geothermal control on flow patterns in the last glacial maximum ice sheet of iceland because it is located both on the mid atlantic ridge and on a mantle plume iceland is a region of intense tectonics and volcanism during the last glaciation the island was covered by an ice sheet approximately 1000 m thick a reconstruction of the ice flow lines based on glacial directional features shows that the ice sheet was partly drained through fast flowing streams fast flow of the ice streams has been recorded in megascale lineations and flutes visible on the currently deglaciated bedrock and is confirmed by simple mass balance considerations locations of the major drainage routes correlate with locations of geothermal anomalies suggesting that ice stream activity was favoured by lubrication of the bed by meltwater produced in regions of high geothermal heat flux similar control of ice flow by geothermal activity is expected in ice sheets currently covering tectonically and volcanically active area such as the west antarctic ice sheet copyright 2000 john wiley sons ltd

self organization in the thermomechanical flow of ice sheets this paper uses a three dimensional thermomechanical ice sheet model to investigate the initiation and evolution of ice streams within ice sheets ice streams are distinguished within general ice sheet flow by their relatively fast of the order of kilometers per year velocity and are thought to be crucial in determining the response of ice sheets to climatic change we show that streaming can arise solely as a consequence of internal feedback between ice sheet flow and temperature inhomogeneities in topography and or geology are not therefore necessary conditions for ice stream development this spatial patterning replaces the large scale surging reported from previous modeling work in one horizontal dimension we suggest that surging is restricted to situations where ice flow is channelized e g valley glaciers and that it is replaced by streaming where horizontal ice flow is unconstrained ice sheets the ice streams in some of our experiments show switching behavior reminiscent of that known from the siple coast ice streams west antarctica the cause appears to be the competition for ice discharge and drainage basin area between adjacent ice streams this may lead to the gradual reduction of a stream s discharge below the threshold necessary to generate enough frictional heat to maintain warm viscous ice at the ice stream bed in this case the stream will stagnate abruptly which then prompts localized ice thickening and the initiation of a new stream in a different location

an appraisal of melting processes and the galapagos hotspot major and trace element evidence abstract the variation of nd sr and pb isotopes in lavas from the galapagos islands is thought to be the result of contamination of a mantle plume by the upper mantle in order to evaluate the control of source mixing on the compositions of the lavas the major and incompatible trace element compositions of parental melts are calculated by normalizing their values in aphyric unaltered lavas to mgo 8 along calculated liquid lines of descent the galapagos lavas have approximately half the worldwide major element compositional range of morbs of all the elements and ratios evaluated only volcano averaged k and la sm correlate with the isotopic ratios this must indicate that differences in the melting process such as depth extent of melting or both overwhelm any compositional difference between the plume and upper mantle several of the volcanoes have exceedingly large ranges in the abundances of incompatible trace elements but the range of isotopic values from single volcanoes is small and best explained by large ranges in the extents of melting inferred parental lavas also have a large range of si and fe concentrations which may result from extraction of the melts over a range of depths element abundances and isotopic ratios have geographic patterns which indicate that magmas from the center of the archipelago are derived by relatively lower average extents of melting deeper average extraction depths and less contribution of plume material in general however the mantle beneath the central islands experienced wide ranges in temperature extent of melting and the melts were extracted from widely different depths the lavas of the western and northern islands are more uniform probably controlled by consistent temperatures in the underlying mantle and depth of extraction these relationships are not explainable in terms of simple two stage melting and mixing models the compositions of parental galapagos magmas are controlled by a complex sequence of processes which have different effects on the isotopic major element and trace element compositions of the magmas

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

growth of an intermittent ice sheet in iceland during the late pliocene and early pleistocene abstract deep sea paleoclimatic records show that the quaternary climate around iceland was and probably still is very sensitive to rapid shifts in north atlantic oceanic circulation studies of several key sections in iceland indicate that similar oscillations are reflected in the late pliocene and early pleistocene terrestrial stratigraphy correlations between six rock sequences in western northern eastern and southern iceland show a fairly distinct trend during the pliocene pleistocene transition indicating the progressive growth of an ice sheet from southeast toward the north and west the correlations are based on k ar dates and paleomagnetic studies a total of 11 glacial horizons are recorded in a stratigraphic column from eastern iceland extending back 6 5 myr in western iceland 7 glacial horizons are preserved in a rock section dated from 7 0 to 1 8 myr and in northern iceland 14 glacial horizons are identified in a section that extends back to 9 0 myr well over 20 glacial horizons have been identified in the stratigraphic column in iceland full scale glacial interglacial cyclicity with regional ice cover is indicated at approximately 2 6 myr a further amplification leading to islandwide glaciations is identified at 2 2 2 1 myr these results show that changes in the ice cover in iceland correlate with the deep sea oxygen isotope records from benthic and planktonic foraminifera as well as ird studies from the north atlantic

