PlanetGeo: The Geology Podcast
PlanetGeo: The Geology Podcast
Yosemite National Park
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Today, we talk about the geology of Yosemite National Park. Specifically, we dive into 4 different aspects:
1- What's the story behind all this granite?
2- Why do these mountains look the way they do? Glaciers
3- What are all the cracks in the granite about?
4- Rockfall!
The formation of granite is taught in a very traditional way. Magma intrudes deep inside the Earth and then cools slowly as the minerals grow larger. Jesse says that's a very debatable and a hotly researched area now. Jesse schools us on how magma is believed to cool and how we know.
Then we get into the glacial part of Yosemite. There are so many glacial features that are perhaps more obvious here than anywhere. We talk about U-shaped valleys, hanging valleys, horns, and aretes.
Exfoliation is so prevalent here that it is impossible to miss. We get into how exfoliation happens including an example that was caught on Camera - Twain Harte Rock.
Lastly, we end with a brief discussion on the prevalence of Rockfall in YNP. There are many rockfalls - some very large that occur with a stunning frequency.
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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.
SPEAKER_01Hey Chris. Jesse Ryman.
SPEAKER_00How are you doing today? Christopher Bullheise, I'm excited, man. I'm not sure. Hold on.
SPEAKER_01I gotta mess with my microphone a second. I know you love that when I do it. So I'm gonna get myself situated right now.
SPEAKER_00There we go. Click it around. If you keep clicking buttons, that's really the most fun when you do that.
SPEAKER_01All right. What's going on today? I'm good. I'm excited about today. This is uh it's an interesting thing we have going on today. We're gonna talk about Yosemite National Park.
SPEAKER_00Yeah, this is a great one. I'm I'm super psyched to think about Yosemite, and I've been thinking about Yosemite a lot recently. But before we do that, let's introduce ourselves briefly. You are Chris Bullheis, a nationally recognized earth science teacher from Michigan, my former high school teacher. I took, what did I take? Ninth grade earth science from you. I was actually in your classroom uh on 9-11. I also took geology and a couple field camps, including a summer science field course from you when I was in high school back in the day, and uh, and we've become pretty good friends. Oh, wait a minute.
SPEAKER_01Don't forget about your your independent study that you did with me, too.
SPEAKER_00So quote unquote, independent study.
SPEAKER_01I was more independent than study, but you know, yes, you were, you lazy turd. Yep. All right, so and you're Dr. Jesse Rymank, and you know it pains me to say it, but you're the doctor in the house. There we go, I like that. Um that's gonna come through in today's episode. Yeah, it's gonna come through in today's episode more than most, I think. But anyway, yeah. Uh you're one of my former students, you went on to get your undergrad at Hope College, and then you went to the University of Alberta in Edmonton, Canada to get your PhD. You did a postdoc at the Carnegie Institute for what three years, Jesse?
SPEAKER_00Yeah, three and a half, four years, yeah.
SPEAKER_01Yeah, three and a half, four years. Took you a while. Yeah, um and now you are a professor of geoscience at the Penn State University. And I was really hoping you guys were gonna pull it out last night with uh Ohio State, but nope. Nope.
SPEAKER_00We're gonna it's you know hopefully our jobs remain intact if the football conclusions.
SPEAKER_01That's right, that's right.
SPEAKER_00No, yeah, and uh this is Planet Geo. We talk about all things earth science, we get to interview some amazing people, and we get to talk about really cool stuff. And this is uh, I don't know, the fourth in a line of things about national parks. We love the national parks here. It might be, right? Yos uh yeah, we've done Grand Tetons, Yellowstone, Grand Canyon. Ah, we've also done glaciers, so this is the fifth one. That's right.
SPEAKER_01Yeah, the fifth one. Okay, cool.
SPEAKER_00So, anyways, we we're gonna talk about yeah, Chris Scottis said, what are we talking about Yosemite for here?
SPEAKER_01Okay, so when we first started talking about this idea of let's say, hey, let's do Yosemite, because I was just there this summer, spent uh, you know, a fair amount of time in Yosemite National Park.
SPEAKER_00Well, you go to Yosemite quite a bit, and this summer was just the most recent out of uh many trips.
SPEAKER_01Yeah, so I was really all fired up about this um because this place is one of the most beautiful, iconic places on the planet. I mean, it is it is just I remember the first time going into the park, and it takes your breath away. It is so different from any place that I'd ever been before. Um, and so I started diving into and organizing, like, all right, how we're gonna talk about the geology of this amazing place. And I just couldn't come up with anything. There was no hook that just stood out. It was it was really, and I don't mean this in any kind of offensive way, and I'm shocked that I'm gonna say this, but the geology was actually kind of it was a little bit boring. I'm like, I I tried working on this. I'm like, how do we how do we make this exciting and so on? So anyway, I I brought that back to you. And you said, well, you know, there is a there's an angle on this though that Yosemite is full of granite, but there's a lot of debate on how this happened with current research. And so we started talking through that, and then it became really interesting.
SPEAKER_00Yeah, I think I mean I'm kind of the exact opposite. I've actually never been to Yosemite National Park, hoping to go this coming summer. But I have a uh a PhD student who's working on rocks from Yosemite right now because the location is such a classic place for studying the rocks that are exposed there, which are called granites, and we're gonna get into. And it's really the focus of a lot of debate about a lot of scientific debate about what the heck a granite represents.
SPEAKER_01So the student, this PhD student, is this person doing like radiometric dating on the granites then? What like what is what what is this person trying to work out?
SPEAKER_00Yeah, we're trying to work out sort of um what the granites looked like and how they formed. And some of the key things that we're gonna talk about today are questions that hopefully, you know, Eric, uh PhD student working here at Penn State, will will sort of tackle in a different way. Um, not so much geochronology, but a lot of geochemistry, a lot of sort of in the weeds geochemistry that we don't need to get into, but interesting.
SPEAKER_01Yeah, yeah, right. Let's not do that.
SPEAKER_00Yeah, yeah.
SPEAKER_01Um, but uh one quick question then. So you want to go there this summer, is that to do research? Then that would be a part of your research, or are you going there with tests?
SPEAKER_00Well, no, not unfortunately, not with tests. She's too busy. But uh no, this is this would be research and fun. The the benefit of my job is I get to pick places I want to go study and uh, you know.
SPEAKER_01That's awesome. That's awesome. Hey, all right, so then are you allowed? How do you do how do you sample?
SPEAKER_00We can get research permits. We have to apply. And you'll be able to collect some stuff then? Yep, we can apply to the federal government or the National Park Service. And usually they're they're pretty um, if you have a valid reason for it, they're they're pretty open to that. Okay, cool. Yeah, maybe you want to come along, Chris. I would love that. I'll hey, I'll uh I'll be your Sherpa. What do you need carried? There we go. We need a pack mule. Uh you're as good as any, I think. I am I'm better than most. So where are we going with this, Chris?
SPEAKER_01Yeah, let's organize what we have going on. So we're gonna begin by talking about the formation of the granite and the current research and the debate, quote unquote debate. Okay. We're gonna talk then about the uplift a little bit just in terms of how the granite got to the surface, and then we're gonna get into like the really interesting part of the geology, the part to me anyway, that the interesting stuff, which deals with the glacial features, the exfoliation of the granite that is all over the place in Yosemite National Park, and then a little bit about the rockfall that is just ever present in Yosemite National Park.
SPEAKER_00Yeah, and we're gonna kind of spend a lot of time talking about this granite part. That's actually the rocks, like how the rocks themselves formed. And then the last couple pieces are the landscape. You know, why does it look so spectacular?
SPEAKER_01Yeah, wait, yeah. Why is it so pretty? Oh man, yeah, that's right. So okay. All right, well, let's go, Jesse. So why don't you go ahead and lead us into this debate about the granite and what's going on?
SPEAKER_00Yeah. So the rocks in Yosemite Valley, the rocks themselves represent Hold on, I'm gonna interject one second, okay?
SPEAKER_01And I'm gonna apologize right up front to the listener because man, uh like uh you're sitting on the edge of your seat right now, and uh you're gonna nerd out on us. This is gonna be a hard one for me. My job in this episode is to corral you. Yeah, yeah.
SPEAKER_00Hopefully you got the leash in hand here. Um because uh you're gonna have to keep me keep me uh from diving too deep here. Okay, so let's frame it here. The rocks themselves, you pick up one of the rocks, and that represents that that rock composition is very common on Earth, and it's kind of what makes Earth unique compared to other planets in our solar system, is this rock called granite. It makes up a lot of the continental crust, uh, it makes you know forms in subduction zone settings like the ancient Sierra Nevadas used to be. And so the they're really unique to Earth, they're really critical rocks to understand how they formed, but there's not a lot of agreement. And I want to say up front here, I don't have an opinion. I'm just gonna try and just present the debate here, and it's a pretty um aggressive scientific debate in some cases. Uh, you could go see people yelling at each other at conferences about this. I'm gonna remain as agnostic as possible here.
SPEAKER_01And you're you're entering into the fray, aren't you?
SPEAKER_00Well, maybe as a little side tack into the fray.
SPEAKER_01I can't this is exciting research, though. That's that's awesome.
SPEAKER_00Well, we'll see.
SPEAKER_01Um, so first of all, you know, I think why this is not well understood, which is actually kind of shocking to me, is because of where these things form. I mean, granites form from magma, okay, and magma is molten material that is like forming, intruding, and cooling miles beneath the surface. So this is not a process that we can, you know, directly observe.
SPEAKER_00Absolutely. And the rocks themselves, they look like they did that. They look like they crystallize from liquid down deep in the earth. The minerals kind of grow and interlock with one another. They make for really hard rocks. They're interlocking and overgrowing one another. They just kind of filled up the space. So the you know, imagine a liquid that cools and crystallizes, it just fills up the space that it's in.
SPEAKER_01But as it's doing this, as it's cooling and crystallizing, the chemistry is always changing, right? I mean, if you think back to our Bowen's reaction series episode.
SPEAKER_00Oh, yeah, good shot on Bowen's reaction series.
SPEAKER_01Yeah, right on, right on.
SPEAKER_00Yep.
SPEAKER_01You know, this order of crystallization, the order of melting, the order of crystallization, and actually both of those play into the story of the way these granites formed.
