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Post by Tweetiepy on Mar 9, 2010 20:13:59 GMT -5
Yeah I know i've been out of it for quite a while and I haven't yet lost my mind. But I have been thinking (never a good idea really) I know that obsidian is volcanic glass, but lava is molten rock (and it looks hole filled & lumpy once cooled) but what is the obsidian? Is it someone's tiffany lamp that melted? Is it window panes that melted in a volcanic eruption? What makes it different than lava? What is it exactly? What makes it NOT lava? **I suck!** :help:
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Post by fishenman on Mar 9, 2010 20:20:34 GMT -5
I think it's lava that cooled so quickly that no gas bubbles happened. Pumice is lava that cooled very slowly, that is why it has holes and obsidian does not.
Geoff
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Post by Woodyrock on Mar 10, 2010 1:45:07 GMT -5
A more exact description would be molten material that cooled so quickly that it did not crystalize, as did basalt, which has much the same composition. It is the lack of crystallization that makes it so easily worked by flaking. Photomicrographs of a broken edge of obsidian show it to be on the order of one molecule thick, thus it so hundreds of times sharper that a razor blade. Woody
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Post by rockrookie on Mar 10, 2010 6:54:48 GMT -5
i would agree agree with Woody . mot because iknow ..... he's very convincing !! Woody . Did you ever play Balderdash ?? ----paul
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Post by Toad on Mar 10, 2010 7:25:28 GMT -5
Obsidian cools quickly, but underground - so not as quickly as what reaches the surface. So you still don't get crystals, but you don't get the rough pock-marked surface either. A happy medium.
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Post by deb193redux on Mar 10, 2010 8:37:24 GMT -5
cooling speed is paramount, but there is still some mineral difference too. Obsidian is almost all silica, while basalt has other minerals/elements including aluminum - much like feldspar. Pumice is a good example. Lava rock like in gardens, is a softer mineral in addition to the bubbles.
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baojiazhi
starting to shine!
Member since October 2008
Posts: 40
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Post by baojiazhi on Mar 10, 2010 9:19:18 GMT -5
Woody's answer has been the best so far. I am a geologist so here are my 2 cents. Obsidian is an extrusive (meaning formed above the ground) volcanic rock. Like Woody said it lacks a crystal structure so it is a glass. If you look at the chemistry of obsidian it has the same chemical make up as a rhyolite. The only real diffrence is that when obsidian cooled it did not have the chance to form crystals so you get the glassy texture. Yes all the silica oxygen aluminum iron ect are still in the obsidian. The dark color in obsidian is from the iron present in the rock. On an interesting side note the snowflakes in snowflake obsidian are places where there was water in the original melt that reacted with the obsidian forming perlite.
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Post by Toad on Mar 10, 2010 12:33:32 GMT -5
Hmmm... Must have been thinking of something else. I thought I read that obsidian was formed just below the surface. Sorry for the bad info.
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Post by Tony W on Mar 10, 2010 16:53:01 GMT -5
Thanks for the info, Baojiazhi. What causes the shiller effects in rainbow, golden sheen, etc. ? Gases? Thanks, Tony
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jimrbto
noticing nice landscape pebbles
Member since April 2007
Posts: 94
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Post by jimrbto on Mar 10, 2010 21:00:03 GMT -5
I think, from past studies, that the snowflakes in obsidian are made by cristobalite and the other schiller and color effects are caused by stratified layering of micro bubbles/inclusions. Jim
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Post by Woodyrock on Mar 11, 2010 1:21:39 GMT -5
Paul......what is Balderdash? Woody
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Post by Toad on Mar 11, 2010 7:41:43 GMT -5
Balderdash is a game where you try to BS people into believing the definition of an obscure word, that you made up, is true.
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Post by johnjsgems on Mar 11, 2010 10:09:14 GMT -5
I'm with Toad. Lava surface flows cool fastest and give us the light weight porous rock for our barbeques. Obsidian has to cool slower. The igneous rocks would cool slower yet.
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baojiazhi
starting to shine!
Member since October 2008
Posts: 40
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Post by baojiazhi on Mar 11, 2010 11:25:37 GMT -5
Scorria the lightweight rock for your barbeque is another type of extrusive volcanic rock. It does tend to cool quickly but there are still distinct crystals present when it forms. You might be able to see little white feldspars present in the stuff in your bbq. It is lightweight due to the air pockets it has due to trapped gases in the original melt. Obsidian has to cool faster due to the lack of crystals. Other extrusive volcanics have crystal structures with the exception of some ash flow tufts that can be composed of mainly little bits of volcanic glass.
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Post by phil on Mar 11, 2010 16:32:47 GMT -5
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