breakup of the newfoundland iberia rift abstract the newfoundland iberia rift is considered to be a type example of a non volcanic rift key features of the conjugate margins are transition zones tzs that lie between clearly continental crust and presumed normal penrose type oceanic crust that appears up to 150 180 km farther seaward basement ridges drilled in the iberia tz consist of exhumed serpentinized peridotite of continental affinity consistent with seismic refraction studies although the boundaries between continental crust and the tzs can be defined with relative confidence there are major questions about the position and nature of the change from rifting to normal sea floor spreading at the seaward edges of the tzs notably drilling of presumed oceanic crust in the young m series anomalies to investigate and potentially resolve these conflicts we analysed the tectonic history and deep pre cenomanian stratigraphy of the rift using seismic reflection profiles and drilling results rifting occurred in two main phases late triassic earliest jurassic and late jurassic early cretaceous the first phase formed continental rift basins without significant thinning of continental crust the second phase led to continental breakup with extension concentrated in three episodes that culminated near the end of berriasian hauterivian and aptian time the first two episodes appear to correlate with separation of continental crust in the southern and northern parts of the rift respectively suggesting that the rift opened from south to north in a two step process the third episode persisted through barremian and aptian time we suggest that during this period there was continued exhumation of subcontinental mantle lithosphere at the plate boundary and that elevated in plane tensile stress throughout the rift caused intraplate extension primarily within the exhumed mantle this rifting may have been interrupted for a time during the barremian when melt was introduced from the southern edge of the rift by plume magmatism that formed the southeast newfoundland ridge and j anomaly ridge and the conjugate madeira tore rise we propose that the rising asthenosphere breached the subcontinental mantle lithosphere in latest aptian earliest albian time initiating sea floor spreading this resulted in relaxation of in plane tensile stress i e a pulse of relative compression that caused internal plate deformation and enhanced mass wasting this aptian event produced a strong rift wide reflection that is unconformably onlapped by post rift sediments that were deposited as a stable sea floor spreading regime was established although previously considered to be a breakup unconformity associated with separation of continental crust the event instead marks the final separation of the subcontinental mantle lithosphere our analysis indicates that interpretation of tectonic events in a non volcanic rift must consider the rheology of the full thickness of the continental lithosphere in addition to spatial and temporal changes in extension that may occur from segment to segment along the rift

predicting yield strengths and effusion rates of lava domes from morphology and underlying topography abstract to investigate the effects of slope on the morphology of lava domes laboratory experiments were conducted with a peg kaolin slurry erupted into cold water on slopes ranging from 0 to 60 in each experiment the slope effusion rate and water temperature were varied producing four distinct morphologies spiny lobate platy and no crust each morphology is correlated with b and slope where b is defined as the ratio of the timescale of solidification to the timescale of advection as the underlying slope increases from 0 to 40 each morphology forms at a lower value of b than it would on a horizontal surface above 40 the transitions between morphologies become independent of slope and the platy morphology is no longer observed once a range of b values for a dome is determined based on its morphology and slope its yield strength or effusion rate can be calculated in the case of active domes observations of the effusion rate can be used to calculate yield strength while for prehistoric domes an assumption of yield strength based on composition is used to determine the effusion rate for active domes we estimate yield strengths of 0 4 7 3 10 5 pa and effusion rate ranges for prehistoric domes of 0 1 330 m 3 s 1 these values compare well with yield strengths of 1 0 2 0 10 5 pa measured in the laboratory and effusion rates of 0 1 40 m 3 s 1 observed during historically active dome eruptions

spine growth and seismogenic faulting at mt unzen japan the concluding episode of activity during the recent eruption of mt unzen october 1994 to february 1995 was characterized by incremental spine extrusion accompanied by seismicity analysis of the seismic record reveals the occurrence of two dominant long period event families associated with a repeating nondestructive source mechanism which we attribute to magma failure and fault controlled ascent we obtain constraints on the slip rate and distance of faulting events within these families that analysis is complemented by an experimental thermomechanical investigation of fault friction in mt unzen dacitic dome rock using a rotary shear apparatus at variable slip rates and normal stresses a power density threshold is found at 0 3 mw m 2 above which frictional melt forms and controls the shear resistance to slip inducing a deviation from byerlee s frictional law homogenized experimentally generated pseudotachylytes have a similar final chemistry thickness and crystal content facilitating the construction of a rheological model for particle suspensions this is compared to the viscosity constrained from the experimental data to assess the viscous control on fault dynamics the onset of frictional melt formation during spine growth is constrained to depths below 300 m for an average slip event this combination of experimental data viscosity modeling and seismic analysis offers a new description of material response during conduit plug flow and spine growth showing that volcanic pseudotachylyte may commonly form and modify fault friction during faulting of dome rock this model furthers our understanding of faulting and seismicity during lava dome formation and is applicable to other eruption modes

volcano dome dynamics at mount st helens deformation and intermittent subsidence monitored by seismicity and camera imagery pixel offsets the surface deformation field measured at volcanic domes provides insights into the effects of magmatic processes gravity and gas driven processes and the development and distribution of internal dome structures here we study short term dome deformation associated with earthquakes at mount st helens recorded by a permanent optical camera and seismic monitoring network we use digital image correlation dic to compute the displacement field between successive images and compare the results to the occurrence and characteristics of seismic events during a 6 week period of dome growth in 2006 the results reveal that dome growth at mount st helens was repeatedly interrupted by short term meter scale downward displacements at the dome surface which were associated in time with low frequency large magnitude seismic events followed by a tremor like signal the tremor was only recorded by the seismic stations closest to the dome we find a correlation between the magnitudes of the camera derived displacements and the spectral amplitudes of the associated tremor we use the dic results from two cameras and a high resolution topographic model to derive full 3 d displacement maps which reveals internal dome structures and the effect of the seismic activity on daily surface velocities we postulate that the tremor is recording the gravity driven response of the upper dome due to mechanical collapse or depressurization and fault controlled slumping our results highlight the different scales and structural expressions during growth and disintegration of lava domes and the relationships between seismic and deformation signals

the challenging retrieval of the displacement field from insar data for andesitic stratovolcanoes case study of popocatepetl and colima volcano mexico abstract despite the ability of synthetic aperture radar sar interferometry to measure ground motion with high resolution application of this remote sensing technique to monitor andesitic stratovolcanoes remains limited specific acquisition conditions characterizing andesitic stratovolcanoes mainly vegetated areas with large elevation ranges induce low signal coherence as well as strong tropospheric artefacts that result in small signal to noise ratio we propose here a way to mitigate these difficulties and improve the sar measurements we derive ground motions for two of the most active mexican stratovolcanoes popocatepetl and colima volcano from the time series of sar data acquired from december 2002 to august 2006 the sar data are processed using a method that combines both persistent scatterers and small baseline approaches stratified tropospheric delays are estimated for each interferogram using inputs from the global atmospheric model narr up to a maximum of 10 rad km these delays are validated using spectrometer data as well as the correlation between the wrapped phase and the elevation the tropospheric effect is removed from the wrapped phase in order to improve the unwrapping process on popocatepetl we observe no significant deformation the colima summit area exhibits a constant subsidence rate of more than 1 cm year centered on the summit but enhanced reaching more than 2 cm year around the 1998 lava flow we model this subsidence considering both a deflating magma source at depth and the effect of the eruptive deposits load

magma degassing buffered by vapor flow through brecciated conduit margins obsidian pyroclasts a common component of rhyolitic tephra preserve a range of volatile contents which has been used to infer syneruptive conditions of magmatic degassing here we show that the textures of obsidian pyroclasts provide information on physical mechanisms of magma flow and degassing along conduit margins obsidian clasts often contain xenoliths sheared bands of lithic powder and textures consistent with magma autobrecciation these features suggest that pyroclastic obsidian primarily forms near conduit walls where magma fragments and reanneals during ascent we use these observations to develop a degassing model for pyroclastic obsidian from the a d 1340 mono craters california eruptions we suggest that degassing was buffered by continual flux of vapor through highly permeable brecciated magma along conduit walls continuous reequilibration of magma with vapor of relatively constant composition not only explains the co 2 h 2 o and d h 2 o data from mono craters pyroclastic obsidian but also requires much lower magmatic co 2 values than the commonly accepted model of closed system degassing taken together the chemical and physical evidence suggests that magma brecciation along conduit walls aids the degassing of ascending rhyolite