SPEAKER_00That's exactly right, Chris. And because that in a traditional interpretation, we get these what are called zoned intrusions. So an intrusion is just a big batch of magma that comes up. And because of Bones Reaction Series, the idea is that the outside of those intrusions, the ones that cooled first, are a different composition than the inside. We kind of get more felsic, more silica-rich, lighter colored rocks on the inside and darker on the outside.
SPEAKER_01Okay, real quick then, I want you to go back. What I th you just defined it, but it it wasn't really clear. Like what is zoning? Okay, you talk about a talk about a crystal, and what does it mean if that crystal is zoned?
SPEAKER_00Yeah, so it's basically like a tree ring. You have stuff that grew, it started from growing on the inside, and then we have uh this record of growth as it grew outward. And so you know, think of it like a tree ring, and you can think of it one individual mineral grain that grows that way, or you can think of it like a magma chamber. It actually happens in the opposite way. Think of a magma chamber um as this big I got an idea.
SPEAKER_01Can I interrupt you here?
SPEAKER_00Yeah, go for it.
SPEAKER_01Like it's it's more like building a wall. If you're building a brick wall, okay? You start building the wall out of one color bricks. Let's say you start with red bricks and and then you you use up all your red bricks, and now you have to go to yellow bricks, and you so you're you're building the wall, but the wall is changing uh its composition as you build it. And that's what happens with zoned crystals, right? The the chemical composition is not the same in the middle of the of the crystal as it is on the outside of the rim of the crystal.
SPEAKER_00Exactly. And then in a magma chamber, it happens sort of the opposite direction. You start growing stuff on the outside because it's this batch of of magma that intrudes big ball of magma sitting down in the earth, it crystallizes from the outside in because the cool stuff is on the outside. So it kind of the cool stuff is one color in composition, the inner stuff, the hotter stuff is is uh, or the stuff that stays hotter longer is lighter colored in a different composition than the outside. So you get this zoning. When you bring that to the surface, cut it in half by erosion, look at that in a sort of map view, you get this bullseye, which is called the zoning there.
SPEAKER_01Yeah, that's cool.
SPEAKER_00So the way that that zoning happens is that the most primitive or the most primary, the sort of magmas that are most similar to their initial composition, start to crystallize first and form on the outside. So they form this rind around the magma chamber, and then it gets more quote unquote differentiated or more felsic or more evolved. Those terms all kind of mean the same thing, more evolved and lighter colored on the inside. So this kind of as it's crystallizing, it's changing composition. The residual stuff is changing composition.
SPEAKER_01Okay. I gotta I have to interrupt you here. Uh, you said a couple things. You said a primitive or primary rock. What does a primitive rock mean?
SPEAKER_00You can think of the earth as this big distillation chamber, and the mantle is our starting material, and the mantle gets distilled in one step to form oceanic crust, the the crust beneath oceans, which is dark colored, which is called basalt. That gets distilled again to form continental crust. So the second distillation step is going from you know stage one to stage two, it's enriching the continental crust in a whole bunch of stuff, just like a distillation thing.
SPEAKER_01Okay, so it's going basically from mafic to more felsic, the more distilled it gets.
SPEAKER_00Exactly. And we use the term primary or primitive to mean a more mafic rock or the starting composition. Whatever it started from, that's kind of primitive or primary. And then it it evolves by this chemical distillation to the sort of evolved compositions.
SPEAKER_01Okay. So again, if you go back to the Bones reaction series, if you take mafic basalt, which is rich in iron and magnesium, and you partially melt it, then you will generate a more felsic or an intermediate composition magma. Exactly.
SPEAKER_00That's what you're talking about. Exactly. Okay. Yep. So that's a distillation step in there. Yep. So Chris, how how do I mean the traditional interpretations of these zoned magma chambers are rocks like Yosemite National Park? You know, can you describe that?
SPEAKER_01Yeah. Yeah, because this is right in my wheelhouse. I mean, this is this is what I teach, right? And you're you're telling me that, hey, yo, this might be uh a little bit off. Yeah, right. You're you you're changing things up on me. So yeah, traditional interpretations are and we've you know, Jesse, we've talked a lot about this. We just did an episode on volcanoes and how the way things are traditionally done sometimes isn't adequate. And that same thing applies to what we're doing here. Traditional interpretations say that you have this big batch of magma that came from way down deep.
SPEAKER_00And we when we say big, we mean freaking enormous. We're talking like a hundred kilometers wide by like 50 kilometers long, uh, you know, and several kilometers deep. I mean a huge volume of magma.
SPEAKER_01Yes, correct. And so this batch of magma comes in, it intrudes the country rock. That's the rock that was there, and that rock then will be usually metamorphosed because there's lots of pressure associated with this, lots of heat, obviously, because we have a bunch of magma, and it just cools slowly. Okay, and that's the traditional interpretation of the way that granites form.
SPEAKER_00And then that single batch of magma does this internal distillation. So it starts out on the outer side, that's how we get this zoning pattern, the sort of dark stuff on the outside, lighter colored rocks on the inside, on a huge scale, hundreds of kilometers long, a big zoned magma chamber becomes a zoned group of rocks, like Yosemite National Park.
