Ice Movement and Glacial Processes

Spec mapping (AQA 7037): Paper 1, §3.1.4 Glacial Systems and Landscapes — geomorphological processes: the operation and outcomes of ice movement (the dynamics of glacial systems); warm- and cold-based glaciers; mechanisms of movement and rates of operation. This lesson explains the flows (transfers) component of the glacial system introduced in the previous lesson, so it links synoptically to systems concepts in §3.1.1 (energy and mass transfer, thresholds), and the velocity controls developed here directly underpin the erosion (§3.1.4) and the climate-change response of glaciers (relevant to §3.1.5 Hazards thinking on surge and glacial-lake outburst risk). The assessment objectives are AO1 (knowledge of named mechanisms and rates), AO2 (applying flow concepts to explain landform development and velocity patterns) and AO3 (interpreting flow-velocity data).

Glaciers are not static features — they are dynamic bodies of ice that move continuously under their own weight. The mechanism and rate of movement are not academic details: they determine directly whether a glacier erodes its bed, how fast it transports debris, and where it deposits. A glacier that slides on basal meltwater can carve a deep trough; one frozen to its bed barely touches the rock beneath. Understanding how ice moves is therefore the hinge between the systems framework and the landforms that follow.

It is worth grasping at the outset just how variable glacier movement is. Ice behaves as a brittle solid near the surface (where it cracks into crevasses) but as a slowly deforming plastic at depth, and it can additionally slide bodily over its bed or be carried on a shearing layer of soft sediment beneath it. Velocities therefore span an enormous range — from a sluggish metre or two per year in cold polar ice, through tens of metres per year for a typical Alpine valley glacier, to several kilometres per year for the fastest Antarctic ice streams and Greenland outlet glaciers, and even metres per day during a surge. The remainder of this lesson explains the mechanisms behind that variation and shows why the answer always comes back to temperature, water and the nature of the bed.

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Why Do Glaciers Move?

Glacier flow is driven by gravity acting on the mass of ice. As snow accumulates in the upper basin, the ice thickens; gravity then drives it from areas of high ice thickness toward areas of low thickness — broadly from the accumulation zone down to the snout. The driving force is the basal shear stress (τ), which can be approximated as:

$\tau = \rho g h \sin\alpha$τ=ρghsinα

where $\rho$ρ is the density of ice (~917 kg/m³), $g$g is gravitational acceleration (9.81 m/s²), $h$h is ice thickness, and $\alpha$α is the surface slope. The equation captures the two key controls neatly: thicker ice ($h$h) and a steeper surface ($\alpha$α) both increase the stress driving flow.

The rate and mechanism of flow therefore depend on:

• Ice thickness — thicker ice generates greater basal shear stress and pressure
• Surface (and bed) gradient — steeper slopes increase the down-glacier component of gravity
• Thermal regime — temperate (warm-based) glaciers move far faster than polar (cold-based) ones because meltwater enables sliding
• Basal conditions — meltwater or deformable till at the bed reduces friction
• Valley geometry — narrow valleys concentrate the flow, raising centre-line velocity
• Mass balance / activity — a steep mass-balance gradient (high accumulation and high ablation) forces a large ice flux through the system, so high-activity maritime glaciers flow fast

These controls help explain the enormous range of observed velocities. Typical valley glaciers move of the order of tens of metres per year (the Athabasca Glacier ~15 m/yr; the Mer de Glace historically ~90 m/yr in its faster reaches). Cold-based polar glaciers may creep only a few metres per year. Fast Antarctic ice streams and Greenland outlet glaciers, by contrast, move hundreds of metres to several kilometres per year — and a surging glacier briefly reaches metres per day. Crucially, velocity also varies in time: most temperate glaciers flow noticeably faster in summer than winter as meltwater reaches the bed, so any single figure is a snapshot of a varying system.

A useful way to summarise the controls is that velocity rises with the driving stress (set by ice thickness and surface slope) and falls with the resisting stress (basal and lateral friction). Anything that increases driving stress (thicker, steeper ice) or reduces resistance (basal meltwater, deformable till, a wide valley) speeds the glacier up. This balance-of-forces view is what underlies the equation $\tau = \rho g h \sin\alpha$τ=ρghsinα and is the conceptual key to the whole topic.

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Mechanisms of Ice Movement

Most glaciers move by a combination of mechanisms; the balance between them is set chiefly by the thermal regime.

