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Some Rocks You SHOULD Know - Metamorphism and Plate Tectonics · PlanetGeo: The Geology Podcast

• 0:02
  SPEAKER_01: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:12
  SPEAKER_00: Are are you uh you doing video too?
• 0:14
  SPEAKER_01: Yes.
• 0:15
  SPEAKER_01: Gonna go straight up.
• 0:16
  SPEAKER_00: Okay.
• 0:18
  SPEAKER_00: Hold on, hold on.
• 0:22
  SPEAKER_01: There we go.
• 0:23
  SPEAKER_01: We've got we've got a zipped up tight bullheist here.
• 0:29
  SPEAKER_00: Always.
• 0:30
  SPEAKER_00: Always.
• 0:31
  SPEAKER_00: Um why why why would you lead in that way?
• 0:34
  SPEAKER_01: I'm leading in that way because I'm just the wheels, they are spinning in this episode.
• 0:40
  SPEAKER_01: I mean, I I don't know.
• 0:41
  SPEAKER_01: My I'm just excited.
• 0:43
  SPEAKER_01: I don't know where this is gonna go.
• 0:44
  SPEAKER_01: It's an interesting topic, and it's kind of something that I haven't thought that deeply about, really.
• 0:49
  SPEAKER_01: And I don't know.
• 0:50
  SPEAKER_01: What do you think?
• 0:51
  SPEAKER_01: Um what kind of bull heights do I have this morning?
• 0:53
  SPEAKER_01: Let me there what kind of bull ice do I have today?
• 0:55
  SPEAKER_00: So that's an interesting question.
• 0:57
  SPEAKER_00: You have uh what kind of Chris do you have today?
• 1:02
  SPEAKER_00: Um Happy Chris, of course.
• 1:04
  SPEAKER_00: Okay, always duh.
• 1:05
  SPEAKER_00: But this episode is about rocks that we really want to teach because they're freaking awesome, but we don't.
• 1:16
  SPEAKER_00: And a little bit about like why that is, but we need to, I think to understand that there's this discussion about pedagogy with the teaching of you know metamorphic rocks.
• 1:29
  SPEAKER_01: Yes, totally.
• 1:30
  SPEAKER_01: Maybe, Chris, would it be okay if we sort of put out a call if you are if you're listening to this and you're a teacher and you've taught geology or you are teaching geology, we really want some feedback on this episode because I don't know where this is gonna go.
• 1:42
  SPEAKER_01: It might end in the proposal to sort of change, fundamentally change how we teach metamorphic rocks in the intro-level classes.
• 1:50
  SPEAKER_01: Do you feel it get that sense, Chris, or not?
• 1:51
  SPEAKER_01: Because this is something I've thought about a little bit, and I'll be honest, I've tried to do this slightly in my class, but it's a really I do a hacked job of it.
• 1:59
  SPEAKER_01: Like I don't do it, it doesn't tease the through line like it really should if I kind of went wholesale in this direction, I suppose, if that makes sense.
• 2:07
  SPEAKER_00: It does make sense.
• 2:09
  SPEAKER_00: I don't think it will.
• 2:10
  SPEAKER_00: It it won't for me.
• 2:12
  SPEAKER_00: I've thought about this a lot, actually, and especially getting ready for this episode.
• 2:17
  SPEAKER_00: But when you talk about these niche kind of metamorphic rocks, which is what this episode is really about.
• 2:23
  SPEAKER_00: Things like blue schist and eclogite and green schist and amphibolite, and these are really awesome rocks.
• 2:30
  SPEAKER_00: And the thing is that I love about them, and I know you're the same way, is that they tell us really specific information.
• 2:36
  SPEAKER_00: Yeah.
• 2:37
  SPEAKER_00: What was the tectonic setting for the formation of these rocks?
• 2:42
  SPEAKER_00: The problem is that they're in some cases and in most of the cases, incredibly rare.
• 2:50
  SPEAKER_00: So that's why I'm not going to change whole scale.
• 2:54
  SPEAKER_00: I can't, in an intro level class, like I can't, I can't do that.
• 2:59
  SPEAKER_00: And I don't think that that's our job, to be honest with you.
• 3:03
  SPEAKER_01: Well, I let me do like just a brief introduction, Chris, and then we can kind of pitch what where we're thinking or sort of clarify what we're what this discussion is going to be like.
• 3:12
  SPEAKER_01: And what we're talking about here is a suite of metamorphic rocks that if you've taken an intro level class, or or really if you've taken a second level, like physical rocks and minerals class, you would have seen these things.
• 3:24
  SPEAKER_01: These are the rocks that make up the phase diagram or the faces diagram.
• 3:31
  SPEAKER_01: Many people would call this the faces or faces diagram for metamorphism.
• 3:35
  SPEAKER_01: And what that means is the metamorphic petrologists back in the day established that there's this pressure-temperature space.
• 3:42
  SPEAKER_01: If you can imagine pressure on the y-axis and temperature on the x-axis, the whole range of earth conditions are on that plot.
• 3:51
  SPEAKER_01: And if you take basalt, take your average run-of-the-mill basalt, you put it at different places in that diagram, so you heat it up to 100 degrees and you know, two kilometers deep in the earth, or you heat it up to 600 degrees and eight or twenty kilometers deep in the earth, that rock, that basalt will change its composition, change its minerals to be a new rock.
• 4:11
  SPEAKER_01: And what that new rock is, the minerals in that new rock are named the rocks we're going to talk about mostly.
• 4:16
  SPEAKER_01: And we name the field, we name the area in the diagram after that new rock, if that makes sense.
• 4:22
  SPEAKER_00: That does make sense.
• 4:23
  SPEAKER_00: But can you really quick because you threw out some terms, you threw out these faces or facies as you sometimes intellectually call them?
• 4:31
  SPEAKER_00: Yeah.
• 4:33
  SPEAKER_00: So I'm just trying to appease our British listeners by calling it faces.
• 4:38
  SPEAKER_00: Love it.
• 4:39
  SPEAKER_00: Let's go ahead, Jesse.
• 4:41
  SPEAKER_00: Let's define exactly what that means.
• 4:43
  SPEAKER_00: Because any, if you get into metamorphic petrology at all, you're gonna get into faces.
• 4:48
  SPEAKER_00: So what does that refer to?
• 4:51
  SPEAKER_00: Is this a a suite of minerals that go along with a set of pressure temperature conditions?
• 4:57
  SPEAKER_00: Like what are we talking about?
• 4:59
  SPEAKER_01: It's it's it it is that that's um it can mean a couple different things depending on the context we're talking about.
• 5:04
  SPEAKER_01: But it it fundamentally is a region of pressure temperature space where a certain suite of minerals are stable.
• 5:11
  SPEAKER_01: And what that means for this context and the rocks that we're gonna talk about is we're gonna start all these rocks, we're gonna take a basalt as the starting rock.
• 5:19
  SPEAKER_01: So that's our starting recipe.
• 5:21
  SPEAKER_01: We're gonna take a basalt.
• 5:22
  SPEAKER_01: And if I heat it up to 400 degrees centigrade and put it about 12 kilometers deep in the earth, it's gonna change into what's called a green schist.
• 5:31
  SPEAKER_01: So we're gonna get the minerals that are gonna form are gonna be stable.
• 5:34
  SPEAKER_01: It's the same recipe, basalt, but it has new minerals in it and it looks totally different, and it's gonna be a green schist.
• 5:40
  SPEAKER_01: And so that's we call the faces the region in pressure-temperature space where green schist is stable, where those minerals are stable with the recipe called basalt.
• 5:50
  SPEAKER_01: That's a facies.
• 5:52
  SPEAKER_01: And you can use that same term faces if you change the composition.
• 5:56
  SPEAKER_01: So if you put a shale into the recipe book and you do the same thing, you can get like garnet or you know, solionite faces, because soliamonite's stable in this pressure-temperature regime with this starting composition called shale.
• 6:09
  SPEAKER_00: So Okay.
• 6:10
  SPEAKER_00: So how come, Jesse, the rocks that we're gonna talk about blue schist, uh, eclogite, green schist, amphibolite, granulite, all these rocks.
• 6:22
  SPEAKER_00: I guess the common denominator with them is the protolith.
• 6:26
  SPEAKER_00: The rock that they came from.
• 6:29
  SPEAKER_00: So these rocks are not as common as their other regionally metamorphosed rocks.
• 6:36
  SPEAKER_00: I mean, a lot of these are schists technically, but things like slate and philite and schist and gneiss, but they a lot of times they have a different protolith, right?
• 6:47
  SPEAKER_00: They have a different parent rock that they existed as before the metamorphism happened.
• 6:53
  SPEAKER_01: Let me just jump in there, Chris, because we talked about these the previous episode.
• 6:56
  SPEAKER_01: The episode where we kind of started this conversation about like teaching these this suite of metamorphic rocks, we talked about schist and philite and slate in the 25 Rocks You Need to Know episode.
• 7:05
  SPEAKER_01: So you can go back and listen to that, or even better yet, you can go to our Camp Geo app, first link in your show notes.
• 7:10
  SPEAKER_01: We have a whole chapter on metamorphic rocks, and we talk about both of these types of compositions there.
• 7:15
  SPEAKER_01: We talk about all the rocks we're gonna talk about today, and we also on there talk about all the rocks we talked about last episode.
• 7:21
  SPEAKER_01: So that's what you're referring to is it is those things if you put shale in as the recipe.
• 7:27
  SPEAKER_01: Now, it's a really, really important question.
• 7:30
  SPEAKER_01: So the reason that the basaltic recipe version of the rocks we're gonna talk about here in a minute are less common than the shale versions or the sediment versions is you kind of gotta think about how plate tectonics works, right?
• 7:44
  SPEAKER_01: Sediments are on the continents, on the edges of the continents, and they're in the continents.
• 7:49
  SPEAKER_01: And so if you deform and metamorphose continents, you're gonna metamorphose a bunch of sediments.
• 7:54
  SPEAKER_01: Where do you get basalt?
• 7:56
  SPEAKER_01: Mostly.
• 7:56
  SPEAKER_01: Sure, there's some on the continents, but you get it a lot in the oceanic crust.
• 8:00
  SPEAKER_01: What happens to the oceanic crust?
• 8:02
  SPEAKER_01: Well, it gets recycled into the mantle.
• 8:04
  SPEAKER_01: So it's getting you know overproduced through subduction, it's getting subducted away and like completely destroyed.
• 8:10
  SPEAKER_01: So at least on the modern Earth, sediments are just trapped on the continents, which is the continents are where geologists walk around.
• 8:18
  SPEAKER_01: If you had geologists walking around on the ocean floor, you'd see more, you know, zeolite and green shist kicking around there, but we don't.
• 8:26
  SPEAKER_01: So that's that's one of the reasons, at least.
• 8:28
  SPEAKER_00: So maybe which gets to the heart of the matter, right?
• 8:31
  SPEAKER_00: Yes, exactly.
• 8:32
  SPEAKER_00: In terms of like why my rock sets have Phylite and Slate and Schist and Gnice as opposed to blue schist and ecologite in these rocks that like most beginning students have never heard of before.
• 8:45
  SPEAKER_01: That yeah, I agree.
• 8:46
  SPEAKER_01: I agree.
• 8:47
  SPEAKER_01: And the question kind of is why.
• 8:48
  SPEAKER_01: So maybe Chris, let's just let's go through these rocks real quickly, and then we can kind of frame the discussion after that or have the discussion or debate maybe even.
• 8:56
  SPEAKER_01: Where should we start with that?
• 8:57
  SPEAKER_00: Well, I think it I think it makes sense to start with blue schist.
• 9:01
  SPEAKER_00: To me, this logically makes the most sense because to me, I want to work from like lower grade to higher grade, and I want to work my way through these tectonic settings.
• 9:11
  SPEAKER_00: If that does that make sense, I agree with you completely.
• 9:13
  SPEAKER_01: Let me interrupt real quick and just define grade.
• 9:15
  SPEAKER_01: You said grade.
• 9:16
  SPEAKER_01: Metamorphic grade is really it's the temperature, but it's the intensity of metamorphism.
• 9:20
  SPEAKER_01: So low grade is low intensity, high grade is high intensity, and high temperature.
• 9:25
  SPEAKER_00: So, Jesse, let's start off by talking about blue schist, because to me it just makes sense to make our way through the tectonic processes.
• 9:35
  SPEAKER_00: So it's a regionally metamorphic rock.
• 9:37
  SPEAKER_00: We talked about that in our last episode.
• 9:40
  SPEAKER_00: This is one that involves significant amounts of heat and pressure.
• 9:45
  SPEAKER_00: So it's a subduction zone related rock where you have oceanic crust, basalt, which is that important protolith that we discussed just a second ago.
• 9:55
  SPEAKER_00: But it has a relatively low geothermal gradient.
• 9:59
  SPEAKER_00: In other words, what that means is that the temperature is not increasing at a high rate with depth.
• 10:06
  SPEAKER_00: It's the opposite of that, actually.
• 10:08
  SPEAKER_00: And so what we get then is that kind of faces, that metamorphic faces that you described earlier, we get relatively high pressure, but lower temperatures.
• 10:21
  SPEAKER_00: And we get these rare minerals that like I don't know, Jesse.
• 10:24
  SPEAKER_00: I minerals like lawsite and aragonite, these are not and jadite, these are not common minerals at all.
• 10:31
  SPEAKER_01: No, right.
• 10:32
  SPEAKER_01: Blue shist, though, is just such a pretty rock.
• 10:35
  SPEAKER_01: It's it's very, very vibrant blue.
• 10:39
  SPEAKER_01: It's like undeniably blue.
• 10:42
  SPEAKER_01: I think it's just a really, really pretty rock.
• 10:44
  SPEAKER_01: And it's a schist, so it has this kind of these undulations in it.
• 10:47
  SPEAKER_01: It's really it's quite fine grained, but it has a foliation, so it's just blue it's a blue schist.
• 10:52
  SPEAKER_01: You pick it up, and you'd be like, if you if I know what a schist looks like, this is a blue schist, right?
• 10:56
  SPEAKER_01: It's very easy to identify.
• 10:58
  SPEAKER_01: And you're right, these minerals are not common.
• 11:01
  SPEAKER_01: They're really only found in blue schist.
• 11:03
  SPEAKER_01: And here's the reason: the reason they're found in blue schist is because blue schist is formed in a really specific pressure-temperature range, as you alluded to, right?
• 11:13
  SPEAKER_01: It's high pressure, but low temperature, relatively low temperature.
• 11:17
  SPEAKER_01: And so it gets thrust down really deep before it has time to heat up.
• 11:22
  SPEAKER_01: So it goes down quicker than it can heat up, really, in the subduction zone system, and it turns into this really unique set of minerals that you really only find in blue shists.
• 11:32
  SPEAKER_01: And it's really powerful that way.
• 11:34
  SPEAKER_01: If you walk across the if you see a blue schist, you know you're near a subduction zone or what used to be a subduction zone.
• 11:40
  SPEAKER_00: Yeah, that's what I love about it.
• 11:41
  SPEAKER_00: It gives us this really specific piece of information.
• 11:46
  SPEAKER_00: But I want to ask everybody listening to this is that why would this kind of rock then be so rare?
• 11:53
  SPEAKER_00: If you just think about that for a second, why would a blue schist be rare?
• 11:57
  SPEAKER_00: Oh, it's this is this it's such a good intermediate kind of subduction zone.
• 12:03
  SPEAKER_01: Because let me interrupt.
• 12:03
  SPEAKER_01: It you know, the starting recipe is basalt, one of the most common rocks on the surface of the earth, like like why is blue shist so rare?
• 12:10
  SPEAKER_01: Great question.
• 12:11
  SPEAKER_01: What would you answer to somebody who came up and said that to you?
• 12:15
  SPEAKER_01: Well Mr.
• 12:15
  SPEAKER_01: Belheit, because Mr.
• 12:17
  SPEAKER_01: Belhais, why is blue shist so rare?
• 12:20
  SPEAKER_00: Well, what happens to this is that it's involved in a subduction zone.
• 12:24
  SPEAKER_00: So if it continues to subduct, it continues to go through more pressure and more heat.
• 12:31
  SPEAKER_00: Then that's where blue schist then will transition into a different rock.
• 12:36
  SPEAKER_00: It'll transition into the next rock we're going to talk about, which is an eclogite, which has a different metamorphic pressure temperature faces.
• 12:45
  SPEAKER_00: So in order to get a blue schist, something has to interrupt this whole process.
• 12:51
  SPEAKER_00: Something has to interrupt the subduction process and kind of exhume this toward the surface.
• 12:58
  SPEAKER_01: Bring it up to the surface quite quickly so that it doesn't what we call retrogress or react back along the pathway because metamorphic rocks can do that.
• 13:06
  SPEAKER_01: They can they can transition between different types of metamorphic rocks, right?
• 13:10
  SPEAKER_01: As it kind of comes back to the surface.
• 13:12
  SPEAKER_00: So, Jesse, I have a question for you.
• 13:14
  SPEAKER_00: So have you seen blue schist in the field before?
• 13:19
  SPEAKER_01: Yes.
• 13:19
  SPEAKER_01: I've been to the the sort of type locality for subduction zone metamorphism, where like a lot of the details were worked out, which is called the Franciscan complex, which, as the name implies, is north of San Francisco.
• 13:30
  SPEAKER_01: It is a it's an exceptional package of rocks because it's basically all of these metamorphs, well, these blue schist and eclogite, these subduction zone metamorphism rocks are everywhere.
• 13:42
  SPEAKER_01: And it's just really beautiful to see.
• 13:44
  SPEAKER_01: You can see some of these blue schists with with little tiny garnets in them.
• 13:48
  SPEAKER_01: Some of them have bigger garnets.
• 13:49
  SPEAKER_01: They're really, really beautiful rocks.
• 13:51
  SPEAKER_01: And they represent this totally cool process.
• 13:54
  SPEAKER_01: The basalt went down, and we're not talking a little ways down, we're talking like 30 kilometers down into the earth in a subduction zone or even deeper, and then came back up, you know, went down 30, 40 kilometers, and then came back up and is now exposed on the surface, kind of bobbed back up the subduction zone system.
• 14:13
  SPEAKER_00: And so I mean Do you know what did that?
• 14:16
  SPEAKER_00: Like what caused that?
• 14:18
  SPEAKER_00: Uh like I've been, I I know what you're talking about.
• 14:20
  SPEAKER_00: I've seen the Franciscan complex too.
• 14:22
  SPEAKER_00: And they're we're gonna allude to this actually a a few more times because we're gonna see other host rocks here.
• 14:28
  SPEAKER_00: But what interrupted this process?
• 14:31
  SPEAKER_00: Do you know?
• 14:31
  SPEAKER_01: Because I don't know.
• 14:32
  SPEAKER_01: It's such a good question, and it's a topic of quite a lot of debate, I think, how these things ultimately get what's called exhumed on the surface.
• 14:40
  SPEAKER_01: And the same thing goes for eclogite, which we'll talk about next.
• 14:43
  SPEAKER_01: But a lot of people call this a it's a French word.
• 14:45
  SPEAKER_01: So we'll forgive us for using a French word here, but melange, the subduction zone melange, which is just this mess of stuff that comes up in a subduction zone system.
• 14:55
  SPEAKER_01: And some people think that these things, it's kind of like a washing machine.
• 14:58
  SPEAKER_01: They kind of get stuck in this bobbing around cycle where it's kind of tumbling around and around and around.
• 15:05
  SPEAKER_00: Exactly.
• 15:05
  SPEAKER_00: Like it kind of gets caught by the downgoing.
• 15:08
  SPEAKER_01: Soft rock.
• 15:08
  SPEAKER_01: Exactly, gets dragged by the downgoing slab, but then, you know, due to density, kind of percolates its way back up and then gets caught and brought back down again.
• 15:16
  SPEAKER_01: And some people would argue that there's sort of cycles to these, but it's kind of in the in the early phases of the subduction zone system.
• 15:24
  SPEAKER_01: Because remember, the oceanic crust sits on oceanic mantle, and that's the tectonic plate that's going down.
• 15:30
  SPEAKER_01: So the oceanic crust can get scraped off the top by the overriding continental plate, basically, depending upon the tectonics of the region.
• 15:38
  SPEAKER_01: So sometimes it can kind of get just get scraped off in the trench system.
• 15:43
  SPEAKER_01: Anyway.
• 15:44
  SPEAKER_00: Well, let's can we move on then to the next one.
• 15:47
  SPEAKER_00: I was just gonna say this rock.
• 15:48
  SPEAKER_01: We're we're at risk of getting really long-winded on these cool rocks here.
• 15:52
  SPEAKER_00: Well, I know, I know, I know.
• 15:53
  SPEAKER_00: But they're so I want to talk about the next one, which I think is the next one in the proper sequence, which is called ecloget.
• 16:00
  SPEAKER_01: Definitely.
• 16:01
  SPEAKER_00: And to me, you raved about the beauty of blue schist, and and and it is, but I love I don't know, I guess green is my favorite color, and so is it the color of Jenny's eyes?
• 16:11
  SPEAKER_00: An absolute no.
• 16:14
  SPEAKER_00: Oh, you should have answered that with some more confidence.
• 16:19
  SPEAKER_00: Well, that was funny.
• 16:21
  SPEAKER_00: No, she Jenny has bicolored eyes, so she's got some brown and some green, so I that's why, Jesse.
• 16:28
  SPEAKER_00: Don't start hitting me.
• 16:29
  SPEAKER_01: I know what I'm talking about.
• 16:30
  SPEAKER_01: Joyce is gonna get on the hotline after that one and give you a cough.
• 16:34
  SPEAKER_01: Give you a hard time.
• 16:35
  SPEAKER_00: Oh, Joyce is sleeping.
• 16:36
  SPEAKER_00: She's been sleeping for the last five minutes right now.
• 16:38
  SPEAKER_00: Oh, yeah.
• 16:38
  SPEAKER_01: As soon as you said blue schist, she's out.
• 16:41
  SPEAKER_00: So take blue schist, shove it down more, and now you're at higher pressures and higher temperatures, and the faces turns into the rock that will form eclogite.
• 16:54
  SPEAKER_00: That's right.
