spatial variation of radar derived basal conditions on kamb ice stream west antarctica radar profiles of bed echo intensity can survey conditions at the ice bed interface and test for the presence or absence of water however extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice we used the relationship between bed echo intensities from constant offset radar data and ice thickness to estimate depth averaged attenuation rates at several locations on and near kamb ice stream kis west antarctica we found values varying from 29 db km 1 at siple dome to 15 db km 1 in the main trunk region of kis in agreement with a previous measurement and models using these attenuation rate values we calculated the relative bed reflectivity throughout our kis surveys and found that most of the bed in the trunk has high basal reflectivities similar to those obtained in the location of boreholes that found water at the bed areas of lower bed reflectivity are limited to the sticky spot where a borehole found a dry bed and along the margins of kis these results support previous hypotheses that the basal conditions at locations like the sticky spot on kis control its stagnation and possible reactivation

effects of birefringence within ice sheets on obliquely propagating radio waves in this paper effects of birefringence on radio waves obliquely propagating though polar ice sheets are examined to facilitate interpretations of bistatic and side looking radar data a formalism applicable for arbitrary radar configurations is developed to predict the returned power from within and beneath the ice sheets that have arbitrary alignments of ice crystals ice fabrics we applied this formalism to a range of ice fabrics found in ice cores and assessed the effects of birefringence in terms of ray path configurations ice fabrics and radar frequency predicted frequency dependence of the bed return power replicates prominent features observed at greenland ngrip ice core site results show that birefringence in ice of 1 km or more thickness with strong weak fabric can reduce the power returned from the bed 2 db or more at frequencies higher than 200 mhz 20 mhz as compared to isotropic ice this suggests that quantitative interpretation of the power returned from the bed requires careful assessment of birefringence almost everywhere over the ice sheets application of this formalism also suggests a radar frequency range usable for attenuation measurements possible effects of fabric on synthetic aperture radar processing and a feasibility of remote sensing of ice fabric

radar signatures beneath a surface topographic lineation near the outlet of kamb ice stream and engelhardt ice ridge west antarctica visible and infrared satellite images reveal numerous lineations on the siple coast region of west antarctica we used 5 mhz ice penetrating radar to probe the interior and the bed of the ice sheet beneath a lineation at the boundary between engelhardt ice ridge and flat ice terrain to the south of the kamb ice stream kis outlet results show curved reflectors that emerge from the bed beneath 600 m thick ice the tops of the reflectors extend about 100 m into the ice above the bed where they become almost horizontal apparent reflectivity of the horizontal section is about 20 db less than that of the bed we conclude that the likely cause of such strong reflection is sea water that was accreted into basal crevasses when the flat ice terrain was floating internal layers are warped downward just downslope from the basal reflectors it is thought that the downwarping was caused by localized basal melting in the past the spatial pattern of downwarping suggests that localized basal melting was stronger on the north side than on the south side of kis apparently ice ocean interactions on the two sides of kis were different

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

extensive winter subglacial water storage beneath the greenland ice sheet w c is a recipient of the nasa earth and space science fellowship d m s is supported by a grant from the nasa cryospheric sciences program h s is supported by grants from the nasa cryospheric sciences and sea level rise programs t t c and r e b are supported by grants from national science foundation nsf and nasa cryospheric sciences s p is supported by the natural environment research council s centre for polar observation

role of transition zones in marine ice sheet dynamics 1 it has been suggested that rapid thinning of the west antarctic ice sheet is due to increased melting under ice shelves caused by a gradual ocean warming shepherd et al 2004 payne et al 2004 showed that such melting could lead to an acceleration of grounded ice flow in this paper we analyze the response of a marine ice sheet to different perturbations near the grounding line using a numerical ice sheet model that takes into account longitudinal stress coupling and grounding line migration at subgrid precision based on a novel technique results show that stress transmission or longitudinal coupling across the grounding line plays a decisive role the grounding line migration is a function of the length scale over which the basal conditions change from frozen to the bed to floating the transition zone we demonstrate that thinning of the ice shelf due to bottom melting has a negligible effect on the grounded ice mass only perturbations at the grounding line or reduction in buttressing of the ice shelf substantially thins the grounded ice sheet inclusion of lateral drag does not alter these results qualitatively marine ice sheets with large transition zones such as ice streams seem highly sensitive to such perturbations compared to ice sheets with small transition zones such as an abrupt ice sheet ice shelf junction

crustal magnetization of the reykjanes ridge and implications for its along axis variability and the formation of axial volcanic ridges we have inverted the sea surface total magnetic field measurements of the reykjanes ridge between 57 n and 63 n for crustal magnetization intensity assuming a uniformly thick magnetic source layer and direction of magnetization corresponding to the geocentric axial dipole field this section of the north mid atlantic ridge 10 mm yr half spreading rate is unique in that it lies within the influence of the iceland mantle plume which causes it to spread obliquely and in the north to resemble morphologically a fast spreading ridge on a regional scale the axial magnetization anomalies of the reykjanes ridge can be divided into three regimes from south to north these regimes correspond to sections of the ridge with different magmatic and tectonic styles which are believed to represent the effects of a pulsing plume beneath iceland the axis south of 59 10 n has the highest magnetization anomaly with the greatest variability and exhibits the characteristics of a slow spreading ridge unaffected by a plume the axis between 59 10 n and 60 50 n has the lowest magnetization anomaly on average and is where we presume the present day plume front is located the absence of large fault scarps and low magnetization are consistent with the idea of a midcrustal magma reservoir where mixing causes the magma to become less highly fractionated and produces less magnetic lavas finally a moderate increase in the magnetization anomaly and degree of faulting occurs north of 60 50 n signaling a gradual recovery in the crustal production and underlying structure after the passage of the plume front on the local scale with a few exceptions the loci of high magnetization anomalies coincide with the axial volcanic ridges notable lateral offsets in axial magnetization and decay in magnitude were found at 57 50 n 58 05 n and 59 12 n comparisons with seismic layer 2a show that variations in the magnetic source layer thickness may account for as much as half of the observed variability in the along axis magnetization there are also indications that the relative age of the extrusives and the degree of faulting and fissuring may be factors affecting the magnetization intensity of volcanic edifices on the seafloor

ice shelf grounding zone structure from icesat laser altimetry 1 we present a technique for investigating the grounding zone gz of antarctic ice shelves using laser altimetry from the ice cloud and land elevation satellite icesat most surface height variability in the gz is easily resolved by the icesat laser s 65 m footprint and 172 m along track spacing comparisons of repeated tracks sampled at different phases of the ocean tide identify the landward and seaward limits of tide forced ice flexure providing gz location and width information for each track using icesat data in the institute ice stream region of southern ronne ice shelf we demonstrate that the location of the gz based on feature identification in satellite imagery or digital elevation models may be in error by several km our results show that icesat will contribute significantly to improving knowledge of gz structure and to studies requiring accurate gz locations e g ice mass balance calculations and ice sheet ocean modeling

predicting radar attenuation within the antarctic ice sheet abstract to better understand the ability of ice penetrating radar to diagnose the subglacial environment from bed returned power we model the englacial radar attenuation of antarctic ice first we use a one dimensional thermo mechanical model to evaluate the sensitivity of the depth averaged attenuation rates to ice temperature as a function of surface accumulation rate geothermal flux and ice thickness we find that attenuation is most sensitive to variations in geothermal flux and accumulation rate when the bed temperature is close to the pressure melting point but even if geothermal flux and accumulation rate remain fixed attenuation can easily vary with ice thickness such high sensitivities show that one should not assume a uniform attenuation rate in the radar data analysis then using ensembles of modeled ice temperatures with different boundary conditions we generate multiple attenuation predictions for the antarctic ice sheet and evaluate the resulting uncertainties the largest contributor to uncertainty in these predictions is the geothermal flux this uncertainty is localized within the deeper half of the ice sheet by combining these temperature ensembles with ice core chemistry data we show that the sea salt adds little to the attenuation but the contribution from acids accounts for 29 inland to 53 coast of the total attenuation we conclude that improving radar diagnosis of the subglacial environment using bed returned power requires both 1 better data interpretation algorithms that account for attenuation variations and 2 better constraints of geothermal flux and bulk chemistry

radar characterization of the basal interface across the grounding zone of an ice rise promontory in east antarctica radar power returned from the basal interface along a 42 km long profile over an ice rise promontory and the adjacent roi baudouin ice shelf dronning maud land east antarctica is analyzed to infer spatial variations in basal reflectivity and hence the basal environment extracting basal reflectivity from basal returned power requires an englacial attenuation model we estimate attenuation in two ways 1 using a temperature dependent model with input from thermomechanical ice flow models and 2 using a radar method that linearly approximates the geometrically corrected returned power with ice thickness the two methods give different results we argue that attenuation calculated using a modeled temperature profile is more robust than the widely used radar method especially in locations where depth averaged attenuation varies spatially or where the patterns of basal reflectivity correlate with the patterns of the ice thickness

temporally stable surface mass balance asymmetry across an ice rise derived from radar internal reflection horizons through inverse modeling ice rises are locally grounded parts of antarctic ice shelves that play an important role in regulating ice flow from the continent towards the ocean because they protrude out of the otherwise horizontal ice shelves ice rises induce an orographic uplift of the atmospheric flow resulting in an asymmetric distribution of the surface mass balance smb here we combine younger and older internal reflection horizons irhs from radar to quantify this distribution in time and space across derwael ice rise dir dronning maud land antarctica we employ two methods depending on the age of the irhs i e the shallow layer approximation for the younger irhs near the surface and an optimization technique based on an ice flow model for the older irhs we identify an smb ratio of 2 5 between the flanks and the ice divide with the smb ranging between 300 and 750 kg m 2 a 1 the smb maximum is located on the upwind side 4 km offset to today s topographic divide the large scale asymmetry is consistently observed in time until 1966 the smb from older irhs is less well constrained but the asymmetry has likely persisted for ka indicating that dir has been a stable features over long time spans

transient glacier response with a higher order numerical ice flow model in this paper a higher order numerical flowline model is presented which is numerically stable and fast and can cope with very small horizontal grid sizes 10 m the model is compared with the results from blatter and others 1998 on haut glacier d arolla switzerland and with the european ice sheet modelling initiative benchmarks huybrechts and others 1996 results demonstrate that the significant difference between calculated basal drag and driving stress profiles in a fixed geometry disappears when the glacier profile is allowed to react to the surface mass balance conditions and reaches a steady state dynamic experiments show that the mass transfer in higher order models occurs at a different speed in the accumulation and ablation areas and that the front position is more sensitive to migration compared to the shallow ice approximation

pitfalls in radar diagnosis of ice sheet bed conditions lessons from englacial attenuation models 1 radar power returned from ice sheet beds has been widely accepted as an indicator of bed conditions however the bed returned power also depends on englacial attenuation which is primarily a function of ice temperature here using a one dimensional attenuation model it is demonstrated that in most cases variations in bed returned power are dominated by variations in englacial attenuation rather than bed reflectivity both accumulation rate and geothermal flux anomalies can interfere with the interpretation with the consequence analytical radar algorithms that have been widely accepted likely yield false delineations of wet dry beds more careful consideration is needed when diagnosing bed conditions spatial patterns of shallow englacial radar reflectors can be used as a proxy for accumulation rates which affect ice temperature and thus returned power i argue that it is necessary to simultaneously interpret the returned power and englacial reflector patterns to improve the bed diagnosis

hundreds of small volcanoes on the median valley floor of the mid atlantic ridge at 24 30 n we have identified 481 seamounts from 6 000 km2 of sea beam swaths of the median valley floor of the mid atlantic ridge mar between 24 and 30 n we find that seamount abundances are an order of magnitude larger on average 80 per 1 000 km2 with relief above 50 m than estimated in previous studies along mid ocean ridges in the pacific1 and south atlantic2 suggesting that at this section of the mar seamount volcanism plays an important part in the formation of new oceanic lithosphere the neovolcanic ridge often observed on the inner valley floor3 appears to be constructed totally of individual coalesced seamounts so that our counts represent a minimum estimate the frequency distribution of summit heights of the mar seamounts is consistent with the exponential model describing the heights of off axis seamounts in the eastern pacific4 6 but with different parameters such a distribution of heights may be a fundamental aspect of seamount volcanism related to lithospheric properties and or magmatic plumbing seamount dominated axial volcanism must create a crustal structure very different from classic fissure fed models