denmark strait during the late glacial maximum and marine isotope stage 3 sediment sources and transport processes abstract the sediment records from north and south of denmark strait cores ps2644 and md99 2323 and on the kangerlussuaq trough mouth fan ktmf md99 2260 are evaluated for the period 12 to 60 cal ka bp with the goal of evaluating 1 the relative roles of the greenland and iceland ice sheets in sediment composition 2 the processes of sediment supply and 3 the relationship if any between the atmospheric derived ice core records and glacial marine sediment history these questions were addressed using counts of coarse 2 mm ice rafted debris ird magnetic susceptibility grain size spectra and sediment mineral composition a significant difference in these variables occurred between ms3 and mis2 with the latter showing small to modest abrupt changes in felsic and mafic minerals and in ird compared to larger and less variable changes during mis2 the dominant bedrock source archived in the cores is the cenozoic basaltic outcrops from east greenland and iceland 60 but there are frequent pulses of felsic rich sediments 17 36 which coincided with greenland stadials there is also a persistent detrital carbonate input the largest shift in most sediment proxies occurred 28 30 cal ka bp reflecting the sustained expansion of the ice sheets the grain size spectra on the snorri drift md99 2323 reflected complex depositional histories involving the supply of poorly sorted glacially derived sediment sand silt and clay and reworking by bottom currents

geologic and topographic controls on fast flow in the laurentide and cordilleran ice sheets ice streams are fast flowing arteries which play a vital role in the dynamics and mass balance of present day ice sheets although not fully understood fast flow dynamics are intimately coupled with geologic topographic thermal and hydrologic conditions of the underlying bed these are difficult observables beneath contemporary ice sheets hindering elucidation of the processes which govern ice streaxn behavior for past ice sheets the problem is antithetic geologic evidence of former ice streams exists but spatim and temporal histories are uncertain however detailed knowledge of bed geology and topography is available in many places we take advantage of this information to compile terrain characteristics relevant to fast flow dynamics in the laurentide and cordilleran ice sheets using seed points where fast flowing wisconsinan ice has been geologically inferred discriminant analysis of a suite of north american geologic and topographic properties yields a concise measure of ice bed coupling strength our analysis suggests that the interior plains and continental shelf regions of north america have low basal coupling relative to areas of variable relief or exposed bedrock in the cordillera and on the canadian shield we conclude that the interior plains and continental shelves are both topographically and geologically predisposed to large scale basal flows i e ice streaxns or surge lobes this result holds independent of whether the mechanism of fast flow is sediment deformation or decoupled sliding over the bed

diamict fans in subglacial water filled cavities a new glacial environment abstract the occurrence of glaciolacustrine diamict fans on the south side of volcanic tuyas in the laugarvatn area iceland is reported these fans are composed of sheets of diamict gravel and sand and contain little or no primary volcanic material they post date the volcanic pile within the tuyas and appear to have been deposited in subglacial water filled cavities confined to the down ice side of the tuyas it is suggested that these subglacial water bodies formed as result of an enhanced geothermal heat flux associated with the tuyas the volume of glacial sediment within the fans implies that either basal sediment loads were high in this sector of the former ice sheet or that ice flow converged towards the tuyas during their formation concentrating basal sediment into the lee side cavity and fan the glaciolacustrine diamict fans add to our knowledge of the landforms and sediments associated with tuyas and may also provide evidence of ice flow convergence at thermal anomalies

