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So... You Think You Know Plate Tectonics? Part 1 · PlanetGeo: The Geology Podcast

0:03
SPEAKER_01: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
0:13
SPEAKER_00: Well, Dr.
0:15
SPEAKER_00: Rymank, how are you doing?
0:17
SPEAKER_00: Christopher, hi.
0:20
SPEAKER_00: Are you in a better mood now?
0:22
SPEAKER_00: Um, so I don't know.
0:23
SPEAKER_00: Not really.
0:24
SPEAKER_00: I don't know what's going on.
0:25
SPEAKER_00: I'm I rarely do I find myself in the mood I'm in.
0:29
SPEAKER_00: I don't know.
0:29
SPEAKER_01: Cheer me up, Jesse.
0:30
SPEAKER_01: I'm so excited to spend an hour with you today.
0:33
SPEAKER_01: This is just about the best.
0:35
SPEAKER_01: Chris sits down and he goes, uh I'm pretty cranky today.
0:40
SPEAKER_01: Oh, good.
0:41
SPEAKER_00: It's great to see you too, man.
0:43
SPEAKER_00: Oh wow.
0:44
SPEAKER_00: How often do I say that?
0:46
SPEAKER_00: Ever, honestly.
0:46
SPEAKER_01: It's true.
0:47
SPEAKER_01: It's pretty rare.
0:47
SPEAKER_01: You're usually in a in a decently good mood.
0:50
SPEAKER_01: So, without much intro here, what the heck are we doing?
0:55
SPEAKER_00: Well, we you and I, I feel like we went back a year and a half in time because we actually had arguments over what we're gonna do now.
1:03
SPEAKER_01: It's still not clear if we're like we might break up over this still.
1:06
SPEAKER_01: I we haven't quite resolved here.
1:09
SPEAKER_00: I am on to something, and you need to just like relax and trust, and we'll all be good.
1:15
SPEAKER_00: But yeah, there was a point where you said, just trust me, just trust me, Jesse.
1:20
SPEAKER_00: I did.
1:22
SPEAKER_00: So we are gonna do a series on plate tectonics, okay?
1:27
SPEAKER_00: And the reason for this, I want to tell everybody like the backstory on this.
1:32
SPEAKER_01: Um I let me just interject here because you you called me in the middle of the day, which you rarely do because your schedule is like back to back to back with classes, but you called me in the middle of the day, which usually means something's up.
1:44
SPEAKER_01: And usually it's like, oh, something's wrong.
1:48
SPEAKER_01: But you were all fired up, and this is what you're all fired up about when you called me in the middle of the day.
1:52
SPEAKER_00: Yeah, so here's what got me going.
1:54
SPEAKER_00: I my blood was really going.
1:56
SPEAKER_00: I was excited and riled, and uh, all of that, all those emotions wrapped up into this thing.
2:02
SPEAKER_00: Well, here's what happened: I introduced like our next unit in my classes.
2:06
SPEAKER_00: What we're gonna do, we're gonna talk about plate tectonics, and I'm really excited because you know, plate tectonics is that was my aha moment when I'm in college and this professor just opens up about the power of the earth.
2:20
SPEAKER_00: I was hooked right there.
2:22
SPEAKER_00: That was my day, that was my moment.
2:24
SPEAKER_00: And so when I get to teach this to my students, I'm super excited about it, right?
2:28
SPEAKER_00: Well, I got this reaction, like they were not impressed.
2:31
SPEAKER_00: Like, oh, Mr.
2:32
SPEAKER_00: Bull Eyes, we know about plate tectonics.
2:35
SPEAKER_00: Like, oh, oh really do you know?
2:38
SPEAKER_01: The vein in your forehead starts to throb a little bit more.
2:42
SPEAKER_00: It's like, what do you mean you know about plate tectonics?
2:44
SPEAKER_00: Like, what do you know about plate tectonics?
2:46
SPEAKER_00: Well, we know what subduction zones are, and we know about you know convergent boundaries and divergent boundaries, and that other one that starts with a T, you know, and uh the just stuff like this.
2:57
SPEAKER_00: I'm like, okay, well, what what else do you know about plate tectonics?
3:01
SPEAKER_00: And I wasn't getting much beyond that.
3:02
SPEAKER_00: And then I'm like, okay, you know, you guys, you got the gist of plate tectonics, you got all the buzzwords, but then I just started asking them questions because plate tectonics is the unifying theory of geology.
3:17
SPEAKER_00: Well, what does it unify?
3:19
SPEAKER_00: Knowing about divergent boundaries and convergent boundaries, that doesn't unify a whole lot.
3:26
SPEAKER_01: So let me let me interject here before you get too wound up because what you sent me-I am excited.
3:31
SPEAKER_01: Yeah, no, you sent me a list of like 15 questions that you had given to your students that were like, prove to me you know plate tectonics.
3:37
SPEAKER_01: And these questions are not easy, uh, they're not super specific either, but they are unifying.
3:43
SPEAKER_01: So at first I was like, Chris, come on, man.
3:45
SPEAKER_01: Uh I don't get it.
3:47
SPEAKER_01: But you hooked me with that, what you just said right there was how you kind of hooked me saying this is unifying theory, and we have to link plate tectonics to all these other parts of the earth system, rocks and minerals and atmosphere and oceans and all these other things link it together.
4:03
SPEAKER_01: Plate tectonics links all those things, and that was the hook into kind of this thing, right?
4:08
SPEAKER_01: Like what we're introing today.
