Ice Fabrics
Fabrics in Ice Thin section
Clear ice vein crosscuts foliation
Faintly aligned fabric of ice crystals + bubbles
defined by bubble clustering
Clear Ice
Vein in t/s
cuts across foliation
irregular edges due to deformation
formed as even walled body metamorphically incorporated into ice
Firn as Sediment Analogue
Sediment on seafloor
Remains porous for some time
Retains connection with seawater
Has geochemical implications
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
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
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
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
Shown by grain size profiles & lattice preferred orientations