fracture patterns at lava ice contacts on kokostick butte or and mazama ridge mount rainier wa implications for flow emplacement and cooling histories abstract cooling lava commonly develop polygonal joints that form equant hexagonal columns such fractures are formed by thermal contraction resulting in an isotropic tensional stress regime however certain linear cooling fracture patterns observed at some lava ice contacts do not appear to fit the model for formation of cooling fractures and columns because of their preferred orientations these fracture types include sheet like ladder like rectangular fracture pattern intermediate pseudo aligned individual column bounding fractures and pseudopillow straight to arcuate fractures with perpendicular secondary fractures caused by water infiltration fractures that form the edges of multiple columns along a single linear fracture despite the relatively common occurrence of these types of fractures at lava ice contacts their significance and mode of formation have not been fully explored this study investigates the stress regimes responsible for producing these unique fractures and their significance for interpreting cooling histories at lava ice contacts data was collected at kokostick butte dacite flow at south sister or and mazama ridge andesite flow at mount rainier wa both of these lava flows have been interpreted as being emplaced into contact with ice and linear fracture types have been observed on their ice contacted margins two different mechanisms are proposed for the formation of linear fracture networks one possible mechanism for the formation of linear fracture patterns is marginal bulging melting of confining ice walls will create voids into which flowing lava can deform resulting in margin parallel tension causing margin perpendicular fractures if viewed from the ice wall these fractures would be steeply dipping linear fractures another possible mechanism for the formation of linear fracture types is gravitational settling pure shear during compression and settling can result in a tensional environment with similar consequences as marginal inflation in addition to this horizontally propagating cooling fractures will be directly influenced by viscous strain caused by the settling of the flow this would cause preferential opening of fractures horizontally resulting in vertically oriented fractures it is important to note that the proposed model for the formation of linear fractures is dependent on contact with and confinement by glacial ice the influence of flow or movement on cooling fracture patterns has not been extensively discussed in previous modeling of cooling fractures rapid cooling of lava by the interaction with water and ice will increase the ability to the capture and preserve perturbations in the stress regime

melting of the glacier base during a small volume subglacial rhyolite eruption evidence from blahnukur iceland abstract although observations of recent volcanic eruptions beneath vatnajokull iceland have improved the understanding of ice deformation and meltwater drainage little is known about the processes that occur at the glacier base we present observations of the products of a small volume effusive subglacial rhyolite eruption at blahnukur torfajokull iceland lava bodies typically 7 m long have unusual conical morphologies and columnar joint orientations that suggest emplacement within cavities melted into the base of a glacier cavities appear to have been steep walled and randomly distributed these features can be explained by a simple model of conductive heat loss during the ascent of a lava body to the glacier base the released heat melts a cavity in the overlying ice the development of vapour escape pipes in the waterlogged permeable breccias surrounding the lava allows rapid heat transfer between lava and ice the formed meltwater percolates into the breccias recharging the cooling system and leaving a steam filled cavity the slow ascent rates of intrusive rhyolitic magma bodies provide ample time for a cavity to be melted in the ice above even during the final 10 m of ascent to the glacier base an equilibrium cavity size is calculated at which melting is balanced by creep closure this is dependent upon the heat input and the difference between glaciostatic and cavity pressure the cavity sizes inferred from blahnukur are consistent with a pressure differential of 2 4 mpa suggesting that the ice was at least 200 m thick this is consistent with the volcanic stratigraphy which indicates that the ice exceeded 350 m in thickness although this is the first time that a subglacial cavity system of this type has been reconstructed from an ancient volcanic sequence it shares many characteristics with the modern firn cave system formed by fumarolic melting within the summit crater of mount rainier washington at both localities it appears that localised heating at the glacier base has resulted in heterogeneous melting patterns despite the different rheological properties of ice and firn similar patterns of cavity roof deformation are inferred the development of low pressure subglacial cavities in regions of high heat flux may influence the trajectory of rising magma with manifold implications for eruptive mechanisms and resultant subglacial volcanic landforms

structures and facies associated with the flow of subaerial basaltic lava into a deep freshwater lake the sulphur creek lava flow north cascades washington abstract the ca 8800 14c yrs bp sulphur creek lava flowed eastward 12 km from the schriebers meadow cinder cone into the baker river valley on the southeast flank of mount baker volcano the compositionally zoned basaltic to basaltic andesite lava entered crossed and partially filled the 2 km wide and 100 m deep early holocene remnant of glacial lake baker the valley is now submerged beneath a reservoir but seasonal drawdown permits study of the distal entrant lava as a lava volume that may have been as much as 180 106 m3 entered the lake the flow invaded the lacustrine sequence and extended to the opposite east side of the drowned baker river valley the volume and mobility of the lava can be attributed to a high flux rate a prolonged eruption or both basalt exposed below the former level of the remnant glacial lake is glassy or microcrystalline and sparsely vesicular with pervasive hackly or blocky fractures together with pseudopillow fractures these features reflect fracturing normal to penetrative thermal fronts and quenching by water a fine grained hyaloclastite facies was probably formed during quench fragmentation or isolated magma water explosions although the structures closely resemble those developed in lava ice contact environments establishing the depositional environment for lava exhibiting similar intense fracturing should be confirmed by geologic evidence rather than by internal structure alone the lava also invaded the lacustrine sequence forming varieties of peperite including sills that are conformable within the invaded strata and resemble volcaniclastic breccias the peperite is generally fragmental and clast or matrix supported fine grained and rounded fluidal margins occur locally the lava formed a thickened subaqueous plug that as the lake drained in the mid holocene was exposed to erosion the baker river then cut a 52 m deep gorge through the shattered highly erodible basalt