4:12
SPEAKER_01: This is gonna be a series of sort of questions about plate tectonics, misconceptions that you come across in the classroom, deeper kind of thought-provoking questions about plate tectonics that are linked together in a series, and uh we don't know how long this series might be.
4:27
SPEAKER_00: That's right.
4:28
SPEAKER_00: We don't know how long it's gonna be.
4:29
SPEAKER_00: I wrote these questions on my lunch period.
4:33
SPEAKER_00: Fuming mad.
4:35
SPEAKER_00: I wasn't, I don't get mad, I was just a little taken aback.
4:41
SPEAKER_00: You know, I was shocked actually, because I don't remember ever having this experience before.
4:45
SPEAKER_00: So I wrote them on my lunch hour, and then my my next class comes in and I get the same reactions.
4:51
SPEAKER_00: So I had these questions prepared.
4:53
SPEAKER_00: I'm like, okay, well, here you go.
4:55
SPEAKER_00: If you know everything about plate tectonics, then prove it, put it on paper, let me know, and then we can just move on.
5:01
SPEAKER_00: I will go to other stuff, I'm good to go.
5:03
SPEAKER_00: But you have to show me that you really know plate tectonics because this is the thing.
5:09
SPEAKER_00: This has me fired up.
5:10
SPEAKER_00: This is geology, man.
5:12
SPEAKER_00: And that's where it came from.
5:15
SPEAKER_00: I'm glad you said that I was able to at least convince you the hook and so on.
5:20
SPEAKER_00: Now, the thing is about this though, is that I know where I want to go with these questions, but I don't know where you want to go with this, and uh so I'm really looking forward to it from that perspective because you come at things from a very different point of view than I do, as you should.
5:39
SPEAKER_00: I mean, we live in really, really different worlds.
5:42
SPEAKER_00: So that's gonna be a kind of a fun thing, I think, to go through this series.
5:47
SPEAKER_00: They're not all my questions.
5:48
SPEAKER_00: I want everybody to understand that.
5:50
SPEAKER_01: The doctor in the house has added his so the really sort of weird questions that are really hard to understand, those are probably mine.
5:58
SPEAKER_01: But what we're gonna do here, Chris, we've kind of broken these apart into rough categories, I guess.
6:03
SPEAKER_01: Some of them are about the rocks that plate tectonics forms, which we're gonna lead into today, at least start today.
6:09
SPEAKER_01: Some of them are about how Earth is unique compared to other planetary bodies and how plate tectonics drives that uniqueness.
6:16
SPEAKER_01: We've got other things like magnetism and how powerful that was to figure out plate tectonics at the beginning, and also how it's useful for understanding the earth and interior structure of the earth.
6:28
SPEAKER_01: Those are just a couple of kind of the general categories of questions about plate tectonics that again unify our understanding of our planet by talking about plate tectonics.
6:38
SPEAKER_01: And we'll probably have some on you know, climate and oceans and the driving forces and how plate tectonics interacts with that type of thing.
6:45
SPEAKER_01: So it's gonna be It's really endless.
6:48
SPEAKER_01: So let's dive into the first part.
6:50
SPEAKER_01: But before we get to that, let's just plug Camp Geo, our conversational textbook.
6:54
SPEAKER_01: If you like Planet Geo, if you like this podcast, but you want to learn the basics of geoscience in a very structured manner with the key images that you need to have integrated within the audio content, but decoupled from it.
7:07
SPEAKER_01: Go to CampGeo, go to that link in your show notes.
7:09
SPEAKER_01: Chris, we're about halfway through building that class and more content is being posted frequently.
7:14
SPEAKER_01: Go there, let us know what you think about it.
7:16
SPEAKER_01: We're really excited that that's out there and um excited to get people using it and uh get some feedback on it.
7:22
SPEAKER_01: So let us know.
7:22
SPEAKER_00: Yeah, I think it's very exciting, so I can't wait to get feedback from everyone out there.
7:27
SPEAKER_00: Jesse, before we start, we need to do a cheers here.
7:30
SPEAKER_00: So grab your whiskey, cheers to you, brother.
7:33
SPEAKER_00: Cheers to you, man.
7:33
SPEAKER_00: All right.
7:34
SPEAKER_01: Here we go.
7:34
SPEAKER_01: All right.
7:35
SPEAKER_01: So we're gonna start with my favorite type of rocks, which are igneous rocks.
7:41
SPEAKER_01: And Chris, the first question on our list here is how does plate tectonics generate the rock basalt?
7:49
SPEAKER_00: Yes, it is.
7:50
SPEAKER_00: Long.
7:50
SPEAKER_01: That is the first question.
7:51
SPEAKER_01: That is the first question.
7:53
SPEAKER_01: Where do we go from there?
7:56
SPEAKER_00: So, my approach to this question, and I want you to jump in and interject, like just interrupt, okay?
8:01
SPEAKER_00: But my approach to how does plate tectonics generate the rock basalt?
8:07
SPEAKER_00: Well, plate tectonics begins at divergent boundaries, running through the middle of oceans.
8:12
SPEAKER_00: You have these mid-ocean ridges, and you have this asthenosphere that is rising up convectively below.
8:22
SPEAKER_01: Asthenosphere is the plastic, the hot plastic type of the mantle.
8:26
SPEAKER_01: It's the deeper part of the mantle, it is the non-plate tectonic part of the mantle, the part of the mantle that convects and moves around, but it is solid.
8:34
SPEAKER_01: That's the asthenosphere, is the word you're saying there.
8:36
SPEAKER_01: Sorry, Chris, go ahead.
8:37
SPEAKER_00: Yeah, it's rock that is solid, but it's soft because it's like right at or right near its melting point, the pressures and temperatures are just right to allow this layer to behave that way.
8:52
SPEAKER_00: So as the asthenosphere rises up underneath the lithosphere, it convectively cools.
8:59
SPEAKER_00: In other words, this asthenosphere reaches the base of the lithosphere, which is the cold, crunchy and brittle.
9:06
SPEAKER_00: It rises up below that, cools, and the lithosphere is pushed apart by this kind of convective flow.
9:13
SPEAKER_00: Anyway, you have this asthenosphere rising up.
9:16
SPEAKER_00: Well, the asthenosphere is a part of the mantle, the upper part of the mantle.
9:20
SPEAKER_00: Well, that's ultramafic rock.
9:23
SPEAKER_00: It's mostly peritotite.
9:25
SPEAKER_00: Ultramafic means super rich in iron and magnesium.
9:30
SPEAKER_00: Well, as this rises up, it partially melts due to the reduction in pressure.
9:37
SPEAKER_00: Remember, it's right at its melting point.
9:39
SPEAKER_00: So if you lower the pressure, some of the minerals are gonna melt.
9:44
SPEAKER_00: Well, what happens then is you take an ultra mafic parent rock that is partially melting, and we're essentially gonna distill it.
9:53
SPEAKER_00: We're gonna create material then, magma, that's gonna be mafic.
10:00
SPEAKER_00: In other words, it's gonna be a little bit more on the felsic side than the parent material.
10:06
SPEAKER_01: Let me just interject that really quick, Chris, and set the stage.
10:09
SPEAKER_01: Ultra mafic is one end member of composition.
10:12
SPEAKER_01: It's rich in magnesium and iron, low in silica.
10:15
SPEAKER_01: Felsic is the other end member of composition that we find on Earth.
10:18
SPEAKER_01: It is high in silica, high in aluminum, potassium, sodium, low in magnesium and iron, and there's a gradient in between there.
10:27
SPEAKER_01: Mafic and intermediate are gradients between fels.
10:31
SPEAKER_01: It goes ultramafic, mafic, intermediate, felsic.
10:34
SPEAKER_01: Those are the four categories of composition that we're talking about here.
10:37
SPEAKER_01: So you're exactly right.
10:38
SPEAKER_01: As this thing upwells, as the mantle, the ultramafic mantle rises up.
10:42
SPEAKER_01: The important point is that it decompresses, it gets to a lower pressure environment faster than it can cool down.
10:49
SPEAKER_01: Because if it cooled at the same rate, then it would actually never melt.
10:53
SPEAKER_01: But it rises up faster than it can cool down.
10:55
SPEAKER_01: It's still extremely slow, but it rises up faster than it can cool down, which means it melts.
11:00
SPEAKER_01: And partially melting, we're talking like seven to fifteen percent melt, maybe twenty or twenty-five in some rare instances, but we're talking about that amount of melting.
11:12
SPEAKER_01: So seven to fifteen percent of the ultramafic mantle is melting, and the melt, the magma is mafic now, which ends up being basalt, the rock in the question.
11:22
SPEAKER_01: How does plate tectonics generate basalt?
11:24
SPEAKER_00: And that's super important, basalt, because it's the most common rock on the surface of the earth.
11:29
SPEAKER_00: You know, the earth is 70 to 75 percent oceans, the ocean floor is made up of 99 basalt, and it's created by this divergence, which is plate tectonics.
11:43
SPEAKER_00: If we didn't have the divergence, we wouldn't have the decompression melting.
11:48
SPEAKER_00: The decompression melting takes an ultramafic parent, generates a mafic magma, and then erupts out, cools and hardens, and forms the rock basalt.
12:00
SPEAKER_00: To me, the most abundant rock on the surface of the earth, it being you know what it is, basalt, makes it important just by default, and it's there because of plate tectonics.
12:16
SPEAKER_00: What do you think?
12:17
SPEAKER_01: I like that.
12:18
SPEAKER_01: I think that's where it's interesting here, is that's how a lot of basalt is formed.
12:23
SPEAKER_01: There is basalt formed in different environments that are not this kind of upwelling thing.
12:26
SPEAKER_01: We have mantle plumes that form basalt, subduction zones, which we'll talk about in the next question, it appears, uh, also form some decent amount of basalt.
12:35
SPEAKER_01: But I think basalt's really interesting as well because a basalt occurs on other planets as well that do not have plate tectonics, but they're typically variations on basalt.
12:46
SPEAKER_01: So the basalt that we're talking about here is kind of particular to Earth in some geochemical indications that are too in the weeds for this to really talk about here.
12:56
SPEAKER_01: But this basalt is a really interesting variety of basalt that is formed at mid-ocean ridge settings.
13:01
SPEAKER_01: And again, just the plates are spreading apart, it's divergent.
13:04
SPEAKER_01: The pulling apart of the plates, stuff up wells to fill the spot that the plates left behind and it melts and forms new plates.
13:12
SPEAKER_01: So it's this kind of cycle uh that forms oceanic crust.
13:15
SPEAKER_00: That's right.
13:16
SPEAKER_00: And if you look at the Atlantic Ocean, if you look at a map of the Atlantic Ocean and you see in the middle of the ocean this kind of S-shaped ridge running through it, the mid-Atlantic ridge, that's where we're talking about.
13:30
SPEAKER_00: Every ocean in the world has something similar to this.
13:34
SPEAKER_00: This is just strikingly obvious because that S-shaped ridge follows the contour of the continents that are on the east and the west of it, you know, Africa and South America being the very striking example of this.