the location of the grounding zone of evans ice stream antarctica investigated using sar interferometry and modelling evans ice stream west antarctica has five tributaries and a complex grounding zone the grounding zone of evans ice stream between the landward and seaward limits of tidal flexing was mapped using sar interferometry the width of the mapped grounding zone was compared with that derived from an elastic beam model and the tidal height changes derived from interferometry were compared with the results of a tidal model results show that in 1994 and 1996 the evans grounding zone was located up to 100 km upstream of its location in the bedmap dataset the grounding line of evans ice stream is subjected to 5 m vertical tidal forcing which would clearly affect ice stream flow

eruptive hummocks building blocks of the upper ocean crust the spreading axis at many slow spreading mid ocean ridges is marked by an axial volcanic ridge in this study we use a combination of high resolution remote sensing methods to elucidate the detailed nature of volcanoes in such a ridge we find that the hummocks described in previous sidescan sonar studies are dome or cone shaped edifices 5 150 m high with diameters of 30 330 m we estimate they form quickly in single eruptions each of which may produce several hummocks hummock collapse is common and hummocks of all heights are prone to failure collapses generally occur down the regional seafloor slope suggesting control by local topography approximately 33 of hummocks lose 40 of their volume by collapse so 12 of all material erupted on the axial volcanic ridge is rapidly converted to talus the higher porosity of these deposits may increase average upper crustal porosity by several percent contributing 0 5 km s 1 to seismic velocity decrease in the upper oceanic crust and may be one of the dominant mechanisms for increasing porosity in upper slow spreading oceanic crust

comparative volcanology of small axial eruptive centers in the mark area we describe selected examples of small volcanic features identified during ocean drilling program odp site surveys in the mark area for which additional observations and sample data have been obtained by manned submersible and or drilling based on both morphologic relations and similarities in chemistry and petrography serocki volcano the site of bare rock drilling on odp legs 106 and 109 appears to be a flat topped megatumulus a large rootless vent developed on a lava flow originating from an adjacent cone located above the true eruptive fissure a linear set of five apparent volcanic centers on the east side of the median valley the eastmark cones differ from one another in morphology and degree of tectonic degradation they formed at different times and from different vents but all were localized by damming of the flows against the steep walls of the linear valley containing them the two northernmost features are probably megatumuli at the terminal ends of lava flows descending the valley walls the other three vents are cones with breached craters and probably were more recently erupted in situ the snakepit hydrothermal area is located near the highest and widest part of a 35 km long ridge that appears to be a major relatively young fissure eruption the associated lavas are among the geochemically most primitive in the mark area but have their counterparts in several subsets of lavas associated with serocki volcano in spite of the morphologic contrasts and large geographic separation between the two features we conclude that a volcano like form geographic proximity and conformance to a linear geometry do not guarantee compositional or temporal coherence nor is the alignment of apparent volcanic centers a reliable guide to underlying tectonic patterns the chemistry and petrography of the serocki and eastmark basalts resemble older adjacent lavas and also conform to subtle along ridge variations this and the relatively old appearance of their flow surfaces indicate that these lavas represent the waning stage of an older magmatic event the more recent eruptions in the snakepit area seem to reflect a renewed influx of the same relatively primitive and presumably hotter magma that initiated the serocki eruptions residual pockets of this hotter magma in the shallow crust may provide the heat that drives the present hydrothermal system

a comparison of volcanic edifices at the reykjanes ridge and the mid atlantic ridge at 24 30 n a detailed analysis of high resolution side scan sonar data shows that the volcanic edifices built within the axial zones of the slow spreading 20 mm yr reykjanes ridge and the slow spreading 25 mm yr mid atlantic ridge are remarkably similar in fact individual seamounts hummocks rounded mounds and hummocky ridges at the reykjanes ridge have nearly identical morphological counterparts at the mid atlantic ridge mar arguing that commonly eruptive conditions are similar at the two ridges the construction of these volcanic edifices typical of slow spreading ridges occurs at the reykjanes ridge despite its proximity to the iceland hot spot and the influence of the hot spot on other characteristics of the ridge axis such as its overall shape an axial high north of about 59 n its shallow axial depths and its thicker crustal section the volume of magma that goes into constructing volcanic edifices at each of 12 segments of the mar and four regions of the reykjanes ridge is calculated using multibeam bathymetry data our results indicate that edifice volume per unit seafloor area equivalent to a uniform thickness over the area of interest is larger at the mar than at the reykjanes ridge on average taking into account spreading rate we then estimate b the average building rate per unit length along the axis which is also larger at the mar we conclude that the same volcanic edifices build the crust at the reykjanes ridge and mar but that their emplacement is more frequent at the mar to explain our results and reconcile them with those of magde and smith 1995 who documented that seamounts are taller and more abundant at the reykjanes ridge than at the mar we consider the controls on diking and eruption within the axial zones of the two ridges we suggest that the combination of slower spreading rate and the oblique geometry of spreading at the reykjanes ridge leads to fewer eruptions which are more widely dispersed across the axial zone than at the mar if this hypothesis is correct the proportion of seamounts modified by subsequent eruptions will be less this might also explain the taller heights of the seamounts if there is less overlap and infilling between edifices

ice shelf tidal flexure and subglacial pressure variations abstract we develop a model of an ice shelf ice stream system as a viscoelastic beam partially supported by an elastic foundation when bedrock near the grounding line acts as a fulcrum leverage from the ice shelf dropping at low tide can cause significant 1 cm uplift in the first few kilometers of grounded ice this uplift and the corresponding depression at high tide lead to basal pressure variations of sufficient magnitude to influence subglacial hydrology tidal flexure may thus affect basal lubrication sediment flow and till strength all of which are significant factors in ice stream dynamics and grounding line stability under certain circumstances our results suggest the possibility of seawater being drawn into the subglacial water system the presence of seawater beneath grounded ice would significantly change the radar reflectivity of the grounding zone and complicate the interpretation of grounded versus floating ice based on ice penetrating radar observations

