River,
Gilgit-Baltistan, northern Pakistan 2017. Several glaciers flow into the Shimshal Valley, and are prone to blocking the river. Khurdopin glacier surged in 2016–17, creating a sizable lake.|375px . The river has started to carve a path through the toe of the glacier. By early August 2017, the lake had completely drained.|375px When a
glacier is experiencing an accumulation input by precipitation (snow or refreezing rain) that exceeds the output by ablation, the glacier shows a
positive glacier mass balance and will advance. Conversely, if the loss of volume (from evaporation, sublimation, melting, and calving) exceeds the accumulation, the glacier shows a
negative glacier mass balance and the glacier will melt back. During times in which the volume input to the glacier by precipitation is equivalent to the ice volume lost from calving, evaporation, and melting, the glacier has a steady-state condition. Some glaciers show periods where the glacier is advancing at an extreme rate, that is typically 100 times faster than what is considered normal, it is referred to as a
surging glacier. Surge periods may occur at an interval of 10 to 15 years, e.g. on
Svalbard. This is caused mainly due to a long lasting accumulation period on
subpolar glaciers frozen to the ground in the accumulation area. When the stress due to the additional volume in the accumulation area increases, the pressure melting point of the ice at its base may be reached, the basal glacier ice will melt, and the glacier will surge on a film of meltwater.
Rate of movement The movement of
glaciers is usually slow. Its velocity varies from a few centimeters to a few meters per day. The rate of movement depends upon the factors listed below: •
Temperature of the ice. A
polar glacier shows cold ice with temperatures well below the freezing point from its surface to its base. It is frozen to its bed. A
temperate glacier is at a melting point temperature throughout the year, from its surface to its base. This allows the glacier to slide on a thin layer of meltwater. Most glaciers in alpine regions are temperate glaciers. •
Gradient of the slope. • Thickness of the glacier • Subglacial water dynamics ==Glacial Terminology ==