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Glacial Movement

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Glaciers may seem stationary, but in fact, glacial movement is always slowly occurring. Its process depends on the thermal characteristics of the glacier. Cold-based glaciers move only by internal deformation (also known as internal flow) while warm-based glaciers move via internal deformation and basal slippage.

Glacial movement definition

It's useful to have a definition of glacial movement before we look more closely at how it happens.

Glacial movement is the way in which a glacier stays in motion (internal deformation, basal slippage). This movement depends largely on the type of glacier (warm, cold, or polythermal-based).

What are warm, cold, and polythermal-based 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.

Glacial Movement Alpine GlacierFigure 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 0oC 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.

What processes cause 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).

Internal deformation

Internal deformation occurs when the weight of the glacial ice causes its ice crystals to deform or 'stretch out'. As a result, the glacier moves downslope very slowly (approximately 1-2cm a day). This process will continue, and adjacent crystals will overlap over one another as the glacier continues its advance.

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.

Enhanced basal creepEnhanced regelation creepExtensional and compressional flow

Enhanced basal creep occurs when there is an obstacle on the path of the glacier, which is less than approximately 1m in width. This causes increased pressure which makes ice plastically deform around the obstacle very slowly. Consequently, the obstacle is almost entirely enveloped as the glacier passes over it.

Enhanced regelation creep occurs when a glacier moves over an obstacle greater than a 1m wide. The increased localised pressure decreases the Pressure Melting Point (PMP) and causes meltwater to form which acts as a lubricant for the ice to move up and over it. After the obstacle is passed, pressure decreases meaning that PMP increases and meltwater once again freezes.

Ice moves more quickly in glacial valleys causing ice to fracture into thick layers. Each layer begins to accelerate more downslope. When it reaches a steeper point on the slope, the layers temporarily separate during their descent. This is called extensional flow. As the valley gets less steep, the fractured ice layers decelerate. As a layer slows down, faster-moving ice layers coming from behind slam into it. This causes the ice layers to compress together, resulting in a thicker ice sheet.

As the name suggests, extensional flow causes the glacier to extend. It resembles a stretching in its longitudinal axis. This results in a decrease in the vertical height, and hence a lower trimline in the valley. The opposite effect is present in compressional flow (an increase in vertical height and hence a higher trimline). This may cause a problem in reconstructing past ice mass extent (see Erosional Landscapes).

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.

It is believed that in Iceland, volcanic activity can lead to a surge event. Equally, seismically active locations with activity such as earthquakes could potentially facilitate a surge.

What factors affect the rate of 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).

Glacial Movement - Key takeaways

  • The thermal characteristics of glaciers such as temperate, cold-based, or polythermal determine the processes by which they move.
  • Numerous physical factors contribute to the rate of glacial movement including lithology, altitude, gravity/gradient slope, ice temperatures, ice thickness (size and mass), and mass balance.
  • Basal slip and internal deformation are the two types of glacial movement.
  • The regions of glaciers determine the potential for certain types of glacial movements, e.g. surges.

Glacial Movement

Glacial movement will generally occur in the accumulation zone and move in the same direction as gravity would dictate. Characteristics of landscapes that have been subject to glacial movement are those with typical glacial landforms including depositional, erosional, and fluvioglacial landforms.

This may be done by using landforms found in relict glacial landscapes to reconstruct past glacial ice movement. Some examples of these include using striations, identifying the stoss and lee slope of a Roche Moutonnée, or perhaps identifying an erratic.

Glacial movement is caused by either basal slippage or internal deformation. Warm-based glaciers are affected by both processes whereas cold-based glaciers only move via internal deformation. Basal slippage can take place by 3 different processes, Extensional / Compressional flow, Enhanced Basal / Regelation Creep, and Surges.

Glacial movement is categorised according to the process that causes glacial movement. If it is enabled due to the presence of basal meltwater, it is known as basal slip. Otherwise, if it is caused by the deformation of ice crystals it is called Internal Deformation.

Final Glacial Movement Quiz

Question

Which process/es cause movement in cold-based glaciers?

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Answer

Internal Deformation

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Question

How is extensional/compressional flow linked to problems with reconstructing past ice mass extent?

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Answer

 It can cause variations in the height of the trimline which reduces the possibility of determining the precise height of the glacier.

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Question

Where do Polythermal glaciers occur?


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Answer

Lower-climate zones e.g. Svalbard, Norway.

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Question

What two aspects of lithology affect glacial movement?


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Answer

Permeability as well as the hardness

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Question

Which processes cause movement in warm-based / temperate glaciers?


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Answer

Basal Slippage: Enhanced basal and regelation creep, Extensional Compressional Flow and Surges; Internal Deformation too.

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Question

What percentage of movement can Basal slippage account for in Warm-based glaciers?


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Answer

75%

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Question

What is the approximate general speed of internal deformation?

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Answer

1-2cm a day

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Question

What speed increase and total forward motion of a glacier can surge account for?


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Answer

Speed increase: 100x, forward motion: 90%

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Question

Name an example glacier where compressional/extensional flow can be observed in the present day


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Answer

Mer De Glac

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Question

Name 3 factors that control glacial movement

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Answer

3 of (Lithology, Altitude, Gravity, Ice temperatures, Mas Balance, Ice thickness)

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Question

How does the permeability of the subglacial surface affect glacial movement?


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Answer

Impermeable surfaces trap meltwater which decreases friction allowing for faster glacial movement, permeable surfaces allow meltwater to escape so there is no lubrication effect and the glacier has no increase in speed or movement.

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Question

Define glacial movement

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Answer

Glacial movement is the way in which a glacier stays in motion (internal deformation, basal slippage.) This movement depends largely on the type of glacier (warm, cold or polythermal-based)

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Question

True or False: glaciers are stationary

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Answer

False

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Question

Warm-based glaciers, also known as temperate glaciers, are most likely to occur in higher or lower altitudes?

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Answer

Lower latitudes

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Question

Where are cold-based glaciers more likely to be found?

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Answer

In higher latitudes

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