Snow into ice. Snow turns into glacial ice because of the pressure from overlying layers of snow. The increased weight compacts the delicate snowflakes and collapses pockets of air. The snowflakes become rounded granules called
firn, which are held loosely together by new ice that acts as a cement. The greater the overlying weight, the greater the amount of compaction and recrystallization that leads to the development of thick slabs of glacial ice.
Wasting and calving. As its mass increases, a glacier begins to move downslope, or flow, under the influence of gravity. It commonly picks up loose rock and sediment or breaks off pieces of the irregular rocky surface. When glacial ice reaches its point of farthest advance, it is wasted, or ablated, through melting. A small amount of the ice evaporates directly into the atmosphere from the warmed surface of the glacier. Large blocks of ice may break off, or calve, from the glacial face and plunge into the water of a lake or ocean as an iceberg. In extremely cold climates, most glaciers lose their ice through calving.
Budget. A glacier's budget is defined as the ratio between ice gained and ice lost. When a glacier gains more volume from new snowfall than it loses from melting, it has a positive budget. This positive growth is reflected by the outward or downslope movement of an advancing glacier because of the increased snow mass at the top, even if the front of the glacier is melting. A glacier with a negative budget loses more volume than it gains and is therefore a receding glacier. A receding glacier may at times still move downslope but cannot in total overtake its more rapid rate of uphill recession from melting. A glacier that has a balanced budget neither advances nor recedes.
Zone of accumulation and zone of wastage. The upper elevations of a glacier that are perennially covered in snow are called the zone of accumulation. The lower portion of the glacier where the ice is lost is called the zone of wastage. The snow line is the irregular boundary between these two zones. The position of the snow line varies according to climatic variations and the glacier's budget. A snow line that moves down the glacier indicates the glacier has a positive budget; a snow line that moves up the glacier reveals a negative budget.
The terminus. The front of the glacier, or its terminus, moves down the valley if a glacier has a positive budget; the reverse is true if the glacier has a negative budget. Colder temperatures are not the only reasons a glacier extends forward. Other causes could be that conditions are wetter and more snowfall is accumulating during the winter months or that the summer is cloudier and cooler. Alternatively, a retreating glacier could mean that the winter months have been drier but just as cold, resulting in less snowpack, or that a sunnier summer has resulted in more wastage. Experts guess that a worldwide decrease in the mean annual temperature of only 4 or 5 degrees centigrade could trigger the onset of another glacial period.