Hart 2011 (10) Flashcards
Prior to the 1980s, what were subglacial tills assumed to be formed by?
What do we now know?
Once the importance of pore-water pressure on subglacial processes was demonstrated, it was suggested that lodgement and deformation till were actually part of a pore-water pressure continuum, which varies temporally and spatially in the form of mobile beds and sticky spots
Till deformation involves movement and reorientation of particles at what scales?
Both microscopic (clast and matrix scale) and macro- scopic (bulk rheology) scales.
What has this paper done?
Developed a new in situ wireless Glacsweb probe to examine the subglacial environment using use probes to investigate the two scales of deformation, rheology and hydro-mechanical interactions at the icebed interface.
In the latter, we focus on the relationship between water pressure (hydrology), case stress and till temperature (mechanical properties).
Glacier motion is composed of…
ice flow, basal sliding and subglacial deformation.
Sediments such as soil or till undergo mixed behaviours of elasticity, viscosity and plasticity during deformation…
At low stresses material deforms elastically and when the stress is removed, the original shape is restored.
However, above a critical stress value (yield stress), the material may deform in a viscous manner, i.e. stress is proportional to strain rate, and the original shape is unrecoverable.
Low water pressures are associated with…
high case stress vari- ability and till temperatures. We suggest that this reflects velocity driven stick-slip events, directly transmitted via the grain structure through a relatively strong till (due to liquid grain bridges). Inter- mediate water pressures are associated with intermediate case stress variability and high till temperatures, reflecting the importance of friction associated with high deformation under these conditions.
High water pressures are associated with…
low case stress variability and low meltwater controlled till temperatures.
It was also shown that as water pressure rose, the till weakened, but as pressures continued to rise and the till reached saturation, it also strengthened in association with dilation.
