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Geomorphology > Glacial landscapes > Flashcards

Glacial landscapes Flashcards

1
Q

importance of glaciers

A
  • landscape development
  • climate change and glacier response
  • water resources (consumption/hydroelectricity using sub-glacial water)
  • glacial hazards
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2
Q

causes of glaciation and deglaciation

A
  • swing from icehouse to greenhouse worlds (100’s of millions of years)
  • mid term fluctuations are superimposed on the longer term cycles (100s of thousands of years) northern hemisphere glacier build up, advance and retreat
  • short term flucturatiosn are superimposed on the mid term cycles (10s to 100s of years) waxing and waning of established glaciers
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3
Q

Factors forcing glaciation

A

isolation: Incidence of Solar radiation

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4
Q

Milankovitch cyclicity

A
  • 23ka precession, angle
  • 41ka obliquity, wobble of earths axis - enhanced seasonal differences
  • 100ka eccentricity, shape of orbit around the sun, circular to elliptical
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5
Q

short term controls on ice extent

A
  • thermohaline circulation changes
  • solar activity
  • volcanic activity
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6
Q

glaciation through time

A
  • icehouse to greenhouse worlds (snowball earth hypothesis)

- Proterozoic: 2.3 - 2.2 billion years ago

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7
Q

Geological time

A

From ancient glaciation to modern

  • Quaternary period , the ice age
  • glacial cold stages interspersed with warm interglacials
  • last interglacial: Eemian 125,00 years ago
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8
Q

Summary of last glaciation

A
  • 2.6 million years of expansion and contraction of glaciers and ice sheets
  • today glaciers cover 10% of the earth with their occurrence being influenced by a number of factors
  • glaciers provide a valuable resource
  • changes in glacial extent affect the earths climate
  • changes in the earths climate affect glacial extent
  • helped shape the landscape of much pf the mid-latitudes on timescales of 10s to millions of years
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9
Q

Glacier morphology and movement importance

A
  • characteristics of ice determines glacier temp
  • temp controls processes of erosion, transport and deposition
  • climate controls annual gains and loss of snow and ice
  • gains and losses influence ice movement (advance and retreat)
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10
Q

formation of glacial ice

A

ice forms by a 5 stage continuous process
- snowflakes
- compaction
- grains
- firn (somewhere between snow and ice)
- glacial ice
(rate of transformation varies with climate: temperature and precipitation regimes

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11
Q

firn to ice transformation

A

this is the stage at which the accumulated snow has the density to form a coherent pack of ice which can least year round

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12
Q

nature of glacier ice

A
  • ice is a polycrystalline material. crystal size and shape vary with depth and history
  • glaciers comprise ice, liquid water, air and debris
  • important attributed of ice are temperature, density and melting point
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13
Q

glacier morphology

A
  • classified according to shape and relationship with underlying topography
  • ice sheets, ice caps, ice fields, ice streams (all dominate topography)
  • outlet glaciers, valley glaciers, Piedmont glaciers, cirque glaciers (all constrained by topography)
  • marine ice masses: ice shelves
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14
Q

first order classification

A
  • ice sheet and ice cap unconstrained by topography (cirque/corrie/cwn)
  • glaciers constrained or controlled by topography
  • marine glaciers: floating
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15
Q

how do glaciers move?

A

combination of:

  • driving forces
  • stress
  • resisting forces
  • strain
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16
Q

glacier motion: driving forces

A
  • surface slope

- weight of ice; basal ice conditions (frozen or slightly melted)

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17
Q

Resisting forces

A
  • strength of glacier ice

- contract between glacier and bed (resisting force may vary dependent on nature of the bed)

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18
Q

Acumulation and ablation

A

equals mass balance

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19
Q

stress

A

measure of how hard a material is being pushed or pulled by external forces

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20
Q

strain

A

measure of the amount of deformation occurring as a result of the applied stress `

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21
Q

type of glacier motion

A
  • internal ice deformation 1: creep
  • internal ice deformation 2: faulting (crevasses)
  • sliding of ice over bedrock/sediment (Water film)
  • sub glacial bed deformation (Weak substrate)
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22
Q

cold-based ice

A
  • ice which is frozen
  • no water present at the glacier bed
  • low erosive potential
  • protects the bed
  • selective erosion
  • most common in polar regions
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23
Q

warm based ice

A
  • water present ice-bed boundary
  • high erosive potential (Slides over/erodes the bed)
  • most common in temperate regions
  • thicker ice masses
  • still below zero
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24
Q

