submarginal debris transport and till formation in active temperate glacier systems the southeast iceland type locality abstract exhaustive sedimentological analysis of freshly exposed subglacial surfaces and moraines in southern iceland provides diagnostic sedimentological signatures of a debris transport pathways through active temperate glacier snouts and b till production in subglacial traction zones dominated by deforming layers three till end members are recognised based on stratigraphic architecture 1 thin and patchy tills over eroded bedrock 2 single push moraines and complexes and 3 overridden moraines or outwash fans typical till thicknesses are 0 10 1 40 m with each till relating to a deformation event driven by the seasonally tuned processes of glacier sub marginal shearing freeze on squeezing and bulldozing clast form trends demonstrate progressive modification towards mature forms in subglacial traction zones with till being clearly differentiated from scree and glacifluvial deposits clast macrofabric strengths are variable rarely matching those of laboratory shearing experiments except where obviously lodged clasts are abundant they also consistently record former glacier flow directions but localized variability is introduced by bedrock protuberances cavity infill clast interference and freshly imported plucked clasts within tills macrofabrics strengthen from lower b horizon to upper a horizon tills but at the outer edges of sub marginally thickening till wedges or push moraines seasonally driven cycles of squeezing flowage freeze on melt out and bulldozing give rise to a range of clast macrofabric strengths as well as superimposed deformation signatures this reflects two extremes of till emplacement including the more mobile flowing and often liquefied matrixes in push squeeze moraines and in contrast the lodgement deformation and ploughing at the thin end of sub marginal till wedges

quantifying global warming from the retreat of glaciers records of glacier fluctuations compiled by the world glacier monitoring service can be used to derive an independent estimate of global warming during the last 100 years records of different glaciers are made comparable by a two step scaling procedure one allowing for differences in glacier geometry the other for differences in climate sensitivity the retreat of glaciers during the last 100 years appears to be coherent over the globe on the basis of modeling of the climate sensitivity of glaciers the observed glacier retreat can be explained by a linear warming trend of 0 66 kelvin per century

erosional feedbacks and the oscillation of ice masses 1 an analytical solution to the development of a glacier or ice sheet in a region where the underlying rock rises steadily shows that the ice mass oscillates in size independefntly of changes in the climate in response to a feedback in which ice thickness is linked to topography by ice induced erosion the rate of vertical movement of the underlying rock uplift and the parameters governing ice erosion ablation and precipitation determine the period of oscillation two dimensional numerical simulations show that the tectonic uplift rate is the most important control on the period of oscillation and that the feedback period is likely to be between 10 and 100 kyr an examination of sediment data from the deep sea drilling project shows evidence of ice mass oscillations in the chugach st elias mountains and in the himalayas with periods consistent with those produced by the proposed mechanism localized changes in glacial extent may not therefore always be the consequence of climate change and care must be taken in interpreting evidence of glacial advance and retreat

geomorphically driven late cenozoic rock uplift in the sierra nevada california geologists have long accepted that the sierra nevada california experienced significant late cenozoic tectonically induced uplift a flexural isostatic model presented here shows however that a large fraction of the primary evidence for uplift could be generated by the lithospheric response to coupled erosion of the sierra nevada and deposition in the adjacent central valley and therefore requires less tectonic forcing than previously believed the sum of range wide erosion and the resultant isostatic rock uplift would have lowered sierra mean elevation by 200 to 1000 meters since 10 million years ago and could also have increased summit elevations during the current period of relief production

glacier longitudinal profiles in regions of active uplift abstract we combine a simplified equation for ice flow with several proposed glacial erosion laws to solve for longitudinal profiles of glaciers in topographic steady state based on prescribed patterns of rock uplift the solutions produce realistic looking glaciers and are consistent with previous numerical results scaling relationships for the dependence of ice thickness and surface slope on rock uplift and climate in the form of an imposed ice flux emerge from the solutions the general patterns of these dependencies are robust the ice thickness is dependent upon the ice flux and inversely dependent upon the rock uplift while the surface slope is inversely dependent upon the ice flux and dependent upon the rock uplift different erosion laws lead to only subtle differences in these slope and thickness dependencies despite the simplicity of the physics a first order match to an actual over deepened fjord profile can be obtained the results provide a theoretical basis for understanding the shape of glacier profiles in tectonically active landscapes can be used to benchmark numerical models of glacial erosion and can readily be incorporated directly into simple models of orogen dynamics

controlled moraine development and debris transport pathways in polythermal plateau icefields examples from tungnafellsjokull iceland a modern landsystems analogue for plateau icefield glaciation is presented based upon tungnafellsjokull in the arid cold interior of iceland landform mapping and intensive clast form analysis on three separate forelands occupied during the little ice age reveal variability in the morphology of moraines and the degree of clast modification which is interpreted as a function of the complex interplay between a the efficacy of glacial erosion b passive versus active debris transfer mechanisms and c the volume of debris provision for ice marginal freeze on mechanisms a steep outlet bounded by rockwalls produced a landform assemblage and debris transport signature typical of valley glaciers whereas two unconfined outlets produced a landform sediment imprint typical of polythermal conditions this comprises subglacial bedforms relating to the initiation of warm based ice by strain heating below icefalls at the plateau edge terminating at arcs of ice cored controlled moraine representative of frozen snout conditions patterns in clast form data on the unrestricted forelands reflect the characteristics of glacier dynamics and thermal regimes at the margin of a plateau icefield where bedrock steps of sufficient size to initiate strain heating and bedrock abrasion and plucking are located only a short distance from the marginal freeze on conditions of a snout creating debris rich folia and controlled moraine additional englacial debris may well have been carried to the glacier margin through the development of ogives and englacial thrusting at the base of the icefall thereby explaining the occurrence of striae on clasts that have otherwise been little modified by glacial abrasion copyright 2010 john wiley sons ltd

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

response of glacial landscapes to spatial variations in rock uplift rate 1 the response of glaciated landscapes to rapid rock uplift driven by tectonic convergence is an important often neglected aspect of proposed interactions between plate tectonic processes and climate change rivers typically respond to more rapid rock uplift in part through increasing channel gradients in contrast the glacial buzzsaw hypothesis suggests that glaciers can erode as quickly as the fastest rock uplift rates 6 10 mm yr without any increase in mean elevations however it has not been established how this is achieved we examined moving window maps swath and longitudinal profiles hillslope relief and hypsometry for glacierized and formerly glacierized basins in areas of spatially variable rock uplift rate in the southern alps new zealand and around nanga parbat pakistan to determine whether glaciers have a specific response to rapid rock uplift the response of these glaciated landscapes to rapid rock uplift 6 10 mm yr comprises 1 modest steepening of the longitudinal profiles in smaller glaciated basins 2 maintenance of shallow downvalley slopes in larger glaciated basins 30 km2 southern alps 100 km2 nanga parbat 3 development of tall headwalls and 4 steepening of the basin as a whole dominated by hillslope lengthening around nanga parbat headwalls several kilometers high constitute 50 of the basin relief at rapid rock uplift rates although glaciers can incise the valley floor swiftly they cannot prevent headwalls from reaching exceptional heights the associated increase in mean distance between cirque heads i e a decrease in drainage density causes regional mean elevation to rise with increasing rock uplift rate however this is much less than the changes in elevation expected in unglaciated ranges