13:48
SPEAKER_00: That's where this process begins.
13:52
SPEAKER_01: Yes, that's exactly right.
13:53
SPEAKER_01: And I think you hit it right on the head there at the end.
13:56
SPEAKER_01: This is the first stage.
13:57
SPEAKER_01: This is the important first part of the process because our second question here how is andesite produced at a subduction zone, is the second question.
14:06
SPEAKER_01: How is andesite, which is an intermediate volcanic rock.
14:10
SPEAKER_01: So this andesite is an intermediate rock type, it's in between Mafic and Felsic compositionally, and it's a volcanic rock, so it came out of a volcano.
14:19
SPEAKER_01: So how is andesite produced at a subduction zone?
14:22
SPEAKER_01: And this is the part of the story where the basalt that we just talked about, it was being birthed at these mid-ocean ridge places, often in the middle of an ocean or somewhere in the ocean.
14:34
SPEAKER_01: Subduction zone is where it goes to die, the basalt or the oceanic crust.
14:39
SPEAKER_01: This is where it's dying.
14:40
SPEAKER_01: Is that where you're going with this question, Chris?
14:42
SPEAKER_01: Is that the kind of where we're headed?
14:44
SPEAKER_00: Yeah, and again, one other thing about basalt that's super important in this process is that it is a high density rock.
14:52
SPEAKER_00: And that's why basalt, then in a subduction zone, which is what you just said, where the plate goes to die, it's where it's gonna dive down beneath another plate.
15:02
SPEAKER_00: Basaltic crust is the one that's involved in this because of its high density.
15:08
SPEAKER_00: And acidic plates don't subduct continental crust or granitic crust, that does not subduct.
15:15
SPEAKER_00: It's gonna be oceanic basalt that is involved in subduction due to its density.
15:20
SPEAKER_00: So I want to pass this off to you actually, Jesse, because I want to hear your take on this.
15:25
SPEAKER_00: Um, I feel like you're the expert here on this process.
15:29
SPEAKER_00: So go ahead and lead us into how plate tectonics forms the rock andesite, which by the way, andesite is named after the Andes mountains.
15:40
SPEAKER_01: And that really helps sort of categorize the rock and put it into a plate tectonics context because the Andes are a classic subduction zone system, one of the classic ones on Earth.
15:51
SPEAKER_01: What's happening is the oceanic plate, in this case, the Pacific Plate, if you imagine South America, you're looking on the west side, there's all the mountains there, all this huge mountain chain down the west side of South America.
16:02
SPEAKER_01: The oceanic plate to the west of South America is diving down underneath of the continent of South America.
16:08
SPEAKER_01: Well, let's go back to that basalt, the mid-ocean ridge setting.
16:12
SPEAKER_01: What happened to that stuff in the meantime?
16:13
SPEAKER_01: It got formed at the East Pacific Rise, that's a mid-ocean ridge setting.
16:17
SPEAKER_01: New basalt was formed, erupted under the ocean.
16:20
SPEAKER_01: It's been sitting under the ocean for a long time.
16:22
SPEAKER_01: It's picked up water.
16:24
SPEAKER_01: Not carrying water molecules and liquid water, but it has picked it up in the minerals.
16:29
SPEAKER_01: The rocks at the surface have been altered, they're forming clays, they're forming a whole bunch of different hydrous minerals that have water in their structure.
16:35
SPEAKER_01: So now this oceanic plate, this basalt, is waterlogged.
16:39
SPEAKER_01: It's a waterlogged plate when it hits South America and it dives down, it's more dense, it starts to dive down.
16:46
SPEAKER_01: And on the modern earth, which is a qualifier that I think we'll get to at some point in this plate tectonics, you know, sort of path we're taking.
16:54
SPEAKER_00: Uh, and the just itching to integrate that in, aren't you?
16:58
SPEAKER_00: And then I want to talk about old rocks.
17:00
SPEAKER_01: I want to talk about old rocks, but I won't.
17:02
SPEAKER_01: I'll hold back.
17:03
SPEAKER_01: But on the modern earth, what happens is that plate dives down in the mantle.
17:07
SPEAKER_01: As it dives down, it goes up in pressure, first of all, but it also goes up in temperature.
17:12
SPEAKER_01: Gets heated up by the mantle around it, but it stays pretty cold for how deep it's diving down.
17:18
SPEAKER_01: It goes up in pressure a lot faster than it heats up.
17:21
SPEAKER_01: So as it goes up in pressure, it reaches a point where those hydrated minerals, the water logged part of the basalt, gets squeezed out.
17:29
SPEAKER_01: They're no longer stable.
17:30
SPEAKER_01: The water in their structure gets kind of ringed out of the rock itself, right?
17:34
SPEAKER_00: And so you kind of it's kind of like wringing out a sponge.
17:37
SPEAKER_00: That's how kind of how I describe it.
17:39
SPEAKER_00: I now I know that when you pick up a waterlogged saturated sponge, you're literally wringing out liquid water, and that's not what you're referring to, but the analogy still works.
17:49
SPEAKER_00: It's getting heated, it's getting squeezed, and the water that's in the mineral structures now is getting squeezed right out.
17:57
SPEAKER_01: Exactly.
17:58
SPEAKER_01: And that water is an extremely powerful molecule.
18:02
SPEAKER_01: I mean, water might there's a strong case to be made that water is the most important molecule on earth because what that water does is when you put it in the mantle, that peritotite that we talked about, the peritotite that melted as it got risen up in mid-ocean ridge settings, that peritotite, if you add water to that thing, it melts.
