How do glaciers move and erode, and what landforms of erosion and deposition do they leave behind?
The glacial system and mass balance; warm- and cold-based glaciers and ice movement; glacial and fluvioglacial processes; and the erosional and depositional landforms of glaciated landscapes.
A focused answer to the AQA A-Level Geography 3.1.4 content on glacial processes and landscape development, covering the glacial system and mass balance, ice movement, glacial and fluvioglacial processes, and the erosional and depositional landforms of glaciated landscapes.
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What this dot point is asking
AQA section 3.1.4 wants you to explain how glaciated landscapes develop: the glacial system and mass balance, how ice moves (warm- and cold-based glaciers), the glacial and fluvioglacial processes that operate, and the erosional and depositional landforms they produce. The reliable marks come from linking a named process to a named landform and from the contrast between ice-laid and meltwater-laid deposits.
The glacial system and mass balance
A glacier is an open system with inputs, stores and outputs.
Over a year the upper zone gains mass and the lower zone loses it; the glacier flows downhill to redistribute the surplus. Many glaciers today have a persistently negative balance and are retreating as the climate warms, a direct link to climate change.
How ice moves
Ice moves in three main ways. Internal deformation (creep) is the slow movement of ice crystals under their own weight, the dominant mechanism in cold-based (polar) glaciers frozen to their bed. Basal sliding occurs in warm-based (temperate) glaciers, where meltwater at the base lubricates movement, making them faster and far more erosive. Rotational flow occurs in corrie hollows, deepening the basin. Warm-based glaciers do most of the world's glacial erosion because basal sliding lets the ice grind its bed.
Glacial and fluvioglacial processes
Glacial erosion works by plucking (meltwater freezes ice onto the bed, pulling away loose, jointed rock as the ice moves) and abrasion (rock embedded in the ice base grinds and scratches the bedrock, leaving striations). Weathering, especially freeze-thaw, supplies angular debris above and around the ice. Transport carries debris supraglacially, englacially and subglacially. Fluvioglacial processes are those of meltwater, which sorts and rounds sediment as it carries and deposits it.
Erosional and depositional landforms
Erosional landforms include the corrie (armchair basin, often holding a tarn), the knife-edge arete between two corries, the pyramidal peak where three or more corries meet, the U-shaped glacial trough (with truncated spurs and hanging valleys), the ribbon lake in an over-deepened trough, and the roche moutonnee (smoothed upstream, plucked and steep downstream).
Depositional landforms divide by agent. Till (deposited directly by ice, unsorted and angular) forms moraines (lateral, medial, terminal, ground), drumlins (streamlined till mounds, steep up-ice, tapering down-ice) and erratics (boulders carried far from their source). Fluvioglacial deposits (laid by meltwater, sorted and rounded) form outwash plains (sandar), eskers (sinuous ridges from subglacial streams) and kames.
Try this
Q1. Define mass balance. [2 marks]
- Cue. Accumulation (snow input) minus ablation (melt and loss); positive advances the glacier, negative retreats it.
Q2. Explain how a glacial trough forms. [4 marks]
- Cue. A valley glacier erodes by plucking and abrasion, widening, deepening and straightening a former V-shaped river valley into a U-shaped trough with truncated spurs.
Q3. Distinguish between plucking and abrasion. [2 marks]
- Cue. Plucking: ice freezes onto and pulls away jointed bedrock. Abrasion: debris embedded in the ice grinds and scratches the bedrock (striations).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 2018 (style)6 marksExplain how glacial erosion produces a corrie.Show worked answer →
A 6 mark "explain" question rewarding a process-to-landform chain (AO1). Snow accumulates in a hollow on a north- or east-facing slope, compacts to firn then ice. Rotational movement of the ice, combined with freeze-thaw weathering above, deepens the hollow.
Plucking (ice freezing onto rock, pulling away loose blocks) steepens the back wall, and abrasion (rock embedded in the ice grinding the floor) over-deepens the basin. A rock lip is left where erosion weakens at the snout. After deglaciation a tarn (lake) often fills the over-deepened basin.
Markers reward naming the processes (plucking, abrasion, rotational movement, freeze-thaw), linking each to a part of the corrie, and the sequence of formation. Top answers note aspect (sheltered, shaded slopes favour accumulation).
AQA 2020 (style)4 marksDistinguish between till and fluvioglacial deposits.Show worked answer →
A 4 mark "distinguish" question (AO1). Till is material deposited directly by ice: it is unsorted (a mix of clay to boulders) and angular, because the ice carried it without rounding or grading. Moraines and drumlins are made of till.
Fluvioglacial deposits are laid down by meltwater: they are sorted (graded by size, as water drops the heaviest first) and rounded, because water transport rounds and grades the particles. Outwash plains (sandar), eskers and kames are fluvioglacial.
Markers reward the contrast on two axes: sorting (unsorted versus sorted) and shape (angular versus rounded), tied to the agent (ice versus meltwater). A named landform for each strengthens the answer.
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Sources & how we know this
- AQA A-level Geography (7037) specification — AQA (2016)