pressure melting point

A
  • as pressure increases, the temperature at which the ice becomes liquid is lowered
  • controls are: atmospheric conditions, geothermal heat flux, frictional heat
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25
Q

ways water enters a glacier

A
  • englacial channels
  • snow melt percolation
  • firn
  • water saturation zone
  • subglacial sediments and channels
  • moulin
  • crevasses
  • groundwater flow
  • supraglcial melt water channels
26
Q

temperate glacier

A

warm ice except seasonally warmed and cooled surface layer

27
Q

cold glacier

A

entirely cold ice

28
Q

polythermal glacier

A

both warm and cold ice

29
Q

erosive processes vary depending on the type and composition of the ice

A
  • ice thickness,
  • temperature (warmed by friction of movement and geothermal heat)
  • water content
    affect erosion considerably
30
Q

erosion

A
  • removal of rock or sediment
  • polishing of rock
  • widening, deepening or lengthening pre-exising topographic features (glacial action)
31
Q

factors that influence rates of erosion

A
  • bedrock composition
  • unconsolidated sediment
  • friction
  • ice flow
  • incorporation of sediment into the ice
32
Q

Key concept of stresses

A 
shear stress (force acting on the bed) and resistance forces (friction and cohesion)
SS>RF = erosion
RF>SS = deposition
33
Q

stress and resistance

A
  • uneven bed: the more uneven , the more the ice has to adapt to be able to flow around protuberances
  • erosion and deposition are both the result of competing stresses in the subglacial environment
34
Q

large scale erosion

A
  • Quarrying: breaking or fracturing the rock (loading, pressure release)
  • Plucking: removal of large blocks of rock from valley sides or bottom by glacial action (freeze thaw activity)
35
Q

small scale erosion

A
  • Abrasion: the rubbing down of rock surfaces surrounding the glacier by debris held within the ice
  • Polishing: smoothing of rock surfaces by very fine-grained materiel held within the ice
36
Q

Erosion controls summary

A
  • Strength of the materials making up the bed measured as frictional strength and cohesion
  • For unconsolidated materials (sediment), water content is important
  • Low  cohesion low
  • High  cohesion/resistance higher
  • For solid bedrock the internal structure and orientation of structures is critical
  • Bedding planes
  • Weathering planes
  • Faults
37
Q

micro-scale

A
  • forms parallel to ice movement
  • superficial erosion: abrasion
  • generally less than 1cm deep, and up to several metres long
  • close inspection show rough base and sides
  • size proportional to rock roughness, load applied
  • generally widen down glacier
  • orientation may vary over short distances
38
Q

meso-scale erosional forms

A
  • streamline bedrock features (Whaleback, Roche Moutonees; crag and tails)
  • steep up slope (resistant bedrock)
  • smooth down-slope
  • ice stream around obstacle
  • quarrying and abrasion with increased pressure to form land forms
39
Q

macro-scale erosional forms

A
  • cirques: compound, cirque complexes, staircase cirques, cirque troughs
  • glacial valleys
40
Q

Process of cirque formation

A
  • initial nivation hollow - nivation cirque - glacial cirque
  • critical threshold (ice is thick enough to achieve internal deformation and basal sliding )
  • floor is abraded and quarried
  • headwall retreats by; freeze thaw, mass failure, rock slope failure
  • enlargement is backwards and downswards
  • theories of formation state that enlargement occurs over time
41
Q

Troughs and Fjords

A
  • these are amongst the largest erosional landforms one earth
  • difficult to attribute all erosion to glaciers. most are multi-process products (Glacial, periglacial, paraglacial, weathering, fluvial erosion)
  • largest are 1000km long, 40km wide and 3.4km deep
42
Q

erosion summary

A
  • Bedrock is eroded by glacial ice (e.g. quarrying (plucking) and abrasion) and/or subglacial meltwater (dependent on subglacial water pressures).
  • The processes of erosion are inherently linked with glacier dynamics and glacier motion e.g. sliding, abrasion and quarrying.
  • Erosional processes, therefore, reflect glacier dynamics and the patterns of stresses and pressures across the subglacial environment.
  • Scale of erosive features varies from microscopic to entire landscapes (Striae to Roches Moutonées, Glacial Valleys)
43
Q

as a glaciers mass balance adjusts

A 
Englacial environment
• Ice: creep, faulting, folding
• Water: percolation, flow
Subglacial environment
• Sliding via meltwater
• Sediment Deformation
• Erosion (Abrasion, Plucking, Quarrying, 
•Deposition
Proglacial environment
• Sediment deposition/accumulation/accretion
• Erosion by meltwater
• Periglacial features
44
Q

entertainment and transport

A
  • sediment is incorporated at many stages
    Accumulation zone
  • supraglacial, remains unaltered
  • englacial/subglacial becomes rounded through abrasion
    Ablation zone
  • transport times redueced
  • transport passively as supraglacial sediment
  • sediment also transported subglacially
  • conveyor belt of sediment
45
Q

