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Ice Fabrics (Mechanical Consequences Ice Evolution (Fabric develops as…
Ice Fabrics
Mechanical Consequences
Ice Evolution
Fabric develops as snow column compacted
Ice crystals assume very particular c-axis orientation
Coarsening with increasing depth
influences mechanical response to strain
Shown by grain size profiles & lattice preferred orientations
Processes snow--> ice
Surface
ρ ~ 550 kg/m3
Firn grains
rearranged
to achieve more dense packing
Greater depth
~ 60-110 m
Plastic deformation
= most important transformation process
(not rearrangement)
ρ > 800 kg/m3
Pores become isolated
i.e. mechanisms primary foliation evolution
Clear Ice
Vein in t/s
cuts across foliation
irregular edges due to deformation
formed as even walled body metamorphically incorporated into ice
Glacial Ice Evolution
Snow deposited
in layers -
very porous
Firn forms
Recrystallised into denser granular material
Partly compacted snow - open & porous
Open porous structure
communicates w/atmos &surface
pores trap air
Glacial ice forms ~ 60-110m
Bubbles pinched off ∴ isolated from atmosphere
Gas pressure now = lithostatic pressure (~overburden)
Climate archive in gas etc.
Ice with isolated bubbles
Fine
grained
rock
Has substantially larger surface area
(vs coarse)
--> Influences all processes reliant on grain boundaries
i.e. favouring metamorphic processes
--> fluid migration occurs along grain boundaries
Fabrics in Ice Thin section
Clear ice vein crosscuts foliation
Faintly aligned fabric of ice crystals + bubbles
defined by bubble clustering
Firn as Sediment Analogue
Sediment on seafloor
Remains porous for some time
Retains connection with seawater
Has geochemical implications