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Flashcards in Glaciated landscapes Deck (108):
1

Glacial system definition

  • A system is a set of interrelated objects comprising components and processes that are connected together to form a working unit or unified whole
  • Glaciated landscape systems are open systems which means that energy and matter can be transferred from neighbouring systems, as an input. They can also be transferred to neighbouring systems as an output
    • Inputs - including kinetic energy from wind and moving glaciers, thermal energy from the Sun and potential energy from the position of material slopes, debris, meltwater and precipitation as well as avalanches
    • Throughputs - which consist of stores, including ice, water and debris accumulations, and flows, including the movement of ice, water and debris downslope under gravity
    • Outputs - meltwater, water vapour, icebergs and debris which leave the glacier

2

System feedback in glaciated landscapes

  • When inputs = outputs, the glacier is in a state of equilibrium
  • When the inputs or outputs of a system change it responds with either positive feedback (which takes it further from its original equilibrium) or negative feedback (which returns it towards its original state until dynamic equilibrium is reached)
  • Negative feedback loops:
    • Decreased temperature → inputs > outputs → positive mass balance →  glacier advances → increase in extent of ablation area → mass balance status improves - new state of equilibrium
  • Positive feedback loops:
    • Increase in atmospheric temperature →  ice in glacier melts faster → glacier ice flows faster →  total amount of ice in glacier decreases → the glacier gets stretched out and thins

3

Energy and sediment flows through glacial systems

  • The energy available to a glaciated landscape system may be kinetic, potential or thermal. Energy enables work to be carried out by the natural processes that shape the landscape
  • The material found in a glaciated landscape is predominantly the sediment found on valley floors in upland areas as well as in glacial lowlands

4

Annual glacier mass balance

  • The glacier mass balance, or budget, is the difference between the amount of snow and ice accumulation and the amount of ablation occurring in a glacier over a one year time period
  • The majority of inputs occur towards the upper reaches of the glacier and this area, where accumulation exceeds ablation, is called the accumulation zone
  • Most of the outputs occur at lower levels where ablation exceeds accumulation, in the ablation zone.
  • The two zones are notionally divided by the equilibrium line where there is a balance between accumulation and ablation
  • There will often be seasonal variations in the budget with accumulation exceeding ablation in the winter and vice versa in the summer
  • *Note* - even when a glacier is retreat, the ice in a glacier may move forwards across the equilibrium line under gravity, so it can appear that a retreating glacier is advancing

5

The influence of wind on glaciers

  • Wind is a moving force and as such is able to carry out erosion, transportation and deposition
  • These aeolian processes contribute to the shaping of glaciated landscapes, particularly acting upon fine material previously deposited by ice or meltwater

6

The influence of precipitation on glaciers

  • Precipitation provides the main input of snow, sleet and rain so is key to the mass balance of a glacier
  • In high latitude glaciated landscape systems, precipitation totals may be extremely low such as Vostok station in Antarctica has a mean annual precipitation total of 4.5mm
  • HIgh altitude locations often have much higher totals, for example Canadian Rockies where over 600mm per annum is typical
  • Precipitation varies seasonally

7

The influence of temperature on glaciers

  • If temperatures rise above 0oC, accumulated snow and ice will start to melt and become an output of the system
  • High altitude glaciers may experience significant periods in the summer months of above zero temperatures and melting
  • In contrast, in high latitude locations, temperatures may never rise above zero and so no melting occurs → this is why ice sheets are so thick in polar regions

8

The influence of rock type on rates of glacial erosion

  • Igneous - formed by the cooling of molten magma from volcanic activity, it is crystalline and usually quite tough (e.g. granite)
  • Sedimentary - formed by sediment being compacted into rock, it has horizontal lines called bedding planes and may contain fossils (e.g. limestone)
  • Metamorphic - formed by altering existing rocks by extreme heat or pressure, it is also crystalline and very tough (e.g. slate)

9

The influence of lithology on rates of glacial erosion

  • Lithology describes the physical and chemical composition of rocks
  • Some rocks such as clay, have a weak lithology with little resistance to erosion, weathering and mass movement, as the bonds between the particles that make up the rock are relatively weak.
  • In comparison, rocks such as basalt, which are made of dense interlocking crystals, are highly resistant and are more likely to form prominent glacial landforms such as aretes and pyramidal peaks
  • Others, such as limestone, are predominantly composed of calcium carbonate. This is soluble in weak acids and so is vulnerable to decay by the chemical weathering process of carbonation, especially at low temperatures

10

The influence of structure on rates of glacial erosion

  • Structure concerns the properties of individual rock types such as jointing, bedding and faulting.
  • Primary permeability - when tiny spaces separate the mineral particles, in rocks such as chalk, absorb and store water  
  • Second permeability - water seeps into rocks, such as Carboniferous Limestone, because of its many joints. These joints are enlarged by solution.
  • The angle of dip of rocks can have a strong influence on valley side profiles.
  • Horizontally bedded strata support steep cliffs with near vertical profiles. Where strata incline, profiles tend to follow the angle of dip of the bedding planes

11

The influence of latitude on the distribution and movement of glaciers

  • Locations at high latitudes most noticeably beyond 66.5o N/S, tend to have cold, dry climates with little seasonal variation in precipitation
  • This is because there is a greater distance of atmosphere for solar radiation to travel through and it is spread over a greater area, making the radiation less effective
  • Glaciated landscapes at such latitudes tend to develop under the influence of large, relatively stable ice sheets (Greenland and Antarctica)

12

The influence of altitude on the distribution and movement of glaciers

  • Locations at high altitudes tend to have higher precipitation inputs due to orographic rainfall, but more variable temperatures and hence more summer melting
  • The air pressure at high altitudes is lower and the air molecules have less kinetic energy as they do work when they expand at higher altitudes, meaning that glaciers can form even at the equator if the altitude is high enough
  • The Pastoruri glacier in Peru lies at an altitude of 5250 m and is just 10oN of the Equator. It is small glacier with a length of 4km

13

The influence of aspect on glacier movement  

  • Aspect has a large impact on microclimates
  • If the aspect of a slope faces away from the general direction of the sun, temperatures are likely to remain below zero for longer, as less solar energy is received, and so less melting occurs.
    • The mass balance of glaciers in such locations will, therefore, tend to be positive, causing them to advance.
    • This has an impact on shaping the landscape because glaciers with a positive mass balance are more likely to be larger, with greater erosive power

14

The influence of relief on glacier movement  

  • The steeper the relief of the landscape, the greater the resultant force of gravity and the more energy a glacier will have to move downslope
  • Where air temperatures is close to zero, it can have a significant influence on the melting of snow and ice and the behaviour of glacier systems

15

The process of diagenesis

  • Glaciers form when temperatures are low enough for snow that falls in one year to remain frozen throughout the year (Snow that survives one summer is known as firn and has a density of 0.4g/cm3)
  • Fresh snow falls on top of the previous year’s snow (fresh snow consists of flakes with an open, feathery structure and a low density of about 0.05g/cm3 )
  • Each new fall of snow compresses and compacts the layer beneath, causing the air to be expelled and converting low density snow into higher density ice.
  • With further compaction by 30-40 subsequent years of snowfall, it becomes true glacier ice with a density of between 0.83 and 0.91g/cm3 which is found at depths of 100m and is blue in colour

16

Constrained glaciers

  • Valley glaciers - tongues of ice confined within valleys in mountainous regions. They are confined by valley sides, and follow the course of existing river valleys or corridors of lower ground
    • They are typically 10-30 km in length, although the in the Karakoram mountains of Pakistan are as long as 60 km
  • Cirque glaciers - small glaciers that occupy a bowl shaped hollow at the head of a glacial valley

17

Unconstrained glaciers

  • Piedmont glaciers - valley glaciers that have spilled out into lowland regions
  • Ice caps - glaciers that cover entire mountainous regions
  • Ice sheets - glaciers that can be over two miles thick and cover whole continents. They are defined as extending for over 50,000 km2, making Antarctica and Greenland the only ones (between them they contain 96% of the world’s ice)

18

Warm-based glaciers

  • High altitude locations
  • Steep relief
  • Basal temperatures at or above pressure melting point
  • Rapid rates of movement, typically 20-200m per year.
  • Not only will the rapid ice movements cause significant erosion and erosional landforms, but the ablation also produces lots of meltwater and so landforms of glaciofluvial origin are also common.

