crust there.
• 25:01
  SPEAKER_01: You know, it's going from the bottom to the top.
• 25:03
  SPEAKER_01: Uh, as we see, that makes sense.
• 25:05
  SPEAKER_01: Self-refining is kind of what you're touching on here.
• 25:08
  SPEAKER_01: So transcrustal, self-refining, melt mush fractionation column.
• 25:11
  SPEAKER_01: You're talking about the self-refining.
• 25:13
  SPEAKER_01: And what is okay, go into that a little bit more now, Chris.
• 25:17
  SPEAKER_01: Self-refining here.
• 25:18
  SPEAKER_02: Uh, self-refining is referring to this natural changing of the magma composition.
• 25:26
  SPEAKER_02: You're blending, you're assimilating felsic and mafic together.
• 25:31
  SPEAKER_01: And you know, like, Chris, one more hold on, really, really let me let me touch on that.
• 25:34
  SPEAKER_01: You're assimilating for sure.
• 25:36
  SPEAKER_01: You're you're like partially melting, it basalt is intruding this power supply, it's melting the crust around it, but you're also crystallizing stuff.
• 25:43
  SPEAKER_01: We are undergoing fractional crystallization.
• 25:45
  SPEAKER_01: So part of the process of going from basalt to basaltic andesite to andesite, or from basaltic trachey andesite to trache andesite to trache daycyte is crystals are forming, olivenes, pyroxines, early forming crystals are forming and they're settling out of the magma chamber.
• 26:00
  SPEAKER_01: You're also assimilating wall rock, and we're changing the composition in one direction.
• 26:05
  SPEAKER_01: So the magma, I always describe it in this classroom, this big lecture hall I teach in as like fill this room with magma and let it cool down.
• 26:12
  SPEAKER_01: Bowen's reaction series, early minerals will start to crystallize, they'll settle down to the floor.
• 26:17
  SPEAKER_01: So now we have six feet of crystals on the floor.
• 26:20
  SPEAKER_01: We have less magma, that magma is a different composition because we've removed some of the low silica, high iron magnesium minerals from the system.
• 26:29
  SPEAKER_02: So you know, it's like eating Lucky Charms, okay, with the Chris Bullheis way.
• 26:35
  SPEAKER_01: In what way is it like eating Lucky Charms?
• 26:38
  SPEAKER_02: So you pour a bowl of Lucky Charms and you have all these colorful little like hardened marshmallows, and then you have the oats, you know, that are around it.
• 26:46
  SPEAKER_02: But Chris Bullheis doesn't eat it that way.
• 26:48
  SPEAKER_02: Chris likes the marshmallows, and so I eat them first.
• 26:53
  SPEAKER_01: Oh, you're a first marshmallow eater.
• 26:55
  SPEAKER_01: Oh man, you don't like savorum, you don't eat it.
• 26:57
  SPEAKER_02: No, because I like them crunchy still.
• 27:00
  SPEAKER_02: So I eat them first, and the more that I eat, you know, I take them out of what's left, and I'm changing the composition of my bowl of cereal.
• 27:10
  SPEAKER_02: Come on, it works.
• 27:11
  SPEAKER_02: This works.
• 27:12
  SPEAKER_01: It does work.
• 27:12
  SPEAKER_01: I I'm just uh I'm I'm just judging you for eating all the charms first and and instead of waiting till the end.
• 27:18
  SPEAKER_01: Oh, what a heathen.
• 27:19
  SPEAKER_01: Um Chris Boyce's you know, bowl of lucky charms is self-refining in the same way that we're self-refining this magma here.
• 27:28
  SPEAKER_02: So but Jesse, hold on, I hold on, let me interrupt again.
• 27:31
  SPEAKER_02: Sorry, I'm sorry, but this is basic geoscience principles here, what we're talking about.
• 27:37
  SPEAKER_02: This is this goes into any intro level class on the evolution of magma.
• 27:43
  SPEAKER_02: You know, we talk about all of this without using the phrases that Wes Hildreth used, you know.
• 27:50
  SPEAKER_02: I think it's this is so clever, though.
• 27:52
  SPEAKER_01: I love it.
• 27:53
  SPEAKER_01: I mean, it's great.
• 27:53
  SPEAKER_01: It encapsulates a whole geological story into one sentence, which is an impressive bit of writing, I must say.
• 27:59
  SPEAKER_01: So if we go back to that, mantle dry basalt, talked about that, power supply, talked about that, transcrustal, talked about that, self-refining, talked about that.
• 28:06
  SPEAKER_01: The last two pieces are melt mush fractionation column.
• 28:10
  SPEAKER_01: And melt mush is a kind of a WTF sort of a phrase.
• 28:15
  SPEAKER_01: Like, what do we mean by melt mush?
• 28:16
  SPEAKER_01: Those seem like two opposite things.
• 28:18
  SPEAKER_01: All it means is it's not pure magma, it's crystals plus magma.
• 28:22
  SPEAKER_01: And this goes kind of to the partial melting thing.
• 28:25
  SPEAKER_01: If you like imagine if you tried to have a magma body in the crust, you know, 10 kilometers down the crust, a solid vat of liquid, a huge vat of liquid, it just wouldn't be stable.
• 28:37
  SPEAKER_01: There's too much pressure down there.
