Systems and processes — AQA A-Level Geography
Test yourself on Systems and processes with AQA A-Level practice questions.
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Systems and processes explained
A glacier operates as an open system driven by climate.
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Inputs (accumulation) primarily include snowfall, avalanches, and wind deposition, adding mass to the upper glacier. Outputs (ablation) involve melting, calving, and sublimation, removing mass at the snout. The glacial budget, or mass balance, is the annual difference between total accumulation and total ablation. If accumulation exceeds ablation, the budget is positive, and the glacier advances. Conversely, a negative budget causes retreat. For example, the Rhone Glacier in Switzerland currently exhibits a strongly negative mass balance, losing significant volume (measured in m³ of water equivalent) annually due to rising global temperatures, leading to rapid snout retreat.
Ablation and accumulation – historical patterns of ice advance and retreat.
The glacial system is driven by the balance between accumulation (inputs like snowfall and avalanches) and ablation (outputs like melting, sublimation, and calving). When accumulation exceeds ablation, the glacial mass balance is positive, leading to ice advance. Conversely, a negative mass balance causes ice retreat. Historical patterns of advance and retreat are linked to long-term climate fluctuations, such as the Pleistocene epochs (characterised by extensive glacial advance) and the Holocene interglacial period (marked by widespread retreat). For example, the Little Ice Age (roughly 1300 to 1850) saw significant glacial advance in the Alps, followed by rapid retreat as global temperatures rose during the industrial era.
Warm and cold based glaciers: characteristics and development.
Glaciers are classified by their thermal regime into warm-based (temperate) and cold-based (polar) types. Warm-based glaciers develop in high-altitude temperate regions like the Alps. Their basal temperature is at or above the pressure melting point, allowing basal sliding and rapid movement (often 20 to 200 m per year), which drives significant erosion and transportation. Cold-based glaciers develop in high-latitude polar regions like Antarctica. Their basal temperature remains well below the pressure melting point, meaning they are frozen to the bedrock. Movement is slow (often less than 2 m per year) and occurs solely through internal deformation, resulting in minimal erosion. Polythermal glaciers exhibit characteristics of both regimes.
Geomorphological processes – weathering: frost action, nivation; ice movement: internal deformation, rotational, compressional, extensional and basal sliding; erosion: plucking, abrasion; transportation and deposition.
Glacial landscapes are shaped by complex geomorphological processes. Weathering includes frost action, where water freezes and expands in rock joints, and nivation, combining freeze-thaw and meltwater removal beneath snow patches. Ice movement occurs via internal deformation (ice crystals sliding) and basal sliding where meltwater lubricates the bed. Movement can be rotational in corries, compressional where ice slows, or extensional where it accelerates. Erosion is driven by plucking, where ice freezes to bedrock and pulls fragments away, and abrasion, where entrained debris acts like sandpaper. Glaciers transport material supraglacially, englacially, or subglacially, depositing it as unsorted till when the ice melts.
Fluvioglacial processes: meltwater, erosion transportation and deposition.
Fluvioglacial processes are driven by the immense power of glacial meltwater, shaping landscapes during deglaciation or seasonal melting. Meltwater streams flow under high hydrostatic pressure within subglacial channels, giving them significant erosive power. Erosion occurs primarily through hydraulic action and abrasion of bedrock by a high suspended load. Meltwater transports vast quantities of sediment, rolling boulders via traction or carrying finer particles in suspension. As discharge decreases, the meltwater loses competence and capacity. This leads to the deposition of stratified, sorted, and rounded sediments, forming distinct fluvioglacial landforms such as eskers and outwash plains, contrasting with unsorted glacial till.
Periglacial features and processes: permafrost, active layer and mass movement.
Periglacial environments are characterised by intense frost action and permafrost, occurring at the margins of glacial ice or in high-latitude regions. Permafrost is ground that remains continuously at or below 0 °C for at least two consecutive years. Above this lies the active layer, which thaws during the brief summer and freezes in winter. This seasonal thawing saturates the active layer because the underlying permafrost is impermeable, leading to distinct mass movement processes. Solifluction is a key mass movement where the waterlogged active layer slowly flows downslope under gravity, creating features like solifluction lobes. Frost creep also contributes to mass movement through the cyclical heaving and settling of soil particles.
