Periglacial processes Flashcards
What is a periglacial environment?
A landscape that undergoes seasonal freezing and thawing, typically on the fringes of past and present glaciated regions
How is a periglacial environment characterised by?
permafrost
What is the climate of a periglacial environment like?
- highest annual temperatures usually between 1-4 degrees
- low precipitation, less than 600mm per year
- below 0 degrees for at least 6-9 months of the year
- summer temperatures does not exceed 18 degrees
What is permafrost?
an area where soil and rock has not risen above 0 degrees for at least 2 consecutive years - PERMANENTLY FROZEN GROUND
How much of the Earth’s surface is currently experiencing permafrost conditions?
25%
What are the three different types of permafrost?
Continuous
Discontinuous
Sporadic
What is continuous permafrost?
Give an example
Large swathes of permafrost ground where the only unfrozen parts are under lakes, seas and rivers
Up to 1500m thick
e.g. Siberia
What is discontinuous permafrost?
Permafrost that is predominantly frozen but there is scattered areas of unfrozen ground (talik)
Mean temps between -5 and -1
What is sporadic permafrost?
- small areas of permafrost in mainly unfrozen ground (talik)
a few m thick
How fast does permafrost develop?
- only a few cm per year
- new layers take thousands of years to develop
What are the layers in permafrost?
Frozen layer - IMPERMEABLE
Active layer
Describe the distribution of permafrost
- Mostly northern hemisphere e.g. Alaska
- continuous permafrost mostly in the Arctic Circle and mountains
- submarine locations e.g. Beaufort Sea
What is the active layer?
the top layer of the soil that thaws in summer, making it saturated and mobile
between 2-5m deep
What is a thaw lake?
Give characteristics
A meltwater sat on the surface of the active layer because water cannot percolate through the impermeable permafrost layer beneath it, or when water fills depressions in thermokarst landscapes
common in poorly drained areas they absorb solar radiation and get bigger and deeper
less than 5m deep, less than 2km across
What is a felsenmeer/blockfield?
The angular rocks that litter the surface of the periglacial landscape that are quickly weathered by extensive freeze-thaw action. Subjected to intense cycles of FTA
means ‘field of rocks’ in German
What are the periglacial processes?
Solifluction
Frost thrust
Frost heave
Frost creep
Ground ice
What is solifluction?
where one section of soil moves faster than the surrounding section
What are the two types of solifluction?
Fast
Slow
What is fast solifluction?
- When the active layer gets heavily waterlogged due to melting, gravity begins to pull it downhill
- The flow downhill produces solifluction lobes on the surface.
- The steeper the slope, the faster the flow, the bigger the lobes
- movement can be up to 10cm/year
What is slow solifluction also known as?
frost creep
What is solifluction lobe?
a long, tongue shaped soil lobe on the side of a slope formed when the active layer of the permafrost moves over solid ground beneath by solifluction
What is frost heave?
the expansion of water in the soil as it turns to ice, forcing the soil to lift, arranging the sediment in the soil
How does frost heave work?
- Freezing occurs from the surface down, helping ice crystals to form in soil pores or as ice needles
- in winter, the surface layer lifts as the water in the soil freezes and expands. Stones are also pushed to the surface
- in summer, the surface layer drops but the stones remain at the surface s they are supported by finer sediment that has dropped to fill in the gaps
What is frost creep?
the slow process of the movement of individual stones downhill, a couple cm a year
How does frost creep work?
- Water in the soil expands as it freezes
- expansion causes soil particles to rise perpendicularly to the ground (frost heave)
- during thaw, particle is pulled downhill vertically due to gravity
(think of the hypotenuse of a right-angles triangle)
What is frost thrust?
When stones brought to the surface by frost heave move/roll down a slope
How does frost thrust work?
- Frost heave creates a dome
- Stones are brought to the surface by frost heave
- They roll down and collect at the bottom of the dome, potentially creating stone circles
What is talik?
The areas of unfrozen ground within permafrost
What are the three different types of talik?
Open
Through
Closed
What is open talik?
a small area of unfrozen ground exposed to the surface
What is a through talik?
a large area of unfrozen ground beneath a small open area
What is a closed talik?
unfrozen ground completely surrounded by permafrost
What is ground ice?
When ice forms underground either as:
PORE ICE:
- forming in the air spaces in the soil between soil and rock particles where water has frozen
NEEDLE ICE:
- thin slivers of ice up to several cm long
- is useful for loosening soil for processes such as frost creep
- forms in moist soil where temps drop below freezing at night
What are the differences in processes between glacial and periglacial areas?
ALTERED BY ICE
Periglacial: within the ground
Glacial: above the ground
LANDFORMS CREATED THROUGH:
Periglacial: FTA and mass movement
Glacial: Erosion and deposition
FOUND:
Periglacial: Outer margins of cold environments
Glacial: High altitudes and latitudes
What are the periglacial landforms?
Felsenmeer/blockfields
Ice wedges
Solifluction lobes/teracettes
Ice lenses
Pingos
Patterned ground
Thermokarsts
Scree slopes
Pro talus rampart
What is an ice wedge?
Give characteristics
a block/chunk of ice extending down towards the permafrost, formed by the seasonal melting and freezing of the active layer in continuous permafrost creating layers
1-2m wide
10m deep
100s years to form
What is an ice lens?
an underground block if ice formed when water in the soil pools and freezes, growing over time due to subsequent freezing and thawing of ice from other landforms e.g. ice wedges
As they increase in size, they cause frost heave, patterned ground, and pingos
What is a terracette?
frost creep but with soil - saturated soil rises perpendicularly when frozen, then drops vertically, making a terraced environment as it moves downslope
What is patterned ground?
when moisture in the soil accumulates in a localised zone and the repeated cycle of freezing, thawing and heaving sorts the stones into distinctive patterns found both inside and outside the permafrost zone
What processes cause patterned ground?
Frost heave
Frost creep
Frost thrust
FTA
Gelifluction
Nivation
How is patterned ground formed?
- Freezing sorts material from rock, then thawing distributes it into shapes such as polygons, circles and stripes
- Frost heave pushed pushes larger particles to the surface, and uplift moves them sideways
- Smaller particles removes by wind/meltwater leaving the large material lying on top of ice wedges, forming the shapes
- Sloping ground causes rocks to roll down forming stripes instead of shapes
What are thermokarsts?
a landscape of uneven topographic depressions and hummocks due to the thawing of ground ice
e.g. Lena and Aldan Rivers in central Yakutia, Russia
e.g. flat, lowland plains of the arctic
What are the characteristics of thermokarst conditions?
irregular hummocky ground interspersed with large, waterlogged hollows
What are the landforms found in thermokarst conditions?
Thaw lakes
Alases
Alas valleys
What are alases?
depressions that are flat floored and steep sided that develop due to the melting of ground ice (e.g. ice wedges) causing ground subsidence
they may contain lakes
they are basically a collection of thaw lakes
What are alas valleys?
when individual alases coalesce, may be 10s of km in depth
they are basically a combination of alases
What are thermokarst conditions impacted by?
Climatic warming
changes of surface insulation characteristics
e.g. more vegetation = more insulation = more melting
e.g. surface erosion reduces underling permafrost insulation
What is a pingo?
ice cored hills found in the Arctic and Sub-Arctic that reach up to 90m high and are covered on the outside by green vegetation
What are the two different types of pingo?
Open
Closed
What is an open pingo?
Give an example of where they are found
A pingo formed when water is forced up the gaps between the permafrost, forming a ice lens that is then pushed above ground
e.g. Greenland, in discontinuous permafrost
What is a closed pingo?
Give an example of where they are found
When a pingo is formed due to the permafrost encroachment on a frozen surface lake, forcing it above the ground
e.g. in the Mackensie Delta in Canada, continuous permafrost
What is a scree slope?
Give an example
When the angular rocks from a blockfield roll down a slope, or when freeze thaw action happens on a slope, and the rocks collect on the foot of the slope
e.g. Wasdale Screes, Scotland
What is a pro talus rampart?
Give an example
When rocks that have rolled down a slope collect away from the foot of the slope as snow/ice has collected at the foot of the slope, meaning that the rocks loosened by FTA will roll across the snow/ice
e.g. Baosbheinn, Highlands of Scotland
What are the associated landforms with water and wind in periglacial environments?
Dry Valley
Loess
What is a dry valley?
a valley that was once occupied by a meltwater stream but now is not
How does a dry valley form?
- SEASONAL:
- in summer, melting causes a meltwater stream
- in winter, the ground will freeze and there will be no meltwater - when the permafrost has all melted, there is no longer an impermeable layer beneath the active layer keeping the water from escaping
- water can infiltrate the ground
What is loess?
Give an example
fine sediment deposited by wind, with no stratifications
- silty/loamy
e.g. The Loess Plateau, North Central China
How does loess form?
glacial erosion of rocks creates fine sediment and dust. It is common in periglacial environments because they are dry and lack vegetation, and sediment is easily moved by winds
What are the main types of loess morphology?
Loess ridges
Loess hills
Loess platforms
Loess tableland
How fast can variations in glacial landforms/landscapes/processes happen?
Rapid
Seasonal
Millennial
Give some rapid variations in glacial landforms/landscapes/processes
- Rock falls and landslides change the profile of a glacial valley
- Volcanic activity causes large scale melting resulting in flooding and lahars
Give some seasonal variations in glacial landforms/landscapes/processes
- Increase in glacier discharge allows more transportation and erosion
- Meltwater streams increase in discharge and have more more erosional force
- Changing volume of meltwater into a pro glacial lake can affect the volume of sediment deposited and therefore the formation of varves
Give some millennial variations in glacial landforms/landscapes/processes
- Modification by moraine deposits by rivers or run off
- U shaped valleys altered by the formation of scree slopes
- Glacial lakes become infilled by sediment from run off
Since when have the glacial landforms been altered by physical processes?
The end of the Pleistocene (last ice age)
- thing are a lot different today than they were 10 thousand years ago
What landforms have undergone post glacial modification? (timescale - millenia)
GLACIAL TROUGH:
- scree slopes
- misfit streams
- alluvial fans
- asymmetric valleys
CORRIES:
- tarns
- arêtes?
How have glacial troughs undergone post glacial modification?
- Scree slopes made and accumulated over a long period of time due to FTA
- Ribbon lakes are long, winding lakes in the bottom of a valley. Misfit streams occur when melting of the glacier leaves a wide valley with a small stream running in my bottom. Moraine gets reworked, deposits are made by the sides, and sediment erodes the valley so they meanders change course, and still doesn’t fit the valley bottom. Lakes may get separated e.g. Brothers Water and Ullswater, LDNP
- Alluvial fans form when the river leaves a small channel into a large open space and carrying capacity is reduced, depositing lots of sediment into the previously glaciated areas
- Asymmetric valleys (valleys with a decrease in a slope angle) form when:
- increased heat/sunlight on one side of the valley
- increased melting
- increased melt water
- increased solifluction
- decreased angle of the slope
- increased slumping
How do corries undergo post glacial modification?
- Tarns form as the ice melts and corrie fills with water. There is then an infilling of sediment as there is increased overland flow, scree, and input from rivers
- Aretes are formed when the glacier melts, then they get eroded and weathered etc, FTA breaks them up, walkers erode them, making them even more jagged