19

Cold-based glaciers

  • High latitude locations
  • Low relief
  • Basal temperatures below pressure melting point, thus frozen to the bedrock
  • Very slow rates of movement, only a few metres per year
  • Limited landscape impact due to a lack of erosion and deposition taking place  

20

Pressure melting point

  • The pressure melting point is the temperature at which ice is on the verge of melting
  • At the surface this is at 0oC, but within an ice mass it will be fractionally lowered by increasing pressure
  • Ice at pressure melting point deforms more easily than ice below it.

21

Factors that influence the movement of glaciers

  • Gravity - the fundamental cause of the movement of an ice mass
  • Gradient - the steeper the gradient of the ground surface, the faster the ice will move if other factors are excluded
  • The thickness of the ice - as this influences basal temperature and the pressure melting point
  • The internal temperatures of the ice - as this can allow movement of one area of ice relative to another.
  • The glacial budget - a positive budget causes the glacier to advance.

22

Basal sliding

Includes a range of processes by which ice moves across its bed, varies by the temperature of ice at the base

  • Slippage - where ice slides over the valley floor as the meltwater has reduced friction between the base of the glacier and the valley floor
  • In rougher areas, ice deforms around large obstacles by creep, and overcomes small obstacles by melting a tiny amount of ice at a time, moving around the obstacle as water and refreezing on the downstream side of the bump (regelation)
  • Basalsliding accounts for 45% of the movement of the Salmon glacier in Canada, but can account for as much as 90% in extreme cases

23

Internal deformation (creep)

Because a glacier is a polycrystalline mass of ice, it deforms under its on weight in a process known as 'creep'

  • Deformation includes
    • Individual crystals re-aligning themselves within the ice mass
    • Crystals melting and refreezing
    • Crystals deforming by motion of the atoms within the crystal
      • This can occur as intergranular or laminar flow
  • Together, these mechanisms allow ice to deform slowly in response to any applied force
  • Deformation occurs on a slope only
    •  When ice moves over a steep slope it is unable to deform quickly enough and so it fractures, forming crevasses - this is extending flow
    • When the gradient is reduced compressing flow occurs as the ice thickens and the following ice pushes over the slower-moving leading ice
  • The Meserve glacier in Antarctica moves only 3-4 m per year at its equilibrium and 100% of this movement is by internal deformation

24

Substrate deformation

Substrate deformation - the substrate underneath the ice deforms due to the weight of the ice, and its movement causes the ice to move too.

  • This process is affected not only by the glacier size and slope, but the material that the substrate is composed of, whether it is thawed or frozen and whether there is high water pressure within it
    • If the substrate is porous, unconsolidated and saturated, it will be weak, and subglacial deformation will be likely to occur

25

Glacial erosion

  • Glacial erosion is the wearing away of the landscape by the action of ice moving over it.
  • As glaciers erode the landscape they are expanding energy and creating a supply of debris which will later be used to create new landforms.
  • Glaciers erode the landscape in two main ways, abrasion and plucking.

26

Abrasion

Rock that has been plucked is carried along at the base of the glacier.

  • This debris embedded in the base and sides of the glacier scoured the bedrock as the glacier moves over it, causing it to be worn away.
  • Fine material will smooth the bedrock to a polished finish, while coarser debris will scratch the bedrock, leaving grooves known as striations or sometimes chips called chatter marks.
  • The process of abrasion produces a fine substance called rock flour which gives the milky appearance to glacial meltwater streams and lakes.

27

Plucking

Plucking occurs as meltwater freezes in joints in the bedrock, enabling the glacier to pull out pieces of loose rock as it moves forward.

  • Plucking is especially effective on the downstream side of well jointed protruding bedrock at the base of the glacier.
    • High pressure on the upstream side of a protrusion causes pressure melting
    • The meltwater flows into joints on the downstream side where pressure is lower and the water refreezes
    • As the glacier moves forward the rock is pulled away as it is held within the mass of the glacier.
      • The entrained material is key to the process of abrasion
    • Melting and freezing of the ice is known as regelation.
  •  

28

Factors affecting the rate of glacial erosion

  • Presence of basal debris
    • Pure ice is unable to carry out any abrasion, therefore basal debris is an essential requirement (up to a point where it produces too much friction which slows down movement)
  • Debris size and shape
    • Debris particles exert a downwards pressure proportional to their weight so larger debris will erode more effectively
    • Angular debris is also more effective as the weight is more concentrated (higher pressure)
  • Relative hardness of basal debris and bedrock
    • Abrasion is most successful with hard, resistant rock debris and weak, soft bedrock. If debris is harder than the bedrock more abrasion will take place
  • Ice thickness
    • The greater the thickness of the ice the greater the pressure, leading to an increased rate of abrasion
    • However if pressure becomes too great friction will increase reducing glacial sliding (thickness normally 100-200m)
  • Basal water pressure
    • A layer of meltwater is vital for basal sliding
    • However if the water pressure is too high it can cause uplift in the glacier, reducing pressure and friction
  • Sliding of basal ice
    • Determines whether abrasion can take place as it moves the embedded debris across the rock surface
    • The more sliding, the more potential there is to erode as more debris is passing along the rock per unit of time
  • Movement of debris to the base
    • Abrasion does not only wear away the bedrock, it also wears away the basal debris
    • Debris needs to be replenished (by glacial erosion and weathering processes) if abrasion is to remain effective
  • Removal of fine debris
    • To maintain high rates of abrasion, rock flour (fine debris) needs to be removed so that the larger particles can abrade the bedrock
    • This is mainly done by meltwater

29

Transportation

The volume of debris that glaciers carry can come from a variety of sources limited to abrasion, plucking, volcanic eruption, rockfall avalanches, debris flows and aeolian deposits.

  • Transported material is classified according to its position in the glacier.
    • Supraglacial - carried on the surface of the glacier, generally fallen from the valley sides.
    • Englacial - within the glacier, generally incorporated by crevasses and deformation.
    • Subglacial - embedded in the base of the glacier, derived from plucking and abrasion.

