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Rock Circles: Geologically Curious Phenomena (GeoShort) · PlanetGeo: The Geology Podcast

• 0:06
  SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
• 0:22
  SPEAKER_00: Chris.
• 0:23
  SPEAKER_00: Jesse, how are you doing?
• 0:26
  SPEAKER_00: I am good, man.
• 0:27
  SPEAKER_00: I'm good.
• 0:28
  SPEAKER_00: We got a fun one today.
• 0:29
  SPEAKER_00: Yeah, we do.
• 0:30
  SPEAKER_00: But first, you're Chris Bullheis.
• 0:32
  SPEAKER_00: You are a nationally recognized, national award-winning high school teacher from Michigan.
• 0:37
  SPEAKER_00: From Hudsonville High School in uh Hudsonville, Michigan.
• 0:41
  SPEAKER_00: It's hard for you to say, wasn't it?
• 0:42
  SPEAKER_00: I know.
• 0:43
  SPEAKER_00: I had to get it out.
• 0:44
  SPEAKER_00: I mean nationally recognized.
• 0:46
  SPEAKER_00: Oh my goodness.
• 0:47
  SPEAKER_00: Your ego is large enough.
• 0:48
  SPEAKER_00: I don't need to continue to make it bigger.
• 0:51
  SPEAKER_01: All right.
• 0:52
  SPEAKER_01: Well, you are Dr.
• 0:53
  SPEAKER_01: Jesse Rymank, and uh you're one of my former students back in the day, a long time ago.
• 0:57
  SPEAKER_00: Yep.
• 0:58
  SPEAKER_01: And uh you went on to get your PhD in geology or geoscience, and you are now a professor of geoscience at Penn State University.
• 1:06
  SPEAKER_00: Penn State, we are.
• 1:07
  SPEAKER_00: It's uh coming up on football season, and seems like everybody's excited about it.
• 1:11
  SPEAKER_01: I think you and I are gonna have to catch a game.
• 1:13
  SPEAKER_00: We might have to this year, right?
• 1:15
  SPEAKER_00: We're gonna get back to normal.
• 1:16
  SPEAKER_00: Totally.
• 1:17
  SPEAKER_01: I hope so.
• 1:18
  SPEAKER_01: I'll make the drive.
• 1:19
  SPEAKER_00: Perfect.
• 1:19
  SPEAKER_00: Can't wait.
• 1:20
  SPEAKER_01: Well, hey, let's go.
• 1:22
  SPEAKER_01: Let's go, man.
• 1:23
  SPEAKER_01: We're gonna talk about something like this.
• 1:26
  SPEAKER_01: Might seem like an odd random topic.
• 1:28
  SPEAKER_01: It's something that's called spheroidal weathering.
• 1:31
  SPEAKER_01: And it's not a really common thing, but it's a it's just a cool thing when you see this in the field.
• 1:37
  SPEAKER_00: Yes, and it's a really cool feature.
• 1:39
  SPEAKER_00: It's shocking.
• 1:40
  SPEAKER_00: If you walk past it on a hike, you're gonna be like, what the hell is going on there?
• 1:44
  SPEAKER_00: Like crazy, right?
• 1:45
  SPEAKER_00: But I think it also there's a couple really important basic concepts that are uh uh teachable with this process of spheroidal weathering.
• 1:55
  SPEAKER_00: So, as the name implies, this is weathering of a rock into a sphere.
• 2:00
  SPEAKER_00: Like that's kind of it, right?
• 2:02
  SPEAKER_00: Um, but let's get into the details real quick.
• 2:05
  SPEAKER_01: Yeah, well, let's first talk about what it's not.
• 2:07
  SPEAKER_01: I think that's important.
• 2:08
  SPEAKER_01: And you were talking about all right, well, Chris, what's the hook gonna be with this?
• 2:11
  SPEAKER_01: Like, what are what are we doing this for?
• 2:13
  SPEAKER_01: Well, in geology, there are two kinds of weathering.
• 2:17
  SPEAKER_01: Okay, there's uh mechanical weathering and then there's chemical weathering.
• 2:21
  SPEAKER_01: And what spheroidal weathering is not, it's not a type of mechanical weathering.
• 2:28
  SPEAKER_01: Mechanical weathering, like uh imagine a fast flowing river and it's it's taking gravel and you know larger particles and rolling them downstream along the bed of a river.
• 2:39
  SPEAKER_01: And the further it goes and the longer this takes place, you're gonna end up with spheres, right?
• 2:43
  SPEAKER_01: You're gonna end up with these rounded pieces of rock.
• 2:47
  SPEAKER_00: That's right.
• 2:48
  SPEAKER_00: And so the little corners get knocked off when Crisple Heis, when you're out there with your rock hammer bashing away at stuff, you are doing mechanical weathering, right?
• 2:55
  SPEAKER_00: That's crystal heist-induced mechanical weathering.
• 2:58
  SPEAKER_01: Right on.
• 2:58
  SPEAKER_01: Yeah.