SPEAKER_01If I can just maybe paint a picture, if we were in the mountains then and we found the contact between the metamorphosed rocks and the granite, and we're at that contact, and we started walking away from the metamorphic rocks then toward the center of the granite, the composition would change, the color of the granite would change.
SPEAKER_00And that's exactly what we see in Yosemite National Park, in some way, is you go from darker rocks on the outside to lighter colored rocks on the inside. There's something in the textures change and the minerals change. So there's a chemical difference across this, which is this quote unquote zoning in here. Now, this is a little bit problematic because there was a bit of a revolution that kind of started around about 2004. Um, there's a couple papers that were published that went and sampled the dark-colored rocks on the outside, the light-colored rocks on the inside of this huge zoned rock complex that is Yosemite National Park. So the outside of the park to the inside of the park, different rock types, they sampled those things and dated them using high precision geochronology. We talked about geochronology before. We won't get into it too.
SPEAKER_01You are just talking a thousand miles per hour. Oh my gosh. All right, all right. Let me you need to hey, let me hit the point. How much coffee have you had this morning? Like, holy crap.
SPEAKER_00It's pure science that's got me excited here.
SPEAKER_01You're getting all revved up.
SPEAKER_00I'm getting all nerdy.
SPEAKER_01Okay. All right, let's let's let's pick it up now, okay?
SPEAKER_00Okay, you're right. Right. Deep breath, deep breath. So the main point is that the outside rocks formed about 10 million years before the inside rocks. Hold on. I want I just want to hear that again. Go ahead. Yeah. The the outside rocks. So the outside rocks formed about 10 million years before the inside rocks. And there's a progression as you work from the outside to the inside and like the five rock types that are exposed there, they all have different ages and they span a 10 million year age range. So each one has a distinct age. One is like 92 million years old, one is like 86 million years old, one is like 84 million years old, the inside one's like 82 million years old. So what's the what's the problem with that? The problem is that that 10 million years is way, way, way too long for this magma to sit there and you just can't have this magma chamber. It's too long for the magma to sit there and cool down, basically.
SPEAKER_01Okay. Um, is it though? Is it too long? Because our listeners and and me, we're sitting here saying, wait a minute, this forms miles beneath the surface. So this is really, really insulated. It's a massive intrusion, okay, and it has miles of rock all the way around it. So this is gonna cool off slowly. Uh like, is it really too long?
SPEAKER_00Yeah, and uh the answer is absolutely it can cool off slowly, but the most conservative estimates for a cooling down of a magma chamber like that are on the order of a million years, not ten million. So this is like ten times too long for even the most conservative estimates for how long a magma chamber like that would take to cool down.
SPEAKER_01Okay. So what's the interpretation then?
SPEAKER_00What's going on? What do they think? So the interpretation here is that this magma chamber is not one big magma chamber. It didn't have we didn't have a hundred kilometers wide by fifty kilometers deep, by uh we didn't have this huge batch of magma sitting there and cooling down for a long period of time. Instead, it occurred in little pulses. So little fits and spurts of magma came into the system progressively over 10 million years. And so this is like a way more long-lived system and a way more sort of active, active meaning like little little blurps of magma.
SPEAKER_01Okay, do you think then if this is the way it's gonna be interpreted and this is then gonna change our thinking on these kinds of things, right? Is that gonna change then a lot of other places?
SPEAKER_00Yes, exactly. And it did. And this was 2004, so this was a while ago. People have gone to similar uh locations, similar zoned plutonic complexes like Yosemite, done the same type of analyses and found mostly the same thing. These things take a long time. Do you have an example? Uh there's one in northern Italy called the Burgel Intrusion that has taken a similar amount of time. Um, there's some in the Andes that have a similar type of thing, and people have actually gone and looked at the volcano, well, the the eruptive products. So this is basically Yosemite is the roots of an ancient volcano. And so people have gone to places like the Andes, where there are actually volcanoes erupting, and um, and they've seen the same kind of thing. These lavas erupt over 10 million years, and so the idea is that, yeah, this is a reasonable uh assessment. So that's kind of the first scientific revolution in how these things formed. We didn't have a big batch of magma sitting there, we had what people call a magma mush, which is basically a bunch of crystals with a little bit of magma in between them.
SPEAKER_01Okay, so you said that was really kind of the first round or the first thing that was going on and started back in 2004. So what's next then? That implies that there's some other thing that's come out of this. What is it?
SPEAKER_00Yeah, so what's come out of this? That there's another, I would say, revolution or hotly debated thing right now going on in in the scientific literature. And this is we know now that these rocks take a long time to intrude. Um, but the question now is about the textures. If you take that rock and look at it, we said earlier that the rock looks like it grew from a liquid magma, that the minerals are interlocking with one another, you know, there's no space in the system, it's very massive, it's very solid, very hard. There's currently a debate about whether those textures are actually magma and magma flowing around. Like if you have a big bunch of magma with some crystals in there and it flows, the crystals will become oriented. In some places, you see that orientation, but there's a whole nother group of people who argue that those textures are not a magma flowing. You can generate some of those textures by having temperature differences, big temperature gradients or big compositional gradients, in that it's all actually what we call solid state. Everything's happening as a solid mass. There's no there's not a lot of liquid in the system when this stuff is going on. So it's a little bit in the weeds there, but they're arguing about you know whether these little pulses of stuff coming in were actually magmas that were flowing around, or whether it's just fluids flowing through rock and doing a lot of alteration, basically.
