7.1 Glacier formation, movement, and dynamics

Glacier Formation and Types

Glaciers form where snow accumulates faster than it melts, year after year, gradually compacting into dense ice masses. These ice bodies then flow under their own weight, carving landscapes and storing enormous volumes of freshwater. Understanding how glaciers form, move, and respond to changing conditions is central to interpreting both ancient and modern Earth surface processes.

Environmental Conditions for Glacier Formation

Two ingredients are required: cold temperatures to preserve snow and ice, and sufficient snowfall to provide raw material. Regions that meet both criteria (polar latitudes, high mountain environments) are where glaciers develop.

The transformation from fresh snow to glacier ice is called firnification, and it happens in stages:

1. Freshly fallen snow (density ~50–70 kg/m³) begins to settle and compact under its own weight.
2. Air pockets between snow crystals shrink as grains round off and pack together. After surviving one melt season, the snow becomes firn (density ~400–830 kg/m³).
3. Continued burial and compaction over decades to centuries squeezes out remaining air. Ice crystals grow and interlock.
4. Once density reaches ~830–850 kg/m³ and air passages seal off into isolated bubbles, the material is considered glacier ice (up to ~917 kg/m³).

This process can take anywhere from a few decades in wet, temperate climates to thousands of years in cold, dry polar regions.

Classification of Glacier Types

There are two broad categories: alpine (mountain) glaciers and continental ice sheets.

Alpine glaciers come in several forms:

• Cirque glaciers occupy bowl-shaped depressions carved into mountainsides. These are often the smallest and most numerous alpine glaciers.
• Valley glaciers flow downhill from cirques into pre-existing river valleys, sometimes extending tens of kilometers.
• Piedmont glaciers form when valley glaciers spill out onto flat lowlands and spread into broad lobes. Malaspina Glacier in Alaska is a classic example.

Continental ice sheets are vastly larger, burying entire landmasses under kilometers of ice. Today only two exist: the Greenland Ice Sheet (~1.7 million km²) and the Antarctic Ice Sheet (~14 million km²). Related forms include:

• Ice caps: dome-shaped ice masses smaller than 50,000 km² (e.g., Vatnajökull in Iceland)
• Ice fields: similar in scale but conform to underlying topography rather than forming a dome

Thermal Regime Classification

A glacier's thermal regime controls how it moves and how it interacts with its bed.

• Temperate (warm-based) glaciers have ice at or near the pressure melting point throughout. Liquid water exists within and beneath the glacier year-round, which makes them highly dynamic and responsive to climate shifts.
• Cold-based (polar) glaciers remain well below freezing from surface to bed. Because the base is frozen to the bedrock, basal sliding is minimal and erosion rates are low. These are typical of interior Antarctica and very high altitudes.
• Polythermal glaciers contain both cold and temperate zones. A common configuration has a cold surface layer overlying a temperate interior and base. Many Arctic glaciers (e.g., Svalbard) fall into this category.

Glacial Accumulation and Ablation

Accumulation Processes

Accumulation is any process that adds mass to a glacier. The primary sources are direct snowfall, avalanches from surrounding slopes, and wind-blown snow deposition. The accumulation zone occupies the upper portion of the glacier, where annual gains exceed losses.

Within the accumulation zone, seasonal snowfall layers create distinct annual bands. Ice core scientists use these layers to date the ice and reconstruct past climate conditions, since trapped air bubbles preserve samples of ancient atmosphere.

Accumulation rates vary with:

• Elevation: higher elevations are generally colder and receive more snowfall
• Topography: windward slopes intercept moisture-laden air and accumulate more snow
• Latitude: polar regions have long, cold accumulation seasons but often low total precipitation, while mid-latitude mountain glaciers may receive heavy snowfall

Ablation Processes

Ablation is any process that removes mass from a glacier. The ablation zone occupies the lower portion, where annual losses exceed gains. You'll often see bare, dirty-looking ice here, along with surface meltwater streams and moulins (vertical shafts that drain meltwater into the glacier interior).

The main ablation mechanisms are:

• Melting: occurs at the surface (driven by warm air and solar radiation), internally (from friction and geothermal heat), and at the base
• Sublimation: ice converts directly to water vapor without melting. This dominates in cold, dry, windy environments like high-altitude tropical glaciers and parts of Antarctica.
• Calving: chunks of ice break off where a glacier terminates in a lake or ocean. This can remove enormous volumes of ice very quickly.
• Wind erosion: minor compared to other processes, but relevant in arid polar settings

Ablation rates depend on air temperature, incoming solar radiation, surface albedo (reflectivity), wind speed, and humidity.

Mass Balance and the Equilibrium Line

The Equilibrium Line Altitude (ELA) is the elevation on a glacier where accumulation exactly equals ablation over a year. Above the ELA, the glacier gains mass; below it, the glacier loses mass.

