7. Dynamics of Snow and Ice in Glacier Basins

This chapter examines the processes that regulate glacier behaviour and how ice masses respond to environmental conditions. Glacier movement is considered and some distinctive features in larger Karakoram glaciers. Rates of movement along main ice streams are typically between 150 and 300 m annually but may be over 1,000 m on large icefalls. Among different styles of ice movement, available observations suggest sliding motion is a large factor in most Karakoram glaciers. The so-called Blockschollen or ‘block motion’ is widely observed, a slab-like movement of large sections of the glacier with sharp lines of shear at the margins. It is typical of relatively gentle as well as steep sections in ablation zones. Strong fluctuations in glacier movement show up in detailed measurements and on all timescales: day to day and minute to minute as well as with season and from year to year. Summer ablation zone velocities are commonly 20–50 % greater than in winter, suggesting sensitivity of the sliding component to meltwater availability. The variety of flow instabilities may reflect the roles of basin steepness and icefalls, debris content and ice thermal conditions. Such ice temperature measurements as are available show that thermal regimes include cold, warm and polythermal types. Verticality is again an important factor and different thermal regimes occur in the same glacier at different elevations. Icefalls are a major but neglected concern, responsible for much of the fastest movement and the larger part of transfers of ice mass vertically. Ablation zone icefalls create environments where rapid modifications may penetrate far into the thinner more broken ice mass and faster responses to climate variability. The great extent of ogives on main ice streams, seasonal forms generated in icefalls, is another indication of their importance. Drivers of ice dynamics, as of mass balance, include processes that commence off-ice, notably vertical mass transfers of snow and ice by wind and avalanche. They intervene in the density, structure and thermal properties of inputs to the glaciers, their timing and spatial organisation. This can be expressed as an ‘input concentration factor’ (ICF), derived from the ratio between conventional accumulation zone areas and steep walls in glacier source zones. The highest concentration values are for Turkestan-type glaciers, lowest for Alpine and Mustagh types are intermediate. Glacier dynamics and responses to climate are shown to involve the whole contributing basin and a range of processes that affect snow and ice, not solely glacier mass balance.