Glacial Mass Balance and Ice Movement

AQA specification section 3.1.4 requires an understanding of glacial systems, including the mass balance of glaciers, the mechanisms of ice movement, and the factors controlling glacial dynamics. This lesson provides the detailed knowledge needed for A-Level examination success.

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The Glacial System

Like coastal systems, glaciers can be understood as open systems with inputs, outputs, stores and transfers.

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.

Mass Balance = Total Accumulation − Total Ablation

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

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

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)