crustal anatomy of a transform continental margin abstract this paper describes new deep seismic reflection results across the major transform margin southwest of the grand banks off eastern canada an interpretation of these results is presented supplemented with previous seismic refraction gravity and magnetic measurements the results show a fairly sharp transition between oceanic and continental regions at the transform margin a narrow zone about 40 km wide separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust faults in this region extend throughout the crust and there is some evidence that a major strike slip fault may extend some tens of kilometres into the upper mantle magmatism may have further modified the crust in this shear zone which is bounded on the oceanic side by a small seamount the continental crust is thinned by a factor of 3 or more in this zone which may be due to erosion or to flow in the lower crust during shearing the oceanic crust does not appear to thin significantly as the transform margin is approached crustal scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two appalachian terranes the meguma and avalon terranes this supports earlier interpretations of a large magnetic lineament in the region as marking this boundary these crustal seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins the picture they provide is different in many respects from that across rifted margins and similar in others in some cases it might be difficult to distinguish between these two classes of atlantic type margins on the basis of crustal structure

petrology and geochemistry of lava and ash erupted from volcan colima mexico during 1998 2005 abstract lava n 8 and bulk ash samples n 6 erupted between july 1999 and june 2005 were investigated to extend time series compositional and textural studies of the products erupted from volcan colima since 1869 in particular we seek to evaluate the possibility that the current activity will culminate in major explosive plinian style event similar to that in 1913 lava samples continue to show relatively heterogeneous whole rock compositions with some significant mafic spikes 1999 2001 as have prevailed since 1976 groundmass sio2 contents continue trends to lower levels that have prevailed since 1961 in the direction of the still lower groundmass sio2 contents found in 1913 scoriae importantly ash samples from investigated vulcanian style explosive eruptions in 2005 are devoid of particles with micro vesiculated groundmass textures such textures characterized the 1913 scoriae signifying expansion of in situ magmatic gas as the propellant of the 1913 eruption all magmas erupted since 1913 appear to have arrived in the upper volcanic conduit system in a degassed state the small to moderate vulcanian style explosive eruptions which have been common since 1999 16 000 events have blasted ash clouds as high as 11 km a s l and sent pyroclastic flows out to distances of 5 km these eruptions do not appear to be powered by expansion of in situ magmatic gas new small lava domes have been observed in the crater prior to many explosive eruptions these plugs of degassed lava may temporarily seal the conduit and allow the build up of magmatic gases streaming upward from below ahead of rising and degassing magma in this interpretation when gas pressure exceeds the strength of the plug seal in the upper conduit an explosive vulcanian style eruption occurs alternatively these explosive eruptions may represent interactions of hot rock and groundwater phreato magmatic

estimation of magma depth for resurgent domes an experimental approach post collapse resurgence is a phenomenon affecting many calderas attributed to a renewed magma rise the process is still poorly understood and the associated source parameters remain poorly constrained a set of experiments has been conducted to gain insight into the structural evolution of caldera resurgent domes a sand plaster mixture was chosen as an analogue for the brittle pile of volcanic rocks and silicone putty simulates the ductile behavior of the intruding magma resurgence is driven by the vertical uplift of the silicone with variable shape and depth similarity conditions are achieved through eight dimensionless numbers which are of the same order of magnitude in both nature and experiments results show that extension due to doming is in many cases accommodated by one axial graben opposite master faults of this graben intersect at depth at the junction with the rising viscous silicone the simplicity of the geometry of the whole analogue system provides equations which allow the estimation of the silicone depth from surface parameters these equations are then used in some field examples to assess the magma depth beneath natural resurgent domes

transitions between explosive and effusive eruptions of silicic magmas water rich silicic magmas may erupt explosively giving rise to massive columns of fragmented ash1 5 or effusively as viscous bubbly lavas to form lava domes6 8 complex cycles of explosive and effusive eruptions are often observed8 12 although these two eruption mechanisms have been modelled independently1 8 the conditions under which a volcano exhibits a particular style of eruption are not known and the transitions between eruption styles are poorly understood by modelling the ascent of magma along a permeable conduit we show here that for small magma chamber overpressures the style of eruption explosive or effusive is dependent on the magma flow rate we also identify a critical overpressure above which only explosive eruptions occur complex eruption sequences follow naturally for example if a shallow magmatic system is supplied with magma at an intermediate flow rate the eruption will be slow and effusive until the chamber over pressure becomes too large an explosive eruption then relieves the overpressure and an effusive eruption style resumes we suggest that monitoring of temporal variations in chamber overpressure for example by measuring ground deformation can be used to assess whether a passively effusing volcano has the potential to erupt explosively

petrology of the 1991 1995 eruption at unzen effusion pulsation and groundmass crystallization abstract effusive eruption of dacite magma 2 1 108 m3 during 1991 1995 formed a lava dome at the summit of unzen volcano japan the effusion rate was highest at the beginning 4 0 105 m3 day 4 6 m3 s and decreased roughly with time to almost zero before this pattern was repeated with a second pulse of magma supply the whole rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance the more mafic the more abundant the phenocrysts the pattern of chemical variation with time shows some difference from that of the effusion rate all phenocrysts in dacite plagioclase hornblende biotite quartz and magnetite show evidence of disequilibrium with melt although a glomerophyric aggregation of phenocrysts suggests coexistence with each other phenocrysts are isotopically heterogeneous from species to species the calculated initial melt composition was rhyodacite and was nearly constant throughout the activity in contrast the bulk phenocryst population is andesite a model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments or xenocrysts by parental rhyodacite magma it is suggested that when effusion rate was high less viscous crystal poor magma exited from the reservoir groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate the higher the effusion rate the more evolved the compositions groundmass crystallinity increased with decreasing effusion rate from 33 to 50 textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top the fe ti oxide temperature 880 780 c was low when the crystallinity was high micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit porosity of dome lavas was lower at lower effusion rates collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low frequency earthquakes phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction when effusion waned degassing and the resultant crystallization proceeded more completely so that the magma became too viscous to flow in the conduit top and behaved as a plug resulting in a temporary halt of effusion in turn groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling the plug was scavenged when rising excess pressure overcame its effective strength then the second pulse of magma supply began strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason

exogenous and endogenous growth of the unzen lava dome examined by satellite infrared image analysis abstract phase 1 of the most recent eruption of unzen volcano japan occurred between 1991 and 1993 and involved the endogenous and exogenous addition of magma to a growing summit lava dome the temporal variation between endogenous and exogenous styles of dome growth has an important bearing on the hazard posed by dome collapse and explosive events however such data are difficult to acquire and have previously only been obtained in a fully quantitative manner for the mount st helens dome in that case using detailed photogrammetry here we investigate these competing dome growth processes at unzen via analysis of their thermal emission signals present in shortwave infrared swir satellite imagery we find that both the endogenous and exogenous growth styles exhibit identifiable swir signatures whose magnitude can be related to their respective magma supply rates we find that the swir radiant area corresponding to endogenous growth is around four times larger than that corresponding to exogenous growth at the same magma discharge rate analysis of time series imagery indicates that like mount st helens unzen experienced a temporally decreasing magma discharge rate during phase 1 as time progressed and the discharge rate fell the growth style of the unzen dome changed from exogenous only through exogenous decreasing and endogenous increasing to endogenous only thus relative growth rates at the endogenous and exogenous areas of the unzen dome appear to be at least partly controlled by the overall magma discharge rate as with mount st helens the cause of this relationship was most likely the formation and strengthening of a brittle crust at the surface vent which impeded exogenous growth more effectively as time elapsed and the discharge rate fell these results suggest the similarity of the processes operating at the unzen and mount st helens domes though important differences remain in that dome growth was basically continuous at unzen whilst at mount st helens growth episodes were interspersed by significant periods of repose