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

mid atlantic ridge volcanism from deep towed side scan sonar images 25 29 n abstract we present deep towed side scan sonar mosaics of the inner valley floor of eight spreading segments at the slow spreading mid atlantic ridge between 25 and 29 n an analysis of these images which well resolve features a few tens of meters in size confirms that the multitude of small seamounts with diameters between 0 5 and 3 km identified on the inner valley floor from previously collected multibeam bathymetry data are volcanically constructed moreover these images reveal that these volcanoes have distinct surface morphologies not evident in the coarser resolution multibeam bathymetry maps 83 of the seamounts have a hummocky bulbous morphology the other 17 have a smooth morphology in addition to near circular seamounts small 1 2 km long volcanic ridges are abundant in our study regions and are not in general seen in the bathymetry maps we combine these new morphological data with existing models for the construction of the shallow oceanic crust to obtain a better understanding of the melt delivery system that builds the distinctive seafloor topography at the slow spreading mid atlantic ridge

lucky strike seamount implications for the emplacement and rifting of segment centered volcanoes at slow spreading mid ocean ridges the history of emplacement tectonic evolution and dismemberment of a central volcano within the rift valley of the slow spreading mid atlantic ridge at the lucky strike segment is deduced using near bottom sidescan sonar imagery and visual observations volcano emplacement is rapid 1 myr associated with focused eruptions and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction this volcanic phase likely requires efficient melt pooling and a long lived crustal magma chamber as a melt source a reduction in melt supply triggers formation of an axial graben rifting the central volcano and the onset of seafloor spreading may eventually split it at lucky strike this results in two modes of crustal construction eruptions and tectonic activity focus at a narrow graben that bisects the central volcano and contains the youngest lava flows accumulating a thick layer of extrusives away from the volcano summit deformation and volcanic emplacement is distributed throughout the rift valley floor lacking a clear locus of accretion and deformation volcanic emplacement on the rift floor is characterized by axial volcanic ridges fed by dikes that propagate from the central axial magma chamber the mode of rapid volcano construction and subsequent rifting observed at the lucky strike seamount is common at other central volcanoes along the global mid ocean ridge system

segmentation volcanism and deformation of oblique spreading centres a quantitative study of the reykjanes ridge abstract new multi beam swath bathymetry charts and high resolution side scan sonar imagery provide detailed evidence for volcanic and magmatic segmentation along the reykjanes ridge by combining these data sets we have quantified some relationships between different orders of segmentation various characteristics of volcanism and changes in tectonism along the ridge axis three orders of segmentation along the reykjanes ridge have been recognised 1 a long wavelength depth variation or swell that is related to the shoaling of the ridge towards iceland 2 intermediate wavelength rises of the order of 40 120 km and 3 a short wavelength volcanic segmentation of the order of 5 30 km the long wavelength depth variation which is probably controlled by the temperature of the underlying asthenosphere has a maximum positive deviation from a predicted depth curve that forms a break in slope at 59 60 n this feature migrated southward along the ridge at between 5 and 10 cm yr at least four times in the past 10 ma the intermediate wavelength rises which are also temporally and spatially unstable are probably related to local rayleigh taylor style asthenospheric instabilities the short wavelength segments have an evolutionary life cycle starting with eruption via fissure volcanoes and progressing by increasing magma flux to the formation of conical and then shield like volcanoes these segments are tectonically dismembered by extensional faulting following a rapid decrease in magma supply we propose a model for the construction of the volcanic layer of the reykjanes ridge which involves a cyclicity of magmatic and tectonic activity

en echelon axial volcanic ridges at the reykjanes ridge a life cycle of volcanism and tectonics abstract new deep towed sidescan records and multibeam bathymetric data along the reykjanes ridge allow us to assess the varying thermal effect of the icelandic hotspot the increasing predominance of faulting speculated to be derived from mantle upwelling and outflow over simple extensional neotectonics is traced from northeast to southwest numerous en echelon volcanic ridges built along the plate boundary are seen in various stages of growth and decay and a cycle of alternating construction and dismemberment is proposed unfaulted short narrow lensoid ridges occasionally in small groups are interpreted as immature and developing axial volcanic ridges broad convex flanked ridges appear to be mature sites of volcanic construction where the magma supply has been maintained narrow bow form ridges occasionally with rectilinear flanks represent axial volcanic ridges which have ceased construction and are in initial stages of disaggregation isolated polygonal blocks generally seen off axis represent the most advanced stage of dismantling of the surface crustal accretionary features while this cyclicity is speculated to be related to periodic magmatic and tectonic focusing and defocusing along the axis due to the restricted magma supply at this slow spreading ridge it is recognised that this needs modelling although the oblique spreading of the reykjanes ridge allows one to clearly distinguish between structures attributable to plate spreading from those related to small scale accretionary processes we propose that the major processes identified in this study are directly applicable to all slow spreading sections of mid ocean ridges

melting and refreezing beneath roi baudouin ice shelf east antarctica inferred from radar gps and ice core data indicate that this reflector pattern is best fit by including basal melting of at least 15 cm a 1 this rate of melting is low compared with rates observed on larger ice shelves in both west and east antarctica ice cores extracted from a rift system close to the ice rise promontory show several meters of marine ice accreted beneath the shelf these observations of low rates of basal melting and limited distribution of accreted marine ice suggest that either antarctic surface water may reach the ice shelf base or that circulation beneath the shelf is likely dominated by the production of high salinity shelf water rather than the incursion of circumpolar deep water implying a weak sub shelf circulation system here many of the ice shelves located along the coast of dronning maud land are like roi baudouin ice shelf characterized by frequent ice rises and promontories therefore it is highly likely that these are also of shallow bathymetry and are subject to similarly weak side shelf basal melting and refreezing

