Basal Sliding

Basal Sliding Processes

  1. Enhanced plastic deformation
  1. Regelation
  1. 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

  1. 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

  1. Ice deforming downhill hits a bedrock bump
  1. Stoss - enhanced stresses

Locally lower ice melt pt on upstream side

∴ induces pressure melting

Result: meltwater flows around/over obstacle

  1. Lee - lower stresses

Lower stresses ∴ melting point = higher

→ Induces refreezing of meltwater

  1. 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)