identification of structural controls in an active lava dome with high resolution dems volcan de colima mexico monitoring the topography of active lava domes is critical for detecting changes that may trigger or influence collapse or explosive activity internal dome structure and conditions are more difficult to elucidate but also play vital roles here we describe the exposure following an explosion of significant scarps in the active dome at volcan de colima mexico that are interpreted as evidence of brittle failure planes and a complex internal dome morphology in the first use of automated 3d computer vision reconstruction techniques structure from motion and multi view stereo sfm mvs on an active volcanic dome we derive high resolution surface models from oblique and archive photographs taken with a consumer camera the resulting 3d models were geo referenced using features identified in a web sourced orthoimage no ground based measurements were required in december 2010 the dome 2 14 106 m3 had a flat upper surface reflecting an overall ductile emplacement regime between then and may 2011 a period of low explosivity was accompanied by a small volume loss 0 4 105 m3 and arcuate steps appeared in the dome surface suggesting the presence of localized planes of weakness the complex array of summit scarps was exposed following a significant explosion in june 2011 and is interpreted to be the surface expression of fault planes in the dome the 1 m resolution dems indicated that the region of greatest volume loss was not coincident with the assumed location of the conduit and that heterogeneity within the dome may have been important during the june explosion

using a discrete element approach to model lava dome emplacement and collapse abstract lava dome collapses can lead to explosive activity and pyroclastic flow generation which makes them one of the most deadly consequences of volcanic activity the mechanisms linked to a collapse are however still poorly understood and very few numerical models exist that investigate the actual collapse of a lava dome after emplacement we use a discrete element method implemented in the modelling software particle flow code to investigate lava dome growth but also go further to test the stability of the dome under the following conditions increased internal pressure switch in extrusion direction caused by partial cooling of the dome and extrusion of lava onto variable underlying topography we initially show the morphology development of a growing lava dome and how the rheological boundary between core and talus evolves throughout the lifetime of a dome and with varied solidus pressures through visualisation of strain accumulation within the lava dome we show superficial rockfall development due to interaction with topography whereas large deep seated failures occur when the dome is exposed to internal overpressures we find that a switch in extrusion direction promotes a transition from endogenous to exogenous dome growth and leads to lava lobe formation we demonstrate that lava dome collapse exhibits many features similar to common landslides and by investigating strain patterns within the dome we can use numerical modelling to understand features that elude field observations

structure of the demerara passive transform margin and associated sedimentary processes initial results from the iguanes cruise he iguanes cruise took place in may 2013 on the r v l atalante along the demerara passive transform margin off french guiana and surinam seismic multibeam and chirp acquisitions were made piston cores were collected for pore geochemistry and sedimentology a mooring was deployed on the sea bottom for 10 months temperature salinity turbidity and current measurements this new dataset highlights the lateral variability of the 350 km long guiana surinam transform margin due to the presence of a releasing bend between two transform segments the adjacent demerara plateau is affected by a 350 km long giant slide complex this complex initiated in cretaceous times and was regularly reactivated until recent times since the miocene contourite processes seem to be active due to the onset of the north atlantic deep water nadw bottom current a main nadw water vein flows towards se eroding slide headscarps and allowing the deposition of contourite drifts numerous depressions looking like comet tails or comet scours record this flow some of those were interpreted before the cruise as active pockmarks pore geochemistry and core analysis do not show any evidence of present day gas seepage

faulting within the mount st helens conduit and implications for volcanic earthquakes the 2004 2008 eruption of mount st helens produced seven dacite spines mantled by cataclastic fault rocks comprising an outer fault core and an inner damage zone these fault rocks provide remarkable insights into the mechanical processes that accompany extrusion of degassed magma insights that are useful in forecasting dome forming eruptions the outermost part of the fault core consists of finely comminuted fault gouge that is host to 1 to 3 mm thick layers of extremely fine grained slickenside bearing ultracataclasite interior to the fault core there is an 2 m thick damage zone composed of cataclastic breccia and sheared dacite and interior to the damage zone there is massive to flow banded dacite lava of the spine interior structures and microtextures indicate entirely brittle deformation including rock breakage tensional dilation shearing grain flow and microfaulting as well as gas and fluid migration through intergranular pores and fractures in the damage zone slickenside lineations and consistent orientations of riedel shears indicate upward shear of the extruding spines against adjacent conduit wall rocks paleomagnetic directions demagnetization paths oxide mineralogy and petrology indicate that cataclasis took place within dacite in a solidified steeply dipping volcanic conduit at temperatures above 500 c low water content of matrix glass is consistent with brittle behavior at these relatively high temperatures and the presence of tridymite indicates solidification depths of more than a million small and low frequency drumbeat earthquakes with coda magnitudes m d 2 slip patches possibly along observed extensive principal shear zones within the fault core but probably not along the smaller riedel shears although our field and structural data are compatible with stick slip models they do not rule out seismic and infrasound models that call on resonance of steam filled fractures to generate the drumbeat earthquakes we suggest that stick slip and gas release processes may be coupled and that regardless of the source mechanism the distinctive drumbeat earthquakes are proving to be an effective precursor for dome forming eruptions our data document a continuous cycle of deformation along the conduit margins beginning with episodes of fracture in the damage zone and followed by transfer of motion to the fault core we illustrate the cycle of deformation using a hypothetical cross section of the mount st helens conduit extending from the surface to the depth of magmatic solidification

influence of porosity and groundmass crystallinity on dome rock strength a case study from mt taranaki new zealand lava domes pose a significant hazard to infrastructure human lives and the environment when they collapse their stability is partly dictated by internal mechanical properties here we present a detailed investigation into the lithology and composition of a 23 rocks show low rock strength 41 mpa and dominantly ductile failure whereas lower porosity rocks 5 23 exhibit higher measured rock strengths up to 278 mpa and brittle failure groundmass crystallinity porosity and rock strength are intercorrelated high groundmass crystal content is inversely related to low porosity implying crystallisation and degassing of a slowly undercooled magma that experienced rheological stiffening under high pressures deeper within the conduit this is linked to a slow magma ascent rate and results in a lava dome with higher rock strength samples with low groundmass crystallinity are associated with higher porosity and lower rock strength and represent magma that ascended more rapidly with faster undercooling and solidification in the upper conduit at low pressures our experimental results show that the inherent strength of rocks within a growing dome may vary considerably depending on ascent emplacement rates thus significantly affecting dome stability and collapse hazards

insights into lava dome and spine extrusion using analogue sandbox experiments abstract lava dome formation is a common process at stratovolcanoes involving the shallow intrusion or extrusion of viscous lava and may lead to the rise of spines spines are protrusions observed to extrude episodically during lava dome growth yet the structural and mechanical factors controlling their formation are only partially understood here we provide new detailed insight into lava dome growth and the production of spines using a novel set of analogue experiments extruding sand plaster mixtures from a fixed diameter conduit under isothermal conditions we trace displacement and strain with photogrammetric methods for precise and detailed monitoring of the extrusion process results show initial dome growth forming a steep sided and flat topped shape through extrusion of new material leading to slumping of oversteepening slopes forming a talus spines are found to protrude at a later stage through the dome surface along discrete circular faults that originate from the conduit walls starting a cycle of spine growth and collapse as our spines only appear after prolonged extrusion we relate their appearance to the compaction and strengthening of material within the conduit we find that spine diameter height and volume are positively correlated with increasing cohesion and therefore material strength the spine diameter was also observed to be smaller or equal to the diameter of the underlying conduit as shear extrusion occurs along vertical to outward dipping fault planes for natural domes our findings imply that spine growth may be the consequence of compaction and densification via porosity loss shearing and or outgassing of conduit magma during ascent more efficient compaction will yield wider and taller spines as a result of increasing rock strength our study further highlights the relevance of analogue experiments in the study of lava domes and spines which remain one of the most hazardous and unpredictable features at dome forming volcanoes worldwide

