ICE Flashcards
1
Q
During the last ice age what was sea level relative to today?
A
In the last ice age sea level was 120m lower than today.
2
Q
Changes in the seasonal and latitudinal patterns of insolation can be explained how?
A
Using gravitational effects of Milankovitch theory:
Eccentricity
Obliquity
Precession
3
Q
Explain what is eccentricity?
A
- How elliptical is the orbit?
- Change in orbit from elliptical to circular
- 100,000 years cyclicity
4
Q
Explain what is obliquity?
A
- How titled is the Earths’ axis of rotation?
- Change in angle relative to the celestial plane
- Varies from 21.8 to 24.4°, currently 23°
- 40,000 years
5
Q
Explain what is precession
A
- How close to the perihelion is the solstice?
- Perihelion is the point where the Earth is closest to the Sun
- Solstice is the longest or shortest day of the year
- 21,000 years
6
Q
An increase in albedo by increase in ice cover will do what?
A
create a positive feeback mechanism, and a continued decrease in T
7
Q
What allows us to infer past climates?
A
Isotope analysis from ice cores:
Ice sheets are O-16 rich and ocean sediments are O-18 rich
(Refer to climate)
8
Q
At what age to ice cores stop?
A
~800,000 yrs
9
Q
What does sea floor sediment analysis show happened at around 800,000 yrs ago?
A
A transition from dominance of Obliquity to dominance of Eccentricity
10
Q
What’s the difference between how glaciology has been and is now being understood?
A
- Was originally Geologically based before becoming Geographical and then finally Mathematical
- Becomes more quantitative with the introduction of classical mechanics into glaciology
11
Q
How is ice deformed?
A
- Ice is a solid but will deform if large stresses are applied over large periods of time via three mechanisms
- Ice deformation (happens all the time)
- Basal Sliding (needs water at glacier base)
- Deformation of basal sediments (needs water at glacier base)
12
Q
How is mathematics applied to ice flow mechanisms?
A
- Algorithms can be applied to describe ice flow mechanisms which can then be put into the Ice Continuity Equation, which allows ice sheets to be modelled
13
Q
What is glacial isostacy?
A
- Crustal subsidence under the weight of the ice sheet above
- Melt this ice and the crust shall rebound through isostasy
14
Q
What are the forms of glacier motion?
A
- Deformation of the ice itself
- Sliding at the bed
- Deforming subglacial substrate
15
Q
How is the ice itself deformed?
A
- It’s a solid but if large stresses are applied over large periods of time it will deform
- Zero Deformation at the base, greatest velocity at the top
16
Q
During the deformation of subglacial substrate where is the maximum deformation take place?
A
- Maximum deformation at the ice-bed interface due to decoupling at the base
- This needs water
17
Q
What are the rheological properties of ice?
A
- Ice was first thought of as a perfect plastic
- Only true for large stresses
- Very little change in strain rate for little change in shear stress
- At a certain yield stress, you obtain enormous levels of strain rate
- Actually approximates to a Newtonian Viscous fluid at low stresses
- Application of shear stress results in linear increase in strain rate
18
Q
What is Glens flow law?
A
- A is related to temperature and impurities (flow law constant)
- A=A0 exp(-Q/RT)
- As T increases, so does A (A is v. sensitive to T)
- n is a constant approximately equal to 3
- t is the effective shear stress (measure of the overall stress regime)
- E* is the effective strain rate
- Doesn’t fully describe the flow rate of ice
- And its not really a law, its a power law
19
Q
What is the shear stress at the bed during pure ice deformation?
A
- Shear Stress at the bed tb
- p is the density of the ice
- g is the acceleration due to gravity
- h is the ice thickness
- a is the angle of the surface slope
20
Q
What assumptions are made during pure ice deformation?
A
- Assumptions made:
- No basal sliding
- Flat bed
- Single value for flow parameters
- No constraints by the valley walls
21
Q
How do we measure the velocity of the ice during pure ice deformation?
