L4: Ground Freezing and Ground Ice

Question 1

What are the micro-scale factors affecting permafrost distribution?

Answer 1

Soil and rock

Moisture and water

Relief and aspect

Surface terrain type (vegetation vs snow)

Question 2

How does the material affect permafrost distribution?

Answer 2

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

Question 3

How does moisture and water affect permafrost distribution?

Answer 3

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

Question 4

How does relief and aspect affect permafrost distribution?

Answer 4

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

Question 5

How does vegetation as a surface type affect permafrost distribution?

Answer 5

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

Question 6

How does snow as a surface type affect permafrost distribution?

Answer 6

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

Question 7

What are temperate (“warm”) glaciers?

Answer 7

At pressure melting point

Question 8

What are polar (“cold”) glaciers?

Answer 8

Just below 0oC at base

Question 9

What is an open talik?

Answer 9

An area of unfrozen ground that is open to the ground surface but otherwise enclosed in permafrost.

Question 10

What is a through talik?

Answer 10

Through talik is unfrozen ground that is exposed to the ground surface and to a larger mass of unfrozen ground beneath it.

Question 11

What is a closed talik?

Answer 11

Unfrozen ground encased in permafrost.

Question 12

Why is it, in continuous permafrost areas, taliks are found under lakes?

Answer 12

Because of the ability of water to store and vertically transfer heat energy.

Question 13

Deep continuous permafrost with no talik =

Answer 13

Very stable, non-dynamic

Question 14

Closed taliks can develop when…

Answer 14

lakes fill in with sediment and become bogs.

They can also form because of groundwater flow.

Question 15

What is zero annual amplitude?

Answer 15

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.

Question 16

Define Cryotic

Answer 16

Subjected to temperatures below 0oC, but not necessarily frozen

Question 17

What is the autumn freeze?

Answer 17

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

Question 18

What is the zero curtain effect?

???

Answer 18

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

Question 19

Zero Curtain Implications

Answer 19

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

Question 20

What dominates unfrozen content?

Answer 20

Grain size

Unfrozen content dominates engineering strength

Question 21

Spring Thaw

Answer 21

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

Question 22

Types of ground ice

Answer 22

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

Question 23

Types of soil ice

Answer 23

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.

Question 24

What is Discontinuous ground ice?

Answer 24

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

Question 25

What is Compact ground ice?

Answer 25

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

Question 26

What is Segregated ground ice?

Complex process but with three main controls:

Answer 26

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)

Question 27

Ground ice segregation

Answer 27

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

Question 28

The role of pore water pressure

Answer 28

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

Question 29

Water to ice =

Answer 29

Release of latent heat (no bonds need to be broken)

Question 30

Ice to water =

Answer 30

Take up of latent heat (needs to break bonds in solid ice)

Question 31

Why is latent heat important in growing segregation ice?

Answer 31

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

Question 32

Ice lenses are perpendicular to…

Answer 32

Heat flux
Usually 1-3cm (but up to 40cm)
Usually single diurnal cycle
Significant contribution to slope sediment transport