666 SEDIMENTARY PROCESSES/Glaciers Figure Mass balance regime of a typical glacier in the tem perate regions of the Arctic (A) Schematic cross section through the glacier, illustrating zones of accumulation and ablation (B) How the volumes of accumulation and ablation vary over the year, assuming steady state conditions Figure An alpine valley glacier, the Mer de Glace, in the French Alps ß 2005, Photograph, M J Hambrey on a glacier where accumulation equals ablation is known as the equilibrium line and is normally expressed as an altitude in metres above sea-level Accumulation exceeds ablation in the upper part of a glacier, whereas the converse is true in the lower part (Figure 5A) Exceptions to this rule occur in parts of Antarctica, where costal snowfall compensates for losses in the middle (drier) reaches of a glacier Glacier Thermal Regime Temperatures within glacier ice vary both vertically and horizontally Variations in temperature at the base of a glacier are known as the basal thermal regime and are determined by the balance between the heat generated at the base of the glacier and the temperature gradient within the ice, which governs the rate at which heat is drawn towards the ice surface (Figure 6) The most important factors in determining the basal thermal regime are ice thickness, ice surface temperature, geothermal heat, and frictional heat Heat is generated beneath a glacier by geothermal heat, which enters the base of the glacier from the Figure The vertical variation in thermal regime through a polar ice sheet rocks beneath, and by frictional heat, which is given off as a result of basal sliding and internal deformation The distribution of ice temperature is vital in understanding the landscapes produced by glaciers because the basal thermal regime is a primary factor controlling the patterns of erosion and deposition