<p>Devitrification is the crystallization of volcanic glass to crystals. Obsidian groundmass -black in color- turns to white with devitrification.</p>
<p>I want to learn why devitrification takes place.</p>
<p>Attached photo, exhibits an example of devitrification, which i had taken in the field. <a href="https://i.sstatic.net/1s6P6.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/1s6P6.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2024-02-22T13:31:09.577Z
Are you sure it's an ignimbrite? I've never heard of obsidian layers within an ignimbrite deposit. Generally obsidian is associated with effusive volcanism (rhyolite lava flows). In such flows, there are often the two facies (obsidian and crystalline rhyolite) coexisting, see the model of Fink (1983) https://doi.org/10.1130/0016-7606(1983)94%3C362:SAEOAR%3E2.0.CO;2 (sometimes at a small scale, see flow banding). Such flows are like this *from the start*, i.e., the rhyolite part is non-glassy from the beginning. But it is true that the obsidian part will tend to devitrify over long timescales.
Jean-Marie Prival
CC BY-SA 4.0
2024-02-28T19:49:45.523Z
There are other facies in Fink (1983). The succession bottom to top is, Basal Breccia, Coarse Pumice, Obsidian, fine pumice, surface breccia. If i see such a succession in the field i will say this is an ignimbrite not a lava flow. In that paper photos are black-white i i couldn't see textures properly. In my field study i saw eutaxitic and vitrophyric textures origining from a pyroclastic density current but i saw a lithofacies inside this flow unit, which has abundant visible crystals  which made me to rethink on Fink 1983. I am considering remelting or mixing.
Muharrem Yavuz
CC BY-SA 4.0
2024-02-29T09:58:52.643Z
Be careful, there are several examples of alleged "rheomorphic ignimbrites" that have later been proven to be extensive, silicic lava flows. The two can actually be hard to distinguish in the field. Here are a few classic papers about this:
- https://doi.org/10.1007/BF00278387
- https://doi.org/10.1016/0377-0273(92)90113-R
- https://doi.org/10.1007/s004450050120
Jean-Marie Prival
CC BY-SA 4.0
2024-02-29T11:05:06.947Z
I found photos from Glass Mountain. The pumice facies there, seem as an extensive rock which all made of pumice. Pumices aren't existing as an pyroclasts inside the rock.  So Little glass mountain has no relation with an ignimbrite as i thought previously. I learned that pumice and obsidian beds can be occured by a lava flow. But one must expect autobrecciation from this lava as similar to basaltic aa flows. Here links of photos https://highway8a.blogspot.com/2015/11/views-from-glass-mountain.html and https://www.americansouthwest.net/california/lava_beds/glass-mountain-obsidian-block_l.html
Muharrem Yavuz
CC BY-SA 4.0
2024-02-29T11:10:09.663Z
It's hard to tell from one picture. It looks like a breccia. Many silicic lava flow have some breccia facies that look like this. Here is a picture I took of Pietre Cotte rhyolite flow (Vulcano, Italy): https://imgur.com/SMdURoW You can see 3 layers, from top to bottom: massive obsidian facies; a layer of big blocks beginning to break/detach from the upper layer (with some nice flow banding); a layer with many small obsidian clasts that we interpret as a basal breccia (not explosive). Could you share the location (coordinates) of your outcrop? I'd love to look at satellite photos of the area.
Jean-Marie Prival
CC BY-SA 4.0
2024-02-29T12:13:04.227Z
I'm inviting you to Turkey. Direct observation will be better. Coordinate of the outcrop is  39° 29.116', 26° 4.654' I want to send you my studies about the region. Are you using this mail address? contact@kipuka.fr
Muharrem Yavuz
CC BY-SA 4.0
2024-02-29T13:16:24.397Z
Haha thanks, I would like to come, for sure! Turkey has some amazing volcanoes (I'd love to visit Nemrut caldeira for instance). But it will be really complicated for me to come... But sure, you can send me your work at this address if you'd like me to take a look. It won't be before April, because March is already filled with many things... And thanks for the location. I can't see much evidence on satellite images. Is it the Ayvacık volcanics? I couldn't find much info about this formation online, but hey, maybe that's why they send you figure out the place! I'd be happy to help if I can.
Jean-Marie Prival
CC BY-SA 4.0
2024-03-01T10:13:36.197Z
It will be great doing field observation with an experienced volcanologist. I am appreciating your visit in April!
Muharrem Yavuz
CC BY-SA 4.0
2024-03-04T10:15:11.917Z
Sorry, you got me wrong... I'd love to come but I just can't. I have two young kids at home, and no more university affiliation to pay for the trip... What I can do is to look at some of your research (photos of outcrops or samples, data, whatever you are willing to share) that you would like to send me by email. In April I will be able to take some time to look at it from my home, on the computer. Some kind of early, unofficial peer-review. :)
Jean-Marie Prival
<volcanology><volcanic-eruption><pyroclastic-flows>
In which conditions ignimbrites devitrifies?
Muharrem Yavuz
# In which conditions ignimbrites devitrifies?

