thickness and extent of the subglacial till layer beneath an antarctic paleo ice stream fast flowing ice streams and outlet glaciers currently account for as much as 90 of the discharge from the antarctic and greenland ice sheets although the deformation of subglacial material has been proposed as the mechanism for this rapid motion such sediment is usually hidden under several kilometers of ice marine geophysical records have allowed reconstruction of the three dimensional thickness of the sedimentary bed beneath a large antarctic paleo ice stream for the first time fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength unsorted sediment averaging 4 6 m thick rapid motion of the paleo ice stream was a result of subglacial deformation within this layer

bedforms of the keewatin ice sheet canada abstract by compiling glacial bedforms on a map that covers most of one sector of the laurentide ice sheet it is possible to make some suggestions about their genesis based largely on spatial relationships it can be concluded that drumlins and ribbed moraine form at the base of actively flowing ice under similar dynamic conditions for either landform to exist however there must have been enough sediment available in the base of the glacier to leave or form a feature large enough to be recognizable the presence or absence of sufficient load is related to the geology of the glacier bed and has little to do with regionally changing dynamics of the ice water system likewise given sufficient load it is evident that whether drumlins formed or whether ribbed moraine formed in a certain area is a function of the physical nature of the load which is again related to geology of the source outcrops whether the physical characteristics come into play after the sediment has been released from the ice and is being reshaped by basal drag streamlining etc or whether the nature of the load while entrained changes the behaviour of the basal part of the ice is unclear physical characteristics of the basal sediment load have apparently promoted internal thrusting of coherent slabs of entrained debris and ice to form ribbed moraine on melting whereas drumlins may reflect moulding of plastic subglacial debris or erosional streamlining of both the unconsolidated glacial substrate and bedrock the observation that many eskers cross drumlin fields at nearly right angles to their orientation suggests that conditions producing streamlining and those pertaining to subglacial drainage are separated in time and circumstance the general occurrence of drumlins and eskers throughout the sediment rich portions of the keewatin ice sheet from zone 1 to its edge is difficult to reconcile with the restriction and intimate association of these forms with ribbed moraine almost exclusively in zone 2 because such a zonal relationship exists to some extent around other ice divides at least in labrador nouveau quebec and newfoundland it seems that some condition changed or existed in this zone throughout or at some specific time during the existence of the keewatin ice sheet possibly around the last ice divides reactivation of the ice sheet by alimentation associated with climatic deterioration may have promoted flow of thin brittle ice this may have shattered the glacier forming stacked thrust plates of ice and debris with subsequent stagnation the thrust plates may have melted out without substantial deformation

striated clast pavements products of deforming subglacial sediment studies of modern glaciers have recently drawn attention to the importance of subglacial sediment deformation to glacier dynamics and processes inferences regarding the probable shear strength of this sediment imply that large clasts may in some cases sink to underlying rigid sediment where abrasion by overlying deforming sediment could occur this scenario provides a formative mechanism for striated clast pavements commonly described from the base of fine grained massive diamictons associated with the late pleistocene laurentide ice sheet such a mechanism indicates that at the time of formation of clast pavements overlying diamictons associated with pavements had a low yield strength 0 5 kpa and were deforming mechanically like a debris flow clast pavements may therefore be an important criterion for recognition of sediments deposited by subglacial deformation transport

lobal interactions rheologic superposition and implications for a pleistocene ice stream on the continental shelf of british columbia abstract data on structures stone striae clast pavements fabrics morphology provenance and matrix grain size of four pre late wisconsinan diamictons are presented from the lower accessible part of an 855 m long sea cliff at cape ball east coast of graham island queen charlotte archipelago british columbia the data indicate that an ice stream crossed the continental shelf from the british columbian mainland to the queen charlotte islands local queen charlotte piedmont ice coalesced with the western edge of the cordilleran ice sheet resulting in northward flow deflection along the east coast of graham island generally consistent structural and stone data indicate that diamictons were deposited as primary subglacial tills and that subglacial dynamics and rheologic conditions were complex two of the tills can be traced continuously over the cape ball section which provides a north south transect oblique to ice flow each of the two tills reveals five major changes in rheology and genesis ranging from a strong lodgement component to dominantly deformation mainly by subglacial squeeze flow true end member tills are rare and in almost all places the tills are hybrids that formed within a continuum of till forming processes commonly involving stone rotation and superposition of structures and rheologic states this could have occurred mainly in response to fluctuating till pore water content along the transect two dimensional stretches of contrasting till types probably represent a three dimensional patchwork of contrasting till rheologies and pore waters if patches of ductily deforming till were common and or widespread they could have accounted for much of the movement of cordilleran ice over glaciomarine mud on the continental shelf when sea level was much lower than present deforming probably saturated muddy till reconstituted glaciomarine would have possessed negligible shear strength resulting in a thin rapidly moving glacier most likely an ice stream that reached cape ball but failed to remove the record of previous glacial events the ice stream probably issued out of the mainland s skeena valley which is aligned with cape ball and is approximately the size of valleys in antarctica that today conduct small outlet glaciers and ice streams evidence for this and other ice streams on the northwestern continental shelf of north america imply that destabilization and rapid decay of the western edge of the cordilleran ice sheet may have been accomplished by a combination of ice streams sea level rise and extensive calving

a model for spiral flows in basal ice and the formation of subglacial flutes based on a reiner rivlin rheology for glacial ice 1 flutes are elongated sediment ridges formed at the base of glaciers and ice sheets in this paper we show that flutes can be the product of a corkscrew like spiral flow in basal ice that removes sediment from troughs between flutes and deposits it at their crests as first suggested by shaw and freschauf in order to generate the type of basal ice flow required for this mechanism the viscous rheology of ice must allow for the generation of deviatoric normal stresses transverse to the main flow direction this type of behavior which is commonly observed in real nonlinearly viscous and viscoelastic fluids can be described by a reiner rivlin rheology here we develop a mathematical model that describes the role of these transverse stresses in generating spiral flows in basal ice and investigate how these flows lead to the amplification of initially small basal topography and the eventual formation of assemblies of evenly spaced subglacial flutes

lava lake surface characterization by thermal imaging erta ale volcano ethiopia the project sviluppo di sistemi di monitoraggio dipartimento di protezione civile di regione sicilia ingv catania section italy the leverhulme trust the bbc and the ethiopian air force

rheological properties of paraffin as an analogue material for viscous crustal deformation the rheological stratification induced by temperature variations within the crust requires analogue materials with temperature sensitive viscosity to properly simulate the change of mechanical properties with depth rheological calibration experiments on commercial paraffin wax with melting point of 53 1 c show that this material is a nonlinear fluid 1 6 n 2 1 for homologous temperatures t tm 0 70 and an approximately newtonian fluid 1 0 n 1 3 for t tm 0 70 the decrease in the stress exponent n is probably associated with the phase transition of paraffin effective viscosity values are strongly temperature dependent varying by nine orders of magnitude from 1010 to 10 pa s in the temperature range between 30 and 52 c the rheological properties of paraffin make this material useful for modelling crustal rheology and tectonic processes when linear dimensions and time scale down by a factor of 106 and 1011 respectively

a multiproxy study of a basal till a time transgressive accretion and deformation hypothesis a two part basal till at knud strand denmark reveals a uniform fabric pattern and strength petrographical composition and clay mineralogy the nature of the contact with the underlying sediments ductile deformation structures partly intact soft sediment clasts small meltwater channels and thin horizontal outwash stringers dispersed in the till indicate both bed deformation and basal decoupling by pressurised subglacial water a time transgressive model is suggested to explain the lack of vertical gradation in till properties in which debris released from the active ice sole is sheared in a thin zone moving upward as till accretion proceeds it is suggested that although strain indicators occur throughout the entire till thickness the deformation at any point of time encompassed the uppermost part of the till only allowing preservation of fragile clasts below the substantial thickness of the till up to 6 m coupled with a much smaller by more than one order of magnitude inferred thickness of the deforming bed suggests that the bulk of till material was transported englacially prior to deposition the lack of petrographical gradation in the till is attributed to effective mixing and homogenisation of material along the ice flow path copyright 2004 john wiley sons ltd

bedform signature of a west antarctic palaeo ice stream reveals a multi temporal record of flow and substrate control abstract the presence of a complex bedform arrangement on the sea floor of the continental shelf in the western amundsen sea embayment west antarctica indicates a multi temporal record of flow related to the activity of one or more ice streams in the past mapping and division of the bedforms into distinct landform assemblages reveals their time transgressive history which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo ice velocity as suggested previously a main control on the bedform imprint is the geology of the shelf which is divided broadly between rough bedrock on the inner shelf and smooth dipping sedimentary strata on the middle to outer shelf inner shelf bedform variability is well preserved revealing information about local complex basal ice conditions meltwater flow and ice dynamics over time these details which are not apparent at the scale of regional morphological studies indicate that past ice streams flowed across the entire shelf at times and often had onset zones that lay within the interior of the antarctic ice sheet today in contrast highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow and may be a truer representation of fast palaeo ice flow in these locations a revised model for ice streams on the shelf captures complicated multi temporal bedform patterns associated with an antarctic palaeo ice stream for the first time and confirms a strong substrate control on a major ice stream system that drained the west antarctic ice sheet during the late quaternary

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

a groove ploughing theory for the production of mega scale glacial lineations and implications for ice stream mechanics mega scale glacial lineations msgls are longitudinally aligned corrugations ridge groove structures 6 100 km long in sediment produced subglacially they are indicators of fast flow and a common signature of ice stream beds we develop a qualitative theory that accounts for their formation and use numerical modelling and observations of ice stream beds to provide supporting evidence ice in contact with a rough scale of 10 10 3 m bedrock surface will mimic the form of the bed because of flow acceleration and convergence in ice stream onset zones the ice base roughness elements experience transverse strain transforming them from irregular bumps into longitudinally aligned keels of ice protruding downwards where such keels slide across a soft sedimentary bed they plough through the sediments carving elongate grooves and deforming material up into intervening ridges this explains msgls and has important implications for ice stream mechanics groove ploughing provides the means to acquire new lubricating sediment and to transport large volumes of it downstream keels may provide basal drag in the force budget of ice streams thereby playing a role in flow regulation and stability we speculate that groove ploughing permits significant ice stream widening thus facilitating high magnitude ice discharge

the composition and internal structure of drumlins complexity commonality and implications for a unifying theory of their formation investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory unifying explanation of their formation this paper presents the first systematic survey of the vast literature on this subject with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain results confirm that investigations are often limited by availability of suitable sediment exposures 40 of studies report data from 5 drumlins and 44 do not specify sample size although borehole data and geophysical techniques can alleviate this problem however it is clear that the constituents of drumlins are incredibly diverse in terms of their composition e g a range of lithologies clast shapes sizes and fabrics structure e g sediments that are sorted homogeneous surface conformable unconformable and evidence of deformation e g ranging from pervasive to non pervasive limited to absent despite this diversity our review leads us to suggest that drumlin composition can be simplified to five basic types i mainly bedrock ii part bedrock part till iii mainly till iv part till part sorted sediments and v mainly sorted sediments this is a potentially significant step in that it reduces the oft cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target the various types can occur within the same drumlin field which leaves us with two possible implications for drumlin formation 1 different types of drumlin are formed seeded by different processes despite being morphologically similar equifinality investigation of drumlin composition may therefore reveal diagnostic processes explanations for these different types of drumlin and we argue that bedrock drumlins are an example 2 a single process occurs across large parts of the ice bed interface to create drumlinised terrain in a variety of sediments investigation of drumlin composition may in this case simply reflect pre existing sediments but importantly the way in which the drumlin forming mechanism modifies is modified by them we argue that the latter simpler explanation applies to all other types of drumlin excluding purely bedrock forms and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 2 sedimentology and stratigraphy mega scale glacial lineations msgls are highly elongate subglacial landforms produced beneath zones of fast flowing ice while qualitative data on their morphology have existed for several decades studies of their composition and sedimentology are comparatively rare sediment exposures along the course of the finnie river in nunavut northern canada provide a window into the internal stratigraphy and sedimentology of msgls formed by the dubawnt lake palaeo ice stream during regional deglaciation of the laurentide ice sheet stratigraphic sections record evidence for an initial advance of ice into the study area followed by ice sheet recession and deposition of glacifluvial and glacilacustrine outwash subsequently the dubawnt lake palaeo ice stream overrode and reworked this outwash subglacially forming an msgl till this till comprises a sandy red diamicton facies forming the core of the msgl ridges and containing variably deformed lenses stringers and rafts of outwash the sedimentology of this diamicton is consistent with an origin as a glacitectonite and hybrid till formed by a combination of non pervasive subglacial sediment deformation and lodgement facies variations from stratified to massive diamicton reflect in turn variations in strain and subglacial transport distance the occurrence of stratified glacifluvial sediments within these ridges and the well preserved nature of many of the sandy inclusions within the diamicton imply relatively short transport distances and incomplete mixing msgls under the dubawnt lake palaeo ice stream formed through a combination of subglacial erosion and deposition this included non pervasive subglacial sediment deformation and the reworking of pre existing sediment depocentres during streaming flow these results highlight the importance of sediment supply to msgl formation with the presence of abundant pre existing sediments which were subsequently overridden being critical to lineation formation

formation of mega scale glacial lineations observed beneath a west antarctic ice stream most discharge from large ice sheets takes place through fast flowing ice streams and their speed is strongly modulated by interactions between the ice and the underlying sediments seismic surveys and investigations through boreholes have revealed a spatial association between fast ice flow and saturated deformable sediments nevertheless our knowledge of the morphology of the interface between ice and sediments is still limited resulting in only rudimentary understanding of the basal boundary conditions beneath ice streams and the generation of subglacial bedforms here we present radar data from the bed of a west antarctic ice stream that reveal the presence of mega scale glacial lineations we combine these data with previously published seismic data and show that these lineations develop in areas of dilatant deforming till and are part of a dynamic sedimentary system that undergoes significant change by erosion and deposition on decadal timescales we find that the mega scale glacial lineations are indistinguishable from those found on beds of palaeo ice streams providing conclusive evidence for the hypothesis that highly elongate bedforms are a characteristic of fast flow regions in ice sheets

a laboratory analog study of the surface morphology of lava flows extruded from point and line sources abstract in laboratory experiments designed to model lava flow processes liquid polyethylene glycol wax is forced through either a small hole or a long narrow slit onto the base of a tank of cold water where it spreads laterally while cooling and solidifying at its surface we observe the surface structure of the flow and its dependence on the flow rate thermal conditions and basal roughness in each case solidification of a crust during spreading gives rise to a number of different surface morphologies each of which forms under a restricted range of conditions the dominant morphologies referred to as pillows rifts transverse folds and marginal levees correspond to features observed on natural lava flows results for radial spreading over a rough base and for spreading in two directions from the line source over both smooth and rough bases complement those reported earlier for radial spreading of solidifying wax from a point source on a smooth base together they indicate a robust dependence of morphology on the distance from the vent at which solid crust begins to form this distance is in turn determined by the extrusion rate the rheology of the liquid wax or magma the reduced gravity the magnitude of the surface heat flux and the amount of cooling required to solidify the flow surface the results also indicate factors influencing the distribution of crust and its deformation and may provide a means by which observations of surface morphology can be used to place constraints on the emplacement conditions of lava flows

comment on shaw j pugin a and young r 2008 a meltwater origin for antarctic shelf bedforms with special attention to megalineations geomorphology 102 364 375 the submarine glacial geomorphology and sedimentology of the cross shelf troughs and the adjacent continental slope around the antarctic peninsula and west antarctica have been the focus of a series of marine geophysical and geological investigations over the last decade e g shipp et al 1999 canals et al 2000 wellner et al 2001 lowe and anderson 2002 o cofaigh et al 2002 dowdeswell et al 2004 heroy and anderson 2005 evans et al 2005 domack et al 2006 mosola and anderson 2006 wellner et al 2006 dowdeswell et al 2006 o cofaigh et al 2007 o cofaigh et al 2008 c o cofaigh j a dowdeswell j evans and r d larter geological constraints on antarctic palaeo ice stream retreat earth surface processes and landforms 33 2008 pp 513 525 o cofaigh et al 2008 and dowdeswell et al 2008 these studies have interpreted sets of characteristic streamlined glacial bedforms and sediments on the shelf which in numerous cases occur in front of modern ice streams as largely the product of fast flowing palaeo ice streams that drained across the shelf during or following the last glacial maximum a key feature of these studies is the observation of highly attenuated bedforms known as mega scale glacial lineations msgl formed in soft sediment on the outer continental shelf these lineations are regarded as key evidence for streaming flow in their recent paper shaw et al present a radically different interpretation of the glacial geomorphology of antarctic cross shelf troughs in terms of catastrophic discharge of subglacial meltwater floods across the shelf in their interpretation msgl are regarded as the product of erosion by turbulent meltwater flow the following comment discusses a number of the key assertions made in their paper

fabric signature of subglacial till deformation breidamerkurjokull iceland the foreland of breidamerkurjokull iceland is the only locality where tills known to have undergone subglacial deformation are exposed till on the foreland has a two tiered structure consisting of a dilatant upper horizon c 0 5 m thick and a compact lower till these horizons correspond to the ductile deforming a horizon and the brittle ductile b horizon observed below the glacier by g s boulton and co workers the relationship between known strain history and a variety of macrofabric elements is examined for these two genetic facies of deformation till the upper horizon exhibits variable a axis fabrics and abundant evidence for clast re alignment reflecting ductile flow and rapid clast response to transient strains in contrast the lower horizon has consistently well organized a axis fabrics with a narrow range of dip values recording clast rotation into parallel with strain axes during brittle or brittle ductile shear the data indicate that till strain history imparts identifiable macrofabric signatures providing important analogues to guide the interpretation of pleistocene tills

the athabasca fluting field alberta canada implications for the formation of large scale fluting erosional lineations abstract current drumlin and fluting hypotheses centred on processes of formation erosion and deposition by subglacial deformation accretion by lodgement glacial erosion and meltwater erosion and deposition rarely receive comprehensive field testing this paper documents the morphology internal structure and landscape associations of the athabasca fluting field alberta and discusses how well these characteristics are explained by postulated fluting forming processes fluting hypotheses are comprehensively tested by this approach and their rejection or acceptance are firmly based on field observation and knowledge of processes using multiple criteria fluting is shown to be erosional and not to be a product of pervasive subglacial deformation or accretion by lodgement morphology landscape associations bed truncation plus glaciofluvial lags and gravel on fluting ridges are inferred to support erosion by meltwater intact primary bedding synsedimentary faults and injection structures very close to the landsurface showing no sign of shear deformation are used to reject erosion by subglacial deformation fluting is shown to be closely associated with tunnel channels and indicates that these channels were overtopped by broad floods that formed the fluting field and eroded anabranching tracts over the length of the alberta plains prior to fluting formation sequences with melt out and lodgement till conduit sediment and boulder lags record net deposition over an extended time period diapirism prior to fluting formation marks a period of extreme pressure gradients which are related to bed separation following fluting formation there was only minimal landscape modification dominated by glaciofluvial and glaciolacustrine deposition not glacial processes this progression is presented as evidence for two very different glacial regimes separated by a short period with diapirism and extensive landscape modification by meltwater erosion we note that the study area was glaciated only once during the late wisconsinian and it follows that glacial sedimentation and landscape modification were confirmed to this single glacial episode

geomorphological criteria for identifying pleistocene ice streams ice streams are critical regulatory mechanisms in contemporary ice sheets it has been inferred that they also had a significant effect on the dynamics of former ice sheets subsequently many people have invoked their widespread occurrence from a variety of formerly glaciated areas hypothesised locations however have often outweighed meaningful evidence this paper addresses the problem using the characteristics of contemporary ice streams as a basis for their identification from former ice sheet beds a convergence of knowledge gained from contemporary ice stream research coupled with theories of glacial geomorphology allows several geomorphological criteria to be identified as suggestive signatures of ice stream activity it is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams the criteria are used to construct conceptual land system models of the beds of former ice streams and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based

