Periglacial geomorphology Flashcards
Periglacial
Result from seasonal thawing of snow in areas of permafrost, the runoff from which refreezes in ice wedges and other structures.
Key elements
- Permafrost
- Dominant process: frost action
- Creation of very distinctive features
defining periglacial
- those in which frost action and permafrost related processes dominate
- located in extra-glacial area at high latitudes and/or altitudes
- periglacial landscapes are usually considered synonymous with permafrost (Frozen ground)
- many are not directly associated with Quaternary or present day ice sheets
processes
- growth/melt of ground ice + temperature induced ground deformation = periglacial disturbance and dives the preglaical debris system
- weathering (mechanical, chemical, biological)
- slope processes (solifluction/genlifluction)
- fluvial action (Arctic nival flood)
- aeolian action (strong winds, low rainfall, little vegetation
extent of perglaciation
- periglacial is defined by a set of processes, not by area or climatic type
- currently 25% of the land surface
- 82% of Alaska and 50% of Canada
- in glacial periods of Quaternary an additional 20-25% of land area was affected
Permafrost
- perennially frozen ground
- one that survives for >2 years
- not permanently because climate changes and the depth and spatial extent of permafrost changes with it
- ground that has remained at or below 0oC for 2 or more consecutive years
arctic permafrost distribution
- effects extend up to thickness of 1000m
- can be; isolated, sporadic, discontinuous, continuous
types of permafrost
Isolated - 0-10% of affected area Sporadic - 10-50% of affected area Discontinuous - 50-90% of affected area Continuous - 90-100% of affected area
Permafrost structure
- surface layers subject to annual freezing and thawing, as seasons progress
- active layer (most importation where processes take place = landforms)
- active layer varies in thickness with climate
- dynamic layer, usually has high water content; leads to freeze that cycles and results in formation of landforms.
Blockfields
- boulder cover
- no soil/vegetation cove visible
- development requires 2 basic processes
- detachment of the block from bedrock
- movement of the block towards the surface
- lower limit linked to upper limit of glacial ice cover (ice protects/erodes: frost weathering/heave above ice)
frost weathering of rock
- breakdown of rock through repeated freezing and thawing
- water in rocks expands by 9% on freezing
- 2 effects: Macrogelivation & Microgelivation
- landforms: frost shattered bedrock, blockfield and other forms of frost weathered detritus
Microgelivation:
small-scale breakdown of rock into particles (Silt, sand) by freezing of water in pores and by formation of ice lenses
- often causes rounding of rock surfaces by granular disaggregation
Macrogelivation
breakdown of rock into angular clasts by water freezing in joints
Upland feature: Patterned ground
- sorted circles or unsorted hummocks, stropes
- common where boulders cover mountain top
- 1-15m in diameter
- vegetated cell
- forming by mass displacement of soil fines (freeze thaw)
- class occupy troughs
freezing model
during freezing, frost penetration is uneven. clasts are heaved upwards to the surface and outwards towards cell margins
Thaw model
during thaw an unstable density configuration set convection cells in the soil
soil convection
warmed (more dense) surface waters descends - melts frozen beneath - undulating lower surface projected to surface
- sorting occurs if soil particles move with water during convection
patterned ground
- some forms grade into others
- as gradient increases, nets becomes elongated forming ovals then stripes
solifluction
slow downslope movement of soil due in cold region to freezing and thawing
frost heave
- vertical rise in position of material ins oil due to volume expansion that accompanies freezing of pore water
- results in net downslope movement because lifting by heave occurs perpendicular to the slope which back-dropping after thawing occur vertically
Gelifluction
- seasonal thaw of the active layer saturates surficial soil as the water connot percolate into frozen soil below
- can result in flow of active layer on slopes as gentle as 2 degrees and results in terrace like lobes
ice wedge polygons
- intense cold = thermal contraction/cracking
