crustal accretion model for iceland predicts cold crust we present a self consistent crustal accretion model of iceland that includes heat and mass transfer between an upwelling melting mantle and an accreting cooling crust this models predicts sub solidus crustal temperatures that are consistent with seismic estimates of the maximum depth of earthquake hypocenters and mid crustal shear wave quality factors and with geothermal estimates of surface heatflow

the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

three dimensional seismic transmission prospecting of the mont dore volcano france summary the three dimensional seismic structure of the mont dore volcano is studied by inversion of the arrival times of seismic waves with this aim two new methods are developed first the arrival times are those of moho reflected waves at a critical distance from artificial sources in different azimuths secondly the inversion uses a technique which does not require the traditional a priori partition of the space into blocks the resulting picture reveals such features as 1 a circular caldera within the basement the rim of which is marked by magnetic anomalies associated with post caldera activity 2 a clear lower limit of the volcano sedimentary sequence under part of the caldera opposed to low velocity anomalies extending deeper beneath another part and which may have been the site of volcanic material transport and 3 eeper heterogeneities possibly related to foundered basement blocks

isolation basins sea level changes and the holocene history of the greenland ice sheet abstract isolation basins are natural topographic depressions that at various times in their history may be connected to or isolated from the sea by changes in relative sea level rsl they provide a valuable source of data for tracking large scale tens of meters changes in rsl over millennia as well as quiet water depositional environments where abrupt changes caused by tsunami iceberg roll or storms may be recorded in this paper we review isolation basins as sources of rsl data with a particular focus on their use in greenland to constrain the holocene history of the ice sheet a new rsl curve from disko fjord west greenland is presented which shows that local ice free conditions were established at c 11 k cal yr bp after which rsl fell rapidly from a marine limit at c 80 m to reach close to present sea level by c 4 k cal yr bp we compare this record with other isolation basin rsl data from six other sites in disko bugt and note a strong northwest southeast differential rebound across the area during the early and mid holocene that reflects variations in ice load history we compare the disko bugt data with other previously published isolation basin rsl records from sisimiut central west greenland nanortalik south greenland and ammassalik southeast greenland rsl fell below present during the early holocene at nanortalik c 10 k cal yr bp and during the mid to late holocene elsewhere before rising to present these differences reflect variations in the timing and amount of greenland ice load change since the last glacial maximum as well as non greenland processes notably the collapse of the laurentide forebulge and also changes in ice equivalent sea level isolation basin data have relatively small age and height uncertainties compared with other rsl indicators enabling them to resolve between different earth and ice sheet models especially during periods of large ice load and rsl change

parameter uncertainty analysis on acoustic response in fluid filled poroelastic media this paper examines the effect of input parameter uncertainty in the prediction of acoustic response in ocean sediment due to the relatively large number of physical parameters in the model and the difficulty of their direct measurement in the ocean environment uncertainty in the predicted acoustic response is expected utilizing a second order taylor series expansion this paper demonstrates a methodology for estimating the output uncertainty in terms of statistical moments from the input uncertainty the methodology is then tied to a practical procedure that uses engineering measurements and empirical relations to determine parameters such as shear modulus bulk modulus and permeability from porosity a related issue of pore size distribution and its effect on acoustic response is also investigated

formation and mechanics of magma reservoirs in iceland summary provided magma flow in a partially molten layer follows darcy s law of flow in porous media it can be demonstrated that magma in the proposed magma layer beneath iceland flows towards regions of reduced crustal thickness in these regions magma accumulates to form partially molten magma reservoirs which are thus located in the magma layer these reservoirs either give rise to surface eruptions or feed totally molten shallow magma chambers located in the crust that may erupt it is argued that the melt fraction varies throughout a reservoir being highest near the top where the reservoir may be totally molten but that the average is 0 25 it is concluded that for individual eruptions the reservoirs behave on the whole as a poroelastic material it is suggested that formation of crustal magma chambers is facilitated by the occurrence of stress barriers that lead to the formation of thick sills while liquid such sills absorb the magma of all dykes that enter them and may evolve into magma chambers ideal sites for stress barriers and thus for magma chambers are formations where individual layers have different elastic properties i e where the crust behaves as a multilayer denoting the bulk volume of the reservoirs by vb the volume of the crustal chambers by v and the volume of lava and dyke material in individual eruptions by ve it is found that vb 5365ve and v 1850ve using the estimated volumes of exposed plutons as a basis it is found that provided the feeder reservoir does not participate in the eruption the volume of individual eruptions from elastic crustal magma chambers in iceland is less than 0 25 km3 usually less than 0 1 km3 and that typical central volcano eruptions should be about 0 02 km3 for eruptions of the order of 1 km3 or larger the reservoirs must supply magma during the eruption it is concluded that from the point of view of volume historical eruptions of typical size could have come from elastic crustal magma chambers

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

propagator matrix for continuously layered porous seabeds the analytical expression of the propagator matrix for elastic waves in a continuously layered porous plasto elastic medium is obtained the biot theory incorporated with the linearized expressions of soil anelasticity is used to model sediments numerical calculations are made for the p sv motion of seabeds excited by shear and compressional waves it is found that the sediment layers have very little effect on the seismic reflection at the sediment rock interface however the excitation of the sediment layer is sensitive to the profile of sediment properties frequency and wave amplitude the dynamic behavior of coarse sand and porous rocks excited by acoustic waves and earthquakes should be modeled by the biot porous solid model in which the fast compressional waves the slow compressional waves and the shear waves are included however excitations of most marine sediment beds by earthquakes may be modeled by the biot solid model in which the slow compressional waves are omitted in the biot porous solid model

intense seismicity during the 2014 2015 barðarbunga holuhraun rifting event iceland reveals the nature of dike induced earthquakes and caldera collapse mechanisms

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

magnetotelluric measurements across the eastern neovolcanic zone in south iceland a magnetotelluric survey consisting of 18 separate soundings was performed in 1982 along a 125 km profile crossing the eastern neovolcanic zone in south iceland although the three dimensional ocean effect may be significant at the longest periods which previous workers have analyzed to study features in the upper mantle beneath iceland our studies suggest that this effect is minimal for periods less than 1000 s at distances greater than 50 km from the coast therefore we are confident that two dimensional interpretations are valid at periods suitable for interpreting crustal resistivities such a two dimensional interpretation presented here indicates the presence of a layer 5 km thick of low resistivity 5 ohm m at the crust mantle interface 10 20 km depth the depth to this layer increases from the center of the neovolcanic zone this feature has been seen elsewhere in iceland by other workers however the present study suggests that east of the neovolcanic zone the low resistivity layer is not present which we argue may be associated with a recent eastward shift of the axial rift zone to a new location within older crust a striking feature of our two dimensional crustal interpretation is that pockets of low resistivity material appear to be present at approximately 8 km depth at the base of the crust beneath the two volcanic centers that we have studied torfajokull and hekla these may be manifestations of local magma chambers

calving signature in ocean waves at helheim glacier and sermilik fjord east greenland abstractwhen glaciers calve icebergs a fraction of the released potential energy is radiated away via gravity waves the characteristics of such waves caused by iceberg calving on helheim glacier in east greenland are investigated observations were collected from an array of five high frequency bottom pressure meters placed along sermilik fjord calving generated tsunami waves were identified and used to construct a calving event catalog calving events are observed to cluster around high and low semidiurnal tides and around high and prior to low semimonthly tides in the postcalving ocean state discrete spectral peaks associated with calving events are observed and they are consistent among all the events a numerical model is used to compute the resonant modes of the fjord and to simulate calving generated ocean waves damped oscillator boundary forcing with 5 to 10 min periods is found to reproduce well the observed properties of calving waves these observations and modeling are relevant for be

catastrophic ice shelf break up by an ice shelf fragment capsize mechanism two disintegration events leading to the loss of larsen a and b ice shelves antarctic peninsula in 1995 and 2002 respectively proceeded with extreme rapidity order of several days and reduced an extensive seemingly integrated ice shelf to a jumble of small fragments these events strongly correlate with warming regional climate and accumulation of surface meltwater supporting a hypothesis that meltwater induced propagation of pre existing surface crevasses may have initiated ice shelf fragmentation we address here an additional subsequent mechanism that may sustain and accelerate the ice shelf break up once it begins the proposed mechanism involves the coherent capsize of narrow less than thickness ice shelf fragments by rolling 90 in a direction toward or away from the ice front fragment capsize liberates gravitational potential energy forces open ice shelf rifts and contributes to further fragmentation of the surrounding ice shelf

basal crevasses on the larsen c ice shelf antarctica implications for meltwater ponding and hydrofracture a key mechanism for the rapid collapse of both the larsen a and b ice shelves was meltwater driven crevasse propagation basal crevasses large scale structural features within ice shelves may have contributed to this mechanism in three important ways i the shelf surface deforms due to modified buoyancy and gravitational forces above the basal crevasse creating 10 m deep compressional surface depressions where meltwater can collect ii bending stresses from the modified shape drive surface crevassing with crevasses reaching 40 m in width on the flanks of the basal crevasse induced trough and iii the ice thickness is substantially reduced thereby minimizing the propagation distance before a full thickness rift is created we examine a basal crevasse 4 5 km in length 230 m in height and the corresponding surface features in the cabinet inlet sector of the larsen c ice shelf using a combination of high resolution 0 5 m satellite imagery kinematic gps and in situ ground penetrating radar we discuss how basal crevasses may have contributed to the breakup of the larsen b ice shelf by directly controlling the location of meltwater ponding and highlight the presence of similar features on the amery and getz ice shelves with high resolution imagery 2012 american geophysical union all rights reserved

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

breakup of the larsen b ice shelf triggered by chain reaction drainage of supraglacial lakes this research is supported by the u s national science foundation under grant geop ant 0944248 awarded to d r m and grants ant 0838811 and arc 0934534 awarded to o v s

ice melange dynamics and implications for terminus stability jakobshavn isbrae greenland 1 we used time lapse imagery seismic and audio recordings iceberg and glacier velocities ocean wave measurements and simple theoretical considerations to investigate the interactions between jakobshavn isbrae and its proglacial ice melange the melange behaves as a weak granular ice shelf whose rheology varies seasonally sea ice growth in winter stiffens the melange matrix by binding iceberg clasts together ultimately preventing the calving of full glacier thickness icebergs the dominant style of calving and enabling a several kilometer terminus advance each summer the melange weakens and the terminus retreats the melange remains strong enough however to be largely unaffected by ocean currents except during calving events and to influence the timing and sequence of calving events furthermore motion of the melange is highly episodic between calving events including the entire winter it is pushed down fjord by the advancing terminus at 40 m d 1 whereas during calving events it can move in excess of 50 103 m d 1 for more than 10 min by influencing the timing of calving events the melange contributes to the glacier s several kilometer seasonal advance and retreat the associated geometric changes of the terminus area affect glacier flow furthermore a force balance analysis shows that large scale calving is only possible from a terminus that is near floatation especially in the presence of a resistive ice melange the net annual retreat of the glacier is therefore limited by its proximity to floatation potentially providing a physical mechanism for a previously described near floatation criterion for calving

laboratory investigations of iceberg capsize dynamics energy dissipation and tsunamigenesis 1 we present laboratory experiments designed to quantify the stability and energy budget of buoyancy driven iceberg capsize box shaped icebergs were constructed out of low density plastic hydrostatically placed in an acrylic water tank containing freshwater of uniform density and allowed or forced if necessary to capsize the maximum kinetic energy translational plus rotational of the icebergs was 15 of the total energy released during capsize and radiated surface wave energy was 1 of the total energy released the remaining energy was directly transferred into the water via hydrodynamic coupling viscous drag and turbulence the dependence of iceberg capsize instability on iceberg aspect ratio implied by the tank experiments was found to closely agree with analytical predictions based on a simple hydrostatic treatment of iceberg capsize this analytical treatment along with the high reynolds numbers for the experiments and considerably higher values for capsizing icebergs in nature indicates that turbulence is an important mechanism of energy dissipation during iceberg capsize and can contribute a potentially important source of mixing in the stratified ocean proximal to marine ice margins

active formation of chaos terrain over shallow subsurface water on europa the galileo spacecraft revealed a number of chaos regions on jupiter s moon europa where the surface terrain appears to have been disrupted from below in many places the surface contains sharp edged blocks or rafts of ice that have at some point been flipped or rotated some characteristics of these regions have been hard to explain such as the fact that the archetypal conamara chaos stands above its surroundings and contains matrix domes schmidt et al apply lessons learned from analogous processes within earth s subglacial volcanoes and ice shelves to an analysis of archival data that suggests chaos terrain forms above liquid water lenses that are perched only 3 kilometres deep within the ice shell the data suggest that ice water interactions and freeze out give rise to the varied morphology of chaos terrains implying that more water is involved than has been previously appreciated for instance the sunken topography of thera macula a large chaos area may indicate that europa is actively resurfacing over a lens comparable in volume to north america s great lakes europa the innermost icy satellite of jupiter has a tortured young surface1 2 3 4 and sustains a liquid water ocean1 2 3 4 5 6 below an ice shell of highly debated thickness1 2 3 4 5 7 8 9 10 quasi circular areas of ice disruption called chaos terrains are unique to europa and both their formation and the ice shell thickness depend on europa s thermal state1 2 3 4 5 7 8 9 10 11 12 13 14 15 16 17 no model so far has been able to explain why features such as conamara chaos stand above surrounding terrain and contain matrix domes10 18 melt through of a thin few kilometre shell3 7 8 is thermodynamically improbable and cannot raise the ice10 18 the buoyancy of material rising as either plumes of warm pure ice called diapirs1 9 10 11 12 13 14 15 or convective cells16 17 in a thick 10 kilometres shell is insufficient to produce the observed chaos heights and no single plume can create matrix domes10 18 here we report an analysis of archival data from europa guided by processes observed within earth s subglacial volcanoes and ice shelves the data suggest that chaos terrains form above liquid water lenses perched within the ice shell as shallow as 3 kilometres our results suggest that ice water interactions and freeze out give rise to the diverse morphologies and topography of chaos terrains the sunken topography of thera macula indicates that europa is actively resurfacing over a lens comparable in volume to the great lakes in north america

reverse glacier motion during iceberg calving and the cause of glacial earthquakes nearly half of greenland s mass loss occurs through iceberg calving but the physical mechanisms operating during calving are poorly known and in situ observations are sparse we show that calving at greenland s helheim glacier causes a minutes long reversal of the glacier s horizontal flow and a downward deflection of its terminus the reverse motion results from the horizontal force caused by iceberg capsize and acceleration away from the glacier front the downward motion results from a hydrodynamic pressure drop behind the capsizing berg which also causes an upward force on the solid earth these forces are the source of glacial earthquakes globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in greenland and antarctica

a computational investigation of iceberg capsize as a driver of explosive ice shelf disintegration potential energy released from the capsize of ice shelf fragments icebergs is the immediate driver of the brief explosive phase of ice shelf disintegration along the antarctic peninsula e g the larsen a larsen b and wilkins ice shelves the majority of this energy powers the rapidly expanding plume of ice shelf fragments that expands outward into the open ocean a smaller fraction of this energy goes into surface gravity waves and other dynamic interactions between ice and water that can sustain the continued fragmentation and break up of the original ice shelf as an initial approach to the investigation of ice shelf fragment capsize in ice shelf collapse we develop a simple conceptual model involving ideal rectangular icebergs initially in unstable or metastable orientations which are assembled into a tightly packed mass that subsequently disassembles via massed capsize computations based on this conceptual model display phenomenological similarity to aspects of real ice shelf collapse a promising result of the conceptual model presented here is a description of how iceberg aspect ratio and its statistical variance the two parameters related to ice shelf fracture patterns influence the enabling conditions to be satisfied by slow acting processes e g environmentally driven melting that facilitate ice shelf disintegration

hawaiian basalt and icelandic rhyolite indicators of differentiation and partial melting in spite of the voluminous basaltic volcanism on the island of hawaii rhyolite is not produced iceland on the other hand exhibits common rhyolitic volcanism amounting to some 10 12 of its surface rocks this contrast is investigated using the fundamental igneous processes exhibited by sheet like hawaiian lava lakes and shonkin sag laccolith in montana highly differentiated residual melts normally reside within inwardly advancing solidification fronts and are generally inaccessible to eruptive processes only when a large initial phenocryst population is present from which a thick basal cumulate can rapidly form is it possible to supply highly differentiated melt into the active i e eruptable portion of the magma chamber although there is protracted control of differentiation at hawaii by settling of olivine further differentiation occurs within the solidification fronts only by repeated transport and holding is it possible to differentiate beyond the critical composition of the leading edge of the solidification front 7 mgo and 51 5 sio2 crystal size distributions csds for hawaii and shonkin sag are used to demonstrate the inferred physical and chemical processes of solidification including the kinetics of crystallization

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

deterministic and stochastic analyses of acoustic plane wave reflection from inhomogeneous porous seafloor the propagator matrix technique for the acoustic plane wave reflection in layered porous anisotropic seafloor is combined with monte carlo simulation to investigate the layering effect of the sediment statistical data from a geologically known site are processed and used as a benchmark realizations of sediment property profiles are generated using a spectral technique a deterministic analysis demonstrates that the amplitude and phase of reflection coefficient is highly dependent on the variation of porosity in a stochastic analysis the effect of randomness is examined it is found that the fluctuation of the sediment physical properties around a mean profile has a significant influence on the seabed reflectivity particularly for the incident angles near and above the critical value frequency behavior of reflectivity is strongly dependent on the correlation length of medium inhomogeneities this behavior is attributed to the interaction among different waves that propagate in the porous sediment layers suggesting the possibility of obtaining the properties using the reflection loss data

dynamic jamming of iceberg choked fjords we investigate the dynamics of ice melange by analyzing rapid motion recorded by a time lapse camera and terrestrial radar during several calving events that occurred at jakobshavn isbrae greenland during calving events 1 the kinetic energy of the ice melange is 2 orders of magnitude smaller than the total energy released during the events 2 a jamming front propagates through the ice melange at a rate that is an order of magnitude faster than the motion of individual icebergs 3 the ice melange undergoes initial compaction followed by slow relaxation and extension and 4 motion of the ice melange gradually decays before coming to an abrupt halt these observations indicate that the ice melange experiences widespread jamming during calving events and is always close to being in a jammed state during periods of terminus quiescence we therefore suspect that local jamming influences longer timescale ice melange dynamics and stress transmission

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

ice shelf disintegration by plate bending and hydro fracture satellite observations and model results of the 2008 wilkins ice shelf break ups abstract satellite remote sensing observations of three break up events in 2008 for the wilkins ice shelf 28 february to 6 march 27 may to 31 may and 28 june to mid july provide unprecedented detail of ice shelf calving during rapid break up the observations reveal that the wilkins break ups occur through a distinctive type of shelf calving which we term disintegration as well as more typical rifting and calving here we focus on the disintegration process which is characterized by repeated rapid fracturing that creates narrow ice edge parallel blocks with subsequent block toppling and fragmentation forming an expanding iceberg and ice rubble mass we use these data to develop and test a model of floating ice plate disintegration in which ice plate bending stresses at the ice front arising from buoyancy forces can lead to runaway calving when free mobile water is available high resolution satellite images and laser altimetry of the first break up event provide details of fracture spacings ice thicknesses and plate bending profiles that agree well with our model predictions we suggest that surface or near surface meltwater is the main pre condition for disintegration and that hydro fracture is the main mechanism brine layers from near waterline brine infiltration can support a similar process but this is less effective unless regional ice stress patterns contribute to the net stress available at the crack tip for fracturing a combination of brine enhanced fracturing and changing internal net extensional stresses was the likely mechanism behind the latter two wilkins events

strong surface melting preceded collapse of antarctic peninsula ice shelf 1 during the austral summer of 2001 02 melting at the surface of larsen ice shelf in the antarctic peninsula was three times greater than the average of five previous summers this exceptional melt event lasted for three months and was followed by the collapse of larsen b ice shelf during which 3 200 km2 of ice shelf surface was lost the strong melting was caused by a persistent atmospheric circulation anomaly which depleted sea ice concentrations in front of larsen ice shelf and transported warm air to the ice shelf throughout the 2001 02 summer this supports the theory that large meltwater fluxes accelerate the retreat of antarctic peninsula ice shelves

biot model of sound propagation in water saturated sand elastic theory of wave propagation and the measured speed of sound in sandy ocean sediments indicate that such sediments are impenetrable to high frequency sound at shallow grazing angles the speed of sound in water saturated unconsolidated sand is in the region of 1700 m s which under the elastic theory of wave propagation gives it a critical grazing angle in the region of 28 at shallower grazing angles refraction is not permitted and total internal reflection is predicted recent experimental measurements contradict this view biot s theory of acoustic propagation in porous sediments is the most likely explanation biot s theory of acoustic propagation as it applies to water saturated sand is reviewed the speed of the slow wave is found to be higher than previously predicted new input parameter values are deduced

thin low velocity zone within the krafla caldera ne iceland attributed to a small magma chamber we examine seismograms from microearthquakes recorded at krafla central volcano in the northern volcanic zone of iceland during a short inflation period in 1988 we produce record sections for propagation both to the north and to the south of the center of the caldera by combining seismograms from many earthquakes the northern record section contains clear evidence of a low velocity zone lvz including a low amplitude diffraction of the p wave as it impinges on top of the lvz and high amplitude reflections from beneath the lvz the absence of clear shear waves indicates that the lvz is at least partially molten and should be interpretated as a magma chamber the magma chamber is less than 1 km thick with a top approximately 3 km beneath the surface we find no evidence for a lvz in the southern part of the caldera

the crustal magma chamber of the katla volcano in south iceland revealed by 2 d seismic undershooting summary results of a 2 d seismic undershooting experiment on the katla central volcano in south iceland are reported large localized traveltime anomalies 0 4 s are observed on an array within the katla caldera the traveltimes are forward modelled using a wavefront tracker developed in appendix a thus non linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity an extreme variation in compressional velocity is required to extend over a significant volume in order to model the data the resulting model is not unique but constraints on the allowable range of velocities 2 5 6 0 km s render the basic features well constrained a clear s wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly low amplitude p waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly the model is interpreted in terms of a magma chamber containing extensively molten rock the magma chamber is shallow with a bottom at a depth of about 1 5 km below sea level 3 0 km below surface and measures about 5 km across the depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved on the other hand the bottom of the chamber is well resolved the chamber is underlain by rocks of average or high velocity for that depth the magma chamber is a persistent feature big enough 10 km3 to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels the chamber is fed by magma fracturing from below the model agrees with phenomenological models of magma chambers in iceland based on geological observations and provides a quantification of those models in terms of depth and size on the other hand it is fundamentally different from recent models of magma chambers at mid ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath iceland this underlines the important effect of the icelandic hotspot on tectonics and volcanism in iceland and implies a substantially different crustal and thermal structure in iceland from that of normal mid ocean ridges

the influence of ice melange on fjord seiches we compute the eigenmodes seiches of the barotropic and baroclinic hydrodynamic equations for an idealized fjord having length and depth scales similar to those of ilulissat icefjord greenland into which jakobshavn isbrae also known as sermeq kujalleq discharges the purpose of the computation is to determine the fjord s seiche behavior when forced by iceberg calving capsize and melange movement poorly constrained bathymetry and stratification details are an acknowledged obstacle we are nevertheless able to make general statements about the spectra of external and internal seiches using numerical simulations of ideal one dimensional channel geometry of particular signifi cance in our computation is the role of weakly coupled ice melange which we idealize as a simple array of 20 icebergs of uniform dimensions equally spaced within the fjord we find that the presence of these icebergs acts to 1 slow down the propagation of both external and internal seiches and 2 introduce band gaps where energy propagation group velocity vanishes if energy is introduced into the fjord within the period range covered by a band gap it will remain trapped as an evanescent oscillatory mode near its source thus contributing to localized energy dissipation and ice melange fragmentation

evaluation of models for the formation of chaotic terrain on europa chaotic terrain on europa is interpreted to be the result of the breakup of brittle surface materials over a mobile substrate two end member models have been proposed for the mobile substrate a liquid water ocean or solid warm ductile ice direct contact between the brittle surface ice and an underlying ocean might be accomplished by melting through europa s ice shell but plausible sources of thermal energy are insufficient to locally melt through the ice shell we calculate that a liquid or partially liquid substrate is probably necessary for tilted blocks to be observed challenging a solid state model we propose that a model in which chaos areas on europa formed over bodies of melt or partial melt within the ice shell may be the best match to observational and thermal constraints such bodies of melt within the ice shell may be triggered by warm diapirs of relatively clean ice that partially melted an overlying layer of lower melting temperature such as ice contaminated with salts

seismogenic magma intrusion before the 2010 eruption of eyjafjallajokull volcano iceland we present relatively relocated earthquake hypocentres for 1000 microearthquakes ml 3 that occurred during the 2 weeks immediately prior to the 2010 march 20 fissure eruption at fimmvor uhals on the flank of eyjafjallajokull volcano in iceland our hypocentre locations lie predominantly in horizontally separated clusters spread over an area of 10 km2 and approximately 4 km below sea level 5 km below the surface seismic activity in the final 4 d preceding the eruption extended to shallower levels 2 km below sea level and propagated to the surface at the fimmvor uhals eruption site on the day the eruption started we demonstrate using synthetic data that the observed apparent 1 km vertical elongation of seismic clusters is predominantly an artefact caused by only small errors 0 01 0 02 s in arrival time data where the signal to noise ratio was sufficiently good to make subsample arrival time picks by cross correlation of both p and s wave arrivals the mean depth of 103 events in an individual cluster were constrained to 3 84 0 06 km epicentral locations are significantly less vulnerable to arrival time errors than are depths for the seismic monitoring network we used within clusters of typically 100 recorded earthquakes most of the arrivals exhibit similar waveforms and identical patterns of p wave first motion polarities across the entire monitoring network the clusters of similar events comprise repetitive sources in the same location with the same orientations of failure probably on the same rupture plane the epicentral clustering and similarity of source mechanisms suggest that much of the seismicity was generated at approximately static constrictions to magma flow in an inflating sill complex these constrictions may act as a form of valve in the country rock which ruptures when the melt pressure exceeds a critical level then reseals after a pulse of melt has passed through this would generate recurring similar source mechanisms on the same weak fault plane as the connection between segments of the sill system is repeatedly refractured in the same location we infer that the magmatic intrusion causing most of the seismicity was likely to be a laterally inflating complex of sills at about 4 km depth with seismogenic pinch points occurring between aseismic compartments of the sills or between adjacent magma lobes as they inflated during the final 4 d preceding the eruption onset between 22 30 and 23 30 utc on 2010 march 20 the seismicity suggests that melt progressed upwards to a depth of 2 km this seismicity was probably caused by fracturing of the country rock at the margins of the propagating dyke subsequently on the morning of the eruption a dyke propagated eastward from the region of precursory seismic activity to the fimmvor uhals eruption site

observing calving generated ocean waves with coastal broadband seismometers jakobshavn isbrae greenland we use time lapse photography modis satellite imagery ocean wave measurements and regional broadband seismic data to demonstrate that icebergs that calve from jakobshavn isbrae greenland can generate ocean waves that are detectable over 150 km from their source the waves which are recorded seismically have distinct spectral peaks are not dispersive and persist for several hours on the basis of these observations we suggest that calving events at jakobshavn isbrae can stimulate seiches or basin eigenmodes in both ilulissat icefjord and disko bay our observations furthermore indicate that coastal land based seismometers located near calving termini e g as part of the new greenland ice sheet monitoring network glisn can aid investigations into the largely unexplored oceanographic consequences of iceberg calving

step wise changes in glacier flow speed coincide with calving and glacial earthquakes at helheim glacier greenland this is the publisher s version also available electronically from http onlinelibrary wiley com

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

rapid collapse of northern larsen ice shelf antarctica in january 1995 4200 square kilometers of the northern larsen ice shelf antarctic peninsula broke away radar images from the ers 1 satellite complemented by field observations showed that the two northernmost sections of the ice shelf fractured and disintegrated almost completely within a few days this breakup followed a period of steady retreat that coincided with a regional trend of atmospheric warming the observations imply that after an ice shelf retreats beyond a critical limit it may collapse rapidly as a result of perturbated mass balance

the role of density in the accumulation of basaltic melts at mid ocean ridges it is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust at the east pacific rise this neutral buoyancy level lies only 100 400 m below the sea floor significantly shallower than the depths 1 2 km of the magma bodies imaged in multichannel reflection data suggesting that other factors must control the collection of melt in these reservoirs the apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis not the buoyancy of melt is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid ocean ridges

a structural glaciological analysis of the 2002 larsen b ice shelf collapse this study provides a detailed structural glaciological analysis of changes in surface structures on the larsen b ice shelf on the antarctic peninsula prior to its collapse in february march 2002 mapped features include the ice shelf front rifts crevasses longitudinal linear surface structures and meltwater features we define domains on the ice shelf related to glacier source areas and demonstrate that prior to collapse the central larsen b ice shelf consisted of four sutured flow units fed by crane jorum punchbowl and hektoria green evans glaciers between these flow units were suture zones of thinner ice where the feeder glaciers merged prior to collapse large open rift systems were present offshore of foyn point and cape disappointment these rifts became more pronounced in the years preceding break up and ice blocks in the rifts rotated because of the strong lateral shear in this zone velocity mapping of the suture zones indicates that the major rifts were not present more than about 20 years ago we suggest that the ice shelf was preconditioned to collapse by partial rupturing of the sutures between flow units this we believe was the result of ice shelf front retreat during 1998 2000 reducing the lateral resistive stress on the upstream parts of the shelf and glacier flow units ice shelf thinning and pre shelf break up glacier acceleration

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

partial melt below iceland a combined interpretation of seismic and conductivity data a combined interpretation of published seismic and electric data is carried out to impose constraints on the state of melting below central iceland these data indicate high electrical conductivity and low vp velocity regionally below 6 km s for the crust mantle transition region while the mantle below shows lower conductivity but still anomalous low velocity the velocity data are treated by utilizing gravity data to quantify the elastic modulus decrease due to partial melt a comprehensive set of theoretical models on elasticity anelasticity and electrical conductivity of partially molten rock is applied to this data set the geometry the fraction and the interconnection of the melt phase are varied systematically the conductivity models are based on published laboratory data on conductivity of solid and melt phases of rocks anisotropy and possible unexplored conductivity mechanisms near the melting temperature are not considered here by means of the melt models the data can be explained most easily as follows in a depth range roughly between 20 and 50 km only a small percentage of melt 2 at the most 5 if melt pockets are also present occurs in thin films possibly with a distribution of aspect ratios and a low degree of interconnection if in the crust mantle transition region zones of low p wave velocity 6 km s are correlated with those of high conductivity 0 08 s m melt fractions between 10 and 20 are likely and the degree of interconnection should be high thin films can be excluded as a dominant melt geometry in that region apart from improved constraints on melt geometry the results are in good agreement with published models based only on conductivity data however they disagree with published models based entirely on seismic data the discrepancy is now resolved by the combined interpretation

high resolution seismic tomography of compressional wave velocity structure at newberry volcano oregon cascade range compressional wave velocity structure is determined for the upper crust beneath newberry volcano central oregon using a high resolution active source seismic tomography method newberry volcano is a bimodal shield volcano east of the axis of the cascade range it is associated both with the cascade range and with northwest migrating silicic volcanism in southeast oregon high frequency 7 hz crustal phases nominally pg and a midcrustal reflected phase travel upward through a target volume beneath newberry volcano to a dense array of 120 seismographs this arrangement is limited by station spacing to 1 to 2 km resolution in the upper 5 to 6 km of the crust beneath the volcano s summit caldera the experiment tests the hypothesis that cascade range volcanoes are underlain only by small magma chambers a small low velocity anomaly delineated about 3 km below the summit caldera supports this hypothesis for newberry volcano and is interpreted as a possible magma chamber of a few to a few tens of km3 in volume a ring shaped high velocity anomaly nearer the surface coincides with the inner mapped ring fractures of the caldera it also coincides with a circular gravity high and we interpret it as largely subsolidus silicic cone sheets the presence of this anomaly and of silicic vents along the ring fractures suggests that the fractures are a likely eruption path between the small magma chamber and the surface

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

iceberg capsize tsunamigenesis calving from the floating termini of outlet glaciers and ice shelves is just the beginning of an interesting chain of events that can subsequently have important impacts on human life and property immediately after calving many icebergs capsize roll over by 90 due to the instability of their initial geometry as icebergs melt and respond to the cumulative effects of ocean swell they can also reorient their mass distribution by further capsize and fragmentation these processes release gravitational potential energy and can produce impulsive large amplitude surface gravity waves known as tsunamis a term derived from the japanese language iceberg capsize tsunamis in greenland fjords can be of sufficient amplitude to threaten human life and cause destruction of property in settlements iceberg capsize tsunamis may also have a role in determining why some ice shelves along the antarctic peninsula disintegrate explosively in response to general environmental warming to quantify iceberg tsunami hazards we investigate iceberg capsize energetics and develop a rule relating tsunami height to iceberg thickness this rule suggests that the open water tsunami height located far from the iceberg and from shorelines where the height can be amplified has an upper limit of 0 01h where h is the initial vertical dimension of the iceberg

sensitivity analysis of flow in multilayered leaky aquifer systems the sensitivity of drawdown prediction to the uncertainty in estimating the hydraulic properties of multiple leaky aquifer systems is studied by first order sensitivity analysis a new set of dimensionless parameters is suggested to reduce the amount of sensitivity coefficient calculations the procedure is illustrated for three aquifer configurations pumped by one well one aquifer with impermeable boundaries theis case one aquifer with two aquitards and two aquifers with one aquitard the results allow one to assess the relative contribution of uncertainty of each hydraulic property on the accuracy of predicting drawdown and to reduce the number of parameters that influence uncertainty the methodology and results may be employed when margins of confidence of the predicted drawdown are needed

seismic reflection studies in long valley caldera california seismic reflection studies in long valley caldera california indicate that seismic methods may be successfully employed to image certain types of features in young silicic caldera environments however nearsurface geological conditions within these environments severely test the seismic reflection method data quality are degraded by static reve t eration and band limiting problems due to these near surface conditions in long valley seismic reflection and refraction methods were used to image both the shallow and deep geothermal aquifers within the area the deep geothermal aquifer the welded bishop tuff was imaged as a fairly continuous reflector across the western moat of the caldera near surface refraction information indicates that there may be a buffed paleochannel system or horst and graben system that could control the shallow geothermal flow pattem high amplitude events observed in a wide angle survey were offginally interpreted as reflections from a contemporary magma body however a migration of the events utilizing the new generalized cellular migration algorithm indicates that these events are probably reflections from the faults of the caldera ring fracture system the reflections may be caused by the high acoustic impedance contrast associated with the juxtaposition of relatively low velocity low density caldera fill against the granite plutons and metasediments of the sierran basement along this fault system

determination of the seabed porosity and shear modulus profiles using a gravity wave inversion summary a previously developed bottom shear modulus profiler bsmp theory yamamoto torii 1986 allows the seabed shear modulus versus depth profile to be extracted by inverting measurements of seabed motion and water wave induced pressures at one point on the seafloor preliminary bsmp experiments trevorrow et af 1988b badiey et af 1988 compared to existing geologic borehold data at two inner continental shelf sites showed that this method can accurately predict the magnitude and depth structure of the sediment shear modulus an improved version of bsmp instrumentation system has been developed and deployed in deeper water at outer continental shelf sites gravity water wave induced bottom pressures are measured in a period band from 7 to 200s whereas measurements of gravity water wave induced ground motion are limited to a period band between 7 and 30 s due to the limited sensitivity of the seismometers used it is also found that burial of a bsmp in the seabed improved the seabed seismometer coupling significantly the bsmp inversions of these data extract the shear modulus versus depth profile of the seabed with a depth resolution of a few meters and penetrating as much as 200m into the seabed the shear modulus of a sediment at a given depth of burial is a unique function of the porosity of the sediment using this relation the porosity versus depth profile of the seabed is calculated from the shear modulus profile obtained by the bsmp inversion excellent agreements are shown between the porosity profiles obtained from the bsmp inversion and the borehole porosity logs the pressure data indicate that the potential penetration depth of the bsmp inversion is about 2 km into the continental shelf and 20 km into the deep seafloor

replenished magma chambers effects of compositional zonation and input rates several recent publications have reported investigations of the dynamical effects and geological consequences of the replenishment of a homogeneous magma chamber by the rapidinjection of hot relatively dense fresh magma the new magmaforms a separate layer at the base of the chamber whereit cools and crystallizes until the density of the residual liquid in the lower layer becomes equal to that of the upperlayer at which time the fluid in the two layers becomes intimately mixed the fluid dynamical aspects of this modelwere confirmed by laboratory experiments using aqueous solutions this paper reports three experimental variations on the basic theme which examine the effects of the rate of injection into homogeneous and compositionally stratified chambers first the rapid injection of a hot dense layer beneath a fluid with a lower temperature and a vertical compositional gradient is considered a series of layers gradually builds up from the base of the upper gradient region while the lower layer cools crystallizes and finally overturns in this situation however the rise of the lower layer fluid is restrained by the density gradient and mixing takes place with only part of the upper fluid second the slow influx of a hot dense liquid beneath a cold less dense homogeneous layer is considered a considerable quantity of the input fluid coolsand crystallizes immediately releasing some lighter fluid to mix with the fluid in the homogeneous layer with time some dense input fluid spreads along the bottom so that locally the situation is the same as the one considered in the previous studies with a local overturning occurring when the density of the lower fluid has decreased sufficiently the deposition of crystals continues and some rather fine chimney like structures can develop which bear a resemblance to the black smokers recently discovered on the sea floor third a combination of the first two variations is studied by the consideration of the slow influx of hot dense liquid beneath a uniformly cooler fluid with a vertical compositional gradient the input fluid is constrained in its rise by the density gradient as the influx continues the additional thermal energy is also confined and causes the crystals to redissolve and build up a lower layer as was observed in the case of a rapid injection beneath a compositional gradient

location of a magma reservoir beneath hekla volcano iceland hekla situated on the western boundary of the eastern volcanic zone in south iceland is one of the most active volcanoes in iceland having erupted 16 times since 1104 repeated electronic distance measurements since its eruption in 1980 81 show significant changes in crustal movement in the aftermath of the eruption such movement on and around volcanoes can be used to monitor movement of magma within and below volcanoes1 3 we show here that these measurements can be interpreted in terms of inflating magma reservoirs with a centre at 8 km depth below the volcano over a period of 10 months the volume of the inflation amounted to 55 106 m3

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

seismic observations of glaciogenic ocean waves micro tsunamis on icebergs and ice shelves seismometers deployed over a 3 year period on icebergs in the ross sea and on the ross ice shelf antarctica reveal that impulsive sources of ocean surface waves are frequent e g 200 events per year in the ross sea in the ice shelf and iceberg covered environment of coastal antarctica the 368 events recorded by our field deployment suggest that these impulsive events are generated by glaciological mechanisms such as 1 small scale calving and edge wasting of icebergs and ice shelf fronts 2 edge on edge closing and opening associated with iceberg collisions and 3 possibly the impulsive opening of void space associated with ice shelf rifting and basal crevasse formation the observations described here provide a background of glaciogenic ocean wave phenomena relevant to the ross sea and suggest that these phenomena may be exploited in the future using more purposefully designed observation schemes to understand iceberg calving and ice shelf disintegration processes

simultaneous inversion for lateral velocity variations and hypocenters in the yellowstone region using earthquake and refraction data a three dimensional upper crustal p wave velocity structure of the yellowstone region was determined by simultaneous inversion using pg refracted arrival times and earthquake generated direct p wave arrival times the velocity structure was modeled by a two layer block configuration with the velocity of the surface layer fixed the inversion was performed on a 14 4 km thick second layer 5 6 blocks using an optimal block size of 21 2 18 3 km and incorporated the method of separation of parameters results show volumes of higher velocities approximately 6 3 km s north of the yellowstone caldera that correlate with gravity anomaly highs and are interpreted to be associated with near surface crystalline basement the upper crust beneath the caldera showed an average velocity of 5 8 km s with prominent low velocity zones of 4 9 and 5 2 km s in the northeast and southwest portions of the caldera respectively the 4 9 km s low velocity layer in the northeast corner of the caldera coincides with a localized 20 mgal bouguer gravity low and the largest area of hydrothermal activity in yellowstone national park relocating earthquakes with the new velocity model showed a systematic migration of epicenters radially toward the caldera by an average of 1 2 km relative to locations using a homogeneous velocity model

the role of cooperative iceberg capsize in ice shelf disintegration disintegration of several ice shelves along the antarctic peninsula demonstrates a mechanism that involves the conversion of a contiguous ice shelf into an expanding plume of ice shelf fragments that spreads rapidly across the ocean surface the growth of surface area and energetic expansion are hypothesized to be driven by gravitational potential energy release associated with iceberg capsize and break up here we investigate this process using a water tank filled with plastic icebergs scaled to represent a laboratory analogue of an expanding plume of ice shelf fragments icebergs our experiments suggest that hydrodynamic pressure within the water separating neighbouring icebergs is sufficient to couple the motion when their separation is comparable to the iceberg size this allows one iceberg s capsize to initiate a domino like effect where the entire array will subsequently capsize in the same direction and expand across the water surface our experimental results motivate the suggestion that cooperative iceberg hydrodynamics is a process that enhances the expansion of ice shelf fragment plumes during ice shelf disintegration

