THE CRYOSPHERE
GLACIERS AS CLIMATE INDICATORS
Fundamentals of glaciology pt1
Fundamentals of glaciology pt2
Fundamentals of glaciology pt3
glacier erosion and transport
depositional environments and landforms
glacial landscapes
MOUNTAIN GLACIERS
overview
the Hindu Kush himalaya
glaci...
Fundamentals of glaciology pt1
- ‘Accumulation’ is the gains/inputs in a glacier system
- Accumulation includes: snowfall, wind-blown drift, avalanches, condensation (rime), freezing
rain/external water
- Ablation is the losses/outputs in a glacier system
- Ablation includes calving, melting, evaporation/sublimation
- Snowfall requires sub-zero temperatures + atmospheric moisture
- The highest accumulation rates occur in mountainous maritime regions
- Such as alaska, western patagonia, west coast of nz south island, and japan
- The lowest rates of accumulation occur in the continent interiors
- Such as central east antarctica
- Local variations in accumulation occur due to wind-blown snow, and avalanches (only
mountainous regions obviously)
- firn/nevé is snow that is partially compressed towards ice - it is found under the snow that
accumulates at the head of a glacier
- The density of F or N is at 400-830kg-m3
- The glacial mass balance of a system depends on its inputs minus its outputs
- Accumulation and ablation together, cause ice to flow and therefore the mass balance to
change
- A “steady state” assumes a state of equilibrium whereby accumulation is matched by ablation
- A steady state can adjust a little over time, allowing for fluctuations in temperature etc.
- The ELA is the equilibrium line altitude - also known as the line/zone of equilibrium
- This line is found directly between the zones of accumulation and equilibrium
- Glacial mechanisms of motion include: Internal ice deformation (UF), sliding at the bed (US),
and shear in underlying deformable sediments (UD)
Fundamentals of glaciology pt2
- Glacier sliding refers to slip between the glacier and its bed
- For frozen bed glaciers, sliding is negligible: less than ~4 mm/year (ignored)
- The factors controlling sliding include bed roughness, amount of debris embedded in base of
glacier, and the quantity, distribution and ‘delivery rate’ of water at the bed
- Regelation is pressure melting - whereby pressure forces ice to melt and refreeze on the
other side of the obstacle
- Enhanced basal creep is due to stress concentration when ice encounters an obstacle
- Many glaciers are underlain by soft unlithified sediments (rather than crystalline bedrock)
- If sediments are deformable, this contributes to glacier flow speeds measured at the surface -
difficult to study due to access, difficult to observe
- If sediment deformation occurs, this allows ice to flow very fast
- Sediment deformation depends on water content (and pressure) of the sediments
- With higher water pressure, the sediment is weaker and less able to resist
deformation
- Factors affecting the melting/freezing temperature of ice include
- Pressure (increases with ice thickness), below 2000m of ice, Melt = -1.27•C
- Impurities (e.g. salinity), sea water freezes at ~-2•C due to salt content
- Surface energy balance
- Geothermal heat flux
- Frictional heat due to ice deformation and sliding
- Milankovitch cycles (eccentricity, precession, obliquity)
Fundamentals of glaciology pt3
, - SW net is controlled by albedo (= a)
- Clean ice = high a
- Dirty ice = intermediate a
- Debris-covered ice = low a
- Geothermal heat flux depends on various factors
- Crustal thickness (thin crust = higher heat flux)
- Tectonic history / proximity to boundary
- Local subglacial topography (locally higher at sides of valleys)
- Average for Greenland is ~60mW m-2, and for Antarctica it is ~50 mW m-2
- Frictional heat can be generated by ice deformation, sliding at the bed, and at the
shear margins
- We can define glaciers by its ‘thermal’ or ‘temperature regime’
- Cold based glaciers only have internal deformation - frozen to the base, eg they are
thin, high latitude glaciers
- Warm based glaciers are at pressure melting point, therefore there is water present.
These are common in mid-latitude ‘maritime’ glaciers
- Polythermal glaciers are a mixture of both cold and warm glaciers
- Sliding, and therefore significant erosion can only occur where the ice is at the
pressure melting point
- Cold-based ice preserves underlying surfaces
Glacier erosion and transport
- Erosion
- Glaciers have a very high capacity for erosion
- Glacial environments produce characteristic landforms
- The thermal regime of glaciers: affected by atmospheric temperature, and also pressure
melting point (base of glacier - pressure of ice)
- Basal sliding - caused by the melting at base of glacier
- Subglacial erosion primarily occurs via two processes - Abrasion and Plucking (Boulton 1979)
- Subglacial erosional formations include: striae and polishing, and chattermarks and gouges
- Striae (striations) are scratches incised into bedrock or clast surfaces; it is a direct evidence of
abrasion (therefore evidence of basal sliding) - direction of ice-flow can be observed if scratch
gets wider (as particle gets more blunt from friction)
- Chattermarks and gouges are crescent-shaped fractures, concave side facing down-ice
(usually a few centimetres across), the spacing is often consistent
- Plucking (also known as quarrying), is extended abrasion processes that lead to isolation of
fragments and rock fracture, it is strongly influenced by pre-existing joints
- Transport
- There are many ways that debris is transported, it depends on glacier configuration and
conditions
- Debris may undergo many different cycles of ‘modification’ due to ice/water/wind/gravity etc.
- This leads to a very large variability in form and appearance - which means it is hard
to understand what caused the different landforms found
- Supraglacial debris entrainment: valley glaciers, ice caps (nunataks, volcanic ash), ice sheets
(nunataks)
- Subglacial debris entrainment: mostly dependent on basal flow, however ‘freeze-on’ process
(regelation) has a huge impact on basal flow. Basal flow affects
- Ice motion - debris content changes rheology
- Rates of subglacial erosion
- Debris transportation distances (eg. beyond glacier margins)
- ‘Freeze-on’? = where basal ice freezes onto the frozen glacier bed
- Frost wedging: weathering of rocks due to repeated freeze-thaw cycle
, - Effect of transport on debris - Active transport versus passive transport (Boulton 1978)
- Active transport in basal transport zone: sediment transported in the basal tractive zone,
abrasion and crushing progressively modifies particles
- Passive transport ‘high-level transport’: supraglacial and englacially transported particles
transported with little or no modification
- Sorting (of sediment) gives evidence of transport, and energy (type).
- Clast morphology gives evidence of transport duration and process - shape (sphericity),
roundness
- Glaciofluvial transport includes suspended load and bedload
- Suspended load: for a given water flow, grains settle at a rate proportional to diameter;
generally finer than sand
- Bedload: larger particles roll/slide across the stream bed
Depositional environments and landforms
- Glacial deposition
- Glacial sediments can be categorised by the processes which deposited them (genetic
classification)
- Glacigenic deposits
- Glaciofluvial deposits
- Gravitational mass movement
- Primary glacigenic deposits - moraines
- Moraines are the key glacigenic ice marginal landform - they help outline the previous
configuration of glaciers, and are diagnostic of particular behaviours
- The main types of ice-marginal moraines are:
- Glaciotectonic moraines
- Annual push moraines
- Dump moraines
- Ablation moraines
- Glaciotectonic landforms - glaciotectonic is the main deformation process at ice margins
- Glaciotectonic structures occur where stress transferred from the glacier is greater than (>)
the strength of the material subjected to stress, which = failure
- Push moraines - pushed from behind (shoving)
- Morphology - asymmetric and arcuate
- Locally, it can be winding, reflecting the prophology of the glacier snout
- Distinctive structure single asymmetric fold, tilted axis dipping up-glacier
- Push moraines can link glacier recession (mass balance) to climate
- Beedle et al. (2009) showed that rate of ice-front retreat correlates to summer
air temperature
- Seasonal push moraine patterns can provide a high-resolution ice-climate
proxy
- Valuable in reconstructing (and understanding the forcing of) recently
deglaciated areas
(Bennett, 2001)
- Dump moraines - ridges of supraglacial and subglacial material that accumulates at a
stationary glacier front that has steep margins, and is then deposited by slump and flow
processes
- Common to temperate glacier systems
- Frontal and lateral examples; often latero-frontal
- Depositional-constructional features
- Material is not deformed (as in glaciotectonic moraines)
- Moraine size depends on the: ice flow velocity, debris content, duration of stationary event
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller ejr22. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $9.69. You're not tied to anything after your purchase.
