3. Periglacial landscapes Flashcards
What is a periglacial landscape?
A landscape that is covered in permafrost that has seasonal temperature variations, where freeze-thaw cycles dominate geomorphic processes
What is permafrost?
Perennially frozen ground overlain by an active layer
What is an active layer?
The near surface in a periglacial environment which seasonally freezes and thaws
Where can periglacial environments be found?
High latitude areas eg Alaska
Continental interiors eg Siberia
Low latitudes with high altitudes eg Andes
What % of Earth’s land surface is covered in periglacial landscapes?
25% (estimated another 25% has also experienced previous periglacial conditions)
What geomorphic processes are important in periglacial landscapes?
Frost heave
Freeze thaw
Ground ice formation
Describe freeze thaw:
Cause by seasonal fluctuations in temperature
The mechanical weathering process caused by water in cracks in rocks to freeze and expand, resulting in rocks breaking into smaller particles
Describe frost heave:
The downslope displacement of soil particles that results from cycles of freeze-thaw
- Stones w finer material heat/cool down quickest as have lowest specific heat capacity
- As temperatures fall water beneath the stones freeze and expand, pushing stones upwards to the surface
- Ground ice can also push fine material upwards producing a domed surface
Describe the formation of ground ice:
During summer melting, water percolate into sub-surface geology where it accumulates below the water table
During winter thus freezes and expands between 9-10%, forcing ground surface to be pushed upwards
Where can periglacial landforms be found?
They can be a feature of current periglacial periods or fossil features in more temperate regions eg UK
What landforms are associated with periglacial environments?
Patterned ground
Pingos
Blockfields
What is patterned ground + example?
The collective term for a number of small-scale features of periglacial environment
Eg Barrow, Alaska
How is pattern ground formed?
Frost heave forces largest stones towards surface which the move radially under gravity, down the domed surface to form a network of stone polygons 1-2 metres in diameter
What is a stone garland?
Elongated polygons that form on a slope
What is a stone stripe?
Occurs on slopes of over 60 degrees where stone polygons lose they shape
What are pingos?
Rounded ice cored hills that can be up to 90m in height and 800m in diameter
What are the two types of pingo?
Open system
Closed system
Describe open system pingos:
Common in east Greenland, found in areas of discontinuous permafrost
Develop in valley bottoms
Growth of the ice core is hydraulic
Describe how open system pingos are formed:
Water seeps from higher-surrounding areas to low-lying areas by gravity (forms a talik)
Talik freezes and expands, pushing overlying ground surface upwards
Describe closed system pingos:
Found in the MacKenzie Delta, Canada, found in areas of continuous permafrost
Develop beneath lake beds where supply of water is from immediate local area
Growth of the ice core is hydrostatic
Describe how closed system pingos form:
Groundwater beneath the lake is trapped between the permafrost below and the frozen lake above, causing it to be insulated and thaw = melted
The saturated talk is compressed by the expanding ice and is under hydrostatic pressure
When lake eventually drains, talik is no longer insulated so freezes and expands, pushing ground surface upwards
What are blockfields?
Large amounts of freeze-thaw action results in large amounts of scree - in flat areas expansive angular boulders form blockfields
What is solifluction?
In summer, large areas of meltwater in upper layers of permafrost cannot be drained away which causes lubrication of the soil and causes ground movement over slopes
What has happened to periglacial landforms as temperatures have risen?
Patterned ground has been colonised by vegetation eg Leedon Tor in Dartmoor is covered in grasses
Pingos collapse when ice cores thaw leaving a circular depression = ognip Eg Llanberis, North Wales
