Lecture 6 - Thermodynamics and Englacial Hydrology

Thermal Regime and Englacial Hydrology

Review of Supraglacial Hydrology (Last Week)

• Supraglacial hydrology is the key link to englacial hydrology because water on the surface is needed to introduce water into the glacier.
• Features include supraglacial streams (rates of flow, meandering) and melt ponds (melt into the ice and store meltwater on a short timescale).
• Snowpack:
  • Unsaturated snow: vertical movement if isothermal (warm) or dry. Models account for irreducible water content, latent heat, and flow routing.
  • Saturated snow: water flows down slope, usually controlled by the snow/ice interface.
• Firn: Water is stored within firn, particularly on ice shelves (firn aquifers), impacting short and long-term water storage and the formation of ponds and streams.

Englacial Hydrology

• Occurs within the glacier ice.
• Features: fractures, moulins, crevasse traces, and englacial streams.
• Focus on englacial water storage on a scale of hours to months (less activity over winter).
• Two types of englacial hydrological systems:
  • Primary Permeability: Water flow between ice crystals on a granular scale (temperate ice).
    • Represents a store and a flow of water.
  • Secondary Permeability: Meltback channels, tubes, capillaries, moulins, and crevasses on a larger scale.

Thermal Properties of Ice

• Ice melts at $0^\circ C$ at atmospheric pressure.
• Under pressure, the melting point decreases (becomes colder).
• Greater depth in a glacier results in greater pressure, thus a lower melting point at the base.
• Pressure Melting Point: The melting point of ice under pressure.
  • Atmospheric pressure: $0^\circ C$
  • Beneath a kilometer of ice: $-0.7^\circ C$
• Impermeable Ice: Ice below the pressure melting point (e.g., $-2^\circ C$ to $-50^\circ C$), where no englacial hydrology occurs.
• Temperate/Warm Ice: Ice at the pressure melting point; any additional heat causes melting and meltwater formation, enabling hydrological features.

Types of Glaciers Based on Thermal Regime

• Temperate Glaciers: Entire ice body is at the pressure melting point (warm/temperate ice).
  • Water movement occurs through primary and secondary englacial permeability.
  • Routing from supraglacial to subglacial systems via crevasses and moulins.
• Cold Glaciers: Entire ice body is below the pressure melting point.
  • Ice is impermeable; no meltwater movement between ice crystals.
  • Frozen to the bed; no subglacial hydrological activity.
• Polythermal Glaciers: Contain both cold and temperate ice.
  • Various configurations (cold ice on the surface, warm ice at the bed, and vice versa).
  • Englacial and subglacial hydrology occur only where temperate ice is present.

Controls on Ice Temperature

• Surface Heat Sources
  • Surface energy balance (solar radiation, latent heat, conduction).
• Basal Heat Sources
  • Friction: Heat released due to glacier movement over the bed.
  • Geothermal Heat: Varies geographically; constant input or major events (e.g., volcanoes).
  • Latent Heat: Exchanges during freezing/melting processes.
• Air Temperature Variations: Penetrate only the top 10-15 meters of the glacier surface.
• Idealized Temperature Distribution:
  • Accumulation Area: Coldest temperatures at the surface (cold snow accumulation), geothermal heating at the bed.
  • Ablation Area: Surface melting removes cold snow layers; warmer temperatures due to movement to lower elevations.
• Advection:
  • Glacier ice doesn't flow as a uniform block; it follows flow lines.
  • Ice sourced from high elevations is transited to the base.
  • Frictional forces increase with movement, potentially overcoming temperature reversal.
• Equilibrium Line:
  • Cold ice at the surface from snow accumulation.
  • Colder ice deeper within the ice body due to advection from upstream.
  • Geothermal and frictional heating warm the base.

Idealized Glacier Thermal Regime

• Accumulation Area: Mostly cold ice with warming towards the bed.
  • Net freezing at the bed.
• Towards the Terminus: Increased friction leads to temperature inversion.
  • Cold ice is advected, and warmer ice is found at the bed.
  • Ice reaches or surpasses the pressure melting point, resulting in hydrology and melting.
• Terminus
  • Ice slows down, decreasing friction.
  • Potential switch back to net freezing at the bed.
  • Hydrology is expected where ice is at the pressure melting point, typically towards the terminus.

Primary Permeability

• Movement between ice crystals at a very small scale.
• Glacier ice formation involves compression and squeezing out of air particles, resulting in connections between ice crystals.
• Temperate glaciers allow permeability between ice crystals.
• Air passages might be sealed off, but melt water can exploit and connect voids between ice crystals.
• Meltwater can exploit passages between ice crystals and form capillaries.
• Movement can be slow with little meltwater or few airspaces, but increases with more meltwater, further exploiting passages and joining air parcels.

Secondary Permeability

• Water moving between ice crystal boundaries causes melt back and connects voids into capillaries.
• Fractures will form when increased pressure from meltwater pushes the ice apart.
• Must have voids or capillaries full of water for hydrostatic pressure against the ice.
• Fractures can be individual or in networks.

Channel Formation

• Water-filled Crevasses: Crevasses form due to glacier movement but don't penetrate to the bed.
  • If filled with meltwater and enough hydrostatic pressure, the base can break through, forming a moulin if constant meltwater flow is present.
• Superglacial Lakes: Large water bodies form on the surface, and pressure can exploit weaknesses in the ice, causing water drainage to the bed.
• In Situ Generation: Fracture networks filled with water force themselves open due to hydrostatic pressure and can form arborrescent networks.
• Fractures to Channels: Channels form from fracture networks due to meltwater flow. Meltwater is warmer than ice, and channels/fractures are full of water, keeping themselves open through melting.
• Downcutting of Supraglacial Streams: Streams cut themselves in over time with warmer water and frictional forces.
  • Leads to cut enclosure, where the ice deforms back over the surface and forms a lid over the channel.
• Moulin Networks: Crevasses are forced open by a large volume of meltwater.

Summary

• Channels have to be kept open through melting and a full water supply to counter pressure from ice.
  Crevasse fields can form on steep bedrock terrain, with streams intersecting and filling them.

Water Flow Dynamics

• Terrestrial Environments: Water flows downhill, and hydraulic potential is dependent on water density, gravity, and elevation.
• Englacial Environments: Ice overburden pressure affects water flow direction.
  • Flow is determined by elevation (downslope movement) and pressure (high to low pressure).
  • Hydraulic potential equation includes water density, gravity, elevation, ice density, gravity, and ice thickness.
  • $Hydraulic Potential = (Water Density * Gravity * Elevation Potential) + (Ice Density * Gravity *(Surface Elevation - Bed Elevation))$

Channel State Influence

• Full Channels: The equation accounts for ice overburden pressure.
  • When Channels are not in contact with the ice, the system is at atmospheric pressure, ice overburden pressure is removed from calculation.
• Channel Dynamics
  • Ice overburden pressure = water pressure: Channels stay open.
  • Ice overburden pressure > water pressure: Channels are squeezed shut.
  • Water pressure > ice overburden pressure: Channels grow.
• Glacier water follows the line of steepest hydraulic potential between elevation and pressure.
• Equipotential lines on the glacier show that steeper slopes for the glacier bed mean the water is transferred within the ice for a longer period of time.

Conduit Characteristics

• Arborescent Nature: Lower pressure leads to efficient melting of the channel wall, sustaining channel openness.
• Channels actively seek and join larger channels to minimize pressure and enhance efficiency.

Recap of Key Points

• Hydraulic Potential: Dependent on the pressure and the elevation.

• Cold Ice = no meltwater and no englacial hydrology. Supraglacial water is routed straight off the surface.

  • Temperate and Polythermal Glaciers -englacial/subglacial hydrology present. Primary and secondary permeability, combining features for larger channels, and hydrofracture routing to the bed.