crustal structure beneath the southern grand banks seismic refraction results and their implications a seismic refraction profile was shot on the southern grand banks using large air gun sources and an array of ocean bottom seismograph receivers a sediment column 1 2 km thick directly overlies paleozoic basement with velocity structure similar to that of the meguma zone of nova scotia the main crustal layer is 27 km thick with seismic velocity of 6 3 km s increasing to about 6 5 km s in the lowest few kilometres complexity is apparent in the crust mantle transition around 32 km depth comparison with deep multichannel reflection data suggests that the increased velocity in the lower part of the crust may be associated with a reflective zone and shows the mohorovicic discontinuity to be delineated by a well defined reflection the absence of a major lower crustal layer of intermediate velocity 7 km s is consistent with observations elsewhere in the region

properties of granular analogue model materials a community wide survey we report the material properties of 26 granular analogue materials used in 14 analogue modelling laboratories we determined physical characteristics such as bulk density grain size distribution and grain shape and performed ring shear tests to determine friction angles and cohesion and uniaxial compression tests to evaluate the compaction behaviour mean grain size of the materials varied between c 100 and 400 m analysis of grain shape factors shows that the four different classes of granular materials 14 quartz sands 5 dyed quartz sands 4 heavy mineral sands and 3 size fractions of glass beads can be broadly divided into two groups consisting of 12 angular and 14 rounded materials grain shape has an influence on friction angles with most angular materials having higher internal friction angles between c 35 and 40 than rounded materials whereas well rounded glass beads have the lowest internal friction angles between c 25 and 30 we interpret this as an effect of intergranular sliding versus rolling most angular materials have also higher basal friction angles tested for a specific foil than more rounded materials suggesting that angular grains scratch and wear the foil most materials have an internal cohesion in the order of 20 100 pa except for well rounded glass beads which show a trend towards a quasi cohesionless c 20 pa coulomb type material the uniaxial confined compression tests reveal that rounded grains generally show less compaction than angular grains we interpret this to be related to the initial packing density after sifting which is higher for rounded grains than for angular grains ring shear test data show that angular grains undergo a longer strain hardening phase than more rounded materials this might explain why analogue models consisting of angular grains accommodate deformation in a more distributed manner prior to strain localisation than models consisting of rounded grains

the association of lava dome growth with major explosive activity vei 4 domehaz a global dataset investigation of the global eruptive records of particular types of volcanoes is a fundamental and valuable method of understanding what style of activity can be anticipated in the future and can highlight what might be expected or unusual in particular settings this paper investigates the relationship between large explosions volcanic explosivity index vei 4 and lava dome growth from 1000 ad to present and develops the domehaz database domehaz contains information from 397 dome forming episodes including duration of dome growth duration of pauses in extrusion extrusion rates and the timing and magnitude vei of associated large explosions major explosive activity when associated with dome growth is more likely to occur before dome growth rather than during or at the end of dome forming eruptions in most cases where major explosive activity has been associated with dome growth the eruptions occurred at basaltic andesite to andesitic volcanoes the most common type of dome forming volcano but a greater proportion of dacitic and rhyolitic dome growth episodes were associated with large explosions high extrusion rates 10 m3 s 1 seem to be associated with large explosions and may inhibit degassing or destabilize existing domes leading to explosive decompression large explosions may alternatively be followed by dome growth which represents the clearing of residual magma from the conduit relationships extracted from the global record can be used to construct probability trees for new and ongoing dome forming eruptions or can be used in conjunction with other types of event trees to aid in forecasting volcanic hazards during a crisis especially for volcanoes where data are sparse

intrusive and extrusive growth of the mount st helens lava dome lava domes grow by a combination of endogenous intrusive and exogenous extrusive addition of magma either as single continuous events1 2 or through a series of relatively small pulses which together build a larger construct3 evaluating the hazards that attend growth of a dome requires an assessment of the processes controlling whether lava is added to its interior or surface because many of these dangers depend on how volatiles are admitted to or released from hot magma if the volatile pressure inside an inflating dome becomes high enough sudden exposure provided by flow front slumping can lead to explosive decompression and the generation of pyroclastic flows4 5 on the other hand lava erupted on the oversteepened surface of a dome may collapse fragment and mix with snow or ice to form pyroclastic flows surges and mudflows6 7 here we provide a quantitative assessment of the partitioning of magma into endogenous intrusions and exogenous lobes using high resolution digital topographic maps of the mount st helens dome derived from aerial photographs taken periodically between 1980 and 1986 endogenous exogenous and total volume production rates follow distinct trends which provide important clues about the nature of eruption mechanisms calculating endogenous and exogenous components for active domes like those recently formed at redoubt volcano may help quantify magma chamber processes and provide another tool for understanding the re equilibration of shallow magmatic systems

coupled subdaily and multiweek cycles during the lava dome eruption of soufriere hills volcano montserrat 1 observations of volcanoes extruding andesitic lava to produce lava domes often reveal cyclic behavior at soufriere hills volcano montserrat cycles with subdaily and multiweek periods have been recognized on many occasions observations clearly show that the period of subdaily cycles is modulated by the multiweek cycle the subdaily and multiweek cycles have been modeled separately as stick slip magma flow at the junction between a dyke and an overlying cylindrical conduit and as the filling and discharge of magma through the elastic walled dyke respectively here we couple these two models to describe the behavior over a period of well observed multiweek cycles with accompanying subdaily cycles from 13 may to 21 september 1997 the coupled model captures well the asymmetrical first order behavior the first 40 of the multiweek cycle consists of high rates of lava extrusion during short period high amplitude subdaily cycles as the dyke reservoir discharges itself the remainder of the cycle involves increasing pressurization as more magma is stored and extrusion rate falls followed by a gradual increase in the period of the subdaily cycles

dynamics of seismogenic volcanic extrusion at mount st helens in 2004 05 the 2004 05 eruption of mount st helens exhibited sustained near equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick slip motion along the margins of the plug as it was forced incrementally upwards by ascending solidifying gas poor magma we formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma plug system and that of a variably damped oscillator modelled stick slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption and they imply that magma pressure never deviated much from the steady equilibrium pressure we infer that the volcano was probably poised in a near eruptive equilibrium state long before the onset of the 2004 05 eruption

fracture and compaction of andesite in a volcanic edifice the failure mode of lava dilatant or compactant depends on the physical attributes of the lava primarily the porosity and pore size and the conditions under which it deforms the failure mode for edifice host rock has attendant implications for the structural stability of the edifice and the efficiency of the sidewall outgassing of the volcanic conduit in this contribution we present a systematic experimental study on the failure mode of edifice forming andesitic rocks porosity from 7 to 25 from volcan de colima mexico the experiments show that at shallow depths 1 km while low porosity 10 lava remains dilatant the failure of high porosity lava is compactant and driven by cataclastic pore collapse although inelastic compaction is typically characterised by the absence of strain localisation we observe compactive localisation features in our porous andesite lavas manifest as subplanar surfaces of collapsed pores in terms of volcano stability faulting in the upper edifice could destabilise the volcano leading to an increased risk of flank or large scale dome collapse while compactant deformation deeper in the edifice may emerge as a viable mechanism driving volcano subsidence spreading and destabilisation the failure mode influences the evolution of rock physical properties permeability measurements demonstrate that a throughgoing tensile fracture increases sample permeability i e equivalent permeability by about a factor of two and that inelastic compaction to an axial strain of 4 5 reduces sample permeability by an order of magnitude the implication of these data is that sidewall outgassing may therefore be efficient in the shallow edifice where rock can fracture but may be impeded deeper in the edifice due to compaction the explosive potential of a volcano may therefore be subject to increase over time if the progressive compaction and permeability reduction in the lower edifice cannot be offset by the formation of permeable fracture pathways in the upper edifice the mode of failure of the edifice host rock is therefore likely to be an important factor controlling lateral outgassing and thus eruption style effusive versus explosive at stratovolcanoes

