The basal ice layer of glaciers and ice sheets

Knight, Peter G.

doi:10.1016/s0277-3791(97)00033-4

Cited by 142 publications

(166 citation statements)

References 85 publications

Supporting

Mentioning

152

Contrasting

Unclassified

Order By: Relevance

“…lOb) r un s para llel to foli ation for at least 100 m , em erg ing from clean ice as a debri s-bearing fo ld , with steep limbs a nd a fold axis gcntly plunging up-g lacier. This ice is coar scg ra ined a nd cl ear, a nd had th e appearance of regelation icc, fo r example the "stratified facies" o f Knight (1997;fi g. 8). Furthermorc, the two -dim ensiona l c1asl fabric shows a preferrcd orientation p a rallel to the foli a ti on, although this is not as strong as fo r debris assoc iatcd with fo ldcd stratification (Fi g. 6c ).…”

Section: Fig 8 Al Odification Ofstratification-fjarallel Debris Laymentioning

confidence: 99%

Debris entrainment and transfer in polythermal valley glaciers

et al. 1999

View full text Add to dashboard Cite

ABSTRACT. M od es of debri s entra inm ent a nd subseque nt transfer in se\'e n "norm a l" a nd five surge-t yp e g lac iers in Sv a lba rd (76-79° N ) a re o utlined in the co ntext of the structura l e\'olutio n o f a glac ier as th e ice deform s durin g fl ow. Three m a in m odes of entra inm e11l a nd tra nsfe r a re inferred fro m structu ra l a nd se dime11l010gical o bsen 'ati ons: ( i) Th c inco rpora ti o n o[ a ng ul a r r oc H a l1 ma te ri a l within th e stra tifi e d sequ ence o f sno w/firn /superimposed ice. Thi s de bri s ta kes a n eng lacia l p a th throug h the g lacier, becoming folded. At th e m a rgins a nd a t th e bo und a ri es offl o w units the stra tifi ed ice including de bris is strongly fo lded , so th at near th e snout the debri s emcrges a t th e surface on the hinges a nd limbs of th e folds, producin g m edi a l mora in es which merge towa rd s the snout. Th e res ulting lin es o f d ebri s a rc tra nsmitted to th e proglac ia l a rea in the fo rm of regula r tra ins of a ng ul a r d e bris. (ii ) Incorpo ra ti o n of debris of b o th supraglae ia l a nd basa l cha racte r within longitudin a l foli ati on. This is pa rticul a rl y evide nt a t the surface of th e glac ier a t th e m a rgins o r a t fl ow unit bo und a ri es. It can be som e tim es dem onstra ted th a t foli ati o n is a product of strong fo lding, since it usua ll y has a n ax ia l pl a na r rel ati o nship with folded stratificati on. Folia ti o n-pa ra ll el debri s thus represe nts a m o re a d\'anced stage of deformati o n tha n in (i). Altho ug h the presence of basa l debri s is pro bl emati c, it is p ro poscd th a t thi s m a teri a l is tig htl y fo lded ice deri\Td fro m th e bed in th e m a nn er ofdi sh a rm onic folding. The readil y defo rmed subglacial sedime11l or bedrock surface represe nts the pl a ne of dcco llement. (iii ) Thrusting, whereby d e bri s-rich basa l ice ( including regelati o n ice ) a nd subg lacia l sedimrnl s a re uplifted i11l0 a n englac ia l pos iti o n, so metimes em e rg ing at th e ice surface. Thi s materi a l is much more va ri a bl e in cha rac ter th a n th at deri\"Cd fr ol11 roc kfa ll s, a nd reOee[s the subsLra te lith ologies; dia micton with stri a ted clas ts a nd sa ndy g ravels a rc th e most commo n fac ies represe nted . Thrusting is a d y na m ic process, a nd in p oly therm a l g lac ie rs is proba bl y linked m a inl y to th e tra nsition fro m sliding to froze n b ed co nd itions. It is n o t th erefo re a so lel y ice-m argin a l o r proglacia l process. INT RODUCTIONM o d ern Arcti c glac ie rs prO\'ide exc ell e nt oppo rtuniti es fo r the stud y of debri s in tra nsport because o f th e combin a ti o n of rel a tivel y clea n ice surfaces, stee p-sided m a rgins a nd , occasion a lly, tidewa te r cliIT secti ons. It is thu s possibl e to d ec ipher how debri s is transported in basal, englac ia l a nd suprag lac ia l positi o ns, pa rticul a rl y if the relaLi o nships be twee n ice structures (foli ati on, fo lding a nd thrusting ) a nd d eb...

show abstract

Section: Fig 8 Al Odification Ofstratification-fjarallel Debris Laymentioning

confidence: 99%

Debris entrainment and transfer in polythermal valley glaciers

et al. 1999

View full text Add to dashboard Cite

show abstract

“…The entrainment of debris at glacier beds, and its subsequent transport, also provides an important supply of material to the margins. Such entrainment, elevation, and transport is likely to be most effective under warm-based conditions, which facilitate the development of debris-rich basal ice sequences (Sharp et al, 1994;Knight, 1997), debris-rich shear planes (Glasser et al, 1998), and injections of debris into basal crevasses (Rea and Evans, 2011). It is suggested that the entrainment of subglacial debris is particularly efficient on the adverse slopes of overdeepenings, where supercooling processes result in the formation of debris-rich basal ice (Cook et al, 2010) and debris is entrained and elevated via shear planes (Swift et al, 2002).…”

Section: Supply Of Debris To Glacier Marginsmentioning

confidence: 99%

A review of topographic controls on moraine distribution

2014

View full text Add to dashboard Cite

Ice-marginal moraines are often used to reconstruct the dimensions of former ice masses, which are then used as proxies for palaeoclimate. This approach relies on the assumption that the distribution of moraines in the modern landscape is an accurate reflection of former ice margin positions during climatically controlled periods of ice margin stability. However, the validity of this assumption is open to question, as a number of additional, nonclimatic factors are known to influence moraine distribution. This review considers the role played by topography in this process, with specific focus on moraine formation, preservation, and ease of identification (topoclimatic controls are not considered). Published literature indicates that the importance of topography in regulating moraine distribution varies spatially, temporally, and as a function of the ice mass type responsible for moraine deposition. In particular, in the case of ice sheets and ice caps ( > 1000 km 2 ), one potentially important topographic control on where in a landscape moraines are deposited is erosional feedback, whereby subglacial erosion causes ice masses to become less extensive over successive glacial cycles. For the marine-terminating outlets of such ice masses, fjord geometry also exerts a strong control on where moraines are deposited, promoting their deposition in proximity to valley narrowings, bends, bifurcations, where basins are shallow, and/or in the vicinity of topographic bumps. Moraines formed at the margins of ice sheets and ice caps are likely to be large and readily identifiable in the modern landscape. In the case of icefields and valley glaciers (10-1000 km 2 ), erosional feedback may well play some role in regulating where moraines are deposited, but other factors, including variations in accumulation area topography and the propensity for moraines to form at topographic pinning points, are also likely to be important. This is particularly relevant where land-terminating glaciers extend into piedmont zones (unconfined plains, adjacent to mountain ranges) where large and readily identifiable moraines can be deposited. In the case of cirque glaciers (< 10 km 2 ), erosional feedback is less important, but factors such as topographic controls on the accumulation of redistributed snow and ice and the availability of surface debris, regulate glacier dimensions and thereby determine where moraines are deposited. In such cases, moraines are likely to be small and particularly susceptible to post-depositional modification, sometimes making them difficult to identify in the modern landscape. Based on this review, we suggest that, despite often being difficult to identify, quantify, and mitigate, topographic controls on moraine distribution should be explicitly considered when reconstructing the dimensions of palaeoglaciers and that moraines should be judiciously chosen before being used as indirect proxies for palaeoclimate (i.e., palaeoclimatic inferences should only be drawn from moraines when topographic controls on mora...

show abstract

“…Basal ice is defined as ice formed by processes occurring at the lower part of glaciers and ice sheets (for comprehensive reviews of basal ice, see Knight (1997) and Hubbard et al (2009)). Basal ice is generally composed of a series of ice layers with distinctive characteristics (basal ice facies) formed by different processes at the ice-bed interface, which are able to incorporate significant amounts of debris into the basal ice.…”

Section: Physical Environmental Conditionsmentioning

confidence: 99%