18:19
SPEAKER_01: Boom, instantly, almost instantly, it melts.
18:21
SPEAKER_01: Partially melts.
18:22
SPEAKER_01: Yes.
18:22
SPEAKER_01: It partially melts, not the whole thing, it partially melts.
18:25
SPEAKER_01: You add water to it, all of a sudden the minerals in there are very unhappy.
18:29
SPEAKER_01: They break down and start to melt.
18:31
SPEAKER_01: And we do this again 15 to 10, 15, 20% partial melting when you add water to this thing, and now we have magma from the mantle that is less dense and rises up and it hits the base of the crust.
18:44
SPEAKER_01: If you partially melt the mantle, ultra mafic, you get basalt mafic.
18:49
SPEAKER_01: That's first distillation step.
18:50
SPEAKER_01: Now we've got all that magma, that basaltic magma, underneath of the thick South American continental crust.
18:57
SPEAKER_01: And the way we get andastite out the top is that basalt sits there and crystallizes partially.
19:04
SPEAKER_01: So it kind of partially crystallizes, it melts some of the continental crust.
19:08
SPEAKER_01: It's very hot, 1500 degrees centigrade, when it hits the crust, which melts a lot of the crust and it integrates a lot of that crustal composition, and it becomes this blended composition between mafic and felsic, which is andesite.
19:21
SPEAKER_01: And that's what we get out of the volcanic chain.
19:24
SPEAKER_01: The stuff that erupts out onto the surface is andesite.
19:29
SPEAKER_01: Did I get close to what you're going for?
19:32
SPEAKER_00: Jesse, that was really well put.
19:33
SPEAKER_00: I loved that explanation.
19:35
SPEAKER_00: Well done.
19:36
SPEAKER_00: I just want to paint a picture for where the water's getting driven off.
19:41
SPEAKER_00: So imagine this subducting plate diving down beneath a continental plate, diving down beneath the west coast of South America at, let's say, a 45 degree angle.
19:53
SPEAKER_00: So this is gonna be well under that continental plate and fairly deep down, where it gets enough heat and enough pressure.
20:04
SPEAKER_00: So this is not a surficial process.
20:07
SPEAKER_00: The water's getting driven off fairly deep beneath that continental plate.
20:12
SPEAKER_00: So then that will rise up into the mantle.
20:15
SPEAKER_00: The water will.
20:16
SPEAKER_00: That's what causes the partial melting, which then rises up further and does this kind of blending that you just described there.
20:24
SPEAKER_01: So andesite is I said before, we were talking about basalt, that basalt is found on other planetary bodies, which it is.
20:32
SPEAKER_01: Most of the Martian crust is basalt of some variety.
20:35
SPEAKER_01: It's basically basalt.
20:37
SPEAKER_01: A lot of the moon is basalt.
20:39
SPEAKER_01: Andesite is extremely rare on other planets, if it's found at all.
20:44
SPEAKER_01: There's been a series of papers the last five years that are documenting andesite-like rocks on other planets, but they're tiny, tiny volumes.
20:53
SPEAKER_01: Like there is very little andesite on any other planet.
20:55
SPEAKER_01: And the reason is because andesite is really one of these unique plate tectonic features.
21:01
SPEAKER_01: Like it's fairly hard to produce andesite, at least how we understand it, in any other tectonic environment apart from plate tectonics.
21:10
SPEAKER_01: Like plate tectonics equals andesite.
21:13
SPEAKER_00: That this distillation process that you described only happens when plate tectonics is an active process.
21:21
SPEAKER_00: Is that a fair statement?
21:22
SPEAKER_01: Absolutely.
21:23
SPEAKER_01: And and you know what we're talking about here, this subduction zone system, it's not a one-and-done thing.
21:30
SPEAKER_01: This is a conveyor belt.
21:32
SPEAKER_01: This oceanic plate that we're describing, we've just described how basalt is formed out of mid-ocean ridge setting, it's pulled apart, it's moved over, and then it's dives down beneath the continent.
21:41
SPEAKER_01: That's a conveyor belt.
21:42
SPEAKER_01: So we can produce shitloads of andesite by this process.
21:46
SPEAKER_01: Like this produces a lot of rocks for a long time.
21:50
SPEAKER_01: I mean, subduction zone systems operate on the west coast of the United States, it operated for about 180 million years pretty much constantly, producing andesite.
22:00
SPEAKER_01: And the intrusive equivalents of andesite, diriges, and gabbros, and granodyrites and tonalites.
22:05
SPEAKER_01: So it produces a ton of this stuff because it's this conveyor belt.
22:10
SPEAKER_01: It's just constantly bringing new waterlogged basalt to the subduction zone, squeezing it out, wringing it out, melting the mantle, adding new stuff to the continent.
22:18
SPEAKER_01: It's a really, really volumetrically dominant process.
22:22
SPEAKER_00: And we have this same process going on in the northwestern part of the United States in Cascadia, where we have from Northern California, Oregon, Washington, and up into Canada, where the Wandafuca plate is diving down, subducting beneath that part of North America, doing this same process that you just described.
22:40
SPEAKER_00: It's generating massive amounts of these rocks, in particular andesite.
22:46
SPEAKER_00: So the same rocks that you're banging around on Mount St.
22:49
SPEAKER_00: Helens or Mount Rainier or Cascadia volcanoes, Jefferson and Hood, and so on, they're the same ones that you would see down in South America along the West Coast, too.
22:58
SPEAKER_00: Same mechanism, same process.
23:00
SPEAKER_00: We have it too.
23:01
SPEAKER_01: And I just want to touch on that again is that we can go back in the rock record and see the presence of these types of settings because we can see these exact types of rocks, andesytes that look exactly like modern andesites going back.
23:13
SPEAKER_01: There's huge pulses of this.
23:15
SPEAKER_01: 1.9 billion years ago, there is andesite in the related intrusive rocks all over the globe found in these belts of 1.9 GA rocks that look a lot like modern subduction zone systems.
23:27
SPEAKER_01: And so this has been going on for a long time on Earth and really constructed the continental crust.
23:33
SPEAKER_01: This is how the continental crust was really made.
23:36
SPEAKER_01: That second distillation step is what gets you from mantle.
23:40
SPEAKER_00: So I want to sorry, sorry to interrupt.
23:42
SPEAKER_00: I want to ask you where is this 1.9?
23:45
SPEAKER_00: You said GA, which is billion-year-old, andesitic rock.
23:50
SPEAKER_00: Where is this?
23:51
SPEAKER_01: This is kind of all over, actually, like the base of the Grand Canyon, the Vishnu Schist.
23:58
SPEAKER_01: There are granitoids that are kind of similar ages of that, that are what we call Kelc Alklin, which is the andesite type, it's the category of rocks that includes andesite.
24:09
SPEAKER_01: There's a whole bunch up in northern Canada, huge belts, one's called the Wapme origin.
24:14
SPEAKER_01: They're all called erogenies because they're subduction zone systems that ended with continental collision, but they're kind of all over.
24:21
SPEAKER_01: If you look at a map, a geological map of any continent, there will be 1.9 GA belts all over the place.
24:32
SPEAKER_01: It's a really interesting thing.
24:34
SPEAKER_00: That is really interesting.
24:35
SPEAKER_00: That's that's awesome.
24:37
SPEAKER_00: All right.
24:37
SPEAKER_00: Hey, way to go, my young sage.
24:39
SPEAKER_00: Nice job.
24:40
SPEAKER_00: See, thank you.
24:41
SPEAKER_00: Thank you.
24:41
SPEAKER_00: I knew I was right.
24:42
SPEAKER_00: Can you just say it, please?
24:44
SPEAKER_00: Not yet.
24:44
SPEAKER_00: Not yet.
24:45
SPEAKER_01: We're not there.
24:46
SPEAKER_01: Maybe when we get to episode six, you'll get a uh you're right on me.
24:50
SPEAKER_01: But I will say I am having fun.
24:52
SPEAKER_00: So let's move to the last question for today in this episode.
24:57
SPEAKER_00: How does plate tectonics explain the formation of granite?
25:03
SPEAKER_00: Now, granite is the second most abundant rock on the surface of the planet.
25:09
SPEAKER_00: So you have oceanic crust, which is primarily made up of basalt.
25:12
SPEAKER_00: You have continental crust, which is, while it's often covered with a real thin veneer skin, kind of like sedimentary rock and other kinds of things, but below that is granite.
25:22
SPEAKER_00: So continental crust is typified by granite, making it the second most abundant rock on the surface of the earth.
25:28
SPEAKER_01: I think let me just modify that slightly, Chris, what you just said.
25:32
SPEAKER_01: If you take the average of the continents, the bulk average of the continents is andesitic, right?
25:37
SPEAKER_01: You know this.
25:38
SPEAKER_01: But it's usually because the idea is that there's a lot of andesite in the continental crust for sure.
25:43
SPEAKER_01: But we have granite and deeper down in the continents, the base of the continents, most people would argue are mafic or basalt.
25:52
SPEAKER_01: So we have this like two-layered structure of continents.
25:55
SPEAKER_01: We have granite up top and basalt down below.
25:57
SPEAKER_01: There's some debate about that structure of the continents, but if you average mafic and felsic, you get intermediate.
26:04
SPEAKER_01: And so the average continental crusts, if you take a global average, it might be anticytic, but there's a lot of granite, and the granite is usually the upper half of the continent crust.
26:12
SPEAKER_01: Continental crust is you know between 45 and 75 kilometers thick.
26:17
SPEAKER_01: So the upper tens of kilometers beneath your feet, if you're listening to this, if you're not sitting on an island somewhere, some beautiful island in Hawaii, if you're sitting on a continent listening to this, the upper 20 kilometers beneath your feet is usually mostly granite.
26:31
SPEAKER_00: So that's good.
26:32
SPEAKER_00: Good point.
26:33
SPEAKER_00: Way to get doctory on me, but just a little nuance, a little bit in the weeds, but yeah, absolutely.
26:38
SPEAKER_00: But it's 100% right.
26:39
SPEAKER_00: All right, so how does plate tectonics produce granite?
26:44
SPEAKER_01: Exactly.
26:44
SPEAKER_01: Let me just review quickly.
26:46
SPEAKER_01: We hit distillation step number one, which was ultra mafic to mafic.
26:49
SPEAKER_01: We hit distillation to step number two, which was mafic through water to intermediate in andesite.
26:56
SPEAKER_01: Now we're gonna get to distillation step number three, which is intermediate to felsic.
27:03
SPEAKER_01: We're getting to that last step, last distillation step.
27:06
SPEAKER_01: This is your like really high, really, really, really aggressive liquor that you've distilled, or like maple syrup that's been super distilled to get super sugary and really distilled.
27:16
SPEAKER_01: It's it's your everclear.
27:17
SPEAKER_01: Everclear.
27:18
SPEAKER_01: This is the everclear.
27:19
SPEAKER_01: That's a good one.
27:20
SPEAKER_01: Granite is the everclear of rocks.
27:22
SPEAKER_01: Um, so granite basically we have to melt continents.
27:27
SPEAKER_01: That's how we get granite is either melting continental crust, partially melting continental crust.
27:32
SPEAKER_01: If you partially melt andesite, you get granite, or you have to take basalt and fractionate the hell out of it, meaning remove 80% of the mass of the magma by crystallization in what you're left with is granite.
27:47
SPEAKER_01: Is that an apt description, Chris?
27:49
SPEAKER_00: Yeah, I think so.
27:50
SPEAKER_00: To me, I would, in terms of the formation of granite, to my students, I'm gonna stay away from fractionation.
27:58
SPEAKER_01: That's fair.
27:58
SPEAKER_00: At this point, because it's not as it's not as important.
28:03
SPEAKER_00: So you correct me if I'm because you're in the this is the the space you live in, but it's not as important of a mechanism as distilling the middle part of the continent.
28:13
SPEAKER_00: That part that you corrected me on, you know.
28:15
SPEAKER_01: You are absolutely right.
28:16
SPEAKER_01: There is granite in the oceanic crust, but it's tiny volumes.
28:20
SPEAKER_01: I mean, it's tiny, tiny, tiny amounts because you have to do so much fractional crystallization to get to granite.
28:25
SPEAKER_01: So it really doesn't matter for this purpose.
28:28
SPEAKER_01: You're right.
28:28
SPEAKER_00: Yeah, for our classes, our intro level classes that we teach, they're not ready for that part of it yet.
28:34
SPEAKER_00: I think it would just be confusing and so on.
28:36
SPEAKER_00: So we can just stick to you know what?
28:39
SPEAKER_00: Take that average composition of the continental crust, that andesite, and let's take it one step further.
28:45
SPEAKER_00: Let's distill it again, let's have magma well up underneath it, partially melt intermediate magma.
28:52
SPEAKER_00: And guess what you get?
28:54
SPEAKER_00: There's a theme here.
28:55
SPEAKER_00: You partially melt intermediate and you generate now felsic magma, which then cools and crystallizes usually inside the earth, not becoming extrusive, stays intrusive, forming the beautiful rock granite.
29:09
SPEAKER_01: Yes.
29:09
SPEAKER_01: Another way that plate tectonics does this is if you take two continents, which are, like we said, on average andesitic, and you smash them together.
29:18
SPEAKER_01: This is what's happening in Tibet right now.
29:20
SPEAKER_01: The Indian continent is hitting the Asian continent, and you have two continents together.
29:25
SPEAKER_01: Chris, you said earlier that continents don't subduct, you're absolutely right.
29:28
SPEAKER_01: Continents do not go down into the mantle.
29:30
SPEAKER_01: What happens?
29:31
SPEAKER_01: One slides under the other one.
29:33
SPEAKER_01: When you have continent that is all of a sudden doubly thick, that stuff gets really hot and starts to melt.
29:40
SPEAKER_01: So there's a lot of granite being produced underneath of Mount Everest, for instance, right now, because the continent is doubly thick and it's heated up and it's melting like mad down there.
29:51
SPEAKER_01: But what is it melting?
29:52
SPEAKER_01: It's melting andesite.
29:53
SPEAKER_01: When you partially melt andesite, you get granite.
29:55
SPEAKER_01: When you partially melt basalt plus andesite mixed together, you get granite out of it.
29:59
SPEAKER_01: So that's another way that plate tectonics generates granite.
30:03
SPEAKER_00: That's a very good point.
30:04
SPEAKER_00: And it is the Indy, the subcontinent of India that is shoving itself underneath, kind of like a sliver underneath Eurasia.
30:13
SPEAKER_00: The reason why is because before those two continents met, India was riding on an oceanic plate, and it was just a typical ocean to continent subduction zone at that point.
30:26
SPEAKER_00: When that subduction zone brought India to Eurasia, it tried to pull it down.
30:32
SPEAKER_00: It's like um, you know, if I I grabbed your wrist and yanked on it, your body's gonna come with it.
30:37
SPEAKER_00: It tried to yank the subcontinent of India down with it into that subduction zone, didn't work, but it it shoved it underneath the Eurasian continent.
30:48
SPEAKER_00: And then now you're talking about, like you said, generating this really hot situation and also lower temperature minerals.
30:56
SPEAKER_00: They melt easier because they're more felsic.
31:00
SPEAKER_01: Absolutely great point.
31:01
SPEAKER_01: Felsic rocks on average melt at much lower temperatures than the mantle.
31:06
SPEAKER_01: You have to get to really high temperatures to melt the mantle, over a thousand degrees centigrade to partially melt it.
31:11
SPEAKER_01: A granite will partially melt at about 700 degrees centigrade.
31:15
SPEAKER_01: So there's a big difference in those temperatures as to what temperatures you can achieve within the crust and within the mantle.
31:22
SPEAKER_01: So that's a great point, Chris.
31:24
SPEAKER_01: That um, you know, the dynamics of how this works.
31:26
SPEAKER_01: And basically, not only do you have to have continents to form granite, but you have to smash continents together to form granite to do this third distillation step, really, in a very general sense.
31:37
SPEAKER_01: So to form large volumes of granite at least.
31:40
SPEAKER_00: And none of that happens without the movement of tectonic plates.
31:45
SPEAKER_00: So these rocks are directly tied to plate tectonics.
31:49
SPEAKER_00: This diversity of rocks is tied to plate tectonics.