• 16:54
  SPEAKER_00: And this has a higher concentration then of garnets and minerals like a clinopyroxene.
• 17:01
  SPEAKER_00: So it's a different host of minerals that are here, but it is absolutely beautiful because it has these, like it's got these other minerals, Jesse, minerals that I love, minerals like kyanite and rootil, and then it does have some quartz in it too.
• 17:18
  SPEAKER_00: And you know, I've collected kyanite and rootil, actually.
• 17:22
  SPEAKER_00: I collected rootil in Arkansas long, long time ago.
• 17:24
  SPEAKER_01: Oh, yeah, yeah.
• 17:25
  SPEAKER_01: Totally cool.
• 17:26
  SPEAKER_01: Not out of uh eclogite there in Arkansas, probably.
• 17:29
  SPEAKER_00: No, but they they it was out of a riverbed, but oh cool.
• 17:32
  SPEAKER_01: Oh, that's but that I'm sure somewhere nearby, exposed nearby.
• 17:35
  SPEAKER_01: Maybe there was uh because root teal is quite common in these things.
• 17:38
  SPEAKER_01: It's the Christmas tree rock, right?
• 17:40
  SPEAKER_01: It's deep, deep, beautiful red garnets and really bright green, sodium-rich clinopyroxines.
• 17:46
  SPEAKER_01: It's just an unbelievably beautiful rock.
• 17:48
  SPEAKER_01: And Chris, we talked about this a lot in the Camp Geo app on plate tectonics.
• 17:54
  SPEAKER_01: This is the most important rock on earth, probably, for operating plate tectonics.
• 17:59
  SPEAKER_01: Eclogite, as soon as it transforms to eclogite, usually there's melting.
• 18:03
  SPEAKER_01: So you between blue schist and eclogite, you lose some melt.
• 18:07
  SPEAKER_01: And as soon as you transform to eclogite, the garnet and the composition of the clinopyroxene is really dense.
• 18:13
  SPEAKER_01: That rock is actually more dense than the mantle, and so it sinks down into the mantle and it drags, it's like an anchor dragging the entire subducting plate with it.
• 18:23
  SPEAKER_01: So it kind of people think of like you know, it lends to the mechanism of plate tectonics.
• 18:28
  SPEAKER_00: It lends to this kind of slab pull.
• 18:30
  SPEAKER_00: Slab pull.
• 18:31
  SPEAKER_01: That's exactly right.
• 18:32
  SPEAKER_01: People think of plate tectonics as stuff shoving continents around.
• 18:35
  SPEAKER_01: Actually, it's eclogite pulling the oceanic crust down.
• 18:39
  SPEAKER_01: That's the main driving force.
• 18:41
  SPEAKER_01: So an eclogite, eclogite, that's the one that does it.
• 18:44
  SPEAKER_01: And it's because the peculiar mineral arrangement, it's more dense than the mantle around it.
• 18:50
  SPEAKER_01: So anyway, we're we're belaboring this, but I've seen it in the Franciscan complex.
• 18:54
  SPEAKER_01: You get it in these areas, you get big cobbles, like big, big boulders.
• 18:57
  SPEAKER_01: They call the blocks.
• 18:58
  SPEAKER_01: Blocks of eclogite, kind of, you've seen them, Chris, you know, in a sea of blue shist, you'll get a big block of eclogite.
• 19:04
  SPEAKER_00: And they're beautiful.
• 19:05
  SPEAKER_00: If you think about how rare a blue schist is, then think about how rare an eclogite is because this is deeper.
• 19:12
  SPEAKER_00: The exhumation process is much more robust.
• 19:16
  SPEAKER_00: It has to be to bring this to the surface.
• 19:18
  SPEAKER_00: Something had to interrupt this.
• 19:19
  SPEAKER_00: And and like it's it's a really rare rock at the surface of the earth, but it is so, so important in telling us what's going on.
• 19:28
  SPEAKER_01: And so diagnostic.
• 19:30
  SPEAKER_01: If you see an eclogite in the field, you know you're at an ancient subduction zone system where the eclogite has come back up.
• 19:37
  SPEAKER_01: So these rocks, blue schist and eclogite, it's hard to really over-emphasize how valuable they are for tracing subduction zone systems.
• 19:46
  SPEAKER_01: I mean, there are people who would argue that you go back and you find the oldest blue schist on earth and you've identified the start of plate tectonics on earth because they're so diagnostic of just plate tectonics in subduction zone systems.
• 19:59
  SPEAKER_01: So they're really, really important.
• 20:01
  SPEAKER_01: Sorry, go ahead.
• 20:02
  SPEAKER_00: No, no, no.
• 20:02
  SPEAKER_00: I just uh you're you're absolutely right.
• 20:04
  SPEAKER_00: But I want to transition then into the next host of rocks that we're gonna talk about, which are green shist, which are more common.
• 20:11
  SPEAKER_01: These are the more common ones.
• 20:12
  SPEAKER_00: Yeah, they are, and I want to talk about why.
• 20:14
  SPEAKER_00: So we're gonna talk about green schist, amphibolite, and then a rock called granulite in that order.
• 20:21
  SPEAKER_00: So let's start, Jesse.
• 20:23
  SPEAKER_01: Let me Chris, can I can I just frame this really quickly?
• 20:26
  SPEAKER_01: Just set this out.
• 20:27
  SPEAKER_01: These rocks form at the same pressure conditions as blue shist eklogite do, but they're hotter.
• 20:33
  SPEAKER_01: I said if you go down 30 kilometers and you're at 200 to 300 degrees, you have a blue schist.
• 20:38
  SPEAKER_01: If you go down 30 kilometers and you're at 400, 450 degrees, you're gonna form a green shist.
• 20:43
  SPEAKER_01: And so it's higher temperature.
• 20:45
  SPEAKER_00: I want to double tap on what you just said, and then I'm gonna have a I have a question for you.
• 20:49
  SPEAKER_00: But you you said, look, these rocks, green shist, amphibolite, and granulite, they form at roughly the same pressures, but they're hotter.
• 20:58
  SPEAKER_00: So what is the difference in the geoscience setting or the tectonic setting in which these happen?
• 21:06
  SPEAKER_01: Dr.
• 21:07
  SPEAKER_01: Primec.
• 21:08
  SPEAKER_01: This is I think it's valuable to go really slowly and let the listener ponder that for a minute because yeah, what would cause that difference?
• 21:18
  SPEAKER_01: Think about that for a second.
• 21:19
  SPEAKER_01: So important, right?
• 21:20
  SPEAKER_01: What would cause these rocks to at the same depth be higher temperature?
• 21:26
  SPEAKER_01: And I think it's best to think what makes Bluchist and Ecloget unique is not that they go down deep.
• 21:32
  SPEAKER_01: I mean, they do go down deep, it's that they go down deep and cool.
• 21:36
  SPEAKER_01: And they go down deep and cool because of subduction zone system, they're actively being carried down.
• 21:41
  SPEAKER_01: So they start cold at the ocean level, right near the surface, and then they get carried down and they go deeper faster than they can heat up.
• 21:50
  SPEAKER_01: Every other type of tectonic setting, you know, if you take stuff in the continents and you smash it together, it's going deep, but it's going deep slower, and so it's heating.
• 22:00
  SPEAKER_01: Heating up as well as going down.
• 22:01
  SPEAKER_01: I mean, we've talked about the geothermal gradient before a lot, and why is the interior of the earth hot?
• 22:06
  SPEAKER_01: The interior of the earth is hot.
• 22:07
  SPEAKER_01: So if you move rocks down slowly, they're going to heat up and get to higher pressure.