Supraglacial transport of debris

A

Glacier surface transport
• This is SUPRAglacial transport
• Common sources of debris are: Rock fall, Avalanche, Debris flow, Airborne debris (aeolian sands, volcanic ash)
• Rock fall is most common and voluminous, but only if
there are rock surfaces above the ice
• Not prevalent on ice sheets or ice caps

46
Q

Basal transport of debris

A
  • debris is derived from abrasion processes and quarrying/plucking
  • entrainment can be of:
    • individual clasts
    • agglomerations of rock flour
    • large rafts of unconsolidated debris
  • there are 2 sub-zones of basal debris transport:
    1) traction zone (contact between ice and its bed)
    2) dispersed zone (sediment becomes incorporated in ice)
47
Q

Traction zone

A
  • very active
  • comminution processes ensure that debris undergoes significant change with distance of transport
  • debris characteristics which change include; texture, roundness, fabric
48
Q

compostion of debris in a glacier

A

Sorted:
- key processes; aeolian (wind), fluvial (Flowing water), still water
- not deposited by glacial ice itself
Unsorted
- key process; ploughing, crushing, plucking,, rock/debris flows, dumping
- deposition by glacial ice can occur

49
Q

Tills

A

characteristics depend on glacial history
- source
- transport
- depositional environment
Types: lodgement, deformation, sub-glacial meltout, supra-glacial meltout, flow and indurated till

50
Q

classification of glacial deposition landforms

A
  • aligned (drumlins)
  • ice marginal (End moraines and kame/kettle topography)
  • fluvioglacial (rivers under and in front of ice)
  • glaciolacustrine (lake systems in front of the ice)
51
Q

Drumlins

A
  • positive forms
  • typically smooth
  • oval/elliptical (approx. aligned along orientation of flow)
  • blunt upstream end and tapered downstream ent
  • 5-50m in height
  • 10-3000m long
  • composed of sediments
  • occur in fields or swarms in lowland locations
52
Q

Active drumlin field in Iceland

A
  • Boulton-Menzies theory 1) sediment accumulated and is built up and shaped at the bed of the glacier, formed by deformation of soft bed, accretion of hills
  • Shaw theory 2) formed by mega floods, erosion of hollows
53
Q

Boulton-Menzies theory of drumlin formation

A
  • subglacial deformation and accretion theory
  • weaker sediment (Finer grained, less well drained) deform more easily
  • stronger sediment forms core, slow deformation
  • weaker sediment transported around the cores, rapid deformation
54
Q

Shaw Hypothesis for drumlin formation

A
  • meltwater theory
  • similar morphology to small sale erosional features, therefore similar processes
  • horseshoe vortices from down steam of obstacles
  • rock drumlins, formed by erosion
  • sediment drumlins, formed when flood is hyper concentrated with sediment
55
Q

problems associated with shaw drumlin theory

A
  • lack of evidence
  • drumlins revealed by ice since LIA have not been associated with this type of melt water mechanism
  • large floods in Iceland today do not produce drumlinised forms
56
Q

Kame topography

A
  • mound or ridge between glacier and hillside
  • supra-glacial, glaciofluvial deposition at ice margin
  • have flatter, regularly sloping surface
  • less chaotic sedimentology (bedding present)
  • more sand in composition (due to fluvial formation)
57
Q

Kettle topography

A
  • depression or hollow
  • subsidence following melt of ice within kame sediment
  • often formed during ice down wastage
58
Q

multiple ridges

A
  • glacier change from positive to negative mass balance
  • advance and retreat
  • pushing up previously deposited moraine ridge
59
Q

Moraines

A
  • can be annual features (push forward winter/surge, retreat back in summer)
  • up to 100s m high
  • following plan form of the snout
  • steep proximal slope
  • gentle distal slope
60
Q

ice margin

A
  • if ice margin is stable a ridge will accumulate

- is ice margin is activity retreating debris will be spread over a wide are and may be no definite land forms

61
Q

Ice marginal moraine ridges

A
  • ridge of debris, often till, but not always
  • processes involved in deposition vary (sliding, rolling, squeezing, bulldozing)
  • extreme limit of ice usually marked by (terminal moraine)
  • still stand during retreat marked by recessional or retreat moraines

Geomorphology (9 decks)

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