Your focus
- Identify the inputs, stores, transfers, and outputs within a glacial system.
- Calculate or interpret glacial mass balance data to determine if a budget is positive or negative.
- Explain how changes in the glacial budget affect the position of the glacier snout over time.
Show all 18 objectives
- Define the processes of ablation and accumulation within the glacial system.
- Explain how the balance between ablation and accumulation determines ice advance and retreat.
- Analyse historical patterns of glacial advance and retreat using specific geological timeframes.
- Define the characteristics of warm-based and cold-based glaciers.
- Explain the role of the pressure melting point in the development of different glacial thermal regimes.
- Compare the mechanisms of movement and erosional potential between warm and cold-based glaciers.
- Describe the mechanisms of frost action and nivation in glacial environments.
- Explain the different types of ice movement and the conditions under which they occur.
- Evaluate the relative importance of plucking and abrasion in shaping glacial landscapes.
- Explain the sources and flow paths of glacial meltwater.
- Describe the processes of fluvioglacial erosion and transportation.
- Analyse the characteristics of fluvioglacial deposition and how they differ from direct glacial deposition.
- Define permafrost and the active layer using precise temperature and temporal criteria.
- Explain the mechanism of solifluction and frost creep in periglacial environments.
- Identify and describe landforms created by periglacial mass movement.
Systems and processes exam tips
Marking Points
- Identify glaciers as open systems with inputs (accumulation), stores (ice mass), transfers (ice movement), and outputs (ablation).
- Define the glacial budget (mass balance) as the net difference between accumulation and ablation over a one-year period.
- Explain the concept of the equilibrium line, where annual accumulation exactly equals annual ablation.
- Relate a positive mass balance to glacial advance and a negative mass balance to glacial retreat, linking this to climate change.
- Define accumulation as the addition of mass to a glacier, primarily through precipitation (snow), wind deposition, and avalanches.
- Define ablation as the loss of mass from a glacier, occurring via melting, sublimation, evaporation, and calving.
- Explain that a positive net mass balance results in glacial advance, while a negative net mass balance results in glacial retreat.
- Describe historical patterns of ice advance and retreat, referencing specific periods such as the Pleistocene glacials, the Holocene interglacial, or the Little Ice Age.
- Explain the concept of the pressure melting point and how it dictates the thermal regime of a glacier.
- Describe the characteristics of warm-based glaciers, including basal temperatures at the pressure melting point, presence of meltwater, and rapid movement via basal sliding.
- Describe the characteristics of cold-based glaciers, including basal temperatures below the pressure melting point, lack of basal meltwater, and slow movement via internal deformation.
- Explain the development of these glaciers in relation to their geographical location, contrasting high-altitude temperate environments with high-latitude polar environments.
- Explain how frost action and nivation weaken bedrock, preparing it for subsequent glacial erosion.
- Distinguish between internal deformation in cold-based glaciers and basal sliding in warm-based glaciers.
- Describe how compressional flow leads to ice thickening and overdeepening, while extensional flow causes thinning and crevassing.
- Detail the mechanisms of plucking and abrasion, linking them to the formation of specific macro and micro landforms.
- Explain the role of hydrostatic pressure in increasing the erosive power of subglacial meltwater streams.
- Describe how fluvioglacial erosion occurs through hydraulic action, abrasion, and attrition.
- Detail the transport mechanisms of meltwater, including traction, saltation, suspension, and solution.
- Explain how a decrease in meltwater discharge leads to the deposition of sorted, stratified, and rounded sediments.
- Define permafrost accurately as ground remaining at or below 0 °C for two or more consecutive years.
- Explain the role of the active layer and its seasonal freeze-thaw cycle.
- Describe how the impermeable nature of permafrost leads to the saturation of the active layer during the summer thaw.
- Explain solifluction as the downslope flow of saturated soil under the influence of gravity.
- Identify and describe resulting periglacial features such as solifluction lobes or terracettes.
Examiner Tips
- 💡Draw and label a systems diagram showing inputs, the equilibrium line, and outputs to support explanations of glacial budgets.
- 💡Use the term 'water equivalent' when discussing changes in glacial mass or volume.
- 💡Reference a specific, named glacier (e.g., Mer de Glace) when discussing recent trends in negative glacial budgets.