on subglacial stone pavements in till four pavements from central and western canada display uneven pavement surfaces abundant tilted and inverted stones reversed lee ends fine crossing striae in various directions weak striae around curves worn facets distributed evenly on rounded stones and till fabric girdles collectively inconsistent with the popular concept of stones firmly held in till being abraded by overriding ice based on these data new models are offered that involve stone rotation in viscously deforming till to produce deformation pavements in the specific model increased pore water pressure under moving ice locally mobilizes previously formed lodgment and or erosional pavements into deformation pavements in the more general model stones released from the sole of a wet based glacier rotate interact and settle into a viscous suspension zone in deforming till according to velocity density and viscosity gradients stone axes and striae settle into various orientations following dewatering and till stiffening conceptu

chambered structures from the ediacaran dengying formation yunnan china comparison with the cryogenian analogues and their microbial interpretation enigmatic chambered structures have been reported forming reef frames in cryogenian interglacial carbonates prior to the commonly acknowledged microbial metazoan reefs at the terminal ediacaran and interpreted as fossils of possible sponge grade organisms a better constraint on the affinity of these structures is partly hindered by few analogues in other time periods this study describes similar structures from peritidal dolostones of the ediacaran denying formation from yunnan china samples were investigated using optical microscopy and three dimensional 3 d reconstruction based on grinding tomography the dengying chambered structures are comparable with cryogenian structures in basic construction but are not frame building and show variations in overall shape and inhabiting facies two dimensional 2 d cross sections show that thin homogeneous micritic laminae are the basic building blocks of the chamber walls thick walls represent parallel accretion of these laminae and thin walls developed from the angular growth of a single lamina or merging of multiple laminae in 3 d space the laminae primarily correspond to continuous surfaces which sometimes contain sub circular holes while a few represent filamentous elements connected to the surfaces the morphological features and growth pattern of the dengying chambered structures indicate that they are likely to be calcified microbial constructions rather than skeletal remains of basic metazoans however aside from the cryogenian and dengying examples comparable chambered constructions with laminae based architecture are yet unknown from other fossil or extant microbialites further work investigating related structures is needed to determine the microbial consortia and controlling environmental factors that produced these chambered structures

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

superimposition of ribbed moraines on a palaeo ice stream bed implications for ice stream dynamics and shutdown the sediments and landforms preserved on palaeo ice stream beds can provide important information about their subglacial conditions and flow mechanisms and the processes accompanying their shutdown in this paper detailed observations of an intriguing subglacial landform assemblage of ribbed moraines superimposed on glacial lineations on the dubawnt lake ice stream bed north west canadian shield are presented including their morphometry internal structure from ground penetrating radar gpr surveys and from glaciogeological analysis and sedimentological characteristics from sediment architecture and lithofacies analysis the observations suggest an abrupt change in ice dynamics that correlates with two phases of glacial landform development this hypothesis is based on evidence from a deformed lodgement till which subsequently underwent brittle deformation and developed prominent thrust shear structures and tension fractures tension fractures are observed in a sediment exposure and thrust structures are observed in gpr surveys where they occur most prominently in the ribbed moraine ridge crests the presence of the fractures and their association with a population of clasts in the till that are orientated with their a axes transverse to the inferred ice flow direction suggests a compressional flow regime it is therefore inferred that the glacial lineations were formed under an extensional flow regime during ice stream activity but that at some point patches of till under the ice stream stiffened through dewatering the subsequent increase in basal shear stress resulted in compressional flow and the development of subglacial thrusting and the building of ribbed moraines we therefore suggest that ribbed moraines may form in areas of compressional flow under ice streams i e sticky spots and or at the transition between slow and fast ice flow along parts of an ice stream the general absence of ribbed moraines on most other palaeo ice stream beds suggests that either these ice streams continued operating during deglaciation or processes other than the development of localized compressional flow sticky spots led to their shutdown e g ice depletion

till deposition by glacier submarginal incremental thickening macro and micro scale sedimentological analyses of recently deposited tills and complex push squeeze moraines on the forelands of icelandic glaciers and in a stacked till sequence at the former younger dryas margin of the loch lomond glacier lobe in scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris rich basal ice by cumulative freeze on processes glacier submarginal processes are dictated by seasonal cycles of refreezing and melt out of tills advected from up ice by a combination of lodgement deformation and ice keel and clast ploughing although individual till layers may display typical a and b horizon deformation characteristics the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record at the macro scale icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings very strong fabrics typify unequivocally lodged clasts despite previous interpretations of these tills as subglacial deforming layers micro morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes and water escape and sediment flowage features are widespread a model of submarginal incremental thickening is presented as an explanation of these data involving till slab emplacement over several seasonal cycles each cycle involves 1 late summer subglacial lodgement bedrock and sediment plucking subglacial deformation and ice keel ploughing 2 early winter freeze on of subglacial sediment to the thin outer snout 3 late winter readvance and failure along a decollement plane within the till resulting in the carriage of till onto the proximal side of the previous year s push moraine 4 early summer melt out of the till slab initiating porewater migration water escape and sediment flow and extrusion repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements

a frozen record of density driven crustal overturn in lava lakes the example of kīlauea iki 1959 lava lakes are found at basaltic volcanoes on earth and other planetary bodies density driven crustal foundering leading to surface renewal occurs repeatedly throughout the life of a lava lake this process has been observed and described in a qualitative sense but due to dangerous conditions no data has been acquired to evaluate the densities of the units involved k lauea iki pit crater in hawai i houses a lava lake erupted during a 2 month period in 1959 part of the surface of the k lauea iki lake now preserves the frozen record of a final incomplete crustal overturn cycle we mapped this region and sampled portions of the foundering crust as well as overriding and underlying lava to constrain the density of the units involved in the overturn process overturn is driven by the advance of a flow front of fresh low density lava over an older higher density surface crust the advance of the front causes the older crust to break up founder and dive downwards into the lake to expose new hot low density lava we find density differences of 200 to 740 kg m3 between the foundering crust and over riding and under lying lava respectively in this case crustal overturn is driven by large density differences between the foundering and resurfacing units these differences lead inevitably to frequent crustal renewal simple density differences between the surface crust and underlying lake lava make the upper layers of the lake highly unstable

extent and basal characteristics of the m clintock channel ice stream abstract the m clintock channel palaeo ice stream drained the northwestern margin victoria island arctic canada of the laurentide ice sheet functioning for 200 years prior to 10 ka bp from its bed imprint it is reconstructed as 720 140 km with a cross sectional area of 100 km2 comparable in size to the hudson strait ice stream it drained an estimated 80 000 km3 of ice enough to significantly affect the shrinking ice sheet and drive the keewatin ice divide south eastwards such discharges likely produced heinrich events and influenced the temperature and salinity of the western arctic ocean ice stream location was controlled by a topographic trough but glaciological regulation rather than properties of the bed controlled margin locations and hence ice stream width the ice stream eroded into pre existing unconsolidated sediments and left a pattern of subglacially produced landforms recording a snapshot view of the bed prior to shutdown frictional shutdown occurred once downcutting through sediments reached bedrock in places high sediment availability appears critical for ice stream functioning with a drift thickness of 50 m required to sustain its 200 year lifespan debris discharge was exceptionally high 73 000 m3 a 1 m 1 this ice stream demonstrates non steady response of ice sheets to climate change and their large impact on mass balance and stability of ice sheets

