Advance of the glacier's terminus occurs when net accumulation is greater than net ablation. This situation is shown in the animation in Figure 11o-5 . Advance of the glacier's terminus occurs when the addition of snow in the zone of accumulation exceeds losses of ice in the zone of ablation . In this animation, the firn line moves forward as the zone of accumulation expands with increasing snowfall. Figure 11o-6 models the condition of glacial retreat.
Glacial retreat takes place when net accumulation is less than net ablation.
Retreat of the glacier's terminus occurs when the addition of snow in the zone of accumulation is less than the losses of ice in the zone of ablation . In this animation, the firn line moves upslope as the zone of accumulation shrinks with decreasing snowfall. Ideally, the rate of flow and movement of the glacier's terminus should be controlled by net accumulation and net ablation. This relationship, however, is not temporally immediated. In many glaciers, there are significant time lags between one year's net accumulation and net ablation and the corresponding movement of the glacier.