Types of Glaciers and Ice Movement

Understanding the diversity of glacial forms and the mechanisms by which ice moves is essential for explaining the landforms created by glacial erosion and deposition. This lesson addresses Edexcel A-Level Geography Enquiry Question 1 (EQ1) by exploring how different glacier types and ice dynamics shape the landscape.

---

Classification of Glaciers

Glaciers are classified by their morphology (size and shape), their thermal regime (temperature characteristics) and their relationship to topography. These classifications are not mutually exclusive — a single glacier may be described using multiple categories.

Classification by Morphology

Type	Description	Scale	Examples
Ice sheet	Vast dome-shaped mass of ice covering > 50,000 km²	Continental	Antarctic Ice Sheet (~14 million km²), Greenland Ice Sheet (~1.7 million km²)
Ice cap	Dome-shaped mass < 50,000 km²; submerges the topography	Regional	Vatnajökull (Iceland, ~7,700 km²), Austfonna (Svalbard)
Ice field	Extensive ice cover that does not submerge the topography; mountain peaks protrude as nunataks	Regional	Juneau Icefield (Alaska), Columbia Icefield (Canada)
Valley glacier (alpine glacier)	A glacier confined within a mountain valley; fed from cirques or icefields	1–100+ km	Aletsch Glacier (Switzerland, 23 km), Mer de Glace (France, 12 km)
Cirque (corrie) glacier	A small glacier occupying an armchair-shaped hollow on a mountainside	< 1 km	Many examples in Snowdonia, Scottish Highlands
Piedmont glacier	A valley glacier that spreads out as a lobe on a lowland plain at the foot of a mountain range	10–100+ km	Malaspina Glacier (Alaska, ~3,900 km²)
Tidewater glacier	A valley glacier that terminates in the sea or a lake; loses mass primarily through calving	Variable	Columbia Glacier (Alaska), Jakobshavn (Greenland)
Outlet glacier	A glacier that drains an ice sheet or ice cap through a topographic channel	10–200+ km	Jakobshavn Isbræ (Greenland), Pine Island Glacier (Antarctica)

Classification by Thermal Regime

The thermal regime of a glacier — the distribution of temperatures within and beneath the ice — is the most important control on how the glacier moves and erodes.

Type	Basal Temperature	Key Characteristics	Typical Location
Warm-based (temperate)	At or near pressure melting point (0°C) throughout	Basal meltwater present; fast movement; high erosion rates; basal sliding dominant	Mid-latitude mountains (Alps, Rockies, Andes, New Zealand)
Cold-based (polar)	Well below pressure melting point	Frozen to bedrock; very slow movement; minimal erosion; internal deformation only	High-latitude ice sheets (interior Antarctica, high Arctic)
Polythermal	Variable — warm-based in some areas, cold-based in others	Complex behaviour; margins may be frozen while interior is warm-based	Svalbard, Greenland outlet glaciers, large ice caps

The thermal regime is determined by:

• Latitude: Higher latitudes = colder air temperatures = more likely cold-based
• Altitude: Higher altitude = colder = more likely cold-based
• Ice thickness: Greater thickness = more insulation from cold surface temperatures + more geothermal heat retained + pressure lowers melting point → more likely warm-based at depth
• Geothermal heat flux: Higher geothermal heat (e.g., Iceland) → warm-based conditions
• Climate: Maritime climates with high snowfall and mild temperatures favour warm-based glaciers

Exam Tip: The warm-based vs cold-based distinction is fundamental to understanding differential erosion rates. Warm-based glaciers are responsible for most erosional landforms (corries, U-shaped valleys, etc.) because they can slide over their bed. Cold-based glaciers are largely protective of the underlying landscape. Always specify the thermal regime when discussing glacial processes.

---

Mechanisms of Ice Movement

Glaciers move through several interconnected mechanisms. The relative importance of each depends on the glacier's thermal regime, thickness, slope gradient and bed characteristics.

1. Internal Deformation (Plastic Flow)

Internal deformation occurs in all glaciers — both warm-based and cold-based. It is the creep of ice crystals under the force of gravity.

• Ice crystals deform and slide past each other along crystal planes (a process called dislocation creep)
• The rate of deformation increases with stress (which is determined by ice thickness and slope gradient) and temperature (warmer ice deforms more readily)
• Movement is fastest at the surface and decreases with depth due to friction at the bed
• Internal deformation alone produces relatively slow movement — typically 1–20 metres per year
• This is the only movement mechanism available to cold-based glaciers because they are frozen to their beds

The relationship between stress and strain rate in ice is described by Glen's Flow Law (John Glen, 1955):

ε̇ = Aτⁿ

Where ε̇ is the strain rate, τ is the shear stress, A is a temperature-dependent constant, and n ≈ 3 for ice. The cubic relationship means that small increases in stress produce disproportionately large increases in deformation rate.

2. Basal Sliding

Basal sliding is the dominant movement mechanism in warm-based glaciers and can account for 50–90% of total glacier motion. It requires meltwater at the glacier bed to lubricate the ice-bedrock interface.

Sub-mechanism	Description
Enhanced basal creep	Ice deforms more rapidly around obstacles on the bed due to increased stress on the upstream side
Regelation	Ice melts under pressure on the upstream side of a bedrock obstacle (pressure lowers the melting point); meltwater flows around the obstacle and refreezes on the downstream side where pressure is lower — effectively allowing the glacier to "creep" past small obstacles
Bed deformation	Where the glacier rests on soft, saturated sediment (till), the sediment itself deforms and flows, carrying the ice with it
Meltwater lubrication	A thin film or pressurised layer of meltwater at the bed reduces friction, allowing the ice to slide more freely

Basal sliding rates are highly variable and depend on:

• Meltwater availability (seasonal — faster in summer)
• Bed roughness and topography
• Ice thickness and slope gradient
• Subglacial water pressure

3. Regelation Creep

Regelation creep (sometimes called pressure melting and regelation) is a specific mechanism by which a glacier overcomes small bedrock obstacles (< ~1 m in size):

1. Ice presses against the upstream (stoss) side of a small bedrock bump
2. The increased pressure lowers the melting point of the ice (the Clausius-Clapeyron relationship: the melting point decreases by approximately 0.072°C per MPa of pressure increase)
3. A thin film of meltwater forms on the upstream side
4. The meltwater flows around the obstacle to the downstream (lee) side
5. On the downstream side, pressure is lower, so the meltwater refreezes, releasing latent heat
6. The latent heat is conducted through the bedrock to the upstream side, sustaining further melting