• 22:12
  SPEAKER_01: So basically, they're not subduction zone system rocks.
• 22:16
  SPEAKER_01: They're more like continental collision zone metamorphism.
• 22:18
  SPEAKER_01: Greenchist and phonemic.
• 22:20
  SPEAKER_00: Modern day analogies would be things like where the Himalayas are being actively built today, where you have the subcontinent of India colliding with the continent of Eurasia, or back further in time, the eastern part of the United States, when Pangaea came together and it formed the Appalachian Mountains.
• 22:39
  SPEAKER_00: Another perfect example for this kind of setting where you had massively thick continental crust, which means you have a lot of pressure, and then the geothermal gradient provides the heat.
• 22:52
  SPEAKER_00: You get the ingredients then for these kinds of rocks.
• 22:56
  SPEAKER_01: Yeah, and I think let's go through the rocks real quick because that's a perfect segue, and we can talk about like specific examples here.
• 23:01
  SPEAKER_01: But green shist, it's the lowest temperature and pressure version of this, but it's actually a really big field in pressure-temperature space.
• 23:09
  SPEAKER_01: So green schist is really common.
• 23:13
  SPEAKER_01: It is the most common of these rocks that we're discussing today.
• 23:16
  SPEAKER_01: They're green because they have chlorite mainly and serpentinite as minerals and some epidote.
• 23:21
  SPEAKER_01: There's some beautiful epidotes around here in just in like northern Maryland, Pennsylvania border.
• 23:27
  SPEAKER_01: There's the South Mountain Bafflith, which has some it's all green schist, fasci metamorphism of basalts, and there's epidote everywhere.
• 23:34
  SPEAKER_01: It's really pretty stuff.
• 23:35
  SPEAKER_01: But those are all green, chlorite, serpentine, group minerals, and epido.
• 23:39
  SPEAKER_01: They're all really green.
• 23:40
  SPEAKER_00: Can I jump in a second, Jesse, on this?
• 23:42
  SPEAKER_00: That the green schist that you're describing is different than an eclogite.
• 23:47
  SPEAKER_00: That's exactly in terms of you know the mineralogy is really different.
• 23:50
  SPEAKER_00: Right.
• 23:51
  SPEAKER_00: The green schist doesn't necessarily have a lot of garnets in it.
• 23:54
  SPEAKER_01: No, never.
• 23:55
  SPEAKER_00: It doesn't have clinopyroxine.
• 23:57
  SPEAKER_00: Chlorite to me, Jesse, is this kind of it resembles a smearing of this beautiful green colour in an otherwise black rock.
• 24:09
  SPEAKER_01: That's a really good word because green schists have that greasy.
• 24:13
  SPEAKER_01: They look greasy, I think, because of that serpentine group minerals, chlorite.
• 24:17
  SPEAKER_01: Epidote even is a little bit greasy.
• 24:19
  SPEAKER_01: And they're they're more of a dark green, like a dark forest green, whereas eclogrite is a really bright, vibrant green, like spring leaf green, kind of like really, really vibrant green.
• 24:30
  SPEAKER_01: So you're exactly right.
• 24:31
  SPEAKER_01: Green schist has this kind of greasy feel to it.
• 24:34
  SPEAKER_01: Now, I mean, green schist can be found in subduction zone systems for sure, but it's much more common in sort of continental collision or regional metamorphic environments where you don't have this low temperature metamorphism.
• 24:46
  SPEAKER_00: Think about what how we described slate and phylite with those regionally metamorphosed rocks that are super common.
• 24:55
  SPEAKER_00: I would put green schist a little bit higher in terms of metamorphic grade than that, but then lower than the next rock that we're going to talk about, which is amphibolite.
• 25:06
  SPEAKER_01: Amphibolite, uh amphibolite's such a fun one.
• 25:08
  SPEAKER_01: Can I talk?
• 25:09
  SPEAKER_01: Can we talk about amphibolite now?
• 25:10
  SPEAKER_00: Yeah, let's go ahead.
• 25:11
  SPEAKER_00: Oh, first green schist, like you said, can be found all over the place.
• 25:15
  SPEAKER_00: In fact, you know, in terms of the way my logic goes, I should probably add greenschist to my rocks.
• 25:22
  SPEAKER_01: And maybe amphibolite.
• 25:24
  SPEAKER_01: Just as far as like metamorphic rocks, they're gonna come across.
• 25:27
  SPEAKER_01: Green schist and amphibolite are they're certainly not uncommon.
• 25:30
  SPEAKER_01: Let's they're not bleeding.
• 25:31
  SPEAKER_01: So I'll tell you this, Jesse.
• 25:32
  SPEAKER_00: I actually do what I do on in my class is I'll lay out my metamorphic rocks, my regionally metamorphic rocks, in an order that makes sense on a big lab table.
• 25:42
  SPEAKER_00: I'll start with like shale or basalt as the protolith, and then I'll go from that to slate and then fillite, and then I have my whole host of rocks that are schists because you have so many different index minerals.
• 25:58
  SPEAKER_00: So many different things.
• 25:58
  SPEAKER_00: And I'll start with what I call often a metabasalt, which is a it's a green schist.
• 26:04
  SPEAKER_01: It's a green schist, yeah.
• 26:04
  SPEAKER_01: That I remember this term, you know, the famous pillows behind the menards were metabasalts.
• 26:10
  SPEAKER_01: They're no longer there, but they're green schist, right?
• 26:12
  SPEAKER_01: They're metamorphosed to green shist faces.
• 26:14
  SPEAKER_01: And so they're they're really quite common.
• 26:16
  SPEAKER_01: I mean, I th I would argue that if you find a basalt that's older than a couple hundred million years old, it's pretty rare that it's a basalt.
• 26:23
  SPEAKER_01: It's most likely going to be a green schist.
• 26:25
  SPEAKER_01: It's most likely going to be slightly metamorphosed and have that greenish kind of color to it.
• 26:29
  SPEAKER_00: But it's hard to teach that.
• 26:31
  SPEAKER_00: And here's why I I find this difficult.
• 26:34
  SPEAKER_01: I agree.
• 26:34
  SPEAKER_00: Because in in terms of pedagogy, we do minerals before we do rocks, because they have to be able to identify minerals in order to identify rocks.
• 26:42
  SPEAKER_00: But finding chlorite as a standalone mineral is very difficult to do.
• 26:47
  SPEAKER_01: Totally.
• 26:48
  SPEAKER_00: Which, and that's the primary mineral of a green shist.
• 26:51
  SPEAKER_01: And it's not important to teach it as a mineral because it's only present in you know green shist rock.
• 26:55
  SPEAKER_00: Like it's it's not the index mineral.
• 26:57
  SPEAKER_00: That's right.
• 26:58
  SPEAKER_01: Yeah, I mean, it's not that uh sort of important.
• 27:00
  SPEAKER_01: Let's cover amphibolite and granulite, because amphibolite's another pretty common one.
• 27:04
  SPEAKER_01: And Chris, you and I have seen probably the most spectacular amphibolites there are, which is the garnet amphibolites in the Gore Mountain mine.
• 27:12
  SPEAKER_01: I mean, huge garnets, right?
• 27:14
  SPEAKER_01: So amphibolite is higher temperature and higher pressure than green schist, but it's mainly amphibole and plagiaris.
• 27:23
  SPEAKER_01: That's the mineralogy of the rock.
• 27:24
  SPEAKER_01: It's black, it's salt and pepper, it's black minerals and plagiarism, white minerals.