- networks of vertical ice wedges shape the polygons which occur below the active seasonal freeze thaw layer
- visible surface formation is a result of the soil slumping above the ice wedges
- may be a few meters to over 100 meters in diameter
- low centered if ice wedge is growing
- high centered if thawing is more prevalent causing stream channels along ice-wedges
Lowland feature: Pingos
- dome shaped hill (when active)
- sub-surface water freezes and expands = dome
- up to 600 m diameter and 50 m high
- as growth progresses, cracks appear in surface
- ice core becomes exposed - melting
- material slumps to edges
- forms ramparts (ridges) with central depression
Pingo formation: hysrostatic
- closed system
- lake underlain by unfrozen sediment in permafrost
- lake darins = exposed to cold temps = permafrost aggrades
- talik freezes as permafrost encroaches forming a core of ice
- core deforms overlying soil = pingo
- found in area of continuous permafrost (Siberia and Canada)
Pingo formation: hydraulic
- open system
- common in area of discontinuous permafrost (Alaska, Svalbard, Greenland)
- requires flowing water beneath permafrost
- flows from high to low pressure
- as freezes forms dome
Thermokarst
- characteristic landforms from thawing of ice-rich permafrost or the melting of massive ground ice blocks
- it is characterised by an irregular topography, with irregular pits and depressions develop by thaw settlement
- perhaps indicative of warming climate
- thaw lakes can appear aligned due to wind/water erosion
Ice Wedge
Active: - modern high-latitude, periglacial areas - mean annual temp below -8 to-6 Fossil: - modern mid-latitude maritime areas
Frost action
- granular disintegration and flaking of rock production of 6 tonnes of sand accumulate in one valley in one winter
- frost sorted patterned ground, regular surface patterns of fine and course debris only on bare ground
- solifluction, downslope movement of debris (under seasonal freezing/frost creep) abover 550m in UK
Aeolian activity
- removal of material from deflation surfaces
- Arkle and An Teallach
- debate: activation by either little ice age or over grazing by sheep
Dominant modern day features
- rock slope failures, mainly paraglacial
- talus slope accumulation
- debris flows
- solifluction deposits
- coversand deposits
- ploughing boulders, most widspread indication of current perglacial activity
Primary concern: release of methane gas
- large uncertainties in the budget of atmospheric methane limit the accuracy of climate change projections
- contributors; industry agriculture wetlands, ruminants, fire, ocean
- climate effects; absorption of infrared radiation, atmospheric warming, increase in methane output = greater effect hat equivalent increase in CO2
Primary concern: release of CO2
- frozen land: anoxic, preservation of organic material
- degradation: oxygenation and decomposition of organic matter
- contributors: industry agriculture wetlands, ruminants, fire, ocean
- climate effects: greenhouse effect
components of arctic permafrost regions
- permafrost degradation and erosion enhance methane release
- extreme scenarios suggest all may have thawed by 2100
climatic change in the arctic
- predicted actic temp 4-8oC
- annual precip increase of 20%
- active layer of permafrost will deepen over broad areas
- roads and pipline disrupted, populations of animal fluctuate, hunting season shorted for native people
- boreal forest will migrate poleward and productivity will decline form drought, insects, disease, fire etc.
positive consequences of future climate warming in cold regions
- Expanded trade routes
- Increased economic potential – oil and gas reserves
- More land suitable for cultivation/farming – longer growing season
- Deferring of next ice age (is this a positive?)
- Fewer deaths/injuries from cold related illnesses
- Increased travel/tourism to previously inaccessible areas
Negative consequences of future climate warming in cold regions
- Extinction of flora and fauna – loss of biodiversity
- Sea level rise - flooding
- Political tensions e.g. competition for oil and gas reserves
- Economic impact: e.g. skiing resorts
- Loss of resources: e.g. fresh water
- Destruction of environmental archives & infrastructure: e.g. permafrost/ice cores
- Instability in the landscape: e.g. rivers and volcanoes