1. Internal Deformation (Plastic Flow / Creep)

Once ice exceeds roughly 30–50 m thickness (generating a shear stress of order ~50–100 kPa), it ceases to behave as a brittle solid and begins to deform plastically. Ice crystals re-orient and glide past one another along their basal crystal planes, so the ice as a whole creeps forward.

• Operates in all glaciers, regardless of thermal regime
• Is the only mechanism in polar (cold-based) glaciers, where the base is frozen to the bed — which is why they move slowly (often only a few metres a year)
• The strain rate rises with depth because shear stress increases downward
• This behaviour is described by Glen's Flow Law (John Glen, 1955):

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

where $\dot{\varepsilon}$ε˙ is strain rate, $\tau$τ is shear stress, $A$A is a temperature-dependent "softness" coefficient, and the exponent $n$n is about 3. The non-linearity is the crucial point.

Exam Tip: Glen's Flow Law tells you the response is non-linear — because $n \approx 3$n≈3, doubling the stress increases the strain rate roughly eightfold ($2^3 = 8$23=8). This is why a small thickening of ice, or a small steepening of the bed, can produce a large change in flow rate.

The temperature dependence of $A$A also explains why warm ice (near the pressure melting point) deforms much more readily than cold ice — temperate glaciers are "softer" as well as able to slide.

2. Basal Sliding

Basal sliding is the movement of the whole glacier over its bed. It requires meltwater at the base to lubricate the contact and reduce friction, so it operates only in temperate (warm-based) glaciers (and the warm-based interiors of polythermal glaciers) where basal ice is at the pressure melting point.

• Can account for 50–90% of total movement in temperate glaciers, especially in summer
• Two sub-processes do the work:
  • Enhanced basal creep (stress concentration) — where ice meets a bump on the bed, stress concentrates on the up-glacier side and the ice creeps faster around the obstacle
  • Regelation slip — on the high-pressure up-glacier side of a small obstacle the pressure melting point is depressed, so ice melts; the meltwater flows around the obstacle and refreezes on the low-pressure lee side, releasing latent heat. First explained in principle by Michael Faraday (1850s) and formalised by John Weertman (1957)

graph LR
    A["High pressure (up-glacier)<br/>PMP lowered → ice MELTS"] --> B["Meltwater flows<br/>around obstacle"]
    B --> C["Low pressure (lee side)<br/>water REFREEZES, releasing latent heat"]

Key Point: Regelation works best around small obstacles (< ~1 m); enhanced creep dominates around large obstacles. Because the two trade off, there is a controlling obstacle size (≈ 0.5 m, Weertman 1957) at which the bed offers the greatest resistance to flow.

3. Subglacial Bed Deformation

Where a glacier rests on saturated, unconsolidated till rather than solid rock, the sediment itself can deform and flow beneath the ice. This was famously measured by Geoffrey Boulton and colleagues beneath Breiðamerkurjökull in Iceland, where instruments showed the till shearing under load.

• Saturated till behaves like a viscous fluid under the weight of overlying ice
• Can contribute the majority of movement (up to ~90%) where it operates
• It is the key to fast ice streams in the West Antarctic Ice Sheet, which slide on deforming marine sediments

The relative contribution of these three mechanisms is not fixed — it shifts with thermal regime, bed material and season. A useful summary is:

Mechanism	Operates in	Requires	Typical share of motion
Internal deformation	All glaciers	Ice thickness > ~30–50 m	Dominant in polar ice; minor in fast temperate ice
Basal sliding	Warm-based only	Basal meltwater at PMP	50–90% in temperate glaciers, peaking in summer
Bed deformation	Glaciers on soft till	Saturated, weak sediment	Up to ~90% over deforming beds (ice streams)

A polar glacier frozen to hard bedrock moves entirely by slow internal creep; a temperate glacier on rigid rock adds rapid basal sliding; and a glacier or ice stream over soft, wet till can be carried mostly by the bed shearing beneath it. Recognising that these mechanisms coexist and trade off — rather than treating a glacier as moving by one process alone — is exactly the nuance that distinguishes a strong answer, and it explains why measured velocities range from a metre or two per year to several kilometres per year across different settings.

