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Geog375Gmphlgy > Periglacial Processes and Landforms > Flashcards

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
Q

Relict pf in NWT, Yukon, Alaska, and offshore

A
  • Relict pf in widespread Beringia, including offshore: Mckenzie Delta, Banks Island, Alaska North Shore, Bering Strait, Siberia North Shore
  • Implications for melting methane hydrates
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26
Q

Implication of melting relict pf

A
  • Melt of methane hydrates

- Strong gh gas release, positive feedback

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27
Q

Active layer

A
  • 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.
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28
Q

Talik

A
  • Zones of perenially unfrozen ground w/in pf

- Common in areas of discontinuous pf and often under deep lakes and rivers

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29
Q

Geothermal flux, Q

A
  • Heat flows from centre of Earth towards surface
  • Rate controlled by thermal conductivity, k
  • Q = delta T (k)
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30
Q

Thermal conductivity, k

A
  • Ability of a medium to conduct heat
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31
Q

Geothermal gradient

A
  • Delta T = Change in temp/change in depth
  • Input to geothermal flux eqn
  • Changes through seasons
32
Q

Base of pf

A
  • When temp at depth = 0C
33
Q

Determining pf thickness

A
  • 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
Q

What happens to pf when air Temp warms?

A
  • 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
Q

Stefan eqn

A
  • 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
Q

Ground ice

A
  • 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
Q

How many types of ice are recognized in Russia vs. N. America?

A
  • Russians recognize 20 ice types (including buried glacial ice)
  • NA only 8 types commonly recognized
38
Q

Why is ground ice important?

A
  • 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
Q

Periglacial landforms and processes

A
  • Pingos
  • Ice wedges and polygons
  • Patterned ground
  • Rock glaciers
  • Thermokarst
  • Drunken forest
  • Peat plateaux
  • Thaw slumps
  • Solifluction
  • Frost heave
  • Frost cracking
40
Q

Pingos

A
  • 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
Q

Closed-system Pingo, Hydrostatic

A
  • 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
Q

Open-system Pingo, Hydraulic

A
  • 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
Q

Pingo formation: confined freezing

- In saturated non-pf envrs

A
  • 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
Q

Pingo formation: confined freezing

- In pf areas

A
  • 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
Q

Closed-system pingo formation

A
  • 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
Q

Open-system pingo formation

A
  • 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
Q

Pingos may show what seasonal effect?

A
  • Annual growth layers from summer-winter layering

- Dilatent cracks

48
Q

Ice wedges and ice-wedges polygons

A
  • 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
Q

What is the most common feature of continuous pf terrain

A
  • Ice wedges and ice-wedge polygons
50
Q

How many sides do ice-wedge polygons tend to have?

A

4 - 7

51
Q

High centre vs. low centre ice wedge polygons

A
  • High centre have water on edges

- Low centre have water in centre (small lakes)

52
Q

Ice wedge formation

A
  • 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
Q

Ice wedge development after n years

A
  • 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
Q

High centre ice wedge polygons form in?

A
  • 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
Q

Ice-wedges as indicators of past climate

A
  • 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
Q

Wedges in arid cold areas

A
  • Sand wedges may show vertical structure like an ice wedge

- Both ice and sand wedges indicate pf but moisture contents differ

57
Q

Remnant Laurentide ice sheet wedges

A
  • 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
Q

Thermokarst involutions

A
  • Churning of upper layer

- Present seds mix w/ melt-out till

59
Q

Other forms of patterned ground

A
  • Sorted polygons
  • Stone circles
  • Stone stripes
  • Mud boils
60
Q

Sorted polygons and stone circles

A
  • 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
Q

Frost-churning

A
  • Slow process

- Forms stone circles and sorted polygons

62
Q

Rock glaciers

A
  • 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
Q

What is the evidence for motion of rock glaciers?

A
  • Flow lobes, evees, arcuate ridges, steep margins, encroaching on vegetation
64
Q

What are the likely origins of rock glaciers?

A
  • 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
Q

Thermokarst

A
  • Subsidence due to thawing of ice-rich pf

- Most common feature of pf terrain

66
Q

How does thermokarst terrain arise?

A
  • Human surface disturbance (roads, pipelines, sewage lagoons, buildings etc.)
  • Forest fires
  • Climate change (imp in arctic where T rapidly rising)
67
Q

Thermokarst and forest fires

A
  • 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
Q

Drunken Forest

A
  • 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
Q

Peat Plateaux

A
  • Flat-topped expanses of frozen peat
  • Several km^2 extent, m’s thick
  • Elevated above surrounding unfrozen areas
70
Q

Why do peat plateaus form?

A
  • 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
Q

Solifluction / gelifluction

A
  • 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
Q

Solifluction

A
  • Form of frost creep accelerated by presence of abundant water
73
Q

Gelifluction

A
  • Solifluction that occurs over pf

- Usually involves faster plug-like flow

74
Q

Retrogressive thaw-flow slides

A
  • 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
Q

Most common form of mass movement and erosive rates in periglacial environments is?

A
  • Retrogressive thaw-flow slides
76
Q

Summary

A
  • 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

Geog375Gmphlgy (9 decks)

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