identification of variable convective regimes at erta ale lava lake abstract during february 17 19 2002 we collected a combined thermal and seismic data set for persistent lava lake activity at erta ale volcano ethiopia these data indicate that the lake cycled between periods characterized by low 0 01 0 08 m s 1 and high 0 1 0 4 m s 1 surface velocities typically lasting tens to hundreds of minutes these periods of high and low velocity motion define periods of vigorous and sluggish convection respectively spectral analysis revealed that vigorous convection periods were characterized by high frequencies and energies in the thermal data and an increased presence of high frequency energy in the seismic data the data show that vigorous periods were characterized by formation of hot short lived plastic crusts and sluggish periods by cooler longer lived brittle crusts here the higher transit velocity across the lake surface from upwelling zones of crust formation to downwelling zones of crust destruction during the vigorous periods decreases the crust lifetime this in turn decreases the total cooling and thickening experienced by a plate of crust moving across the lake surface two scenarios can be envisaged to explain such convection cycles the first relates variable convection rates to changes in the volume flux and rheology of magma entering the lake in the second cyclic convection is set up by the generation of convective instabilities within the lake in this case cooling of a surface layer generates a slow moving viscous increasingly dense convection layer at the lake surface which is consumed and replaced during overturn

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

ice sheet extent of the antarctic peninsula region during the last glacial maximum lgm insights from glacial geomorphology we present marine geological and geophysical data for the antarctic peninsula that call for a larger ice sheet reconstruction during the last glacial maximum lgm than suggested by previous studies such glacial reconstructions are important for quantifying post lgm sea level rise and providing boundary conditions for general circulation models megascale glacial lineations on the continental shelf surrounding antarctica provide documentation for grounded ice that was streaming swath bathymetry data nbp02 01 reveal lineations in each of the major glacial troughs except for smith trough which features grooves and bedrock drumlins 0 5 3 km spacing 1 20 elongation ratios on crystalline bedrock we place the maximum extent of ice at the seaward limit of the lineations at or near megascale glacial lineations imaged in this study and other areas surrounding antarctica including the ross sea pine island bay have a consistent morphology with elongation ratios of 80 1 and spacings of 200 600 m mode of 300 m in contrast lineations bundles canals et al 2000 in the gerlache boyd trough exhibit larger spacing 1 5 km and the upstream portions 7 km are carved directly into bedrock rather than till as suggested by previously unpublished air gun 120 hz seismic data these features have yet to be dated however they are believed to have been active during the lgm radiocarbon dates in glaciomarine sediments foraminifera and organic matter from seven glacial troughs indicate that megascale glacial lineations were formed during the lgm other geomorphic features such as the grounding zone wedge in the northwestern weddell sea formed when the ice sheet was retreating the initial retreat of the grounded ice from the outer shelf occurred by 18 500 cal yr b p the inner shelf was mostly ice free by 13 000 cal yr b p this is significantly earlier than numerical models which suggest antarctic deglaciation began at 12 000 cal yr b p this also suggests the peninsula area contributed to global sea level rise associated with meltwater pulse mwp 1a

subaqueous mass flow origin for lower permian diamictites and associated facies of the grant group barbwire terrace canning basin western australia the intracratonic canning basin is western australia s largest sedimentary basin 400 000 km2 and has experienced repeated episodes of phanerozoic extension and subsidence resulting in deposition of a number of first order megasequences a major phase of basin extension and sedimentation grant group occurred in the late carboniferous early permian when australia lay at high palaeolatitudes facies analysis of 5000 m of drill core from 25 continuously cored wells in grant group strata on the fault bounded barbwire terrace in the northern canning basin identified three facies associations fas these record the predominance of fault generated subaqueous mass flow and sediment reworking the lowest association fa i up to 355 m thick rests unconformably on tilted older strata and consists of coarse grained subaqueously deposited sediment gravity flow facies these include fault generated breccias massive and graded sandstones and conglomerates deposited by turbidity currents and diamictites generated by mixing of different textural populations during downslope remobilization fa i is overlain abruptly by relatively fine grained deposits of fa ii up to 140 m thick which consist of laminated to thin bedded mudstone and sandstone turbidites recording an abrupt increase in relative water depths in turn these facies coarsen upwards and are transitional into shallow water swaley cross stratified and rippled sandstones of fa iii up to 125 m thick the overall stratigraphic succession probably records an initial phase of faulting and accommodation of coarse sediment fa i a subsequent phase of rapid subsidence increasing water depths and sediment underfilling fa ii and finally a regressive phase of shoreface progradation the occurrence of rare striated clasts in fa i suggests reworking of glacial sediment but no direct glacial influence on sedimentation can be identified

formation of bands and ridges on europa by cyclic deformation insights from analogue wax experiments 1 we perform a set of analogue wax experiments aimed at understanding the processes and conditions that lead to lineament formation on europa we heat a layer of wax from below and cool it from above so that a solid layer of wax forms above a molten layer simulating either the liquid ocean or the actively convecting part of the ice shell the solid layer of wax is then deformed periodically simulating the cyclic strain caused by tides or by the satellite s nonsynchronous rotation we find that the formation and morphology of lineaments that develop in the solid wax layer are governed by two main dimensionless parameters the ratio of the cooling timescale for the solid layer to the deformation timescale and the dilation quotient which is defined as the magnitude of net extension during one cycle of deformation divided by the amplitude of the periodic deformation for 1 features form that qualitatively resemble bands and ridge complexes on europa we conclude that diurnal tides may be responsible for the formation of bands on europa if the conducting layer of ice is a few hundred meters thick and is underlain by a convecting layer of ice features resembling double ridges can form if the ice shell has a thickness of a few kilometers and periodic strains are caused by europa s nonsynchronous rotation

