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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.
As further layers build up, the pressure increases, and at about 1500 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.
In Greenland, a sequence of collaborative projects began in the 1970s with the Greenland Ice Sheet Project; there have been multiple follow-up projects, with the most recent, the East Greenland Ice-Core Project, expected to complete a deep core in east Greenland in 2020.
Because the rate of snowfall varies from site to site, the age of the firn when it turns to ice varies a great deal.
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.
Hand augers can be rotated by a T handle or a brace handle, and some can be attached to handheld electric drills to power the rotation.
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.
These data can be combined to find the climate model that best fits all the available data. Coastal areas are more likely to include material of marine origin, such as sea salt ions.
Greenland ice cores contain layers of wind-blown dust that correlate with cold, dry periods in the past, when cold deserts were scoured by wind.
The bubbles disappear and the ice becomes more transparent.