on the lithospheric seismic structure of reykjanes ridge at 62 5 n a long reversed refraction line on the southeast flank of reykjanes ridge subparallel to the axis is modeled with synthetic seismograms these are calculated using the reflectivity method the data have been recorded by two ocean bottom seismometers during the reykjanes ridge iceland seismic program 1977 rrisp77 experiment the observed amplitudes are characterized by a low at around 80 km range and a pronounced maximum at around 100 km distance here the apparent velocity increases from about 8 km s to at least 8 6 km s in both directions the modeling demonstrates that most of the observed features can be described by a laterally homogeneous structure except for layer 2 which thickens markedly toward iceland the structure of the lower crust seems to be similar to that of the bay of islands ophiolite newfoundland there is an indication of a low velocity zone at the base of the crust which may be caused by partial serpentinization of the olivine component of the gabbros at that depth the moho seems to be modeled best by a discontinuity about 10 km below the water surface in the uppermost mantle there is a very slight velocity increase from 8 to 8 25 km s it is underlain by a transition zone to velocities as high as 8 6 8 9 km s at about 20 km depth this could possibly be explained by velocity anisotropy perhaps together with a petrological boundary the direction of maximum velocity of the deep anisotropic layer should be parallel to the ridge axis this could be caused by a pipe flow from the iceland mantle plume which is possibly indicated by the shear wave structure at greater depth

paleofluvial and subglacial channel networks beneath humboldt glacier greenland the identification of subglacial drainage systems can inform our understanding of past and present hydrological processes landscape evolution and ice dynamics here we present evidence from satellite imagery digital elevation models and radio echo sounding data for a series of channelized networks with contrasting paleofluvial and subglacial origins beneath humboldt glacier northern greenland a 250 km long dendritic paleofluvial channel network beneath the northern portion of humboldt is interpreted as a preglacial feature roughly linear channels beneath the southern portion of humboldt which display a similar distribution to tunnel valleys found on the beds of former ice sheets are likely to have been eroded by subglacial meltwater routed along the ice sheet bed we suggest that basal meltwater is actively being routed down both the paleofluvial and subglacially formed channel networks to the coast inheritance of the preglacial channel network may have influenced the present day location and dynamics of humboldt glacier and enhanced selective erosion at its down glacier end

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

investigating tidal flexure on an ice shelf using kinematic gps the positions of ice stream grounding zones are uniquely sensitive to changes in the mass balance of the ice sheet present methods for locating groundingzone features are either imprecise or require considerable effort in interpretation and so are of little value for change studies we present a new method which uses the kinematic gps technique to locate the position of the limit of tidal flexure the method involves the collection of at least two surface elevation profiles along the same track through the grounding zone at different times during the tidal cycle the elevation profiles obtained coincide upstream of the limit of flexure but diverge downstream of the limit of flexure subtracting the profiles produces a tidal def1ection profile which shows directly the response of the ice shelf to the tidal forcing we present two examples of the use of this method both on rutford ice stream antarctica the first is across the grounding zone and shows that the method is capable of measuring grounding positions to around 200 m precision the second taken across an active shear margin shows a tidal deflection profile with an absence of steps that would indicate the presence of fracture planes penetrating from the ice base to sea level

the northern mid atlantic and reykjanes ridges spreading center morphology between 55 50 n and 63 00 n seamarc ii and hydrosweep acoustic imagery and bathymetry reveal that the northern mid atlantic ridge mar and reykjanes ridge between 55 50 n and 63 00 n are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from iceland the northern 100 km of the mar 55 50 n 56 47 n is oriented 005 perpendicular to the spreading azimuth and contains four third order spreading segments separated along strike by right and left stepping axial discontinuities the boundary between mar and reykjanes ridge occurs at 56 47 n where the bight transform fault offsets the neo volcanic axis 15 km right laterally across a single linear fault oriented 092 this is the only transform fault between 55 50 n and 63 00 n and is inferred to have generated the bight fracture zone the reykjanes ridge lies between the bight transform and iceland s reykjanes peninsula and is oriented 30 oblique to the spreading orthogonal direction axial faults and volcanic systems on the reykjanes ridge are arranged in a right stepping en echelon pattern and display strikes intermediate between the spreading orthogonal direction and the strike of the ridge which is compatible with experimental models of oblique extension spreading segments on the northern mar and reykjanes ridge are defined structurally and bathymetrically at the smallest scale the volcanic axis is segmented into individual volcanic systems 4 45 km long which are separated along strike by intervolcano gaps or offsets these features are analogous to fourth order spreading segments documented elsewhere on the mar axial volcanic systems are superimposed on intermediate wavelength 13 65 km axial topographic highs that constitute third or second order spreading segments segment boundaries occur at axial depth maxima which correspond to offsets in the volcanic axis four primary characteristics of the spreading axis axial valley depth axial boundary fault throw relief along the neovolcanic axis and degree of inter segment structural discontinuity decrease toward the north this variation is consistent with higher crustal temperatures near iceland due to either thicker crust or elevated mantle temperatures

a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes 1 a new three dimensional thermomechanically coupled ice sheet model is developed contrary to the majority of three dimensional ice sheet models shallow ice approximation higher order stress gradients longitudinal and transverse are accounted for in the force balance equations the horizontal velocity field is determined from the force balance equations in their derivative form elliptical equation using the method presented by pattyn 2000 2002a the model is solved on a regular grid in the horizontal and an irregular grid in the vertical and is numerically stable basic experiments include the european ice sheet modeling initiative eismint benchmarks for largescale ice sheet models and a comparison with the saito blatter ice sheet model including higher order stress gradients saito et al 2003 detailed calculations of ice flow over three dimensional bedrock perturbations showed the validity of the higher order solution the model is capable of simulating the evolution of an ice stream within the ice sheet and shows important aspects of observed ice stream features such as the surface flattening and the importance of side drag the simulation of the ice flow over a subglacial lake results in a flattening of the surface a local velocity increase over the lake and a deviation of the ice flow from the main flow direction features which are also observed at lake vostok antarctica index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3230 mathematical geophysics numerical solutions keywords higher order ice sheet model thermomechanical ice stream subglacial lake citation pattyn f a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes j geophys res 108 b8 2382 doi 10 1029 2002jb002329 2003