variations in felsic versus mafic sources in the western nordic seas during mis 1 to mis 4 abstract the goal of this paper is to employ quantitative methods to first describe the spatial variations in the quantitative non clay and clay mineral composition of seafloor surface sediments secondly use those data to describe the temporal variations in mineral compositions of nine cores that extend variously between the present day and 85 ka bp and thirdly describe regional variations in the transport of felsic versus mafic rich glacial marine sediments at four sites close to denmark strait k mean clusters of recent seafloor samples n 156 distinguished four mineral facies which characterize a range of mafic to felsic compositions these samples serve as a training set to identify the down core mineral facies in eight cores n 1053 the degree of provenance mixing is measured by fuzzy clustering on an unsupervised cluster analysis classification decision trees validate the mineral facies only 6 misclassified and indicate that variations in the weight of pyroxene are of primary importance in defining cluster membership mineral compositions identify two end member core sites jm07 174gc greenland sea and lo9 18 reykjanes ridge while the cores north and south of the greenland faroes ridge cores ps2644 md99 2274 2280 and 2323 are a mixture of mafic rich and felsic rich mineral facies variations in magnetic susceptibility and quartz wt exhibit coherent runs of values above or below their medians in three of the cores occurred indicating that these cannot be ascribed to random variations p

large scale ice moulding a discussion of genesis and glaciological significance abstract landsat images reveal a previously unsuspected large scale pattern of streamlining within drift which comprises a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain consideration of the genesis and glaciological significance of such landforms leads to a number of conclusions it is suggested that mega scale glacial lineations were formed under conditions of fast ice flow and their presence may thus record former locations of ice streams or surge events many lineations of varying scales display a pattern that reveals that they were not formed in sub marginal positions extensive sets of lineations must have been formed approximately synchronously thus indicating that lineation generation occurs over a wide range of glaciodynamic conditions from sub marginal positions to interior portions of ice sheets subglacial deformation of tills has been widely invoked to account for rapid glacier motion criteria for identifying such tills are restricted to structures viewed in cross section and it is thus hard to assess how widespread the processes of deformation may have been lineations provide a surface marker of structure that records subsequent deformation from other ice flow events it is argued that the palaeo flow record including cross cutting patterns provides the surface or plan expression of subglacial deformation the degree of modification to pre existing lineations should permit the identification of zones favoured by deformation and thus assist in an assessment of how pervasive deforming bed processes were in influencing ice sheet dynamics

basin geometry and salt diapirs in the flinders ranges south australia insights gained from geologically constrained modelling of potential field data abstract the adelaide basin in australia is a complex of late neoproterozoic to early cambrian rift and sag basins which was inverted during the cambro ordovician delamerian orogeny the deposition of evaporitic sediments during the earliest stage of basin development in the late neoproterozoic willouran age played a major role in the subsequent tectonic evolution of the basin previous studies have shown that early mobilization vertical transport and withdrawal of the evaporites influenced the sedimentation during the late neoproterozoic and early cambrian the evaporites also influenced deformation during the inversion of the basin and the development of the delamerian fold and thrust belt however the control exerted by basement structures in the deposition of the evaporitic beds and the role of these tectonic structures in the later inversion of the basin have been poorly constrained in this work we use a combination of published and original geological observations along with the interpretation of potential field datasets total magnetic intensity and bouguer anomaly data to better constrain the geology of the basin at depth we construct two and a half dimensional forward models of the potential field data along selected profiles across the adelaide basin these models are constrained by the geology at the surface drill hole data and measured petrophysical properties specific gravity and magnetic susceptibility we achieved the best fit between observed and modelled potential fields with a model favouring thick skinned deformation where the diapirs are not randomly distributed but located directly above basement penetrating normal faults furthermore these normal faults were probably active during the sedimentation of the late neoproterozoic and early cambrian sediments and underwent partial or total inversion during the cambro ordovician delamerian orogeny mobilisation and withdrawal of the evaporites was therefore initiated and facilitated by the extension coeval with the opening of the basin and continued during the subsequent delamerian orogeny