glaciovolcanism at volcan sollipulli southern chile lithofacies analysis and interpretation magma ice meltwater interactions produce diverse landforms and lithofacies reflecting the multitude of factors that influence glaciovolcanism including both magmatic e g composition eruption rate and glacial e g ice thickness thermal regime conditions this is exemplified by the walls of the partly ice filled summit caldera of volcan sollipulli a stratovolcano in southern chile which include lithofacies from eruptions of a wide range of magma compositions beneath or in contact with ice here we analyse these lithofacies and hence propose new interpretations of the eruptive and glacial history of sollipulli the facies include a thick laterally extensive sequence of fragmental glaciovolcanic deposits comprising massive mafic lava pillow bearing hyaloclastite overlain by sills and then hyaloclastic debris flow deposits similar to dalsheidi type sequences the distribution and thickness of these units indicate an unusual abundance of magma meltwater interaction for an arc stratovolcano in temperate latitudes perhaps due to eruptions beneath a thick ice cap coherent lava coulees domes lobes and stacks of basaltic andesite trachydacite composition are present around the top of the caldera rim these display morphologies and fracture patterns on caldera facing margins that indicate that the caldera was filled with ice when these lavas were erupted the lithofacies characterised in this study demonstrate the diversity of glaciovolcanism that is possible at arc stratovolcanoes capped by temperate ice or with ice filled calderas and the potential for uncertainties in inference of the palaeoenvironmental conditions of their emplacement

scaling of columnar joints in basalt 1 we describe field work analysis and modeling of columnar joints from the columbia river basalt group this work is focused on the regions around the grand coulee snake river and columbia gorge which form parts of this unusually homogeneous and very large sample of columnar basalt we examine in detail the scaling relationship between the column width and the size of the striae and relate these quantitatively to thermal and fracture models we found that the column radius and stria size are proportional to each other and inversely proportional to the cooling rate of the lava near a flow margin our results put observational constraints on diffusive thermal models of joint formation deeper than a few meters into a colonnade our measurements are consistent with a simple advection diffusion model of two phase convective cooling within the joints regardless of the direction of cooling this model allows an accurate comparison of igneous columnar jointing and joints due to desiccation in laboratory analog systems we also identify a new length scale in which wavy columns can appear in some colonnades the mechanisms leading to the wavy columns are likely related to those underlying similar wavy cracks in 2 d analog systems

ductile opening mode fracture by pore growth and coalescence during combustion alteration of siliceous mudstone opening mode fractures with blunt tips and large maximum apertures are characteristic of clinker that formed by combustion alteration of siliceous mudstone these fractures are inferred to result from pore growth and coalescence with initially circular pores inherited from the diatomaceous protolith circular pores grow preferentially in an en echelon arrangement and coalesce to elongate pores and blunt tipped fractures by thinning and rupture of bridges between pores coalescence of overlapping en echelon pores causes fracture propagation in a zig zag path that is considered indicative of significant inelastic deformation outside the immediate vicinity of the fracture tips this process of fracture formation by void growth and coalescence is inferred to result from solution mass transfer and possible bulk melt movement during partial melting of clinker chemical mass transfer provides a mechanism for extensive inelastic deformation in the surrounding host rock concurrent with fracturing that is considered characteristic of ductile fracture the preferred elongation of coalescing pores and local rupture of pore bridges is explained by a tensile sintering stress due to the thermodynamic tendency of the system for energy minimization of solid and liquid surfaces it is suggested that ductile fracture processes that are accompanied by extensive inelastic deformation lead to opening mode fractures with large apertures in a variety of crustal settings including metamorphic and magmatic systems as well as in reactive diagenetic environments thus affecting fracture controlled transfer of heat and mass and the rheology of the earth s crust

ridge forming ice bounded lava flows at mount rainier washington large 0 3 4 km3 andesite and dacite lava flows at mount rainier washington sit atop or are perched along the sides of high ridges separating deep valleys early researchers proposed that these ridge forming lavas flowed into paleovalleys and displaced rivers to their margins entrenchment of the rivers then left the lavas atop ridges on the basis of exceptional flow thickness ice contact features and eruption age measurements we propose that the lavas flowed beside and between valley glaciers that filled the adjacent valleys in the pleistocene when the glaciers retreated the flows were left high on the adjacent ridges these lavas were never situated at valley floors and do not represent products of reversed topography instead ridge forming and perched lava flows at mount rainier and at many other high stratovolcanoes illustrate the ability of ice to dam deflect and confine flowing lava

entablature fracture types and mechanisms entablature is the term used to describe zones or tiers of irregular jointing in basaltic lava flows it is thought to form when water from rivers dammed by the lava inundates the lava flow surface and during lava meltwater interaction in subglacial settings a number of different fracture types are described in entablature outcrops from the burfell lava and older lava flows in jorsardalur southwest iceland these are striae bearing column bounding fractures and pseudopillow fracture systems that themselves consist of two different fracture types master fractures with dimpled surface textures and subsidiary fractures with curved striae the interaction of pseudopillow fracture systems and columnar jointing in the entablature produces the chevron fracture patterns that are commonly observed in entablature cube jointing is a more densely fractured version of entablature which likely forms when more coolant enters the hot lava the entablature tiers display closely spaced striae and dendritic crystal shapes which indicate rapid cooling master fracture surfaces show a thin band with an evolved composition at the fracture surface mineral textures in this band also show evidence of quenching of this material this is interpreted as gas driven filter pressing of late stage residual melt that is drawn into an area of low pressure immediately preceding or during master fracture formation by ductile extensional fracture of hot partially crystallised lava this melt is then quenched by an influx of water and or steam when the master fracture fully opens our findings suggest that master fractures are the main conduit for coolant entering the lava flow during entablature formation