- 💡Use the mass balance equation to structure answers explaining why a glacier is advancing or retreating.
- 💡Reference specific historical periods, such as the Little Ice Age, to provide concrete evidence for historical patterns of advance and retreat.
- 💡Clearly distinguish between seasonal variations in mass balance and long-term historical trends.
- 💡Always link the thermal regime of a glacier to its dominant mechanism of movement (basal sliding versus internal deformation).
- 💡Use the concept of the pressure melting point to explain why meltwater exists at the base of temperate glaciers despite sub-zero surface temperatures.
- 💡Compare the erosional power of warm and cold-based glaciers to evaluate their impact on landscape development.
- 💡Use specific terminology such as 'compressional flow' and 'extensional flow' when describing changes in glacier gradient.
- 💡Link weathering processes like nivation directly to the early stages of corrie formation to demonstrate synoptic understanding.
- 💡When discussing erosion, always mention the role of basal meltwater and entrained debris in facilitating abrasion.
- 💡Emphasise the seasonal and diurnal variations in meltwater discharge when explaining depositional processes.
- 💡Clearly distinguish between the characteristics of sediments deposited directly by ice and those deposited by meltwater.
- 💡Use the concepts of competence and capacity to explain why meltwater deposits sediment as its energy decreases.
- 💡Always include the specific timeframe of at least two consecutive years when defining permafrost to secure full marks.
- 💡Use annotated diagrams to show the relationship between the active layer, the permafrost table, and solifluction lobes on a slope.
- 💡Link the seasonal temperature changes directly to the saturation of the active layer to explain why mass movement occurs.
Common Mistakes
- Stating that a retreating glacier moves backwards; correct by explaining that the ice always flows downhill under gravity, but the snout melts faster than ice is replenished.
- Confusing accumulation with advance; correct by noting accumulation is an input process (e.g., snowfall), whereas advance is the physical extension of the snout due to a positive budget.
- Ignoring sublimation as an output; correct by including sublimation (direct phase change from solid ice to water vapour) alongside melting and calving in ablation zones.
- Confusing glacial retreat with the ice physically moving backwards; correct by explaining that the ice continues to flow downhill under gravity, but the snout melts faster than it is replenished.
- Assuming ablation only means melting; correct by including sublimation, evaporation, and calving as significant ablation processes.
- Viewing historical patterns as short-term weather events; correct by attributing historical advance and retreat to long-term climatic shifts like Milankovitch cycles.
- Stating that cold-based glaciers do not move at all; correct by explaining that they do move, but slowly, through internal deformation rather than basal sliding.
- Assuming the pressure melting point is a constant 0 °C; correct by explaining that the melting point of ice decreases as overlying pressure increases.
- Confusing the geographical development zones; correct by associating warm-based glaciers with high-altitude temperate regions and cold-based glaciers with high-latitude polar regions.
- Confusing plucking with abrasion; plucking involves freezing to and pulling rock away, whereas abrasion is the sandpaper effect of entrained debris.
- Stating that glaciers only move by sliding; correct this by explaining that internal deformation is a key movement mechanism, especially in cold-based glaciers.
- Assuming glacial deposition is always sorted; correct this by noting that direct glacial deposition (till) is unsorted and unstratified, unlike fluvioglacial deposits.
- Confusing glacial till with fluvioglacial deposits; correct this by stating that fluvioglacial deposits are sorted and stratified, whereas till is unsorted.
- Assuming meltwater only flows on the surface; correct this by explaining that subglacial and englacial channels carry significant volumes of meltwater under pressure.
- Stating that meltwater erosion is identical to river erosion; correct this by highlighting the role of high hydrostatic pressure in subglacial environments.
- Error: Defining permafrost as permanently frozen water. Correction: Define permafrost as ground (soil or rock) that remains at or below 0 °C for at least two consecutive years, regardless of its water content.
- Error: Confusing solifluction with standard soil creep. Correction: Specify that solifluction requires a saturated active layer overlying an impermeable permafrost boundary, whereas soil creep occurs in non-periglacial environments.
- Error: Stating the active layer is permanently frozen. Correction: Explain that the active layer is the top layer of soil that thaws in the summer and freezes in the winter.