Periglacial 1: landforms and processes Flashcards
What are periglacial environments?
Cold non-glacial environments
What are periglacial environments characterised by?
Continuous freezing and thawing of landscapes in conjunction with seasonal climate variations
What is permafrost?
any part of the ground that has been frozen for 2 consecutive years plus
What is not necessary for permafrost but is commonly found near it?
Common to have water/ice present but its not necessary for defining permafrost environments because they are defined as a thermal state and not a physical or material state
Describe the structure of permafrost areas from top to bottom
Active layer
Supra-permafrost talik
Intra-permafrost talik and closed talik
What is a supra-permafrost talik?
an unfrozen part of the ground in a permafrost environment that resides above or a the boundary between the permafrost layers and the active layers
What is an intra-permafrost and a closed talik?
Intra-permafrost talik is an unfrozen part of the ground well below the boundary between the active layer boundary. A closed talik is similar but is usually a lot smaller and so is also surrounded completely by permafrost
What are the 3 types of permafrost layer defined by coverage of permafrost?
Continuous = pervasive permafrost that comprises 90-100% area of the ground Discontinuous = Continuous permafrost that is broken up by taliks meaning that the ground is only made up of 90-50% permafrost. Sporadic = isolated blocks of permafrost that comprise 10-50% of the ground area
Outline the distribution of permafrost across the earth’s surface
Underlies 26% of the earth’s surface. It is commonly found in mountain and alpine environments
Describe the relationship between permafrost distribution and latitude
The permafrost layer increases with distance from the equator
Describe the thermal regime of the permafrost layer
The temperature increases with depth to a point and the starts to increase after an undefined point
Explain the permafrost thermal regime
The temperature at the surface is controlled by the sun’s radiation that obviously has a bigger impact at the surface/shallower depths but then the temperature at the bottom of the permafrost layer has a greater influence of the geothermal heat flux
What defines the boundaries of the permafrost layer?
Where the temperature starts to exceed 0 degrees celsius at both the surface and base. Anything in between is the permafrost layer
What is ground ice?
Ice formed in freezing or frozen ground
Where might ground ice be particularly prevalent?
In sporadic permafrost layers
What are the two types of ground-ice?
- Pore ice = forms within the pores of the ground and cements the ground matrix
- Segregated ice = bodies of pure ice that form by migration of liquid water in to freezing surfaces.
Where does segregated ground ice normally occur?
Usually occurs where unfrozen water exists nearby and permafrost aggrades
What are the different forms of ground ice, which is most common?
- Ice lenses (most common)
2. Massive ice beds
What are ice lenses?
Thin horizontal beds of ice
What are massive ice beds?
What the tin says - massive bodies of ice
What are the 3 ways that massive ice beds are formed?
- convergence of ice lenses in to one body
- water in a talik will be closed off and formed in to a massive body of ice under hydrostatic pressure
- Burial of glacier ice that survives underground
What are Pingos?
Conical domes of deformed ice
How are pingos formed?
- Lake underlain by talik
- Lake drains and exposes talik
- Permafrost layer accumulates on top of the new lake floor
- Hydrostatic pressure forces water to move upwards as the talik is forced to shrink
- Water in the talik has been forced up enough above the surface and to form a massive ice bed
What are collapsed pingos?
The ice cores within a pingo thaws and collapses leaving the bump to collapse and therefore leaving a circular rimmed crater behind
What are palsas?
Essentially pingos that form under peat environments. An ice lense does not experience thawing during summer because of peat layer above which acts as an insulator. The lense over successive seasons will grow and expand (heave)
What differentiates palsas from pingos?
They are usually not as amplified in shape and size.
Describe the shape and size of palsas
1-10m high, low relief, circular rim
What are ice wedges?
Downward-facing wedges in the permafrost layer of pure ice
How are ice wedges formed?
Thermal cracking of permafrost layer during winter allows meltwater to trickle down the cracks during the summer seasons. During the next winter season the meltwater will then freeze against the cold surrounding permafrost layer. Over time, the wedge will weaken the layer and consequently the process will repeat to increase the size of the crack and consequently the ice wedge.
What are ice wedges polygons and how are they formed?
There will be a collection of ground cracks arranged in a polygon shape across a landscape that will allow ice wedges below this polygon to form. The ice wedges will form around these polygon sides. The soil within the arrangement of the polygons on the surface will then slump down where it might then be infilled by a lake
What are ice wedge clasts?
The infilling of melted ice wedges with sediment after the ice wedges melt.
What are ice wedge clasts evidence of?
Where former annual mean air temperature was <6 degrees celsius as this is the level for continuous permafrost which would be ideal conditions for these ice wedges to form. Since then they have melted to allow the clasts to form
What are thermokarst landforms/landscapes?
These are landscapes where the ground has failed across a region underlain by ground ice that has since slumped and the ground that was underlain by ground ice has not slumped thereby revealing where the ground ice is
What has been happening in the Arctic related to thermokarst landscapes?
The arctic has been warming and so has made thermokarst landscapes more prevelant and visible
What are the two types of soil based on frost action?
- frost-susceptible = ice lenses formation is common through ice segregation
- non-frost-susceptible = pore ice is only frost that can form in these sol types
What determines whether the soil is frost-susceptible or not?
Size of voids in soil - function of grain size distribution. If silt content is 10% or more then this will facilitate capillary water flow between sand particles. This increased water flow will allow more water to be present and therefore more ice to exist in the layers
What is the silt content likely to be in frost-susceptible soils?
greater or equal to 10%
What is frost heave?
the upward and outward movement of the ground surface due to the freeze/thaw process of ice in the sol
What is a large influence upon frost heave?
Frost-susceptibility of the ground/soil
What does frost heaving permit?
down slope movement and formation of surface features and landforms (e.g. pingos and ice wedges)
What are the two types of frost weathering?
Microgelivation = small scale breakdown of sediment by freezing that occurs in pores and formation of ice lenses Macrogelivation = breakdown in to angular clasts through inter-joint freezing
What are two landforms that can occur as a result of frost-weathering?
Frost shattered bedrock = breakdown in to alligned sections of bedrock (looks like bread)
Blockfields = detatched rocks from block that were previously from the block as the block moves toward the surface.
How can sediment be sorted in to patterns as a result of frost heave?
Ground heave forces coarse sediment out the ground and allows fine material to simultaneously subside down in to the ground. The coarse sediment does not subside back down because it is too coarse as it rolls down the hills of the mounds produced in these patterns
What sort of sediment patterns can be produced?
circles, stripes, ovals, steps
What is solifluction and how does it occur
slow downslope movement of soil due to freeze thawing of water logged ground following heave and creep of the ground