Mass balance is the net difference between total accumulation and total ablation over a given time period (usually one year).

• Positive mass balance (accumulation > ablation): the glacier thickens and tends to advance.
• Negative mass balance (ablation > accumulation): the glacier thins and tends to retreat.

The ELA shifts in response to climate. Warmer temperatures or reduced snowfall push the ELA upward, shrinking the accumulation zone. If the ELA rises above the glacier's highest point, the entire surface is in the ablation zone and the glacier will eventually disappear.

Mass balance measurements are one of the most direct indicators of glacier health and are critical for predicting future water resources in glacier-fed river basins.

Glacier Movement Mechanisms

Glaciers move through two main mechanisms: internal deformation and basal sliding. Most glaciers use some combination of both.

Internal Deformation

Internal deformation (also called creep) occurs because ice behaves as a plastic material under sustained stress. The weight of overlying ice generates shear stress, causing ice crystals to slowly deform and slide past one another.

The rate of internal deformation depends on:

• Ice thickness: thicker ice means greater pressure and faster deformation
• Surface slope: steeper slopes increase the driving stress
• Ice temperature: warmer ice (closer to the melting point) deforms more readily because crystal bonds are weaker

This relationship is formalized in Glen's Flow Law:

$\dot{\epsilon} = A\tau^n$

where $\dot{\epsilon}$ is the strain rate (how fast the ice deforms), $A$ is a temperature-dependent flow parameter, $\tau$ is the applied shear stress, and $n$ is the creep exponent (typically ~3 for glacier ice). The exponent of 3 means that small increases in stress produce large increases in flow rate, which is why thick, steep glaciers move so much faster than thin, flat ones.

The velocity profile across a glacier is not uniform. Ice moves fastest at the surface and along the centerline, while friction with the bed and valley walls slows flow near the margins and base.

Basal Sliding

Basal sliding occurs when a layer of meltwater at the glacier bed reduces friction, allowing the ice to slide over bedrock or sediment. This mechanism is only possible in temperate or polythermal glaciers where the base reaches the pressure melting point. Cold-based glaciers, frozen to their beds, experience little to no basal sliding.

Basal sliding rates are influenced by:

• Subglacial water pressure and distribution
• Bed roughness and rock type
• Whether the bed is "hard" (bedrock) or "soft" (deformable sediment)

Weertman's theory explains how ice moves past obstacles on the bed through two processes working together:

1. Regelation: on the upstream side of a small obstacle, pressure raises and ice melts. Water flows around the obstacle and refreezes on the downstream (low-pressure) side. This is most effective for small obstacles.
2. Enhanced plastic flow: ice deforms around larger obstacles through creep. This is more effective for large obstacles.

Some glaciers exhibit stick-slip behavior, alternating between periods of slow creep and sudden rapid movement. This is linked to fluctuations in subglacial water pressure: when water pressure rises, effective friction drops and the glacier lurches forward.

Glacier Surges and Velocity Variations

Glacier surges are dramatic episodes of rapid movement, with velocities increasing by factors of 10 to 100 compared to normal flow. Surges are often triggered by reorganization of the subglacial drainage system. When an efficient channel system collapses into a distributed, high-pressure water film, basal friction drops and the glacier accelerates.

Glacier velocity varies in both space and time:

• Spatially: fastest at the center and surface, slowest near the bed and valley walls
• Seasonally: many temperate glaciers speed up in spring and summer when meltwater reaches the bed, then slow down in winter
• Over longer periods: changes in ice thickness, slope, bed conditions, and thermal regime all modulate flow speed

Factors Influencing Glacier Dynamics

Climate and Mass Balance

Mass balance is the primary driver of long-term glacier behavior. Temperature controls melt rates, ice viscosity, and the potential for basal sliding. Precipitation patterns determine how much snow feeds the accumulation zone.

Climate change affects glaciers through rising temperatures, shifting precipitation patterns, and extended melt seasons. Globally, most glaciers are currently experiencing sustained negative mass balance. The World Glacier Monitoring Service reports that the average glacier has lost roughly 28 meters of ice thickness (water equivalent) since 1970, with losses accelerating since the 1990s.

Topography and Glacier Geometry

The shape of the landscape exerts strong control over glacier behavior:

• Valley orientation and width influence how much solar radiation the glacier receives and how freely ice can flow.
• Glacier hypsometry (the distribution of area across different elevations) determines how sensitive a glacier is to ELA shifts. A glacier with most of its area concentrated near the ELA will respond dramatically to even small climate changes.
• Overdeepened basins (bedrock depressions carved below the downstream lip) can slow retreat by trapping thick ice, but they can also promote proglacial lake formation, which accelerates calving.
• Steep, narrow valleys tend to produce fast-flowing glaciers; broad, gentle terrain produces slower, more diffuse ice flow.

Feedback Mechanisms and System Interactions

Several feedback loops amplify or dampen glacier change: