Latest / PlanetGeo: The Geology Podcast / How is Obsidian Formed? (GeoShort)

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How is Obsidian Formed? (GeoShort) · 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_01: All right, let's do some introductions, Chris.
• 0:16
  SPEAKER_01: You are Chris Bohois, high school science teacher extraordinaire in the great state of Michigan.
• 0:21
  SPEAKER_01: I am Jesse Ramick, geoscience professor at Penn State, and your former student.
• 0:27
  SPEAKER_01: And this is Planet Geo, a podcast where we talk about some amazing aspects of our planet and why they matter.
• 0:32
  SPEAKER_01: Welcome to Planet Geo, Geo Shorts.
• 0:35
  SPEAKER_01: Today we are going to cover a common misconception that we hear in the geosciences a lot.
• 0:40
  SPEAKER_01: And uh, Chris, you hear this one in your classroom quite frequently.
• 0:43
  SPEAKER_01: So what's the misconception we're talking about?
• 0:46
  SPEAKER_00: The misconception is that obsidian forms from an instant cooling of a rock like basalt, which is a very dark-colored rock that forms from lava.
• 0:56
  SPEAKER_00: That's the misconception.
• 0:57
  SPEAKER_00: Like instant cooling in water of a mafic basaltic lava flow.
• 1:02
  SPEAKER_01: Yeah, okay.
• 1:03
  SPEAKER_01: That's this is a good one.
• 1:04
  SPEAKER_01: This is a good one.
• 1:05
  SPEAKER_01: So, those of you out there that don't know what obsidian is, you probably maybe have heard the term before, but it's it's a really pretty rock.
• 1:11
  SPEAKER_01: It's one of these rocks.
• 1:12
  SPEAKER_01: I mean, I love obsidian.
• 1:13
  SPEAKER_01: Chris, I'd imagine you do too.
• 1:14
  SPEAKER_00: Oh, obsidian is one of my favorites.
• 1:16
  SPEAKER_00: I'm a I'm a big fan of obsidian for sure.
• 1:18
  SPEAKER_01: It's beautiful, but it's also used, you know, is used by the Native Americans for arrowheads because it makes these really nice sharp edges.
• 1:25
  SPEAKER_01: It's very glassy in texture, it's usually dark colored.
• 1:28
  SPEAKER_01: It's big in the sort of mineral collector, rock collector kind of community.
• 1:31
  SPEAKER_01: You know, people often assume that obsidian is formed by lava going into water and cooling really, really quickly.
• 1:38
  SPEAKER_01: But that's not the case, right, Chris?
• 1:40
  SPEAKER_00: It's not necessarily the case.
• 1:42
  SPEAKER_00: You know, the misconception comes from you know, basalt is black, um, Hawaii has a lot of basalt, it flows into the ocean, and when it flows into the ocean, you get instant cooling and you get obsidian.
• 1:53
  SPEAKER_00: But that's not really not what happens.
• 1:55
  SPEAKER_00: Obsidian actually happens from the opposite kind of lava and from a chemical standpoint, right?
• 2:01
  SPEAKER_00: It forms from what we call in geology rhyolite, which is a very light-colored rock.
• 2:07
  SPEAKER_00: Okay, it's like yellowish or whitish or pinkish, even.
• 2:11
  SPEAKER_00: In fact, yellowstone.
• 2:12
  SPEAKER_00: Yellowstone is named Yellowstone because most of the rock inside the caldera is yellow, because it's all rhyolite.
• 2:18
  SPEAKER_00: Everywhere you look, it's rhyolite, rhyolite, rhyolite.
• 2:21
  SPEAKER_00: So that's the kind of stuff that obsidian forms from, because that kind of lava is so thick and sticky and viscous that when it comes out onto the surface, it cools quickly, not instantly, but quickly.
• 2:38
  SPEAKER_00: But the lava's so sticky and thick that crystallization can't happen.
• 2:44
  SPEAKER_00: And so as it cools, it cools as glass.
• 2:47
  SPEAKER_01: That's right.
• 2:48
  SPEAKER_01: So glass is a physical term, which means that there's no crystals in it, there's no specific minerals in it.
• 2:53
  SPEAKER_01: It is just elements that are all just locked in this amorphous mix of stuff.
• 2:59
  SPEAKER_01: There's no crystals, there's no quartz.