some geological implications of average quaternary glacial conditions analyses of quaternary landscape evolution tend to focus on events associated with culminations of glacial and interglacial ages but for most of quaternary time environments were intermediate in character average quaternary glacial conditions based on assessment of the marine isotope record approximated those near the stage 2 stage 1 transition and during substages 5b and 5d isotope stages 3 and 4 and substages 5a d lie within one standard deviation of the mean value under average glacial conditions ice sheets lying over northern north america and europe were much more contracted than their full glacial counterparts and the distribution of mountain glaciers reflected a snowline depression of some 500 m geomorphic processes operating under average quaternary conditions contributed importantly to landscape evolution examples of landscapes that may represent such average conditions include cirques and fluvial deposits of the pacific northwest the fjords and strandflat of western norway and atolls of the tropical oceans by examining the geologic record from the perspective of average conditions rather than those of climatic extremes added insight can be gained regarding the evolution of quaternary landscapes

climatic controls of western u s glaciers at the last glacial maximum abstract we use a nested atmospheric modeling strategy to simulate precipitation and temperature of the western united states 18 000 years ago 18 ka the high resolution of the nested model allows us to isolate the regional structure of summer temperature and winter precipitation that is crucial to determination of the net mass balance of late pleistocene mountain glaciers in this region of diverse topography and climate modeling results suggest that climatic controls of these glaciers varied significantly over the western u s glaciers in the northern rocky mountains existed under relatively cold july temperatures and low winter accumulation reflecting anticyclonic easterly wind flow off the laurentide ice sheet in contrast glaciers that existed under relatively warmer and wetter conditions are located along the pacific coast south of oregon where enhanced westerlies delivered higher precipitation than at present between these two groupings lie glaciers that were controlled by a mix of cold and wet conditions attributed to the convergence of cold air from the ice sheet and moisture derived from the westerlies sensitivity tests suggest that for our simulated 18 ka climate many of the glaciers exhibit a variable response to climate but were generally more sensitive to changes in temperature than to changes in precipitation particularly those glaciers in central idaho and the yellowstone plateau our results support arguments that temperature depression generally played a larger role in lowering equilibrium line altitudes in the western u s during the last glacial maximum than did increased precipitation although the magnitude of temperature depression required for steady state mass balance varied from 8 18 c only the sierra nevada glaciers required a substantial increase in precipitation to achieve steady state mass balance while glaciers in the cascade range existed with decreased precipitation

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

positive degree day factors for ablation on the greenland ice sheet studied by energy balance modelling ice ablation is related to air temperature by the positive degree day factor variations of the positive degree day factor in west greenland are studied using an energy balance model to simulate ablation under different conditions degree day factors for simulated and measured ice ablation at nordbogletscher and qaman rssup sermia agree well with values around 8 mm d 1 c 1 degree day factors for snow are less than half those for ice energy balance modelling shows that degree day factors vary with summer mean temperature surface albedo and turbulence but there is only evidence of large positive degree day factors at lower temperatures and with low albedo 0 3 the greatest effect of albedo variations 0 3 0 7 is at lower temperatures while variations in turbulence have greater effect at higher temperatures current models may underestimate runoff from the greenland ice sheet by several tenths because they use a degree day factor for melting ice that is too small for the colder parts of the ice sheet i e the upper ablation area and the northerly margin

numerical simulations of glacial valley longitudinal profile evolution glaciers shape alpine landscapes they broaden valley bottoms enhance local valley relief generate multiple steps overdeepen valley floors and cause tributary valleys to hang these distinctive glacial signatures result from 10 4 10 5 yr of erosion during which swings in climate drive advances and retreats of alpine glaciers we use a numerical model of glacial erosion to explore the development of the longitudinal profiles of glaciated valleys the model is driven by the past 400 k y of variable climate because both sliding speed which dictates abrasion rate and water pressure fluctuations which strongly modulate quarrying rate should peak at the equilibrium line altitude ela we expect the locus of most rapid erosion to follow the transient ela simulations of a single glacial valley show rapid flattening of the longitudinal profile inclusion of a tributary glacier creates a step immediately downvalley of the tributary junction that persists over multiple glaciations and commonly leaves the tributary valley hanging steps and overdeepenings result from an increase in ice discharge immediately below the tributary junction which is accommodated primarily by increased ice thickness and hence sliding rate the size of the step increases with the ratio of tributary to trunk ice discharge while the height of a hanging valley reflects the difference in the time integrated ice discharge in tributary and trunk valleys and therefore increases as the discharge ratio decreases

distribution and inferred age of exfoliation joints in the aar granite of the central swiss alps and relationship to quaternary landscape evolution abstract exfoliation joints are well known natural fractures limited to near the ground surface relatively few details however are known about their distribution and age in the swiss alps exfoliation joints follow the landscape surface at the time of their formation the age of the associated landscape feature then provides a maximum age of exfoliation joints while landscape forms can change through time exfoliation joints preserve elements of former landscape morphologies by their undisturbed orientations the grimsel region of the central alps is well suited for analyzing the impact of erosional episodes and accompanying stress changes on exfoliation joint formation in granitic rocks mapping above and below ground revealed that exfoliation joints are widespread and occur between valley bottoms and mountain crests within glacial inner and hanging u shaped trough valleys glacial cirques and steep mountain crests and predominantly fluvial landforms gently inclined linear slopes above the inner trough valleys narrow inner valley gorges and steep v shaped side gullies based primarily on their geometric properties at the ground surface three exfoliation joint types were distinguished in our study area 1 closely spaced

