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Thread: Pyroclastic flow

  1. #1 Pyroclastic flow 
    Forum Professor sunshinewarrior's Avatar
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    I’ve put this in Earth Sciences because that’s where pyroclastic flow appears to be most used.

    My understanding (though the wiki I’ve linked seems to suggest otherwise), was that pyroclastic flow was characterised by a combination of solids and heated gas, and it was in that context that I wanted to raise this question.

    Say that I (hypothetically, and anyway it was in a different country where it is allowed and I was younger – in fact not yet of age so I can’t be held responsible..) had some experience of using hash. Now as the article suggests, a common method for use involves preparing “for smoking by heating it with a flame for a couple of seconds, producing some bubbling or sizzling. It then softens and can be crumbled into tiny pieces or formed into shapes to obtain maximum surface area when burning.”

    Here’s my question, if anyone has enough chemistry/geology/whatever to tell. When one ‘roasts’ hash, is the subsequent crumbly nature due to something akin to pyroclastic flow? Does the heating raise the temperature of trapped gases/volatiles in the hash, thereby reducing the structural integrity of the solid?

    Admins/mod, if this has in any way violated the terms of use, please feel free to delete, and accept my apologies.

    cheer

    shanks


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  3. #2  
    Forum Junior Zitterbewegung's Avatar
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    Ummmmmmm......not quite.

    Pyroclastic flows in general are basically an avalanche of "acidic" (in geological terms, not chemically speaking i.e. high silica content and thus high viscosity + high content of trapped volatile components) lava ejected from a strato-vulcano. This is basically a self-amplifying process as after the lava got ejected the pressure is of course released. This in turn causes the bubbles of volcanic gasses trapped inside the lava chunks to expand. They go more or less "popp" and release more hot gasses and decreasing the avérage size of the lava chunks effectively creating a self sustaining air cushion under the pyroclastic flow. This cussion accelerates the downhill mtion of the avalanche sucking in even more air. Did you ever see a dust avalanche in the mountains? More or less like this but a little bit hotter

    To give you a better conception: Try standing behind a jet powering up it's engines to the max. On top of this jet is a truck loaded with tons of slag directly from a blast furnace. The truck dumps the slag in the exhaust of the throttle-full-open turbine while you are just a couple of feet away.

    Have fun.


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  4. #3  
    Forum Professor sunshinewarrior's Avatar
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    Quote Originally Posted by Zitterbewegung
    Ummmmmmm......not quite.
    But but but...

    Quote Originally Posted by Zitterbewegung
    They go more or less "popp" and release more hot gasses and decreasing the avérage size of the lava chunks effectively creating a self sustaining air cushion under the pyroclastic flow. This cussion accelerates the downhill mtion of the avalanche sucking in even more air.
    It's exactly that sort of air cushion effect that I'm talking about. Obviously neither the temperatures nor pressures are comparable, but is there nothing analogous about them at all?
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  5. #4  
    Forum Junior Zitterbewegung's Avatar
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    It's exactly that sort of air cushion effect that I'm talking about. Obviously neither the temperatures nor pressures are comparable, but is there nothing analogous about them at all?
    Well, in one case you have some resin that is not really easy to crumble when cold. The pyroclastic flow consists of a high silica melt interspersed with allready cooled chuncks in it. I do not see a common property here or do I miss the point? :?
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  6. #5  
    Forum Professor sunshinewarrior's Avatar
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    Quote Originally Posted by Zitterbewegung
    It's exactly that sort of air cushion effect that I'm talking about. Obviously neither the temperatures nor pressures are comparable, but is there nothing analogous about them at all?
    Well, in one case you have some resin that is not really easy to crumble when cold. The pyroclastic flow consists of a high silica melt interspersed with allready cooled chuncks in it. I do not see a common property here or do I miss the point? :?
    Perhaps I was led astray by University Challenge - one of the bonus question sets consisted of asking about the difference between three types of flowing mixtures - gas + liquid, liquid + solid, and gas + solid, which last they said was the characteristic of pyroclastic flow. It was the idea of a flowing combination of gas and solid that got me thinking.
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  7. #6  
    Forum Isotope Bunbury's Avatar
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    A flowing combination of gas plus solid is what goes on in a circulating fluidized bed boiler furnace and in any number of pneumatic conveying systems. That University Challenge question seems open to a broad range of answers. (I'm amazed it's still on. Just checked - it started in 1962.)
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  8. #7  
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by Bunbury
    (I'm amazed it's still on. Just checked - it started in 1962.)
    This is a reincarnation. Jeremy Paxman instead of Bamber Gascoine. I think the old one was on ITV, not BBC also, but I may be wrong.

    I could answer almost none of the questions in 1962, now I find many of them ridiculously easy. I must have learned something in four and a half decades.
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  9. #8  
    Forum Professor sunshinewarrior's Avatar
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    So pyroclastic can't be used as a catch-all term for all solid+gas flows or substances. Bother. I shall probably call them pyroclastics anyway - it's catchy.
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  10. #9  
    Forum Junior Zitterbewegung's Avatar
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    Ummmpfff.....now I get it. Long week, no brain left. Good example by bunburry btw.
    I love deadlines. I like the whooshing sound they make as they fly by
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  11. #10  
    Forum Isotope Bunbury's Avatar
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    Quote Originally Posted by sunshinewarrior
    So pyroclastic can't be used as a catch-all term for all solid+gas flows or substances. Bother. I shall probably call them pyroclastics anyway - it's catchy.
    Now, a pyroclastic coprolite - that would be something to see.
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  12. #11  
    Forum Professor sunshinewarrior's Avatar
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    Quote Originally Posted by Bunbury
    Quote Originally Posted by sunshinewarrior
    So pyroclastic can't be used as a catch-all term for all solid+gas flows or substances. Bother. I shall probably call them pyroclastics anyway - it's catchy.
    Now, a pyroclastic coprolite - that would be something to see.
    Well... if you ever visit London...
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