30

Nivation

Nivation is a glacial process that is not easily classified as erosion or weathering

  • This complex process is thought to include a combination of freeze-thaw action, solifluction, transport by running water and, possibly, chemical weathering
  • Nivation is thought to be responsible for the initial enlargement of hillside hollows and the incipient development of corries

31

Deposition

  • Glaciers deposit their load when their capacity to transport material is reduced
    • This usually occurs as a direct result of ablation during seasonal periods of retreat or during de-glaciation
    • However, material can also be deposited during advance or when the glacier becomes overloaded with debris
  • All material deposited during glaciation is known as drift
    • This can be subdivided into till, which is material deposited directly by the ice, and outwash, which is material deposited by meltwater
    • The latter is also known as glacio-fluvial material
  • It is estimated that glacial deposits currently cover about 8% of the Earth's surface
    • In Europe they cover almost 30% and are mainly material left by earlier ice masses that have since retreated
    • East Anglia has deposits that are up to 143 m thick, however in the Gulf of Alaska they each up to 5000 m thick

32

Till

  • There are two types of glacial till
    • Lodgement till - this is material deposited by advancing ice
      • Due to the downward pressure exerted by thick ice, subglacial debris may be pressed and pushed into existing valley floor material and left behind as the ice moves forward
      • This may be enhanced by localised pressure melting around individual particles that are under significant weight and pressure
      • Drumlins are the main example of landforms of this type
    • Ablation till - this is material deposited by melting ice from glaciers that are stagnant or in retreat, either temporarily during a warm period or at the end of the glacial event
      • Most glacial depositional landforms are of this type
  • Both types of till typically have three distinctive characteristics
    • Angular or sub-angular in shape - this is because it has been embedded in the ice and has not been subjected to further erosion processes, particularly by meltwater, which would make it smooth and round
      • Although it may have been altered in an earlier period of erosion by meltwater, before being entrained, transported and deposited by glaciers
    • Unsorted - when glaciers deposit material, all sizes are deposited en masse, together
      • When water deposits material, it loses energy progressively and deposits material in size-based sequence
    • Unstratified - glacial till is dropped in mounds and ridges rather than in layers, which is typical of water-borne deposits

33

Weathering

The in situ denudation of rock by physical or chemical processes

34

Frost shattering

  • Rainwater enters cracks in the rocks on the valley sides during the day.
    • As the temperature drops at night the water freezes and its volume increases by 9% which exerts pressure on the rock, creating cracks.
    • During the day the ice thaws and meltwater can then penetrate deeper into the rock.
    • As this process repeats the cracks widen and eventually pieces of rock break off to form scree which covers the slopes of glaciated valleys.

35

Dilation

  • When overlying pressure on bedrock is removed, such as when a glacier begins to melt during deglaciation, the underlying rock expands (dilates) and fractures parallel to the surface.
    • This weakens the rock and makes it more susceptible to future subglacial erosion or frost shattering.

36

Chemical weathering

  • Decay of rock is the result of chemical weathering, which involves chemical reactions between the elements of the weather and some minerals within the rock
    • It may reduce the rock to its chemical constituents or alter the chemical and mineral composition of the rock
    • Chemical weathering processes produce weak residues of different material that may then be easily removed by erosion or transportation processes
  • The rate of most chemical reactions is faster when temperature is higher, so most chemical weathering processes are most effective in warm or hot climatic conditions

37

Oxidation

  • Some minerals in rocks react with oxygen, either in the air or in water
  • Iron is especially susceptible to this process
    • It becomes soluble under extremely acidic condition and the original structure is destroyed
    • It often attacks the iron-rich cements that bind sand grains together in sandstone

38

Carbonation

  • Rainwater combines with dissolved carbon dioxide from the atmosphere to produce a weak carbonic acid
    • This reacts with calcium carbonate in rocks such as limestone to produce calcium bicarbonate, which is soluble
  • This process is reversible and precipitation of calcite happens during evaporation of calcium rich water in caves to form stalactites and stalagmites

39

Solution

  • Some salts are soluble in water
    • Other minerals, such as iron, are only soluble in very acidic water, with a pH of about 3
  • Any process by which a material dissolves in water is known as solution, although mineral specific processes, such as carbonation, can be identified

40

Hydrolysis

  • This is a chemical reaction between rock minerals and water
    • Silicates combine with water, producing secondary minerals such as clays
    • Feldspar in granite reacts with hydrogen in water to produce kaolin

41

Hydration

  • Water molecules added to rock minerals create new minerals of a larger volume
    • This happens to anydrite, forming gypsum
  • Hydration causes surface flaking in many rocks, partly because some minerals also expand by about 0.5% during the chemical change as well because they absorb water

42

Biological weathering

  • Biological weathering may consist of physical actions such as the growth of plant roots or chemical processes such as chelation by organic acids
  • Although this, arguably, does not fit with the precise definition of weathering, biological processes are usually classed as a type of weathering
    • Certainly the effects are very similar to some of the physical and chemical processes even if it may be difficult to directly relate them to the weather
  • In glacial environments, plant and animal activity may be severely limited by the low temperatures and so these mechanisms may be of very little significance

43

Biological weathering - tree roots

  • Tree roots grow in cracks or joints in rocks and exert outward pressure
    • This operates in a very similar way and with similar effects to freeze-thaw
  • When trees topple, their roots can also exert leverage on rock and soil, bringing them to the surface and exposing them to further weathering
  • Burrowing animals may have a similar effect

44

Biological weathering - organic acids

  • Organic acids produced during decomposition of plant and animal litter cause soil water to become more acidic and react with some minerals in a process called chelation
  • Blue-green algae can have a weathering effect, producing a shiny film of iron and manganese oxides on rocks

45

Mass movement

The downward movement of slope material under the influence of gravity that is not accompanied by a moving agent such as water or ice.

  • It occurs when there is a decrease in the shear strength of a slope or an increase in the shear stress acting on a slope which disrupts the slope equilibrium causing a slope failure.
  • Recently deglaciated valley sides experience mass movement because erosion has steepened them and therefore made them unstable to the point where the shear stress exceeds the shear strength of the rock sheet.

46

Rock fall

  • This occurs on slopes of 40° or more, especially if the surface is bare, when rocks may become detached from the slope by physical weathering processes.
    • These then fall to the foot of the slope under gravity.
    • Transport processes may then remove this material, or it may accumulate as a relatively straight, lower angled scree slope.

47

Slides

  • These may be linear, with movement along a straight line slip pane, such as a fault or a bedding plane between layers of rock, or rotational, with movement taking place along a curved slip plane (slumps).
  • In glaciated landscape systems, slides may occur due to a steepening or undercutting of valley sides by erosion of the base of the slope, adding to the downslope forces.
  • Slumps are common in weak rocks like clay, which also become heavier when wet, adding to downslope force.

48

Corries

Steep sided hollows in a mountain or valley formed by glacial erosion

  • Start in a sheltered, gently sloping hollow on the shady side of a mountain where snow accumulates and the process of nivation deepens the hollow further by a mixture of freeze-thaw action, chemical action and solifluction
  • Should this snow fail to melt in summer subsequent layers of snowfall will compress it into neve and ultimately ice - a corrie glacier is thus formed
  • The headwall is eroded predominantly by plucking and the floor by abrasion, deepening the corrie as the ice moves downhill under gravity. The flow of ice in the cirque glacier can be called rotational flow.
    • Weathering of exposed rock faces by frost shattering generates debris which falls onto the ice and down the headwall crevasse (bergschrund) providing material for abrasion by the glacier
    • Seasonal and daily melting allows water to flow down the bergschrund which by nivation causes disintegration of rocks at the bottom of the glacier.
    • Summer meltwater lubricates the base of the glacier.
    • As the volume of ice increases and pressure release sheeting takes place, blocks are plucked from well jointed rocks and the corrie is over deepened.
  • When the ice melts, an armchair shaped hollow with a steep-sided backwall remains. The hollow may  be occupied by a small lake (tarn, llyn or corrie lake eg Red Tarn by Helvellyn)

49

Aretes

Knife edge ridges found between two corries

  • An arete starts as a ridge between two adjacent hollows on a mountainside
  • The hollows are then deepened to form Corries and the sides of the ridge become steeper
  • The sides of the ridge lie above the ice so they are exposed to frost shattering and become sharper until they form a prominent ‘knife edge’
  • Examples of aretes include Striding edge above Red tarn.

50

Pyramidal peaks

Angular, sharply pointed mountain peaks which results from the cirque erosion due to multiple glaciers diverging from a central point

  • They often start as high altitude ground (often a nunatak above the ice)
  • The steepening of corrie headwalls on all sides together with frost shattering and subsequent mass movement on the headwalls leaves a pyramid shaped peak at the center, surrounded by steep headwalls and aretes
  • Examples of pyramidal peaks include Mount Snowdon in North Wales and the Matterhorn in Zermatt.

51

Glacial troughs

U-shaped valleys with flat floors and steep sides

  • They generally start with a pre glacial V shaped river valley
  • As the erosive power of the glacier deepens the base and sides of the valley a characteristic U shape is formed
  • A marked break of slope on the valley side generally indicates the depth of the glacier
    • This break in the slope is called a bench or shoulder
  • Post glaciation the river that originally formed the V shaped valley is relatively small compared to the size of the U shaped valley
    • This river is clearly not large enough to have eroded such a massive landform so it is called a misfit stream
    • An example of a misfit stream would be the River Duddon in the Lake district

52

Roche Moutonnees

Small bare outcrops of rock shaped by glacial erosion, with one side smooth and gently sloping and the other steep, rough, and irregular

  • Roche Moutonnees are formed when a lump of bedrock protrudes above the underlying rock
  • As the glacier moves over it, on the Stoss side (uphill) abrasion takes place, forming a smooth and polished surface and striations
  • Pressure builds up on the Stoss end, leading to pressure melting
  • The water flows downhill through joints and bedding planes to the Lee side of the Roche Moutonne where the pressure is lower so the water refreezes in the joints and bedding planes
  • Plucking then takes place, leaving the Lee side with its characteristic jagged and steep side

53

Hanging valleys

A valley which is cut across by a deeper valley or cliff

  • Before glaciation tributary streams fed the main river channel
  • These rivers formed their own tributary valleys
  • However during glaciation the main valley would have been over deepened
  • After this glacier melts this tributary valley is left ‘hanging’ above the main trough, with its stream cascading as a waterfall above the edge
  • Examples of hanging valleys include the valley formed by the Corrie Glacier that formed Cwm Idwal.

54

Truncated spurs

Ridges that descend towards a valley floor or coastline from a higher elevation, ending in an inverted-V face

  • Before glaciation, truncated spurs were the interlocking spurs of the river valley
  • The glacier, being more powerful and less flexible than the river truncates them, leaving a line of steep cliffs
  • The southern slope of Blencathra in the Lake district, as well as the North side of the Nant Ffrancon Valley hold examples of truncated spurs

55

Ribbon lakes

A long and narrow, finger-shaped lake, usually found in a glacial trough

  • They require a glacial trough to be blocked at one end for example by a moraine, to create a natural dam that allows long thin lakes such as Windermere to form behind it
  • Many Ribbon lakes today such as the one that used to cover the base of the Nant Ffrancon Valley have been filled in by progressive sedimentation as weathered debris accumulates in the lake from surrounding slopes
  • Ribbon lakes normally formed on impermeable bedrock and the hollow that they are formed in may have been deepened due to weaker lithology in that area

56

Striations

Rock that has been plucked is carried along at the base of the glacier, scouring the glacier. Coarser debris will scratch the bedrock, leaving the bedrock with grooves known as striations.

57

Ellipsoidal basins

Major erosional landforms created by ice sheets. There are many in North America created by the Laurentide ice sheets.

58

Moraines

  • Moraines are ridges or humps of glacial till which can be either terminal (marking the farthest position of the glacier's snout), lateral (along the side of the valley) or medial (running along the centre of the valley and formed by the confluence of two glaciers)
    • Additionally recessional moraines are deposited as the snout of the glacier retreats
  • Examples of Moraines in the UK include the Escrick and York Moraines, deposited at the end of the Devensian period
    • Escrick found 6 miles south of York, forms a long ridge of laminated clays
    • York moraine runs south-east towards Stamford Bridge, composed of sands, clays and gravels

59

Terminal moraines

  • A terminal moraine is a ridge of till extending a cross a glacial trough
    • They are usually steeper on the up-valley side and tend to be crescent shaped, reaching further down-valley in the centre
    • These landforms mark the position of the maximum advance of the ice sheet and were deposited at the glacier’s snout
    • Their crescent shape is due to the position of the snout; further advance occurs at the centre of the glacier, as there is no friction with the valley sides
    • The steeper up-valley side is the result of the ice behind supporting the deposits and making them less likely to collapse
    • The Franz Josef glacier in New Zealand has left a terminal moraine 430 m high

 

60

Lateral moraines

  • A lateral moraine is a ridge of till running along the edge of a glacial valley
    • The material accumulates on top of the glacier having been weathered from the exposed valley sides
    • As the glacier melts or retreats, this material sinks through the ice to the ground and is deposited
    • A lateral moraine left by the retreating Athabasca glacier in Canada is 1.5 km long and 124 m high

 

61

Recessional moraines

  • Recessional moraines are a series of ridges running transversely across glacial troughs and which are broadly parallel to each other and to the terminal moraine
    • They are found further up the valley than the terminal moraine
    • They form during a temporary still-stand in retreat
    • These temporary pauses are rarely prolonged; thus recessional moraines seldom exceed 100 m in height

 

62

Drumlins

  • They are streamlined hills of glacial till, usually around 50 m height and 500m long, formed under a fast moving ice sheet
  • They have a long axis indicating the direction of flow of the glacier, a steep upstream (stoss) end and a narrower tail facing downslope (lee slope).
  • They often occur in groups called swarms, for example in the vale of Eden, Cumbria, forming a ‘basket of eggs topography’
  • The drumlin would have been deposited when the glacier became overloaded with sediment

63

Erratics

  • Erratics are large rocks carried within ice sheets
  • They differ from the type of rock generally found in that area and are often found hundreds of miles from their source
  • They are generally stripped from the landscape by plucking or have fallen from the sides of a valley due to freeze thaw action
  • These erratics are carried by the ice sheet for thousands of miles, they are then deposited either at the snout of the Ice sheet or when the sheet melts
  • For example Norber erratics in the Yorkshire dales are blocks of Silurian shale perched on top of younger Carboniferous limestone plinths
    • They were deposited by an ice sheet towards the end of the Devensian period
    • They have protected the limestone pavement from carbonation whilst the surrounding limestone has been removed by carbonation for the last 18,000 years at a rate of 25mm per 1000 years.

64

Till sheets

  • Glacial till sheets are a uniform blanket of glacial deposits in a low lying area
  • They generally cover areas at the margins of former ice sheets
  • The bedrock surface during glaciation may have been highly eroded but it is concealed by a thick cover of till, sands and gravels deposited by the glacier
  • Glaciation often leads to confused surface geology as the deposited till may have been carried from hundreds of miles away
  • Tills are unsorted, unconsolidated and unstratified

65

Snowdonia - location

  • Location: Snowdonia is in the North West of Wales, just south east of the Isle of Anglesey and the Irish sea
  • The closest town, Bangor, is 10 km away
  • It has an approximate latitude and longitude of 53 degrees North, 4 degrees West
  • The location of Snowdonia is intriguing as it is one of the more southerly examples of Glaciation in the UK
  • Snowdonia was able to sustain glaciers all the way until the Loch Lomond stadial 10,000 years ago due to its relatively high altitude, containing the highest peak in England and Wales
  • Additionally its position on the West coast of and its subsequent maritime climate increased precipitation, fuelling glaciation when temperatures were cool enough.