segmentation of the mid atlantic ridge between 24 n and 30 40 n the mid atlantic ridge mar extends for over 12 000 km from iceland to the bouvet triple junction in the south atlantic most of our knowledge of the geology of this slow spreading centre comes either from widely spaced low resolution echo sounding lines or from detailed studies of small areas using deep towed instruments or submarines high resolution multibeam bathy metry1 2 has been insufficient to define the variability of accretionary processes along this plate boundary here we present the results of a recent investigation of a 800 km long section of the mar carried out using the multibeam echo sounder seabeam3 analysis of these data reveals the scale and nature of the segmentation of the mar between the kane fracture zone 24 n and latitude 30 40 n

spatial variation of englacial radar attenuation modeling approach and application to the vostok flowline 1 constraining the spatial variation of englacial radar attenuation is critical for accurate inference of the spatial variation of the englacial and basal properties of ice sheets from radar returned power here we evaluate attenuation models that account for spatial variations in ice temperature and chemistry and test them along the flowline that passes through the vostok ice core site antarctica the simplest model often used but rarely valid assumes a uniform attenuation rate everywhere along the flowline so that total attenuation is proportional to ice thickness the next simplest model uses spatially varying temperatures predicted by an ice flow model and assumes uniform chemistry additional models account for spatially varying chemistry using englacial stratigraphy we find that the roundtrip attenuation to the bed can easily differ by 10 db or more between the uniform attenuation rate model and models that account for variable ice temperature such differences are sufficient to confound the delineation of dry and wet beds also including spatial variations in chemistry produces smaller differences 10 db but the magnitude of these differences depends on the relative importance of dry and wet deposition of impurities in the past accounting for dry deposited impurities requires ice flow modeling and results in larger differences from all other models which assume uniform chemistry or wet deposition only these results indicate that modeling the spatial variation of attenuation requires a spatially varying temperature model in order to infer bed conditions from bed returned power accurately and that both ice core data and radar stratigraphy are also strongly desirable

building the crust at the mid atlantic ridge the morphology of the sea floor at mid ocean ridge spreading centres provides a key to understanding how ocean crust is constructed images of the axial zone of the slow spreading mid atlantic ridge obtained at a range of spatial scales show that crustal construction is complex and highly variable reflecting an underlying three dimensional variability in magmatic tectonic and hydrothermal processes

evolution of derwael ice rise in dronning maud land antarctica over the last millennia ice rises situated in the ice shelf belt around antarctica have a spatially confined flow regime with local ice divides beneath the divides ice stratigraphy often develops arches with amplitudes that record the divide s horizontal residence time and surface elevation changes to investigate the evolution of derwael ice rise dronning maud land antarctica we combine radar and gps data from three consecutive surveys with a two dimensional full stokes thermomechanically coupled transient ice flow model we find that the surface mass balance smb is higher on the upwind and lower on the downwind slopes near the crest the smb is anomalously low and causes arches to form in the shallow stratigraphy observable by radar in deeper ice arches are consequently imprinted by both smb and ice rheology raymond effect the data show how arch amplitudes decrease as along ridge slope increases emphasizing that the lateral positioning of radar cross sections is important for the arch interpretation using the model with three rheologies isotropic with n 3 4 5 and anisotropic with n 3 we show that derwael ice rise is close to steady state but is best explained using ice anisotropy and moderate thinning our preferred albeit not unique scenario suggests that the ice divide has existed for at least 5000 years and lowered at approximately 0 03 m a 1 over the last 3400 years independent of the specific thinning scenario our modeling suggests that derwael ice rise has exhibited a local flow regime at least since the mid holocene

a new isostatic model for the east pacific rise crest we have developed a new isostatic model for the east pacific rise epr that explains the origin of both the axial topographic high and the rise crest gravity anomaly our model involves a thin elastic plate with a constant thickness crust that is broken at the ridge crest we assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly we have used this model and two conventional isostatic models to examine the isostasy of three segments of the epr at 9 l4 n 6 11 s and 16 21 s rather than modeling individual profiles we have stacked summed the gravity and bathymetry data in each area and computed a single 200 km long mean mirrored profile for each different ridge segment these stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly in all three areas the best fitting model parameters are similar and give average compensation depths of about 6 7 km below the seafloor near the base of the oceanic crust and an upper limit on the flexural rigidity of the plate that increases from about 1018 n m at the ridge axis to about 3 1019 n m 5 km from the axis and 1020 n m 25 km or more from the rise crest these results indicate that the epr crest is compensated at much shallower depths than suggested by previous investigators our results are consistent with the presence of a small low density magma chamber in the lower crust although at least part of the compensating root for the axial high must extend into the upper mantle we interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished

magnetic properties of archean gneisses from the northeastern north china craton the relationship between magnetism and metamorphic grade in the deep continental crust

high variability of climate and surface mass balance induced by antarctic ice rises ice rises play key roles in buttressing the neighbouring ice shelves and potentially provide palaeoclimate proxies from ice cores drilled near their divides little is known however about their influence on local climate and surface mass balance smb here we combine 12 years 2001 12 of regional atmospheric climate model racmo2 output at high horizontal resolution 5 5 km with recent observations from weather stations ground penetrating radar and firn cores in coastal dronning maud land east antarctica to describe climate and smb variations around ice rises we demonstrate strong spatial variability of climate and smb in the vicinity of ice rises in contrast to flat ice shelves where they are relatively homogeneous despite their higher elevation ice rises are characterized by higher winter temperatures compared with the flat ice shelf ice rises strongly influence smb patterns mainly through orographic uplift of moist air on the upwind slopes besides precipitation drifting snow contributes significantly to the ice rise smb the findings reported here may aid in selecting a representative location for ice coring on ice rises and allow better constraint of local ice rise as well as regional ice shelf mass balance

mapping the grounding zone of the ross ice shelf antarctica using icesat laser altimetry we use laser altimetry from the ice cloud and land elevation satellite icesat to map the grounding zone gz of the ross ice shelf antarctica at 491 locations where icesat tracks cross the grounding line gl ice flexure in the gz occurs as the ice shelf responds to short term sea level changes due primarily to tides icesat repeat track analysis can be used to detect this region of flexure since each repeated pass is acquired at a different tidal phase the technique provides estimates for both the landward limit of flexure and the point where the ice becomes hydrostatically balanced we find that the icesat derived landward limits of tidal flexure are in many places offset by several km and up to 60 km from the gl mapped previously using other satellite methods we discuss the reasons why different mapping methods lead to different gl estimates including instrument limitations variability in the surface topographic structure of the gz and the presence of ice plains we conclude that reliable and accurate mapping of the gl is most likely to be achieved when based on synthesis of several satellite datasets