landform assemblage in isidis planitia mars evidence for a 3 ga old polythermal ice sheet abstract the floor of isidis planitia a giant impact basin located close to the martian equator exhibits a landform assemblage nicknamed thumbprint terrain made of arcuate ridges aligned cones isolated cones cone fields associated with a peripheral network of sinuous ridges linear depressions and mounds from a new comprehensive mapping initiative of these landforms and from comparisons with terrestrial analogues ribbed moraines dirt cones kettle holes eskers tunnel valleys and moraine plateaux we demonstrate that this distinctive assemblage is a glacial landsystem inherited from the presence of a massive polythermal ice sheet over the basin during the hesperian the flow of the ice sheet was controlled by its basal thermal regime wet based conditions led to the formation of arcuate ridges and aligned cones in most parts of the basin while a negative geothermal anomaly due to impact related crustal thinning was responsible for cold based conditions in its central part where only isolated cones and cones fields are present sinuous ridges linear depressions and mounds at the basin margins are interpreted as relicts of a radial network of subglacial channels which drained the glacial meltwater produced within the interior of the ice sheet across its cold based periphery

fast glacier flow ice streams surging and tidewater glaciers fast flowing glaciers are unusual but important jakobshavns glacier in western greenland illustrates this point figure 1 considered to be the world s fastest glacier its maximum observed flow rate measured in midsummer near the floating terminus is 8360 m yr 1 lingle et al 1981 except for a tidal influence near the terminus the flow is believed to be fairly steady bindschadler 1984

evolution of galapagos magmas mantle and crustal fractionation without assimilation summary of helium isotopic data from the gala pagos islands and dredged samples from the gala pagos platform data from graham et al 1993 and kurz et al 1993 data are normalized to the atmospheric value r ra most gala pagos samples are intermediate betweenmorb value of 8 1 and the fernandina value 22 material in those magmas the exception is floreana structure first the depth to the magma chambers mayisland which ranges from 14 5 to 7 7 ra it is possible be due to the rate at which magma is supplied from thethat the low value at floreana re ects assimilation of mantle as the volcano is carried away from the focus ofmaterial with excess radiogenic 4 he by a magma with the hotspot the volcanoes in a juvenile growing stage 3 he 4 he ra 14 most floreana lavas contain abundant of evolution those that have had numerous historicdisaggregated debris from ultrama c xenoliths bow eruptions are dominated by lavas with mgo con geist 1992 a potential source of helium contamination centrations between 5 and 6 e g reynolds geist 1995 a narrow deep caldera characterizes the juvenilestage e g cerro azul wolf and fernandina and iteventually broadens and shallows because of lling with

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 2 application to the hudson strait ice stream episodic exportations of ice rafted debris to the north atlantic in the late pleistocene suggest quasiperiodic ice streaming or surging activity on the northeastern margin of the laurentide ice sheet much of this efflux of ice may originate from an ice stream issuing from hudson strait and tapping into core regions of the laurentide in hudson bay labrador and the foxe basin applying the continuum mixture theory outlined by marshall and clarke this issue we model the thermomechanical evolution of the hudson strait ice stream in a three dimensional finite difference model of the laurentide ice sheet our simulations focus on internal dynamics of the ice stream under thermal regulation of basal flow we find surge cycles of stream activity interspersed with quiescent periods where the ice stream is frozen to the bed modeled surge durations vary from 105 to 3260 years while surge periodicities range from 585 to 22 410 years with pervasively warm or cold internal temperature distributions in the ice ice streams can also establish modes of permanent activity or inactivity under thermal regulation our most vigorous ice streams produce peak values of approximately 0 03 sv of freshwater flux to the north atlantic from basal meltwater and iceberg production associated ice stream velocities in this maximum case approach 6700 m yr 1 the total ice volume mobilized in a single surge event is equivalent to a global sea level rise of 0 04m in the most tranquil surge and almost 0 6m in the most extreme case these velocities and sea level impacts are an order of magnitude less than those predicted by macayeal 1993a b and only our most exuberant streams approach the iceberg flux estimates of dowdeswell et al 1995 we propose that the sediment load of icebergs emanating from hudson strait in a surge event may exceed expectations from contemporary icebergs

magellan strait part of a neogene rift system the magellan strait joins the atlantic and pacific oceans separating tierra del fuego from southernmost continental south america the strait cuts both the andean cordillera and the magellan foreland basin other arms of the sea intrude the axial zone of the magellan basin these depressions have long been interpreted as glacial valleys on the basis of landsat images digital topography and field data we interpret the depressions as rifts or half rifts in general active rifts developed within foreland basins are unusual but in patagonia they are consistent with regional deformation and its plate tectonic setting during the neogene

