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Thread: Volcanic Rock and Ash Questions

  1. #1
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    Volcanic Rock and Ash Questions

    I have some questions about the products of "dry" (as in non-molten) eruptions.

    Mainly about pumice and that red and/or black lava rock that looks like it wanted to be pumice when it started out, but is a lot denser.

    Now I know not all pumices are the same. In fifth grade school most of us had that cool teacher who shows you the rocks that float on water and all that and that pumice was completey innocuous to handle.

    Yet when I was a greenhouseman we used pumice stones that imitated the heavier lava rocks to make walls and banks for trade show displays and you absolutely had to wear leather gloves to handle these pumice stones. These pumice stones had quarter inch on a side shiney flat inclusions the first pumice stones lacked. But still very friable. Like having razor blades pieces hidden in dry bread.

    One time I warned one of our decorators sharply against picking one of these pumice stones up bare handed. He looked at me like I grew a second head, picked up the pumice piece bare handed and pushed it hard into place in the mozaic of the bank we were building.

    Thirtyfive stitches of damage in seven cuts to his right hand alone. AND he was flying to Hawaii for a two week vacation that afternoon. I really felt bad about not physically stopping him but thought that would have been a bit much. Before hand.

    So my questions regarding pumice, (and cinder is it?) are

    Are pumices closer to open cell foam or closed cell foam?

    What are the production differences between safe to handle pumice vs oh dear Lord, no don't handle that pumice? Both were classic grey, but the dangerous stuff had dense non-fluffy black streaks in it. Both still floated.

    What do you really call the red and black lava rocks used in construction and where do they fit in the order of pumice-like effluvia?

    I saw deep valley in what looked like a dry plain once. The walls of the valley showed that what I thought was "hole" was actually the original pre-eruption ground level and that for as far as the eye could see everything was buried under several hundred feet of breadbox sized rocks of that denser than pumice pumice-like rock. Explained why it was a dry plain if nothing else.

    My question in this case is:

    How deep does volcanic cinder have to be before it starts crushing down like snow into glacial ice?

    I would have to think pretty deep given the light weight and structural strength of cinder compared to snow.

    If it does crush down without going into a liquid phase, where does the trapped pockets of gas go? One big pocket somewhere? I wonder that about the glacial ice as well.

    That's all I have at the moment. I know I have more.

  2. #2
    Mainly about pumice and that red and/or black lava rock that looks like it wanted to be pumice when it started out, but is a lot denser.
    If it's dark, it's probably scoria. If it's lighter, possibly cinders. Truth be told though, I'm not sure there's a hard and fast distinction between the types so long as one floats and the other doesn't, though if I'm wrong I welcome correction.

  3. #3
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    Don, you may be asking about tephra or welded tuff:
    http://en.m.wikipedia.org/wiki/Tephra
    http://en.m.wikipedia.org/wiki/Tuff

    But I'd love to see some pictures, even though, in your case they'd be severely devalued, probably only worth 60, 70 words...retail.

  4. #4
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    Thanks Romanus; Grapes.

    Grapes, that tuff link was really informative. Thanks again.

  5. #5
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    I wonder. If it were non-fluffy black streaks, could it have been a type of obsidian? Then your pumice came with free shards of glass. That would explain the hand damage too.

  6. #6
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    Don, if you find yourself on the eastern part of your state, you'll be near some excellent examples of tuff. In particular, the Bishop tuff, formed from the eruption of the Long Valley caldera. The Bishop tuff shows the variations of consolidation that occurs with ash falls, depending on depth. It goes from very pumicey to hard, welded.

    http://en.wikipedia.org/wiki/Bishop_Tuff

    There's also some other locations in the area that are interesting, such as the Mono-Inyo craters.

    http://en.wikipedia.org/wiki/Mono%E2%80%93Inyo_Craters

    I spent some time in these areas during volcanology class field trips.

  7. #7
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    Yes, the shards must have been some sort of glass, shattered, and re-welded.
    http://en.wikipedia.org/wiki/Apache_tears
    The wiki article on obsidian points out that an obsidian edge can be shaper than surgical steel instruments:
    http://en.wikipedia.org/wiki/Obsidian

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    Of note: As flintknappers, one of my buddies and I made piles of obsidian chips. Being both frugal and inventive, we took them to the forge in an attempt to save some digging sweat, and if possible, cast the results into knapping preforms. Turns out melting obsidian releases a lot of gas - the result was pumice (maybe not exactly, but glass foam in any event).

  9. #9
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    Ah ha!

    Two things I didn't have I learned.

    Diagenesis and connate fluids. The wiki articles on those filled a lot of voids.

    (From links in both Geonuc's and Grapes' links.)

  10. #10
    Turns out melting obsidian releases a lot of gas - the result was pumice (maybe not exactly, but glass foam in any event).
    That's really cool; I saw something like it in a book illustration some years ago, which showed re-melted rhyolite (or some other felsic rock). The regular rock was nondescript, but when re-melted in a crucible it foamed up and out of it like whipped cream. It's just kind of amazing that you can take an extrusive rock that's been cold and solid for thousands, perhaps millions of years, and remelt it to find that it still has a lot of unfinished business in volatiles. For a converse example, when scientists remelted samples of lunar rocks, they found out that the resulting lava was thinner and runnier than any known terrestrial flow, probably due to a combination of low silica and a virtual absence of volatiles.

  11. #11
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    Quote Originally Posted by LookingSkyward View Post
    Of note: As flintknappers, one of my buddies and I made piles of obsidian chips. Being both frugal and inventive, we took them to the forge in an attempt to save some digging sweat, and if possible, cast the results into knapping preforms. Turns out melting obsidian releases a lot of gas - the result was pumice (maybe not exactly, but glass foam in any event).
    I had trouble understanding this, because obsidian forms extrusively--it's hot in the open already before it cools. Turns out (I read my own wiki links), obsidian is metastable, and degrades over time. It can take on water and form perlite, which behaves like you describe: http://en.wikipedia.org/wiki/Perlite

    Did you rinse your chips?

  12. #12
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    No, but they had been sitting outside getting the western Washinton natural rinse - maybe I'll try a low-temp kiln dry... I still have plenty of chips...

  13. #13
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    Next question:

    Geodes. (Yes, I read the wiki first. )

    My father lived in Roseburg in the Willaimette Valley of Oregon.

    Dad plowed a quarter of his half acre back yard to plant a small field of corn and uncovered three geodes the size of bowling balls and about a dozen smaller ones. All amethyst bearing ones. Though he only had the big ones cut Dad presumed they all had a similar chemistry. (After retiring as a policeman he started prospecting and formed a...whatever a group of prospectors are called.)

    Where would these source from? This is deep top soil they were plowed out of and I can't see a glacier being kind to a geode at all.

  14. #14
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    Quote Originally Posted by BigDon View Post
    Where would these source from? This is deep top soil they were plowed out of and I can't see a glacier being kind to a geode at all.
    They're formed in volcanic deposits, which pretty much comprises the whole of the mountain range to east of the Willamette. So I imagine they wound up in the valley by way of erosion off the Cascades.

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    Wouldn't this be an unusually high concentration of them?

  16. #16
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    Hard to say. Maybe your father's field was downstream from a particularly rich source of the geodes.

    Or, maybe a geologist years ago was transporting his haul of geodes back to his lab when he was waylaid by ne'er-do-wells, who took his valuables and money but dumped the rocks in a nearby field.

    Or maybe it was aliens.


  17. #17
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    For a classification scheme one can refer to a publication by the British Geological Survey at:

    http://www.bgs.ac.uk/bgsrcs/

    Volume one, pages 8-10, lays it out for volcaniclastic rocks.

    It's a "tuff" read.

    [Did someone say "ice"?]

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