a 220 km2 recently erupted lava field on the east pacific rise near lat 8 s seamarc ii records show a 220 km2 area of unusually high reflectivity on the east pacific rise epr near lat 8 s which we interpret to be a recently erupted lava field this is the only segment of the epr exhibiting high reflectivity over such a large area in our continuous seamarc ii coverage from lat 8 n to 18 n and 3 s to 23 s the lavas appear to issue from the axial neovolcanic zone between lat 8 15 and 8 17 s the lava field extends up to 18km off axis and inundates scarps up to 100 m high on the basis of measurements of scarp heights just outside areas where they are buried we estimate the average thickness of the field to be 70 20 m thus the volume of the lava field is 15 4 km3 in comparison the average annual volcanic budget for the earth is approximately 4 5 km3 and the largest historic basaltic eruption from the icelandic laki fissure in 1783 was approximately 12 3 km3 when did the eruption occur comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the epr suggest that the lava field has little or no sediment cover and is very young this is also one of the only areas on the epr where earthquakes occur that do not appear to be caused by transform faulting if the earthquakes are related to the lava field then major eruptions may have occurred in 1964 1965 and 1969 a large helium plume near lat 13 s may be caused in part by eruptions of the lat 8 s lava field this is consistent with estimates of currents at 2 3 km depth which are such that a plume generated at lat 8 s would eventually appear off axis at lat 13 s our observations suggest that this spreading segment which is bounded by small ridge axis discontinuities at lat 8 05 s and 8 47 s may have been the most volcanically active segment along the epr during the past 25 yr

bed radar reflectivity across the north margin of whillans ice stream west antarctica and implications for margin processes surface based ice penetrating radar profiles were made across the active north margin the snake of the upper part of whillans ice stream formerly ice stream b branch b2 west antarctica at three locations low frequency about 2 mhz and the ground deployment of the radar allowed penetration through the near surface zone of fracturing to detect internal layering and bed reflection characteristics on continuous profiles spanning from the slow moving ice of engelhardt ridge well into the chaotic zone of the shear margin internal layers were tracked beneath the chaotic zone where they are warped but remain continuous the energy returned from internal layers showed no systematic changes associated with the transition from the undisturbed surface of the slow moving ice into the fractured surface of the shear margin thus indicating little effect from the surface crevasses on the penetration of the radar signal based on this calibration of the near surface effects and corrections for path length spreading and attenuation we examine the spatial variation of bed reflectivity low bed reflectivity found under engelhardt ridge extends under the chaotic zone of the margin into fast moving ice we argue that the fast motion in a band along the margin is mediated by processes other than deformation of thick dilated till that is the source of lubrication allowing fast motion in the interior of the ice stream

a comparison of rift zone tectonics in iceland and hawaii observations of eroded volcanic rift zones indicate that dikes in iceland are typically several times thicker than those in hawaii geodetic and seismic observations of active rifts however suggest that dike heights in the two regions are similar provided the elastic properties of the rift zones are the same this implies that dikes are intruded with higher driving pressures magma pressure minus compressive stress perpendicular to the dike plane in iceland than hawaii a second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in iceland from this it can be infered that a lower percentage of dikes breach the surface in iceland than in hawaii thus although dikes in iceland are intruded with higher driving pressures they possess lower absolute magma pressures than in hawaii these differences can be interpreted in terms of the tectonic settings in the two regions in iceland a steady remote extension reduces the horizontal stress perpendicular to the rift zone allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available in hawaii a more continuous magma supply on the timescale over which the dike induced stresses are relaxed and perhaps a greater role for intrusions in driving long term rift extension ensure that the rift compressive stress is not relaxed significantly before the next dike is intruded thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur if the mechanism for relaxing the rift compressive stress were less efficient still then an even higher percentage of dikes would erupt and at times the rift zone trend could become an unfavorable orientation for dike intrusion such might be the case at mauna loa which lacks large rift zone faults and fissures and possesses numerous radial vents outside its two main rift zones

seamount volcanism at the reykjanes ridge relationship to the iceland hot spot the axial zone of the reykjanes ridge is covered with small 0 5 3 km in diameter volcanoes that pile together to form larger axial volcanic ridges this style of volcanism is similar to that at the mid atlantic ridge mar and may be common to slow spreading ridges despite proximity of the reykjanes ridge to the iceland hot spot in this study we quantitatively investigate the population of seamounts in three study areas at the reykjanes ridge areas a and b are centered at 62 n and 60 n respectively area c is centered at 58 n and is located south of the transition in ridge morphology from an axial high to an axial graben using multibeam bathymetry data 541 seamounts summit height h 50 m were identified in the three areas and their size and shape statistics were compiled additionally 105 seamounts in areas b and c were recognized in deep towed side scan images and their surface morphologies hummocky or smooth were recorded on the basis of estimated population parameters we find that seamounts at the reykjanes ridge are more abundant 310 20 per 103 km2 on average than at the mar between 24 and 30 n 200 10 per 103 km2 significant along axis variations exist at the reykjanes ridge however which are not simply related to distance from the hot spot area b has nearly twice the abundance of seamounts as either area a or area c variation in the characteristic height of the seamount population is also observed between the reykjanes ridge 68 2 m and the mar 58 2 m but no significant variation is found between our three study areas a dramatic change in seamount surface morphology occurs between areas b and c there are no side scan data from area a area c has 78 hummocky seamounts similar to the proportion observed at the mar while area b has 83 smooth seamounts on the basis of these results we present a conceptual model for building the shallow crust at the slow spreading reykjanes ridge that takes into account the possible influence of the iceland hot spot on the crustal melt delivery system and its influence on variables that control seamount abundances sizes shapes and surface morphologies in this model we suggest that the increased seamount production and proliferation of smooth seamounts in area b may be associated with a pulse of hot spot material in the form of asthenosphere of higher temperature that has recently affected area b

