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This article:

Anderson, C. H., C. J. Behrens, G. A. Floyd, and M. R. Vining. 1998. “Crater Firn Caves of Mount St. Helens, Washington.” Journal of Cave and Karst Studies 60: 44–50.

discusses, among other things, density of the ice that forms the walls of the fumarolic ice caves in the crater of Mt St Helens.

It says:

The [St Helens summit crater] caves are called firn caves because their density ranges from 0.55 to 0.82 g/cm3...

but also:

The ice density in September 1994 at the base of this crevasse was 0.85 g/cm3. The ice density in the lowest cave passage was 0.86 g/cm3.

As far as I know, 0.86 is greater than 0.82. Right!?

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There's no contradiction. Firn is an intermediate form between snow and ice, with a density between that of snow and ice.

The frozen fluffy stuff that falls from the sky -- that's not ice, it's not firn. It's snow. It's also not very dense, typically less than 0.5 grams/cm3. As the snow sits and gets compressed by snow atop it, some of the gas gets squeezed out. The snow changes from ice to firn. The authors are using density between 0.55 and 0.82 grams/cm3 as the definition of firn. As the firn sits even longer, it changes from firn to ice. Note that solid frozen water (no air pockets whatsoever) has a density of about 0.934 grams/cm3. Compressed former snow with a density of 0.82 or higher acts a lot like ice. It is ice, not firn.

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  • $\begingroup$ Thanks for your reply. You're right that they're using 0.82 as the firn / glacial ice cutoff. The problem I have is that first they say the St Helens caves are entirely firn, and then later they say they measured the density at 0.86. It might have been unclear in my original question that both quotes refer specifically to the St Helens caves, so I edited it to emphasize that. $\endgroup$ – foobarbecue May 16 '15 at 23:13
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    $\begingroup$ Re * first they say the St Helens caves are entirely firn* -- I just read the article. That is not what they say. They said just the opposite. They noted the gradual transition from snow to firn to ice in the firn ice field and in the caves. $\endgroup$ – David Hammen May 17 '15 at 0:44
  • $\begingroup$ Right... except for the part where they say "The caves are called firn caves because ice density ranges from 0.55 to 0.82 g/cm3.", which appears to contradict the rest of the article, prompting my question. What do you think "The caves" refers to here? $\endgroup$ – foobarbecue May 17 '15 at 0:57
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    $\begingroup$ You are reading too much into that sentence. $\endgroup$ – David Hammen May 17 '15 at 1:00
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    $\begingroup$ @DavidHammen "You are reading too much into that sentence." - story of my life $\endgroup$ – Isopycnal Oscillation May 17 '15 at 1:15

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