geology and petrology of floreana island galapagos archipelago ecuador abstract floreana is the sixth largest of the galapagos islands and lies at the extreme southern margin of the galapagos platform in sharp contrast to the other major islands floreana lacks a well developed volcanic center but instead is dominated by pyroclastic vents and derivative ash and cinder deposits the majority of exposed lavas are xenolith bearing alkali olivine basalts that constitute a coherent suite referred to as the main series the flank series comprises a group of isolated vents on the southern flank of the island which erupted xenolith poor basaltic lavas that are generally poorer in incompatible trace elements and richer in plagioclase paleomagnetic measurements supported by k ar age determinations record a relatively limited timespan for subaerial volcanism shield development reached maximum dimensions by 1 0 ma with the final phases of pyroclastic activity at an end by 0 3 ma main series rocks are phenocryst poor and typically contain clusters of xenocrysts in an extremely fine grained granular matrix petrogenetic modelling indicates they represent products of partial melting of a lree enriched spinel lherzolite source little modified by subsequent fractional crystallization porphyritic flank series lavas experienced rather different ascent histories and may have resided in shallow level magma chambers prior to eruption isotopic geochemical and petrographic data demonstrate that flank series lavas are not cogenetic in any simple way with main series flows floreana lavas contain a diverse suite of ultramafic xenoliths al augite group megacrysts are likely to be cognate and comagmatic with host alkali olivine basalts transition metal contents of megacrysts and host lavas however preclude clinopyroxene fractionation as a mechanism of magmatic differentiation it is proposed that megacrysts were products of passive crystallization and were never effectively separated from host magmas cr diopside group nodules may have crystallized at very high temperatures from advanced partial melts and are less likely to bear any direct genetic connection to host basalts

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

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

climate and ice sheet mass balance at the last glacial maximum from the genesis version 2 global climate model abstract at the last glacial maximum lgm about 21 000 calendar years ago 21 ka bp the overall mass balance of the laurentide and eurasian ice sheets should have been close to zero since their rate of change of total ice volume was approximately zero at that time the surface mass balance should have been zero or positive to balance any iceberg iceshelf discharge and basal melting but could not have been strongly negative in principle this can be tested by global climate model cgm simulations with prescribed ice sheet extents and topography we describe results from a suite of 21 ka bp simulations using a new global climate model genesis version 2 0 a with ssts prescribed from climap 1981 and predicted by a mixed layer ocean model and with ice sheets prescribed from clih the ice 4g peltier 1994 and climap 1981 reconstructions in common with previous gcm simulations using mixed layer oceans substantial cooling over and around antarctica occurs due to normal gcm dynamics and polar sea ice feedback without recourse to changes in thermohaline circulation we find slightly enhanced cooling and cloudiness over low latitude land masses compared to over ocean especially over the andes which decreases the disagreement with land based data and suggests that tropical climap ssts may only be 1 2 c too warm the gcm used here is well suited for ice sheet mass balance studies because i the surface can be represented at a finer resolution than the atmospheric gcm ii an elevation correction accounts for spectral distortions of the atmospheric gcm topography iii a simple post processing correction for refreezing of meltwater is applied and iv the model s precipitation and mass balances for present day greenland and antarctica are realistic however for all reasonable combinations of ssts and ice sheet configurations the predicted annual surface mass balances of the lgm laurentide and eurasian ice sheets are implausibly negative possible reasons for this discrepancy are discussed including increased ice age aerosols higher climap like ice sheet profiles in the few thousand years preceding lgm and a surge of the southern laurentide just before lgm to fleetingly produce the ice 4g profile at 21 ka bp

origin of espanola island and the age of terrestrial life on the galapagos islands geological studies of espanola hood island galapagos ecuador indicate that the island had a subaerial rather than a submarine origin because the younger lava flows are dated at 3 million years espanola has apparently existed as an island for at least that long thus terrestrial life may have existed or arrived on the galapagos islands at least 3 million years ago more than twice as long as had been assumed

