Editing Ice core (section)

== Structure of ice sheets and cores ==
{{see also|Ice-sheet dynamics}}
[[File:Taku glacier firn ice sampling.png|thumb|Sampling the surface of [[Taku Glacier]] in Alaska.  There is increasingly dense [[firn]] between surface snow and blue glacier ice.|alt=A scientist in a pit of snow]]

An ice core is a vertical column through a glacier, sampling the layers that formed through an annual cycle of snowfall and melt.<ref name="Alley-2000-2" /> As snow accumulates, each layer presses on lower layers, making them denser until they turn into [[firn]]. Firn is not dense enough to prevent air from escaping; but at a density of about 830&nbsp;kg/m<sup>3</sup> it turns to ice, and the air within is sealed into bubbles that capture the composition of the atmosphere at the time the ice formed.<ref name="Alley-2000" /> The depth at which this occurs varies with location, but in Greenland and the Antarctic it ranges from 64&nbsp;m to 115&nbsp;m.<ref name="Talalay-2016-6">{{harvnb|Talalay|2016}}, p. 263.</ref> Because the rate of snowfall varies from site to site, the age of the firn when it turns to ice varies a great deal. At [[Summit Camp]] in Greenland, the depth is 77 m and the ice is 230 years old; at [[Dome C]] in Antarctica the depth is 95 m and the age 2500 years.<ref>{{Cite book|title=Paleoclimatology: Reconstructing Climates of the Quaternary|last=Bradley|first=Raymond S.|publisher=Academic Press|year=2015|isbn=978-0-12-386913-5|location=Amsterdam|page=138}}</ref> As further layers build up, the pressure increases, and at about 1500&nbsp;m the [[crystal structure]] of the ice changes from hexagonal to cubic, allowing air molecules to move into the cubic crystals and form a [[clathrate]]. The bubbles disappear and the ice becomes more transparent.<ref name="Alley-2000">{{harvnb|Alley|2000}}, pp. 48–50.</ref>

Two or three feet of snow may turn into less than a foot of ice.<ref name="Alley-2000" /> The weight above makes deeper layers of ice thin and flow outwards. Ice is lost at the edges of the glacier to [[icebergs]], or to summer melting, and the overall shape of the glacier does not change much with time.<ref>{{harvnb|Alley|2000}}, pp. 35–36.</ref> The outward flow can distort the layers, so it is desirable to drill deep ice cores at places where there is very little flow. These can be located using maps of the flow lines.<ref>{{Cite book|title=Glaciers|last=Knight|first=Peter G.|publisher=Stanley Thornes|year=1999|isbn=978-0-7487-4000-0|location=Cheltenham, UK|page=[https://archive.org/details/glaciers0000knig/page/206 206]|url=https://archive.org/details/glaciers0000knig/page/206}}</ref>

Impurities in the ice provide information on the environment from when they were deposited. These include soot, ash, and other types of particle from [[Wildfire|forest fires]] and [[volcano]]es; isotopes such as [[beryllium-10]] created by [[cosmic ray]]s;
[[micrometeorite]]s; and [[pollen]].<ref name="Alley-2000-2">{{harvnb|Alley|2000}}, pp. 71–73.</ref> The lowest layer of a glacier, called basal ice, is frequently formed of subglacial meltwater that has refrozen. It can be up to about 20 m thick, and though it has scientific value (for example, it may contain subglacial microbial populations),<ref>{{Cite report|url=https://websites.pmc.ucsc.edu/~tulaczyk/Report_I_V.pdf|title=FASTDRILL: Interdisciplinary Polar Research Based on Fast Ice-Sheet Drilling|last1=Tulaczyk|first1=S.|last2=Elliot|first2=D.|last3=Vogel|first3=S.W.|last4=Powell|first4=R.D.|last5=Priscu|first5=J.C.|last6=Clow|first6=G.D.|year=2002|publisher=2002 FASTDRILL Workshop|page=9|access-date=17 June 2017|archive-date=4 September 2017|archive-url=https://web.archive.org/web/20170904110904/https://websites.pmc.ucsc.edu/~tulaczyk/Report_I_V.pdf|url-status=dead}}</ref> it often does not retain stratigraphic information.<ref>{{Cite book|title=Environmental Contaminants: Using Natural Archives to Track Sources and Long-Term Trends of Pollution|last1=Gabrielli|first1=Paolo|last2=Vallelonga|first2=Paul|publisher=Springer|year=2015|isbn=978-94-017-9540-1|editor-last=Blais|editor-first=Jules M.|location=Dordrecht, Netherlands|page=395|chapter=Contaminant Records in Ice Cores|display-editors=et al.}}</ref>

Cores are often drilled in areas such as Antarctica and central Greenland where the temperature is almost never warm enough to cause melting, but the summer sunlight can still alter the snow. In polar areas, the Sun is visible day and night during the local summer and invisible all winter. It can make some snow [[Sublimation (phase transition)|sublimate]], leaving the top inch or so less dense. When the Sun approaches its lowest point in the sky, the temperature drops and [[hoar frost]] forms on the top layer. Buried under the snow of following years, the coarse-grained hoar frost compresses into lighter layers than the winter snow. As a result, alternating bands of lighter and darker ice can be seen in an ice core.<ref>{{harvnb|Alley|2000}}, pp. 43–46.</ref>