(Asked by: username_0 on 2024-02-18)

Devitrification is the crystallization of volcanic glass to crystals. Obsidian groundmass -black in color- turns to white with devitrification.

I want to learn why devitrification takes place.

Attached photo, exhibits an example of devitrification, which i had taken in the field. [![enter image description here](https://i.sstatic.net/1s6P6.png)](https://i.sstatic.net/1s6P6.png)

**Tags:** volcanology, volcanic-eruption, pyroclastic-flows

> **Comments:**
> **username_1**: Are you sure it's an ignimbrite? I've never heard of obsidian layers within an ignimbrite deposit. Generally obsidian is associated with effusive volcanism (rhyolite lava flows). In such flows, there are often the two facies (obsidian and crystalline rhyolite) coexisting, see the model of Fink (1983) https://doi.org/10.1130/0016-7606(1983)94%3C362:SAEOAR%3E2.0.CO;2 (sometimes at a small scale, see flow banding). Such flows are like this *from the start*, i.e., the rhyolite part is non-glassy from the beginning. But it is true that the obsidian part will tend to devitrify over long timescales.  
> **username_0**: There are other facies in Fink (1983). The succession bottom to top is, Basal Breccia, Coarse Pumice, Obsidian, fine pumice, surface breccia. If i see such a succession in the field i will say this is an ignimbrite not a lava flow. In that paper photos are black-white i i couldn't see textures properly. In my field study i saw eutaxitic and vitrophyric textures origining from a pyroclastic density current but i saw a lithofacies inside this flow unit, which has abundant visible crystals  which made me to rethink on Fink 1983. I am considering remelting or mixing.  
> **username_1**: Be careful, there are several examples of alleged "rheomorphic ignimbrites" that have later been proven to be extensive, silicic lava flows. The two can actually be hard to distinguish in the field. Here are a few classic papers about this: - https://doi.org/10.1007/BF00278387 - https://doi.org/10.1016/0377-0273(92)90113-R - https://doi.org/10.1007/s004450050120  
> **username_0**: I found photos from Glass Mountain. The pumice facies there, seem as an extensive rock which all made of pumice. Pumices aren't existing as an pyroclasts inside the rock.  So Little glass mountain has no relation with an ignimbrite as i thought previously. I learned that pumice and obsidian beds can be occured by a lava flow. But one must expect autobrecciation from this lava as similar to basaltic aa flows. Here links of photos https://highway8a.blogspot.com/2015/11/views-from-glass-mountain.html and https://www.americansouthwest.net/california/lava_beds/glass-mountain-obsidian-block_l.html  
> **username_1**: It's hard to tell from one picture. It looks like a breccia. Many silicic lava flow have some breccia facies that look like this. Here is a picture I took of Pietre Cotte rhyolite flow (Vulcano, Italy): https://imgur.com/SMdURoW You can see 3 layers, from top to bottom: massive obsidian facies; a layer of big blocks beginning to break/detach from the upper layer (with some nice flow banding); a layer with many small obsidian clasts that we interpret as a basal breccia (not explosive). Could you share the location (coordinates) of your outcrop? I'd love to look at satellite photos of the area.  
> **username_0**: I'm inviting you to Turkey. Direct observation will be better. Coordinate of the outcrop is  39° 29.116', 26° 4.654' I want to send you my studies about the region. Are you using this mail address? contact@kipuka.fr  
> **username_1**: Haha thanks, I would like to come, for sure! Turkey has some amazing volcanoes (I'd love to visit Nemrut caldeira for instance). But it will be really complicated for me to come... But sure, you can send me your work at this address if you'd like me to take a look. It won't be before April, because March is already filled with many things... And thanks for the location. I can't see much evidence on satellite images. Is it the Ayvacık volcanics? I couldn't find much info about this formation online, but hey, maybe that's why they send you figure out the place! I'd be happy to help if I can.  
> **username_0**: It will be great doing field observation with an experienced volcanologist. I am appreciating your visit in April!  
> **username_1**: Sorry, you got me wrong... I'd love to come but I just can't. I have two young kids at home, and no more university affiliation to pay for the trip... What I can do is to look at some of your research (photos of outcrops or samples, data, whatever you are willing to share) that you would like to send me by email. In April I will be able to take some time to look at it from my home, on the computer. Some kind of early, unofficial peer-review. :)  

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## 0 answers