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Patterns of Ice Flow

Extending and Compressing Flow

A glacier's velocity changes along its length as the bed gradient changes — analysed mathematically by John Nye (1952):

Flow Type	Bed Gradient	Ice Behaviour	Landform Implications
Extending flow	Bed steepens	Ice accelerates, thins and stretches; transverse crevasses open at the surface	Erosion enhanced — rock steps, over-deepened rock basins
Compressing flow	Bed flattens	Ice decelerates, thickens and is compressed	Deposition/transport favoured; debris carried toward the surface along shear planes

This alternation matters for landforms: over-deepened basins (which later hold ribbon lakes) tend to form under extending flow on steepened sections, while debris is brought up to the surface along thrust planes under compressing flow near the snout.

graph LR
    A["Steep section<br/>EXTENDING FLOW<br/>Ice stretches & thins<br/>Crevasses open<br/>Erosion dominant"] --> B["Gradient eases<br/>COMPRESSING FLOW<br/>Ice thickens & shears upward<br/>Crevasses close<br/>Transport to surface"]
    B --> C["Next steepening<br/>EXTENDING FLOW"]

Rotational Movement

In corrie (cirque) glaciers and the heads of valley glaciers, ice slides along a curved, pivoting path — rotational slip, described by W. Vaughan Lewis (1949) from Norwegian cirques. Because the curved motion concentrates pressure and abrasion at the base of the hollow, it produces the characteristic over-deepened corrie floor, while erosion weakens toward the front, leaving a raised lip (threshold) — the rock bar that later impounds a tarn.

Velocity Profiles

A glacier does not move as a rigid block; velocity varies in three dimensions.

Cross-profile (across the valley): ice moves fastest at the centre line and slowest at the margins because friction with the valley walls retards the edges. The shear between fast centre and slow margins opens marginal (lateral) crevasses.

Long-profile (down-glacier): velocity tends to peak around the equilibrium line, where the flux of ice passing through is greatest (all the ice gained above must pass this point on its way to be lost below), and slows toward the thinning snout.

Depth profile (through the ice): the surface moves fastest, the bed slowest.

• In a temperate glacier a large fraction of surface motion is carried by basal sliding, so the velocity-depth curve shows a marked jump at the bed.
• In a polar glacier all motion is by internal deformation, so velocity rises smoothly from (near-zero at) the frozen bed to a maximum at the surface.

Exam Tip: Be ready to sketch and annotate a velocity-depth profile and contrast temperate vs polar glaciers. The diagnostic feature is the basal jump under temperate ice (sliding) versus the smooth gradient under polar ice (deformation only). Label axes and annotate the reason for each feature.

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Crevasses — the brittle response to flow

The upper ~30 m of a glacier behaves as a brittle solid (above this depth, pressure forces plastic deformation that closes fractures). Where the ice is stretched faster than it can deform, it fractures, opening crevasses. Crevasses are therefore a surface read-out of the stresses driving flow, and their pattern tells you what the ice beneath is doing:

Crevasse type	Where it forms	What it indicates
Transverse	Across the glacier where the bed steepens	Extending flow — ice accelerating and stretching longitudinally
Marginal (lateral)	Near the valley sides, angled up-glacier	Shear between fast centre-line ice and slow margins
Splaying / radial	Where the glacier widens (e.g., onto a piedmont lobe)	Lateral spreading of the ice
Bergschrund	At the head of a corrie/valley glacier	Where moving ice pulls away from the static ice/rock of the back wall

Crevasses are not just hazards for mountaineers; they are geomorphologically and hydrologically important. They allow surface meltwater to penetrate the ice via moulins (near-vertical shafts), delivering water to the englacial and subglacial systems — which, as we will see, is central to both sliding and surging. Where compressing flow dominates (the bed flattening), crevasses close, and debris carried up along shear planes can be released at the surface. Reading a glacier's crevasse field is thus a practical way to map zones of extending and compressing flow.

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Subglacial Hydrology — the hidden control on speed

Because basal sliding depends on water at the bed, the plumbing of the glacier exerts a first-order control on how fast it moves. Subglacial water is organised into two contrasting drainage systems, and the switch between them governs velocity:

• Channelised (efficient) drainage — water concentrated into a few large Röthlisberger (R-) channels melted up into the ice or Nye channels cut into bedrock. This system drains water quickly at low pressure, so it keeps basal water pressure down and the glacier relatively slow. It tends to develop later in the melt season as channels enlarge.
• Distributed (inefficient) drainage — water spread across the bed in a thin film, linked cavities or porous till. This system drains slowly, so water pressure stays high, friction is reduced across a large area, and the glacier slides faster. It dominates early in the melt season and after sudden inputs.