stress gradient coupling in glacier flow i longitudinal averaging of the influence of ice thickness and surface slope for a glacier flowing over a bed of longitudinally varying slope the influence of longitudinal stress gradients on the flow is analyzed by means of a longitudinal flow coupling equation derived from the vertically cross sectionally integrated longitudinal stress equilibrium equation by an extension of an approach originally developed by budd 1968 linearization of the flow coupling equation by treating the flow velocity u vertically averaged ice thickness h and surface slope in terms of small deviations u h and a from overall average datum values u 0 h 0 and 0 results in a differential equation that can be solved by green s function methods giving u x as a function of h x and x x being the longitudinal coordinate the result has the form of a longitudinal averaging integral of the influence of local h x and x on the flow u x u x u0 2l l 0 ln n nh n l exp x x l dx where the integration is over the length l of the glacier the operator specified deviations from the datum state and the term on which it operates which is a function of the integration variable x represents the influence of local h x x and channel shape factor x at longitudinal coordinate x on the flow u at coordinate x the influence being weighted by the influence transfer function exp x x l in the integral the quantity l that appears as the scale length in the exponential weighting function is called the longitudinal coupling length it is determined by rheological parameters via the relationship l 2h n n 3n where n is the flowlaw exponent n the effective longitudinal viscosity and n the effective shear viscosity of the ice profile n is an average of the local effective viscosity n over the ice cross section and n 1 is an average of n 1 that gives strongly increased weight to values near the base theoretically the coupling length l is generally in the range one to three times the ice thickness for valley glaciers and four to ten times for ice sheets for a glacier in surge it is even longer l 12h it is distinctly longer for non linear n 3 than for linear rheology so that the flow coupling effects of longitudinal stress gradients are markedly greater for non linear flow the averaging integral indicates that the longitudinal variations in flow that occur under the influence of sinusoidal longitudinal variations in h or with wavelength are attenuated by the factor 1 1 2 l 2 relative to what they would be without longitudinal coupling the short intermediate and long scales of glacier motion raymond 1980 over which the longitudinal flow variations are strongly partially and little attenuated are for 2l 2l 20l and 20l for practical glacier flow calculations the exponential weighting function can be approximated by a symmetrical triangular averaging window of length 4l called the longitudinal averaging length the traditional rectangular window is a poor approximation because of the exponential weighting the local surface slope has an appreciable though muted effect on the local flow which is clearly seen in field examples contrary to what would result from a rectangular averaging window tested with field data for variegated glacier alaska and blue glacier washington the longitudinal averaging theory is able to account semi quantitatively for the observed longitudinal variations in flow of these glaciers and for the representation of flow in terms of effective surface slope values exceptions occur where the flow is augmented by large contributions from basal sliding in the ice fall and terminal zone of blue glacier and in the reach of surge initiation in variegated glacier the averaging length 4l that gives the best agreement between calculated and observed flow pattern is 2 5 km for variegated glacier and 1 8 km for blue glacier corresponding to l h 2 in both cases if l varies with x but not too rapidly the exponential weighting function remains a fairly good approximation to the exact green s function of the differential equation for longitudinal flow coupling in this approximation l in the averaging integral is l x but is not a function of x effects of longitudinal variation of l are probably important near the glacier terminus and head and near ice falls the longitudinal averaging formulation can also be used to express the local basal shear stress in terms of longitudinal variations in the local stope stress with the mediation of longitudinal stress gradients

glacial cirque formation in northern scandinavia passglaciaren is a small cirque glacier in the kebnekaise massif northern sweden it is frozen to its bed over more than 70 of its area and under present climatic conditions has little effect on cirque formation more favourable conditions for cirque glacier erosion during the holocene are of short duration assuming similar conditions during previous interglacials it is suggested that forms such as the passglaciaren cirque developed mainly during the initial phases of glacials when they were part of networks of large valley glaciers or of a small warm based mountain centred ice sheet passglaciaren has been examined in order to evaluate its erosive capacity and its association with the subglacial cirque morphology the methods used are radar surveys and direct ice temperature measurements erosion is restricted to a small section of the glacier bed at present resulting in only partial deepening of the cirque and erosion of the backwall in cold arid regions with extensive permafrost small cirque glaciers are largely frozen to the bed and therefore cannot contribute significantly to cirque formation in such regions glacial erosion by larger temperate glaciers is more likely to be the major cause of cirque excavation

glacial steady state topography controlled by the coupled influence of tectonics and climate glaciers and rivers are the main agents of mountain erosion while in the fluvial realm empirical relationships and their mathematical description such as the stream power law improved the understanding of fundamental controls on landscape evolution simple constraints on glacial topography and governing scaling relations are widely lacking we present a steady state solution for longitudinal profiles along eroding glaciers in a coupled system that includes tectonics and climate we combined the shallow ice approximation and a glacial erosion rule to calculate ice surface and bed topography from prescribed glacier mass balance gradient and rock uplift rate our approach is inspired by the classic application of the stream power law for describing a fluvial steady state but with the striking difference that in the glacial realm glacier mass balance is added as an altitude dependent variable from our analyses we find that ice surface slope and glacial relief scale with uplift rate with scaling exponents indicating that glacial relief is less sensitive to uplift rate than relief in most fluvial landscapes basic scaling relations controlled by either basal sliding or internal deformation follow a power law with the exponent depending on the exponents for the glacial erosion rule and glen s flow law in a mixed scenario of sliding and deformation complicated scaling relations with variable exponents emerge furthermore a cutoff in glacier mass balance or cold ice in high elevations can lead to substantially larger scaling exponents which may provide an explanation for high relief in high latitudes

the influence of alpine glaciation on the relief of tectonically active mountain belts a surface processes model incorporating large scale fluvial processes local hill slope processes and glacial erosion is used to investigate the effect of glaciation on the relief of a fast growing narrow orogen the results of several simulations show that glaciers concentrate erosion near peaks fluvial erosion is reduced downstream of glaciers glaciers with frozen bases can lead to an increase in relief and the net affect of glaciation in the absence of widespread basal freezing is to reduce relief

using inherited cosmogenic 36cl to constrain glacial erosion rates of the cordilleran ice sheet cosmogenic 36 cl cl ratios measured from glacially eroded bedrock provide the first quantitative constraints on the magnitude rate and spatial distribution of glacial erosion over the last glacial cycle of 23 36 cl cl ratios 8 yield exposure ages that predate the well constrained deglaciation of the puget lowland washington and are inferred to result from 36 cl inherited from prior exposure during the last interglaciation where ice did not erode enough rock 1 80 2 95 m to reset 36 cl cl ratios to background levels surfaces possessing inherited 36 cl

positive feedbacks associated with erosion of glacial cirques and overdeepenings the principal points of water input to a glacier are the bergschrund in cirques and crevasse fields lower on the glacier crevasse fields commonly occur over convexities at the heads of overdeepenings in glacier beds the amplitude of subglacial water pressure fluctuations is large just down glacier from these points of water input erosion by quarrying is likely in such areas erosion is thus inferred to be localized on the headwalls of cirques and overdeepenings in the case of overdeepenings this leads to a positive feedback process in which a perturbation in the bed causes crevassing at the surface resulting in erosional forces that accentuate the perturbation when subglacial water flows up an adverse bed slope leading out of a cirque or overdeepening much of the viscous energy dissipated is used to warm the water to keep it at the pressure melting temperature as the ice thins and the pressure decreases in such situations subglacial conduits are maintained by high water pressures rather than by melting of conduit walls in the limit water pressures apparently become so high that water is forced out along the ice bed interface and the conduits collapse the products of erosion are then no longer flushed out and a protective till layer accumulates by limiting erosion on such adverse bed slopes this till layer controls the geometry of these overdeepened basins

strong feedbacks between hydrology and sliding of a small alpine glacier 1 we report the spatial and temporal pattern of sliding on the 7 km long bench glacier alaska using five continuously recording gps antennas following motion of the surface ice distributed at 1 km spacing along the glacier center line we documented surface ice motion over 50 days during summer 2002 surface speeds in two previous winters constrain the motion component associated with ice deformation allowing isolation of the sliding speed history we observed two speedup events bracketing 2 weeks of steady slow sliding the first event was not associated with a meteorological trigger was more subtle than the second and propagated up glacier at a rate of several hundred meters per day the second event coincided with a warm up valley wind which triggered considerable melt of the glacier surface sliding speeds in this event reached 0 3 m d 1 and began almost simultaneously at all sites in the ablation area both the horizontal and vertical displacement time series can be explained by growth and collapse of cavities in the lee of bumps in the bedrock bed cavities grow during rapid sliding and decay by viscous creep we posit that effective pressure averaged over some large area of the bed is inversely proportional to the sliding speed this effective pressure then controls the collapse rate of cavities whose dimensions are estimated from a plausible stepped bed geometry this model explains well the horizontal and vertical surface displacement history through the first event and beginning of the second event the vertical record demands a substantial and abrupt drop in water pressure that departs from the posited sliding effective pressure relationship we argue that this pressure drop reflects establishment of efficient subglacial drainage also manifested in a nearly simultaneous step increase in water discharge in the exit stream the establishment of an efficient conduit system terminates sliding its maintenance inhibits further sliding over the remainder of the summer

hillslope glacier coupling the interplay of topography and glacial dynamics in high asia 1 high asian glacial landscapes have large variations in topographic relief and the size and steepness of snow accumulation areas associated differences in glacial cover and dynamics allow a first order determination of the dominant processes shaping these landscapes here we provide a regional synthesis of the topography and flow characteristics of 287 glaciers across high asia using digital elevation analysis and remotely sensed glacier surface velocities glaciers situated in low relief areas on the tibetan plateau are mainly nourished by direct snowfall have little or no debris cover and have a relatively symmetrical distribution of velocities along their length in contrast avalanche fed glaciers with steep accumulation areas which occur at the deeply incised edges of the tibetan plateau are heavily covered with supraglacial debris and flow velocities are highest along short segments near their headwalls but greatly reduced along their debris mantled lower parts the downstream distribution of flow velocities suggests that the glacial erosion potential is progressively shifted upstream as accumulation areas get steeper and hillslope debris fluxes increase our data suggest that the coupling of hillslopes and glacial dynamics increases with topographic steepness and debris cover the melt lowering effect of thick debris cover allows the existence of glaciers even when they are located entirely below the snow line however slow velocities limit the erosion potential of such glaciers and their main landscape shaping contribution may simply be the evacuation of debris from the base of glacial headwalls which inhibits the formation of scree slopes and thereby allows ongoing headwall retreat by periglacial hillslope processes we propose a conceptual model in which glacially influenced plateau margins evolve from low relief to high relief landscapes with distinctive contributions of hillslope processes and glaciers to relief production and decay

allometry and cirque evolution a deductive analysis of the processes responsible for cirque formation leads to a model that describes cirque development through time in this model cirque volumes increase as the third power of time although observed variations in cirque size also reflect pre glacial erosion as well as the average rate of surface recession lowering the deductive findings are extended via analysis of 23 cirques from a part of the sangre de cristo mountains south central colorado the assumption that glacier occupance times are proportionately adjusted to topoclimatic situation leads to the conclusion that average rates of cirque erosion exhibit at least a fourfold possibly as much as an order of magnitude range within a given massif the notion that cirque development rates depend on bed rock resistance is supported by a statistically significant inverse relationship between estimated erosion coefficients and average bed rock joint spacing allometric analysis of a 15 cirque subsample the cirques were chosen because they exhibit similar erosion susceptibility lends credence to the hypothesis that headwall recession has been more rapid than cirque floor lowering within the study area however allometric enlargement cannot by itself account for observed values for the ratio cirque length cirque depth the results imply that pre glacial topography strongly influences observed variability in glacial morphometry

numerical modeling of glacial erosion and headwall processes in alpine valleys abstract how rock relief is developed by glacial erosion in alpine landscapes is a key question in understanding the controls on mountain range topography any generation of relief by glaciers at the highest elevations in mountain ranges can affect the distribution of precipitation in alpine landscapes creating feedbacks between glacier mass balance and glacial erosion relief production at high elevation mountain crests is addressed using a numerical model that incorporates glacier dynamics and subglacial erosion as well as relevant hillslope processes blowing snow from a valley bounding plateau and avalanching onto the accumulation zone of the glacier is included in the model furthermore temperature dependent frost cracking is used as a proxy for headwall backwearing the parameterization of subglacial erosion produces about 500 m of erosion over the 400 ka of each numerical simulation in steady and varying climate scenarios valley floors flatten as headwalls increase in height and retreat headward over time headwall slopes increase in simulations in which quarrying is explicitly incorporated rates of quarrying are high near the base of the headwall increasing headwall length and slope as a result of valley floor lowering high rates of subglacial erosion and production of headwall relief also result when elastic flexural uplift occurs in response to mass unloading small overdeepenings form at the base of the headwall in steady climate simulations when the glacier is small at the end of the model run while the final profiles are relatively insensitive to the erosion rule used quarrying is most effective near the head of the glacier whereas rates of abrasion reflect the instantaneous pattern of integrated ice discharge or ice flux in simulations in which cirques are formed the equilibrium line altitude is hundreds of meters above the cirque floor however the time averaged horizontal position of the equilibrium line corresponds well with the downvalley position of the cirques

the chemistry and mineralogy of ferromanganese encrustations on rocks from the sierra leone rise equatorial mid atlantic ridge and new england seamount chain abstract chemical and mineralogical compositions of ferromanganese oxide coatings on rocks dredged from the new england seamounts the sierra leone rise and the mid atlantic ridge near the equator have been determined in an investigation of regional differences in atlantic ferromanganese deposits most encrustations are clearly of hydrogenous origin consisting mainly of todorokite and mno 2 but several recovered from the equatorial fracture zones may be hydrothermal accumulations mn fe ratios are similar to those observed in nodules and encrustations elsewhere in the atlantic generally falling below unity except for samples from the new england seamounts cu 0 02 0 2 is lower than in other atlantic deposits ni contents vary from 0 064 to 0 478 the latter value being found near the st paul s fracture zone a large difference in co is observed between the northwest and equatorial atlantic 0 048 0 135 new england seamounts 0 355 1 010 near the equator ferromanganese coatings on phosphatized limestones from the sierra leone rise being richest in co differences in the chemistry of the water column and in growth rates of the ferromanganese coatings may be important in producing this regional contrast in composition fine scale changes in element abundances within the encrustations indicate that the nature of the substrate has little influence on compositional variations