SPEAKER_01Okay. So basically, and and cut me off if I'm wrong, but when if you take like these these hot superheated fluids and circulate them through hot minerals, it can cause change. And it can cause some melting too, like localized melting.
SPEAKER_00Exactly. So it can cause localized melting, or more importantly, it might not even do melting. It might just be a totally solid state mineral transformation. So it could chemically overprint the mineral without ever dissolving it and making a magma, if that makes sense.
SPEAKER_01We're we're in the weeds with some chemistry here, but yeah, let's let's we need to stay out of that, I think, a little bit. But let's uh I want to ask you though. So coming out of this discussion, in a traditional geology class, we're taught that with an igneous rock, the number one factor that controls the texture is the rate of cooling. Is that changing?
SPEAKER_00No, at a general level, that remains true. Obasol, okay.
SPEAKER_01So let me explain what I meant. Yeah. Just for the listener a second. So what I mean is basically igneous rocks, the minerals grow larger if they cool slowly. Minerals need time to grow. So with slow cooling, you get a coarse grained, or the minerals get big enough and they interlock with each other, but they're big enough to see without magnification, unaided eye. And and rocks that form from lava, which are outside, they cool much faster. The they cool faster, the minerals then don't have time to grow, and it stays what we call a fine-grained texture or an aphinetic texture. That's ch is that gonna change.
SPEAKER_00No, that's not that's not gonna change too much. I think it's more an argument about the big crystals and whether the big crystals can grow sort of in place, basically, or whether they need to grow from a magma chamber. And I have to add a qualifier in here, Chris. You know, I love my qualifiers. Is that out? I'm not in this debate. Uh there is a debate. I'm kind of painting two end member sides of it. There's a bunch of people in the middle, two sides don't agree.
SPEAKER_01Um aren't you gonna enter the debate though with your research?
SPEAKER_00Kind of. We're gonna try and stay on the side.
SPEAKER_01And um I I think you know both sides are are I I can't wait to go to a conference and listen to you speak and get yelled at by somebody else.
SPEAKER_00I I think I'll I'll try and stay out of that. Maybe send a student up there to get yelled at. But um going to a conference and watching people yell at each other about this is quite funny. Oh, it's it's really interesting.
SPEAKER_01I'll tell you what, I've seen that. I've seen that at GSA before. Oh my gosh.
SPEAKER_00Like it it gets heated. It gets heated, yeah, it gets heated. Lots of ideas. And I think so. Let me just kind of so I I know that I'm in the weeds here with the with the science of it, and I'm kind of excited about it. But it is important because we're talking about temperature differences here in these systems and what these systems actually look like. And there are lots of what are called IOCG or deposits, iron, copper, and gold deposits that form above these types of magmatic systems with a lot of hot fluids circulating around and concentrating gold and copper. So there's a lot of important deposits that occur in these types of locations. And so that understanding how the magma system is working and what that is, like what the magma system looks like, is really important for understanding how those types of deposits are formed, as just one example. So there's some actually like take-home sort of real-world um, you know, meaning behind this or or interest, yeah.
SPEAKER_01Because if you have hot fluids flowing through this rock, so these precious metals like you know, copper and and gold, they're pretty soluble in hot, salty water. And so as they flow through these like vast volumes of rock, they're selectively dissolving these elements then. And then on the margins, when it cools off and the conditions change, they get concentrated and precipitated there. That's what you're saying.
SPEAKER_00Exactly. Yep, that's exactly right. Suddenly the conditions will change in that fluid. Boom, they dump out all those interesting and important metals, and then we can go mine them uh efficiently and economically.
SPEAKER_01So there we go. Hey, listen, planet Geo at its best right here. You know, what are we doing? We're explaining like these super important resources and how they form, and that then shapes where we go look for this stuff.
SPEAKER_00Absolutely. And so we're talking about the roots of that system, right? Like, where is the temperature coming from, where are the fluids coming from? It's all related to how these granites form. So when you're walking across Yosemite National Park, which I've never done, I can't wait to go do, but you can walk across so you can see the rocks, you can see these big, massive white feldspar crystals. There's a big debate about how those form. Like that is not well understood at all. And I think it's a really interesting and important thing to understand that like we don't really understand those minerals in some ways. That's really cool and really exciting. We'll have to go, Chris. We'll we'll have to go to your summit.
SPEAKER_01I can't, but Jesse, I'll tell you what, that would be an absolute riot of it.
SPEAKER_00Okay, let's let's do it. Let's do it next summer. Let's uh we'll go collect some samples. We'll bring the we'll we'll buy a little uh remote mic or a little uh you know on-site mic that we can buy. We can record a little episode while we're standing on the rocks sampling. Let's do it. I got the mic. Yeah, I'm all set, I'm ready to go.
