Basal Sliding
Basal Sliding Processes
- Enhanced plastic deformation
- Regelation
- Cavitation
Only process capable of causing dramatic variability basal sliding rates on short timescales (weeks/seasonal)
1+2 - Enable basal sliding but cannot cause short term ∆ bs rates
1. Enhanced Plastic Def
As ice flows/deforms over this rough bed,
Glacier beds = rough
i.e. lotas undulations & protrusions
→ Basal stresses=enhanced on upglacier side of undulations
∴ Ice deforms more easily
Stoss side induces > average basal shear stresses
Implications
Ice deforms around basal obstacles much faster
than if used Glens Flow Law w surface slope / thickness
Why ?
Glens flow law exponent > 3
∴ VERY sensitive to basal stress ∆
Enhanced plastic def
= internal deformation process operating @ bed
Driver: bed topog heterogeneity
Basal topog promotes τ enhancement on stoss side bumps
Most effective around large hummocks
Proportional to
bedrock obstacle size
Bigger bump → more τb enhancement → faster ice def ∴ sliding
Usc = τb . a
a = hummock height
- Regelation
= process of melting & refreezing ice melt moving around bedrock obstacles
Requires ice @ pressure melting pt
Most effective
around small bumps
∴ only occurs in warm-based glaciers
Usr = τb / a
Inversely proportional
to bedrock obstacle size
Regelation ice
Thinly laminated basal ice
Often debris rich
Layers of refrozen meltwater
Regelation Process
- Ice deforming downhill hits a bedrock bump
- Stoss - enhanced stresses
Locally lower ice melt pt on upstream side
∴ induces pressure melting
Result: meltwater flows around/over obstacle
- Lee - lower stresses
Lower stresses ∴ melting point = higher
→ Induces refreezing of meltwater
- Refreezing releases latent heat
heat = conducted through bump to stoss
∴ fuels further melt...
Regelation importance
important for promoting erosion @ bed
As ice refreezes, water incorporates erosion products @ bed into ice
∴ exposes new bed
Which bs process?
around obstacles > 1m
Ice flows via
enhanced plastic def
around obstacles < 1m
Ice flows via regelation
Smaller obstacles - exert less stress
& greater heat transfer poss
Bed roughness
Intermediate obstacles (~0.5 - 1m)
offer greatest resistance to basal sliding
Why?
Not large enough for enhanced prlastic def (> 1m)
Not small enough to induce regelation
∴ In absence of other factors
Basal sliding rate = f ( bed roughness / obstacle size & frequency)