the geomorphic significance of rock glaciers estimates of rock glacier debris volumes and headwall recession rates in west greenland to obtain knowledge on rockwall retreat rates at sites with rock glaciers an inventory of about 400 individual rock glacier sites in west greenland was carried out by means of field studies and using aerial photographs and topographic maps a number of sites with ice cored moraines beyond normal glaciers were included in the analysis for comparative purposes the estimated debris volume was compared with the size of the source headwall the result of the inventory suggests that the presence of rock glaciers indicates extraordinary high retreat rates for rock free faces above especially headwalls above glacier derived rock glaciers are exposed to high weathering rates and thereby presumably delimit critical ranges regarding altitude for headwall retreat rates the relation of such a critical altitudinal range to other regional concepts such as glaciation level gl and glacier equilibrium line is discussed from this comments are also made on the relation between various permafrost zones and the typical regional climate within the maximum headwall weathering altitude at sites with active rock glaciers finally the potential importance of this range of maximum weathering for understanding the long term evolution of high relief cold climate landscapes is emphasised

controls on spatial variations of glacial erosion in the qilian shan northeastern tibetan plateau abstract understanding interactions between topography and glacial erosion is a key to advancing knowledge of mountain landscape evolution and is often hampered by the lack of quantitative data for glacial erosion here we examine spatial variations in glacial erosion in the qilian shan using a power law model y axb fit to cross profiles of glacial valleys we further analyze the impacts of glacial dynamics topography and lithology on the degree of glacial erosion using qualitative and quantitative methods although the distribution of glacial erosion is controlled by the combined effects of glacial dynamics topography and lithology average glacier size or ice flux and mean slope above the equilibrium line altitude are dominant controls in addition the topographic data analyzed through a b fr diagram suggests two opposite types of glacial valley cross profile development temperate maritime glacier valleys are mainly dominated by overdeepening while continental glacier valleys are mainly dominated by a widening process although this varies with lithology

holocene rockwall retreat in svalbard a triple rate evolution volumetric calculations of slope deposits direct measurements of rockwall retreat and chronological control based on lichenometry provide a wide range of rockwall retreat rates in svalbard 0 1580 mm ka 1 that appears consistent with previous evaluations from other arctic areas in northwest and central spitsbergen 79 n a triple rate rockwall retreat is suggested for the last two millennia very slow biogenic flaking 2 mm ka 1 moderate retreat due to frost shattering 100 ka 1 and rapid retreat associated with post glacial stress relaxation c 1000 mm ka 1 examination of the distribution of various processes indicates that the holocene retreat of most rockwalls has not exceeded one or two metres bedrock conditions appear to be the main control on retreat rates the massiveness of igneous and metamorphic outcrops widespread in arctic shield areas largely accounts for the slowness of rockwall retreat which on these lithologies is primarily due to chemical and biological processes more rapid rates are usually associated with stress relaxation following glacial surges or with local frost susceptibility of bedrock often where faulting has induced high joint density at such sites rockwall retreat rates are of the same order of magnitude as those reported from alpine areas 1000 3000 mm ka 1 where both bedrock weakening due to tectonic stresses and the greater height of steep rockwalls account for the more rapid rockwall retreat rate 1997 by john wiley sons ltd

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

testing a glacial erosion rule using hang heights of hanging valleys jasper national park alberta canada 1 in most models of glacial erosion glacier sliding velocity is hypothesized to control rates of bedrock erosion if this hypothesis is correct then the elevation difference between hanging and trunk valley floors the hang height should be dictated by the relative sliding velocities of the glaciers that occupied these valleys by considering sliding velocity to be proportional to balance velocity and using mass continuity hang height is expressed in terms of glacier catchment areas slopes and widths which can be estimated for past glaciers from the morphology of glacial valleys these parameters were estimated for 46 hanging valleys and their trunk valleys in three adjacent regions of jasper national park the variability in valley morphology can account for 55 85 of the hang height variability if erosion rate scales with balance velocity raised to a power of 1 3 this correspondence is in spite of spatial variations in glaciation duration snow accumulation rates and other variables that likely affected hang heights but cannot be readily estimated and so are not included in our formulation thus it appears that balance velocity and by extension sliding velocity if the two are proportional may be a reasonable control variable for assessing erosion rate

the geology of pluto and charon through the eyes of new horizons nasa s new horizons spacecraft has revealed the complex geology of pluto and charon pluto s encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection with a crater retention age no greater than 10 million years surrounding terrains show active glacial flow apparent transport and rotation of large buoyant water ice crustal blocks and pitting the latter likely caused by sublimation erosion and or collapse more enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic and ridges with complex bladed textures pluto also has ancient cratered terrains up to 4 billion years old that are extensionally faulted and extensively mantled and perhaps eroded by glacial or other processes charon does not appear to be currently active but experienced major extensional tectonism and resurfacing probably cryovolcanic nearly 4 billion years ago impact crater populations on pluto and charon are not consistent with the steepest impactor size frequency distributions proposed for the kuiper belt

seasonal changes in the morphology of the subglacial drainage system haut glacier d arolla switzerland dye tracing techniques were used to investigate the glacier wide pattern of change in the englacial subglacial drainage system of haut glacier d arolla during the ablation seasons of 1990 and 1991 analysis of breakthrough curve characteristics indicate that over the course of a melt season a system of major channels developed by headward growth at the expense of a hydraulically inefficient distributed system by the end of the melt season this channel system extended at least 3 3 km from the snout of the 4 km long glacier and drained the bulk of supraglacially derived meltwater passing through the glacier the upper limit of the channel system closely followed the retreating snowline up glacier rates of headward channel growth reached c 65 m d 1 although these rates decreased in the upper 1 km of the glacier where snowline retreat exposed a patchy firn aquifer it appears that the removal of snow with its high albedo and significant water storage capacity from the glacier surface resulted in a dramatic increase in the volume of runoff into moulins and in the peakedness of daily runoff cycles this induced transient high water pressures within the distributed drainage system which caused it to evolve rapidly into a channelised system it is therefore likely that at a local scale channel growth occurred down glacier from moulins and that the overall up glacier directed pattern of channel formation was caused by the retreating snowline exposing new moulins and crevasses to inputs of ice derived meltwater damping of diurnal melt inputs by storage in the firm aquifer accounts for the slowing of channel growth in the upper glacier 1998 john wiley sons ltd