• 27:30
  SPEAKER_00: Can I interrupt you one second, Jesse?
• 27:31
  SPEAKER_00: Because like I want to just say that when when you say amphibole, I think most people associate a more common mineral with that as hornblend.
• 27:39
  SPEAKER_00: Yeah, yeah, yeah.
• 27:40
  SPEAKER_01: Sorry.
• 27:40
  SPEAKER_01: I yep.
• 27:40
  SPEAKER_00: Oh, horn blend.
• 27:41
  SPEAKER_00: No, no, no.
• 27:42
  SPEAKER_00: That's that's it's totally fine.
• 27:43
  SPEAKER_00: Yeah.
• 27:43
  SPEAKER_00: Horn blend is a type of amphibole.
• 27:47
  SPEAKER_00: Amphibole is a mineral group.
• 27:48
  SPEAKER_00: It's kind of like muscovite and biotite are part of the mineral group, the micas.
• 27:53
  SPEAKER_00: Horn blend is the same as that for amphibole.
• 27:56
  SPEAKER_00: So just all right.
• 27:57
  SPEAKER_00: So sorry to interrupt you.
• 27:58
  SPEAKER_01: No, no, no.
• 27:59
  SPEAKER_01: That's a really that's a really good point.
• 28:00
  SPEAKER_01: I mean, it's mostly most often it's very much hornblend and plagiarclase.
• 28:04
  SPEAKER_01: Very little quartz in it because again, you started with basalt.
• 28:07
  SPEAKER_01: There's not much like free quartz around or free silica around there.
• 28:10
  SPEAKER_01: Chris, we've seen these with massive garnets in them.
• 28:14
  SPEAKER_01: So you can have garnet.
• 28:15
  SPEAKER_01: You can have garnets.
• 28:16
  SPEAKER_00: The size of a baby's head, just the size of a baby's.
• 28:20
  SPEAKER_00: We have garnets that are that large.
• 28:23
  SPEAKER_01: They're amazing.
• 28:24
  SPEAKER_01: Unbelievably big.
• 28:25
  SPEAKER_01: So you can have garnet and think of if you just change the recipe of the basalt, so you just some basalts have more iron in them.
• 28:31
  SPEAKER_01: That iron will go into forming garnet.
• 28:33
  SPEAKER_01: So you just change the recipe of the basalt slightly, and you can get garnets or not garnets in this amphibolite.
• 28:39
  SPEAKER_01: And so amphibolite's a really it's quite a common metamorphic rock, at least it's not as rare as eclady or blue schist again.
• 28:45
  SPEAKER_01: But it's kind of that's correct, you know, this is in the five to seven hundred degree range and pretty deep, sort of the the center of the continental crust.
• 28:55
  SPEAKER_01: And then we get into if we go continue higher pressure and really higher temperature, we're gonna get to a rock that I really love called granulite, uh, which is just a very cool rock.
• 29:07
  SPEAKER_00: Yeah, this is a high grade, regionally metamorphosed rock, which what I mean by that again is I mean high temperature and relatively really high pressure, also.
• 29:18
  SPEAKER_00: And it's called granulite.
• 29:20
  SPEAKER_00: This is kind of a like I love it when geology makes sense, and and to me, this term makes sense.
• 29:25
  SPEAKER_00: You know, it's called granulite because the the mineral grains, they're usually granular, they're usually the same size, which is not true of amphibolite.
• 29:35
  SPEAKER_00: So they're usually the same size and they're equidimensional, and it's very like I think symmetrical in that kind of way, if that makes sense.
• 29:44
  SPEAKER_01: That's totally right.
• 29:45
  SPEAKER_01: And the way that you get grain sizes like that, where often we think of this as having what we call 6120 connections, where basically three minerals come together to form a triple junction.
• 29:56
  SPEAKER_01: It's kind of the most stable.
• 29:57
  SPEAKER_00: Think of columnar joints and so on.
• 30:00
  SPEAKER_01: It's the most stable configuration, thermodynamically stable.
• 30:03
  SPEAKER_01: The way that you get that is by having melt around and then removed.
• 30:08
  SPEAKER_01: And so granulite is a rock that has had melt removed from it.
• 30:13
  SPEAKER_01: So you've lost melt.
• 30:14
  SPEAKER_01: It's been melted.
• 30:15
  SPEAKER_01: It's the sponge after it's been squeezed out.
• 30:18
  SPEAKER_00: And this is on the precipice of you know, gosh, this is almost igneous.
• 30:24
  SPEAKER_00: You know, it's on that kind of squishy boundary, if you will.
• 30:27
  SPEAKER_01: It's what's left behind after you've made an igneous rock, is really what it is.
• 30:31
  SPEAKER_01: And granulite is just such a cool rock because it's usually this is what's in the deep crust.
• 30:37
  SPEAKER_01: If you drill down, let's say the continental crust is 40 kilometers thick, the bottom 10 kilometers are almost certainly all going to be granulite, or the vast majority of it's gonna be granulite because it's so deep, it's so hot, it's had all the good stuff removed.
• 30:50
  SPEAKER_01: All the goodies have been removed, squeezed out of it, and all you're left with is this really stable, what we call residual stuff that's everything, every all the good stuff's been melted out, and it's just what's left behind.
• 31:03
  SPEAKER_01: It's just a cool rock.
• 31:05
  SPEAKER_00: So, Jesse, correct me if I'm wrong here, but this is not going to be really uncommon rock because it's a crustal rock.
• 31:16
  SPEAKER_00: Although deep in the crust, it's still a crustal rock.
• 31:19
  SPEAKER_00: And so you can think about it, how does it get to the surface in a manner similar to the way that granite can get to the surface?
• 31:28
  SPEAKER_01: Yeah, it's less like shouldering its way through the crust like magma will do, but basically you get a package of crust that can get tilted.
• 31:36
  SPEAKER_01: Like basically, think of India, the subcontinent of India.
• 31:40
  SPEAKER_01: The bottom half of India is granulite for all intents and purposes.
• 31:43
  SPEAKER_01: Like it's been tilted.
• 31:44
  SPEAKER_01: The whole package of the continent has been tilted on its side, exposed its legs, its calves, up above, you know, bobbed up.
• 31:53
  SPEAKER_01: It's like crisple heights going swimming, and those big old calves kind of float to the surface.
• 31:58
  SPEAKER_00: Oh, so now I have cankles.
• 32:00
  SPEAKER_00: Is that what you're saying, right?
• 32:01
  SPEAKER_00: I didn't say cankle too.
• 32:02
  SPEAKER_00: Did you really just like oh my gosh, you painted a really not a good image right there?
• 32:07
  SPEAKER_01: I'm just thinking Chris Bell heist in his bathtub.
• 32:10
  SPEAKER_01: Gets in, lays back, and his legs just kind of float up outside of his control.
• 32:19
  SPEAKER_00: All right.
• 32:19
  SPEAKER_01: I like that visual.
• 32:20
  SPEAKER_01: I thought that was quite a good one.
• 32:22
  SPEAKER_00: Do you know?
• 32:22
  SPEAKER_00: I'm gonna try to move on as quickly as we can.
• 32:25
  SPEAKER_00: Um so question where have you, what's your favorite place that you've seen granulate?
• 32:32
  SPEAKER_01: You know, I saw it was a really um transformational experience for me, but as an undergrad, we went and did some mapping in southwest Sweden in a big metamorphic terrain, and we were walking across granulites.
• 32:42
  SPEAKER_00: Um, you're with Dr.
• 32:43
  SPEAKER_00: Hansen, right?
