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Ice Deformation Mechanisms (Dominant mode ice def (= Slip on basal 0001…
Ice Deformation Mechanisms
Dominant
mode ice def
=
Slip on basal 0001 plane
= deformation of crystal by mvmt of an edge dislocation
i.e. half plane added/inserted into lattice
Stress & creep accomodation
Propagation of dislocations through crystal lattice
Fabric development
in ice
--> as lattice shape is rearranged
Causes
plastic deformation
of individual crystals ∴ material itself
Dislocation
= linear defect in a crystal lattice i.e. rock / ice
allow shear at lower stress than that needed for a perfect crystal structure
Ice mass subjected to body forces
reflect gravitational potential
experienced by ice mass
causes creep motion
Dislocation glide
(slip)
= migration of dislocation through lattice
How ?
bond reattaches on adjacent plane
allows dislocation migration
atomic bond breaks on one plane
Crystal shape changes
without
mechanical fracture/loss crystal structure
Basal plane
⊥ C-axis
c axis = A vertically oriented crystal axis, usually the principal axis; the unique symmetry axis in tetragonal and hexagonal crystals
Ice - Fabric development
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
Orientations correspond to c-axis orientation of individual crystal
Colour coding
corresponds to diff ice crystal orientations
Apply load to ice @ different temperatures
shear ice btwn 2 applied loads with opposing motion
Microstructures emerge during shear deformation
Temp
Influence
Ice deformation mechanisms
--> vary significantly with temperature
Temp wrt melting temp affects deformation
Colder temps (-15°C)
Marked grain size reduction in sheared body
Warmer temps
(-2°C)
(Closer to melting pt)
Undulose extinction
= early sign lattice deformation (bending)
Shear zone is distinct
n.b. Limited shortening possible via basal slip