Permafrost and Preglacial Environments Flashcards
Define preglacial environment
- Those that are cold but not glacial
- Characterised by seasonal/perennial frozen ground conditions
Define Freeze-thaw
- Processes drive geomorphic change on land surface
- Driven by seasonal or dismal temperatures varying around 0 degrees
Define permafrost
- Soil or bedrock that is perennially frozen over long timescales (over 2 years and to considerable depth)
- Characterised by significant accumulation of frozen water in soil and sediment
How much of earths surface is affected by permafrost and freeze thaw processes?
- Permafrost affects 26%
- Freeze thaw processes affect 36%
Where is temperature lowest?
- Temp is lowest close to the surface and increases with depth
How does temp control permafrost thickness?
- Mean annual surface temp, ground conductivity and geothermal heat flux
- Exceptions occur under thermal disturbances – e.g. water bodies and climate change response time
How does surface albedo play a role in temp?
Dark vegetation absorbs more short-wave solar radiation and thus increases temp
Why is frozen ground significant?
- Drives changes in mechanical and hydrological properties of ground
- Highly susceptible to changes in climate surface conditions - most permafrost exist at temps below 0
- Permafrost stores GHGs - as it defrosts, the gases are being released
- Melting permafrost releases large quantities of fresh water into rivers and oceans - increasing sea level rise
Outline polar permafrost
- Polar permafrost - occurs in high latitudes with cold temp
- 82% of Alaska, 50% of Canada
Outline Alpine permafrost
- Occurs due to cold temp in high altitude areas
- Mainly on Tibetan Plateau, European Alps and Rockies
Outline subsea permafrost
- Its extent is not well known
- Either low temp of subsea or remnant of past colder conditions
Define isotherm
A line on a map connecting points having the same temperature at a given time or on average over a given period
Define sporadic permafrost
Patches of permafrost existing below 1 degree isotherm
Define taliks
Unfrozen regions within or below permafrost
Describe hydrology in permafrost regions
- Groundwater flow restricted by frozen ground
- Infiltration is restricted
- Rapid run off because permafrost retards percolation
Define gelifraction
The mechanical breakup and churning of rock or soil due to repeated freezing and thawing of water within its cracks
How is segregated ice formed?
Migration of water to freezing front by suction causes formation of segregated ice, allowing further growth of ground ice
What are involutions?
- Forms when ice-rich ground thaws
- Disruptions of sedimentary structure
- Often used as diagnostic for past permafrost conditions
- Permafrost/preglacial ground ice features
What are ice wedges?
- V-shaped bodies of ground ice formed from cracking and expansion during freeze-thaw cycles
- Permafrost/preglacial ground ice features
What is an alas?
- Small irregular shaped thaw lakes and depressions
- Caused by the melting of massive ground ice
- Permafrost/preglacial ground ice features
What are ice wedge polygons
- Form when ice wedges join up
- Currently, millions of square km of earth’s surface
- Permafrost/preglacial ground ice features
What are pingos?
- Ice cored mounds up to 50m high and 500m in length
- Top of mound often becomes cracked as ping grows
- Two types
1) Hydrostatic (closed system)
2) Hydraulic (open system) - Ground ice feature
What is patterned ground?
- Characteristic feature of both permafrost and periglacial environments;
- Cryoturbation processes sort active-laer sediments by texture
- Precise mechanisms poorly known, but thought to be some form of convectional rotation
- Sorted circles and polygons on flat land
- Stone garlands and stripes on sloping ground
What are the 4 key processes of mass wasting in preglacial environments?
1) slope fracture
2) gelifraction
3) creep and flow processes
4) nivation
- Range of landforms result
- Pronival ramparts, ploughing boulders and rock glaciers