north american ice sheet build up during the last glacial cycle 115 21 kyr the last glacial maximum lgm outline and subsequent retreat pattern 21e7 kyr of north americanice sheets are reasonably well established however the evolution of the ice sheets during their build upphase towards the lgm between 115 and 21 kyr has remained elusive making it difficult to verifynumerical ice sheet models for this important time interval in this paper we outline the pre lgm icesheet evolution of the laurentide and cordilleran ice sheets by using glacial geological and geomorphologicalrecords to make a first order reconstruction of ice sheet extent and flow pattern we mappedthe entire area covered by the laurentide and cordilleran ice sheets in landsat mss images andapproximately 40 of this area in higher resolution landsat etm images mapping in aerial photographsadded further detail primarily in quebec labrador the cordilleran region and on baffin island our analysis includes the recognition of approximately 500 relative age relationships from crosscuttinglineations together with previously published striae and till fabric data these are used as the basis forrelative age assignments of regional flow patterns for the reconstruction of the most probable ice sheetevolution sequence we employ a stepwise inversion scheme with a clearly defined strategy for delineatingcoherent landforms swarms reflecting flow direction and configuration and linking these topreviously published constraints on relative and absolute chronology our results reveal that icedispersalcentres in keewatin and quebec were dynamically independent for most of pre lgm time andthat a massive quebec dispersal centre rivalling the lgm in extent existed at times when the sw sectorof the ice sheet had not yet developed the oldest flow system in eastern quebec labrador atlanticswarm had an ice divide closer to the labrador coast than later configurations a northern keewatin central arctic ice sheet existed prior to the lgm but is poorly chronologically constrained there is alsoevidence for older and more easterly cordilleran ice sheet divide locations than those that prevailedduring the late wisconsinan in terms of ice sheet build up dynamics it appears that residual ice capsafter warming phases may have played an important role in particular the location and size of remnantice masses at the end of major interstadials i e ois 5c and 5a must have been critical for subsequentbuild up patterns because such remnant uplands may have fostered much more rapid ice sheetgrowth than what would have occurred on a fully deglaciated terrain the ice sheet configuration duringstadials would also be governed largely by the additional topography that such residual ice constitutesbecause of inherent mass balance topography feedbacks

architectural element analysis applied to glacial deposits internal geometry of a late pleistocene till sheet ontario canada deposits left by continental ice sheets are characterized by sedimentological complexity and stratigraphic heterogeneity but stratigraphic descriptions of such deposits and resulting first generation facies models are still based primarily on one or two dimensional borehole or outcrop data reconstruction of depositional environments hydrogeological investigations of pleistocene glacial deposits and hydrocarbon exploration in pre pleistocene glaciated basin fills require a more detailed understanding of the form and heterogeneity of lithofacies sequences in three dimensions architectural element analysis is used widely by sedimentologists for categorizing internal stratigraphic heterogeneity in sandstones particularly those of fluvial origin this paper demonstrates the first application of architectural element analysis to glacial deposits such as tills outcrop borehole and a broad range of subsurface geophysical data were collected from a thick 60 m till sheet present across an 80 km 2 study area near toronto canada the till sheet is not homogeneous but is composed of three distinct architectural elements and associated lithofacies viz diamict elements interbeds of subglaciofluvial sediments and glaciotectonically deformed zones application of architectural element analysis to these subglacial strata provides insights into the origin of drumlin bedforms and subglacial processes below the laurentide ice sheet and creates a framework for understanding ground water and contaminant movement in underlying aquifers

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

the yield strength of subliquidus basalts experimental results yield strength is an important property of particle fluid suspensions in basaltic lavas that crystallize during flow emplacement the onset of yield strength may result in threshold transitions in flow behavior and flow surface morphology however yield strength crystallinity relations are poorly known particularly in geologic suspensions where difficulties of experimental and field measurements have limited data acquisition in the subliquidus temperature range here we describe two complementary experimental approaches designed to examine the effect of particle shape on the low shear yield strength of subliquidus basalts the first involves melting cubes of holocrystalline basalt samples with different initial textures to determine the temperature crystallinity at which these samples lose their cubic form these experiments provide information on the minimum crystal volume fractions 0 20 0 35 required to maintain the structual integrity of the cube the second set of experiments uses suspensions of corn syrup and neutrally buoyant particles to isolate the effect of particle shape on yield strength development from these experiments we conclude that the shape is important in determining the volume fraction range over which suspensions exhibit a finite yield strength as anisotropic particles may orient during flow the effect of particle shape will be controlled by the orientation distribution of the constituent particles we find that the so called excluded volume can be used to relate results of experiments on anisotropic particles to those of suspensions of spherical particles recent measurements of yield strength onset in basaltic melts at crystal volume fractions near 0 25 are consistent with our observations that crystal frameworks develop at low to moderate crystal volume fractions when crystals are anisotropic e g plagioclase we further suggest that conditions leading to yield strength onset at low crystallinities include rapid cooling increased crystal anisotropy heterogeneous nucleation which promotes extensive crystal clustering and large cluster anisotropy and static conditions random crystal orientations

emplacement and inflation of pahoehoe sheet flows observations and measurements of active lava flows on kilauea volcano hawaii inflated pahoehoe sheet flows have a distinctive horizontal upper surface which can be several hundred meters across and are bounded by steep monoclinal uplifts the inflated sheet flows we studied ranged from 1 to 5 m in thickness but initially propagated as thin sheets of fluid pahoehoe lava generally 20 30 cm thick individual lobes originated at outbreaks from the inflated front of a prior sheet flow lobe and initially moved rapidly away from their source velocities slowed greatly within hours due to radial spreading and to depletion of lava stored within the source flow as the outward flow velocity decreases cooling promotes rapid crustal growth at first the crust behaves plastically as pahoehoe toes form after the crust attains a thickness of 2 5 cm it behaves more rigidly and develops enough strength to retain incoming lava thus increasing the hydrostatic head at the flow front the increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow resulting in uniform uplift of the entire sheet flow lobe initial uplift rates are rapid flows thicken to 1 m in 1 2 hours but rates decline sharply as crustal thickness increases and as outbreaks occur from the margins of the inflating lobe one flow reached a final thickness of nearly 4 m after 350 hr inflation data define power law curves whereas crustal cooling follows square root of time relationships the combination of data can be used to construct simple models of inflated sheet flows as the flow advances preferred pathways develop in the older portions of the liquid cored flow these pathways can evolve into lavatube systems within a few weeks formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1 2 orders of magnitude slower if advance of the sheet flow is terminated the tube remains filled with lava that crystallizes in situ rather than draining to form the cave like lava tubes commonly associated with pahoehoe flows inflated sheet flows from kilauea and mauna loa are morphologically similar to some thick icelandic and submarine sheet flows suggesting a similar mechanism of emplacement the planar sheet like geometry of flood basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly instantaneous emplacement as literal floods of lava

observations and temperatures of io s pele patera from cassini and galileo spacecraft images abstract pele has been the most intense high temperature hotspot on io to be continuously active during the galileo monitoring from 1996 2001 a suite of characteristics suggests that pele is an active lava lake inside a volcanic depression in 2000 2001 pele was observed by two spacecraft cassini and galileo the cassini observations revealed that pele is variable in activity over timescales of minutes typical of active lava lakes in hawaii and ethiopia these observations also revealed that the short wavelength thermal emission from pele decreases with rotation of io by a factor significantly greater than the cosine of the emission angle and that the color temperature becomes more variable and hotter at high emission angles this behavior suggests that a significant portion of the visible thermal emission from pele comes from lava fountains within a topographically confined lava body high spatial resolution nightside images from a galileo flyby in october 2001 revealed a large relatively cool

reconstruction of palaeo ice sheets the use of geomorphological data the article discusses the nature of the glacial inversion problem which is defined as the extraction of time slice ice sheet flow patterns from the patchy and partly overprinted landform record present in former ice sheet areas a coherent inversion model for derivation of flow patterns and interior ice sheet configuration from geomorphological data is presented glacial landscapes are classified according to the three criteria of internal age gradients presence or absence of meltwater traces aligned to flow traces and basal condition frozen bed thawed bed inferred from morphology the inversion model uses landscapes classified accordingly spatially delineated into fans as input data relative chronologies at fan intersections are used to sort fans in a relative age stack that can be linked to stratigraphic dating information