SPEAKER_01All right, hey, so let's transition now. Okay, so we talked about this. That's the the interesting part of the you know, the geology of all of this just massive granite that you get in Yosemite. So this granite forms really deep inside the earth, miles below the surface. So after it cooled and and hardened and crystallized and did what it what we just talked about, then it has to get brought to the surface. So, you know, much more recently, anywhere from what what what, Jesse, 25 to 35-ish million years ago.
SPEAKER_00Yeah, somewhere around there, you see we've seen estimates of 40, we've seen estimates of 25 or even 10, so somewhere around that that many million years ago.
SPEAKER_01This gradually then gets brought to the surface. And so all of the metamorphic rocks that were formed due to this intrus these intrusions that happened, most of those metamorphic rocks are gone. All of that country rock that kind of like encapsul encapsulated this this intrusion.
SPEAKER_00And all the volcanic rocks that were erupted on top of this thing, the ancient volcano system that these that we're seeing the roots of right now are all gone as well.
SPEAKER_01Most of it is gone. There's there just you have to really be looking for this stuff in order to find it. So it's all been removed.
SPEAKER_00Yeah, and I want to make one extra point here is that the stuff we're talking about with the Yosemite National Park applies to most of this here. Nevada's there, all the way down to like Mono Pass and Mount Whitney. They all have the same kind of composition of rocks because they're the same routes to this ancient volcanic system.
SPEAKER_01Yeah, it's it's absolutely uh unbelievable in size, it's hard to comprehend. So, all right, the granite then eventually gets it's exposed at the surface, the overburden is gone, it's still going on today, they're still being uplifted, you know, and this is a slow but continuing process. And and so then we can transition into a much more recent part of the geological story of of Yosemite, and that's the glacial stuff that happened.
SPEAKER_00And the glacial stuff, I mean, this is like uh uh the type locality for glacial features. I mean, maybe the Alps are probably the same thing, but it's beautiful, and the glaciers really sculpted the landscape and made it what it is today, right, Chris? So, what are the most prominent features of glaciers flowing through here?
SPEAKER_01Yeah, the features with this place, they stand up and they smack you right in the face. I mean, if you if you're standing anywhere around Yosemite Valley and you don't recognize a U-shaped valley, then something's wrong. You know, it's it's really, really clear.
SPEAKER_00And that's a pretty self-explanatory term, a U-shaped valley. It's a valley that's shaped like a U. And the important point here is that a glacier cuts a U, a river cuts a V into a valley. So a river cuts down at a really narrow part, makes a V, a glacier scrapes away the sides as well, and so it cuts a big U.
SPEAKER_01Yep. It basically glaciers, these alpine or valley glaciers, what they do is they're gonna widen and straighten a river valley. And and it's just so obvious. These valleys are immense uh in in width and and breadth and everything else, and they're straight. And so that's the you know, that's one of the things that is just striking you. And the other thing, or another thing that's really striking with Yosemite are all of these hanging waterfalls that you get, like Bridlevale Falls, Yosemite Falls, they're all over the place.
SPEAKER_00And so what Chris explain to me the hanging, why why a hanging waterfall? What's going on there?
SPEAKER_01A hanging valley is well, imagine this this whole landscape is just full of ice, right? And and you have these, just like with rivers, there are tributary rivers that these small rivers that contribute their water to the larger system. You have the same thing with glaciers. So you have big uh valley glaciers and and smaller valley glaciers, and the small valley glaciers merge in with the big one. Well, small glaciers carve smaller, you know, and and not as deep U-shaped valleys. So when you melt all the ice away, you have this glacier, this valley glacier, this U-shaped valley that gets truncated or cut off by the main trunk, and and it's just hanging above the main valley.
SPEAKER_00That's right. And the difference between river tributaries, which kind of flow into uh, you know, a small river flows into a big river and they just kind of merge, it's relatively um, there's not usually a waterfall, not necessarily a waterfall there, but with a U-shaped valley, because the big trunk of the glacier is cutting the sides so much more, it basically the small tributary flows in and it flows into only the top. So you have this huge cliff, this vertical cliff on the big part of the glacier.
SPEAKER_01On the valley wall of the big valley glacier.
SPEAKER_00Yes, exactly, because that part of the glacier is so much bigger, cuts so much deeper, and also cuts out the sides as well.
SPEAKER_01Yeah. So now all the glaciers are gone. The rivers have returned to the valleys, right? And so when these rivers flow over the lip of that cliff, it forms these spectacular waterfalls. And you know, that's it's one of the main attractions of the place, is all the all these waterfalls that are just absolutely amazing.
SPEAKER_00So stunningly beautiful.
SPEAKER_01I can't wait. I can't wait to go see them. Yeah, oh man. You know, you have um also a lot of horn peaks that that form and and horns form when you have these like several glaciers that are moving away from a central high point, they're scouring this out. And you know, Jesse, basically glaciers make mountains beautiful. You know, non-glaciated mountains would not be as angular and carved and jagged and cut up like they like these are. And and so that's what these horn peaks, uh, like I think of uh Cathedral Peaks up by Tuolumni, that part of the park. It's just uh they're amazing, just unbelievable. Yeah. So those are one more. Oh, I want to talk about one more. Okay, uh because there's a famous place in Yosemite, it's called Clouds Rest. And Clouds Rest stands when you're on top of this mountain, you're higher up than Half Dome, and you have a great vantage point of Half Dome off in the distance. Okay, and it's an arette, which is this uh sharp knife-like ridge that separated two valley glaciers that were basically um close to parallel to each other. And so they as they scoured their valley walls, they left this like narrow, sharp knife-like ridge, and it's called Clouds Rest. And uh, it's so it's amazing. Uh so cool. We went there this summer up to the top of Clouds Rest, and uh Bella, my daughter, her boyfriend, had never like you talk about getting thrown into the lion's den. It's like your first mountaineering experience with the bull heistes.
