Periglacial Processes and Landforms Flashcards
Why is permafrost important?
- Covers 50 percent of Canada
- Much development expected in area (mines, pipelines, shipping etc.)
- Thermokarst, huge geotechnical implications for infrastructure
- Climate change, erosion, etc.
- Ecosystem ‘dominos’
Ecosystem dominos
- Arctic ecosystems sensitive, fairly simple w/ few trophic levels
- Active layer of permafrost holds key to life, most readily disturbed layer
Geotechnical considerations
- Ice-rich pf highly sensitive to thermal disturbance
- Modern construction standard maintain thermal eq.
- Raise buildings on stilts, thick gravel bases, thermosiphons on pipelines
- Gravel to insulate roads
Thermosiphons
- On pipelines to maintain thermal eq. of pf
- Cold liquids circulate w/o a mechanical pump
Transportation considerations
- Highways are dark and readily thaw pf causing heaves (dark=absorb energy)
- Many communities, petroleum fields and mines only have winter ice road access = short transportation window
Winter and utilities
- Water supply and sewage treatment
- Surface water shallow/ easily contaminated
- Water below pf (100s m) difficult and expensive to find/move
- Sewage lagoons rare and waste often dumped into waterways
- Sewage difficult to treat due to long freezing winters
Alaska pipeline
- 1300km long, built 1975-1977
- Built on sliders in case ground moves, protect from pf
Climate change and pf
- pf contains massive volumes of frozen methane, a powerful gh gas
- pf thaws, releases methane, sets up positive feedback
Periglacial environment definitions
- Original: Climatic and geomorphic conditions of areas peripheral to the pleistocene ice sheets and glaciers
- Current: envrs in which frost action and permafrost-related processes dominate
Current definition of Periglacial envr
- Where frost action and permafrost-related processes dominate
- Wide range of cold envrs regardless of proximity to glacier
- High-latitude tundra envrs, and some below tree line
- High-altitude envrs and some coastal w/ cold ocean currents
What are the 2 diagnostic criteria of periglacial envrs?
- Freezing and thawing of ground
- Presence of perennially frozen ground
- 1 or both must be met
- Note: Periglacial does not require permafrost. pf may be too deep or used to exist or migrated away
Periglacial basic defn
- Cold landscapes dominated by frost action and/or permafrost processes
- Areas that are cold for long lengths of time
- May or may not be near glaciers
Proglacial basic defn
- Ice-marginal conditions
- Must be near glaciers
Paraglacial basic defn
- ‘Non-glacial processes directly conditioned by glaciation’
- Unstable envrs that persist after deglaciation
- Geomorphic processes w/ slow relation times
- Eustatic sea level is a force of this (glacial isostasy)
- Can include periglacial processes
What is permafrost
- Ground that remains below 0C for more than 2 years
- Continuous MAAT less than -6C, Discontinuous less than -3C (extensive, sporadic, isolated)
- Presence of ice is critical for geomorph development
Different types of pf
- Continental
- Alpine/montane
- Subsea
How much pf in Canada?
- 1/2 of Canada w/ 1/3 in the continuous zone (less than -6C MAAT)
- 22 percent of exposed landmass in N. hemisphere
How does subsea permafrost form?
- Glacier in the past that grounded into ocean
- or
- Lower sea level at end of ice-age, coastal plains w/ pf were above SL but are now below as SL rose, therefore now subsea
Controls on permafrost
- Mainly governed by air temp
- Others:
- Snow cover
- Vegetation
- Water
- Time (eg relict pf offshore)
Snow cover influence on pf
- Insulator and high albedo = less pf
- Dry northern prairies vs snowy quebec/ labrador
Vegetation influence on pf
- Insulator in summer, keeps cold = more pf
- But traps snow in winter, keeps warm = less pf
Water influence on pf
- Saturated ground much harder to freeze than dry
Mapping of in Canada
- Federal and territorial gov’s
- Industry: mining, forestry, power, pipelines, roads, buildings, infrastructure, research etc.
Pf and ground ice
- High ice content in Continuous pf
- Decrease in ice content from Extensive Discontinuous to Sporadic Discontinuous, to Isolated patches
Relict pf in NWT, Yukon, Alaska, and offshore
- Relict pf in widespread Beringia, including offshore: Mckenzie Delta, Banks Island, Alaska North Shore, Bering Strait, Siberia North Shore
- Implications for melting methane hydrates
Implication of melting relict pf
- Melt of methane hydrates
- Strong gh gas release, positive feedback
Active layer
- Top layer of ground that thaws and refreezes each year
- Thinnest in polar regions, thicker in South
- Frost Depth determined at end of summer
- Plants grow, water flows, etc.
Talik
- Zones of perenially unfrozen ground w/in pf
- Common in areas of discontinuous pf and often under deep lakes and rivers
Geothermal flux, Q
- Heat flows from centre of Earth towards surface
- Rate controlled by thermal conductivity, k
- Q = delta T (k)
Thermal conductivity, k
- Ability of a medium to conduct heat
Geothermal gradient
- Delta T = Change in temp/change in depth
- Input to geothermal flux eqn
- Changes through seasons
Base of pf
- When temp at depth = 0C
Determining pf thickness
- Determine geothermal flux (simple if k is known)
- Q = Delta T(k) = (Change in T/ Change in depth) x (k) = (T2/z2) x (T1/z1) x (k)
- Depth to permafrost base, z1 = T2/Q (k), where T2 = Temp at top of active layer, z2 is 0 depth at top of active layer
What happens to pf when air Temp warms?
- Geothermal gradient is approx. constant so warming air shifts gradient to the right and if all else is equal, base of al drops and base of pf rises
- Base of pf becomes shallower
- Active layer depth becomes thicker
- Essentially, pf thins from both sides
Stefan eqn
- Active layer thickness
- Ground surface temp during the thawing season, thermal conductivity, duration of thaw season and latent heat of fusion (normalized for soil density and moisture content)
- Zal = sq. root [2Tgkt(Qi)]
- Where Tg = ground surface temp, t = duration of thaw season
- Volumetric latent heat of fusion, Qi = Latent heat of ice x dry density of soil x (total moisture content x unfrozen water content)
Ground ice
- Ice that forms in freezing and frozen ground
- Not necessary condition of pf but will be there if water present, but in highly variable amounts
- ice exposures rare and short-lived
How many types of ice are recognized in Russia vs. N. America?
- Russians recognize 20 ice types (including buried glacial ice)
- NA only 8 types commonly recognized
Why is ground ice important?
- May constitute 40-60 percent by vol. of upper 10m of pf
- Most info comes from boreholes, mining, excavations
- pf thaw and associated melt of ground ice has terrain stability and geotechnical implications
Periglacial landforms and processes
- Pingos
- Ice wedges and polygons
- Patterned ground
- Rock glaciers
- Thermokarst
- Drunken forest
- Peat plateaux
- Thaw slumps
- Solifluction
- Frost heave
- Frost cracking
Pingos
- Ice-cored conical mounts
- Typical of tundra flats
- Up to 60m high, 300m wide, core of pure ice, dilation cracks common, may eventually rupture at top
- Mainly along Arctic coastal plain (Tuktoyuktuk as approx. 1350), relict pingos widespread (Including Saudi Arabia)
- Diagnostic of pf envr
- Two types, closed and open
Closed-system Pingo, Hydrostatic
- Form on flat terrain under Hydrostatic pressure, enhanced by cryostatic processes
- Areas of Continuous pf w/ impermeable layer at depth (e.g. closed talik)
- Usually form in drained thaw lakes/former stream beds
Open-system Pingo, Hydraulic
- Form under hydraulic/ artesian pressures
- Areas of discontinuous pf
- Typically in valleys at the base of slopes by artesian pressure
- Relatively common in Yukon (approx. 400), AK, Greenland,, due to high relief and coarse soils w/ high hydraulic conductivity
Pingo formation: confined freezing
- In saturated non-pf envrs
- Water freezes at top first, expands upwards (no resistance)
- Approx. 10cm of surface ice acts like a lid, forces freezing downwards
- results in downwards ‘pore water expulsion’
Pingo formation: confined freezing
- In pf areas
- Ice can’t expand downwards b/c there is a boundary, so water moves laterally
- Water in confined taliks (e.g. old lake bed) can’t escape laterally
- High cryostatic pressure develops
- Massive ice growth heaves surface seds upwards
Closed-system pingo formation
- Initiate in confined talks below lakes and streams
- If thaw lake drains, saturated seds exposed to atm, freezing progresses from all sides, talik gets smaller and pore pressure increases
- High cryostatic pressure forces pore water out from talik, expelled water freezes upwards into a massive ice lens
- Freezing expands upwards along the path of least resistance, seds heave upwards
Open-system pingo formation
- Result from gw flowing from outside source, such as upslope aquifer, driven by hydraulic pressure
- Pf in valley inhibits flow, ice lens forms, expands upwards towards lower surface pressure
- Open system pingos have no limitations on amount of water available, unless aquifer freezes
Pingos may show what seasonal effect?
- Annual growth layers from summer-winter layering
- Dilatent cracks
Ice wedges and ice-wedges polygons
- Most common feature of continuous pf terrain
- Useful for paleoenvironmental reconstruction
- Contraction cracks in-fill w/ water that freezes and subsequently re-cracks and the cycle continues
- In plan view, result is polygonal features w/ 4-7 sides
What is the most common feature of continuous pf terrain
- Ice wedges and ice-wedge polygons
How many sides do ice-wedge polygons tend to have?
4 - 7
High centre vs. low centre ice wedge polygons
- High centre have water on edges
- Low centre have water in centre (small lakes)
Ice wedge formation
- W yr 1: crack forms
- W-S yr1: Spring, snow melts, water infiltrates crack, freezes b/c crack is w/in pf
- S yr1: Active layer develops, surface above crack thaws
- yr2: winter, ice wedge cracks again, process repeats, Spring snowmelt fills crack and refreezes, Summer active layer development may drain off top water
Ice wedge development after n years
- yr n: many years, wedge develops
- Generally young ice in centre, older at edges
- Incorporates organic matter which can be dated (C-14)
- Often ridges of soil build up on rims as wedge expands, outward forming ‘low-centre’ ice wedge polygons
High centre ice wedge polygons form in?
- Coarse-grained soils where ice wedge tops are deep
- Peats where relatively thin organic cover above wedge provides less insulation and develops a deeper active layer
Ice-wedges as indicators of past climate
- Relicts preserved as casts or pseudomorphs
- Often in-filled w/ sand
- Ice wedge casts tend to fill from base up
- Arid, cold areas sand wedges may preferentially develop over ice wedges
Wedges in arid cold areas
- Sand wedges may show vertical structure like an ice wedge
- Both ice and sand wedges indicate pf but moisture contents differ
Remnant Laurentide ice sheet wedges
- Supra-glacial Melt-out till on top insulates ice below from thawing
- Vertical ice structures
- Sand wedges from cold-dry period w/ wind blown sand
- Current holocene ice wedges may from in upper till layer above Laurentide remnants
Thermokarst involutions
- Churning of upper layer
- Present seds mix w/ melt-out till
Other forms of patterned ground
- Sorted polygons
- Stone circles
- Stone stripes
- Mud boils
Sorted polygons and stone circles
- In active layer
- Centre of fine-grain material surrounded by circular coarse gravel border (1-5m deep)
- Reflect long term frost churning (100yr cycles) of fines
- Troughs at the course fine interface suggest down-ward motion at margins
- Coarse more susceptible to frost shattering and heave
Frost-churning
- Slow process
- Forms stone circles and sorted polygons
Rock glaciers
- Mix of rock and ice that slowly flow down slope
- Angular rock w/ interstitial ice
- Lobate or tongue-shaped w/ obvious evidence of motion
- Associated w/ alpine pf (debris prevents thaw)
What is the evidence for motion of rock glaciers?
- Flow lobes, evees, arcuate ridges, steep margins, encroaching on vegetation
What are the likely origins of rock glaciers?
- Initiated as debris-covered glaciers, often in cirque basins
- But true periglacial origin may occur where pore water gradually freezes w/ in debris-covered pf
Thermokarst
- Subsidence due to thawing of ice-rich pf
- Most common feature of pf terrain
How does thermokarst terrain arise?
- Human surface disturbance (roads, pipelines, sewage lagoons, buildings etc.)
- Forest fires
- Climate change (imp in arctic where T rapidly rising)
Thermokarst and forest fires
- Widespread and uncontrolled in N
- May result in sudden pf degradation in discontinuous zone
- Forest fires burn increase active layer thickness b/c of heat and removal of insulating organics and shade
- Leads to thermokarst, slumps, slides etc.
Drunken Forest
- Consequence of thermokarst
- Typical where active layer is moist, pf is ice-rich and discontinuous
- Common in black spruce forests (wet and cold)
- Trees don’t look straight and tall, ground has lakes and ponds
Peat Plateaux
- Flat-topped expanses of frozen peat
- Several km^2 extent, m’s thick
- Elevated above surrounding unfrozen areas
Why do peat plateaus form?
- Low thermal conductivity of peat promotes pf growth and inhibits summer melt
- Ice lenses elevate peat above water table, further lowers thermal conductivity (positive feedback)
- Change in veg from wetland sedges/willows to dry lichen/feathermoss/ sphagnum
Solifluction / gelifluction
- Slow gradual down-slope movement of saturated soil and rocks
- Can occur on shallow slopes (1-2 degrees)
- Produces lobes w/ steep leading edges 1-6m high (also sheets, benches and ‘streams’)
Solifluction
- Form of frost creep accelerated by presence of abundant water
Gelifluction
- Solifluction that occurs over pf
- Usually involves faster plug-like flow
Retrogressive thaw-flow slides
- Initiate along river/ocean banks where erosion exposes buried ground ice
- Melting saturates soil, begins flow, exposes more ice
- Results in steep arcuate headwall above low angle slope of thawed flowing debris
- Scarps may keep retreating until headwall runs out of ground ice or becomes debris covered
Most common form of mass movement and erosive rates in periglacial environments is?
- Retrogressive thaw-flow slides
Summary
- pf distribution governed primarily by air temp and latitude but local factors exert significant influence
- Seasonally thawed active layer above pf
- Ice-rich pf extremely susceptible to thaw
- Many landforms unique to pf environments
- Permafrost thaws, ground ice melts
