Geomorphology Flashcards
Importance of meltwater landforms:
1.
2.
3.
- Main ablation product (maintains equilibrium), lubricating sliding and hence controls flow.
- Removes debris and some erosion
- Can be used to (a) reconstruct paleo-ice sheets; (b) properties of subglacial hydrological system.
How may meltwater enter and travel through subglacial drainage?
Supraglacial melting below ELA –> self organisation into streams –> water enters crevasses/moulins –> englacial/subglacial flow –> can hydrofracture to bed for basal flow.
How is meltwater flow governed?
Driven by hydraulic head, so water runs at right-angles to hydropotential surface.
- driven by slope and thickness of ice, fracturing (crevasses) can also control..
Outline drainage style ‘R - channels’:
Channels or conduits cut into ice often englacial or at bed.
Modulated by: heat/turbulence (keeps open) and ice pressure (closing)
R-channels: sediment load and landforms
Ability to carry high sed load/large clasts due to overlying ice pressure.
landforms: sediment deposition and eskers form
Outline esker formation:
Storrer et al., 2014: ‘ ‘
4 types:
Storrer e.a (2014)- ‘elongate ridges of glacifluvial sand and gravel’
Outline:
- water flow subsides, debris deposited in conduit, subglacially, englacially and supra.
- ice then retreates - leaving ‘fossil’ of different forms:
1. tunnel fill (fills englacial or sub)
2. ice-channel fill (fills supra)
3. Segmented tunnel fill (fill intermittently during pulse retreat)
4. Beaded eskers (subaqeuous fans during pulse retreat, beading occurs with sedimentary fan)
Internal composition of eskers:
well sorted layers
rounded clasts –> sig distance.
diverse series of beds (rapid to slow deposition), multiple layers = changing flow
variety of materials: locally, with a poorly sorted sand/gravel core surrounded by depositional structures (reveal cyclic sequences of water flow e.g. seasonal/annual)
can be deformed - indicates subsidence during melt.
Distribution of eskers:
- sub-parallel to ice flow
- variable shape/size
- bradied - shorter
- single ridges - longer (100s km)
- can climp uphill (pressure)
What did Storrer e.a (2013) find about esker distribution?
- Mapped all eskers across Canada from Landsat.
Characteristics: - fragmentary pattern, sub parallel to parallel, direction of flow.
Size and shape: - can be traced to reach 1000s of km.
Controls on esker spacing:
- Pressure: Low = steady state. Bigger then channel - lower the pressure. - idea of competing
- Channels - lower water pressure - smaller channels widened under high pressure.
- Distributed/permeable bed –> widely spaced & larger channels
- Near margin = smaller spacing as discharge higher and potential gradient higher.
- some level of self organisation –> creating a typical spatial pattern.
Controls on esker occurrence:
e.g. Clark and Walder (1994) - Esker dist formed by Laurentide and Scandinavian Ice Sheets
- relationship with substrate
- close correlation between hard bedfrock and esker occurrence
- suggest not always formed on deformable bed.
- consistent with theory that deformable bed = distributed (as esker = channel)
- occurrence largely where modelled subglacial flow is low and away from paleo ice streams
- preservation potential is difficult when fast-flowing dist networks due to erosive force. - so may get areas with less water
Hypotheses for esker formation and hydrological conditions:
- High flow
- Low flow
- High flow - a low sediment supply more likely to get single simple esker. Channel expands faster than deposition
- Low Flow - channels trying to melt through water but high sediment load builds - drainage diverts - sediment deposits - drainage diverts = braided system. New channels and esker at every diversion.
Esker formation: time transgressive (accumualtes) VS synchronous (at once)
Thought to be time transgressive - especially when beaded. But not definite and on what time scale..
How may eskers be used to infer/reconstruct retreat patterns?
- Indicate subglacial drainage beneath ice sheet
- sub/parallel to ice surface slope indicate flowlines.
- as time transgressive - indicators of retreat pattern.
- More melt = more channels = greater density/amount channels.
therefore more frequent during ice margin recession and climatic warming.
What did Storrar et al., (2014) find regarding channelisation of subglacial drainage and deglaciation of Laurentide?
- Theory
- Method
Some argue increased melt flux may be counteracted (ice dynamics) by evolution of drainage system - i.e. channel widening (BUT OBSERVATIONS DIFFICULT)
- during deglaciation area of the bed subjected to less efficient system - could preclude dynamic instabilities (surge/streaming)
- thought to be the same manner as alpine glaciers coping with increased meltwater (seasonally) BUT on much larger timescale.
Method: mapped 20,000 eskers to reconstruct evolution of system during final glaciation (13-7 kyr BP)
What did Storrar et al.,(2014) find:
- final demise
- system
- density
- pattern
- retreat rates
- substrate
- final demise - linked to surface mass balance due to limiting potential for flow instabilities (surge/stream)
- system traced up to 760km
- density - 1.24 per 100km but range 0.05-3.31
- pattern - more branching ‘dendritic’ systems found, which diverge away from positions of major ice divides. but no eskers at final divide positions.
- retreat rates - 100-200 m yr-1 from 13-9.5 kyr BP. increased rapidly between 9.5-9 kya BP. –> thought to be associated with pronounced warming in N.hem between 12-9.5 kya, thus negative surface mass balance and increased melt water.
retreat rates after warming & increased meltwater: 0-100m yr-1 (8.5 kya) to over 400 m yr-1 (7.5 kya) - substrate - local
What did Drews et al., (2017) discover about actively forming eskers:
- Large basal obstacles found beneath AIS (radar)
- Interpreted as esker ridges - depositional conduits.
- size increases towards grounding line where max deposition rates (widening and slower flow)
Outline ‘kame’ according to:
Livingstone et al., (2009)
Benn and Evans, (1998)
Livingstone et al., (2009) - irregularly shape mound of sand/gravel/till that accumulates in a depression on a retreating glacier, which is then deposited on land surface upon further melting.
Benn and Evans (1998) - evolves where large quantities of debris reworked by supra/englacial drainage systems during final stage of glacial wastage. common in hummocky terrain
Outline ‘kame’ significance to subglacial meltwater landform geomorphology:
- a series of kame can document periodic retreat and surface lowering of glacier.
- complex mode of deposition caused by ice stagnation and down wasting.
- topographic inversions: occur which lead to distinctive positive relief landforms and represents an end product of deposition and ablation. = can result in kettle holes, hollows, flat topped hills and discontinuous hills.
- kame belts associate with meltwater drainage of ice mass, and therefore not in isolation of other glaciofluvial/lacustrine landforms which are linked in a temporal/spatial continuum.
conclusion: kame belts demonstrate a polygenetic topography, and a time-transgressive evolution with sedimentation controlled by enlarging glacier karst.
Give an example of ‘kame’ in the UK:
- e.g. Brampton Kame belt, Cumbria - one of the largest glaciofluvial complexes (44km2) - represents major depositional episode during advanced stages of recession of the late Devensian BIIS.
- distinctive landforms due to topographic inversion and formation of Lake Carlisle.
Meltwater landforms ‘p-forms’ or ‘plastically moulder forms’:
- wide variety - smooth structures
- scale - ms
- formation - uncertain, appear in variety of settings (.e.g debris-rich basal ice, saturated till, subglacial meltwater under high Pw, ice-water mixes)
- abrasion effects - potholes and structures
- striations - some moulding occuring.
- doesnt follow topography - pressure control.
What are r-channels controlled by:
‘The balance between channel enlargement by viscous heating and closure by ice deformation when the channels are water-filled reflects their size and water pressure’
Provide a summary of Anderson and Fretwell (2008) on N-channels/Nye channels:
- channels incised into the underlying bedrock by glacial motion in the context of a glacial
- found to operate over repeated glacial cycles over millions of years, demonstrated by its vast size and organisation.
- in the context of Marguerite Bay, a paleo IS area.
- Series of 3 geomorphic zones, erosional in character and floored by crystalline bedrock. Some evidence (channels) suggest connectivity between zones likely through well organised subglacial drainage network.
- Channels incise 200-300m into bedrock highs and anastomosing channels occur in deeper portions of the basins 10s m deep.
- presence of drumlins/flutes parallel to flow orientation indicate fast flow in outer zone.
- zones indicate combined action of glacial ice and melt water in shaping the seascape. overall trend of ice-sculpted features strongly influenced by tectonic fabric of the region.
Summarise Sharp et al., 1989 on lee side cavities (small scale subglacial drainage): Location/where: Method: Findings: Conclusions:
Location/where:
- Glacier de Tsanfleuron a former subglacial drainage system: 4km2 plateau glacier on N. Rhine Valley, Switzerland.
- system with large cavities in lee of bedrock steps connected by n-channels (limestone bedrock), exposed through deglaciation.
Method:
- Kamb (1987) cavity hydraulic model to predict transition between drainage network configuration and quantify effective basal pressures. Argues surges caused by switch from channelised (prior) to cavity system with lower pressures.
Findings:
- geometry of tunnels considered stable and running at atmospheric effective pressures.
- low pressure & cavity - associated with fast flow behaviour (surges)
- system appears to adjust to varying discharge by varying water pressure and total cross sectional area of flow by altering the number of channels connecting cavities.
- 0.1 - 0.2m W, 0.1m deep, 1.5m intervals, parallel.
- n-channels connect cavities.
Conclusions:
- surface derived meltwater evacuated by network of step cavities linked by n-channels. covers 51% glacier bed.
- 51-83% of length in channels. BUT cavities major role in lateral dispersion of flow.
Give an example of a large scale subglacial meltwater channel case study:
Rampton (2000): Subglacial meltwater flow in southern Slave province, NW Canada.
- Location/Context:
- Findings:
- Conclusions:
Location/Context:
- glaciofluvial meltwater corridors marked by scoured bedrock, irregular and transverse gravel ridges, drumlins, potholes and eskers among others.
- First noted St. Onge (1984) in Coppermine area, regular intervals and up to 1km wide. Associated with glaciofluvial sediment.
Findings:
- corridors found in abundance/large areas - attributed to large regional floods permitting subglacial erosion of till by high velocity, turbulent meltwater under high Pw and meltwater transport/deposition. (High energy)
- various landform assemblages attest to high pressure & turbulent sheet flow, broad streams and channels.
- large water sources: geothermal, sliding, viscous thermal energy OR supraglacial melting.
- bedrock scoured at bottom (appears in eskers, boulder/sediment patches in corridors, erosional forms on bedrock surface)
Conclusions:
- tunnel valleys/channels - eroded into bedrock 100’s m W, 10’s km L, 10-100’s deep. semi-regular, incised. - lots of time and power required.
- Formation: uncertain, big event, lake that drains slowly cuts large system? (a) repeated low magnitude outwash floods, (b) gradual/steady-state erosion or (c) combination.
- defo subglacial due to landforms e.g. eskers, outwash fans (high energy needed for this transport/deposition)
What does Greenwood et al., (2016) have to say in his subglacial hydrology review paper regarding regime’s for landform development:
Catastrophic drainage events
EVIDENCE of process resolving large scales:
- Labyrinth Mountains, Ant - 50m deep depressions, boulders, large potholes, scours.
- Large wash of water up and over cutting anastomising network.
- argued to imply one large flooding event/rapid discharge peaks.
What does Livingstone and Clark, (2016) have to say in his subglacial hydrology review paper regarding regime’s for landform development:
gradual erosion (Laurentide ice sheet)
- depositional fan, indicating time transgressive growth.
- Individual valleys evolve upstream (gradual)
- hypothesised that composite features evolved during a glacial episode or over multiple glaciations.
Impact of large scale subglacial meltwater drainage networks on ice dynamics:
evidence: Lelandais et al., (2018)
- analogue model of ice flow, subglacial hydrology and sedimentary-geomorphic processes.
- modelled IS more dynamic when outburst events (decoupling)
- decelerate when water reorganised into channels –> then tunnel valleys (partial recoupling)
- ice stream surge/migrate when low discharge maintained
- ice stream switch off when sufficient drainage network established by tunnel valleys.
- tunnel valley development may be crucial in stabilizing potions of ice sheets during periods of climate change.
Outline main aspects of Lateral Meltwater Channels:
- form on edges of ice, half ice-half land.
- upon retreat = leaves lateral channels on valley sides, used to reconstruct thinning history.
- PLEISTOCENE lateral channels used frequently as evidence of cold based ice.
Syverson and Mickelson (2008) on ‘Lateral Meltwater Channels’:
- formed upon rapid thinning of temperate glacier margin, Burrows Glacier, Alaska. (maritime)
- observed <40yrs along margins
- nested groups or singular channels, subparallel to land surface.
- formed with ablation rates 4-9.5m/yr
- nested channels - good proxies for slope of ice margin, eroded into the sandy till.
- a ‘perched’ water table associated with high precip and ablation is a saturated zone separated from underlying unsaturated area by a low permeability layer.
= perched table meant water flowed along margins eroding lateral channels until water entered subglacial system. - at rates of 0-8 channels a year.
what are the controls on lateral meltwater channel formation?
controlled by:e.g.
land-surface slope - optimal = 5-10 degrees, if less 5* high ablation rates dont permit time to erode. (Burrows)
ablation rates,
substrate erodibility
discharge.
too steep = ice margin doesnt migrate laterally as ice thins, so nested channels dont form.
Outline key aspects of proglacial meltwater channels:
- Large dimensions due to high discharge and sediment load during peak flow.
- Largest channels cut by GLOFs. Big lakes at margin of glacier that drain, and can rapidly erode and cut large channels. e.g. Newtondale Spillway, N. York Moors.
- can reconstruct lake extent, drainage routes, ice margins.
