7.1 Glacier formation, movement, and dynamics

Glaciers, nature's icy sculptors, shape our planet's surface through their formation and movement. These massive ice bodies form in cold regions where snow accumulates faster than it melts, gradually transforming into dense glacier ice through compaction and recrystallization.

Glacial dynamics involve a delicate balance between accumulation and ablation processes. As glaciers flow under their own weight, they carve landscapes and respond to climate changes, playing a crucial role in Earth's water cycle and climate system.

Glacier Formation and Types

Environmental Conditions for Glacier Formation

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• Glaciers form in areas where snow accumulation exceeds snow melt over many years (polar regions, high mountain environments)
• Temperature and precipitation play critical roles in glacier formation
  • Cold temperatures preserve snow and ice
  • High snowfall provides material for glacier growth
• Transformation of snow to glacier ice occurs through compaction and recrystallization (firnification)
  • Snow densifies under its own weight
  • Air pockets between snow crystals decrease
  • Ice crystals grow and interlock

Classification of Glacier Types

• Two main types of glaciers alpine (mountain) glaciers and continental ice sheets
• Alpine glaciers include:
  • Cirque glaciers (bowl-shaped depressions on mountainsides)
  • Valley glaciers (flow down valleys, often originating from cirques)
  • Piedmont glaciers (spread out at the base of mountains)
• Continental ice sheets cover vast areas (Greenland, Antarctica)
  • Further divided into ice caps and ice fields
  • Ice caps dome-shaped, covering less than 50,000 km²
  • Ice fields irregular topography, typically larger than ice caps

Thermal Regime Classification

• Temperate glaciers remain at pressure melting point throughout
  • Contain liquid water year-round
  • Highly responsive to climate changes
• Polythermal glaciers have both cold and temperate ice
  • Complex internal structure
  • Found in various climatic regions
• Cold-based glaciers remain below freezing throughout
  • Typically found in polar regions or very high altitudes
  • Limited basal sliding due to frozen bed

Glacial Accumulation and Ablation

Accumulation Processes

• Accumulation adds snow and ice to a glacier
  • Primary sources snowfall, avalanches, wind-blown snow deposition
• Accumulation zone located in upper part of glacier
  • Maintains positive mass balance
  • Snow transforms into ice through compaction and metamorphism
• Seasonal variations create annual layers within glacier
  • Used to determine glacier age and past climate conditions (ice cores)
• Accumulation rates vary depending on:
  • Elevation (higher elevations typically receive more snowfall)
  • Topography (windward slopes often accumulate more snow)
  • Latitude (polar regions have longer accumulation seasons)

Ablation Processes

• Ablation removes snow and ice from glacier
  • Melting (surface, internal, and basal)
  • Sublimation (direct transition from ice to water vapor)
  • Calving (ice breaking off into water bodies)
  • Wind erosion (particularly in arid, cold environments)
• Ablation zone located in lower part of glacier
  • Characterized by exposed ice and negative mass balance
  • Often features surface streams and moulins
• Ablation rates influenced by:
  • Air temperature
  • Solar radiation
  • Albedo (reflectivity) of glacier surface
  • Wind speed and humidity

Mass Balance and Equilibrium Line

• Equilibrium Line Altitude (ELA) separates accumulation and ablation zones
  • Represents point of zero net balance on glacier
  • ELA position fluctuates with climate changes
• Mass balance difference between accumulation and ablation over specific time period
  • Positive mass balance leads to glacier advance
  • Negative mass balance results in glacier retreat
• Mass balance measurements crucial for:
  • Assessing glacier health
  • Monitoring response to climate change
  • Predicting future glacier behavior and water resources

Glacier Movement Mechanisms

Internal Deformation

• Internal deformation (creep) results from weight of overlying ice
  • Causes plastic flow within glacier body
• Rate of internal deformation influenced by:
  • Ice thickness (greater thickness increases deformation)
  • Surface slope (steeper slopes accelerate flow)
  • Ice temperature (warmer ice deforms more easily)
• Glen's Flow Law describes relationship between stress and strain rate in glacier ice
  • $\dot{\epsilon} = A\tau^n$
  • $\dot{\epsilon}$ strain rate, $A$ flow parameter, $\tau$ shear stress, $n$ creep exponent
• Velocity profiles show faster movement at surface and center of glacier
  • Friction with bed and valley walls slows ice near boundaries

Basal Sliding

• Basal sliding occurs when meltwater at glacier bed reduces friction
  • Allows glacier to slide over bedrock
• Influenced by:
  • Presence and distribution of subglacial water
  • Bed roughness and lithology
  • Sediment characteristics at glacier base
• Weertman's theory of basal sliding combines:
  • Regelation (pressure melting and refreezing around obstacles)
  • Enhanced plastic flow (ice deformation around obstacles)
• Stick-slip behavior observed in some glaciers
  • Alternating periods of slow and rapid movement
  • Related to changes in subglacial water pressure

Glacier Surges and Velocity Variations

• Glacier surges periods of unusually rapid glacier movement
  • Often associated with changes in subglacial hydrological system
  • Can increase velocities by factors of 10-100
• Velocity of glacier movement varies spatially and temporally
  • Faster flow typically observed in center and surface of glacier
  • Seasonal variations related to meltwater availability
• Factors affecting glacier velocity:
  • Ice thickness and surface slope
  • Subglacial water pressure
  • Bed characteristics (hard vs. soft beds)
  • Thermal regime of glacier

Factors Influencing Glacier Dynamics

Climate and Mass Balance

• Mass balance primary driver of glacier dynamics
  • Represents difference between accumulation and ablation
  • Positive mass balance glacier advance
  • Negative mass balance glacier retreat
• Temperature affects glacier dynamics by:
  • Influencing melt rates
  • Altering ice viscosity
  • Affecting potential for basal sliding
• Precipitation patterns impact:
  • Accumulation rates
  • Distribution of snow and ice across glacier surface
• Climate change alters glacier dynamics through:
  • Shifts in temperature and precipitation patterns
  • Accelerated melting
  • Changes in glacier extent and volume

Topography and Glacier Geometry

• Topography plays crucial role in glacier dynamics:
  • Influences flow direction and velocity
  • Affects formation of crevasses and other glacial features
  • Determines glacier hypsometry (area-altitude distribution)
• Glacier geometry factors:
  • Ice thickness variations
  • Surface and bed slopes
  • Valley width and shape (U-shaped vs. V-shaped)
• Topographic controls on glacier response:
  • Overdeepened basins may delay glacier retreat
  • Steep slopes can accelerate ice loss during retreat

Feedback Mechanisms and System Interactions

• Albedo feedback:
  • Decreased snow cover exposes darker ice or rock
  • Lower albedo increases absorption of solar radiation
  • Accelerates melting and further reduces albedo
• Meltwater-dynamics feedback:
  • Increased meltwater production enhances basal sliding
  • Faster flow can lead to increased crevassing and ablation
• Debris cover effects:
  • Thin debris layer accelerates melting (lowered albedo)
  • Thick debris layer insulates ice and reduces melting
• Proglacial lake formation:
  • Can increase calving and accelerate glacier retreat
  • Alters local climate and affects glacier mass balance
• Isostatic rebound:
  • Uplift following deglaciation can affect regional climate
  • May influence glacier distribution and dynamics over long timescales