• 2:59
  SPEAKER_01: And it doesn't, mechanical weathering is just as the name implies, it doesn't involve a change in chemistry, just taking rocks and breaking them up into smaller and smaller pieces.
• 3:07
  SPEAKER_01: That's mechanical weathering.
• 3:08
  SPEAKER_00: Chemical is, as the name implies, it's chemistry.
• 3:10
  SPEAKER_00: This is the rock being dissolved.
• 3:12
  SPEAKER_00: The rock being changing.
• 3:15
  SPEAKER_00: Yes, there's water involved.
• 3:16
  SPEAKER_00: This is dissolving the rock in in parts and uh leaving other parts untouched.
• 3:22
  SPEAKER_00: And so spheroidal weathering is chemical weathering.
• 3:25
  SPEAKER_00: This is a a f uh one type of chemical weathering that's going on.
• 3:29
  SPEAKER_01: It is, but it's also an interesting intersection between mechanical weathering and chemical weathering.
• 3:34
  SPEAKER_01: Now, let me explain what I mean.
• 3:35
  SPEAKER_01: So, in order to get spheroidal weathering, something has to happen first.
• 3:39
  SPEAKER_01: You need to have this, like oftentimes it's granitic rock, this rock that's granite, okay, light colored, loaded with quartz and feldspar and things like that.
• 3:48
  SPEAKER_01: And it needs to be jointed or cracked, and it needs to be cracked in a kind of a special kind of way where these cracks in the rock intersect at nearly right angles.
• 4:00
  SPEAKER_01: You end up with these rocks that are rectangular shape or square roughly.
• 4:05
  SPEAKER_01: So that's the mechanical weathering part.
• 4:07
  SPEAKER_01: It's broken, but it's broken in a unique kind of way.
• 4:11
  SPEAKER_01: Now, this isn't really uncommon because you know, where does granite form, Jesse?
• 4:16
  SPEAKER_00: Well, it happens in many mountain belts, it happens uh in places where tectonic stresses are going on.
• 4:21
  SPEAKER_01: And it's deep though, right?
• 4:23
  SPEAKER_01: It's not this isn't at the surface.
• 4:24
  SPEAKER_01: Granite forms deep inside the earth.
• 4:26
  SPEAKER_01: But here we have granite exposed at the surface.
• 4:29
  SPEAKER_01: So all this pressure is gone, the weight of this overburdened rock is gone, and so it's quite common then as the rock expands.
• 4:38
  SPEAKER_01: Rock can't just stretch.
• 4:39
  SPEAKER_01: And so as it expands, it'll often like pressure will build up, build up, build up, and then it just cracks.
• 4:46
  SPEAKER_00: Yes, you know, and and pressure building up, this is tensional pressure, this is expansion pressure.
• 4:51
  SPEAKER_00: So the rock was under pressure, it was compact.
• 4:54
  SPEAKER_00: As it's coming up to the surface, it's kind of releasing that pressure and it's breaking apart because it's being expanded under the release of pressure, right?
• 5:02
  SPEAKER_00: Exactly.
• 5:03
  SPEAKER_01: Okay, did you like my sound effect there?
• 5:04
  SPEAKER_01: That was a good one.
• 5:04
  SPEAKER_01: I didn't know I could do that.
• 5:05
  SPEAKER_00: Yeah, I can't quite reproduce it.
• 5:08
  SPEAKER_01: So you have these this jointed mechanically weathered rock.
• 5:13
  SPEAKER_01: Now comes the intersection of mechanical weathering and chemical weathering.
• 5:17
  SPEAKER_01: So what's going on?
• 5:19
  SPEAKER_01: How do you take these blocks and change them into spheres?
• 5:23
  SPEAKER_00: So this is the key to a very basic geometry concept, which is the surface area compared to the volume of a shape.
• 5:32
  SPEAKER_00: And basically, what's happening here, if you go back to your mechanical weathering, this let's picture this shard of rock, let's say it has a bunch of angles, it's tumbling down a stream, those corners are gonna be knocked off mechanically, right?
• 5:44
  SPEAKER_00: That makes sense.
• 5:45
  SPEAKER_00: Those are easier to break off.
• 5:47
  SPEAKER_00: The same kind of principle applies to chemical weathering.
• 5:51
  SPEAKER_00: The corners or a sharp edge has a lot of surface area and not much volume to it.
• 5:57
  SPEAKER_00: There's not much depth.
• 5:58
  SPEAKER_00: So any chemical reactions that are taking place on the surface of that are gonna eat into it really quickly and therefore eat it away really quickly.
• 6:06
  SPEAKER_00: And this means that you're gonna take this cube or this rectangle of rock, and the edges are being chemically dissolved by groundwater flowing through there.
• 6:17
  SPEAKER_00: We've talked about chemical erosion in previous episodes in the context of why the oceans are salty, but but this is chemical erosion occurring on the surfaces.
• 6:26
  SPEAKER_01: You're right.
