Glacial landscapes Flashcards
importance of glaciers
- landscape development
- climate change and glacier response
- water resources (consumption/hydroelectricity using sub-glacial water)
- glacial hazards
causes of glaciation and deglaciation
- 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
Factors forcing glaciation
isolation: Incidence of Solar radiation
Milankovitch cyclicity
- 23ka precession, angle
- 41ka obliquity, wobble of earths axis - enhanced seasonal differences
- 100ka eccentricity, shape of orbit around the sun, circular to elliptical
short term controls on ice extent
- thermohaline circulation changes
- solar activity
- volcanic activity
glaciation through time
- icehouse to greenhouse worlds (snowball earth hypothesis)
- Proterozoic: 2.3 - 2.2 billion years ago
Geological time
From ancient glaciation to modern
- Quaternary period , the ice age
- glacial cold stages interspersed with warm interglacials
- last interglacial: Eemian 125,00 years ago
Summary of last glaciation
- 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
Glacier morphology and movement importance
- 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)
formation of glacial ice
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
firn to ice transformation
this is the stage at which the accumulated snow has the density to form a coherent pack of ice which can least year round
nature of glacier ice
- 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
glacier morphology
- 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
first order classification
- ice sheet and ice cap unconstrained by topography (cirque/corrie/cwn)
- glaciers constrained or controlled by topography
- marine glaciers: floating
how do glaciers move?
combination of:
- driving forces
- stress
- resisting forces
- strain
glacier motion: driving forces
- surface slope
- weight of ice; basal ice conditions (frozen or slightly melted)
Resisting forces
- strength of glacier ice
- contract between glacier and bed (resisting force may vary dependent on nature of the bed)
Acumulation and ablation
equals mass balance
stress
measure of how hard a material is being pushed or pulled by external forces
strain
measure of the amount of deformation occurring as a result of the applied stress `
type of glacier motion
- 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)
cold-based ice
- ice which is frozen
- no water present at the glacier bed
- low erosive potential
- protects the bed
- selective erosion
- most common in polar regions
warm based ice
- water present ice-bed boundary
- high erosive potential (Slides over/erodes the bed)
- most common in temperate regions
- thicker ice masses
- still below zero
pressure melting point
- as pressure increases, the temperature at which the ice becomes liquid is lowered
- controls are: atmospheric conditions, geothermal heat flux, frictional heat
ways water enters a glacier
- englacial channels
- snow melt percolation
- firn
- water saturation zone
- subglacial sediments and channels
- moulin
- crevasses
- groundwater flow
- supraglcial melt water channels
temperate glacier
warm ice except seasonally warmed and cooled surface layer
cold glacier
entirely cold ice
polythermal glacier
both warm and cold ice
erosive processes vary depending on the type and composition of the ice
- ice thickness,
- temperature (warmed by friction of movement and geothermal heat)
- water content
affect erosion considerably
erosion
- removal of rock or sediment
- polishing of rock
- widening, deepening or lengthening pre-exising topographic features (glacial action)
factors that influence rates of erosion
- bedrock composition
- unconsolidated sediment
- friction
- ice flow
- incorporation of sediment into the ice
Key concept of stresses
shear stress (force acting on the bed) and resistance forces (friction and cohesion) SS>RF = erosion RF>SS = deposition
stress and resistance
- 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
large scale erosion
- 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)
small scale erosion
- 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
Erosion controls summary
- 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
micro-scale
- 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
meso-scale erosional forms
- 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
macro-scale erosional forms
- cirques: compound, cirque complexes, staircase cirques, cirque troughs
- glacial valleys
Process of cirque formation
- 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
Troughs and Fjords
- 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
erosion summary
- 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)
as a glaciers mass balance adjusts
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
entertainment and transport
- 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
Supraglacial transport of debris
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
Basal transport of debris
- 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)
Traction zone
- very active
- comminution processes ensure that debris undergoes significant change with distance of transport
- debris characteristics which change include; texture, roundness, fabric
compostion of debris in a glacier
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
Tills
characteristics depend on glacial history
- source
- transport
- depositional environment
Types: lodgement, deformation, sub-glacial meltout, supra-glacial meltout, flow and indurated till
classification of glacial deposition landforms
- 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)
Drumlins
- 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
Active drumlin field in Iceland
- 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
Boulton-Menzies theory of drumlin formation
- 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
Shaw Hypothesis for drumlin formation
- 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
problems associated with shaw drumlin theory
- 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
Kame topography
- 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)
Kettle topography
- depression or hollow
- subsidence following melt of ice within kame sediment
- often formed during ice down wastage
multiple ridges
- glacier change from positive to negative mass balance
- advance and retreat
- pushing up previously deposited moraine ridge
Moraines
- 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
ice margin
- 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
Ice marginal moraine ridges
- 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
