Ice Movement and Glacial Processes

Glaciers are not static features — they are dynamic bodies of ice that move continuously under the influence of gravity. The mechanisms and rates of glacier movement directly determine the nature and intensity of glacial erosion, transport, and deposition. Understanding how ice moves is therefore essential for explaining the landforms and landscapes that glaciers create.

---

Why Do Glaciers Move?

Glacier movement is driven primarily by gravity. Ice accumulates in the upper part of the glacier (accumulation zone), increasing the mass and weight of the ice body. This mass exerts a downslope force due to gravity, causing the glacier to flow from areas of high ice thickness toward areas of lower ice thickness — generally from higher to lower elevations.

The rate and mechanism of flow depend on:

• Ice thickness — thicker ice creates greater pressure at the base
• Gradient of the bed — steeper slopes increase gravitational force
• Thermal regime — temperate glaciers move faster than polar glaciers
• Basal conditions — the presence of meltwater at the base reduces friction
• Valley geometry — narrow valleys concentrate flow, increasing velocity

---

Mechanisms of Ice Movement

There are several distinct mechanisms by which glaciers move. Most glaciers move by a combination of these processes.

1. Internal Deformation (Plastic Flow / Creep)

When the pressure from overlying ice exceeds approximately 50 metres of ice thickness (creating a shear stress of about 50 kPa), the ice crystals begin to deform plastically. Individual ice crystals re-orient and slide past one another along crystal planes.

• Occurs in all glaciers, regardless of thermal regime
• Is the only mechanism operating in polar (cold-based) glaciers, where the base is frozen to the bedrock
• The rate of deformation increases with depth due to greater pressure (Glen's Flow Law, John Glen, 1955)
• Glen's Flow Law states that the strain rate of ice is proportional to the applied stress raised to the power of approximately 3: ε̇ = Aτⁿ (where n ≈ 3)
• This means a small increase in stress produces a disproportionately large increase in flow rate

Exam Tip: Glen's Flow Law is a key concept. Remember that the exponent n ≈ 3 means the relationship is non-linear — doubling the stress increases the strain rate by a factor of approximately 8 (2³ = 8).

2. Basal Sliding

Basal sliding occurs when the entire glacier slides over the bedrock surface. It requires meltwater at the base to act as a lubricant, reducing friction between the ice and the rock.

• Only occurs in temperate (warm-based) glaciers where basal ice is at or near the pressure melting point
• Can account for 50–90% of total glacier movement in temperate glaciers
• Two sub-processes are involved:
  • Enhanced basal creep — ice deforms more rapidly around obstacles on the bed due to increased stress on the upstream side
  • Regelation — ice melts on the high-pressure upstream side of a bedrock obstacle, flows around it as meltwater, and refreezes on the low-pressure downstream side. This process was first described by Michael Faraday (1850) and later elaborated by John Weertman (1957)

graph LR
    A["High pressure<br/>(upstream side)<br/>Ice melts"] --> B["Meltwater flows<br/>around obstacle"]
    B --> C["Low pressure<br/>(downstream side)<br/>Water refreezes"]
    style A fill:#6db3f2
    style C fill:#a8d8ea

Key Point: Regelation is most effective around small obstacles (less than about 1 m). For larger obstacles, enhanced basal creep is the dominant process. Together they produce an optimum obstacle size (about 0.5 m) at which resistance to flow is greatest — the controlling obstacle size (Weertman, 1957).

3. Subglacial Bed Deformation

Where the glacier rests on unconsolidated sediment (till) rather than solid bedrock, the sediment itself may deform and flow. This was demonstrated by Geoffrey Boulton and Andrew Hindmarsh (1987) beneath Breiðamerkurjökull glacier in Iceland.

• The saturated till behaves as a viscous fluid under the weight of the overlying ice
• Can contribute significantly to glacier velocity — in some cases accounting for up to 90% of movement
• Particularly important for ice streams in the West Antarctic Ice Sheet

---

Patterns of Ice Flow

Compressing and Extending Flow

The velocity of a glacier varies along its length in response to changes in bed gradient: