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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.
Impurities in the ice provide information on the environment from when they were deposited.
It can be up to about 20 m thick, and though it has scientific value (for example, it may contain subglacial microbial populations), 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 sun can still alter the snow.
In polar areas, the sun is visible day and night during the local summer and invisible all winter.
These include soot, ash, and other types of particle from forest fires and volcanoes; isotopes such as beryllium-10 created by cosmic rays; micrometeorites; and pollen.
The lowest layer of a glacier, called basal ice, is frequently formed of subglacial meltwater that has refrozen.
Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice.
Some volcanic events that were sufficiently powerful to send material around the globe have left a signature in many different cores that can be used to synchronise their time scales.
With the aid of a tripod for lowering and raising the auger, cores up to 50 m deep can be retrieved, but the practical limit is about 30 m for engine-powered augers, and less for hand augers.
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.
Soviet ice drilling projects in Antarctica include decades of work at Vostok Station, with the deepest core reaching 3769 m.
Numerous other deep cores in the Antarctic have been completed over the years, including the West Antarctic Ice Sheet project, and cores managed by the British Antarctic Survey and the International Trans-Antarctic Scientific Expedition.
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The bubbles disappear and the ice becomes more transparent.