glacier velocities from time lapse photos technique development and first results from the extreme ice survey eis in greenland automated digital cameras were installed in may june 2007 beside major west greenland marine terminating glaciers as part of the extreme ice survey eis eis cameras began imaging the lowest 4 km 2 of the glacier at hourly intervals throughout sunlit periods of the year this study presents the development of techniques for quantifying glacier velocity from a single camera perspective a multi image multi chip matching procedure yields higher matching skill than conventional matching and facilitates false match rejection via a clustering scheme the matching of motionless on land features facilitates compensating camera motion ray projection to a known terrain elevation allows the assigning of scale to convert pixel displacements to velocity units with the 10 2 megapixel camera system velocities on relatively fast glaciers can be resolved at distances up to 4 km at a distance of 2 km a demonstrated precision of 0 5 pixels yields a 0 5 m footprint size daily velocities indicate progressive multi day velocity accelerations associated with calving deceleration trends are associated with the regrowth of resistive stress after major calving the higher observation frequency available to terrestrial photogrammetry indicates higher observed intra seasonal velocity range than observable by the at best weekly satellite snapshots

geochemistry dispersal volumes and chronology of holocene silicic tephra layers from the katla volcanic system iceland at least 12 silicic tephra layers silk tephras erupted between ca 6600 and ca 1675 yr bp from the katla volcanic system have been identified in southern iceland in addition to providing significant new knowledge on the holocene volcanism of the katla system which typically produces basaltic tephra the silk tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts with one exception the silk tephras have a narrow compositional range with sio2 between 63 and 67 geochemically they are indistinguishable from ocean transported pumice found on beaches in the north atlantic region although they differ significantly from the silicic component of the north atlantic ash zone one naazo volumes of airborne silk tephra range from 0 05 to 0 3 km3 we present new isopach maps of the six largest layers and demonstrate that they originate within the katla caldera the apparently stable magma system conditions that produced the silk tephras may have been established as a consequence of the eruption of the silicic component of naazo ca 10 3 ka and disrupted by another large scale event the tenth century ad eldgja eruption ca 1 ka despite the current long repose silicic activity of this type may occur again in the future presenting hitherto unknown hazards copyright 2001 john wiley sons ltd

seasonal and interannual variations in ice melange and its impact on terminus stability jakobshavn isbrae greenland we used satellite derived surface temperatures and time lapse photography to infer temporal variations in the proglacial ice melange at jakobshavn isbrae a large and rapidly retreating outlet glacier in greenland freezing of the melange covered fjord surface during winter is indicated by a decrease in fjord surface temperatures and is associated with 1 a decrease in ice melange mobility and 2 a drastic reduction in iceberg production vigorous calving resumes in spring typically abruptly following the steady up fjord retreat of the sea ice ice melange margin an analysis of pixel displacement from time lapse imagery demonstrates that melange motion increases prior to calving and subsequently decreases following several events we find that secular changes in ice melange extent character and persistence can influence iceberg calving and therefore glacier dynamics over daily to monthly timescales which if sustained will influence the mass balance of an ice sheet

glacier fjord and seismic response to recent large calving events jakobshavn isbrae greenland 1 the recent loss of jakobshavn isbrae s extensive floating ice tongue has been accompanied by a change in near terminus behavior calving currently occurs primarily in summer from a grounded terminus involves the detachment and overturning of several icebergs within 30 60 min and produces long lasting and far reaching ocean waves and seismic signals including glacial earthquakes calving also increases near terminus glacier velocities by 3 but does not cause episodic rapid glacier slip thereby contradicting the originally proposed glacial earthquake mechanism we propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving

instability of the antarctic ross sea embayment as climate warms collapse of the antarctic ice sheet since the last glacial maximum 18 000 years ago is most pronounced in the ross sea embayment which is partly ice free during antarctic summers thereby breaching the o ring of ice shelves and sea ice surrounding antarctica that stabilizes the ice sheet the o ring may have vanished during early holocene 5000 to 3000 b c roman 1 to 400 a d and medieval 900 to 1300 a d warm periods and reappeared during the little ice age 1300 to 1900 a d we postulate further collapse in the embayment during the post 1900 warming may be forestalled because east antarctic outlet glaciers nail the ross ice shelf to the transantarctic mountains so it can resist the push from west antarctic ice streams our hypothesis is examined for byrd glacier and a static ice shelf using three modeling experiments having plastic viscous and viscoplastic solutions as more data and improved modeling became available observed crevasse patterns were not reproduced a new research study is needed to model a dynamic ross ice shelf with all its feeder ice streams outlet glaciers and ice calving dynamics in three dimensions over time to fully test our hypothesis the required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf calving must then successively pull the outlet glacier nails so collapse of the marine west antarctic ice sheet proceeds to completion

from geology to geoacoustics evaluation of biot stoll sound speed and attenuation for shallow water acoustics a procedure for estimating acoustic wave velocity and attenuation in ocean sediment using a minimum amount of geological and geotechnical data is demonstrated first the biot stoll theory is presented next various asymptotic formulae for the attenuation coefficient are derived for high low and intermediate frequencies these expressions clearly isolate the effects of intergranular coulomb friction and fluid viscous dissipation on the attenuation of shear and compressional waves under the constraint of a minimum amount of geological and geotechnical information a sequence of empirical equations is compiled to convert basic data such as blow count number from a standard penetration test or shipboard density into sediment geoacoustic properties as a demonstration two well known field cases the atlantic generating station ags site and the atlantic margin coring amcor 6010 site are examined by incorporating the uncertainty involved in the data collection the estimated geoacoustical parameters are provided with a standard deviation

synthetic seismograms for marine sediments and determination of porosity and permeability we present numerical simulations of vertical seismic profiles vsps of marine sediments the theoretical seismograms which are computed for vertically incident waves in flat layered poroelastic media include the effects of dispersion and attenuation predicted by biot theory according to biot theory fast and slow compressional waves are excited and there is mode conversion at the interfaces we include this effect in the calculation of reflection and transmission coefficients as an energy loss mechanism through the slow compressional waves finally we examine the spectral ratio method for determining porosity and permeability from synthetic seismogram data analytical expressions for the velocity and specific attenuation are found based on the weak frame approximation once the frequency dependent velocity and specific attenuation are calculated the porosity and permeability of marine sediments can be determined by using the proposed weak frame approximations spectral ratio calculations on syntheti

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis