Contrasting glacier behavior over deformable and non-deformable beds Gaute Lappegard

Photo: Jürg Alean

Photo: National Snow and Ice Data Center

Movie courtesy: UNIS

Photo: Jürg Alean

Glaciers on deformable and non-deformable beds Deformable bedNon-deformable bed Ice streams Surging glaciers Valley glaciers Ice sheets Valley glaciers Ice sheets

Temperature control on basal processes T z Pressure melting point, T M If T Bed < T M : no/few active basal processes Photo: Michael Hambrey T M (z) = z T M (1000) = ºC

Deformation of multilayered structures A) Glacier/bedrockC) Glacier/water/bedB) Glacier/sediments Driving stress: τ d = ρ g h sin α α

Glacial beds have different capabilities of handling water Photo: Frank Wilschut No outlet streams Porous media saturated aquifer Surface water tunneled into a few outlet stream For both beds: The diurnal variability of melt water input can force diurnal velocity changes Photo: Roger J. Braithwaite

Non-deformable bed: High flux hydraulics Photo: Michael Hambrey R-channels: Melt enlargement and creep closure in competition Flowing water generates heat Channel enlargement into the ice Creep closure due to deformable ice Seasonal and diurnal geometry evolution Steady-state: inverse pressure-discharge relation arborescent structure low surface-to-volume ratio courtesy: U.H. Fischer

Kamb, 1987 Non-deformable bed: Low flux hydraulics

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Kamb, 1987 Non-deformable bed: Low flux hydraulics Distributed system: High water pressure Low flux Proportional discharge-pressure relation Non-arborescent structure Large surface-to-volume ratio courtesy: U.H. Fischer

Lappegard et.al., 2005 A non-deformable bed is kept clean by the hydraulic systems A p w is low B p w is high C p w is low Hubbard et.al., 1995

Deformable bed: Darcian flow, canals and R-channels Thin sediment layers can not transport large fluxes of water the drainage capacity will be exceeded by the water supply water will start flowing along the ice- till interface R-channel canal For small surface slopes (<0.1) water will drain in canals of high water pressure eroded into the sediments For large surface slopes (>0.1) water will drain in R-channels eroded into the ice

Water pressure influence on sliding and bed deformation Blake et.al, 1994 Glaciers on both deformable and non-deformable beds can respond temporally with increased velocity to a rapid increase in water pressure Effective pressure is defined as: p e = p i – p w p e - indicates level of buoyancy (if p e = 0, the glacier floats!) p i - applied load (ice overburden) p w – either water pressure in the drainage system or porewater pressure of the till

Ice flow Sliding on non-deformable bed: The controlling obstacles Water at the ice-bedrock interface smoothens the bed Fowler, 1987 From fig.: p e (a) > p e (b) > p e (c) For a given basal shear stress sliding, u b, increases when the effective pressure, p e, decreases Sliding inversely related to the effective pressure: u b ~ τ b p p e -q The drag on the ice is generated by obstacles not drowned

Sliding on deformable bed: Controlled by porewater pressure Small scale roughness absent Drag by particles/rocks reduced significantly due to deforming till Shear stress from the ice transmitted to the till Sliding depends on till properties as porosity: n = n ( p e ) shear strength: τ f = τ f ( p e ) both functionally dependent on p e Dilatancy shear thickening i) No free water available porewater pressure decreases shear strength increases ii) Free water available water volume increases shear strength decreases

Deformable bed: Porewater pressure experiment Iverson et.al., 2003

Sliding on deformable bed: Controlled by porewater pressure Low ice flow due to: High sediment strength discourage sediment deformation Sliding and ploughing porewater pressure High ice flow due to: Low sediment strength encourage sediment deformation Dilatation and transition to pervasive ductile flow High ice flow due to: Decoupling and reduction of basal deformation rates Ice flow

Erosion on non-deformable bed Photo: Michael HambreyPhoto: Jürg Alean Photo: Tom Lowell

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Landforms on deformable bed Courtesy: D. Robinson Streamlined subglacial bed forms (drumlins, flutes and Rogen moraines) explained by an instability in the laminar flow of ice over a deformable substrate (Hindmarsh (1998), Fowler (2000))

Glaciers on deformable and non-deformable beds Deformable bedNon-deformable bed Bed displacement Sliding, deformation, free-slip Hydraulics Darcian flow, canals and R-channels Hydraulics Linked cavities and R-channels Bed displacement Sliding Landforms streamlined forms (drumlins) Landforms Roches moutonnées, U-valleys