• 6:27
  SPEAKER_01: The most common mineral on the planet is feldspar.
• 6:30
  SPEAKER_01: And feldspar is uh rather unstable at the surface of the earth, and so it changes chemically into a clay mineral.
• 6:40
  SPEAKER_01: And that's what's happening with this spheroidal weathering is the feldspar in the rock is turning into clay.
• 6:46
  SPEAKER_01: The rate happens fastest on the corners, followed by the edges, followed by the surface.
• 6:53
  SPEAKER_01: So think about if you had a block of wood and you sanded more on the corners, then a little bit less on the edges, and then less yet on the surfaces, you're gonna take a block of wood and just by sanding it that way, turn it into a sphere.
• 7:09
  SPEAKER_00: That's right.
• 7:10
  SPEAKER_00: And so once you get to the sphere, then it just gets to be a smaller and smaller and smaller sphere.
• 7:15
  SPEAKER_00: And this spheroidal weathering can also be called onion peel weathering or concentric weathering, because once you get to a sphere, then you're at the surface area to volume minimum for a shape, and it's just going to get smaller and smaller and smaller.
• 7:30
  SPEAKER_00: Once you've taken a rectangle or a square and made it into a sphere, it's just going to get it to become a smaller sphere by this chemical weathering process that is spheroidal weathering.
• 7:40
  SPEAKER_01: And you know, as the feldspar turns into clay, the clay then gets wet and it expands because clay soaks up water and it expands and it just kind of spalls off like a like an onion.
• 7:52
  SPEAKER_00: So as you break down the feldspars chemically, then the rock just falls apart and it just crumbles.
• 7:57
  SPEAKER_00: So when you're walking, if you ever hike across a rock, and this can happen in granite, most commonly, basalts, other igneous rocks like that, it can also happen in sort of massive sedimentary rocks like sandstones or quartzites sometimes.
• 8:10
  SPEAKER_00: Uh, but if you ever see this, that looks like the rock is just breaking apart like an onion, this is spheroto weathering, and you can think, oh, that's really cool.
• 8:18
  SPEAKER_00: This is a sphere, and you know, think about the surface area to volume ratio of objects, think about mechanical weathering in a stream, think about Crisple Heist out there mechanically weathering rocks with his rock hammer.
• 8:29
  SPEAKER_00: Uh, you know, it's a super fun thing to think about, and it's beautiful.
• 8:32
  SPEAKER_00: It is.
• 8:32
  SPEAKER_01: You know, by the way, I don't have a I don't usually use a rock hammer, I use the sledgehammer.
• 8:36
  SPEAKER_00: Oh, you use your fists, you use your fists in your big bald head.
• 8:40
  SPEAKER_00: Is that what you're doing?
• 8:41
  SPEAKER_00: Bashing that against a rock.
• 8:42
  SPEAKER_00: I know you're out there with you trying to get that limestone to deal with your digestion issues and stuff like that, aren't you?
• 8:49
  SPEAKER_01: That's just smart.
• 8:50
  SPEAKER_01: That's just smart.
• 8:52
  SPEAKER_01: Yeah, that's right.
• 8:53
  SPEAKER_00: That's right.
• 8:54
  SPEAKER_00: Uh-huh.
• 8:54
  SPEAKER_01: All right.
• 8:55
  SPEAKER_01: So anyway, spherota weathering, really, really cool thing.
• 8:59
  SPEAKER_01: Um, when you see this in the field, it's just you and I have seen this.
• 9:02
  SPEAKER_01: We saw Colorado.
• 9:03
  SPEAKER_00: It was Colorado, right?
• 9:05
  SPEAKER_01: Yeah, I think it was.
• 9:06
  SPEAKER_01: Yeah.
• 9:06
  SPEAKER_01: Yeah.
• 9:06
  SPEAKER_01: Near Pike's Peak, I think, but I'm not positive on that.
• 9:09
  SPEAKER_01: I but I think it was near Colorado Springs.
• 9:11
  SPEAKER_00: Yeah, so you get you get it commonly in areas with granite, you get it commonly where there's a decent amount of rain or a particular type of weathering style.
• 9:20
  SPEAKER_00: It's not super common, but you you do see it.
• 9:22
  SPEAKER_00: If you're out hiking around, you're gonna run across it eventually.
• 9:25
  SPEAKER_00: And you know, I've seen it in California, a couple places out east as well, in the sandstones.
• 9:30
  SPEAKER_01: But it yeah, it's cool.
• 9:31
  SPEAKER_01: Yeah, I would encourage you to go ahead and Google image through rotov weathering, and you'll get a clear picture as to what we're trying to describe here then and how it happens.
• 9:40
  SPEAKER_00: Yeah.
• 9:44
  SPEAKER_00: Hey, there we go.
• 9:44
  SPEAKER_00: That's a wrap.
• 9:45
  SPEAKER_00: That's a Geo short.
• 9:46
  SPEAKER_00: Okay.
• 9:47
  SPEAKER_00: See you next week.