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

stress gradient coupling in glacier flow ii longitudinal averaging of the flow response to small perturbations in ice thickness and surface slope as a result of the coupling effects of longitudinal stress gradients the perturbations u in glacier flow velocity that result from longitudinally varying perturbations in ice thickness h and surface slope are determined by a weighted longitudinal average of h h and where h and are influence coefficients that control the size of the contributions made by local h and to the flow increment in the longitudinal average the values of h and depend on effects of longitudinal stress and velocity gradients in the unperturbed datum state if the datum state is an inclined slab in simple shear flow the longitudinal averaging solution for the flow perturbation is essentially that obtained previously kamb and echelmeyer 1985 with equivalent values for the longitudinal coupling length l and with h n l and n where n is the flow law exponent calculation of the influence coefficients from flow data for blue glacier washington indicates that in practice differs little from n whereas h can differ considerably from n 1 the weighting function in the longitudinal averaging integral which is the green s function for the longitudinal coupling equation for flow perturbations can be approximated by an asymmetric exponential whose asymmetry depends on two asymmetry parameters and where is the longitudinal gradient of l dl dx the asymmetric exponential has different coupling lengths l and l for the influences from up stream and from down stream on a given point of observation if is in the range 1 5 2 2 as expected for flow perturbations in glaciers or ice sheets in which the ice flux is not a strongly varying function of the longitudinal coordinate x then when dl dx 0 the down stream coupling length l is longer than the up stream coupling length l and vice versa when dl dx 0 flow thickness and slope perturbation data for blue glacier obtained by comparing the glacier in 1957 58 and 1977 78 require longitudinal averaging for reasonable interpretation analyzed on the basis of the longitudinal coupling theory with 4l 1 6 km up stream decreasing toward the terminus the data indicate n to be about 2 5 if interpreted on the basis of a response factor 0 85 derived theoretically by echelmeyer unpublished for the flow response to thickness perturbations in a channel of finite width the data contain an apparent indication that the flow response to slope perturbations is distinctly smaller in relation to the response to thickness perturbations than is expected on a theoretical basis i e h n n 1 for a slab this probably indicates that the effective l is longer than can be tested directly with the available data set owing to its limited range in x

erosion rate of a younger dryas cirque glacier at krakenes western norway an erosion rate of 0 5 to 0 6 mm a 1 is deduced for a younger dryas cirque glacier the erosion period of 700 a was determined from the laminated glaciolacustrine sediments in a small lake just outside the end moraine of the cirque and radiocarbon dates obtained below and above these sediments the volume of eroded bedrock was calculated from measurements of the deposited sediments of which the end moraine the glaciofluvial delta and the glaciolacustrine sediments are most important with a constant erosion rate the cirque could have formed in 83 to 125 000 a

a theoretical model of the fracture of rock during freezing we present a mathematical model for the breakdown of porous rock by the growth of ice within cracks the model is founded upon well established principles of fracture mechanics and recent advances in soil physics ijong with the assumption that progressive crack growth results from water migrating to ice bodies in cracks much as water migrates to ice lenses in freezing soil our model predicts crack growth rates compatible with empirical data calculations for a granite and a marble indicate that sustained freezing is most effective in producing crack growth when temperatures range from 4 c to 15 c at higher temperatures thermodynamic limitations prevent ice pressure from building up sufficiently to produce significant crack growth at lower temperatures the migration of water necessary for sustaining crack growth is strongly inhibited in hydraulically open systems in which pore water pressure remains near atmospheric pressure during the freezing process crackgrowth rates during continuous cooling will generally be greatest at low rates of cooling less than 0 1 0 5 c h at higher rates of cooling the influx of water to growing cracks is significantly inhibited the model delineates clearly the role of material parameters elastic moduli fracture mechanical properties grain size and shape and crack size environmental conditions temperature temperature gradient water pressure and time in frost damage to rocks our calculations along with recent experimental work on water migration in freezing rocks fukuda 1983 lead us to question the widely accepted importance of two phenomena freezing of water in sealed cracks and freeze thaw cycling in the fracture of rock exposed to natural freezing conditions

testing a numerical glacial hydrological model using spring speed up events and outburst floods 1 we describe a numerical model of a glacial hydrologic system that reproduces the salient features of two hydrological and ice motion events commonly observed on temperate alpine glaciers these include the pattern of ice sliding and water discharge characteristic of spring speed up events and the triggering and structure of outburst floods from ice marginal lakes these behaviors result from strong internal feedbacks between water storage in englacial macro pores or an ice marginal lake ice motion by sliding and deformation and a subglacial hydrologic system consisting of a slow flow linked cavity network and a fast flow centerline conduit

alpine landscape evolution dominated by cirque retreat despite the abundance in alpine terrain of glacially dissected landscapes the magnitude and geometry of glacial erosion can rarely be defined in the eastern kyrgyz range of central asia a widespread unconformity exhumed as a geomorphic surface provides a regional datum with which to calibrate erosion as tectonically driven surface uplift has progressively pushed this surface into the zone of ice accumulation glacial erosion has overprinted the landscape with as little as 500 m of incision into rocks underlying the unconformity distinctive glacial valleys display their deepest incision adjacent to cirque headwalls the expansion of north facing glacial cirques at the expense of south facing valleys has driven the drainage divide southward at rates of as much as two to three times the rate of valley incision glacial erosion rules based on ice flux incompletely explain this dominance of cirque retreat over valley incision local processes that either directly sap cirque headwalls or inhibit erosion down glacier appear to control at least initially alpine landscape evolution

spatial and temporal evolution of rapid basal sliding on bench glacier alaska usa we measured the surface velocity field during the summers of 1999 and 2000 on the 7 km long 185 m thick bench glacier alaska usa in the spring of both years a short lived pulse of surface velocity 2 4 times the annual mean velocity propagated up glacier from the terminus at a rate of 200 250md 1 displacement attributable to rapid sliding is 5 10 of the annual surface motion while the high velocity event comprised 60 95 of annual basal motion sliding during the propagating speed up event peaked at 6 14 cm d 1 with the highest rates in mid glacier continuous horizontal and vertical gps measurements at one stake showed divergence and then convergence of the ice surface with the bed as the velocity wave passed with maximum surface uplift of 8 16 cm high divergence rates coincided with high horizontal velocities suggesting rapid sliding on the up glacier side of bedrock steps initiation of the annual speed up event occurred during the peak in englacial water storage while the glacier was entirely snow covered basal motion during the propagating speed up event enlarges cavities and connections among them driving a transition from a poorly connected hydrologic system to a well connected linked cavity system sliding is probably halted by the development of a cnduit system

hypsometry of glaciated landscapes hypsometry frequency distribution of elevations is often used to characterize landscape morphology traditionally in the context of the degree of uvial dissection recently the hypsometry of glaciated regions has been used to infer how rates of glacial erosion compare with tectonic uplift rates however many factors other than tectonics can also exert a major in uence on the hypsometry of a glaciated landscape resulting in a wide variety of hypsometries using examples from the eastern sierra nevada california the western sangre de cristo range colorado and the ben ohau range new zealand we demonstrate that all else being equal the hypsometries of neighbouring basins can indicate the relative degree of glacial modi cation in each a selection of drainage basins from the rocky mountains shows that the position of the equilibrium line altitude ela within the drainage basin relief is a dominant variable in determining the hypsometry of a glaciated basin this is a non linear effect once the ela falls to some critical level the glaciers scour deeply below the ela causing a noticeably different hypsometry the hypsometry of an arbitrary region encompassing many drainage basins can disguise the variation present in the hypsometries and thus landforms of the individual basins unique local circumstances such as the presence of a mountain ice eld waiho basin southern alps substantial hanging valleys avalanche creek glacier national park a narrow outlet canyon sawmill creek sierra nevada and isolated geologic structures baker creek sierra nevada can have a major impact on the hypsometry of an individual basin copyright 2004 john wiley sons ltd