SPEAKER_00Um yeah, it's a pretty intense group of people.
SPEAKER_01So anyway, we're we're just walking up this this, you know, there's no room to fall or anything. You know, it's you don't you have maybe two or three feet, right? That's all you have.
SPEAKER_00You're pretty exposed, right?
SPEAKER_01You're you're exposed on both sides. Jenny's walking around with her phone out, you know, and and Bella's boyfriend Brandon is just absolutely petrified. He's got the old shuffle going on. He's he wanted to just get down on his hands and knees and crawl. He was so scared.
SPEAKER_00So it that's the exposure on a rett's can be uh quite intense. It is, it's amazing.
SPEAKER_01Um, yeah, so that's it's another just amazing feature of the gym. And then all the lakes, almost all the lakes in Yosemite are glacial.
SPEAKER_00So that means that they're they basically are water that has filled up places where glaciers were eroding out, basically.
SPEAKER_01Yeah, they plucked out the bedrock and left these depressions, you know, and and it's really kind of a cool thing, uh, this evolution of the landscape, then, if you will, because uh, you know, these lakes then that are glacial in origin, which is not, you know, this didn't this wasn't too far in the past, right? Geologically speaking, this is a blink of an eye. So these lakes now are filling up with sediment, you know, and see that and you can see that if you if you know a little bit about the geology as you're walking through this backcountry there, you can see these wet areas that that used to be a lake, but it got filled up with sediment.
SPEAKER_00Very cool. All right, Chris. So those are the landscape features which are dominated by glacial features or rocks that have been eroded out by glaciers. What other sort of spectacular geological things can we see in Yosemite?
SPEAKER_01Yeah, I one of the just most striking, like this is textbook uh example of it's called exfoliation, these exfoliation joints.
SPEAKER_00And this is a weathering feature, this is rocks breaking down on the surface.
SPEAKER_01Right. And the process ties right into what we've talked about already, you know, that granite forms really, really deep and it has all this rock around it. Okay, and then it that granite then eventually gets uplifted and exposed at the surface. But all of that overburden, that weight of all that rock has gradually been removed. And so if you take this confining, squeezing force and you remove it over time, then what does the granite do? Well, it expands. Okay. The stuff that's at the surface is expanding, but it's expanding differentially. Uh like I want to uh see if this makes sense.
SPEAKER_00Yeah, I think it it kind of relaxes, I guess. It's under pressure, it's under this intense pressure, and then it just kind of relaxes.
SPEAKER_01It does, it wants to expand, but the granite that's deeper down is relaxing, but not as much because it has granite on top of it yet. It still has that confining weight on it, right? So you get this differential expansion and the surficial granite. Go ahead.
SPEAKER_00Let me let me interject like a thing here. It because we're talking about this, doesn't necessarily make intuitive sense to a lot of people, but imagine, you know, putting a big rock on your chest, right? I don't know, put a put a steel weight or something like that. You're in the weight room, put a big plate on your chest, right? You feel that now. If you put more on, you're gonna feel that more. That's a lot of pressure, right? So underneath of the earth, these rocks are under a lot of pressure, and that's not intuitive all the time. So the deeper down you go, the more pressure these rocks experience, the more what are what's called confining pressure, they are squeezed by the weight of stuff above them and the weight of stuff around them as well. So when you bring rocks from down deep up to the surface, that pressure is released and it's released kind of gradually, and the rock relaxes down.
SPEAKER_01Right. But it's rock. And rock can't just expand, it can't just like take a deep breath. You know, you inhale a lot of air and your chest kind of heaves, right? That's what the granite is trying to do, but it can't just do that because it's solid, brittle rock. So it's building up this pressure, this expansion, and it builds up, builds up, builds up, and eventually it just cracks, it spalls off.
SPEAKER_00But it's building up tensional pressure, which is it's trying to relax, but it can't like it can't just, as you say, ooze out. It has to break. So it relaxes by breaking off little slabs, right? And those slabs are really beautiful and they add a lot of texture to something like half dome, right? Where you can see these like layers kind of peeling off. It's like an onion ring, right? You're peeling off the onion ring slowly as these layers get relaxed and break off.
SPEAKER_01This has actually been documented just outside of Yosemite National Park at a play place called Twain Heart Rock. And it is these these people were just Oh, is this this video that was uh circulating YouTube a couple years ago?
SPEAKER_00This is amazing. I forgot about it. It really is.
SPEAKER_01They got it on their cell phones. We were walking around on this rock, this outcrop, this granite, right? And they heard it start to snap, crackle, and pop. They just kind of like this, just kind of t t t, you know. And so they get their phones out and they start recording this.
SPEAKER_00It's it's actually pretty loud. Like it's like oh boom, boom.
