Glacial Systems - Glacial Inputs, Processes and Outputs

In geography a system can be summarised as a series of linked inputs, stores, transfers and outputs through which energy and material are cycled and subject to a range of physical processes. Material, or in the case of glacial debris, is moved between stores by geomorphological processes (erosion, weathering, transportation and deposition). In glacial systems these stores are typically landforms and the ice itself. Debris is transferred between these stores by the glacial flow. An output of this movement of debris is the redistribution of rock material into depositional features, as well as meltwater.

Glacial inputs, processes and output

Inputs

The most significant input into the glacial system is the ice itself which accumulates high up in mountain corries through snowfall and avalanches. However, as with other landscape systems, at the heart of the glacial system is the movement of sediment: debris enters the glacier through rock fall (of weathered material) and erosion of glacier paths, valley sides and bedrock. Temperature acts as a component, driving the growth or retreat of the glacier, determining its size and velocity and thus, the quantity and extent of debris redistribution. Gradient is the other key component influencing the speed of glacier movement and ice flow.

Processes

The key processes at work in the glacial system are those of weathering, erosion, transportation and deposition. The amount and rate of weathering and erosion depends on a range of factors including the temperature, local geology, slope gradient, velocity and weight of the glacier, its thickness and the size of its load. Much of the erosion is dependent upon movement of the ice mass. This can occur in a number of ways depending on the type of glacier (warm-based or cold-based).

• Basal sliding: occurs only in warm glaciers where temperatures are such that significant meltwater lubricates the contact between the base layers of glacier ice and the bedrock. The meltwater occurs either as a result of internal tunnels transferring it via surface moulins (vertical tunnels), or increased pressure as the basal ice encounters resistant bedrock, which may reach the pressure melting point that causes basal ice to melt.
• Internal deformation: occurs predominantly in cold glaciers where gravity and the pressure of ice in the accumulation zone causes ice crystals to slide over each other in a series of parallel planes in a ‘crumpling’ deformation. This can result in deep crevasses at the surface.
• Extensional flow: where the gradient becomes steeper the ice moves faster ‘stretching’ the ice mass and becoming thinner through a series of fractures which form crevasses at right angles to the direction of flow.
• Compressional flow: where the gradient becomes less steep or the ice encounters a major obstacle the ice mass slows, backs up, crevasses close and there are thrust fractures as the ice mass compresses. The increased thickness of ice exerts greater pressure on bedrock and can result in more extensive pressure erosion.

The main erosional processes at work within the glacial system are:

• Abrasion – rock fragments and debris carried by the glacier scrape the rock below/adjacent to the ice flow in the valley with which it has contact leaving striations (scratches) in the bedrock and incorporating the eroded rock into the ice mass, from where it can cause erosion subsequently.
• Plucking – meltwater flowing along the base of the glacier freezes to the bedrock, fragments of which are ripped out by the moving glacier.
• Rotational scouring – as layers of rock debris accumulates on the surface of a glacier from valley sides, over successive winters it becomes embedded within the compressed ice. Bands of slowly rotating frozen rock are then scraped over the bedrock as the glacier moves downhill.

As with coastal and fluvial systems, weathering also plays a key role in the glacial landscape:

• Nivation – hollows form under the emerging glacier as a result of the freeze-thaw cycle and mass wasting. Over time these may enlarge and start to form corries (see erosional landforms below).
• Frost action – an umbrella term for freeze-thaw processes where meltwater percolates into cracks and freezes causing fissures to expand under pressure of ice and, with repeated cycles, shatter the surrounding rock.
• Pressure release/dilation – where the variable weight of glacier ice on top of bedrock can cause fractures to open up, expand and extend deeper.

Warm-based glaciers where the glacier is close to melting point tend to be more active and as such more developed erosional landforms are found here. In contrast cold-based glaciers are generally frozen to their beds and incur less movement/energy and therefore erosion.

Outputs

As glaciers melt and retreat they lose energy and deposit debris and rock fragments, known as moraine and till. Meltwater also flows through the glacier and exits the snout from the zone of ablation. Many of the world largest rivers originate in glacial landscapes, for example the Ganges, Indus and Yangtze. They may carry vast quantities of pulverised rock ranging in size from glacial rock flour in suspension to outwash plains of sand, gravel and boulders.