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Post by vegasjames on May 8, 2023 0:25:47 GMT -5
Since you know a lot about basalt, I have a question I was thinking about the other day that I keep wondering about and maybe you can answer. So we know that obsidian is a volcanic glass, and glasses are formed from a very rapid cooling that prevents crystal formation. Even metals can be made in to metallic glasses by cooling them from a molten state rapidly enough. So, what I recall from what we were taught in school the lava get thrown from the volcano causing it to rapidly cool forming obsidian, and I guess flows could possibly be cooled rapidly enough such as under an ice cap or something to form obsidian. We also know that water is a much more efficient and faster heat transfer than air. Therefore, if lava flying through the air can form obsidian as we were taught in school then why doesn't lava from volcanic eruptions underwater form obsidian as the water theoretically should cool the lava faster than air? Instead, in Hawaii they end up with pillow basalt instead and the only volcanic glass I am aware of formed in Hawaii is Pele's hair. There are other volcanic glasses other than obsidian, such as tachylite.
People will often call all volcanic glasses obsidian which is not correct. Obsidian flows have a high content of silica and a high viscosity which means they cool over very fast. Lava cooling in the air does not become obsidian.
If you just take normal basaltic lava and dump it in water you do get a volcanic glass, just not obsidian. ![]()  (examples of pillow basalt in NE Washington)  (Example of volcanic glass formed from basalt contacting water) (Example of rapid cooling. The brown is palagonite, then glassy tachylite and then basalt) I am aware there are various forms of volcanic glass. I was thinking about writing an article on this for our club newsletter as I have been in debates with some members who think that the only volcanic glass is obsidian.
What I am still trying to figure out though, is why is the lava that forms under water in Hawaii still called a basalt, more specifically pillow basalt instead of forming obsidian due to rapid cooling. I saw the same thing in Iceland where a lot the volcanic flows occurred under glaciers and ice fields, yet obsidian is so hard to find there.
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Post by 1dave on May 8, 2023 0:32:23 GMT -5
The difference is what it is made of. Obsidian has a high SiO2 content ~ 75%. The others have much less.
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Post by vegasjames on May 8, 2023 0:36:18 GMT -5
While we're on the subject, do you know what peanut obsidian actually is ? Cuz it sure as hell doesn't seem like obsidian! Peanut obsidian is actually perlite with feldspar spherules stained by iron.
Perlite is another volcanic glass. It is said to be formed from the reaction of obsidian and water leaving a high water content in this volcanic glass.
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Post by Deleted on May 8, 2023 6:53:55 GMT -5
There are other volcanic glasses other than obsidian, such as tachylite.
People will often call all volcanic glasses obsidian which is not correct. Obsidian flows have a high content of silica and a high viscosity which means they cool over very fast. Lava cooling in the air does not become obsidian.
If you just take normal basaltic lava and dump it in water you do get a volcanic glass, just not obsidian. ![]() (examples of pillow basalt in NE Washington) (Example of volcanic glass formed from basalt contacting water) (Example of rapid cooling. The brown is palagonite, then glassy tachylite and then basalt) I am aware there are various forms of volcanic glass. I was thinking about writing an article on this for our club newsletter as I have been in debates with some members who think that the only volcanic glass is obsidian.
What I am still trying to figure out though, is why is the lava that forms under water in Hawaii still called a basalt, more specifically pillow basalt instead of forming obsidian due to rapid cooling. I saw the same thing in Iceland where a lot the volcanic flows occurred under glaciers and ice fields, yet obsidian is so hard to find there.
You just said why, it is the rapid cooling under water that causes basalt to form pillows vs. obsidian flows where the whole flow is highly viscous, doesn't flow far and cools in place. The content of the flows is different. Both are igneous extrusive lava but have different make ups. The silica content of obsidian can be between 60-85% vs. the Grande Ronde Basalt flows which covered much of eastern Washington is about 54% this means that flows were very low viscosity and flowed very far.
The cooling is just an effect of the content.
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Post by vegasjames on May 8, 2023 8:04:31 GMT -5
I am aware there are various forms of volcanic glass. I was thinking about writing an article on this for our club newsletter as I have been in debates with some members who think that the only volcanic glass is obsidian.
What I am still trying to figure out though, is why is the lava that forms under water in Hawaii still called a basalt, more specifically pillow basalt instead of forming obsidian due to rapid cooling. I saw the same thing in Iceland where a lot the volcanic flows occurred under glaciers and ice fields, yet obsidian is so hard to find there.
You just said why, it is the rapid cooling under water that causes basalt to form pillows vs. obsidian flows where the whole flow is highly viscous, doesn't flow far and cools in place. The content of the flows is different. Both are igneous extrusive lava but have different make ups. The silica content of obsidian can be between 60-85% vs. the Grande Ronde Basalt flows which covered much of eastern Washington is about 54% this means that flows were very low viscosity and flowed very far.
The cooling is just an effect of the content.
1Dave mentioned the silica content, which made a lot more sense, so I looked up obsidian formation and found this which shows that obsidian can form from air cooling or water cooling. geology.com/rocks/obsidian.shtml Therefore, from what I can gather the actual answer is the silica content of the lava, especially since they say obsidian and rhyolite, which forms from a viscous, high silica lava flow are both very rich in silica. So, it appears that in order to form obsidian vs rhyolite or basalt, etc, that the lava must first have a sufficient silica content in addition to the rapid cooling. As where slower cooling would form rhyolite and lower silica would form other rocks such as basalt. |
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Post by youp50 on Jun 12, 2023 15:23:26 GMT -5
Different amygdaloids were copper bearing in the Keeweenaw to Ontonagon mines. I coveted a find that had a banded blue/red agate in an amygdaloid pocket. A friend found/sold it to a deep pocket collector.
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