SPEAKER_01It's a violent boom, but but no, but what was leading up to it though, this kind of like you could feel it, it was moving, it was it was about ready to break, you're right it's kind of like if you take a stick and bend it before it breaks, it does this kind of snap, snap, snap, you know, and you know it's about to break, and that's what was going on with this rock. And um, so anyway, it exploded, it exfoliated right in front of them, and they got it all uh recorded also.
SPEAKER_00And they it's a it's a violent thing. I mean, it is like bam, and there's dust everywhere, and the rock is actually breaking. I mean, huge, like huge flake of rock is breaking.
SPEAKER_01Yeah, if you Google exfoliation and twain heart rock, you just can't miss it. It's a minute long.
SPEAKER_00That's right. And the exfoliation also uh makes cool climbing routes uh in Yosemite as well, right? Like a lot of the cracks that people climb in Yosemite are all exfoliation joints, these cracks in the rock that are that pieces are exfoliating off of the central part of the rock unit.
SPEAKER_01And any image that you see of Half Dome that looks like like a layered onion, you know, and that's that's all exfoliation joints. Um, and you just can't get or you can't get away from it in Yosemite. It is literally everywhere, these exfoliation joints. And it's it's kind of cool too. You know, if you're walking across this the surface of an exfoliated joint and you take a hiking pole and you like kind of bang it on the rock, you can hear that it sounds hollow below the rock.
SPEAKER_00Yeah, yeah, that's super cool. So cool. So that's exfoliation, and that's everywhere in Yosemite. What about the last one, Chris? Here, uh rock fall, which kind of goes hand in hand here a little bit with this.
SPEAKER_01It kind of does. Yeah, there's so much rock fall in Yosemite, be and a lot of it is because of these exfoliation joints. The rock is literally detached, right? Well, if you take detached rock, put it on a steep slope, and then add in water. Okay, whether you have water that lubricates the rock or whether you have water that is freezing and thawing, freeze, thaw, freeze, thaw, uh, this kind of ice wedging thing can kind of pry the rock loose and it slides down slope, and then it comes off the cliff of you know, and it lands in the big U-shaped valley below. That's a rock fall. The very common in Yosemite. Actually, I've never witnessed a rock fall in Yosemite, but there is a rock fall on average, one every five days.
SPEAKER_00Wow, that's and these things are aggressive when you hear that. I mean, this is rock, huge amounts of rock falling straight down and hitting the valley floor, and it reverberates around. And yeah, I've I've seen some again, there's videos.
SPEAKER_01Again, if yeah, if you Google rockfall in Yosemite, the amount of video clips that will show up is truly amazing.
SPEAKER_00I mean, it's a it's a very common thing. And these last two, exfoliation and rockfall, and really glacial features as well. This is the you know, one side of this battle on Earth between erosion and tectonic forces. So tectonic forces are trying to push mountains up, erosion and weathering are trying to knock them down as much as possible. And so these these last two exfoliation and rockfall are really representations of this the environment, the atmosphere, the surface environment, trying to knock down mountains. Yeah.
SPEAKER_01Yeah, that's that's a good point. And one of the coolest things, I will also want to touch on this with the rockfall, these slabs come cascading down into the valley below, and before they hit ground, the amount of air that is being shoved out from underneath the slab of rock is enough to snap off these massive trees.
SPEAKER_00Really? It's oh wow, that's cool.
SPEAKER_01Oh, yeah. It is unbelievable. It the the wind that comes off.
SPEAKER_00The wind from the wind escaping. Wow, that's really cool. Okay, that's interesting. So you see like a rock fall with it, it looks like a bomb around it, like trees are knocked down around it because the air got compressed and shoved out of the way. That's super cool.
SPEAKER_01It's one of the ways that they assess rock fall uh potential hazard is by looking at the trees and how old the trees are. They're able to say, well, this area it doesn't have a lot of rock falls versus an area like this because look at all the trees that have been snapped off. And you know, so they're kind of going back in time.
SPEAKER_00Now that is totally cool. That's a cool thing. I did not know that. That's awesome. Very cool. All right, Chris. Well, I think that's a wrap here uh for Yosemite National Park. And all I want to say is I can't wait to go.
SPEAKER_01Uh it is uh you'll never forget the first time. It's uh it's just one of those things, it's an emotional experience. I don't know how else to say it.
SPEAKER_00It's uh incredible. It's amazing. I'm excited to go there. Um I love the science that's going on there. Uh it's a really exciting place. And and I think just walking around, you can look at the rocks, you can look at the glacial features. It's it's just one of the places where geoscience, the processes of geoscience are right in front of you. There's you cannot escape them, and they're so dead obvious, right? It's a textbook place for many, many reasons. Yep, you got it. All right, well, hey, that's a wrap. All right, man. So that's a wrap on Yosemite, and actually tune in next week because we're gonna talk about some hiking recommendations, right, Chris? You you've been there a lot, you've hiked all over Yosemite, and uh, we're gonna go through places, certain hikes you can go to see these features that we've just talked about. So tune in next week for that. All right, sounds good. You can find us on social media. We are at Planet Geocast, and please do like, subscribe, give us a review, and share with your friends. We would really love that.