ice cap erosion patterns from bedrock 10be and 26al southeastern tibetan plateau quantifying glacial erosion contributes to our understanding of landscape evolution and topographic relief production in high altitude and high latitude areas combining in situ be 10 and al 26 ana

influence of a glacial buzzsaw on the height and morphology of the cascade range in central washington state usa abstract analysis of climatic and topographic evidence from the cascade range of washington state indicates that glacial erosion limits the height and controls the morphology of this range glacial erosion linked to long term spatial gradients in the ela created a tilted planar zone of 373 cirques across the central part of the range peaks and ridges now rise 600 m above this zone hypsometric analysis of the region shows that the proportion of land area above the cirques drops sharply and mean slopes 30 indicate that the areas above the cirques may be at or near threshold steepness the mean plus 1 relief of individual cirque basins 570 m corresponds to the 600 m envelope above which peaks rarely rise the summit altitudes are set by a combination of higher rates of glacial and paraglacial erosion above the ela and enhanced hillslope processes due to the creation of steep topography on the high precipitation western flank of the cascades the dominance of glacial and hillslope erosion at altitudes at and above the ela may explain the lack of a correspondence between stream power erosion models and measured exhumation rates from apatite u th he thermochronometry

features of glacial valley profiles simply explained 1 glacial occupation of alpine valleys results in a distinct signature in the long valley profile including steepening of the profile in the headwaters flattening at lower elevations and a step in the profile at the convergence of headwater tributaries we present analytic results for glacial erosion patterns by making the following assumptions 1 the initial profile is linear 2 the width of the valley is uniform 3 the annual mass balance varies linearly with elevation 4 the glacier at any time is quasi steady 5 the erosion rate is proportional to ice discharge per unit valley width and 6 glacial erosion rates far exceed fluvial erosion rates a steady glacier under these conditions would erode a parabolic divot in the longitudinal valley profile with its maximum depth coinciding with the down valley position of the equilibrium line altitude ela the calculated flattening of the valley floor down valley of the ela and the steepening of it up valley captures the essence of the glacial signature when a reasonable probability distribution of elas is allowed the predicted erosion peaks at 30 40 of the down valley distance to the glacial limit and the pattern merges smoothly with the steeper fluvial profile downstream of the glacial limit profiles of mass balance that are capped at a maximum value produce shorter glaciers and a slight asymmetry in the expected erosion pattern only when we mimic the tributaries of glacial headwaters by specifying a valley width distribution do we obtain the upper step observed in many valley profiles

valley asymmetry and glacial versus nonglacial erosion in the bitterroot range montana usa theories that propose feedbacks among climate tectonics and surface processes commonly assume that erosion is enhanced by glacial activity indeed studies have shown that glaciers appear to limit the elevation of mountain ranges however comparisons between rates of glacial and nonglacial erosion are difficult to make ideally such comparisons must hold precipitation and lithology constant while only varying the erosional regime located in a climatic transition zone during the pleistocene the east west trending valleys of the bitterroot range present an opportunity for a quantitative analysis of glacial and nonglacial erosion because the north facing sides of the valleys were glaciated whereas the south facing slopes were not the different erosional regimes operating on either side of the valleys created strongly asymmetric ridges ridgelines separating the east west trending valleys have been pushed southward by glacial headwall retreat such that ridge to valley distances are 50 greater on the north facing slopes than on the south facing slopes in addition mean hillslope angles are 6 lower on the glaciated slopes than on the unglaciated slopes and calculations of geophysical relief suggest that on average glaciers have removed nearly twice as much rock as nonglacial processes finally we conclude that although rates of vertical incision by glacial processes in the bitterroot range were more rapid than nonglacial processes the dominant geomorphological impact of glaciers was lateral erosion by headwall retreat

glacial hydrology and erosion patterns a mechanism for carving glacial valleys article i nfo article history most glacial erosion models assume that erosion rates are proportional to ice sliding velocity while recent studies have shown that water plays a major role in modulating sliding velocities the impact it might have on erosion rates is still unclear here we incorporate subglacial hydrology into a glacial erosion model that is based on a sliding rule our results explicitly highlight that adding subglacial hydrology has profound impacts on the temporal and spatial patterns of glacial erosion first it suggests that erosion might mainly occur during melting seasons when subglacial water pressure is large and effective pressure is low i e before the channels fully develop and reduce the water pressure at the ice bedrock interface which promotes sliding and erosion second the distribution of erosion is bimodal with a peak at the equilibrium line altitude and as we demonstrate enhanced erosion at much lower altitudes within the ablation area where water due to melting abounds this has important implications because it explains why glacial erosion can simultaneously

a numerical model of landform development by glacial erosion the jagged peaks knife edged ridges and steep sided valleys of alpine landscapes are well known products of glaciation yet little is known about how the dynamics of ice flow and glacial erosion give rise to such landforms by linking a finite element model for ice flow through a valley with a model for glacial erosion we have simulated the gradual evolution of one of the most striking of glacial landforms the u shaped valley incorporating realistic glacier sliding laws and spatial variations in rock resistance our simulations illustrate how ice flow erosion patterns and topography interact in the long term evolution of glacial landforms in addition our results indicate for the first time that the formation of a steady state u shaped valley takes approximately 105 years

rockfall activity from an alpine cliff during thawing periods abstract rockfall activity during thawing periods was investigated by collecting rockfall debris fallen on the snow covered talus slope in a cirque in the japanese alps near surface rock temperature was also monitored on the cirque wall maximum rockfall activity occurs on average about 10 days after the meltout of the cirque wall the intensive activity is rarely associated with precipitation events or diurnal frost cycles on the rockface a thermal conduction model suggests that rockfalls at this site occur most frequently when the seasonal thawing front penetrates to a depth of about 1 m in the bedrock while the freeze thaw penetration controls the maximum dimensions of detachable rock mass joint spacing on the rockface affects the size distribution of rockfall debris boulder falls resulting from seasonal frost weathering are considered to be the most important process responsible for the contemporary modification of the cirque wall the rockfall volume during a thawing period is usually 1 3 m3 which is equivalent to the rate of cirque wall retreat of the order of 0 01 mm year 1 however big boulder falls that occur once per decade can increase the retreat rate significantly

the response of two icelandic glaciers to climatic warming computed with a degree day glacier mass balance model coupled to a dynamic glacier model a degree day glacier mass balance model is coupled to a dynamic glacier model for temperate glaciers the model is calibrated for two outlet glaciers from the hofsjokull ice cap in central iceland it is forced with a climate scenario that has recently been defined for the nordic countries for the purpose of outlining the hydrological consequences of future greenhouse warming the scenario for iceland specifies a warming rate of 0 25 c per decade in mid summer and 0 35 c per decade in mid winter with a sinusoidal variation through the year the volume of the glaciers is predicted to decrease by approximately 40 over the next century and the glaciers essentially disappear during the next 200 years runoff from the area that is presently covered by the glaciers is predicted to increase by approximately 0 5 m a 1 30 years from now due to the reduction in the volume of the glaciers the runoff increase reaches a flat maximum of 0 5 2 0 m a 1 100 150 years from now and levels off after that the predicted runoff increase leads to a significant increase in the discharge of rivers fed by meltwater from the outlet glaciers of the ice cap and may have important consequences for the operation and planning of hydroelectric power plants in iceland