• 32:44
  SPEAKER_00: Dr.
• 32:45
  SPEAKER_01: Hansen from Hope, and um, he's an expert in these sort of this type of metamorphism, and we were walking across mapping them, and it was just you walk across here and you're like, holy crap, this this rock has literally lost 30% of its volume to a magma.
• 33:00
  SPEAKER_01: Like it is, it's the other half of a granite.
• 33:02
  SPEAKER_01: This is the other part of it.
• 33:03
  SPEAKER_01: Like, this is so cool.
• 33:05
  SPEAKER_00: That's really interesting.
• 33:06
  SPEAKER_00: So, another follow-up question: did you come across this at all in your Northwest studies, Northwest Territories of Canada?
• 33:13
  SPEAKER_01: Did you come across so I haven't seen not in that part of the Northwest Territories?
• 33:18
  SPEAKER_01: Where I have seen them up there is they get carried up in the the Kimberlite pipes that bring diamonds up, the sort of volcanic eruptions.
• 33:24
  SPEAKER_01: They'll because it takes samples all the way through as it's erupting up through the crust, it'll sample granulites from there.
• 33:30
  SPEAKER_01: And actually, there's a PhD student who was working on granulites at Penn State, and actually one of our listeners has asked about these.
• 33:37
  SPEAKER_01: But um, from the little basalt eruptions out of southwest United States in the in the sort of basin and range environment, these little small volume magmas can bring up pieces of granulite from the lower crust there.
• 33:50
  SPEAKER_01: And this is one way to really form granulite, is if the continent starts spreading apart, basically the mantle underneath is super hot and comes up to the surface and heats up the bottom of the crust really quickly.
• 34:03
  SPEAKER_01: And that will do this high temperature melting basically that can produce granulite.
• 34:08
  SPEAKER_01: So you can kind of bake the whole bottom half of the of the continent with a mantle plume coming up, will just bake the bottom half of the continent, form tons of granulite.
• 34:17
  SPEAKER_01: So that's the other way you get granulite is in these xenoliths that come up to the surface.
• 34:22
  SPEAKER_00: So there we go with that different host of rocks from green schist to amphibolite to granulite, which is increasing metamorphic grade, but in more of a continent to continent convergent boundary as opposed to subduction zone, where we talked about blue schist and eclogite.
• 34:42
  SPEAKER_00: So, Jesse, let's round this episode out real quick with the final rock here called zeolites.
• 34:50
  SPEAKER_00: I've collected some zeolites in Copper Harbor, in really like as far north as you can go in and still be in the state of Michigan.
• 35:02
  SPEAKER_01: These are um these are low, really low grade, kind of hydrothermal metamorphism, I'd say, like really low pressure and temperature.
• 35:10
  SPEAKER_01: And people, I think the the gem collecting community will have heard of zeolite minerals.
• 35:15
  SPEAKER_01: It's a miner, zeolite is a mineral group, but if you take basalts and if you have air pockets, what we call vesicular basalts, where there's little air pockets, you slightly metamorphose.
• 35:26
  SPEAKER_00: And this is can I interrupt you a second, Jesse?
• 35:28
  SPEAKER_00: So the basalt is it commonly has these air pockets because this is an extrusivigneous rock.
• 35:34
  SPEAKER_00: So it's degassing at the surface, and as it's cooling, it will have these big air pockets that are preserved within the rock, and that's where these zeolite minerals kind of reside.
• 35:47
  SPEAKER_01: That's exactly right.
• 35:48
  SPEAKER_01: That's exactly right.
• 35:48
  SPEAKER_01: So if you take that those zeolites, they just basically precipitate out of hot fluids circulating through the rock.
• 35:55
  SPEAKER_01: And it's kind of metamorphism.
• 35:57
  SPEAKER_00: It's it's it's not you know, it's not like it doesn't look like a metamorphic rock.
• 36:01
  SPEAKER_00: Not really, honestly, at all.
• 36:02
  SPEAKER_00: It's just been filled with my experience.
• 36:04
  SPEAKER_00: No, I totally.
• 36:05
  SPEAKER_01: That's right.
• 36:05
  SPEAKER_01: So, but you're right.
• 36:06
  SPEAKER_01: Uh that's like the the sort of lower end of the of the spectrum here.
• 36:10
  SPEAKER_00: Jesse, are these are zeolites associated with petroleum at all?
• 36:15
  SPEAKER_01: Oh, good question.
• 36:16
  SPEAKER_01: The minerals sometimes are not in basalts, but in other types of rocks, where you get there's some mineral deposits, especially up in like the Yukon, where there's a lot of oil and gas.
• 36:27
  SPEAKER_01: You get calcite precipitating with zeolite minerals, and you also get a little sort of hydrocarbon in these little pockets of shales and conglomerates and quartzites and stuff like that.
• 36:38
  SPEAKER_01: So, yes, they can be certainly.
• 36:40
  SPEAKER_01: It's that it's that temperature range.
• 36:42
  SPEAKER_01: Like oil maturations like in the 100 to 200 degrees range.
• 36:46
  SPEAKER_01: That's what we're talking about with zeolites, is low temperature stuff.
• 36:48
  SPEAKER_01: Okay, we talked about blue shistorites.
• 36:50
  SPEAKER_00: So we probably should have put zeolites up top because it really didn't follow our flow.
• 36:55
  SPEAKER_01: Maybe, but here's what it does deep for us, Chris.
• 36:57
  SPEAKER_01: It brings us full circle to talk about why.
• 37:00
  SPEAKER_01: Let's let's talk about the pedagogy here for like five minutes, because I think it's a really interesting question.
• 37:05
  SPEAKER_01: So we talked about well, zeolites, the really low, low pressure, low temperature.
• 37:09
  SPEAKER_01: We talked about blue shist eclogite, which are high pressure and relatively low temperature.
• 37:14
  SPEAKER_01: Then we talked about green schist, amphibolite, and granulite, which are medium pressure, medium temperature, let's put it that way.
• 37:20
  SPEAKER_00: Different tectonic settings, yes.
• 37:21
  SPEAKER_01: All start with the same recipe, have different tectonic settings, and dramatically different rock types that you get out of it.
• 37:28
  SPEAKER_01: So let me pitch this to you, Chris, and you can tell me what where I'm sort of thinking about it wrong, right?
• 37:33
  SPEAKER_01: You said, and every other, myself included, everybody else who teaches the intro to physical geology, like the basic geology class, teaches the rocks we talked about last episode: slate, phylite, schist, gneiss, migmatite, right?
• 37:49
  SPEAKER_01: Like that's the sequence, which is just increasing metamorphic grade.
• 37:52
  SPEAKER_01: This sequence here, we get to say the same recipe, but we also get to say tectonic setting, and we get to explain how subduction zones have lower temperatures than other types of metamorphic settings.
• 38:08
  SPEAKER_01: And I recognize that the rocks, eclogite, blue schists, granulite, they're uncommon.
• 38:14
  SPEAKER_01: You're really not gonna see them out in the field.
• 38:17
  SPEAKER_01: But do you think that teaching this sequence of rocks that we've just talked about might give students a better way to understand the earth even though they're learning rare rock types, or learn plate tectonics, maybe, even though it's rare rock types?
• 38:32
  SPEAKER_01: Or is it just too deep?
• 38:33
  SPEAKER_01: I I I like I I don't know.
• 38:34
  SPEAKER_01: I okay because I I I have a thought on it.
• 38:37
  SPEAKER_01: I do I teach the the basic ones, then the ones we all teach, right?
• 38:40
  SPEAKER_01: But I also layer these on.
• 38:42
  SPEAKER_01: I spend an extra lecture talking about I say, okay, schist, phylite, slate.
• 38:47
  SPEAKER_01: That's if you have shale as a starting composition.
• 38:49
  SPEAKER_01: If you change the recipe, you get green schist, amphiblite, granulite, but also wait, if you change pressure and temperature, you get blue schist eclogite.
• 38:57
  SPEAKER_01: And I don't, I kind of do a frankly a half-assed job of it, and it doesn't quite land, I don't think.
• 39:02
  SPEAKER_01: And I wonder what would happen if I did, you know, if I went the full way and just said, I'm only teaching these ones for metamorphism.
• 39:10
  SPEAKER_00: Yeah.
• 39:11
  SPEAKER_00: Thoughts.
• 39:12
  SPEAKER_00: So uh first my thought is that a lot of these are the same kind of rock where we talked about slate, philate, schist.
• 39:20
  SPEAKER_00: Because a lot of these are schists, these are just parsing it out.
• 39:24
  SPEAKER_00: Like let's get into the weeds now on our schists here at play, and these index minerals and these faces that exist within a schist.
• 39:35
  SPEAKER_00: So we kind of are, I don't think it's fruitful for an intro-level class to get into these kinds of weeds.
• 39:44
  SPEAKER_00: These are deep weeds.
• 39:45
  SPEAKER_01: These are you think so.
• 39:47
  SPEAKER_00: I think they are.
• 39:48
  SPEAKER_00: I think for a petrology class it's totally appropriate.
• 39:52
  SPEAKER_00: But I also think like the value of going out and seeing a hornblende schist or a biotite schist or you know, things like this is you can go almost anywhere to where you have mountain bill in the western US and you can see the rocks that we talked about in our previous episode.
• 40:09
  SPEAKER_00: That's a powerful thing.
• 40:11
  SPEAKER_00: And you can talk about that formation in a very general sense, but I think because we'd have to go, Jesse, we'd have to go everywhere, we'd have to go here and then thousands of miles away to see a blue schist and an eclogite and so on, unless we just spend our time at the Franciscan complex.
• 40:26
  SPEAKER_01: And no, it's a good point.
• 40:28
  SPEAKER_01: Uh uh one of our fa one of my fellow faculty members described it just recently as, you know, we have all the good stuff.
• 40:34
  SPEAKER_01: We have volcanoes, we have earthquakes, we have all the exciting, cool, important stuff.
• 40:39
  SPEAKER_01: We just need to do a better job of selling it to our fellow human beings that we need to know this stuff because it's so important and so unique and and it makes our planet, as you say, totally cool.
• 40:51
  SPEAKER_01: So I just I think that's part of the sales pitch for geoscience that we just need to do a better job of, frankly, highlighting the value of understanding plate tectonics because it tells us where mineral deposits are gonna be.
• 41:02
  SPEAKER_01: Uh it's just so important, it's just as simple as that.
• 41:05
  SPEAKER_01: So I totally agree.
• 41:06
  SPEAKER_01: I totally agree with everything, everything you're saying there.
• 41:08
  SPEAKER_01: And and in that light, you know, talking about the rocks that you're gonna see most as you wander around the earth is is makes a lot of sense, you know, and people are very rarely gonna stumble across blue schists.
• 41:20
  SPEAKER_01: So even if it helps you understand subduction zones a bit better, I I see your point.
• 41:24
  SPEAKER_01: It is second level, it's definitely deeper than you have time to go through in one semester class, for instance.
• 41:31
  SPEAKER_00: So yeah, I I think so because it's hard enough with just understanding that we need to be able to identify rocks because it tells us a story, but we can't spend an inordinate amount of time on that because we have other important things to do.
• 41:47
  SPEAKER_00: That's true.
• 41:48
  SPEAKER_01: We gotta talk about glaciers, we gotta talk about streams, like all these other you're you're totally right.
• 41:52
  SPEAKER_00: It's just it's a matter of talk about climate, we gotta talk about resources, we gotta talk these other things that the environmental aspect, water quality, totally, groundwater contamination, and you know, things like this that are super important.
• 42:06
  SPEAKER_01: Absolutely.
• 42:06
  SPEAKER_01: Well, okay, Chris, I've got one quite good story for you and an episode proposal for you.
• 42:11
  SPEAKER_01: Actually, before we were even talking about doing this episode, our good friend, friend of the pod, Jackie Faraday, who we've had on a couple times, who studies the origins of the universe and all of humans, she was traveling and was sending me photos of rocks and asking me, what the heck?
• 42:26
  SPEAKER_01: How did this form?
• 42:27
  SPEAKER_01: And she was looking at green schist.
• 42:29
  SPEAKER_01: She's looking at green schists in a place that has subduction zones.
• 42:32
  SPEAKER_01: And she's like, Why are all these rocks green?
• 42:34
  SPEAKER_01: It's totally cool.
• 42:34
  SPEAKER_01: And then there was a she had a like a quartz vein, and so I was explaining, I was just quickly, you know, over text describing a quartz vein.
• 42:41
  SPEAKER_01: And you know what she said to me?
• 42:42
  SPEAKER_01: You know, I was mentioning quartz and chlorite and things like this, and she goes, Oh, but I study quartz and endstatite and foresterite in the atmospheres of extrasolar objects, so I totally get it.
• 42:52
  SPEAKER_01: And I was like, now I'm thinking we gotta have her on to talk about this.
• 42:56
  SPEAKER_01: What does that mean, Jackie?
• 42:57
  SPEAKER_01: And what did you think about looking at your first green schist out in the field?
• 43:02
  SPEAKER_00: Don't you think?
• 43:03
  SPEAKER_00: Oh, sh she'll let you know.
• 43:05
  SPEAKER_00: Oh, yeah.
• 43:05
  SPEAKER_01: But don't you think we should have Ron talk about this?
• 43:07
  SPEAKER_00: You had me when you said Jackie Fardy.
• 43:09
  SPEAKER_00: So I was I was all over.
• 43:11
  SPEAKER_01: I mean, just the greatest.
• 43:12
  SPEAKER_01: She's just the greatest.
• 43:14
  SPEAKER_01: All right, well, we'll I'll uh I'll see if she wants to come on.
• 43:16
  SPEAKER_01: She can tell us about her geologizing out in the world a little bit too.
• 43:20
  SPEAKER_00: Oh, she'll let us know.
• 43:21
  SPEAKER_00: Yeah.
• 43:21
  SPEAKER_00: She'll let us know.
• 43:22
  SPEAKER_00: Totally.
• 43:22
  SPEAKER_01: She's already she said she's got some theories about how quartz veins form, so we can uh we can hear about those.
• 43:27
  SPEAKER_01: It'd be great.
• 43:28
  SPEAKER_00: All right.
• 43:28
  SPEAKER_01: I'd love to hear them.
• 43:29
  SPEAKER_01: Oh man, so good.
• 43:31
  SPEAKER_01: Hey, Chris, I don't know.
• 43:32
  SPEAKER_01: What do you think?
• 43:32
  SPEAKER_01: Is that a wrap?
• 43:33
  SPEAKER_01: That's a wrap.
• 43:34
  SPEAKER_01: Cool.
• 43:34
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• 43:52
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• 43:55
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• 44:01
  SPEAKER_01: Just, you know, as you're driving in your car or you're going on a walk and we're describing things like Chris, this pressure, temperature, plot, green shifts, where these things are in this plot.
• 44:10
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• 44:12
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• 44:16
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• 44:17
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• 44:25
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