Glacial Mass Balance and Ice Movement

Spec mapping (AQA 7037): Paper 1 (Physical), §3.1.4 Glacial systems and landscapes — the glacier as an open system, its mass balance (glacial budget), and the mechanisms and rates of ice movement that this budget drives. This depth lesson is the glacial counterpart of the coastal sediment-cell lesson: it treats ice as a conserved, flowing quantity whose budget governs whether a glacier advances or retreats, and whose flow mechanism governs how (and how fast) it erodes. It draws directly on the systems framing of §3.1.1 (inputs/outputs/stores/flows, feedback, equilibrium) and underpins all the erosional and depositional landform content of the following lessons — a glacier can only carve a corrie or build a moraine if it is moving, and how it moves is set by its thermal regime and budget. Assessment objectives: AO1 (mass-balance and flow mechanisms), AO2 (applying budget/thermal reasoning to a named glacier), AO3 (manipulating and evaluating mass-balance, velocity and ELA data).

The conceptual ambition mirrors the coastal half of the course: stop describing glaciers and start accounting for them. A glacier is a throughput system in which ice is gained at the top, conveyed downslope, and lost at the bottom; the difference between gain and loss (the mass balance) sets the position of the snout, while the thermal regime (warm- vs cold-based) sets the mechanism and rate of the conveyor. Master those two control variables — budget and thermal regime — and every subsequent landform, velocity and response-to-climate question follows.

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The Glacier as an Open System

Like the coast, a glacier is an open system exchanging both matter (ice/snow/meltwater) and energy with its surroundings, with inputs, outputs, stores, transfers and feedback.

flowchart LR
  IN[INPUTS<br/>snowfall, avalanche,<br/>windblown snow, rime] --> STORE[STORES<br/>ice, firn, meltwater]
  STORE --> TR[TRANSFER<br/>ice flow downslope]
  TR --> STORE
  STORE --> OUT[OUTPUTS<br/>melting, sublimation,<br/>calving, evaporation]
  STORE -. negative feedback .-> IN

Inputs

• Snowfall — the primary input; snow accumulates and is compressed into firn (compacted granular snow) and eventually glacial ice
• Avalanches — transfer snow and ice from surrounding slopes onto the glacier surface
• Wind-blown snow — redistribution of snow by wind, often filling hollows and accumulating on lee slopes
• Rime ice — freezing of supercooled water droplets on contact with the ice surface

Outputs

• Ablation — the loss of ice and snow from the glacier through:
  • Melting — surface, internal and basal melting
  • Sublimation — direct conversion of ice to water vapour (important in cold, dry environments)
  • Calving — breaking off of icebergs where glaciers reach the sea or a lake
  • Evaporation — minor loss from the glacier surface

Stores

• Ice — the main store, which can be thousands of metres thick in ice sheets
• Firn — intermediate between snow and ice, found in the accumulation zone
• Meltwater — stored within, beneath and on the surface of the glacier

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Mass Balance (Glacial Budget)

The mass balance (or glacial budget) is the difference between accumulation and ablation over a year — the glacial equivalent of the coastal sediment budget, and governed by the same mass-conservation logic.

$b_n = c - a$bn​=c−a

where $b_n$bn​ is the net (specific) mass balance, $c$c is total annual accumulation and $a$a is total annual ablation, conventionally expressed in metres of water equivalent (m w.e.) so that snow and ice of different densities are directly comparable. The net balance of a whole glacier is the sum of the strongly positive balance high in the accumulation zone and the strongly negative balance low in the ablation zone:

$B = \int (c - a)\, \mathrm{d}A$B=∫(c−a)dA

Healthy temperate glaciers have specific balances of the order of ±1–2 m w.e. yr⁻¹; a sustained negative $B$B of even −0.5 m w.e. yr⁻¹ (typical of many present-day Alpine and Svalbard glaciers) drives steady thinning and retreat.

The Accumulation Zone

• Located in the upper part of the glacier, typically at higher elevations
• Snowfall exceeds melting — net gain of mass
• Snow is compressed through successive winters into firn and then ice
• The transformation from snow to ice takes approximately 25–50 years in temperate glaciers and hundreds to thousands of years in polar glaciers

The Ablation Zone

• Located in the lower part of the glacier, typically at lower elevations
• Melting and other ablation processes exceed accumulation — net loss of mass
• Bare ice is exposed in summer
• Meltwater streams develop on the surface and drain through the glacier

The Equilibrium Line Altitude (ELA)

The Equilibrium Line Altitude (ELA) is the elevation on the glacier where accumulation exactly equals ablation over a year. It is also known as the firn line.

Condition	Effect on ELA	Glacier Response
Warming climate	ELA rises	Glacier retreats
Cooling climate	ELA falls	Glacier advances
Increased snowfall	ELA falls	Glacier advances
Decreased snowfall	ELA rises	Glacier retreats

The ELA is a critical indicator of glacier health:

• If the ELA rises above the summit of the mountain, the glacier will eventually disappear
• Current global warming is causing ELAs to rise worldwide, leading to widespread glacier retreat

A useful diagnostic derived from the ELA is the accumulation–area ratio (AAR) — the fraction of the glacier's area lying above the ELA (i.e. in the accumulation zone). A glacier in steady state typically has an AAR of around 0.5–0.6; a markedly lower AAR (a shrunken accumulation zone) signals a glacier out of balance and retreating, while a higher AAR signals advance. Because AAR can be read from a single end-of-summer satellite image (the snowline marks the ELA), it is a powerful, low-cost monitoring tool across remote mountain ranges. The ELA and AAR together convert the abstract budget into a mappable quantity — exactly the kind of resource an AO3 question exploits.

The ice flux through the ELA also explains glacier dynamics: every unit of ice gained above the ELA must, in steady state, be transported down through the ELA and lost below it. The ELA is therefore the line of maximum ice discharge and, in valley glaciers, often of maximum velocity — the throughput is greatest where the conveyor must move a whole glacier's annual accumulation past one cross-section. This links the budget (this lesson's first half) directly to the flow mechanisms (its second half): the budget sets the work the flow must do.

Positive and Negative Mass Balance

Positive mass balance:

• Accumulation > Ablation
• The glacier stores more ice than it loses
• The glacier thickens and may advance
• The snout (terminus) extends to lower elevations

Negative mass balance:

• Ablation > Accumulation
• The glacier loses more ice than it stores
• The glacier thins and retreats
• The snout recedes to higher elevations

Seasonal Variation

In temperate glaciers, mass balance shows a clear seasonal cycle:

• Winter: Accumulation dominates — snowfall adds mass, temperatures are too low for significant melting
• Summer: Ablation dominates — temperatures rise, melting occurs, and the glacier surface shrinks
• The net balance is the sum of winter accumulation and summer ablation

The shape of the annual balance curve is itself informative. Plotting cumulative mass as it builds through winter and is drawn down through summer, the winter balance (the peak) and the summer balance (the subsequent drawdown) can be read separately, and their difference is the net annual balance. A glacier can move into deficit either because accumulation falls (drier winters, a higher ELA) or because ablation rises (warmer summers, a longer melt season) — and distinguishing which is driving a retreat matters, because they imply different climatic causes and different futures. In the maritime mid-latitudes (Norway, the Alps) glaciers are typically summer-ablation-sensitive (warm summers dominate the budget), whereas some high-latitude glaciers are more winter-accumulation-sensitive. Diagnosing the dominant term from a balance record is a sophisticated AO3 move that goes beyond simply noting the sign of the net balance.

Exam Tip: Be prepared to interpret mass balance graphs showing cumulative or annual balance. A declining cumulative balance indicates a glacier in long-term retreat.

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Glacier Types

Understanding glacier types is essential for explaining differences in movement and erosion.

Classification by Temperature

Type	Temperature	Characteristics	Example
Temperate (warm-based)	At or near pressure melting point throughout	High rates of movement; active erosion; meltwater present at base	Alps, Norway
Polar (cold-based)	Well below pressure melting point	Very slow movement; limited erosion; frozen to bed	Antarctica interior
Polythermal	Mix of warm and cold zones	Variable behaviour	Svalbard glaciers

Classification by Morphology

Type	Description	Example
Cirque (corrie) glacier	Small glacier occupying an armchair-shaped hollow	Red Tarn, Lake District
Valley glacier	Glacier confined within a valley, fed by cirque glaciers	Mer de Glace, French Alps
Piedmont glacier	Valley glacier that spreads out on a lowland plain	Malaspina Glacier, Alaska
Ice cap	Dome-shaped ice mass covering an upland area	Vatnajökull, Iceland
Ice sheet	Continental-scale ice mass (> 50,000 km²)	Antarctic, Greenland

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Mechanisms of Ice Movement

Glaciers move through several mechanisms, the relative importance of which depends on glacier type, temperature, slope, and thickness.

1. Internal Deformation (Plastic Flow / Creep)

Ice is a crystalline solid that behaves as a non-Newtonian (pseudo-plastic) fluid under sustained stress: individual ice crystals slip along their basal planes (intracrystalline creep) and reorganise (recrystallisation), so the whole mass deforms slowly under its own weight.

• Occurs once ice thickness exceeds the critical ~30–50 m needed for the basal shear stress to overcome the yield stress of ice.
• The relationship is strongly non-linear — Glen's flow law gives the strain rate $\dot{\varepsilon}$ε˙ as proportional to stress $\tau$τ raised to a power $n \approx 3$n≈3:

$\dot{\varepsilon} = A\,\tau^{n}, \quad n \approx 3$ε˙=Aτn,n≈3

• Because $n \approx 3$n≈3, a small increase in driving stress (from greater ice thickness or slope gradient) produces a large increase in deformation rate; the temperature-dependent factor $A$A means warmer ice deforms far more readily (cold polar ice is stiff and sluggish).
• The velocity profile is fastest at the surface and decreases with depth as deformation accumulates downward — the surface "outruns" the base.
• This is the only movement mechanism in cold-based glaciers (frozen to the bed), which is why they move slowly (often < 1–2 m yr⁻¹) and erode little.

2. Basal Sliding

• The glacier slides over its bed on a thin film of meltwater
• Only occurs in warm-based (temperate) glaciers where the base is at the pressure melting point
• Accounts for the majority of movement in temperate glaciers (up to 80% of total velocity)
• Rate depends on:
  • Amount of meltwater at the base
  • Roughness of the bed
  • Subglacial water pressure

3. Regelation

• A specific sub-mechanism of basal sliding by which warm-based ice melts on the high-pressure stoss side of a small (< ~1 m) obstacle and refreezes on the low-pressure lee side, "flowing" past it as water and re-forming as ice.
• Only possible at the pressure melting point, so confined to warm-based ice; it is the micro-process behind the asymmetry of roches moutonnées. (Developed in full in the thermal-regime section below.)

4. Bed Deformation (Subglacial Till Deformation)

• If the glacier rests on soft, saturated sediment (till), the sediment itself deforms and flows, carrying the ice along on a deforming bed.
• Can contribute the majority of velocity in fast ice streams (e.g. the Antarctic ice streams draining into the Ross Ice Shelf move largely on deforming till) and beneath ice sheets resting on glacial deposits.
• Crucial for landform genesis — a deforming till bed is implicated in the streamlining of drumlins (lesson 8).

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Thermal Regime: Warm-Based vs Cold-Based Glaciers — the Master Control

The single most important control on how a glacier moves and erodes is whether its base reaches the pressure melting point (PMP) — the temperature at which ice melts under the prevailing pressure (which falls below 0 °C as pressure rises, so thick ice can have liquid water at its base even when sub-zero).

Property	Warm-based (temperate)	Cold-based (polar)
Basal temperature	At PMP — meltwater present	Well below PMP — frozen to bed
Movement	Basal sliding + bed deformation + creep	Internal creep only
Velocity	Fast (tens–hundreds m yr⁻¹)	Very slow (often < 2 m yr⁻¹)
Erosion	Vigorous (abrasion + plucking)	Minimal (protects the bed)
Meltwater	Abundant; fluvioglacial activity	Scarce
Setting	Alps, Norway, Iceland, valley glaciers	Antarctic interior, high Arctic

A polythermal glacier is warm-based in its thick interior but cold-based at its thin margins and surface (typical of Svalbard). The decisive geomorphic consequence is that warm-based glaciers do almost all the world's glacial erosion: only where basal meltwater is present can the glacier slide (the precondition for abrasion) and can refreezing in lee-side joints occur (the precondition for plucking). Cold-based glaciers, frozen to their beds, preserve the landscape beneath them — which is why ancient, delicate landforms survive under parts of the Antarctic and Greenland ice sheets. Always diagnose the thermal regime before discussing a glacier's erosional work; it is the hinge on which the whole of §3.1.4 turns.

Crucially, thermal regime is not fixed: a glacier can switch regime as it thickens or thins, because thicker ice both insulates the bed (trapping geothermal heat) and depresses the pressure melting point, so a cold-based glacier that thickens may become warm-based — and a warming glacier that thins may do the reverse. This means a glaciated landscape can record episodes of erosion (warm-based phases) separated by phases of preservation (cold-based phases), and the same ice mass can be erosive in its thick centre and protective at its thin edges simultaneously. The practical upshot for reconstructing past glaciation is that the absence of glacial erosion does not prove the absence of ice — cold-based ice can blanket a landscape without marking it — a subtlety that has reshaped interpretations of the Cairngorms and high-Arctic plateaux, where pre-glacial tors and weathered surfaces survive beneath former ice cover.

Regelation — sliding past obstacles