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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.
The bubbles disappear and the ice becomes more transparent.
The weight above makes deeper layers of ice thin and flow outwards.
Below this depth, electromechanical or thermal drills are used.
Because the rate of snowfall varies from site to site, the age of the firn when it turns to ice varies a great deal.
The proportions of different oxygen and hydrogen isotopes provide information about ancient temperatures, and the air trapped in tiny bubbles can be analysed to determine the level of atmospheric gases such as carbon dioxide.
Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past.
Ice cores have been studied since the early 20th century, and several cores were drilled as a result of the International Geophysical Year (1957–1958).
Depths of over 400 m were reached, a record which was extended in the 1960s to 2164 m at Byrd Station in Antarctica.