Ice sheets and glaciers are essentially very simple: they build-up slowly over time through the addition and preservation of snow fall in the accumulation zone. This occurs above what is known as an equilibrium line, an altitude where the accumulation equals the snow and ice lost. Below this altitude, ice sheets and glaciers ablate, through melting, evaporation or ice berg calving into lakes or oceans. In Antarctica, ice berg calving is the the main mechanism of mass loss.
Over time, the snow turns into ice through what is known as the sintering or firnification process. Here, the increase in pressure from overlying snow effectively squashes into dense ice, preserving a record of the conditions at the time of snowfall and often trapping atmospheric gases as bubbles.