influence of extrusion rate and magma rheology on the growth of lava domes insights from particle dynamics modeling abstract lava domes are structures that grow by the extrusion of viscous silicic or intermediate composition magma from a central volcanic conduit repeated cycles of growth are punctuated by collapse as the structure becomes oversized for the strength of the composite magma that rheologically stiffens and strengthens at its surface here we explore lava dome growth and failure mechanics using a two dimensional particle dynamics model the model follows the evolution of fractured lava with solidification driven by degassing induced crystallization of magma the particle dynamics model emulates the natural development of dome growth and rearrangement of the lava dome which is difficult in mesh based analyses due to mesh entanglement effects the deformable talus evolves naturally as a frictional carapace that caps a ductile magma core extrusion rate and magma rheology together with crystallization temperature and volatile content govern the distribution of strength in the composite structure this new model is calibrated against existing observational models of lava dome growth results show that the shape and extent of the ductile core and the overall structure of the lava dome are strongly controlled by the infusion rate the effects of extrusion rate on magma rheology are sensitive to material stiffness which in turn is a function of volatile content and crystallinity material stiffness and material strength are key model parameters which govern magma rheology and subsequently the morphological character of the lava dome and in turn stability degassing induced crystallization causes material stiffening and enhances material strength reflected in non newtonian magma behavior the increase in stiffness and strength of the injected magma causes a transition in the style of dome growth from endogenous expansion of a ductile core to stiffer and stronger intruding material capable of punching through the overlying material and resulting in the development of a spine or possibly inducing dome collapse simulation results mimic development of a megaspine upon the influx of fresh magma which leads to the re direction of magma flow creating a new shear zone and the switching of dome growth from one side to the other our model shows similar dome growth dynamics as observed at soufriere hills volcano montserrat indicating a strong correlation between extrusion rate and its subsequent effect on mechanical properties and variations in magma rheology

thermal uplift and erosion across the continent ocean transform boundary of the southern exmouth plateau thermal evolution of the continental lithosphere at a continent ocean transform margin is examined using a two dimensional heat conduction model all heating is assumed to result from the emplacement of new oceanic ridge against the continent the assumed initial continental temperature gradient is probably best applied to regions having experienced finite duration periods of rifting lasting tens of millions of years a deep seismic reflection profile across the southern paleo transform margin of the exmouth plateau northwest australia tests the predictions of the model using this reflection profile it is estimated that up to 3 5 km of sediments have been eroded from the continental rim diminishing to almost no erosion at 60 km from the continent ocean transform boundary this trend and these values for erosion can be matched approximately using the model under the condition of local isostasy a finite difference scheme is employed where the surface elevation is the result of the competing 1 thermal uplift up 2 surficial erosion down and 3 local isostatic rebound up in response to the erosion the model predicts that most of the erosion ceases by 40 ma after ridge emplacement and that of the order of 1000 km3 eroded sediments are shed for every 10 km of transform length

the structure of composite volcanoes unravelled by analogue modelling a review abstract volcanic edifices are unstable accumulations of eruptive and intrusive products with complex internal structures a documentation of these structures facilitates the assessing of the various hazards associated with volcanoes e g flanks instabilities and eruptive events however structures are not easily assessed in the field as volcanoes are regularly covered by eruptive products and are rapidly eroded in the case of recent and active volcanoes internal structures are generally extrapolated from the external morphology of the edifice which can be documented using digital elevation models dem and geodetic data interpretation of such data greatly benefits from analogue models which are idealised representations of reality used to test the impact that various processes may have on the shape of a volcanic edifice this short review focuses on the contribution of analogue models to our comprehension of the internal structure of composite volcanic cones the other volcanic morphologies e g caldera are excluded from this review as they have distinct structure and evolution that have already been thoroughly reviewed the reviewed models were used to investigate the impact that regional faults magma injections and ductile substratum among other processes have on the internal structure and surface morphology of many volcanic edifices these models were also used to locate unstable flanks and eruption sites

video observations of beach cusp morphodynamics beach cusps are a common feature of steep reflective and intermediate beaches although many observations of beach cusp spacing exist there are few observations quantifying the incipient formation evolution and eventual destruction of these features beach cusp morphodynamics were analyzed using a 3 year dataset of video images collected at tairua beach new zealand twenty four beach cusp episodes were selected to monitor the cycle of beach face changes from planar to the appearance of the cusp patterns and back to planar again observations show that beach cusp disappearance can be ascribed not only to the erosive influence of storms but also to the persistence of accretionary conditions leading to the infilling of beach cusp bays and the development of an alongshore continuous berm we also report observations of changes in beach cusp spacing over time which can be attributed to the merging of adjacent cusps within the cusp field with the overall cusp spacing re adjusting to accommodate the disappearance of a horn although the self organization theory provides a better fit to the data and theory we were unable to conclusively refute any of the mechanisms causing beach cusp formation since both existing theories subharmonic standing edge wave and self organization can predict the trend in the observed beach cusp spacing these observations show that initial beach cusp formation primarily occurs around 70 of the episodes analyzed under mildly accretionary conditions and when accretion persists the pattern disappearance is likely to occur as a result of bay infilling to form a featureless berm

solidifying bingham extrusions a model for the growth of silicic lava domes in a previous study of the effects of cooling and solidification on flows issuing onto a horizontal plane and spreading under gravity we considered the case of a viscous fluid that solidifies to form a thin surface crust with a finite yield strength in that case the coupling of solidification and viscous stresses in the flow led to a sequence of flow regimes or styles of flow and crustal deformation here we study the spreading from a small source of a plastic material having a yield strength before cooling in this case the fluid again begins to freeze as it spreads radially under gravity and forms a dome having a surface crust which is stronger than the extruded fluid if cooling is sufficiently rapid compared to gravity driven spreading the flow is found to be controlled by solidification the flow again takes on one of a number of flow regimes depending on the pace of solidification relative to the rate of lateral flow or extrusion rate however these flow regimes are quite different from those for the viscous extrusions implying that the internal yield stress has a strong influence on the behaviour styles of flow ranged from inflation of an axisymmetric dome to irregular extrusion of lateral lobes and vertical spines these qualitatively different regimes have much in common with the eruption styles of volcanic lava domes produced by effusion of extremely viscous silicic magmas which may possess a yield strength and the model provides information about the factors influencing the morphology and hazards of such volcanic flows

fumarole monitoring with a handheld infrared camera volcan de colima mexico 2006 2007 abstract this paper describes a methodology for the monitoring of fumarole temperatures at medium ranges 6 km using a handheld infrared camera wavelength range 8 13 5 m as a relationship between fumarole temperatures gas flux and volcanic activity has been demonstrated by a number of studies fumarole temperature data has a potential use as a monitoring tool volcan de colima is an andesitic stratovolcano with a 300 m diameter summit crater formed by the destruction of the 2004 lava dome by a series of explosions in 2005 between january 2006 and august 2007 sequences of thermal images were recorded from a viewpoint 6 km to the north during regular 24 48 hour monitoring excursions the temperatures of fumaroles on the crater rim and the ground surface on the volcano s flanks were measured a methodology was developed to remove data affected by clouds or volcanic water vapour based on rates of temperature change and scatter within the data for the remaining data it is demonstrated mathematically that at this range typical variations in atmospheric transmissivity will affect the apparent temperatures by 2 c while a 25 change in fumarole heat flux would change it by 5 10 c the mean night time apparent temperature of the fumaroles was calculated for each excursion and showed an irregular decline over the 19 month period subtracting the radiant heat flux of flank rocks from those of the fumaroles removes seasonal variations and gives the clearest view of trends in the fumarole heat flux a sharp drop in fumarole temperature during february 2007 coincided with the emergence of a lava dome in the crater the declining fumarole temperature is interpreted to reflect decreasing gas flux from the crater in line with a change in eruptive regime from frequent small ash rich explosions to slow effusion of lava

