Glaciers exist because there are areas, generally at high elevations or in polar latitudes, where snow fall during the winter exceeds melt (and other losses) during the summer. This results in net accumulation, and this part of the glacier is thus called the accumulation area (Figure 3.1). As each snow layer is buried, the pressure of the overlying snow causes compaction, and movement of molecules in the liquid and vapor phases results in snow metamorphism. Snow that is more than a year old, and has thus been altered by these processes, is called firn. The end result of the firnification process, normally after several years, is solid ice.

Where there are lower elevations to which this ice can move, gravitational forces drive it toward these areas. This eventually brings the ice into places where the annual melt exceeds snow fall. Here, all of the winter snow and some of the underlying ice melts during the summer. This is called the ablation area. The line separating the accumulation and the ablation areas at the end of a melt season is called the equilibrium line. Along the equilibrium line, melt during the just-completed summer exactly equaled net snow accumulation during the previous winter.

In this chapter, we first discuss the transformation of snow to ice, and show how the processes involved result in a physical and chemical stratigraphy that, under the right circumstances, can be used to date ice that is thousands of years old.