A
Us = Ub + (2A / (n+1)) (pg sinα)n hn+1
- Ub is 0 (assume no basal sliding)
- Us proportional to h4
- Us proportional to α3
22
Q
How does the velocity of ice change across an ice sheet?
A
- Zero Velocity of ice at the origin (ice divide)
- As you increase distance from the ice divide, velocity will increase pseudo-exponentially
- A particle of ice will take 150,000 years to travel from 50 km to the end of the ice
- Most of this time is spent getting to 300 km
23
Q
Why is basal sliding so important, and the flow of ice isn’t as simple as demostrated by Glens flow law?
A
- In valleys, ice movement is constrained at the valley walls meaning flow is slower here
24
Q
Describe the effects of water on basal sliding
A
- Distribution and pressure of water at glacier beds are the most important factors in regulating short term velocity fluctuations and glacier surge cycles
- Increased water velocity after increased precipitation increases water pressure
- Leads to enhanced sliding
25
By what processes does basal sliding occur?
Regelation
Enhanced plastic deformation
26
How does regelation occur?
* Ice is assumed to be at its pressure melting point
* Comes into contact with a subglacial obstacle (e.g. Roche Moutonnée)
* Stoss side
* provides greatest resistance, increased pressure
* This lowers pressure melting point
* causes melting (needs heat)
* Under pressure occurs on the lee side, increasing pressure melting point and heat is transferred over to the stoss side
* causes freezing of migrated meltwater (releasing latent heat)
27
What features do we see on bumps that have seen regelation processes occur over them? e.g. on Roche Moutonnée
* Shallow stoss and sharp lee side
* sliding on stoss = striations, polishing on surface
* freezing on lee = plucking and frozen artefacts
28
How is the velocity due to regelation shown?
It can be shown that the velocity due to regelation (Ur) equals:
Ur ∝ tb/a
Most relevant for bumps \< ~ 50 cm in length and for ice at the melting point.
Any greater than 50cm and it gets harder to transfer heat
29
How does Enhanced plasic deformation occur and what is the velocity due to enhanced plastic deformation?
With sufficient stress, all ice deforms plastically
Velocity: _Up ∝ τb∙a_ for bumps greater than 100cm in length
τb is the basal shear stress
a is the hummock amplitude (height or length)
Velocity = Strain Rate x Distance
Thus the velocity increases as the ice flows around the obstacle
30
What are the controls over basal sliding?
* Bedrock roughness
* Subglacial water pressure
* Thermal regime
* Bed type
31
How does bedrock roughness have a control over basal sliding?
There is a theoretical bedrock size which inhibits sliding
Unified Sliding law based on roughness at this scale:
Ut ∝ τb/R4
Ut is the total sliding velocity
R is the bedrock roughness equal to a/λ
Explains sliding velocity variations between glaciers on different substrates
Still predicts constant sliding for a given bedrock roughness
Some other factor must be causing temporal variability
Needs ice at reletively warm T but don't need to apply role of water to bed
32
What are the controls of subglacial water pressure over basal sliding?
Firstly, subglacial waater pressure is denoted the symbol Pw
The effective pressure (N or Peff) is equal to the weight of the ice (pgh) - Pw
* N falls as Pw rises
* Forcing bedrock seperation where local ice pressures are lowest
* Lee sides of bedrock hummocks
* Cavitites are formed
* Sliding is enhanced
* total basal friction is reduced
* increased stresses at remaining pinning points, where stresses are already highest
33
How do we measure velocity of basal sliding under the influcnce of subglacial water pressure?
Ut ∝ tbp/Nq
tb is the basal shear stress
N is the effective pressure
p and q are exponents derived from a variety of field and laboratory studies.
2 \< p \< 8
1\< q \<6
34
In what ways can water at the bed influence sliding velocities?
* Submergence of small bed roughness elements.
* Causes Ice base to be much smoother than normal
* Increasing the local stress in areas of ice-bed contact
* Encourages enhanced plastic deformation
* The hydraulic jack mechanism
* Pressurised water exerts a force against the up facing ice
35
What is the thermal regime of an ice body effect basal sliding?
* Warm based ice masses linked to sliding
* Most ice masses are partially warm based
* Cold ice deforms slowly
* Still deforms under enhanced stresses
* Regelation requires ice at the pressure melting point
* May occur in cold ice but very slowly
36
How will bed type effect how the ice moved accross it?
* Basal Sliding occurs over bedrock
* 80% of area underlying former North American and Eurasian ice sheets are soft sediment capable of deforming themselves
37
Why is it difficult to model basal sliding?
there is no universally applicable sliding law
key basal properties are poorly known
38
In sediment deformation, there are two horizons - what are the characterists of the two horizons?
* Bed horizon A above horizon B
* Horizon A characterised as follows:
* In contact with the glacier
* Deformation takes place here
* Saturated with water
* Lose sediment with pores filled with water
* Dilated Sediment
* Horizon B is too stiff to deform due to brittleness and over compaction.
39
How much of the forward motion of a glacier is due to the displacement of sub-glacial sediments?
80-95%
40
What controls the fluctuations in pore pressures in sub-glacial sediment?
the production of surface melt water of the overlying glacier
melt water percolates down into sub-glacial sediment
41
What is the effective pressure in subglacial sediments?
N = ρigH - Pw
42
If unlithified sediment is present beneath the ice column and the base of the ice is at its PMP, how is the horizonal velocity at the top of the deforming till modelled?
Ub = hbKb (τb- τ\*)/N2
Kb is the till deformation softness
Ub is the horizontal velocity at the top of the deforming till
τ\* is the till yield strength
Simplifying this yields the following:
Ub ∝ N-2
Ub ∝ τb - τ\*
43
What are the characteristics of the Whillians Ice Stream?
* Surface velocity of 50 to 850 m yr-1
* Large surface crevasses in place
* Ice deformation ~ 10 m yr-1
* Deformation of 6m of water saturated sediments
* High porosity, characteristic of deforming till
* All water supplied from basal melting with a distributed supply
* Water saturated at a high pressure
* Sediment originates upstream
44
What are the characteristics of the Kamb Ice Stream?
* Ice surface velocity 5m yr-1
* Buried surface crevasses
* Flowed rapidly in the past
* Crevassing ceased 30 to 130 years ago
* Reduced water pressure
* Collapse of dilatant till
* Switched off 175 years ago
* The reason for this is unknown
* This is indicated by there being no crevasses present on the surface
* They’re buried by snowfall
* Measuring this allows time to be calculated
45
Why do we see differing features in the Whillians and Kamb Ice Streams?
differences in features due to rerouting of water from Kamb Ice Stream to Whillans Ice Stream. This is otherwise known as ‘water piracy’
This has also allowed for accumulation on top of the Kamb ice stream while elevation on the Whillians ice stream decreases
46
How would one measure total glacier velocity?
Total velocity = ice deformation + basal sliding + sediment deformation
Basal sliding processes result in the greatest sliding velocitites
47
What are the glacial erosion processes?
* Abrasion and Plucking (several forms of this, subglacial process)
* Rock fracturing (subglacial)
* Pressure Release and Dilation (subglacial and subaerial)
* Frost shattering (subaerial)
* Result in the formation of block fields
* Water enters fracture and expands as it freezes
48
What is abrasion and what controls does it have over it during glacial erosion processes?
* Process by which debris in a glacier scrapes or wears away underlying geology
* Controls
* Presence and concentration of basal debris
* Sliding velocity of glacier
* Transportation of debris to bedrock
* Ice thickness
* Presence of water
* Lithology/composition of basal debris
* Size and shape of basal debris
* Effectiveness of the removal of eroded debris
49
How does plucking take place?
* Ice becomes welded to the bedrock
* Stresses induced from the overriding ice extenuate weaknesses within the rock
* Failure of the rock occurs
* Resulting clast is attached to the basal ice
50
What are the characteristics of a Roche Moutonnée?
* Shallow upstream side where glacier abrades
* Steep down-stream side where glacier plucks
51
What are the glaci-erosional landforms?
* Aerial Scouring (Abrasion)
* Glacial Trough (Abrasion)
* Domes and Whaleback forms (Abrasion)
* Striation (Abrasion)
* Groove (Abrasion)
* Polished surface (Abrasion)
* Reigel
* Cirque
* Crag and Tail
52
What are the characteristics of Aerial scouring?
(Abrasive)
low amplitude
irregular relief
53
What are the characteristics of a glacial trough?
* U-shaped valleys
* Fjords (glacier retreats and the sea fills the resulting valley floor. This forms a narrow, steep sided inlet (sometimes deeper than 1300 metres) connected to the sea.
* Associated hanging valleys and truncated spurs (A blunt-ended, sloping ridge which descends from the flank of a valley)
54
What are the characteristics of Domes and Whaleback forms?
* Similar to Roche Moutonnée
* No plucking on down glacier side
55
What are the most common of the glaci-erosional features?
Striations from abrasion
56
What is a groove?
An enlargement of singular striations
57
How is a polished surface formed?
from abrasion from very fine debris in the glacier
58
What is a Reigel?
A rock barrier perturbation caused by a band of resistant rock
59
What is a cirque?
Forms where the glacier initally forms = good indicator of this
"armchair" shaped
combinations of abrasion on floor, and rock fracturing (crushing) and plucking along walls.
60
How is a Crag and Tail formed and what are its characteristics?
Erosion (abrasion) of upside face, deposition over downside face
Sharp upstream slope + shallow downstream slope
e.g., Castle Rock, Edinburgh - flow around big volcanic plug and deposit on other side
61
What landforms are formed subglacially and parallel to ice flow?
Drumlins
62
How do drumlins form and what is their form?
* Form parallel to ice flow
* Elongated plan with long axis parallel to ice flow due to extensional regime within ice flow as it travels downhill
* Blunt end facing upstream
* Vary in size from 10s to 100s of meters long and meters to 10s of meters high
* Very common feature
63
What significance do striations have in the UK?
Striations give evidence for warm based glaciers in Britain
64
How are ribbed moraines formed?
* Form transverse to ice flow
* Formation influenced by basal topography
65
Explain what's happening in the diagram.
* Downhill flow of glacier results in drumlin formation
* Extending flow
* Transition zone as topography levels
* This region contains both drumlins and moraines
* Uphill motion of the glacier results in moraine formation
* Form perpendicular to ice flow
* elongated plan with long axis perp. to ice flow due to compressional regime within the ice flow as it travels uphill
66
Why is modelling required for **ice sheet reconstructions**?
Terrestrial glacial geology can be used to tell where glaciers were and the direcction etc in which they flowed but modelling is required to differentiate between maximum and minimum ice reconstructions to obtain the true expanse of ice sheets in the past
67
In UK, we have an extensive distribution of drumlins indicating.....?
* direction of ice flow
* that the ice sheet was warm based in many places
this provided important evidence for Boulton to reconstruct the last British Ice Sheet
68
In ice sheet reconstructions what is indicative of direction of ice flow?
striations, drumlins, flutes, Roch Moutonnée’s, crag and tail, lateral morraines and erratics
69
In ice sheet reconstructions what is indicative of the centre of the ice sheet?
cirques and other mountain glacial landforms
70
In ice sheet reconstrctions what is indicative of the dynamics of the ice sheet?
* Indicated by subglacial landforms and erosion
* Indicated by subglacial water activity, both mechanical and chemical
71
Ice sheet maximum and temporal pattern of ice sheet decay can be calculated how?
* Relatively from cross cutting platforms
* Absolutely from geochronological dating of sediment deposits (e.g., carbon dating, thermo-luminescence, chlorine dating).
72
How to raised beaches form and what is their significance?
* When a load is exerted on the lithosphere, it depresses into the asthenosphere in accordance with Archimedes’ Principle.
* The asthenospheric material (assume is a very viscous fluid) is displaced to accommodate the depression (isostatic subsidence).
* This action happens slowly because of the viscous nature of the deforming substance. When the ice is removed (this happens very quickly compared with glacial loading), the lithosphere and asthenosphere respond by the process of isostatic uplift.
* The ice will be removed long before isostatic equilibrium is reached, thus, the shorelines of land masses which experienced glacial loading may also experience shoreline uplift.
* The raised beaches which form, indicate information about past relative sea-levels and tectonic dynamics relative to ice loading
73
Explain the components of relative sea level. (not crucial)
* Relative sea level has two components
* Glacial isostasy
* Eustatic sea level change
* These interact with one another directly and become involved in other feedbacks
* Time dependent function comprises of three components
* Restrained rebound
* Postglacial uplift
* Residual uplift
74
Uplift analysis can do what?
By analysing the rates and pattern of uplift, we can determine information about:
* Determine the centre of ice loading
* Determine timing of deglaciation
* Determine the maximum ice thickness
75
Glaci-Marine sedimentation can be divided into what areas?
* Glaci-Marine sedimentation can be divided into 4 main areas
* Lakes
* Fjords
* Continental shelf
* Deep sea environments
76
Explain Glaci-Lacustrine sedimentation.
* During the last ice age, many ice sheets and glaciers lay over the paths of rivers and lakes
* rivers may have existed prior to glaciation, or have origins from glacial meltwater
* Lakes existing prior to the glaciation are reffered to as ‘ice dammed’ lakes
* Sediments are transported to the lake *predominantly* by the glacier or ice sheet
* We know such lakes existed because of the sedimentary sequences that were deposited within them
77
What does the analysis of Glaci-Lacustrine sediments provide?
* Analysis of this sediment provides evidence for:
* Glacier extent
* Rate of sediment supply from the glacier ice
* Dynamics of the ice sheet at its margin
78
What are Trough Mouth Fans (TMF), and what information do they provide?
* Glacial troughs associated with large sedimentary fans at their mouths
* Located on the continental shelf break
* Sediment is deposited at trough-mouth edge and then gently flows down through gravity slides
* Glacial fan systems provide information on:
* Extent of ice masses
* Dynamics of ice sheets
* Timing of ice sheet growth and decay
* Information on past glaciations not available from terrestrial geology
79
How much isostatic depression would you get with a 1000m thick ice sheet?
900kgm3 ice
3300 kgm3 asthenos/mantle
900/3300 X ice thickness = isostatic depression
isostatic depression = 900/3300 X 1000 = ~273m isostatic depression
Antarctica 4km thick = ~1091m isostatic depression
80
Why should we perhaps not build nuclear power plants on the E coast of the UK and Scotland?
Trough Mouth Fans (TMF) in offshore Norway hold a lot of sediment, and if they destabalize and cause a sub-marine sedimentary flow then that makes the UK susceptible to tsunamis
81
Daniel, what can you tell me about Ice Rafted Debris?
* Found within deep-sea regions bordering glaciated continents
* Flux of sediments derived from icebergs
* Diminish with increasing distance form the ice-sheet margin
* Preserved well due to low energy environment of the deep oceans
* e.g. dropstones
82
Ice rafted debris can be used to provide information of what?
* Rate of iceberg production
* Insight into local ocean water conditions
83
What is the spatial distrobution of Ice Rafted Debris indicative of?
* Increase of ice rafted material indicative of increased iceberg production and/or increase in the volume of sediment carried by icebergs
* Decrease in ice rafted material indicative of a pause in iceberg production and/or a decrease in volume of sediment carried by iceberg
84
What are Heinrich layers, and what are they interpreted to represent?
* There is a significant coverage of (Ice Rafted Debris) IRD’s in the North Atlantic that exist within a number of unique layers
* These layers were interpreted to represent (first interpretation widely accepted as the best):
* An increase in ice berg production, acting to drain the ice dome
* An increase in iceberg debris content
* A hiatus in clean iceberg production
* These layers represent Heinrich events!
85
How is ice gained and lost from an ice sheet at its margin?
* Binge
* Temperature of ice sheet is less than melting value (Bed is frozen)
* Ice motion via internal deformation
* Ice accumulation greater than ice flux
* Ice sheet thickens
* Bing/Purge transition
* Ice sheet thickens to a point at which the base becomes wet
* Sliding then becomes possible
* Basal sliding begins to contribute to ice motion
* Purge
* Ice motion due to internal deformation and basal sliding
* Flux of ice greater than accumulation
* Ice is lost to margin as icebergs
* Purge/Binge Transition
* Ice sheet thins so much that it no longer insulates the bed from cold surface
* Bed becomes frozen
* Sliding ceases
* Flux of ice is less than accumulation
* Ice berg calving reduces
* Regrowth of the ice sheet takes place
86
What is the cyclicity of the Heinrich event model?
7000 years
The same as for the Binge-Purge model
87
What is the cyclicity for the Binge-Purge model?
7000 years
The same as for the Henrich event model
88
During the last glacial maximum, what was the state of the Antarctic ice sheet in comparison to today?
* Thicker + broader ice sheet in W Antarctica
* Thinner + broader ice sheet in E Antarctica
* air T was lower so less moisture in the atmosphere, less accumulation of snow, and so was thinner than today
89
How much of the worlds glaciated surface does the Grenland ice sheet account for?
10%
90
Why is the Greenland ice sheet still there?
It is surrounded by topography that acts as a cage and so the warm ocean is only in contact with little parts of the ice sheet through small outlets
a ring of cliffs keeps the ice intact
91
How thick is the center of the Greenland ice sheet?
3200m thick
92
How is ice lost on the Greenland icesheet?
* Ablation in the South (the removal of snow and ice from a glacier or iceberg by melting or evaporation)
* Iceberg calving at outlet glaciers in the East and West
93
The accumulation rate on the Greenland ice sheet is dependent on what conditions?
* Distance to the North Atlantic
* Moisture Source
* Storm Tracks
94
Where do we see the least and most accumulation on the Greenland ice sheet?
* Greatest accumulation in the South-East
* Least accumulation in the North-East
95
What change in T will cause all the Greenland ice to melt?
* A 3°C in temperature by 2100 will melt all the ice in Greenland
* This will cause a 7m sea level rise
96
What can Ice Sheet Modelling replicate?
* Replicate large scale surface elevation
* Replicate Ice velocity features
97
What will happen to the Antarctic ice sheet in future?
West Antarctica shrinking due to ocean-led melting
East Antarctica growing due to warming-induced snowfall increases
98
How did ice form over the Barents sea?
* Model assumes zero ice at 30,000 years ago
* Shortly afterwards permanent sea ice over Barents Sea; ice caps on islands
* Sea ice and ice caps thicken to a continuous ice-sheet ice-shelf system
* Further thickening results in grounding of ice shelves, and the switching on of ice streams, lubricated by marine sediments
99
Proportionally, what has contributed to anthropogenic sea level rise?
* 50% of rising sea level is due to thermal expansion of the oceans
* Remaining 50% is from melting ice coverage on the planet
* More than 250 million people live within 5 miles of the coastline
* Low atmospheric pressure can also cause a rise in sea level
100
What extreme events have occured due to increasing global temperature?
* Extreme flooding (Tewkesbury, 2007)
* Heatwaves (France, 2003)
* Hurricane Katrina (New Orleans)
* Extreme Winter Weather (across the UK, 2014)
More storms during climate change because putting in more energy + moisture into atmosphere = more activity
101
Why is the Arctic experiencing the greatest signs of global warming? What are these signs?
* Due to polar amplification of the signal
* Results in glacier retreat, and reduction in sea ice and permafrost
* Permafrost are areas of ground that are frozen completely all year round
* Permafrost normally acts as stable base to build buildings and ice roads
* When melted, soft, wet, unstable ground causes buildings to crack + cut off villages due to loss of ice rds
* Acts as a cap to methane stored beneath
* Releasing this methane will cast a shadow over anthropogenic influences
* Often referred to as a time bomb of methane
102
What will happen when sea ice is melted?
Decrease in albedo, gives positive feedback - Earth absorbs more solar radiation = gets warmer
103
Sea level surges are a function of what things?
* Level of the sea
* Low atmospheric pressure
* Storm tracts (pushing and funnelling of water through surface winds)
104
What is the 8.2 kyr event?
* After the what will be the Grate Lakes were carved out by lobe formation from bed lubrication at the margins of the Laurentide Ice Sheet (in-sheet dynamism), proglacial lakes formed from the water coming off the sheet.
* Red River Lobe decay formed lake Agassiz, the largest lake known to ever exist.
* At 8.2 ka, Lakes Ojibway and Agassiz decanted their water into oceans within two days causing catastrophic flooding and drainage
* The vast amounts of fresh water flooded into the Atlantic modulated the thermohaline process
105
What contributed to most of the global sea-level fall during the last ice age?
The presence of the Laurentide Ice Sheet in North America, the second largest (to Antartica) in the world at 18 ka
106
What geological evidence is there for large ice sheets in North America during the last Ice Age?
* Lots of geological evidence.
* Moraines, and in particular those from ice sheet lobes
* Proglacial lakes (big ones)
* Isostatic uplift
* Ice sheet reconstructions possible based on geology.
107
Explain Marine Ice Sheet Instability in West Antarctica.
* E.g. Amazon Bay region
* Ice sheet rests below the ocean surface
* This means the ice sheet is in contact with the ocean water
* ice sheets even has a massive reversing bed slope that results in topography going down to 2.5km below the level of the sea
* This is not stable
* As grounding line begins to retreat inland with increasing temperature it will become rested on deeper bed
* Warm water comes in over continental shelf to the grouding line
* Melts ice, accelerating the rate of ice retreat due to the unstable situation
(Reversing of the groudning line might get held up by normal bedslopes due to the undulating topography, but there is a dominant reversing slope)
108
How do we know what ice sheets are doing?
* **Satellite radar altimetry**, employed since 1992
* **Satellite laser altimeter** or ICESat, 2002 – 2010
* **Satellite gravimeter** or **GRACE**, employed since 2003
* All agree that Antarctica is losing ice sheet mass
* Ice Stream C (Kamb Ice Stream) switched off and so is gaining mass due to accumulation. Water diverted to Ice stream B.
109
Is sea level change the same across the globe?
* No
* Ice sheets have massive gravitational attraction
* Close to melting ice masses sea level will fall due to decrease in gravitational attraction to the ice
* Therefore, in the UK we are not too concerned about the melting of the Greenland ice sheet becasue we're in a neutral (Goldilocks) zone of the gravity effects
* But the effects on the thermohaline circulation will effect us in the UK
* And unfortunetely for us, the Antarctic Ice Sheet is by far the biggest store of ice, and we're going to see the biggest influence of its melting in the Northern hemisphere
110
Growth of the Antarctic Ice Sheet, you're being silly? Really?
As T gets warmer, more moisture gets put into the atmosphere, there'll be more ppt on the surface and maybe E Antarctica will gain some mass
But future of E Antarctic ice sheet is very hard to model.
However, observations sugest that there'll be no arctic sea ice by 2020; and Alpine glaciers will be largely gone by 2100.
111
Can we stop sea level from rising in the near future?
No, because very best case scenario is a 2oC warming, so we need to think **mitigation** and **adaptation**!
Possible sea level rise of 1m by 2100
112
What are the four types of moraines and how are they formed?
Four types formed through deposition
* **Marginal moraines**, forming a complex of end moraines
* **Proper Subglacial moraines**
* Under thick ice
* Away from the ice front
* Occurs at the transition between cold and warm bed glacier
* Ice under compression here
* Formation by active basal ice. tectonic processes within the ice
* folding of basal layers,
* stacking of debris rich basal layers against obstacles to glacier flow
* followed by melt-out
* Filling of open crevasses by supraglacial debris
* Filling of basal crevasses by subglacial debris
113
How much may sea level rise by 2100?
Possibly 1m