the glaciovolcanic evolution of the copahue volcano andean southern volcanic zone argentina chile abstract glaciovolcanism produces distinctive features that are useful paleoclimate proxies for the distribution of past ice sheets and glacier extent the copahue volcano located in the andean southern volcanic zone argentina chile is an active composite volcano known to have glaciovolcanic features such as lava bodies with glassy margins and anomalous cooling fractures however the emplacement conditions of these products and the influence of pleistocene glaciations on the evolution of the copahue volcano remains poorly understood in this contribution we propose a model for glaciovolcanic evolution of the copahue volcano based on the analysis interpretation and mapping of its products ten lithofacies are described on the eastern flank of copahue volcano exhibiting several examples of glaciovolcanism the evolution of the copahue volcano can be divided into two main sequences the ancient sequence s1 and the young sequence s2 separated by a major erosive phase the s1 early middle pleistocene late pleistocene consists of an initial subaerial effusive stage followed by a major glaciovolcanic stage during which a thick ice cap existed and the edifice grew beneath an englacial lake with the eventual formation of a lava fed delta the s2 late pleistocene present is defined by mainly effusive activity during periods of glacial advance and retreat recorded by an alternation of unconfined subaerial lavas and ice confined lavas the evolution of the copahue volcano therefore indicates two glaciations in the copahue caviahue area during the late pleistocene in contrast to a single glaciation based on the glacial history in the area we associate the first glaciation with the end of marine isotope stage 3 57 29 ka and or the last glacial maximum period 26 5 19 0 ka and the second less extensive glacial period with the antarctic cold reversal 14 5 12 9 ka

column on column structures as indicators of lava ice interaction ruapehu andesite volcano new zealand abstract lava flows of the mangawhero formation ca 15 60 ka on ruapehu volcano erupted during the last glaciation in a distal flow lobe at tukino on the east side of the mountain small secondary columns 10 20 cm thick have formed on the sides of large rectangular primary 0 5 3 m thick cooling columns thick 10 m zones of such small columns form a lateral and basal outer rind of the lobe as they do not mark glassy zones of quenching these secondary columns are interpreted as being formed by a second cooling event at temperatures below the boundary between the low creep and elastic regimes 600 c by rapid influx of copious amounts of water temperature drops deduced from extensional strains of the two sets of columns were used to gauge the viability of such a two stage process absence of reliable data on andesite contraction coefficients was overcome by using a sliding scale to assess a large range of values the estimates indicate that two stage chilling is feasible after flowing across relatively ice poor terrain the lava flow must have interacted with a valley glacier that provided water for further chilling the already formed primary columns and formation of the outer rind small columns given this evidence for lava ice interaction it is likely that prominent thick flows elsewhere in the mangawhero formation may have been constrained to their ridge top locations by ice conditions similar to those described by lescinsky and sisson lescinsky d t sisson t w 1998 ridge forming ice bounded lava flows at mount rainier washington geology 26 351 354

geochemistry of historical age silicic tephras in iceland the major element chemistry of nine silicic tephras of historical age from iceland is assessed as a key step in the development of the recent tephrochronology of the north atlantic region the tephras include the largest such layers produced by each of the five central volcanoes hekla o raefajokull eyjafjallajokull torfajokull and askja since the ninth century ad h 1104 o1362 e 1821 landnam tephra c 870 a 1875 and four other tephras h 1158 h 1510 h 1947 o1727 the determination of grain discrete major element chemistry of the glass fraction is a fundamental stage in the identification and correlation of tephra and allows links to be made between icelandic source areas with precise dating evidence and distal deposits elsewhere in the north atlantic region although major element data can be used to discriminate between tephra layers produced by the different central volcanoes on its own it cannot be used to identify all the holocene layers produc

electric study of fracture anisotropy at falkenberg germany electric and electromagnetic methods have been applied for mapping subsurface fractures and the directional dependence of in situ electric parameters at the hot dry rock site at falkenberg germany this study includes the determination of several anisotropy parameters like the mean longitudinal and transverse resistivity components m x1d4c1 and t respectively the anisotropy coefficient and the strike angle terrain conductivity measurements using the technique of frequency domain electromagnetic induction reveal a dominant anomaly strike of east south east west north west nearly parallel to the fracturing strike of n110 with increasing distance from the central borehole the mise a la masse potential differences exhibit a transition from a direct to a paradoxical relationship to the resistivity anisotropy induced by the fracturing these observations are explained using a model for an ellipsoidal fracture the qualitative interpretation of the sounding data of schlumberger and crossed square

the emplacement of an obsidian dyke through thin ice hrafntinnuhryggur krafla iceland an eruption along a 2 5 km long rhyolitic dyke at krafla volcano northern iceland during the last glacial period formed a ridge of obsidian hrafntinnuhryggur the ridge rises up to 80 m above the surrounding land and is composed of a number of small volume lava bodies with minor fragmental material the total volume is 0 05 km3 the lava bodies are flow or dome like in morphology and many display columnar jointed sides typical of magma ice interaction quench fragmented lower margins indicative of interaction with meltwater and pumiceous upper surfaces typical of subaerial obsidian flows the fragmental material compromises poorly sorted perlitic quench hyaloclastites and poorly exposed pumiceous tuffs lava bodies on the western ridge flanks are columnar jointed and extensively hydrothermally altered at the southern end of the ridge the feeder dyke is exposed at an elevation 95 m beneath the ridge crest and flares upwards into a lava body using the distribution of lithofacies we interpret that the eruption melted through ice only 35 55 m thick which is likely to have been dominated by firn hrafntinnuhryggur is therefore the first documented example of a rhyolitic fissure eruption beneath thin ice firn the eruption breached the ice leading to subaerial but ice firn contact lava effusion and only minor explosive activity occurred the ridge appears to have been well drained during the eruption aided by the high permeability of the thin ice firn which appears not to have greatly affected the eruption mechanisms we estimate that the eruption lasted between 2 and 20 months and would not have generated a significant jokulhlaup 70 m3s 1

the origin and evolution of breakouts in a cooling limited rhyolite lava flow understanding lava flow processes is important for interpreting existing lavas and for hazard assessments although substantial progress has been made for basaltic lavas our understanding of silicic lava flows has seen limited recent advance in particular the formation of lava flow breakouts which represent a characteristic process in cooling limited basaltic lavas but has not been described in established models of rhyolite emplacement using data from the 2011 2012 rhyolite eruption of puyehue cordon caulle chile we develop the first conceptual framework to classify breakout types in silicic lavas and to describe the processes involved in their progressive growth inflation and morphological change by integrating multiscale satellite field and textural data from cordon caulle we interpret breakout formation to be driven by a combination of pressure increase from local vesiculation in the lava flow core as well as from continued supply via extended thermally preferential pathways and a weakening of the surface crust through lateral spreading and fracturing small breakouts potentially resulting more from local vesiculation than from continued magma supply show a domed morphology developing into petaloid as inflation increasingly fractures the surface crust continued growth and fracturing results in a rubbly morphology with the most inflated breakouts developing into a cleft split morphology reminiscent of tumulus inflation structures seen in basalts these distinct morphological classes result from the evolving relative contributions of continued breakout advance and inflation the extended nature of some breakouts highlights the role of lava supply under a stationary crust a process ubiquitous in inflating basalt lava flows that reflects the presence of thermally preferential pathways textural analyses of the cordon caulle breakouts also emphasize the importance of late stage volatile exsolution and vesiculation within the lava flow although breakouts occur across the compositional spectrum of lava flows the greater magma viscosity is likely to make late stage vesiculation much more important for breakout development in silicic lavas than in basalts such late stage vesiculation has direct implications for hazards previously recognized from silicic lava flows enhancing the likelihood of flow front collapse and explosive decompression of the lava core

on the role of mechanical energy in maintaining subglacial water conduits at atmospheric pressure recent theo retical studies of glacier hydrology have assumed that subglacial conduits are completely filled with water under steady state conditions this however is not necessarily the case where discharges are larger than a few tens of liters per second and the down glacier slope of the bed is more than a few degrees the potential energy released by water descending this slope may be capable of melting the walls of a subglacial conduit many times faster than the conduit can close by plastic flow of the ice as a result the pressure in such tunnels may normally be atmospheric or possibly even at the triple point pressure if there is no open connection to the glacier surface simple calcu lations suggest that such pressures in subglacial conduits may be more common than herclofore anticipated the positions of such open conduits may be unstable to small per turbations in discharge or ice veloci ty this is because the mechanical energy available in excess of that needed to balance closure can instead offset the gene ral flow of the ice cond ui ts can thus trend diago nally across the direction of ice aow if an increase in the angle which such a conduit makes with the ice flow direction also results in an increase in slope of he conduit more mechanical energy will become available resulting in a positive feedback process subglacial channels at atmospheric pressure may inauence the origin and morphology of certain glacial landforms such as eskers and plastically molded features resume sur le role de l inergie newtonnienne dam le maintien du riseau aquiflrl sous glaciaire cl la pression almosph irique de recentes etudes theoriques d hydrologie glaciaire ont suppose que le reseau sous glaci aire clait corn pletement rempli d eau en condition d cquil ibre stable ceci neanmoins n est pas necess airement le cas lorsque les debits sonl superieurs a quelques dizaines de litres par seconde et que la pente du lit du glacier est superieure a quelques degres i energie potentielle mise en oeuvre par i eau descendant celle pente peut ire capable de fondre les parois d un chenal sous glaciaire beaucoup plus vite que le chenal ne peut se refermer par le auage plastique de la glace il en resuhe que la pression dans de tels chenaux peut nor rnalement etre atrnospherique

snow contact volcanic facies and their use in determining past eruptive environments at nevados de chillan volcano chile studies of the eruptive products from volcanoes with variable ice and snow cover and a long history of activity enable reconstruction of erupted palaeoenvironments as well as highlighting the hazards associated with meltwater production such as jokulhlaups and magma water interaction existing difficulties include estimation of ice snow thicknesses and discrimination between ice and snow contact lithofacies we present field evidence from the cerro blanco subcomplex of nevados de chillan stratovolcano central chile which has erupted numerous times in glacial and non glacial periods and most recently produced andesitic lava flows in the 1861 1865 eruption from the santa gertrudis cone on the northwest flank of the volcano the main period of lava effusion occurred during the winter of 1861 when the upper flanks of the volcano were reportedly covered in snow and ice the bases and margins of the first lava flows produced are cut by arcuate fractures which are interpreted as snow contact features formed when steam generated from the melting of snow entered tensional fractures at the flow base in contrast the interiors and upper parts of these flows as well as the overlying flow units have autobrecciated and blocky textures typical of subaerial conditions due to insulation by the underlying lava similar textures found in a lava flow dated at 90 0 0 6 ka that was emplaced on the northwest flank of cerro blanco are also inferred to be ice and snow contact features these textures have been used to infer that a small valley glacier overlain by snow existed in the santa gertrudis valley at the time of the eruption such reconstructions are important for determining the long term evolution of the volcano as well as assessing future hazards at seasonally snow covered volcanoes

interactions between lava and snow ice during the 2010 fimmvorðuhals eruption south central iceland 1 the 20 march 12 april basaltic effusive eruption at fimmvor uhals southern iceland was an important opportunity to directly observe interactions between lava and snow ice the eruption site has local perennial snowfields and snow covered ice and at the time of eruption it was covered with an additional 1 3 m of seasonal snow syn eruption observations of interactions between lava and snow ice are grouped into four categories 1 lava advancing directly on top of snow 2 lava advancing on top of tephra covered snow 3 snow ice melting at lava flow margins and 4 lava flowing beneath snow based on syn and post eruption observations in 2010 11 we conclude that the features seen in the lava flow field show only limited and localized evidence for the influence of snow ice presence during the eruption estimated melting rates from radiant and conductive heating at the flow fronts are too slow on the order of 5 m hr to allow for complete melting of snow ice ahead of the advancing lava flows at least during periods of observed rapid lava advance rates 15 55 m hr thus we conclude that during those periods which largely established the aerial extent of the lava flow field lava advanced on top of snow that this likely was the predominant mode of lava emplacement for much of the eruption is supported by many syn eruption field observations examination of the lava flows subsequent to the eruption has so far only found subtle evidence for interactions between lava and snow ice for example locally lava flows have fractured and are collapsing or have developed marginal rubble aprons from melting of snow banks that were partly covered by lava flow margins

the formation of columnar joints produced by cooling in basalt at staffa scotland columnar jointing in basaltic lava flows on the island of staffa nw scotland was studied using a combination of field mapping and measurement of column dimensions sample petrology and measurements of plagioclase crystal size distributions csds interpreted using theoretical models of cooling four different lava flow units were measured and column ordering was assessed using the hexagonality index and relative standard deviations of column side length top area and internal angle upper and lower colonnades consist of dominantly 5 6 and 7 sided columns with a hexagonality index value very similar to that of giant s causeway and other basaltic columnar jointed localities csds from samples at different heights within one colonnade were used to infer the propagation of the solidus isotherm which was consistent with a convective cooling mechanism within the colonnade interior sample petrology and csd measurements suggest that entablature can form both by the interaction of propagating joint sets and flooding of the flow surface by water and the most widely exposed unit on staffa shows evidence of both mechanisms operating on the same flow crystal size distribution measurements can provide a useful tool for field interpretation of lava flow cooling mechanisms

conductive cooling of lava columnar joint diameter and stria width as functions of cooling rate and thermal gradient abstract much evidence exists that column diameter and stria width of columnar joints in lava flows vary inversely with cooling rate t t and thermal gradient t x at the point of fracture as the flow cools some evidence suggests that lava flows cool conductively a conductive cooling model yields cooling rate and thermal gradient through space and time for cooling igneous bodies both for extrusive and intrusive settings thermal gradient and cooling rate are then linked to stria width and column diameter with faster cooling leading to narrower columns and steeper gradients to narrower striae this model shows large regions within a body where the ratio of thermal gradient to cooling rate is nearly constant and predicts that the ratio of stria width to column diameter is also nearly constant this prediction is consistent with field observations both in this and other works the model also suggests that if conductive cooling dominates then joint spacing i e column diameter should increase inward from the margins of the body as should stria width

phenocryst hosted melt inclusions record stalling of magma during ascent in the conduit and upper magma reservoir prior to vulcanian explosions soufriere hills volcano montserrat west indies the mechanics of explosive eruptions influence magma ascent pathways vulcanian explosions involve a stop start mechanism that recurs on various timescales evacuating the uppermost portions of the conduit during the repose time between explosions magma rises from depth and refills the conduit and stalls until the overpressure is sufficient to generate another explosion we have analyzed major elements cl s h2o and co2 in plagioclase hosted melt inclusions sampled from pumice erupted during four vulcanian events at soufriere hills volcano montserrat to determine melt compositions prior to eruption using fourier transform infrared spectroscopy we measured values up to 6 7 wt h2o and 80 ppm co2 of 42 melt inclusions 81 cluster between 2 8 and 5 4 wt h2o 57 to 173 mpa or 2 7 km suggesting lower conduit to upper magma reservoir conditions we propose two models to explain the magmatic conditions prior to eruption in model 1 melt inclusions were trapped during crystal growth in magma that was stalled in the lower conduit to upper magma reservoir and during trapping the magma was undergoing closed system degassing with up to 1 wt free vapor this model can explain the melt inclusions with higher h2o contents since these have sampled the upper parts of the magma reservoir however the model cannot explain the melt inclusions with lower h2o because the timescale for plagioclase crystallization and melt inclusion entrapment is longer than the magma residence time in the conduit in model 2 melt inclusions were originally trapped at deeper levels of the magma chamber but then lost hydrogen by diffusion through the plagioclase host during periodic stalling of the magma in the lower conduit system in this second scenario which we favor the melt inclusions record re equilibration depths within the lower conduit to upper magma reservoir

fractures in a trachyandesitic lava at oraefajokull iceland used to infer subglacial emplacement in 1727 8 eruption abstract we present detailed field observations of cooling fractures in a small volume trachyandesitic lava informally named the slaga lava on the south west flank of oraefajokull volcano south east iceland columnar joints pseudopillow fracture systems and curved platy jointing occur in the lava whose exposed section is approximately 600 m in length and generally 2 3 m in thickness columnar jointing may occur at the base of flow lobes whereas pseudopillow fracture systems occur throughout the lava in an outer glassy fractured carapace and curved platy fractures occur in the centres of larger flow lobes pseudopillow fracture systems composed of a single master fracture and multiple subsidiary fractures formed normal to the master fracture are of two types g type pseudopillow fracture systems have very narrow striae chisel marks on their master fractures indicative of rapid cooling and brittle fracture sr type pseudopillow fracture systems display alternating smooth and rough master fracture surface textures evidence of alternating brittle and ductile fracture propagation mechanisms subsidiary fractures in both types show curved striae on their fracture surfaces which enable the determination of fracture propagation directions pseudopillow fracture systems are thought only to form in the presence of water including water caused by the melting of ice and snow the curved platy fractures display prominent river lines and may have resulted from cooling contraction post emplacement degassing flow deflation or shearing in the flow against the outer solid crust of the flow during inflation due to recent advances in the understanding of the formation mechanisms of pseudopillow fracture systems they and the other fractures present in the flow can be used to reconstruct the cooling environment the lava is inferred to have been emplaced within subglacial drainage channels incised into or beneath a thin alpine type glacier with coolant infiltrating the lava radially from all sides and ponding or draining of water at or along the lava base we link this flow to the 1727 8 eruption of oraefajokull volcano on the basis of historical records which describe lava effusion in this approximate location and striking chemical compositional similarity between 1727 tephra and the slaga lava

observations and numerical model simulation of a partialy trapped lee wave over the welsh mountains abstract a large amplitude lee wave event detected in radiosonde ascents during a field experiment in the welsh mountains is described and wave characteristics are deduced the synoptic flow pattern accompanying this wave field is used to initialize the u k meteorological office s nonhydrostatic mesoscale model with a 3 km horizontal grid length in an attempt to simulate the gravity wave response over the welsh mountains after 4 h of integration time the resulting flow held is quasi study and exhibits a fairly regular wave field with a dominant wavelength at about 22 km and nearly vertical phase lines verification is achieved by comparing rate of ascent variations with model vertical velocity variations along the trajectory of a radiosonde the agreement is very good the vertical structure equation for linear gravity waves is solved using the smoothed wind and temperature fields obtained from the sondes a leaky resonance is identified at the dominant wavelength of the model simulation limited co

detailed multidisciplinary monitoring reveals pre and co eruptive signals at nyamulagira volcano north kivu democratic republic of congo this paper presents a thorough description of nyamulagira s january 2010 volcanic eruption north kivu democratic republic of congo based on a combination of field observation and ground based and space borne data it is the first eruption in the virunga volcanic province that has been described by a combination of several modern monitoring techniques the 2010 eruption lasted 26 days and emitted 45 5 106 m3 of lava field observations divided the event into four eruptive stages delimited by major changes in effusive activity these stages are consistent with those described by pouclet 1976 for historical eruptions of nyamulagira co eruptive signals from ground deformation seismicity so2 emission and thermal flux correlate with the eruptive stages unambiguous pre eruptive ground deformation was observed 3 weeks before the lava outburst coinciding with a small but clear increase in the short period seismicity and so2 emission the 3 weeks of precursors contrasts with the only precursory signal previously recognized in the virunga volcanic province the short term increase of tremor and long period seismicity which for example were only detected less than 2 h prior to the 2010 eruption the present paper is the most detailed picture of a typical flank eruption of this volcano it provides valuable tools for re examining former mostly qualitative descriptions of historical nyamulagira eruptions that occurred during the colonial period

subglacial to emergent basaltic volcanism at hloðufell south west iceland a history of ice confinement abstract hlo ufell is a familiar 1186 m high landmark located about 80 km northeast of reykjavik and 9 km south of the langkjokull ice cap in south west iceland this is the first detailed study of this well exposed and easily accessible subglacial to emergent basaltic volcano eight coherent and eleven volcaniclastic lithofacies are described and interpreted and its evolution subdivided into four growth stages i iv on the basis of facies architecture vents for stages i ii and iv lie along the same fissure zone which trends parallel to the dominant nne ssw volcano tectonic axis of the western volcanic zone in this part of iceland but the stage iii vent lies to the north and is probably responsible for the present n s elongation of the volcano the basal stage i is dominated by subglacially erupted lava mounds and ridges which are of 240 m maximum thickness were fed from short fissures and locally display lava tubes some of the stage i lavas preserve laterally extensive flat to bulbous steep glassy surfaces that are interpreted to have formed by direct contact with surrounding ice and are termed ice contact lava confinement surfaces these surfaces preserve several distinctive structures such as lava shelves pillows that have one flat surface and mini pillow

complex circular subsidence structures in tephra deposited on large blocks of ice varða tuff cone oraefajokull iceland several broadly circular structures up to 16 m in diameter into which higher strata have sagged and locally collapsed are present in a tephra outcrop on southwest oraefajokull southern iceland the tephra was sourced in a nearby basaltic tuff cone at var a the structures have not previously been described in tuff cones and they probably formed by the melting out of large buried blocks of ice emplaced during a preceding jokulhlaup that may have been triggered by a subglacial eruption within the oraefajokull ice cap they are named ice melt subsidence structures and they are analogous to kettle holes that are commonly found in proglacial sandurs and some lahars sourced in ice clad volcanoes the internal structure is better exposed in the var a examples because of an absence of fluvial infilling and reworking and erosion of the outcrop to reveal the deeper geometry the ice melt subsidence structures at var a are a proxy for buried ice they are the only known evidence for a subglacial eruption and associated jokulhlaup that created the ice blocks the recognition of such structures elsewhere will be useful in reconstructing more complete regional volcanic histories as well as for identifying ice proximal settings during palaeoenvironmental investigations

lava and ice interaction at stratovolcanoes use of characteristic features to determine past glacial extents and future volcanic hazards structures resulting from lava and ice interaction are common at glaciated stratovolcanoes during summit eruptions at stratovolcanoes meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers eroding the ice and enlarging preexisting glacial drainages as a result eruptions in this environment have produced few catastrophic floods lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker filling the available space and producing steep sided bodies with smooth bulbous contact surfaces quenching of lava against ice or by water forms small scale features such as tensional fractures and glass as the amount of meltwater in contact with the lava increases the type and abundance of smaller scale features become similar to those produced during subglacial eruptions into meltwater lakes identification of large and small scale lava ice contact features in the field can be used to reconstruct paleoglacial extent and combined with geochronology of lavas to determine past paleoclimate an understanding of lava ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes

asynchronous little ice age glacial maximum extent in southeast iceland the little ice age lia maximum glacial extent of 13 glaciers located in se iceland was dated by lichenometry to check for intraregional variations different lichenometric approaches were applied to date maximum lia moraines and they all showed high variability between glaciers according to the extreme value theory and bayesian approach lia advances in the region occurred in or around a d 1740 1760 a d 1810 1820 and a d 1840 1880 with confidence intervals of between 8 and 21 years the dates were correlated with geomorphic characteristics of glacier tongues hypsometric and slope parameters can be considered as determining factors in the variability of glacier timing during the lia as previously observed in norway and in the alps in terms of timing results obtained in se iceland were similar to those obtained for other glacier regions around the north atlantic

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

pseudopillow fracture systems in lavas insights into cooling mechanisms and environments from lava flow fractures abstract detailed field observations of structures within the flow front of a holocene trachyandesite lava from snaefellsnes iceland are presented the lava provides exceptional three dimensional exposure of complex brittle and ductile deformation textures that record processes of lava fracture and quenching driven by external water the flow front interior is characterised by structures consisting of a large metre scale curviplanar master fracture with many smaller centimetre scale subsidiary fractures perpendicular to the master fracture such structures have previously been recognised in a range of lava compositions from basalt to dacite and called pseudopillows or pseudopillow fractures we propose the term pseudopillow fracture systems to emphasise the consistent package of different fracture types occurring together all documented occurrences of pseudopillow fracture systems are in lavas that have been inferred to interact with an aqueous coolant i e liquid water ice or snow we use fracture surface textures and their orientation in relation to flow banding to identify three distinct types of master fracture and two types of subsidiary fractures master fracture surface textures used to identify fracture mechanisms include chisel marks striae cavitation dimples river lines and rough smooth fracture surface textures these indicate both brittle and ductile fracture happening on different types of master fracture chisel marks on subsidiary fractures indicate comparative cooling rates cooling directions and isotherm orientations at the time of fracture we propose a model for pseudopillow fracture system formation taking into account all the various fracture types textures and fracture propagation mechanisms and discuss their implications for interaction mechanisms between lava flows and external coolants

surface morphology of columnar joints and its significance to mechanics and direction of joint growth columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes new observations show that each band contains a single plumose structure and thus represents an individual crack or joint segment formed during a discrete growth event analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies columnar joints form by nucleation and growth of new cracks on the edges of older cracks each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face where thermal stress is concentrated inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle ductile transition of lava outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava cracks often diverge slightly from the planes of previous cracks probably because of spatial and temporal changes in directions of local principal stresses mechanical interaction causes a diverging crack to overlap curve toward and usually intersect the previous crack behind its edge leaving a blind tip that points in the overall growth direction of the columnar joints this and other directional criteria are applied to determine joint growth patterns in several lava flows of the western united states in two tiered and multi tiered flows downward growing columnar joints usually meet upward growing joints well below the middle of the flows which indicates very rapid cooling of upper portions relative to lower portions this supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows whereas conduction is probably the dominant cooling mechanism at the bases

comparison of rev size and tensor characteristics for the electrical and hydraulic conductivities in fractured rock the representative elementary volume rev is a critically important concept in fractured rock investigations as it tells us at what scale the fractured domain can be represented by an anisotropic tensor as opposed to requiring the details of each individual fracture for modelling purposes whereas the rev size and corresponding tensor characteristics for the hydraulic conductivity k in fractured rock have been the subject of numerous previous investigations no studies to date have focused on the electrical conductivity this is despite the fact that geoelectrical measurements are arguably the most popular means of geophysically investigating fractured rock typically via azimuthal resistivity surveying where the observed electrical anisotropy is commonly used to infer hydraulic characteristics in this paper we attempt to fill this void and present a systematic numerical study of the impacts of changes in fracture network properties on the rev size and equivalent tensor characteristics for both the electrical and hydraulic conductivities we employ a combined statistical and numerical approach where the size of the rev is estimated from the conductivity variability observed across multiple stochastic fracture network realizations for various domain sizes two important differences between fluid and electric current flow in fractured media are found to lead to significant differences in the rev size and tensor characteristics for and k these are the greater importance of the matrix in the electrical case and the single power instead of cubic dependence of electric current flow upon aperture specifically the rev for the electrical conductivity will always be smaller than that for the hydraulic conductivity and the corresponding equivalent tensor will exhibit less anisotropy often with notably different principal orientations these findings are of key importance for the eventual interpretation of geoelectrical measurements in fractured rock where we conclude that extreme caution must be taken when attempting to make the link to hydraulic properties

subglacial intermediate volcanism at kerlingarfjoll iceland magma water interactions beneath thick ice the products of andesite and dacite glaciovolcanism at kerlingarfjoll are unlike others previously described in the literature three sequences of lithofacies are described and interpreted here the andesitic deposits at campsite gully are divided into massive vitriclastic lapilli tuff with intrusions fluidal clast bearing vitriclastic lapilli tuff and stratified pumice rich vitriclastic lapilli tuff at tindur andesitic eruptions produced contorted pillow fragment breccia and the clast supported lithic breccia finally pillow lava with intrusions and crudely bedded vitriclastic lapilli tuff of dacitic composition are described at haraldur abundant vitriclasts the presence of pillow lavas hackly fracturing of lava bodies and lack of oxidation of clasts demonstrate that each of these lithofacies formed in the presence of abundant water this contrasts with all other descriptions of subglacially erupted intermediate magmas which are characterised by jointed and glassy lava flows and domes with a marked scarcity of fragmental material the kerlingarfjoll sequences therefore demonstrate that it is possible for intermediate magmas to generate and interact with significant volumes of water at the base of a glacier preliminary estimates of volatile contents in glassy clasts correspond to quenching pressures equivalent to 500 m water or 550 m ice this is consistent with eruption beneath an ice sheet that was thick enough to overwhelm the underlying topography and where meltwater drainage was controlled by the morphology of the glacier surface it is argued that the drainage of water due to steep topography and or thin and fractured ice as opposed to thermodynamic considerations is the most likely explanation for the absence of evidence for significant magma water interaction in previously described instances of intermediate glaciovolcanism the apparent low viscosity of the kerlingarfjoll magmas may relate to relatively high eruption temperatures and or the inhibition of degassing of the magma due to high ambient pressures and the consequent limitation of groundmass crystallisation

determining subsurface fracture characteristics from azimuthal resistivity surveys a case study at nsawam ghana we conducted azimuthal resistivity surveys ars using the square array configuration to characterize the subsurface fractured rock mass at selected farmland sites in nsawam district ghana this study is the first of its kind in ghana and it provides useful information for future hydrological studies of the area where groundwater is suspected to be contaminated as a result of the indiscriminate use of pesticides and fertilizers by local farmers we estimate the fracture orientation fracture porosity and coefficient of anisotropy of the fractured rock mass at the selected sites from the azimuthal resistivity measurements the specific surface area is estimated from field geological mapping of outcrops high correlations exist between the specific surface area and the real and imaginary parts of the measured resistivity of the fractured rock mass fractures at localities with relatively high values of coefficient of anisotropy possess relatively high fracture porosity and relatively low specific surface area and are thus more likely to be intensely fractured and permeable results from this integrated geological and geophysical study indicate two dominant fracture directions in the study area with other minor orientations that may influence groundwater and contaminant transport the dominant orientations of the fracture systems at kitase are northwest southeast in the northern part and northeast southwest in the southern part at amanfrom the fractures are oriented northwest southeast and at nsakye they are northeast southwest these sources of information from a noninvasive geophysical method are useful in assessing the transport properties of the fractured rock mass in the study area

products of an effusive subglacial rhyolite eruption blahnukur torfajokull iceland we present field observations from blahnukur a small volume 400 m thick lithofacies associations attest to complex patterns of volcano ice interaction erosive channels at the base of the subglacial sequence are filled by both eruption derived material and subglacial till which show evidence for deposition by flowing meltwater this suggests that meltwater was able to drain away from the vent area during the eruption much of the subglacial volcanic deposits consist of conical to irregularly shaped lava lobes typically 5 10 m long set in poorly sorted breccias with an ash grade matrix a gradational lavabreccia contact at the base of lava lobes represents a fossilised fragmentation interface driven by magma water interaction as the lava flowed over poorly consolidated waterlogged debris sets of columnar joints on the upper surfaces of lobes are interpreted as ice contact features the morphology of the lobes suggests that they chilled within conically shaped subglacial cavities 2 5 m high avalanche deposits mantling the flanks of blahnukur appear to have been generated by the collapse of lava lobes and surrounding breccia a variety of deposit characteristics suggests that this occurred both prior to and after quenching of the lava lobes collapse events may have occurred when the supporting ice walls were melted back from around the cooling lava lobes and breccias much larger lava flows were emplaced in the latter stages of the eruption columnar joint patterns suggest that these flowed and chilled within subglacial cavities 20 m high and 100 200 m in length there is little evidence for magma water interaction at lava flow margins which suggests that these larger cavities were drained of meltwater as rhyolite magma rose to the base of the glacier the nature of the subglacial cavity system played an important role in governing the style of eruption and the volcanic facies generated we present evidence that the cavity system evolved during the eruption reflecting variations in both melting rate and edifice growth that are best explained by a fluctuating eruption rate

intrusion of basalt into frozen sediments and generation of coherent margined volcaniclastic dikes cmvds coherent margined volcaniclastic dikes cmvds are described for the first time from askja dyngjufjoll iceland these dikes display continuous coherent glassy margins 5 cm thick and have a variety of clastic interiors comprising vitric tephra pillow and pillow fragment bearing lapilli tuffs cmvds are interpreted to form when basaltic dikes interact with subglacial ice cemented volcaniclastic sediments formation requires chilling of dike margins drainage of the dike interior and flooding of the cavity by meltwater followed by non explosive mingling of magma clastics and water evidence for ice cementation includes minimal interaction with the host 3 mm peperite local pillowed dike margins and the preservation of continuous glassy coherent margins along the edge of a drained dike cavity cmvds should be present at other volcanic centers with ice cemented sediments including the martian cryosphere

insights on lava ice snow interactions from large scale basaltic melt experiments quantitative measurements of interactions between lava and ice snow are critical for improving our knowledge of glaciovolcanic hazards and our ability to use glaciovolcanic deposits for paleoclimate reconstructions however such measurements are rare because the eruptions tend to be dangerous and not easily accessible to address these difficulties we conducted a series of pilot experiments designed to allow close observation measurements and textural documentation of interactions between basaltic melt and ice here we report the results of the first experiments which comprised controlled pours of as much as 300 kg of basaltic melt on top of ice our experiments provide new insights on 1 estimates for rates of heat transfer through boundary layers and for ice melting 2 controls on rates of lava advance over ice snow 3 formation of lava bubbles i e limu o pele by steam from vaporization of underlying ice or water and 4 the role of within ice discontinuities to facilitate lava migration beneath and within ice the results of our experiments confirm field observations about the rates at which lava can melt snow ice the efficacy with which a boundary layer can slow melting rates and morphologies and textures indicative of direct lava ice interaction they also demonstrate that ingestion of external water by lava can create surface bubbles i e limu and large gas cavities we propose that boundary layer steam can slow heat transfer from lava to ice and present evidence for rapid isotopic exchange between water vapor and melt we also suggest new criteria for identifying ice contact features in terrestrial and martian lava flows