Glacial ice structure

  • Thread starter Tanja
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  • #1
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Main Question or Discussion Point

I'm not familiar with earth science, in particular ice formation in glaciers and I hope you can bear my rather stupid question:
Is the change of a "normal" ice layer into glacial ice with frozen air bubbles a continous transition or a spontaneous reaction?
Someone told me that the air bubbles inside the snow flake in an ice layer start to freeze. How can air, this mixture of nitrogen, oxygen, carbon dioxide, ... actually freeze? It was a professor of meteorology, who told me that on a winterschool and I tend to believe him but I fail in understanding him.
Are there any other changes in the ice solid structure (due to the high pressure), besides that effect of frozen air? Something like a phase transition into a solid-crystal structure? I mean the ice appears to be blue and that might be an indicator of a different absorption effect or a change into a crystal structure.
 

Answers and Replies

  • #2
Skyhunter
Tanja,

The air bubbles are not frozen. They are merely trapped by the ice surrounding them. Glacial ice for this reason is less dense than ice formed by freezing liquid water.

The blue tint is from the absorption of red light by the water molecule as a result of it's quantum vibrational modes. The same reason that lakes, oceans, and rivers appear blue.
 
  • #3
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Someone told me that the air bubbles inside the snow flake in an ice layer start to freeze. How can air, this mixture of nitrogen, oxygen, carbon dioxide, ... actually freeze?
Must be a misunderstanding. The ice and snow at the ice sheets remain frozen allmost all the time. However as snow accumulates yearly the lower layers start to compress under the pressure and form crude icy snow or "firn" with air moving freely around. As the pressure increases at about 80-90 meters depth the ice grains become solid ice, trapping the air in bubbles.

See also: http://www.crrel.usace.army.mil/sid/Summit/background.htm [Broken]
http://www.carleton.edu/departments/GEOL/Links/AlumContributions/blueice/firn.html
 
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  • #4
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Hello Skyhunter,

thanks for your answer.
But there's one question left
Why does water in oceans, lakes and the glacial ice change its bleuish color if you look at it from different angles? If the color's origin are water quantum vibrational modes rather than light scattering it shouldn't depend on the angle. I might be wrong.

And does this vibritional frequency change as soon as ice turns into glacial ice?
 
  • #5
Skyhunter
Looking at it from different angles is changing the angle of the reflected light you are observing. Since there is only a slight absorption in the visible bandwidth the blue hue is easily negated or intensified by the angle of light.
 
  • #6
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Up, that was a fast answer.
Ok, you nearly convinced me. Another last question: Why is water in my water glas not blue. And I had a look at a deep water tank that didn't appear to be blue at all.
And if it's a slight blue absoprtion, why are some small lakes and small pieces of glaciers very blue? You can see, that I still stick a bit to the pure reflection theory :-).
 
  • #7
Skyhunter
And does this vibritional frequency change as soon as ice turns into glacial ice?
Not to my knowledge. It is derived from the shape of the water molecule and it's magnetic dipole. To change the frequencies that the molecule responds to would necessitate changing the shape of the molecule. Although pure H2O does not exist in nature, so the composition of the ice could determine it's hue.

Each atom has it's own magnetic field. When atoms organize into molecules their magnetic fields overlap and interact. The strength of the bonds determine how a molecule vibrates.

http://www.lsbu.ac.uk/water/index2.html" [Broken]
 
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  • #8
Skyhunter
Why does water in oceans, lakes and the glacial ice change its bleuish color if you look at it from different angles? If the color's origin are water quantum vibrational modes rather than light scattering it shouldn't depend on the angle. I might be wrong.
From my previous source.
Water is almost perfectly transparent to 'visible' light, a property which is made good use of by photosynthesis and allowing production of both biomass and oxygen. Water is very slightly blue in color [131]c as overtone and combination vibrational absorption bands (albeit far less intense, see above [130]) extend through the red part of the visible spectrum with a small peak at 739 nm and shoulder at 836 nm, both varying somewhat with temperature [268] plus a smaller fourth overtone of the v1:v3 stretch at 606 nm, and very small fifth overtone (at 514 nm) and combined overtone (at 660 nm) bands. This absorption spectrum of water (red light absorbs 100 times more than blue light), together with the five-times greater scattering of blue light over red light, contributes to the blue color of lake, river and ocean waters. Colloidal silica may contribute to the outstanding blue color of certain, often hydrothermal, pools and lakes [372]. Ice is also blue [159] for similar reasons but liquid D2O does not absorb in the red region (as the absorption band is shifted into the infrared) and is blue solely because of the light scattering effect [159].
We were both partially correct. It is a combination of both.
 

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