• 28:39
  SPEAKER_01: So we basically end up having this network of crystals with little bits of melt in between them, and that's what we refer to kind of as a magma mush or a crystal mushroom system.
• 28:49
  SPEAKER_02: You know, that would be very easy for us to see uh from a geophysics standpoint.
• 28:54
  SPEAKER_01: Hey, and this is going back to like Diana Roman season one of Planet Geo when we were talking about this with her.
• 28:59
  SPEAKER_01: Um about you know, seismologists we rarely we don't see pure melt, or we it's the seismologists don't see pure melt.
• 29:07
  SPEAKER_01: Yeah.
• 29:07
  SPEAKER_02: Jesse, this goes back to what you've always said.
• 29:12
  SPEAKER_02: What is a magma chamber?
• 29:14
  SPEAKER_02: And the analogy that we've often used is a magma chamber is more like a a vase on the middle of a kitchen table, a centerpiece, and it's full of marbles.
• 29:25
  SPEAKER_02: And then you have a little you can pour some water in there, and the water just sits in the pore spaces and so on.
• 29:30
  SPEAKER_02: And that's more I think a a better description of what a magma chamber actually looks like, you know?
• 29:36
  SPEAKER_02: And then you go back to this this heat machine, this basalt, the power source for this is that hot basalt.
• 29:42
  SPEAKER_02: So you get this injection of fresh heat into this super hot sponge-like chamber, and that can stimulate then melting and exactly the feeding.
• 29:54
  SPEAKER_01: Okay, so that leads to our next thing, which is fractionation column.
• 29:57
  SPEAKER_01: And fractionation column, we already alluded to fractionation.
• 30:01
  SPEAKER_01: This is crystal fractionation, the magma's going from basalt to day site progressively, and we're getting little eruptions of the different kinds here.
• 30:08
  SPEAKER_01: And the fractionation column is just this is a big transcrustal thing.
• 30:12
  SPEAKER_01: So this makes sense.
• 30:13
  SPEAKER_01: Like if you take liquid from the mantle and you transit it through the crust, and it takes some time to do that, you're bringing in all this heat, assimilation, fractionation.
• 30:22
  SPEAKER_01: Of course, things are gonna change.
• 30:24
  SPEAKER_01: Of course, you're gonna kind of go, you're gonna have basalts, you're gonna have trachea day sites, you're gonna have all these compositions in between, and it's kind of gonna be a little bit random.
• 30:31
  SPEAKER_01: So, this last image, this image number four here, is kind of showing this is a really, really schematic view, but this is kind of showing this preemptive phase, the lead up to the eruption that we're talking about, this process.
• 30:45
  SPEAKER_01: We've got this magma chamber forming down deep, and it is a mantle-derived basaltic power supply that is transiting the crust, it's self-refining on the way, magma, melt, mush, fractionation column.
• 30:58
  SPEAKER_01: I think that's the prelude.
• 31:00
  SPEAKER_01: That's the story that is the lead up, the prelude to this eruption.
• 31:04
  SPEAKER_02: And it gets this is so idealized that we're looking at right here.
• 31:08
  SPEAKER_02: And and you know, with as it's transiting, you might get some shallower chambers beneath the surface that are in place higher up in the crust, and the higher up they are in the crust, the more they're gonna assimilate felsic materials, which makes the magma stickier and viscous.
• 31:23
  SPEAKER_01: Hold on, Chris, don't get carried away.
• 31:25
  SPEAKER_01: That's next episode, baby.
• 31:26
  SPEAKER_01: That's the next thing.
• 31:27
  SPEAKER_01: We're this is just the lead up.
• 31:28
  SPEAKER_01: Let's let's let's hold off on that.
• 31:32
  SPEAKER_01: You're exactly right.
• 31:33
  SPEAKER_01: You talked about the the transition from basaltic to kind of dominantly rhyolytic eruptions around two point, you know, around two million years ago.
• 31:40
  SPEAKER_01: That's what's next episode, the the transition.
• 31:43
  SPEAKER_01: And I think we've we're very clearly now pointing in that direction.
• 31:47
  SPEAKER_01: We're saying we've got all this basalt, this basalt today said eruptions.
• 31:50
  SPEAKER_01: We talked about the power supply aspect, and that is a big cliffhanger towards the lead up to the eruption, I think, in the tectonic model for this area.
• 31:59
  SPEAKER_02: So well, Jesse, real quick, what the you know, the takeaway from from this is that Long Valley Caldera parallels is such a good model, Jesse, to talk about a broader geologic concept, which is how does magma traverse the crust, what happens along the way, and how does magma evolve as it as it crystallizes and assimilates and changes composition.
• 32:26
  SPEAKER_02: Um, and and so you can dramatically change the composition of magma over time.
• 32:32
  SPEAKER_01: Yeah, super interesting, super important point, and really I think builds this story beautifully to what we're gonna talk about next, which is a massive eruption um coming out of Long Valley Caldera.
• 32:43
  SPEAKER_01: So, hey, if you're listening to this, you can find the images we're talking about both on YouTube and on our Camp Geo Mobile app.
• 32:49
  SPEAKER_01: Check that out.
• 32:49
  SPEAKER_01: First link in the show notes.
• 32:50
  SPEAKER_01: Send us an email plantgeocast at email.com and follow us on all the social music at plantgeocast.
• 32:56
  SPEAKER_01: Cheers.