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

a meltwater origin for antarctic shelf bedforms with special attention to megalineations the geomorphology of troughs crossing the antarctic shelf is described and interpreted in terms of ice stream hydrology the scale of tunnel channels on the inner shelf and the absence of sediment at their mouths are taken to infer catastrophic drainage drumlins on the inner and outer shelves with pronounced crescentic and hairpin scours are also interpreted as products of catastrophic flow gullies and channels on the continental slope and turbidites on the rise and abyssal plain point to abundant meltwater discharge across the shelf attempts to explain this morphology and sedimentology in terms of release or discharge of meltwater by pressure melting strain heating darcian flow or advection in deforming till are shown to be unrealistic we suggest that meltwater flow across the middle and outer shelves might have been in broad turbulent floods which raises the possibility that megascale glacial lineations msgl on the shelf might originate by erosion in turbulent flow this possibility is explored by use of analogs for msgl from flood and eolian landscapes and marine environments an extended discussion reflects on objections that stand in the way of the flood hypothesis

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 1 size shape and spacing from a large remote sensing dataset abstract mega scale glacial lineations msgls are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly flowing ice streams knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses this paper presents the results of a remote sensing survey of 46 000 bedforms on the dubawnt lake palaeo ice stream bed focussing on a central transect of 17 038 that includes highly elongate bedforms previously described as msgls within this transect lineations exceed 10 km in length max 20 km and 23 have elongation ratios 10 1 max 149 1 highly elongate features are interspersed with much shorter drumlin like features but longer bedforms are typically narrower suggesting that their length develops more quickly than or at the expense of their width bedforms are broadly symmetrical in plan form and have a preferred lateral spacing of 50 250 m which implies a regular rather than random pattern of corrugations comparison with drumlins reveals that the more attenuated msgls simply extend the tail of the distribution of data rather than plotting as a separate population taken together this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity but existing hypotheses of msgl formation are either not supported or are insufficiently developed to explain our observations rather we conclude that under conditions of rapid ice flow msgls attain their great length relatively quickly decades through a probable combination of subglacial deformation which attenuates ridges and erosional processes that removes material from between them

reconstruction of the west antarctic ice sheet in pine island bay during the last glacial maximum and its subsequent retreat history abstract marine geological and geophysical studies were conducted to determine the late quaternary glacial history of the west antarctic ice sheet in pine island bay antarctica the data set includes sea floor imagery in the form of multibeam swath bathymetry and deep tow side scan sonar surveys distribution of sedimentary deposits and basal surface conditions are revealed through intermediate resolution seismic data and chirp 3 5 khz sub bottom profiles sedimentary deposits were sampled with piston cores and calcareous material was dated using ams radiocarbon techniques results from the study indicate the ice sheet was grounded on the outer shelf during its maximum extent sometime in the past thick ice flowing slowly and resting on crystalline bedrock was influenced by significant amounts of subglacial meltwater on the inner shelf thinner but more rapidly flowing ice characterized the seaward extent of the ice sheet which was grounded on a sedimentary substrate deglaciation occurred in two phases beginning soon after the last glacial maximum the outer most extension of the ice sheet retreated gradually pausing on a bathymetric high on the middle shelf by 16 000 14c years ago between 16 000 and 12 000 14c years ago ice sheet retreat resumed and may have proceeded rapidly to reach a present day position by 10 000 14c years ago

sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system abstract the extent and style of shear deformation in sediments beneath modern glaciers and the geological evidence for such deformation in deposited sediments are reviewed new evidence is presented from beneath a modern glacier of the spatial and temporal patterns of water pressure fluctuation and of time dependent patterns of deformation in sediments it is concluded that in most experimental sites beneath soft bedded modern glaciers deformation is a significant or major contributor to glacier movement and the resultant discharge of till is large enough to make sediment deformation a major till forming process particular modes of deformation facilitate incorporation of underlying material into the till whilst the capacity of a deforming till to absorb strain can protect the underlying strata from deformation leading to the commonly found relationship where till overlies other strata with a sharp planar interface it is argued that the almost ubiquitous occurrence of drumlins on the beds of former ice sheets is a reflection of the widespread occurrence of sediment deformation beneath them with important implications for the coupling of ice sheet flow and bed properties it is argued that the mechanical behaviour of the subglacial system is not simply determined by till properties but largely controlled by the subglacial water pressure regime determined by the nature of subglacial drainage results of field experiments show how the nature of the basal hydraulic system can play a vital role in controlling the coupling between the glacier and till deformation processes they show that rapid glacier advances can produce undrained loading of sediments that effective pressure may increase either upwards or downwards in a till according to the direction of drainage and that interstitial water pressures in subglacial sediments can show large and rapid variations producing strong variations in the rate and distribution of strain and in the partitioning of basal movement between sliding and deformation

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

origins and implications of zigzag rift patterns on lava lakes abstract the distinctive rift patterns observed on newly formed lava lakes are very likely a product of interaction between heat transfer cooling of lava and deformation of the solid crust in response to applied stresses one common pattern consists of symmetric zigzag rifts separating spreading plates zigzags can be characterized by two measurable parameters an amplitude a and an angle between segments that make up the zigzags similar patterns are observed in analog wax experiments in which molten wax acts as cooling and solidifying lava we perform a series of these wax experiments to find the relationship between a and the cooling rate we develop a model to explain the observed relationships is determined by a balance of spreading and solidification speeds the amplitude a is limited by the thickness of the solid wax crust theoretical predictions agree well with experimental data this enables us to scale the model to basaltic lava lakes if zigzag rifts are observed on the surface of lava lakes and if physical properties of the lava crust can be measured or inferred by other means measurements of and a make it possible to calculate crust spreading velocity and crust thickness