extreme frictional processes in the volcanic conduit of mount st helens usa during the 2004 2008 eruption abstract the 2004 2008 eruption of mount st helens saw the extrusion of seven high viscosity spines and formation of discrete shear zones along the conduit margin at spine 7 this shear zone consists of four structurally distinct layers the outer surface gouge l1 crosscuts a dark banded layer l2 which grades into a moderately sheared layer l3 and undeformed rock l4 inside the spine field observations porosity measurements geochemistry mineralogy microstructure crystal size and shape distribution kinetic properties and magnetic analyses chart the evolution of deformation processes and products throughout the eruption gouge formation was concomitant with characteristic microseismic drumbeats at depths 0 5 1 km in addition the seismic record shows two larger earthquakes with similar seismic signatures in august 2006 which we conclude represent larger slip amounts along the conduit margin of spine 7 extensive slip resulted in frictional heating on the order of several hundreds of degrees melting the highly viscous crystalline ascending magma plug and forming a pseudotachylyte high ambient temperatures in the conduit resulted in near equilibrium melting and slow recrystallisation thus impeding the development of signature pseudotachylyte characteristics and hindering identification thus frictional melting and recrystallisation in ascending magma plugs may be a common but unidentified phenomena at composite volcanoes worldwide

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

transform continental margins part 2 a worldwide review we present a global inventory of transform continental margins based on both a worldwide comparison of continent ocean boundary identifications with oceanic fracture zones traces and a compilation of published regional studies this inventory increases the number of identified transform margins from 29 to 78 these margins represent 16 of continental margins in cumulative length and 31 of non convergent margins we include morphological data published upper crustal sections continent to ocean transition locations and published moho shape data in the new database this review confirms that continent to ocean transitions are sharper at transform margins than at divergent margins it also emphasizes the structural diversity of transform margins associated with one third of transform margins we define marginal plateaus as a new type of relief that corresponds to a flat but deep surface inside the continental slope and that may be inherited from crustal thinning prior to transform faulting transform margin initiation appears to be favoured along propagating oceans and within cold and thick lithospheres

velocity structure of reflective lower crust beneath the grand banks of newfoundland a seismic refraction experiment using air guns and ocean bottom seismometers was carried out on the southern grand banks in order to study the velocity structure of highly reflective lower crust observed on a previous deep multichannel reflection profile the data were analyzed by interactive forward modeling of travel times and amplitudes wide angle normal moveout analysis and downward continuation of slant stacks laterally varying structure was used in the final stage of modeling the results indicate an upper crust with a p velocity of 6 0 6 2 km s a zone of high velocity gradient from about 19 to 24 km depth and a lower crustal velocity of 6 6 6 8 km s moho is at 29 km the depth of the transition zone is uncertain by 2 or 3 km and its detailed velocity structure is not well resolved however the top of the zone corresponds approximately to the onset of reflectivity while the lower crustal layer is characterized by more coherent and continuous reflections a thin 1 km localized high velocity around 7 2 km s layer is tentatively correlated with one of the reflectors in the lower crust bulk anisotropy of the lower crust is less than 5 the presence of well defined high velocity layers and the lack of anisotropy tend to support mafic intrusion as the mechanism for reflectivity in the lowest part of the crust however there is no evidence of a thick high velocity underplated layer at the base of the crust

the role of tides in beach cusp development 1 field measurements of morphology and swash flow during three episodes of beach cusp development indicate that tides modulate the height and cross shore position of beach cusps during rising tide beach cusp height decreases as embayments accrete more than horns and the cross shore extent of beach cusps decreases during falling tide beach cusp height increases as embayments erode more than horns and cross shore extent increases a numerical model for beach cusp formation based on self organization extended to include the effects of morphological smoothing seaward of the swash front and infiltration into the beach reproduces the observed spacing position and tidal modulation during rising tide water particles simulating swash infiltrate preferentially in embayments causing enhanced deposition during falling tide exfiltration of water particles combined with diversion of swash from horns causes enhanced erosion in embayments smoothing of beach morphology in the swash zone seaward of the swash front and in the shallow surf zone accounts for most of the observed tidal modulation even in the absence of infiltration and exfiltration despite the qualitative and in some cases quantitative agreement of the model and measurements the model fails to reproduce observed large deviations of horn orientation from shore normal some aspects of beach cusp shape and deviations from the basic tidal modulation possibly because of the simplified parameterization of cross shore sediment transport and the neglect of the effects of sea surface gradients on flow index terms 4546 oceanography physical nearshore processes 4255 oceanography general numerical modeling 3220 mathematical geophysics nonlinear dynamics keywords beach cusp tides infiltration exfiltration erosion

chemical and isotopic composition of fumarolic gases and the so2 flux from volcan de colima mexico between the 1994 and 1998 eruptions volcan de colima is the most active volcano in mexico including a significant effusive eruption in 1991 and a weak explosive event in 1994 geochemical monitoring of the volcano started in january 1996 with collection of fumarolic gases approximately every 3 months from two sites in the summit crater z3 800 c and z2 400 c chemical compositions of volcanic gases from the z2 fumarole showed a peak in s cl in the middle of 1997 14 15 months before the start of the 1998 lava eruption high temperature volcanic vapors from z3 changed progressively in water isotopic composition and showed a weak trend of increasing hcl hf and a peak in s cl at the same time starting from the summer of 1997 the volcanic water became gradually enriched in deuterium which probably indicated an increasing contribution from deep less degassed magma a negative correlation between d and cl content for the z3 fumarolic gases is explained by shallow degassing of the colima hot lava plug this hypothesis is consistent with published data on cl contents and solubilities in magmas and supported by the observed positive trends of s cl and hcl hf with d for the z3 fumarole until september 1998 the so2 flux from the volcano was at a low level 100 tons day and often below the detection limit of the instrument 30 tons day a sharp increase in the so2 flux was recorded 1 month before the eruption started it jumped from 400 50 tons day to 1600 tons day 2 days before new lava emerged on 20 november 1998 the so2 flux correlated well with seismicity

low cost volcano deformation monitoring optical strain measurement and application to mount st helens data summary this paper describes an innovative method of volcano deformation measurements applied to camera images taken from the 2004 2008 eruption period at mount st helens dome growth was thought to be characterized by sustained near linear rates of a solid dacite plug through spatial digital image correlation dic analysis of the camera images new evidences arise that the deformation and strain rate of the spine was more complex dic yielded cumulative and incremental displacements strain and shear planes at decimetre resolution it was found that dome extrusion rates are highly non linear decelerating prior to partial collapse followed by a pronounced dome extrusion increase and direction change associated processes have been identified through dic such as shallow landslides and reworking of talus apron material the work highlights the strengths of camera strain monitoring and illustrates that dome growth and collapse is a very dynamic process complexly interplaying with the surrounding

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

persistent summit subsidence at volcan de colima mexico 1982 1999 strong evidence against mogi deflation this paper re examines recent ground deformation measurements at volcan de colima mexico to test the hypothesis that the observed movements are in response to pressure changes within a sub volcanic magma chamber as suggested for other volcanoes by mogi 1958 earthq res inst 36 99 134 measurements of vertical ground deformation across the summit dome complex of volcan de colima from a precise levelling network between 1982 and 1999 together with vertical and horizontal displacements derived from dual frequency gps measurements in 1994 and 1997 show continuous subsidence the deformation pattern derived from the levelling shows that subsidence increases towards the summit the closest stations to the summit 1 1 km distant show a mean subsidence rate of 5 mm per year compared to the reference station at 2 3 km distance which may be subsiding itself vertical displacements of individual stations since 1982 show that the subsidence has been fairly continuous the gps stations which are distributed more widely and include some close to the edge of the active dome confirm summit subsidence the largest measured value a decrease of 280 mm or 93 mm subsidence per year was obtained at the edge of the dome horizontal displacements measured during 1994 1997 also show the largest values at the summit but these are much smaller than the vertical displacements with the maximum rate of 23 mm per year recorded close to the dome it is conclusively shown that these measured movements cannot be due to deflation of a buried mogi source as vertical and horizontal displacements for some stations are in contrary directions to those predicted by the model and there is no consistent pattern to the horizontal movements we attribute the measured deformation to downslope creep settling and compaction of the edifice gravitational spreading or a combination of these processes

shallow crustal mechanics from volumetric strain data insights from soufriere hills volcano montserrat volumetric strain data from the 29 july 2008 vulcanian explosion of the soufriere hills volcano provide an excellent opportunity to probe the mechanical properties of the volcanic edifice and the shallow crust beneath montserrat we use finite element analysis to constrain the geometry of the shallow plumbing system as well as edifice and crustal mechanical properties for mechanically plausible pressure drops associated with dilatational volumetric strains recorded during the explosive activity our results from both forward and inverse models indicate a conduit radius of 40 m and a length of 1500 m and hence much larger conduit dimensions than previously suggested in order to fit the syneruptive volumetric strain data for a conduit pressure drop of 10 mpa the conduit needs to be surrounded by a halo of mechanically compliant rocks for which the best fit models indicate a width of 600 m and a young s modulus of 1 gpa young s moduli for the main edifice and the shallow crust up to 1000 m below sea level are found to be 10 gpa our best fit inverse model predicts a syneruptive radial conduit contraction by 0 24 m with a corresponding volume loss of 0 1 mm3 implying only partial emptying of the conduit upon the explosion this study demonstrates the critical role of edifice and shallow crustal mechanics for strain partitioning on montserrat our findings may have implications for the assessment of conduit processes at other dome building volcanoes

morphodynamics of nearshore rhythmic sandbars in a mixed energy environment sw france i mapping beach changes using visible satellite imagery abstract this paper presents a new method to analyze the morphology and migration of shallow water sandbanks based on the retrieval of maps from high resolution spot satellite imagery this approach was applied to the study of intertidal ridge and runnel systems and subtidal crescents that border the southwest coast of france maps were obtained from 16 spot images recorded between 1986 and 2000 ridge and runnel shapes with regard to a reference level were delineated using a watercolor reflectance code parameterized and validated with field data crescent plan shapes which appear on the images due to water transparency or breaking induced foam were directly extracted the spatial maps show that in conformity with field surveys the mean alongshore spacing of intertidal systems and crescents range from 370 146 m variability is indicated by standard deviation to 462 188 m and from 579 200 to 818 214 m respectively several couples of images also show that ridge and runnel systems and crescents move in the longshore drift direction southward by about 2 4 3 1 and 1 m day 1 respectively alongshore migration rates of intertidal systems are confirmed by field surveys whilst crescent dynamics cannot be validated because there is no in situ data available to complete these measurements an analysis of the influence of wave climate on both the shape and movements of these rhythmic sedimentary patterns is proposed in a companion paper

self organization mechanisms for the formation of nearshore crescentic and transverse sand bars the formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow water equations and on a simplified sediment transport parameterization by assuming normally approaching waves and a saturated surf zone rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed two types of bedforms appear one is a crescentic bar pattern centred around the breakpoint and the other herein modelled for the first time is a transverse bar pattern the feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations basically the waves stir up the sediment and keep it in suspension with a certain cross shore distribution of depth averaged concentration then a current flowing with against the gradient of sediment concentration produces erosion deposition it is shown that inside the surf zone these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro morphodynamic input conditions to which of the patterns develops it is suggested that crescentic bar growth would be favoured by high energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment in intermediate conditions mixed patterns may occur

internal fabric development in complex lava domes abstract viscous lava extrusions were modeled using plaster of paris with admixed magnetite dust which served as a tracer of the internal anisotropy of magnetic susceptibility fabric in model lava domes used analogue material showed pseudoplastic behavior and yield strength level proportional to increasing mixing ratio of plaster powder and water a series of models ranging from simple gravity flows to complex lava domes showing combined endogenous and exogenous growth were created by intrusion of plaster into a sandbox the similarity of model bodies is compared with natural lava domes on the basis of dynamic scaling analysis growth dynamics exogenous growth and internal fabric development in natural lava domes is critically discussed using the experimental results

a deep seismic transect from hovgard ridge to northwestern svalbard across the continental ocean transition a sheared margin study new seismic refraction data were collected across the western svalbard continental marginoff kongsfjorden ny lesund during the cruise leg ark15 2 of rv polarstern the use ofon and offshore seismic receivers and a dense airgun shot pattern provide a detailed viewof the velocity structure of svalbardos continental interior the continent ocean transition and oceanic crust related to the northern knipovich ridge and the molloy ridge the caledonian central and western terranes of svalbard are not distinguishable on the basisof seismic velocity structure below a 7 to 8 km thick paleozoic sedimentary cover the crystallinecrust reveals a three layer structure with seismic velocities ranging between 6 1 and 6 9 km s the geological suture between the terranes is imperceptible the middle and upper crust belowthe tertiary forlandsundet graben shows striking low velocities this can be related to theearly paleozoic convergent transpressive movements between svalbard and northern greenland followed by an extensional relaxing phase we argue that a brittle fractured rock formation ispresent below the graben which also buries a sedimentary paleozoic core the continent ocean transition can be classified as a sheared margin formed at the continentalpart of the spitsbergen fracture zone moderate crustal thinning is achieved only to the westof the low velocity zone below the forlandsundet graben this leads to the assumption thattranstensional rift movements since oligocene were decoupled from the central terraneof svalbard the moho dips with an angle of 45 eastwards at the continent ocean transitionthat exhibits higher seismic velocities of 7 2 km s on the continental side these can beinterpreted as minor mantle derived intrusions probably induced by convection due to thejuxtaposition of cool continental and hot oceanic lithosphere the oceanic crust generated at the knipovich ridge and the molloy ridge is thin 2 to 4 km compared to the global mean and is characterised by the absence of oceanic layer 3 theseobservations can be ascribed to conductive cooling of the ascending mantle due to theextremely low divergence rate and the neighbouring cool continental crust the underlyingmantle is slightly serpentinized below the knipovich ridge segment reflected by low seismicvelocities of 7 7 km s a thicker sequence of syn and post rift sediments and sedimentaryrocks are observed on the molloy ridge oceanic segment which results from greater subsidencerelative to the knipovich ridge segment

stretching and subsidence rifting of conjugate margins in the north atlantic region subsidence on rifted conjugate continental margins around the north atlantic is analyzed to derive the amount and areal distribution of stretching in the crust and in the lower lithosphere during continental rifting study areas are the grand banks and orphan basin regions of the eastern canadian continental margin and the goban spur and galicia bank regions off western europe in all areas maps of synrift and postrift sediment thickness and bathymetry were used to derive maps of post and synrift subsidence a two layer lithospheric stretching model with independent amounts of stretching in the crust and in the lower lithosphere was assumed to be applicable with the rifting history approximated by several instantaneous episodes of extension this model was used to derive estimates of stretching at all points on a 0 05 geographical grid where subsidence values were available within the study regions the models are constrained with seismic measurements of crustal thickness the results imply that pure shear stretching predominates at a lithospheric scale while simple shear is more localized laterally and confined to the crust in places there is significant decoupling between crustal and mantle stretching near the continent ocean boundary final continental breakup may be localized on one side of the rift between conjugate margin pairs rather than symmetrically located total extension of the margins is compatible with that estimated from normal fault geometries and indicates that the continent ocean boundary has been extended up to 350 km seaward of its original position which should be considered in plate kinematic reconstructions