the miocene galapagos ash layer record of integrated ocean drilling program legs 334 and 344 ocean island explosive volcanism during plume ridge interaction drilling at integrated ocean drilling program site u1381 on the cocos ridge offshore costa rica recovered 67 primary miocene ca 8 ma to ca 16 5 ma marine fallout ash layers geochemical volcanological and geological criteria link these ashes to plinian eruptions that carried ash to at least 50 450 km from the galapagos hotspot these ash layers are the first documentation of highly explosive miocene galapagos hotspot volcanism this volcanism is bimodal with two thirds of the tephra layers generated by basaltic magmas glass compositions 57 wt sio2 and one third by rhyolitic magmas the temporal distribution of the tephra layers inferred from sediment accumulation rates calibrated by 40ar 39ar and biostratigraphic ages reveals a distinct increase in eruption frequency and hence increased volcanic activity of the galapagos hotspot after 14 ma which we interpret in the context of dynamic interaction between the galapagos plume and spreading ridge

effects of subglacial geothermal activity observed by satellite radar interferometry we use one day synthetic aperture radar sar interferograms from data of the earth remote sensing satellites ers 1 and ers 2 to study ice flow and uplift of two surface depressions within the vatnajokull ice cap iceland the ice cauldrons are created by melting at sub glacial geothermal areas meltwater accumulates in a reservoir under the cauldrons over 2 to 3 years until it drains in a jokulhlaup under the ice dam surrounding the reservoir the ice surface in the depressions drops down by several tens of meters during these draining events but rises again as ice flows into the depressions until a jokulhlaup occurs again using sar interferograms we quantify an uplift rate of about 2 to 18 cm day within the jokulhlaup cycle varying with the surface slope of the depressions the uplift rate is high during the first months after a jokulhlaup when the cauldron is relatively deep with steep slopes but the uplift rate decreases as the cauldron is gradually filled a simple axisymmetric model simulating the ice flow into one of the depressions describes quantitatively the filling rate of the cauldron and qualitatively the shape of the ice flow field the best fit model has an ice flow law parameter a0 that is about one order of magnitude lower than typically estimated for temperate glaciers

glacial removal of late cenozoic subglacially emplaced volcanic edifices by the west antarctic ice sheet local maxima of the horizontal gradient of pseudogravity from closely spaced aeromagnetic surveys over the ross sea northwestern ross ice shelf and the west antarctic ice sheet reveal a linear magnetic rift fabric and numerous subcircular high amplitude anomalies most of these anomalies have sources that probably resulted from late cenozoic volcanism some of these volcanic structures penetrate the neogene sediments beneath the deglaciated continental shelf and are present at the base of the present grounded ice sheet and beneath the ice shelf geophysical data indicate two or three youthful volcanic edifices at widely separated areas beneath the sea and ice cover in the west antarctic rift system in contrast we suggest glacial removal of edifices of volcanic sources of many more anomalies magnetic models controlled by marine seismic reflection and radar ice sounding data allow us to infer that glacial removal of the associated late cenozoic volcanic edifices probably debris comprising pillow breccias and hyaloclastites has occurred essentially concomitantly with their subglacial eruption removal of unconsolidated volcanic debris erupted beneath the ice is probably a more appropriate term than erosion given its fragmented ice contact origin the exposed volcanoes may have been protected from erosion by the surrounding ice sheet because of more competent rock or high elevation above the ice sheet glacial removal may be the general case exposed late cenozoic volcanic peaks and outcrops consisting primarily of flows which erupted during antarctic glacial conditions since approx 30 ma may be the exceptions the volume of the exposed volcanoes is small in contrast to the much greater volume 106 km3 of late cenozoic magmatic rock remaining at volcanic centers beneath the continental shelf ross ice shelf and west antarctic ice sheet

numerical reconstruction of a soft bedded laurentide ice sheet during the last glacial maximum we used a numerical ice sheet model to reconstruct the north american laurentide ice sheet during the last glacial maximum our model simulates ice sheet conditions that can be specified experimentally as either a rigid substrate hard bed or a wet deformable till soft bed basal sliding is excluded we use geologic records of former basal ice sheet processes to prescribe the distribution of hard and soft beds our reconstruction of the laurentide ice sheet is significantly lower in ice surface height and contains less ice volume than the climap maximum reconstruction in contrast our reconstruction agrees well with the ice 4g reconstruction both in height and volume because the ice 4g reconstruction is based on the inversion of relative sea level data whereas our reconstruction is based on glacial geology and ice mechanics this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the laurentide ice sheet that is most consistent with observations of relative sea level change and other geodynamic considerations

magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics the volcanic rift zone in iceland is characterized by elongate volcanic systems consisting of tension fractures normal faults and volcanic fissures where most of the volcanotectonic activity takes place most volcanic systems develop central volcanoes the formation of which is still poorly understood there is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes in the central volcanoes eruptions are frequent typically one every several hundred years of small volume normally less than 0 1 km3 and fed mainly by thin average thickness of 0 5 m inclined sheets outside central volcanoes the eruptions are rare typically one every several thousand years but of large volume typically more than 1 km3 and fed mainly by thick average thickness of 4 5 m subvertical dikes using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi infinite plate these empirical relations as well as the formation of specific central volcanoes in the volcanic systems can be explained once a cavity like magma chamber has formed its existence in the rift zone concentrates tensile stress this stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system which partly explains the formation of central volcanoes and their high eruption frequencies this concentration also gives rise to a local stress field that encourages injection of small volume sheets in all directions and at various dips from their source magma chamber the magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma through inclined sheets toward a limited area at the surface where the central volcano gradually forms the inverse relationship between eruption frequency and eruption volume when the central volcano is compared with other parts of the volcanic system is also partly due to the trap like nature of the magma chamber the chamber is normally much smaller than its source mantle reservoir so that a single magma flow through a dike from the reservoir lasting perhaps many years may trigger tens of magma flows through sheets from the chamber many of which would reach the surface in the central volcano

geology of santa fe island the oldest galapagos volcano abstract santa fe island was a volcanic center when it emerged 3 9 0 6 m y ago later upfaulting of a horst along the central axis of the island dominates its present morphology santa fe is made up of evolved transitional lavas that are not related by fractional crystallization alone source heterogeneties differing degrees of melting or open system magma chambers may explain the observed trace element variations santa fe baltra and espanola make up a geologic subprovince in the central galapagos they are older than the other islands and their lavas are compositionally similar at the time of their emergence the three islands were in a tectonic setting similar to that of the young western and central galapagos island

time averaged paleomagnetic field at the equator complete data and results from the galapagos islands ecuador 1 we present here the complete paleomagnetic laboratory results from a collection of approximately 1500 oriented cores from all 16 of the galapagos islands ecuador collected by allan cox in 1964 1965 but nearly all previously unpublished the islands are located in the eastern pacific ocean within 1 4 of latitude from the equator and range in age from historically erupted to 3 ma mostly determined by published k ar and 3he isotopic dating the number of sites collected on each island ranges from 1 to 28 for a total of 186 after combining duplicate site mean directions 149 are used for an overall mean direction and 8 represent excursions and one reversal path divided by geomagnetic polarity chron 110 site means are brunhes or jaramillo normal polarity 27 are matuyama reversed polarity and 12 are gauss both polarities we have completed the magnetic cleaning that was commenced in the late 1960s secondary mostly viscous magnetizations were nearly all removed by alternating field demagnetization at 10 mt we have used the so called blanket cleaning method generally at 10 mt all sites were in basalt flows and gave good paleomagnetic results none was rejected in toto and only a few core specimens were magnetically unsatisfactory nearly all sites had eight independently oriented cores and within site angular standard deviations of directions range from 1 to 8 we used both fisher and bingham statistics to analyze the data and found that many of the direction populations are strongly elongate along the paleomagnetic meridian while the corresponding virtual pole vgp populations are essentially circularly distributed the paleomagnetic poles calculated as the means of vgps are as follows brunhes and jaramillo north latitude 86 9 east longitude 245 1 and 95 confidence radius a95 1 9 matuyama latitude 87 2 longitude 158 2 and a95 3 8 gauss latitude 83 0 longitude 204 7 and a95 7 0 these paleomagnetic poles do not differ significantly from one another but the brunhes and jaramillo combined pole is significantly near sided with respect to the galapagos as is the overall mean pole which is at latitude 87 1 and longitude 227 6 with a95 1 7 omitting the excursion and reversal path data the overall angular standard deviation of vgps is 11 7 with lower and upper 95 confidence limits of 10 8 and 12 7 respectively in good agreement with previously published values for near equatorial sampling latitudes

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock