L4: Ground Freezing and Ground Ice Flashcards
What are the micro-scale factors affecting permafrost distribution?
Soil and rock
Moisture and water
Relief and aspect
Surface terrain type (vegetation vs snow)
How does the material affect permafrost distribution?
Colour changes albedo variation and amount of incoming radiation reflected (10% to 30%).
Specific heat (how much heat is required to increase a certain volume by a certain amount) and thermal conductivity (how quickly heat will move through something).
Infiltration and evaporation rates (moisture affects thermal properties).
- How wet or dry a material is may affect albedo (wet = dark)
- Determines how much water can get in via infiltration and evaporation
- Water = heat
How does moisture and water affect permafrost distribution?
Affect specific heat, heat flux, rate of freeze, depth of thaw, basal thaw etc.
Running water gives thermal erosion (constant replenishing warm relative to frozen permafrost).
Standing water insulates against low air temperatures, and is a heat store (heat isn’t being replenished but will still remain warm, 2-3m deep won’t freeze to bottom, water is densest at 4oC) (Permafrost shadow- talik under the water)
Water body insulates ground beneath its centre to a depth equal to its diameter (threshold for talik underneath) (the bigger the lake the deeper the permafrost shadow)
If diameter>pf depth then no permafrost beneath water
How does relief and aspect affect permafrost distribution?
Aspect affects input of radiation.
- In northern hemisphere if you’re in a south facing = as much as 4x more incoming radiation.
Aspect affects input of precipitation and snow drifting (wind bearing slopes)
Altitude affects ground temperature
Slope angle controls snow and runoff (steep slope = less now and rapid runoff) (gradual slope = more snow and more infiltration)
Since permafrost reacts to small imbalances, relief has strong affect (including asymmetry)
Permafrost top/base parallel ground surface
How does vegetation as a surface type affect permafrost distribution?
Variety of vegetation (lots of colours, more/less moisture, organic material etc)
Vegetation acts as vital insulator (trap air and the thermal conductivity of air is low) (protect the ground from cold air temperatures and prevent the loss of heat outwards).
Vegetation influences infiltration, snow retention, and evapotranspiration
Vegetation, micro-relief and permafrost interact strongly but subtly
How does snow as a surface type affect permafrost distribution?
Snow insulates ground from low temperatures, and adds moisture
Don’t need a lot of snow to make a significant impact- Permafrost does not grow if snow>40cm
1.50m snow provides total insulation
Glaciers thus insulate underlying ground
A temperate (“warm”) glacier base is at pressure melting point
Polar (“cold”) glaciers are just below 0oC at base
What are temperate (“warm”) glaciers?
At pressure melting point
What are polar (“cold”) glaciers?
Just below 0oC at base
What is an open talik?
An area of unfrozen ground that is open to the ground surface but otherwise enclosed in permafrost.
What is a through talik?
Through talik is unfrozen ground that is exposed to the ground surface and to a larger mass of unfrozen ground beneath it.
What is a closed talik?
Unfrozen ground encased in permafrost.
Why is it, in continuous permafrost areas, taliks are found under lakes?
Because of the ability of water to store and vertically transfer heat energy.
Deep continuous permafrost with no talik =
Very stable, non-dynamic
Closed taliks can develop when…
lakes fill in with sediment and become bogs.
They can also form because of groundwater flow.
What is zero annual amplitude?
Point below which temperature does not change from year to year- deeper than the active layer.
The depth of material that is going to be impacted is above this.
Define Cryotic
Subjected to temperatures below 0oC, but not necessarily frozen
What is the autumn freeze?
Soil dry and runoff minimum at end of Summer (dry as it’s ever going to get) (some areas may be wet some may be dry = freeze differently)
Freezing commences at surface and spreads downwards from surface as heat lost
Base of surface freezing zone is the frost table- 2 sides freezing (bottom up and top down- whole active layer freezes and two may meet) (material squeezed between two freezing fronts can become mobile under pressure)
As Active Layer heat balance becomes negative, permafrost table rises
What is the zero curtain effect?
???
When ice freezes below 0oC
As temperatures go up you use latent heat and as you come down you release latent heat.
May be due to release or absorption of latent heat in phase change from water to ice +/or dissolved minerals in soil moisture.
Thawing is much quicker than freeze back- penetration of heat is fast
Zero Curtain Implications
The zero curtain delay to freezing suggests a“transitional freezing zone” or “artificially unfrozen zone”.
With good moisture supply the zone between frost table and permafrost table may remain unfrozen for months.
Since fine materials enhance moisture content, which in turn encourages zero curtain effect, clay may be used to manage resistance to freezing
What dominates unfrozen content?
Grain size
Unfrozen content dominates engineering strength
Spring Thaw
Thaw penetrates progressively down from surface
Thaw penetration much faster than freeze up
Excess water released to standing water or runoff
Complete thaw often takes place within 4-6 weeks
Types of ground ice
Discontinuous ground ice
Compact ground ice
Segregated ground ice
Soil ice
Vein ice- occurs where you get thermal contraction cracking- forms ice wedge polygons
Intrusive ice- where the water is under pressure that freezes (e.g hydrostatic pressure)
Buried ice- has become covered by something (e.g mass movement) and this will preserve it
Types of soil ice
Pore ice- in place freezing- water within pores that freezes.
Segregated ice- most important in terms of processes- movement within the system and the potential for more ice to grow.
Needle ice- crystals of ice parallel to heat flow- microprocesses.
What is Discontinuous ground ice?
Stick the system together, where you have got bits of rock you can get rock that sticks like glue (greater cohesion) (interstitial ice)
- Very coarse material is free- draining and loses most of its water
- Even complete freezing fails to fill the pore space
- Freezing expansion does little to disrupt the soil structure
- Increasing ice content generally leads to greater cohesion
What is Compact ground ice?
Medium pore size material, and saturated macro-pore size (no migration of water, water in the pores freezes and can expand).
As pore spaces are full, freezing phase change produces expansion (around 9% is free to expand)
Sediment swells in winter and settles in summer
Freezing increases sediment cohesion in winter, but reduces it in summer
What is Segregated ground ice?
Complex process but with three main controls:
Rate of freeze (i.e. heat flux through soil) (really quick = pore ice, no time for water to move through system)
Moisture supply within soil (indicated by pore water pressure- surface tension within the pores that allows water to move through)
Particle size (influencing pore size and surface tension) (too small = pore water pressure too high) (too big = not enough surface tension to hold the water)
Ground ice segregation
Repeated over many years, this process can produce major ice forms
Slow freezing creates largest segregations
Varied terrain or material creates differential segregation, which may create landforms
The role of pore water pressure
Need just the right amount (e.g silt)
Grain size influences pore water pressure
Fine materials (silt) give highest pore pressures
With high pore pressure water migration can continue and frost table remains static
Lower pressure inhibits water migration and frost table falls
Water to ice =
Release of latent heat (no bonds need to be broken)
Ice to water =
Take up of latent heat (needs to break bonds in solid ice)
Why is latent heat important in growing segregation ice?
Air temperature drops below freezing = net loss of heat from soil
Water migrates to the frost table and freezes there.
Latent heat of fusion released as water freezes.
Freezing front doesn’t penetrate any further = migration of water for ice lenses.
Cold beats hot eventually
Ice lenses are perpendicular to…
Heat flux
Usually 1-3cm (but up to 40cm)
Usually single diurnal cycle
Significant contribution to slope sediment transport