• 3:01
  SPEAKER_01: They're frozen in time.
• 3:02
  SPEAKER_01: Exactly.
• 3:03
  SPEAKER_01: And it's just there's no crystalline structure to it.
• 3:05
  SPEAKER_01: So that's what glass is.
• 3:07
  SPEAKER_00: It's almost as if the ions, as they're migrating to form crystals, just can't get there and they're just frozen right in time.
• 3:14
  SPEAKER_01: That's right.
• 3:15
  SPEAKER_01: So they form a glass, and it's it's too sticky, and it's too sticky because the chemical composition of it is uh, well, it doesn't matter the details here, but it forms the all these chain silicates in the magma.
• 3:26
  SPEAKER_01: It doesn't matter.
• 3:26
  SPEAKER_01: The chemical composition of it makes it really thick and really sticky, such that elements cannot move around in it to form minerals.
• 3:34
  SPEAKER_01: Go back to the Hawaii analogy.
• 3:36
  SPEAKER_01: When you have basaltic lava that flows into the ocean, the outer part of that crystallizes really quickly, and it does form a little bit of glass there, but most of it forms basalt because the ions can migrate really quickly throughout that basalt.
• 3:50
  SPEAKER_00: That composition of the lava is that lava is so thin and so runny.
• 3:54
  SPEAKER_01: It's thin and runny, and you can form crystals, you can form actual minerals in it.
• 3:58
  SPEAKER_01: Whereas in obsidian, you can't.
• 4:00
  SPEAKER_00: So when you go to places like Yellowstone or the Newberry volcanics in Oregon, um, where that lava was so sticky, so the edges of the lava flow is very common to have a fair amount of obsidian formed, which is a very, very cool rock.
• 4:16
  SPEAKER_01: That's right.
• 4:16
  SPEAKER_01: Obsidian is found a lot out in the southwest United States, where these old volcanics out near the Grand Canyon, anywhere there's these sort of younger volcanic rocks around that erupted this particular kind of magma, which is high silica content magma, rhyolites and dacites will form obsidian flows.
• 4:33
  SPEAKER_01: And they're beautiful rocks.
• 4:34
  SPEAKER_01: I mean, they're really spectacular.
• 4:35
  SPEAKER_01: I love looking at obsidian, it's beautiful stuff.
• 4:38
  SPEAKER_01: So, all right, Chris, what is Snowflake Obsidian?
• 4:42
  SPEAKER_00: Snowflake Obsidian comes from the just the way it looks.
• 4:45
  SPEAKER_00: You have this black obsidian that has these like white or gray radiating crystals that come out from it, and that's an interesting thing because obsidian is actually chemically unstable.
• 4:56
  SPEAKER_00: It's so unstable, actually, that old obsidian on the planet doesn't exist.
• 5:00
  SPEAKER_00: It's very young rock, almost universally.
• 5:04
  SPEAKER_00: When it hits the weathering surface of the earth, obsidian begins to crystallize.
• 5:09
  SPEAKER_00: And that's different because obsidian is volcanic glass, you know, and so it's it's it doesn't have any crystals, and it begins to crystallize, but it doesn't happen at the same rate throughout the rock.
• 5:18
  SPEAKER_00: Instead, you get various locations within the rock that start to crystallize, and they kind of radiate out from clusters into these white and gray, and it's it forms a mineral that's called crystalite.
• 5:30
  SPEAKER_00: Right.
• 5:30
  SPEAKER_00: So, in other words, snowflake obsidian forms from crystallization and weathering after the rock actually formed.
• 5:37
  SPEAKER_01: Yeah, that's right.
• 5:38
  SPEAKER_01: Very interesting, very curious stuff.
• 5:40
  SPEAKER_00: So to highlight, it's formed from really sticky lava that comes out of the surface.
• 5:45
  SPEAKER_00: So the edges of lava flows, the outer edges of lava domes, where lava flows meet glaciers and so on, you're gonna get obsidian if it's thick enough and sticky enough.
• 5:56
  SPEAKER_00: So there we go.
• 5:58
  SPEAKER_00: That's a wrap.
• 5:59
  SPEAKER_00: We thank you all for tuning in.
• 6:01
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• 6:06
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• 6:18
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• 6:22
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• 6:24
  SPEAKER_01: Take care.