Ice Deformation Mechanisms

Dominant
mode ice def

= Slip on basal 0001 plane

Stress & creep accomodation

Propagation of dislocations through crystal lattice

Fabric development in ice
--> as lattice shape is rearranged

Dislocation

= linear defect in a crystal lattice i.e. rock / ice

Causes plastic deformation of individual crystals ∴ material itself

Ice mass subjected to body forces

reflect gravitational potential experienced by ice mass

causes creep motion

= deformation of crystal by mvmt of an edge dislocation

i.e. half plane added/inserted into lattice

Dislocation glide (slip)

= migration of dislocation through lattice

How ?

  1. bond reattaches on adjacent plane
  1. allows dislocation migration

Crystal shape changes without mechanical fracture/loss crystal structure

  1. atomic bond breaks on one plane

Basal plane
⊥ C-axis

c axis = A vertically oriented crystal axis, usually the principal axis; the unique symmetry axis in tetragonal and hexagonal crystals

allow shear at lower stress than that needed for a perfect crystal structure

Ice - Fabric development

  1. Glacial ice

c-axes = randomly oriented

newly formed from firn

2. As ice
starts to flow

Basal planes start to glide

Most common ice deformation mechanism on lattice scale

C-axes start to reorient ∴ forming a fabric

Ice stress response

Ice crystals reorient themselves to minimise strain

∴ Crystallographic axes rotate
--> toward compression (stress) direction

Fabric development shear experiment

Results

Stereoplots

Apply load to ice @ different temperatures

Colour coding

corresponds to diff ice crystal orientations

Microstructures emerge during shear deformation

shear ice btwn 2 applied loads with opposing motion

Temp
Influence

Ice deformation mechanisms
--> vary significantly with temperature

Colder temps (-15°C)

Temp wrt melting temp affects deformation

Marked grain size reduction in sheared body

Warmer temps (-2°C)

(Closer to melting pt)

Undulose extinction
= early sign lattice deformation (bending)

Orientations correspond to c-axis orientation of individual crystal

Shear zone is distinct

n.b. Limited shortening possible via basal slip