morphology temperature and eruption dynamics at pele abstract the pele region of io has been the site of vigorous volcanic activity from the time of the first voyager i observations in 1979 up through the final galileo ones in 2001 there is high temperature thermal emission from a visibly dark area that is thought to be a rapidly overturning lava lake and is also the source of a large sulfur rich plume we present a new analysis of voyager i visible wavelength images and galileo solid state imager ssi and near infrared mapping spectrometer nims thermal emission observations which better define the morphology of the region and the intensity of the emission the observations show remarkable correlations between the locations of the emission and the features seen in the voyager images which provide insight into eruption mechanisms and constrain the longevity of the activity we also analyze an additional wavelength channel of nims data 1 87 m which paradoxically because of reduced sensitivity allows us to estimate temperatures at the peak locations of emission measurements of eruption temperatures on io are crucial because they provide our best clues to the composition of the magma high color temperatures indicative of ultramafic composition have been reported for the pillan hot spot and possibly for pele although recent work has called into question the requirement for magma temperatures above those expected for ordinary basalts our new analysis of the pele emission near the peak of the hot spot shows color temperatures near the upper end of the basalt range during the i27 and i32 encounters in order to analyze the observed color temperatures we also present an analytical model for the thermal emission from fire fountains which should prove generally useful for analyzing similar data this is a modification of the lava flow emission model presented in howell howell r r 1997 icarus 127 394 407 adapted to the fire fountain cooling curves first discussed in keszthelyi et al keszthelyi l jaeger w milazzo m radebaugh j davies a g mitchell k l 2007 icarus 192 491 502 when applied to the i32 observations we obtain a fire fountain mass eruption rate of 5 1 10 5 kg s 1 for the main vent area and 1 4 10 4 kg s 1 for each of two smaller vent regions to the west these fire fountain rates suggest a solution to the puzzling lack of extensive lava flows in the pele region much of the erupted lava may be ejected at high speed into the fire fountains and plumes creating dispersed pyroclastic deposits rather than flows we compare gas and silicate mass eruption rates and discuss briefly the dynamics of this ejection model and the observational evidence

observations of the effect of wind on the cooling of active lava flows 1 we present the first direct observations of the cooling of active lava flows by the wind we confirm that atmospheric convective cooling processes i e the wind dominate heat loss over the lifetime of a typical pahoehoe lava flow in fact the heat extracted by convection is greater than predicted especially at wind speeds less than 5 m s and surface temperatures less than 400 c we currently estimate that the atmospheric heat transfer coefficient is about 45 50 w m 2 k 1 for a 10 m s wind and a surface temperature 500 c further field experiments and theoretical studies should expand these results to a broader range of surface temperatures and wind speeds

thermal conditions beneath parts of the last cordilleran ice sheet near its centre as inferred from subglacial till associated sediments and bedrock subglacial sediments and bedrock associated with the last fraser glaciation were studied in a mountain valley and on a mountain plateau ca 1200 m elevation near the south central spreading centre of the last cordilleran ice sheet in order to gain information on subglacial thermal conditions our study indicates that as ice advanced it was probably initially coupled to or dragged along a largely frozen and immobile substrate that fractured as glaciation proceeded subglacial till was deposited mainly by lodgement in most places however eld evidence suggests that as till thickened and pore water content increased the till experienced ductile deformation at some sites there is evidence that pore water increased to a point where till owed as a viscous slurry while eroding entraining and transporting pieces of soft bedrock in places brittle deformation was superimposed on ductily deformed till a transition that we associate with subsequent till dewatering and sti ening a deformable substrate would have contributed to extending ice ow thus increasing ow velocity while decreasing ice surface pro les if this situation was wide spread then the interior of the cordilleran ice sheet would have consisted of local accumulation areas that were drained by rapidly owing glaciers ice streams this is supported by previously recorded ice ow patterns inferred from large scale ice ow indicators and by other evidence of rapid ice movement along the southern fringe of the ice sheet 2000 elsevier science ltd and inqua all rights reserved

the effect of multiple pre existing weaknesses on formation and evolution of faults in extended sandbox models abstract high resolution three dimensional seismic data show that fault distribution and evolution in rift basins can be very complicated and inconsistent with the classic model of fault formation and propagation fault development in the presence of multiple pre existing weaknesses is still not clearly studied trying to understand the role of multiple pre existing weaknesses in sand models two series of experiments were prepared in the first series of models the mechanical properties of pre existing weaknesses were the same but their orientations were different whereas in the second series of models both mechanical properties and orientations were different the basic characteristics of fault formation and evolution in a rift basin revealed by the models are as follows 1 weakness reactivated faults faults that formed directly along and above the pre existing weaknesses and weakness related faults faults which are not located above the pre existing weaknesses but have similar orientation form much earlier than weakness unrelated faults the order of formation of different weakness reactivated faults is determined by the relative reactivation tendency factors fa of the weaknesses 2 weakness reactivated faults and weakness related faults develop mainly along or parallel to pre existing weaknesses in contrast weakness unrelated faults develop nearly perpendicular to the extension direction 3 a complicated fault system can form during a single extension phase in the presence of multiple pre existing weaknesses and 4 the mode of fault formation and propagation and the increase in length and number of faults have close ties to the nature of pre existing weaknesses fault displacement and length do not have a power law relationship for the majority of faults and there is no unique relationship for different kinds of faults in addition to providing new insights to the formation of complex fault systems the experiment results presented here could be reasonably explained using the reactivation tendency theory they can also be applied to the complicated fault system in rift basin

dynamics of mount nyiragongo lava lake inferred from thermal imaging and infrasound array abstract lava lakes provide a direct observation window into processes which usually remain hidden such as magma convection and outgassing dynamics we here report a coupled analysis of thermal infrared footage and infrasound array recordings at mount nyiragongo d r congo and derive a conceptual model of the lava lake s convective system and outgassing mechanism we suggest that surface flow results from a horizontal pressure gradient at the surface of the lake driving the crust from high pressure regions where hot upwelling magma impinges the surface to low pressure regions where cold downwelling magma pulls away from the surface the ascending current of this convection cell carries gas pockets which once at the surface are dragged across the lake into downwelling sinks such sinks are characterized by persistent chaotic bubble bursting spattering whose intensity and position are tracked from infrasound array analysis fluctuations of these are observed but have not been correlated with oscillations of the lava lake level nor with the variations of surface velocities both recorded from infrared footage we also report the activity of a new eruptive vent which opened early 2016 near an inner circular fracture of the crater s third terrace we show that the vent s activity was intermittent alternating between explosive strombolian activity and effusive activity the latter produced lava flows which spread on the crater s last terrace before cascading into the active lava lake although no significant change in the lake behavior was witnessed while the new eruptive vent was active increased attention should be addressed as this new activity could reflect over pressurization of the shallow magmatic system the variety of phenomena captured by this study complements and expands observations reported at other low viscosity lava lakes chiefly kilauea hawai i and erta ale ethiopia despite nyiragongo s more vigorous convective regime where multiple convective cells can operate simultaneously we suggest that the mechanisms controlling the surface motion and outgassing are similar at all three systems pointing to generic processes governing the dynamics of low viscosity lava lakes