66

Snowdonia - brief glacial history

  • The Glacial landforms seen in Snowdonia today are a product of glacial events Glaciers occupied in the Quaternary period
  • During the most recent Devensian period North wales would have been covered by an ice cap
  • The firnline lay below 300 metres, and 2000 km2 of North Wales was an accumulation zone
  • The Welsh Ice sheet centred in the Merioneth region and would have buried all but the largest peaks which protruded out of the ice (nuntaks)
  • The Welsh ice sheet was at its maximum development around 18,000 years ago
  • However glaciers only disappeared from deep recesses 10,000 years agocirqu
  • Once the main ice sheet melted the Ogwen Valley would have had three main valley glaciers which were fed by many smaller Corrie glaciers

67

Snowdonia - moraines

  • In Cwm Idwal several types of moraines are present
  • Terminal Moraines can be found at the mouth of the corrie and lake, just before the land falls down to the Nant Ffrancon valley below
  • Hummocky Moraines can be found on the west and east sides of Llyn Idwal
  • Lateral Moraines also line the sides of the corrie, parallel to the flow of the cirque glacier that previously filled the cwm
    • These lateral Moraines are 300 metres longs and have a classic triangular cross section

68

Snowdonia - corrie

  • Cwm Idwal is located in the Glyderau range in Snowdonia
  • It has an unusual, linear shape for a Corrie 1.5km in length, 1km wide
  • It has a considerably lower altitude than the other Corries in the range (375m)
  • The Cwm’s orientation is 038°, the floor lithology is predominantly tuff whilst the wall lithology is comprised of tuff and basalt.

69

Snowdonia - glacial trough

  • The Nant Ffrancon Valley is a perfect example of a glacial trough
  • It was carved by a glacier moving Northwest towards Anglesey
  • There is a notable step in the valley floor, with a drop of 100m in elevation at the Waterfall Rhaeadr Ogwen
  • On its sides are a number of a number of truncated spurs, which have left vertical crags such as Clogwyn y Gribin

70

Snowdonia - roche moutonnee

  • In the Nant Ffrancon Valley a number of roche moutonnees were left behind after the last glacial period
  • An example of a roche moutonnee in the valley is one that can be seen just above Rhaeadr Ogwen

71

Snowdonia - ribbon lake

  • Llyn Ogwen is a ribbon lake lying in the ogwen valley
  • It lies within a glacial hollow
  • This deepening was caused by a number of factors, including the weakness of the lithology, the thickness of the ice and the gradient of the valley.

72

Snowdonia - pyramidal peak

  • Snowdon Yr Wyddfa is a pyramidal peak made up by five main ridges
  • 10 cirques can be seen on the flanks of Snowdon
  • The two largest cirques, Clogwyn and Du’r Arddu face North West
  • The Pyramidal peak is formed where the Arêtes (Crib Goch, Y Lliwedd, Cwm Brwynog and Crib y Ddysgl) meet
  • On the East face of Snowdon, between Crib Goch and the Y Lliwedd arête are two llyns, within two separate corries, Glaslyn and Llydaw

73

Snowdonia - arete

  • Crib Goch, a famous arete on Snowdon leads directly to the summit
  • The arête was formed due to the three Corries, Cwm Gaslyn and Cwm Glas and Cwm Uchaf. The Glacier within Cwm Gaslyn flowed approximately east to west, whilst the two cirque glaciers flowing from Cwm Glas and Cwm Uchaf flowed perpendicular to Cwm Gaslyn, from the south to the North
  • As Cwm Gaslyn was next to Cwms Glas and Uchaf, between them an Arete, made up of Tuff was formed

74

Snowdonia - location influencing glaciation

  • The Location of Snowdon, and its relatively large size has considerably affected its formation
    • Due to its wide base (80km2) it was not overrun by the ice sheet
    • This means that it was predominantly shaped by alpine glaciation, leading to the formation of it’s distinct summit pyramid, many corries and steep ridges
  • The location of Cwm Idwal has also affected its formation as the cwm is considerably deeper than other cwms in the Glyderau range (the altitude of its floor is 375m, compared to Cwm Clyd (660m))
    • This is because Cwm Idwal lies at the lowest part of the Glyderau range, this meant that it was low enough to allow ice to spill over from the neighboring Llanberis Pass, increasing the volume of ice flowing into the Cwm
    • This was further accentuated by the rotational flow of the Cneiford Icefall

75

Snowdonia - aspect influencing glaciation

  • The aspect of the Corries surrounding Snowdon have significantly affected the mountain’s form
  • Seven of its cirques have an easterly aspect, these cirques are less pronounced and deep
  • Its three remaining cirques are larger, with Clogwyn and Du’r Arddu, the two largest and most pronounced cirques on the mountain facing North-west

76

Snowdonia - lithology affecting glaciation

  • The lithology of Snowdon is predominantly made up of tuff
  • This hard rock is more likely to be shaped into the dramatic features that characterise Snowdon’s shape, as the hardness of the rock allows narrower aretes to be formed
  • The lithology of the Ogwen valley actually led to its over deepening, the valley is predominantly made up of mudstone and siltstone
  • However this was one of a number of factors that enabled the formation of Llyn Ogwen, other causes of the overdeepening include large numbers of tributary glaciers and compressional flow
  • The Lithology of the Nant Ffrancon Valley has also shaped its formation
  • There is a noticeable step in the valley, with a drop of around 100m at the waterfall Rhaeadr Ogwen, this is because the rock in this part of the valley is especially resistant to erosion
  • The Lithology of Cwm Idwal also allowed it to be deeper than its neighboring cwms as the cwm is located on a line of geological weakness called a syncline

77

Snowdonia - landform interreleationships

  • The Moraines deposited in Cwm Idwal, Cwm Idwal itself and the Nant Ffrancon Valley are all closely related
    • The Moraines were deposited by the same glacier that formed Cwm Idwal
  • The Moraines also dam a further landform in the cwm - Llyn Idwal - which was also formed by the same cirque glacier
  • Cwm Idwal is also closely related to the Nant Ffrancon Valley as it forms a hanging valley, adjacent to the Nant Ffrancon Valley
  • Streams coming from Cwm Idwal contributed to the sedimentation of Llyn Ogwen in the Nant Ffrancon
  • Additionally, ice from Cwm Idwal would have eventually reached the main Nant Ffrancon Valley during glaciation 18,000 years ago, so the same ice that deepened Cwm Idwal would have also helped to form the main Nant Ffrancon Glacial Trough
  • The Glacier that flowed down the Nant Ffrancon also formed the roche moutonnees found in the valley
  • Finally, both the Pyramidal peak, Snowdon Yr Wyddfa and the arete, Crib Goch are closely related
    • The Pyramidal peak of Snowdon is actually comprised of Crib Goch as well as four other Aretes
    • Additionally the cirque glacier that formed Cwm Gaslyn on the flanks of Snowdon was also important in the formation of Crib Goch as it flowed perpendicular to two Cwm Glas and Uchaf, leading to the formation of a sharp arete

78

Snowdonia - weathering over time

  • The Moraines deposited in Cwm Idwal have gradually been weathered over time by Freeze thaw weathering, they have been worn down over the last 10,000 years
  • Directly after glaciation, the features of Snowdon were considerably less pronounced
  • Over the last 10,000 years the predominant processes that have shaped Snowdon are weathering and mass movement, predominantly freezethaw and rock fall
  • These processes have led to Aretes becoming narrower and the back walls of Corrie's such as Cwm Gaslyn becoming steeper
  • For example the arete on Snowdon, Crib Goch has been steepened considerably, leading to the formation of large scree slopes of rockfall from the arete forming on its flanks
  • The sides of the Nant Ffrancon Valley are also littered with scree slopes caused by weathering post glaciation
  • After Glaciation the back wall of Cwm Idwal was also affected by weathering processes, especially on the back wall around the Devils Kitchen (Twll Ddu) and the Idwal slabs, especially steep parts of the back wall.

79

Snowdonia - sedimentation over time

  • In the Nant Ffrancon Valley, it is suspected that at some time after the last glacial period, there was a ribbon lake present in the valley
  • This is evident due to the Valleys flat floor. Over thousands of years, this lake was filled by sediments from rivers flowing from the neighboring mountains, as well as Cwms such as Cwm Idwal

80

Snowdonia - filling of ribbon lakes over time

  • The Ribbon Lake Llyn Ogwen, despite being enabled by erosion during the last glacial period, is actually a post glacial feature
  • Following glaciation, the over deepened Ogwen glacial trough was filled by meltwater and rainfall

81

Minnesota - introduction

  • Over the last 2 million years (the Quaternary period) the vast Laurentide ice sheet up to 4km deep covered much of Canada and Northern USA
  • It advanced and retreated many times due to changes in the climate, and transformed the landscape as it did so
  • The most recent advance of the Laurentide ice sheet started around 100,000 years ago and was called the Wisconsin glaciation

82

Minnesota - geology

  • Although situated in northern USA, the landscape of Minnesota is part of the Laurentian Shield
    • Minnesota's oldest rocks lie in alternating belts in the northern half of the state and much of thr Minnesota River Valley
    • The belts are of volcanic and sedimentary rocks; granitic rock materials lie in the areas between the belts
  • Whilst the majority of the North of Minnesota is made of alternating bands of granite (igneous) and gneiss (metamorphic) rock, there is an area towards the Arrowhead region with early Proterozoic sedimentary rock underlying it which had been exposed by previous tectonic tilting of the landscape, meaning that these shales were much less resistant to erosion than the surrounding volcanic rock and there is now a large ellipsoidal basin in the Arrowhead region which contains lake Vermilion

83

Minnesota - glaciation

  • During the Wisconsin glaciation four ice lobes ( Des Moines, Wadena, Rainy & Superior) from the Laurentide ice sheet advanced and retreated a number of times across Minnesota
  • These lobes transported large quantities of glacial till over across the state and the different origins of lobes resulted in tills with different characteristics and material, and therefore it was possible to determine where each lobe flowed
  • Striations preserved in outcrops of gneiss and greenstone also indicate the former direction of flow of the ice lobes.

84

Minnesota - erosional impact

  • The massive erosional impact of the Laurentide ice sheet wore down many of the mountains in Minnesota and the highest peaks are now only between 500-700m with Eagle Mountain the highest at 701m
  • The erosive action of the ice sheet lowered the landscape so much in some parts than an ellipsoidal basin was formed
    • This basin now contains thousands of lakes such as the Upper and Lower Red lakes
  • As the lobes of ice advanced they abraded striations in bare rock outcrops of gneiss and greenstone, their alignment indicating the direction of ice advance
  • The far southeast of the state was not extensively covered by the ice sheet and so retains a more varied landscape of steep hills and deep valleys
    • Most of the ribers draining this area are tributaries of the Minnesota and Mississippi Rivers
  • The erosional impact of the Laurentide ice sheet was therefore considerable, however continental ice sheet erosion does not produce the spectacular landforms associated with valley glaciers and alpine glaciation

85

Minnesota - depositional impact

  • The Wadena Lobe advanced from the Hudson Bay area in the North to just south of Minneapolis, depositing a till sheet with reddish iron-rich sediments characteristic of the red sandstone and shales it was derived from
    • It formed the Wadena Drumlin Field which spans Wadena, Otter Tail and Todd counties and the drumlins have long axes trending southwest
    • It also deposited the Alexandria moraine and formed a set of terminal moraines which extend from northwest St Cloud into the Twin Cities

 

  • The Rainy and Superior Lobes advanced from the northwest as far as Minneapolis and left behind a coarse-textured till with fragments of basalts, gabbro, granite, red sandstone, slate and greenstone strewn across the northwest of Minnesota as far as the Twin Cities

 

  • The Des Moines Lobe advanced from the east till which is tan to buff coloured and clay-rich and calcareous, which reflects the shale and limestone it passed over
    • Part of the sheet in southwest Minnesota is over 160m deep
    • In the south there is a good example of an end moraine ate Prairie Coteau

86

Minnesota - proglacial lakes

  • As glaciers retreat, large amounts of meltwater are released which may be trapped, either by an ice or moranic dam
  • This results in the formation of large features known as proglacial lakes
  • The edge of the Laurentide ice sheet and its associated lobes dammed the natural drainage of the area, blocking the meltwater from its natural northward passage to the sea, creating a number of pro-glacial lakes, the largest of which was Lake Agassiz

 

  • At its largest, Lake Agassiz covered 440,000km2 and in places it was 400m in depth
  • The water from the lake overflowed at Brown’s Valley and as it did, it cut a deep spillway channel known as the Traverse Gap
  • The river that drained Lake Agassiz was called Glacial River Warren and formed a wide valley in which the present day Minnesota River flows
  • Warren was 300ft deep and several miles wide, and left behind fertile silt deposits in what is now the red river valley
  • The lake Agassiz led to the formation of the wide river valley by providing vast quantities of water to cut through the land

 

  • Modern day lakes such as Lake Vermilion (25 km across) and the Upper and Lower Red Lakes (both 39km at longest axes)  still exist in the ellipsoidal basin in the Arrowhead region of Minnesota
  • Are over 11,900 lakes larger than 10 acres in size in the area.

87

Minnesota - changes over time

  • Some landforms, such as drumlins and terminal moraines, have been eroded over time by processes such as rockfall and frost shattering as well as by human erosion by footpaths cutting over them
  • Lakes may change as the climate does, growing or shrinking depending on quantities of precipitation
  • Lakes may also become shallower due to silt being deposited in them by the rivers that lead into them
  • Valley such as the Red River Valley continue to be eroded by the misfit rivers that now exist in them.

88

Glacio-fluvial transportation and deposition

  • Glacio-fluvial is a term which refers to the processes, sediments and landforms produced by water flowing on, in and/or under glaciers and away from glacier snouts
    • Consequently, glacio-fluvial environments may occur in supraglacial, englacial, ice-marginal and proglacial locations of alpine and continental glaciers
  • Proglacial glacio-fluvial environments are often transitional to fluvial environments when the processes, sediments and landforms become dominated by non-glacial tributary inflows rather than the annual rhythm of melting ice
  • In glacio-fluvial environments, material is transported as load and deposited on the bed when there is a decrease in the carrying capacity of the stream

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Outwash plains

A flat expanse of sediment in the pro-glacial area

  • As meltwater streams gradually lose energy as they enter lowland areas beyond the ice front, they deposit their load
  • The largest material is deposited nearest the ice front and the finest further away
    • Outwash plains are typically drained by beaded streams
    • These are river channels subdivided by numerous islets and channels.
  • Debris-laden braided streams lose water at the end of the melting period and so carry less material
    • This material is deposited in the channel, causing it to divide.
  • Braiding begins with a mid-channel bar which grows downstream
    • Discharge decreases after a flood or a period of snow melt, causing the coarsest particles in the load to be deposited first
  • As discharge continues to decrease, finer material is then added to the bar, increasing its size
  • When exposed at times of low discharge, channel bars are stabilised by vegetations and become more permanent features
  • The river divides around the island and then re-joins
  • Unvegetated bars lack stability and often move, form and reform with successive flood or high-discharge events
    • Seen in the jokulhlaup of E15 in 2010, which had an estimated discharge of over 2500 cumecs through the Markafljot valley, depositing over 200,000 tonnes of sediment (even moving a 17 metre boulder weighing 4000 tons)

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Eskers

Long, sinuous ridges composed of stratified sand and gravel laid down by glacial meltwater

  • Material is deposited in subglacial tunnels as the supply of meltwater decreases at the end of the glacial period
  • Sub-glacial streams may carry huge amounts of debris under pressure in confined tunnels at the base of the glacier
  • Some argue that deposition occurs when the pressure is released and meltwater emerges at the glacier snout
  • As the glacier snout retreats, the point of deposition will gradually move backwards - a potential explanation for why some eskers are beaded
  • However others argue that the beads are simply the result of the greater load carried by summer meltwater
  • Example - the Trim esker near Dublin is one of a group of twelve in the area - it is 14.5km long and between 4 and 15m high

91

Kames

Hills composed of stratified and sorted gravel laid down by glacial meltwater

  • Delta Kames
    • Some are formed by englacial streams emerging at the snout of the glacier
      • They lose energy at the base of the glacier and deposit their load
    • Others are the result of supraglacial streams depositing material on entering ice-marginal lakes, losing energy as they enter the static body of water.
    • Some also form as debris filled crevasses collapse during ice retreat
    • Example - Kames are widespread in East Lothian, Scotland

 

  • Kame Terraces - ridges of material running along the edge of the valley floor
    • Supraglacial streams on the edge of the glacier pick up and carry lateral moraine which is later deposited on the valley floor as the glacier retreats
    • The streams form due to the melting of ice warmed in contact with the valley sides as a result of friction and the heat-retaining properties of the valley-side slopes
      • Although they may look like moraines, they are composed of glacio-fluvial deposits which have been rounded and sorted
    • Example - Kingsdale valley of the Yorkshire Valley, a kame terrace 2km in length

92

Kettle holes

A hollow created when buried blocks of glacier ice melt out

  • A kettle hole is formed by blocks of ice that are separated from the main glacier - perhaps the ice front stagnated or retreated or perhaps ice blocks were washed out from the glacier during a glacier flood or jökulhaup
  • If conditions are right, the isolated blocks of ice then become partially or wholly buried in outwash
  • When the ice blocks eventually melt they leave behind holes or depressions that fill with water to become kettle hole lakes
  • In freshly deglaciated areas, such as along the south coast of Iceland, kettles form obvious small lakes in the outwash plains
  • In Scotland, they may be preserved as isolated small lakes, or deep water-filled depressions in boggy areas that were once the low-lying outwash plains
  • Many kettles have been infilled with sediments, especially peat, during the Holocene period
  • There are kettle holes 5 metres in diameter and 2 metres deep in the Solheimajokull outwash area

93

Modification of glacio-fluvial landform over time

  • Repeated advance and retreat modify and alter the appearance of landforms
  • The landforms are also subject to weathering, erosion and colonisation by vegetation in postglacial times.
  • As temperatures continue to rise, further melting and retreat of glaciers results in the production of more meltwater and thus a greater expanse and accumulation of outwash material in pro-glacial zone.
  • Kames and eskers will be exposed in greater number and of greater length during this continued retreat
    • Solheimajokull is currently retreating at a rate of 100m/year
  • As temperatures increase, so does the growing season for vegetation
    • Exposed outwash material tends to become colourised over time, first by mosses and lichens and then by grasses, flowering plants and shrubs
  • Outwash plains are often subject to vegetation build-up, and when the river has dried up they can be used for farming due to their fertile deposits
  • Whilst eskers may have vegetation grow on them, they also become less pronounced due to weathering and soil creep, as well as often having roads and paths built on them

94

Frost heave

  • The upward dislocation of soil and rocks by the freezing and expansion of soil water
  • Frost push occurs when cold penetrates into the ground
    • Large stones become chilled more rapidly that the soil
    • Water below such stones freezes and expands, pushing up the stones
  • Frost pull can then alter the orientation of a large stone causing it to stand upright
    • This occurs when stones become frozen to the ground above, pulling them up as it expands
    • It only happens when ice creeps downwards from the surface
    • The growth of ice crystals on the upper part and the drying of the soil around the lower part cause the stone to be pulled into a more vertical inclination

95

Periglacial weathering

  • Repeated cycles of frost-shattering result in the iconic angular and fractured rocks seen in periglacial landscapes
  • Up to a point, the slower the rate of cooling causes the greatest quantity of fracturing stress

96

Patterned ground

  • There are 5 different forms of patterned ground - circles, nets, steps, stripes and polygons
  • First, stones are brought upwards by frost pull whilst at the same time frost push occurs as the finer material beneath them becomes frozen
  • The stone remains uplifted when the thaw sets in as fine sediments move in and take the place of the ice
  • Repeated processes bring the stones to the surface, where frost thrust (lateral movement of ground material) causes the stones to move outwards via frost heave (vertical movement)
    • This forms circles which provide the basis for all other patterns
  • The up doming of the ground causes larger stones to roll outwards under the effect of gravity, while finer sediment remains central and raised in the circle

97

Pingos

  • Dome shaped hills that are commonly up to 500m in diameter and up to 50m in height
  • They are characterised by permafrost and a seasonally changing active layer
  • At the core of the pingo is an ice lens of varying size, and the surface layer is made of soil often topped with vegetation
  • The surface can also contain cracks as a result of ground swelling
  • There are two types of pingo, open and closed systems:

 

  • Closed System Pingos: typical in low lying areas where permanent and continuous permafrost can be found
    • Over winter, groundwater in the sediment beneath thermokarst lakes (within the talik) can be trapped by ice from the lake's surface as the lake freezes, and permafrost advances through the ground
    • This decrease in temperature causes this groundwater to freeze into an ice lens, which grows over time as water freezes to the ice lens due to the increase in hydrostatic pressure
    • This causes the sediment above to bulge upwards into the characteristic pingo shape and may even cause dilation cracks

 

  • Open System Pingos: Occur in areas of discontinuous permafrost where there are interspersed areas of permafrost and talik
    • The active layer continually freezes and melts year on year above the permafrost layer and talik
    • Over winter, as the active layer freezes down, water can become trapped between the descending freezing plane of the active layer and the permafrost that surrounds it
    • This promotes the growth of an ice lens which pushes the land up above it as it expands
    • Water underneath the permafrost can move through the talik between the permafrost areas because of capillary action (the movement of water through the soil) and hydraulic pressure
    • This water migrates to the ice lens and freezes, swelling the ground above further and creating dilation cracks