31:53
SPEAKER_00: You know, I think to wrap up, Jesse, this episode, because I think we're there, there is a theme, right?
31:59
SPEAKER_00: If you partially melt ultramafic, you get mafic.
32:04
SPEAKER_00: You partially melt mafic, you get intermediate.
32:07
SPEAKER_00: And you partially melt intermediate and you get felsic.
32:11
SPEAKER_00: And then, of course, those corresponding rock types.
32:14
SPEAKER_01: And that's a process that happens on Earth because of plate tectonics.
32:20
SPEAKER_01: On other planetary bodies, we'll talk about this in another episode on this theme, but on other planetary bodies you get basalt, but you don't get that second and third distillation step, at least not in large volumes that you get on Earth.
32:32
SPEAKER_01: You don't get 30% of the surface of the earth covered in continents because again, subduction zones, this is conveyor belt.
32:39
SPEAKER_01: It's great at this creating this second distillation step.
32:43
SPEAKER_01: So plate tectonics is why we have continents.
32:46
SPEAKER_00: And we haven't gotten to yet how do sedimentary rocks tie into plate tectonics?
32:50
SPEAKER_00: How do metamorphic rocks tie into plate tectonics?
32:52
SPEAKER_00: So we have to add to this diverse list, you know?
32:55
SPEAKER_01: We actually had those on the list for this episode, and uh we're not gonna get there.
33:00
SPEAKER_01: Let's cut it off at that and we'll come back to those because again, like you said before, this is the unifying theory.
33:07
SPEAKER_01: We're gonna talk about how all this stuff is unified by plate tectonics, all the different parts of the Earth system, and we've covered the one category of rocks, the igneous rocks.
33:15
SPEAKER_01: We have sedimentary metamorphic to go, we have all sorts of other stuff to hit under the unifying theory theme.
33:22
SPEAKER_00: But the one thing that is important to me that I want to drive home is that I think all too often students think of these rocks as standalone things.
33:31
SPEAKER_00: Oh, okay, there's granite, there's basalt, there's andesite, and this other, all this other rich array of rocks, right?
33:37
SPEAKER_00: Without understanding what they really mean.
33:40
SPEAKER_00: Do you know what I mean by that?
33:42
SPEAKER_00: It's this like skin level understanding of it, but plate tectonics takes it deeper.
33:48
SPEAKER_01: Yeah, it's the difference between, and this is something I've I try and drive home in my class via some of the questions, and I think you've gotten a little exposure to this during Camp Geo when I ask you the questions at the end of each episode of Camp Geo.
34:01
SPEAKER_01: I tend to ask questions that are not just regurgitate the definition to me kind of questions.
34:07
SPEAKER_01: It's put it in context and interpret.
34:09
SPEAKER_01: Like if you go out there and you see an andesite, what does that tell you?
34:12
SPEAKER_01: Like, I want to know do you know what that is?
34:15
SPEAKER_01: And it's difficult because you have to have, like, we've talked about this before, the physical geology class or the intro to geology class that you teach and that I teach.
34:24
SPEAKER_01: That's a difficult class to teach because in order to get all these concepts, you kind of have to have all the names, you have to know all the definitions, and you have to have this linking, this idea, this plate tectonic framework to link them all together in order to do this really complicated interpretation step.
34:40
SPEAKER_01: And that's one of the difficulties of teaching this class, and one of the difficulties that we went through going through this Camp Geo conversational textbook that we built.
34:49
SPEAKER_01: So I think this is kind of a similar exercise in some ways, is that yeah, I think you're right.
34:53
SPEAKER_00: By the way, the questions at the end of each Camp Geo episode, that is my absolute favorite part because I never know what you're gonna ask me, and then sometimes you just come up with ridiculous stuff.
35:04
SPEAKER_01: I it's just yeah, sometimes they're a little bit out there.
35:08
SPEAKER_01: I think sometimes they're okay, and sometimes they're great, and sometimes they're complete garbage.
35:12
SPEAKER_01: But you know, you gotta throw it against the wall.
35:14
SPEAKER_00: They give me a they give me a glimpse into how your mind works, and sometimes I really don't want to know.
35:18
SPEAKER_00: It's a scary place.
35:21
SPEAKER_00: Dr.
35:21
SPEAKER_00: Jesse works.
35:23
SPEAKER_01: Imagine being here all the time.
35:24
SPEAKER_01: Oh man.
35:25
SPEAKER_00: No, no, that's that's tough.
35:27
SPEAKER_00: It'd be tough.
35:28
SPEAKER_00: I feel bad for Tess.
35:29
SPEAKER_01: Yeah, oh man.
35:30
SPEAKER_01: Well, Chris, I think that's a wrap.
35:31
SPEAKER_01: We should wrap it up here on the part one, and uh, you know, we're gonna keep coming back to this.
35:35
SPEAKER_01: Maybe not all in consecutive order.
35:37
SPEAKER_01: We'll bounce around, but we'll come back to this theme more and more in the next couple months here.
35:42
SPEAKER_01: So, with that, you can follow us on all the social medias.
35:44
SPEAKER_01: We're at Planet Geocast.
35:46
SPEAKER_01: Go to our website, planetgeocast.com.
35:50
SPEAKER_01: There you can subscribe, you can follow us, you can learn about us, you can support us, follow the links in the show notes, and share Planet Geo with your friends.
35:58
SPEAKER_01: That's right.
35:59
SPEAKER_00: Thanks for listening.
36:00
SPEAKER_00: Cheers.
36:01
SPEAKER_01: Cheers, I'm gonna do it.