Periglacial Processes and Landforms Flashcards

1
Q

Why is permafrost important?

A
  • 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’
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2
Q

Ecosystem dominos

A
  • Arctic ecosystems sensitive, fairly simple w/ few trophic levels
  • Active layer of permafrost holds key to life, most readily disturbed layer
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3
Q

Geotechnical considerations

A
  • 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
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4
Q

Thermosiphons

A
  • On pipelines to maintain thermal eq. of pf

- Cold liquids circulate w/o a mechanical pump

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5
Q

Transportation considerations

A
  • 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
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6
Q

Winter and utilities

A
  • 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
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7
Q

Alaska pipeline

A
  • 1300km long, built 1975-1977

- Built on sliders in case ground moves, protect from pf

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8
Q

Climate change and pf

A
  • pf contains massive volumes of frozen methane, a powerful gh gas
  • pf thaws, releases methane, sets up positive feedback
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9
Q

Periglacial environment definitions

A
  • 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
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10
Q

Current definition of Periglacial envr

A
  • 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
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11
Q

What are the 2 diagnostic criteria of periglacial envrs?

A
  • 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
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12
Q

Periglacial basic defn

A
  • 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
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13
Q

Proglacial basic defn

A
  • Ice-marginal conditions

- Must be near glaciers

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14
Q

Paraglacial basic defn

A
  • ‘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
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15
Q

What is permafrost

A
  • 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
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16
Q

Different types of pf

A
  • Continental
  • Alpine/montane
  • Subsea
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17
Q

How much pf in Canada?

A
  • 1/2 of Canada w/ 1/3 in the continuous zone (less than -6C MAAT)
  • 22 percent of exposed landmass in N. hemisphere
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18
Q

How does subsea permafrost form?

A
  • 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
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19
Q

Controls on permafrost

A
  • Mainly governed by air temp
  • Others:
  • Snow cover
  • Vegetation
  • Water
  • Time (eg relict pf offshore)
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20
Q

Snow cover influence on pf

A
  • Insulator and high albedo = less pf

- Dry northern prairies vs snowy quebec/ labrador

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21
Q

Vegetation influence on pf

A
  • Insulator in summer, keeps cold = more pf

- But traps snow in winter, keeps warm = less pf

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22
Q

Water influence on pf

A
  • Saturated ground much harder to freeze than dry
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23
Q

Mapping of in Canada

A
  • Federal and territorial gov’s

- Industry: mining, forestry, power, pipelines, roads, buildings, infrastructure, research etc.

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24
Q

Pf and ground ice

A
  • High ice content in Continuous pf

- Decrease in ice content from Extensive Discontinuous to Sporadic Discontinuous, to Isolated patches

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25
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
26
Implication of melting relict pf
- Melt of methane hydrates | - Strong gh gas release, positive feedback
27
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.
28
Talik
- Zones of perenially unfrozen ground w/in pf | - Common in areas of discontinuous pf and often under deep lakes and rivers
29
Geothermal flux, Q
- Heat flows from centre of Earth towards surface - Rate controlled by thermal conductivity, k - Q = delta T (k)
30
Thermal conductivity, k
- Ability of a medium to conduct heat
31
Geothermal gradient
- Delta T = Change in temp/change in depth - Input to geothermal flux eqn - Changes through seasons
32
Base of pf
- When temp at depth = 0C
33
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
34
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
35
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)
36
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
37
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
38
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
39
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
40
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
41
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
42
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
43
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'
44
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
45
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
46
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
47
Pingos may show what seasonal effect?
- Annual growth layers from summer-winter layering | - Dilatent cracks
48
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
49
What is the most common feature of continuous pf terrain
- Ice wedges and ice-wedge polygons
50
How many sides do ice-wedge polygons tend to have?
4 - 7
51
High centre vs. low centre ice wedge polygons
- High centre have water on edges | - Low centre have water in centre (small lakes)
52
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
53
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
54
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
55
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
56
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
57
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
58
Thermokarst involutions
- Churning of upper layer | - Present seds mix w/ melt-out till
59
Other forms of patterned ground
- Sorted polygons - Stone circles - Stone stripes - Mud boils
60
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
61
Frost-churning
- Slow process | - Forms stone circles and sorted polygons
62
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)
63
What is the evidence for motion of rock glaciers?
- Flow lobes, evees, arcuate ridges, steep margins, encroaching on vegetation
64
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
65
Thermokarst
- Subsidence due to thawing of ice-rich pf | - Most common feature of pf terrain
66
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)
67
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.
68
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
69
Peat Plateaux
- Flat-topped expanses of frozen peat - Several km^2 extent, m's thick - Elevated above surrounding unfrozen areas
70
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
71
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')
72
Solifluction
- Form of frost creep accelerated by presence of abundant water
73
Gelifluction
- Solifluction that occurs over pf | - Usually involves faster plug-like flow
74
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
75
Most common form of mass movement and erosive rates in periglacial environments is?
- Retrogressive thaw-flow slides
76
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