pleistocene relief production in laramide mountain ranges western united states gently sloped summits and ridges collectively referred to as summit flats are abundant in many laramide ranges in the western united states the erosion rate of summit flats is 10 m m y on the basis of the concentrations of cosmogenic radionuclides because erosion rates in valleys between summit flats are an order of magnitude faster relief within these ranges is currently increasing by about 100 m m y if summit flat erosion is slower than rock uplift driven by the isostatic response to valley erosion then this relief production could result in increased summit elevations the mean depth of material eroded from a smooth surface fit to existing summit flats varies from 280 to 340 m in four laramide ranges based on geographic information system gis analyses of digital elevation models this erosion would result in a maximum of 250 300 m of rock uplift assuming airy isostasy however because the laramide ranges examined here are narrow relative to the flexural wavelength of the lithosphere erosionally driven rock uplift is limited to 50 100 m over the past several million years summit erosion would approximately offset this rock uplift therefore we conclude that summit elevations have remained essentially constant even though several hundred meters of relief has been produced on the basis of valley and summit erosion rates and the average depth of erosion we estimate that relief production in laramide ranges began at ca 3 ma we hypothesize that this relief production was climatically driven and was associated with the onset or enhancement of alpine glaciation in these ranges

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

climatic limits on landscape development in the northwestern himalaya the interaction between tectonism and erosion produces rugged landscapes in actively deforming regions in the northwestern himalaya the form of the landscape was found to be largely independent of exhumation rates but regional trends in mean and modal elevations hypsometry frequency distribution of altitude and slope distributions were correlated with the extent of glaciation these observations imply that in mountain belts that intersect the snowline glacial and periglacial processes place an upper limit on altitude relief and the development of topography irrespective of the rate of tectonic processes operating

short term velocity variations and basal coupling near a bergschrund storglaciaren sweden distance measurements using an automated electronic distance measurement system in the north cirque of storglaciaren sweden during the summer of 1989 revealed a diurnal variation in glacier speed amplitude and timing of the diurnal cycle correlate well with the timing and intensity of the diurnal temperature cycle indicating that speed responds to variations in daily melt with a lag of approximately 4 h one 2 d period of non diurnal velocity variations corresponded with a large rainfall event finite element modeling suggests that these velocity variations must be closely related to water inputs in the cirque rather than to longitudinal coupling with lower parts of the glacier

scaling the teflon peaks rock type and the generation of extreme relief in the glaciated western alaska range 1 parts of the alaska range alaska usa stand in prominent exception to the glacial buzzsaw hypothesis which postulates that terrain raised above the ela is rapidly denuded by glaciers in this paper we discuss the role of a strong contrast in rock type in the development of this exceptional terrain much of the range is developed on pervasively fractured flysch with local relief of 1000 1500 m and mean summit elevations that are similar to modern snow line elevations in contrast cretaceous and tertiary plutons of relatively intact granite support the range s tallest mountains including mt mckinley or denali at 6194 m with 2500 5000 m of local relief the high granitic peaks protrude well above modern snow lines and support many large glaciers we focus on the plutons of the denali massif and the kichatna mountains to the west we use field observations satellite photos and digital elevation data to demonstrate how exhumation of these plutons affects glacier longitudinal profiles the glacial drainage network and the effectiveness of periglacial processes in strong granite steep smooth valley walls are maintained by detachment of rock slabs along sheeting joints these steep walls act as low friction surfaces teflon efficiently shedding snow simple scaling calculations show that this avalanching may greatly enhance the health of the modern glaciers we conclude that in places such as denali unusual combinations of rapid tectonic uplift and great rock strength have created the highest relief in north america by enhancing glacial erosion in the valleys while preserving the peaks

time scale for adjustment of glaciers to changes in mass balance the length of time tm over which a glacier responds to a prior change in climate is investigated with reference to the linearized theory of kinematic waves and to results from numerical models we show the following tm may in general be estimated by a volume time scale describing the time required for a step change in mass balance to supply the volume difference between the initial and final steady states the factor f in the classical estimate of m l u where i is glacier length and u is terminus velocity has a simple geometrical interpretation ft is the ratio of thickness change averaged over the full length i to the change at the terminus although both u and f relate to dynamic processes local to the terminus zone the ratio f u and therefore tm are insensitive to details of the terminus dynamics in contrast to conclusions derived from some simplified kinematic wave models a more robust estimate of tm independent of terminus dynamics is given by tm h b where h is a thickness scale for the glacier and b is the mass balance rate negative at the terminus we suggest that tm for mountain glaciers can be substantially less than the 1o2 103 years commonly considered to be theoretically expected

numerical modeling of the development of u shaped valleys by glacial erosion the steep sided valleys and overdeepened basins of alpine landscapes are well known products of glaciation yet relatively little is known about how the dynamics of ice flow and glacial erosion interact to give rise to such landforms by linking a finite element model for ice flow through a glacier cross section with an erosion model it is possible to investigate the development of one of the most striking glacial landforms the u shaped valley in addition to providing a detailed understanding of landform development such modeling provides a way to test current understanding of the controls on glacial sliding and erosion to simulate valley development i first model flow through an initial glacier cross section and calculate the glaciological parameters that govern erosion i then numerically simulate erosion to produce a modified transverse profile for which a new flow field and erosion pattern are computed a number of iterations permits examination of the progressive transformation of cross section form which can be compared with field data model predictions of the cross section flow field are in close accord with data from the athabasca glacier and include marked lateral variations in sliding velocity with an erosion law dependent on basal velocity the model predicts the rapid transformation of a v shaped cross section into a recognizably glacial form over a time period on the order of 10 4 yr and the eventual development of a steady state quasi parabolic glacier cross section better agreement with empirical data from glaciated valleys is obtained by including temporal variations in ice discharge in order to mimic the characteristics of 100 000 yr glacial cycles the high discharge phase dominates form development and at low discharges cross section form is essentially inherited from the central part of the form that developed during the preceding high discharge phase

entrainment at cold glacier beds here we present measurements of the gas content and isotopic composition of debris rich basal layers of a polar glacier meserve glacier antarctica which has a basal temperature of 17 c these measurements show that debris entrainment has occurred without alteration of the glacial ice and provide the most direct evidence to date that active entrainment occurs at the beds of cold glaciers without bulk freezing of water entrainment at subfreezing temperatures may have formed the u shaped trough containing meserve glacier in addition to possibly allowing some cold based glaciers to be important geomorphic agents entrainment at subfreezing temperatures provides a general mechanism for formation of the dirty basal layers of polar glaciers and ice sheets which are rheologically distinct and can limit the time span of ice core analyses furthermore accumulating evidence suggests that geomorphologists should abandon the assumption that cold based glaciers do not slide and abrade their beds