Glacial Movement

Types of glaciers

There are many ways of classifying ice masses, and one of the most important classifications is the categorisation according to a glacier’s thermal characteristics. This classification is fundamental to the understanding of glacial movement as we will see further on in this article. There are three different thermal classifications of ice masses:

Warm-based glaciers

Also known as temperate glaciers, these glaciers occur in lower latitudes, often outside the polar regions such as the Alps or sub-arctic areas. Their surface temperatures fluctuate above the melting point during the summer months and below zero during the winter months. The rest of the ice mass is usually fluctuating at about 0°C.

Due to the increased pressure of the overlying ice, the pressure melting point increases which allows water to exist in lower temperatures where it would otherwise be frozen. Consequently, the subglacial regions are often populated with meltwater.

Sometimes glaciers may be located in areas where geothermal energy output is present. If there is a significant output of geothermal energy into the base of the glacier then the pressure in the subglacial region will increase and this further contributes to the increase of meltwater presence. Increased meltwater in the subglacial regions lubricates the contact of the ice mass to the glacial bed, which significantly reduces friction and allows for greater movement.

Figure 1: A diagram of an alpine glacier. Wikimedia Commons.

Cold-based glaciers

These are found in higher latitudes (e.g. Greenland or Antarctica) and aren’t subject to seasonal variation like glaciers found in lower latitudes (such as warm-based glaciers). The average temperature of the Ice is usually well below 0^oC due to extreme surface temperatures. These temperatures stay consistent despite any geothermal output and though there may be high amounts of pressure, little to no meltwater will be present though in summer months there may be small volumes of surface melt.

Polythermal glaciers

These are hybrid glaciers. There is substantial warmth in the subglacial layers which can form a wet environment, whereas the glacial margin is more reminiscent of cold-based glaciers. Many glaciers are cold-based in upper regions and warm based in lower regions when they extend to lower climate zones, one notable example of this is Svalbard in Norway.

Process of glacial movement

The processes that cause glacial movement are dependent on the thermal characteristics of the glacier. There are two types of glacial movements known as Basal Slippage and Internal Deformation. Warm-based / temperate glaciers move via all of the below processes whereas cold-based glaciers move only by Internal deformation (also known as internal flow).

Processes that cause glacial movement: Internal deformation

Processes that cause glacial movement: Basal slippage

Basal slippage accounts for 75% of glacial movement in warm-based glaciers. The pressure from the ice mass decreases the melting point of ice (recall the concept of the Pressure Melting Point). Consequently, meltwater will form on the base and decrease the friction between the glacier and the base which enables the glacier to move more rapidly over the bedrock. Basal slip can be subdivided into three different processes.

Processes that cause glacial movement: Surges

In these short-lived events, a glacier can advance substantially, moving up to 100x faster than normal. When a glacier moves over weak/unconsolidated rock, meltwater mixes with the sediment to create the conditions necessary for the sediment to deform under the weight of the glacier which causes the glacier to surge downslope at extremely fast rates. This may account for up to 90% of a glacier’s total forward motion.

Factors affecting glacial movement

Six main factors control the rate of glacial movement:

Gravity/gradient slope

A steeper gradient causes the effects of the glacier's weight to act far more significantly on the glacier’s movement. A steeper gradient enables faster movement of the glacier, less steep gradients lead to slower movements. It’s important to note that gravity and weight are the fundamental causes for glacial movement as without it none of the other factors would be relevant and there would be zero glacial movement.

Lithology

In temperate zones, movement is faster over impermeable surfaces rather than permeable. This is because impermeable surfaces don’t allow meltwater to escape and it stays at the base of the glacier. This decreases friction which allows the glacier to advance at faster rates. On permeable surfaces, it is possible for basal meltwater to be lost through percolation and therefore it will not be present to reduce friction, and hence the glacier will not move as fast. Furthermore, rocks more prone to erosion, such as sandstone or heavily fractured metamorphic rock, are more prone to deformation and therefore don’t inhibit the movement of the glacier as much as strong and hard-to-erode rocks do. Therefore glaciers on “softer” rocks generally move faster.

Altitude

Higher altitudes are likely to face greater precipitation and lower temperatures. Greater precipitation of snow and hail can cause an increase in mass balance, possibly leading to a positive mass balance and therefore increasing the rate of glacial advance.

Ice temperatures

In some extremely cold environments, ice could freeze and attach to bedrock. Consequently, the glacier may only move by subglacial internal deformation which is slower compared to glaciers whose ice sits at higher temperatures. This is because the potentially increased basal meltwater can decrease friction and increase the rate of glacial movement.

Ice thickness (size and mass)

Greater size and mass of ice can decrease the Pressure Melting Point at the base of the glacier and cause an increase in meltwater which allows for increased rates of glacial movement. On the other hand, the heavier the ice mass the more force is needed to move the glacier and hence greater size and mass could lead to slower movement.

Mass balance

Increased mass balance can cause an increased rate of advance of the glacier. If there is a negative mass balance the glacier may retreat while if the glacier is in equilibrium the glacier will neither advance nor retreat (remaining stationary).

Evidence of glacial movement on landscape

There is plenty of evidence on the landscape that glacial movement has occurred. Glacial movement can be demonstrated by different glacial landforms. There are both erosional and depositional landforms created by the movement of glaciers, forming these vast glacial landscapes. As glaciers erode by plucking, abrasion, and freeze-thaw weathering, they can create landforms such as corries, arêtes, pyramidal peaks, u-shaped valleys, and truncated spurs. Glaciers can also transport material as they move, creating landforms such as drumlins, erratics, and moraines. Some landforms, like striations (scratches on the rock below from sediment carried inside the glacier), can even show the direction in which the ice was moving in.

Fig. 2 - evidence of striations on the landscape.