GLACIAL MOVEMENT

It is the movement of glaciers which allows them to erode and transport materials, forming distinctive glacial landscapes. The idea of mass balance helps us to understand that when there is a positive mass balance and accumulation exceeds ablation, the glacier may advance. Likewise where there is a negative mass balance and ablation exceeds accumulation the glacier may retreat. However, ultimately the movement of a glacier is forwards and downslope due to gravity. Therefore even a 'retreating' glacier may be seen to move forward under the force of gravity.

(i) The nature of ice

Ice can act in two very different ways. In response to sudden movements, ice can be very brittle resulting in shearing and the formation of crevasses. In contrast, when under steady pressure, ice is able to bend or deform plastically.

(ii) Variations in glacial movement

Glaciers do not move in a uniform fashion:

• Rates of movement are fastest in the centre (less frictional effects) and on the surface (due to laminar flow)
• Rates of movement are slowest at the sides and the base of a glacier due to frictional effects.

(iv) Cold and Warm Based Glaciers

Glaciers can be divided into Cold (Polar) glaciers and Warm (Temperate) glaciers. The main difference between the two is the basal termperature.

COLD BASED GLACIERS

• examples include Greenland and Antarctica
• due to very cold temperatures accumulation and ablation are low.
• temperatures remain at 0oC throughout and the PMP (Pressure Melting Point - see below) is not reached - the glacier therefore moves by internal flow rather than basal slippage (due to the lack of meltwater)
• The glacier is frozen to the bed and without an increase in pressure there is no melting - movement is therefore very slow (1-2cm per day) and there is little erosion.
• MAIN METHOD OF MOVEMENT: INTERNAL FLOW / INTERNAL DEFORMATION

WARM BASED GLACIERS

• examples - the Alps / Norway
• base of glacier is around the Pressure Melting Point (PMP) temperature - therefore there is often meltwater created
• These glaciers are more active and mobile and able to carry out erosion.
• The presence of meltwater acts as a lubricant and glacier flow can be rapid - can be 2-3 metres per day.
• MAIN METHOD OF MOVEMENT: BASAL SLIPPAGE (Regelation Slip; Creep; Extending and Compressing FLow; Surges)

(v) Mechanisms of Movement

The type of glacier movement is largely dependent on the basal temperature of a glacier.

1. MOVEMENT IN COLD-BASED GLACIERS:

As these glaciers are frozen to their beds, there is no basal slippage. Movement of cold based glaciers is through INTERNAL FLOW / DEFORMATION. This is where ice deforms and moves plastically.

• LAMINAR FLOW - movement of individual parallel layers of accumulation
• INTERGRANULAR FLOW - ice crystlas are orientated in the direction of the glaciers movement and therefore they gradually slide past each other.

2. MOVEMENT IN WARM-BASED GLACIERS:

The presence of meltwater in warm-based glaciers acts as a lubricant and BASAL SLIPPAGE is the main mechanism of movement. Basal slippage may invovle a combination of the following:

• REGELATION SLIP - where basal ice is forced against a rock obstacle, the PMP may be reached due to the increase in pressure. Meltwater is released at the base, enabling the ice to move over the obstacle. The meltwater quickly refreezes on the other side due to a reduction in the PMP.
• CREEP - when ice comes across an obstacle, the increase in pressure can cause the ice to behave like plastic and the ice 'deforms' to flow around the obstacle.
• EXTENDING AND COMPRESSING FLOW - the rates of flow of a glacier can be controlled by the gradient of the underlying bedrock. Where there is an increase in the gradient, the ice accelerates and becomes thinner (extending flow) - crevasses flow as the ice acts in a brittle way due to the quicker movement. Where the gradient becomes more gentle, the ice decelerates and teh ice becomes thicker (compressing flow).

GLACIAL SURGES - these are periods of very rapid glacial movement in which there is a large increase in velocity due to excessive build up of subglacial metlwater. They often result in the surface being pitted by crevasses due to the rapid movement. Glacial surges can result in forward movements of up to 300m a day. A surging glacier on Disko Island, West Greenland was found to have advanced more than 10km down a valley in 4 years. Glacial surges, may be triggered by events such as earthquakes or even avalanches. The Vatnajökull ice cap experienced extensive subglacial melting as a result of a volcanic eruption beneath it during October 1996. This resulted in a glacial outburst flood (jökulhlaup)

SEE THE REFERENCES BELOW FOR A MORE DETAILED OUTLINE OF PROCESSES OF GLACIAL MOVEMENT.

Lesson Resources (Handouts and Extra Links)

Handouts / Lesson Resources:

Glacier Movement - Class notes sheet

Also see Waugh (2000) "An Integrated Approach (3rd Ed)" p.108-109 and Palmer & Yates (2005) "Advanced Geography" p.325-329

Suggested Background Reading:

Geobytes Glaciation Web (see section on Glacier movement)

Glacial Systems (including overview of glacial flow (external link)

Fundamentals of Physical Geography (external link)

Glacial Movement (an excellent overview - including factors affecting rates) (external link)

How do Glaciers Move? (external link)

Animation of Glacier Flow (external link)

Glaciers and Glaciation (includes a summary of types and rates of movement) (external link)

Effects of glacial surges (external link)

How Glaciers Form and Flow (external link)

Movement of Glaciers (external link)