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Ice Deformation Microscale (Ice Fabric Development (Grain boundary…
Ice Deformation Microscale
Shear experiment changes
Ice crystals subjected to shear at varying temperatures
Crystals exhibit:
Grain size reduction
Distinct shear zone
Undulatory extinction
Fabric development
Fabric
Initially random c-axis distribution evolves to preferred/aligned orientation
Bimodal distribution - 2 maxima
Shear zone centre - c-axis clustering esp. pronounced
Strong clustering crystallographic orientation around N-S
Def @ microscale involves:
Grain size reduction
Fabric development
Temperature plays important role in rock & ice deformation
Fabric
= result of material aligning crystals to minimise resistance to deformation
i.e. achieve energetically optimal state to minimise stress
Temp
Influence
Deformation mechanisms
vary significantly with temp
(Temp wrt
melting temp)
Colder temps (-15°C)
Marked grain size reduction in sheared body
Warmer temps
(-2°C)
(Closer to melt pt)
Undulose extinction
= early sign lattice deformation (bending)
Shear zone is distinct
Ice Fabric Development
Grain boundary migration
Lattice distortion
Kinks (& slip lines)
New subgrains form
1. Grain Boundary Migration
Grain boundary 'eats' into neighbouring crystal
∴ minimises dislocations in neighbouring crystal
2. Lattice Distortion
Lattice starts to bend
affects how light passes through minerals
Alters interference colours
shown by gradient in birefringence colour
Dynamic recryst
New subgrains form via dynamic recrystallisation
= Recovery & crystallisation
Strain accomodation
Limited shortening possible by basal slip
Limited void space in ice
Once aggregate cannot shorten further
Dislocations accumulate in lattice
Raising its internal free energy
Recovery process
∴ Forming domains with lower internal energy
Crystal reduces internal energy by
--> rearranging dislocations into subgrain walls
Dynamic Recrystallisation Mechanisms
Bulging
Subgrain Rotation Recrystallisation
Grain Boundary Migration
Mechanism = f ( temp, strain rate etc )