How and why do Glaciers move?

Glaciers are large masses of ice that are continuously changing and may be perceived as an open system with inputs and outputs. They take up 10% of the world’s total land area and are usually found in Polar Regions like Antarctica and Greenland and places where there are high altitudes and low latitudes.
Fresh falling snow is made up of tiny ice crystals and after it lands it changes into solid ice. Newly fallen snow is light and porous, but as the snow becomes buried by more snow, the snowflakes change into granular ice called firn. As time passes the firn gets buried further causing the weight above it to compress it into solid ice. The force of gravity and the pressure of the ice cause the glacier to move downhill. The ice mass will stay stationary until it reaches a certain amount of thickness and when the ice reaches this amount of thickness, the pressure becomes great enough to cause the glacier to deform and move. The ice will move away from where there is the most pressure (thickest part of the glacier).
Glaciers can retreat or advance depending on the amount of snow accumulation and ablation that occurs from year to year. If accumulation exceeds ablation, then the mass of the glacier will increase which causes the glacier to advance. If the snout moves to an area of lower latitude ablation will increase and eventually equilibrium will be re-established. Retreating and advancing of glaciers are very slow processes and takes hundreds of years. However, when a glacier retreats rapidly, the movement can be seen over a few months and is shown by the Glacier Bay, Alaska.
Movement is much faster at the centre of the glacier as there is less friction between the ice and the valley which it flows down. Movement of ice along the base of the glacier is slower due the amount of friction created. Glaciers move in two ways; basal slip is when the glacier move along the ground on a thin layer of melt water. They move by creep, which is when the ice undergoes plastic deformation and is blocked by large obstacles. These large obstacles increase the stress in the ice and cause the ice to become plastic further. Another way they move is by internal deformation which is movement within the glacier ice which was caused by stresses applied by the force of gravity. This results in the formation of crevasses which are formed when ice stretches faster than it can flow.
Movement of ice depends on whether it’s warm or cold and this depends on the PMP (pressure melting point). PMP is usually 0 ̊c on the surface of the glacier but it can change by the weight of the ice. Temperate glaciers have a higher temperature and therefore can create melt water which facilitates the movement of the glacier. Polar glacier’s temperature is always below 0 ̊c so no melt water is created. Temperate glaciers move much faster where the melt water acts as a lubricant reduces friction. Polar glaciers move slower than temperate glaciers as there is no melt water present and they are usually frozen to their beds.

A glacier moves because the pressure created by its own weight causes it to deform and slide which is why thinner glaciers flow slower than thicker glaciers. There are two types of flow that contribute to the movement of glaciers: extensional and Compressional flow. Extensional flow is when the velocity increases steadily in the zone of accumulation and the down valley ice is being pulled away from the up valley ice. Compressional flow is when the glacier slowly moves upwards as the upper valley is continuously pushing against the down valley ice. When there is a decrease in gradient, the velocity falls and the ice thickens, compressional flow occurs and vice versa for extensional flow.

In conclusion, there are many factors that influence the rate of movement of a glacier. The higher temperatures in a glacier, the more melt water is produced therefore allowing the glacier to flow faster as the melt water acts as a lubricant. The higher the gradient, the faster the glaciers moves as there is an increase in gravity. Lastly, the thicker the glacier the faster it will move as there is an increase in PMPxs. Movement is faster over impermeable surfaces compared to permeable surfaces in temperate zones as the basal melt water is not being taken up by the ground allowing it to flow.