Accumulation adds snow and ice to a glacier, while ablation removes mass through melting, sublimation and calving.
Where annual accumulation exceeds ablation, compacted snow becomes firn and then glacial ice under the pressure of later snowfall.
Gravity moves this ice downslope by internal deformation and basal sliding, with faster movement where ice is thick, the slope is steep and meltwater reduces friction.
Key Idea
A glacier is both a store and a flow within a system, so its size reflects the balance between inputs, transfers and outputs.
Glacial erosion requires moving ice that contains rock debris, not low temperature alone.
Erosion Deepens and Widens Glacial Valleys
Plucking occurs when meltwater enters joints, freezes onto rock and allows moving ice to remove loosened blocks.
Abrasion occurs when debris embedded in basal ice scrapes the bedrock, smoothing surfaces and cutting parallel striations that indicate ice-flow direction.
Rotational erosion beneath a corrie or cirque overdeepens its basin, while freeze-thaw weathering steepens the back wall and a rock lip remains near the outlet.
When cirques erode back on several sides of a mountain, the intervening ridges sharpen into arêtes and a pyramidal peak may remain at their intersection.
A glacier truncates interlocking spurs and removes weaker rock, transforming a narrow V-shaped river valley into a wider U-shaped trough with steep sides and a broad floor.
Transport and Deposition Create Unsorted Landforms
Supraglacial debris travels on the ice surface, englacial debris travels within the ice and subglacial debris moves at the glacier bed.
Till is deposited directly by melting ice and is generally unsorted because the glacier carries particles ranging from clay to boulders together.
Meltwater deposits outwash beyond the ice margin and sorts sediment because faster water carries larger particles than slower water.
Lateral moraine forms along glacier margins, medial moraine forms where tributary glaciers join and terminal moraine marks a former maximum ice extent.
Erratics are transported rock fragments whose geology differs from the local bedrock, allowing past ice routes to be reconstructed.
Landforms Preserve Evidence of Past Ice
The orientation of striations, the provenance of erratics and the position of moraine together provide stronger evidence of former ice movement than any one feature alone.
Postglacial rivers, weathering and slope failure can modify glacial landforms, so present appearance must be interpreted as the result of several processes operating over time.
Exam technique
Explain each landform through a sequence of process, material movement and resulting shape.
Distinguish till deposited directly by ice from sorted outwash deposited by meltwater.
Glacial Processes Form a Connected System
Climate controls accumulation and ablation, mass balance controls ice thickness and movement, and movement controls the glacier’s capacity to erode, transport and deposit material.
A complete explanation therefore links atmospheric conditions to ice dynamics and then to the landforms visible after the glacier retreats.
Active recall
How do plucking and abrasion differ?
Explain how a cirque can develop into an arête or pyramidal peak.
Why is till usually unsorted while outwash is sorted?
How can moraine and erratics be used to reconstruct former ice movement?