surveying the flanks of the mid atlantic ridge the atlantis basin north atlantic ocean 36 n new seismic reflection and okean 9 5 khz sidescan sonar data were used to characterise the seismic stratigraphy and structure of the region south of the azores islands north atlantic ocean the new data provide constraints on the tectono sedimentary evolution of deep ocean basins located on the flanks of slow spreading ridges south of the azores thick sedimentary sequences 1 0 s two way travel time correlated with units in the madeira abyssal plain odp sites 950 952 overlay an irregular basement generated by a combination of volcanic and tectonic processes three main cenozoic depositional events are recognised and signed by distinct seismic megasequences the moderate deformation of the sedimentary units and the structural fabric of the underlying oceanic crust shown on seismic and sidescan data suggest the existence of a complex tectonic setting south of the azores illustrating this complex setting is the change from a basement with prominent abyssal hills west of 30 w to a segmented oceanic crust denoting an extensional transtensional tectonic regime east of the latter meridian

oblique rifting in a slow spreading ridge in oceanic rifts the spreading direction is usually nearly perpendicular to the ridge axis but at the reykjanes1 and mohns2 4 ridges this direction is highly oblique 30 to 40 to the axis these ridges possess axial valleys containing oblique raised features highs corresponding to second or third order ridge axis discontinuities5 at the reykjanes ridge searle and laughton1 observed small highs which they interpreted as volcanic domes here we present a fault map of the mohns ridge obtained from image processing of seabeam data we argue that the oblique highs in this ridge are of tectonic origin the axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading we describe laboratory experiments on brittle ductile models which support this interpretation in which stretching at 30 to a deformable band reproduces the en echelon horst and graben pattern of the mohns ridge

submarine explosive volcanism in the southeastern terceira rift sao miguel region azores abstract morphologic studies with sonar data and in situ observations of modern eruptions have revealed some information suggesting how submarine volcanic cones develop but the information only addresses the modern surfaces of these features here we describe a study combining morphological data with high resolution seismic reflection data collected over cones within the southeastern terceira rift a succession of deep basins volcanic bathymetric highs and islands e g sao miguel representing the westernmost part of the eurasian nubian plate boundary the cones 252 are distributed in depths down to 3200 m and exhibit an average diameter of 743 m an average slope of 20 and heights mainly between 50 and 200 m the cones are here classified into three different categories by physiographic or tectonic setting we find no particular morphometric differences in cone shapes between these areas first numerous cones located at the submarine flanks of sao miguel s sete cidades and fogo volcano are considered to be parasitic structures second in the southeast of the island they form a superstructure possibly reflecting an early submarine stadium of a posterior subaerial stratovolcano third some cones are controlled by faults mostly in a graben system southwest of the island high resolution multichannel seismic data indicates that the graben cones evolved synchronously with the graben formation bottom currents then probably removed the surficial fine grain size fraction leaving rough surface textures of the cones which backscatter sonar signals strongly in the data recorded here however a young cone investigated in detail is characterized by a smooth surface a marked increase of internal stratification with increasing distance from the summit and upwards concave flanks others exhibit central craters suggesting an explosive than an effusive evolution of these structures the morphological characteristics of these submarine cones show that they have similar sizes and shapes to cinder cones onshore sao miguel

on the numerical computation of temperature in an ice sheet quadrature solutions for temperature fields in the central regions of ice sheets where flow is by internal deformation are presented and compared with numerical computations and the european ice sheet modelling initiative eismint benchmark standards the solution appears to be more accurate than the eismint benchmark standards for thermomechanically uncoupled temperature distributions the ability of finite difference and pseudo spectral methods to reproduce this solution is considered errors in the eismint benchmark are larger than expected the possibility that they could arise from inaccurate evaluation of the vertical velocity is considered and found to be unlikely formulae for computing the vertical velocity in jenssen s coordinate are complicated and may lead to programming errors a simple form of the heat transport equation in the coordinate for flow by sliding and internal deformation is derived arid presented and it is shown how this form is particularly suitable for finite element computations it is also shown that only two quadratures are necessary to compute the vertical and horizontal heat transport and the ice flux

the role of acids in electrical conduction through ice 1 electrical conduction through meteoric polar ice is controlled by soluble impurities that originate mostly from sea salt biomass burning and volcanic eruptions the strongest conductivity response is to acids yet the mechanism causing this response has been unclear here we elucidate conduction mechanisms in ice using broadband dielectric spectroscopy of meteoric polar ice cores we find that conduction through polycrystalline polar ice is consistent with jaccard theory for migration of charged protonic point defects through single ice crystals except that bulk dc conduction is impeded by grain boundaries neither our observations nor modeling using archie s law support the hypothesis that grain boundary networks of unfrozen acids cause significant electrolytic conduction common electrical logs of ice cores by electrical conductivity measurement ecm or dielectric profiling dep and the attenuation of radio waves in ice sheets thus respond to protonic point defects only this response implies that joint interpretation of electrical and chemical logs can determine impurity partitioning between the lattice and grain boundaries or inclusions for example in the greenland ice core project grip ice core from central greenland on average more than half of the available lattice soluble impurities h cl nh4 create defects understanding this partitioning could help further resolve the nature of past changes in atmospheric chemistry

structure and petrology of the reykjanes ridge between 62 55 n and 63 48 n geophysical and petrologic data are presented on the northern extent of the reykjanes ridge as it nears iceland the crest of the reykjanes ridge is a small belt of rough topography superimposed on an elevated plateau the rough topography consists of a series of en echelon ridges immediately to the southwest of the reykjanes peninsula the ridges strike about n35 e and apparently represent a rotational adjustment toward the n40 e trends on iceland to the south the en echelon ridges strike about n25 e magnetic profiles place the peak of the magnetic anomaly either directly below or along the eastern side of the topographic highs eight distinctive en echelon volcanic systems swarms are identified and are structurally and petrologically indistinguishable from those of the western reykjanes ridge petrologically three rock types picrite basalts olivine tholeiites and tholeiites were recovered these form distinctive groups which match those described from the western reykjanes peninsula

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data