98

Modification of periglacial landforms over time

  • Thawing Permafrost: Thawing permafrost can cause ground slumping as the volume of the ground decreases as it contracts due to the melting of the ice
  • The ground will then collapse in on itself
  • This can cause landforms such as thermokarst lakes and creates an uneven, hummocky landscape
  • Patterned ground is likely to be covered in vegetation, with the patterns becoming less distinct due to erosion and weathering whilst the rocks are broken up more
  • Pingos over time may have vegetation grow on them, as well as collapsing into ohnips which can become habitats for badgers

99

Alaska - area and reasons for drilling

  • Prudhoe Bay – Northern Alaska, vast oil deposits found in 1968
  • Extracted in Prudhoe Bay, transported along the Trans Alaskan Pipeline which is 1,300km long and transports up to 1.4 million barrels per day
  • Prudhoe Bay has reserves of 3000 million barrels
  • USA needs oil – consumed 6.95 billion barrels in 2014 and demand has risen since 2013 (after fall during the recession of 2008)
    • 40% of its oil is from imports which is a concern for the US government due to the imbalance of trade and the political implications of not having energy security
  • Exploration of oil fields in Alaska is permitted by the government and there is a 95% chance of finding over 16 billion barrels of oil in Area 1002 of the Arctic

100

Alaska - impacts on periglacial system

  • Material flows affected by use of gravel pads – gravel is extracted from stream and river beds and used as an insulating base layer for road construction. The loss of gravel from the river system alters the rate at which gravel is transported and deposited downstream. It also affects the equilibrium between erosional and depositional processes in the river system.
  • Hydrological processes are also affected - in a glacial outwash aquifer near Palmer, groundwater levels fell by more than 1m in an area extending over 2km from the site of extraction
  • Energy flows are affected by the release and burning of gas during drilling. Some gases are burnt in a process called flaring, which releases mainly carbon dioxide into the atmosphere. Others, including methane, are vented without burning into the atmosphere. Both carbon dioxide and methane are significant greenhouse gases that contribute to an enhanced greenhouse effect, with higher levels of terrestrial radiation being trapped in the lower atmosphere, raising temperatures.
  • Energy flows are also affected by the production of heat from the extraction and transportation of oil, as well as from the associated infrastructure. An urban heat island in Barrow, Alaska meant that temperatures were on average 2.2°C higher than in the surrounding rural area and a maximum difference of 6°C was measured on a very calm day. Heat from domestic heating systems in poorly insulated buildings is a big contributor to the heat island effect, and there was a strong correlation between temperature difference and oil production rates in the nearby oilfield. Energy released into the environment by human activities also affects geomorphic processes with 9% fewer days of temperature fluctuation around 0°C since recorded drilling began

101

Alaska - effect of changes on periglacial landscape

  • The heat released by building and infrastructure can lead to the thawing of permafrost and a longer period of melting of the active layer. If the building itself is constructed directly onto the ground surface, some of the heat produced by heating systems may be transferred through the floor to the ground, melting the permafrost.
    • This can result in subsidence and increase the mobility of the active layer, allowing a type of mass movement called solifluction to take place. The downslope movement of the thawed active layer results in the formation of solifluction lobes, tongues of debris, at the base of slopes when the moving material loses energy on a lower gradient.

102

Alaska - consequences of changes

  • Thermokarst is a landscape dominated by surface depressions due to the thawing of ground ice.
  • It is typified by extensive areas of hummocky ground interspersed with waterlogged hollows.
  • Depressions may fill with water to form shallow that lakes, usually less than 5m deep and 1-2km wide.
  • On a larger scale, alases are flat-floored, steep sided depressions ranging from 5 to 50m in depth and 100m to 15km in length.
    • They develop from widespread thawing of ground ice causing large-scale subsidence. Again, these depressions may contain lakes.
    • Where several alases combine, alas valleys may form, which may be many tens of kilometres in length.

103

Grand Dixence - background

  • Grande Dixence Dam is the highest gravity dam in the world
  • It was constructed in the 1960s to provide Switzerland with hydroelectric power and has undergone numerous additions and alterations, most recently in 2010
  • The total costs of the time were approximately 1600 million Swiss Francs.

104

Grand Dixence - the dam

  • The dam itself is 285 metres high and each year it stores over 400 million m3 of water
  • The dam is 200 metres wide at its base and just 15 metres wide at the top
  • To make the foundation soil watertight there is a deep grout curtain, which surrounds the dam extending onto each side of the valley
  • Aggregates for the dam were obtained locally from deposits of moraines in adjacent valleys. It has a catchment area of just over 350 km2, half of which is from 35 glaciers that provide seasonal meltwater
  • The Lac des Dix was created behind the dam and four pumping stations send water through 100 km of tunnels into the reservoir
    • One of its key aims is to optimise the water level so that there is maximum availability before heavy demand periods
    • It is able to maximise profits by offsetting the cost of pumping water into the Lac des Dix in the summer against income from generating energy during the winter

105

Grand Dixence - energy

  • The water stored behind the Grande Dixence dam drives the turbines in four power stations, including those at Fionnay and Nendaz, with a combined capacity of 2000 GWh annually - enough to power 400,000 Swiss households
  • The Grande Dixence operates by storing glacial meltwater during the summer and then using it to generate electricity during the high demand period of the winter
  • About one-third of Swiss electricity comes from storage power stations and another quarter from run-of-the-river schemes
  • The Swiss government has long had a strong environmentally aware energy policy
    • Less than 5 per cent of electricity production comes from fossil fuels, with nuclear providing the rest.

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Grand Dixence - impact on the environment

  • The environmental impact of the project has been minimised, partly to ensure that the area remains an attractive environment for walkers, cyclists and hikers.
  • The pumping stations and power plants are largely built underground or are well concealed to retain the aesthetics of the location. Indeed, tourism to the locality has been enhanced and there are now guided tours and helicopter rides above the dam available to visitors.
  • The reduced flow in the Borgne River, a tributary of the Rhone, below the dam has resulted in higher concentrations of pollutants at Les Haudere, from both agricultural and domestic sources.

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Grand Dixence - impact on the glacial system

  • Of the water available at Grande Dixence, 85 percent is used mainly for electricity generation (a small amount is also used to fulfil a demand for electricity caused by tourism in summer). The other 15 per cent is used to deal with the problems of sedimentation. When water is stored behind the dam, the lack of flow means a loss in energy and the deposition of sediment load behind the dam at rates of 20-40cm/year.
  • Sediments concentrations are > 300mg/l above the dam, 20-50 mg/l just below the dam, and <20mg/l 2km downstream of the dam.
  • To solve this problem, some of the water in the reservoir is used to purge the sediment - flushing it out and moving it downstream. At these times the water has high levels of turbidity and sediment concentrations of up 20,000 mg/l.

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Grand Dixence - impact on river channels

  • The trapping of sediment behind the dam leads to very clear water being returned into the natural river channels below the power stations. This has excess energy as none is being used to transport sediment and results in increased channel erosion.
  • The lack of discharge in the below-dam rivers means that some virtually dry up in the summer. Meanwhile there has been significant contraction in the size of the channels, and the scale of contraction increases with distance downstream. The amount of sediment eventually flowing into Lake Geneva has halved since the construction of the dam.
  • In the Val d’Herens, however, there is a risk of sudden and unexpected flooding when excess stored water has to be released. This has hindered both tourist use and development along the valley floor, although the local residents receive significant revenues from it and so are still strongly in favour of the scheme.