Glaciation Flashcards
4 periglacial landforms
Ice wedges
Patterned ground
Pingos
Loess
Ice wedge polygons
- downward narrowing masses of ice between 2-3m wide at base and extend below ground up to 10m
- formed by refreezing of active layer during winter causing soil to contract and cracks open. During melting in summer cracks fill with meltwater and sediment then freeze the following winter, widening and deepening the crack
Patterned ground
- covers features like: nets, polygons, steps and stripes
- formed by movements resulting from frost action:
- frost push caused by hydrostatic pressure propels stone upwards, whilst frost heave causes stones to migrate outwards to form circles - this provides basis for all patterns
- up doming of circle crested by heaving mean larger stones roll outwards due to gravity, leaving finer sediment in centre
- as a result, a stone polygons are elongated into nets/stripes which has clear relationship between type of patterned ground and slope angle
- beyond 30 degrees angle, patterned ground can no longer form and rock avalanches may occur
Pingos
-ice cored hills between 30-70m high and diameter between 100-500m
- growth of ice cores forces up overlying sediments, causing dilation cracks, once the ice core is exposed it melts which causes top of Pingo to collapse, forming a crater
- most Pingos circular in shape
- smaller Pingos tend to have curved top
- large usually have exposed ice at top and melting of this often forms crater which are sometimes filled with water to cause a lake
7 macro features (<1km)
- Cirque/corrie/cŵn
- Arête
- Pyramidal peak
- Truncated spur
- U-shaped valley
- Hanging valley
- Ribbon lake
4 meso features (10m-1km, usually found in macro features)
- Whalebacks
- Crag and tail
- Roche moutonneees
- Knock an’ Lochan
3 micro features (few metres and less)
- Chatter marks
- Striations
- Crescentic gouges
Milankovitch theory
(Long term climate change)
Earths orbit varies every 100,000 years. Elliptical to more circular
Means amount of solar radiation in summer and winter changes, impacts seasonal changes
Low eccentricity = low seasonality = glaciation, as a result temps at high latitudes in northern hemisphere decrease
Axil tilt = obliquity
(Long term climate change)
Varies between 21.8-24.4 degrees, does this over 41,000yr timescale
Impacts intensity of light received at the poles, so effects seasonality
Less tilt = warmer winters + cooler summers = glaciers don’t melt and advance = more solar radiation reflected so variations in ice volume.
Low obliquity = low seasonality so promotes glacation
Precession - wobble
(Long term climate change)
Earth wobbles on it’s axis over 21,000 year cycle, changing when earth is nearest to sun, increasing variation of irradiation of northern hemisphere
This impacts summer and winter temps which impact ice volume.
Glacation favored when direction of tilt means n. Hemisphere summers are at the largest earth-sun distance
Compressional flow
Ice mass thickens as slope gradient reduces and movement slows.
Potential for erosion increases
Extensional flow
Ice mass thins and movement increases when slope gradient gets steeper.
Erosion potential decreases
Factors controlling glacier movement
Lithology
Altitude
Slope angle
Size/thickness
Mass balance
Ice temp
Processes contributing to glacier movement
Basal sliding
Internal deformation
Regelation creep
Extensional and compressional flow
Rotational flow?
Basal sliding
- temp water freezes at decreases under pressure
- glacier moves and exerts pressure so there’s melting at base
- meltwater acts as lubricant allowing glacier to move
Regelation creep
- glacier encounters big obstacles
- this increases pressure on ice and ice melts under pressure due to PMP, so meltwater forms
- melt water acts as lubricant so glacier can move over and around obstacles
Internal deformation
- most common movement in cold based
- ice crystals change shape and position due to pressure
- they move and slide past each other, allowing glacier to slowly move
Glacial processes: corrie
Macro
Snow accumulates in nivation hollow. Builds up year after year, snow gets compressed into ice.
Freeze thaw weathering and plucking of back wall steepens and deepens bottom
Ice eventually melts, can create lake
Glacial processes: arete, pyramidal peak
Arête = 2 corries side by side
Pyramidal peak: 3+ corries erode on mountain
Glacial processes: u shaped valley
Glacier carves through pre-glacial mountain valley, widening and deepening it
Looks like stream that doesn’t fit the valley
Glacial processes: Truncated spur
Pre-glacial interlocking spurs of river valley get widened and deepened by glacier.
Abrasion is dominant process and plucking, so sides of valley eroded to remove mountain spurs by cutting them off
Glacial processes: hanging valleys
Main glacier erodes trough wider and deeper than smaller glacier joinin in.
As it retreats, leaves large trough with hanging valleys along side, can have waterfalls on them
Glacial processes: Ribbon lake
After deglaciation, water fills hollows in glacial trough, often on impermeable ground
Depositional features
Medial moraines
Terminal moraines
Lateral moraines
Recessional
Drumlins
Erratics
Till plains
Recessional moraines
Marks position where glacier gradually melted away
Terminal moraines
Debris dumped at glacier’s snout, marks maximum position of ice
Medial moraines
Produced when 2 valley glaciers meet to form one larger glacier.
Their lateral moraines merge and run down middle of larger glacier
Lateral moraines
Along edge of valley glaciers where ice meet valley wall
Erratics
Rocks transported from source region to area of different geology
What show evidence of ice movement direction
Crag and tail ; angle of long axis shows direction of movement, tail points down stream
Erratics ; match rock found to source to see where ice came from
Drumlins ; stops points upstream, lee points downstream
Drumlins
Oval mounds caused by glaciers dropping basal debris load due to friction.
Found in clusters
Till plains
Where large section of ice detaches from glacier and melts, depositing debris, forms large plain of unsolved till
Types of till
Boulder clay - blankets underlying topography
Ablation till - deposited by melting ice, usually has some evidence of deposition by running meltwater
Lodgement till - deposited under ice, usually sturctureless
