What is a glacier? “Glaciers are masses of ice and granular snow formed by compaction and recrystallization of snow, lying largely or wholly on land and showing evidence of past or present movement.” Matanuska Glacier, Alaska. Foto: Lachniet (1997)
From snow to ice Snow falls as crystals onto glacier surface, – σ (density) = 0.07 to 0.18 g/cm 3. Firn: Crystals lose shape to become coarse snow via melting of edges – and pack more closely – σ = 0.55 g/cm 3 Glacier Ice forms as firn is buried and begins recrystallization – porosity decreases – metamorphic ice crystals increase in size – σ = 0.8 to 0.9 g/cm 3.
Snow Metamorphism Time of metamorphism varies by climate Cold, polar glaciers with low accumulation = slower periods (up to decades) Warmer, temperature glaciers with higher accumulation = faster (weeks)
Glacier Mass Balance Mass balance is the sum of all mass inputs (+) and outputs (-) – positive – the glacier expands – negative – the glacier retreats Equilibrium Line – Zone between accumulation and ablation is the – Equilibrium Line Altitude (ELA) rises with negative mass balance and vice versa
Accumulation Accumulation is concentrated at high elevations and more polar latitudes – Snowfall – Avalanches – Rain that freezes on glacier – Superimposed ice: re-frozen snowmelt – Blowing snow
Ablation Ablation is the loss of mass – Melting – Evaporation (sublimation) in semi-arid climates – Wind erosion – Calving of large blocks of ice into lakes and ocean
Primary Stratification Snow accumulates in annual layers Visible in stratigraphy by changes in porosity, dirt content Quelccaya Ice Cap - Peru http://www.ngdc.noaa.gov/paleo/slides/slideset/2 0/20_401_bslide.html
Glacier Movement Glacier ice deforms under its own weight and gravity to ‘flow’ downvalley Glaciers move via – 1) Internal deformation via plastic flow (aka “creep”) – 2) basal sliding – 3) bed deformation
Creep Creep is the process by which ice crystals are dislocated along shear planes Happens more rapidly in warmer glaciers
Foliation Secondary feature Formed from ice shear Alternating clear blue and bubble-rich white ice Note unconformity The Ice Palace in the Aletschgletcher, Jungfraujoch, Swiss Alps, Lachniet 2011
Basal Sliding Some glaciers move by slipping along basal water films Water source – Meltwater, geothermal and pressure melting – Not enough water – no sliding – Just right water – sliding – Too much water – development of subglacial drainage
Bed Deformation Ice Movement Deforming Bed Bering Glacier – Fenster site, Lachniet (1998)
Subglacial Sediment Deformation As much as 90% of down- valley movement in some glaciers results from bed deformation Subglacial sediments are very wet with high pore water pressures (low effective stress)
Velocity = Creep + sliding + bed deformation Figure 9-11A B C
Your consent to our cookies if you continue to use this website.