test of edge wave forcing during formation of rhythmic beach morphology 1 this paper reports on concurrent measurements of evolving rhythmic beach morphology and the associated water motion on a macrotidal coarse sandy beach beach cusps with a spacing of 19 m developed over a 3 day period of low energy swell after which they were destroyed by energetic wind waves the beach cusps were clearly accretionary features and developed mainly during the rising tide detailed hydrodynamic measurements were conducted at three locations in the swash and inner surf zone in the vicinity of a developing cusp horn and revealed that energetic standing mode zero subharmonic edge waves were not present during the initiation of the beach cusps specifically analysis of the hydrodynamic data indicated that 1 the amount of subharmonic energy was limited 10 of the total energy 2 none of the computed energy spectra exhibited a significant peak at the subharmonic frequency 3 the cross shore velocity amplitude at the subharmonic frequency was 2 3 times larger than the longshore velocity amplitude 4 the longshore current motion at the subharmonic frequency was barely coherent between adjacent locations in the surf zone and 5 the time series of pressure and cross shore current were in phase at either side of the developing cusp horn it is concluded that at least during this event of beach cusp formation energetic standing edge waves are not a prerequisite to initiate the development of rhythmic beach morphology

stratovolcano growth by co eruptive intrusion the 2008 eruption of tungurahua ecuador 1 volcanic edifices are constructed by a combination of erupted material and internal growth we use the l band satellite alos to produce insar measurements at tungurahua volcano ecuador we find a maximum of 17 5 cm of uplift on the upper western flank between december 2007 and march 2008 coincident with an eruption in february 2008 the deformation can be modeled using an ellipsoidal or sill like source within the edifice the models require an elongated aspect ratio with a length of 4 6 km the intruded volume of 1 2 106 m3 is roughly equivalent to the bulk erupted volume of 1 5 106 m3 and together the values are roughly equal to the long term magma flux the question remains whether this uplift is permanent thus contributing to the internal growth of the edifice or temporary in which case the magma will be released in a future eruption

pathways for degassing during the lava dome eruption of mount st helens 2004 2008 the ability of volatiles to escape rising magma regulates the explosivity of a volcanic system during silicic lava dome eruptions strain localization at the conduit margin occurs during magma ascent creating a damage halo with implications for gas escape here we report the first systematic study of permeability network anisotropy across the marginal shear zone of the a d 2004 2008 lava dome at mount st helens washington state usa the results show increasingly large permeability anisotropy of as much as four orders of magnitude over 4 m moving from the interior of the spine through the damage halo we find the permeability to be essentially isotropic in the spine interior but highly anisotropic in the damage zone and fault core our examination of the dome rocks reveals that the permeability anisotropy depends strongly on the presence of vertically oriented shear layers here we show that the rate of escape of volatiles will be several orders of magnitude higher vertically through a conduit margin shear zone than horizontally into the conduit wall

morphological and structural changes at the merapi lava dome monitored in 2012 15 using unmanned aerial vehicles uavs abstract dome building volcanoes undergo rapid and profound topographic changes that are important to quantify for the purposes of hazard assessment however as hazardous lava domes often develop on high altitude volcanoes that exhibit steep sided topography it is challenging to obtain direct field access and thus to analyze these morphological and structural changes merapi volcano in indonesia is a type example of such a volcano as soon after its 2010 eruption a new lava dome developed this dome was partially destroyed during six distinct steam driven explosions that occurred between 2012 and 2014 here we investigate the topographic and structural changes associated with these six steam driven explosions by comparing close range photogrammetric data obtained before and after these explosions to accomplish this we performed two uav campaigns in 2012 and 2015 by applying the structure from motion sfm technique we are able to construct three dimensional point clouds assess their quality by comparing them to a terrestrial laser scanning tls dataset and generate high resolution digital elevation models dems and photomosaics the comparison of these two dems and photomosaics reveals changes in topography and the appearance of fractures in the 2012 dataset we find a dense fracture network striking to the nnw sse in the post eruptive 2015 dataset we see that this nnw sse fracture trend is much more strongly expressed we also detect the formation of aligned and elongated explosion craters which are associated with the removal of over 200 000 m3 of dome material most of which 70 was deposited outside the crater region therefore this study suggests that the locations of the steam driven explosions at merapi volcano were controlled by the reactivation of preexisting structures moreover some of the newly developed and reactivated fractures delineate a block on the southern slope of the dome which could become structurally unstable and potentially lead to rock avalanche hazards this study therefore demonstrates the significance of characterizing structural fingerprints during the development of lava domes and exemplifies the value of topographic and fracture mapping which is becoming increasingly feasible when using uavs even on high and steep stratovolcanoes

imaging the 2013 explosive crater excavation and new dome formation at volcan de colima with terrasar x time lapse cameras and modelling abstract the summit region of steep volcanoes hosting lava domes often displays rapid geomorphologic and structural changes which are important for monitoring the source region of hazards explosive crater excavation is often followed by new lava dome growth which is one of the most dynamic morphometric changes that may occur at volcanoes however details of these crater formations and the ensuing new dome growth remain poorly studied a common problem is the lack of observational data due to hazardous field access and the limited resolution of satellite remote sensing techniques this paper describes the destructive constructive crater activity at volcan de colima mexico which occurred between january and march 2013 the crater geometry and early dome formation were observed through a combination of high resolution terrasar x spotmode satellite radar images and permanently installed monitoring cameras this combined time lapse imagery was used to identify ring shaped gas emissions prior to the explosion and to distinguish between the sequential explosion and crater excavation stages which were followed by dome growth by means of particle image velocimetry the digital flow field is computed from consecutive camera images showing that vertical dome growth is dominant at the beginning the upward growth is found to grade into spreading and a lateral growth domain after approximately two months of gradually filling the excavated craters with new magma the dome overflows the western margin of the crater and develops into a flow that produces block and ash flow hazards we discuss and compare the observations to discrete element models allowing us to mimic the vertical and lateral growth history of the dome and to estimate the maximum strength of the bulk rock mass moreover our results allow a discussion on the controls of a critical dome height that may be reached prior to its gravitational spreading

concepts in gravel beach dynamics abstract the dominant processes in gravel beach dynamics are reviewed highlighting some common themes which unify the various components of the gravel beach system the repercussions of which impart on how gravel beach dynamics might be understood conceptually in particular gravel beach dynamics are thought to be highly dependent on the temporal and spatial variation in grain size and the continual adjustments made by an active beach step both of which act not only as the expression of changing morphodynamic conditions but also as a controlling influence morphodynamics the notion that the exchanges on beaches between the hydrodynamics sediment transport and morphological change takes the form of reciprocal relationships which are mediated through feedback mechanisms in such a way that they cannot be thought of or studied independently is not a new one yet it appears that for the gravel beach morphodynamics must be re defined to describe conditions where variations in sediment size are thought to deserve parity rather than as merely a sequent entity or boundary condition morpho sedimentary dynamics is a phrase coined to intuit such cause and effect detailing the co evolution of morphology hydro hydraulics and sediment properties whilst acknowledging causative pluralism feedbacks and multiplier effects this is the recommended conceptual framework within which to crystallise thought and organise further research for the gravel beach essentially it increases the minimum number of parameters needed to describe the state of the gravel beach as a physical system therefore it is advised that simplicity will be most expedient in our future modelling efforts if complexity is to be adequately encapsulated

observations of nonlinear runup patterns on plane and rhythmic beach morphology 1 application of nonlinear forecasting and bispectral analysis to video observations of runup over cuspate topography shows that these alongshore patterns in the morphology are accompanied by changes to the fundamental behavior of the runup time series nonlinear forecasting indicates that at beach cusp horns the behavior of swash flow is more predictable and global meaning that characteristics of individual swash events are well represented by the behavior of the time series as a whole conversely at beach cusp bays the behavior of swash flow is less predictable and more local meaning that the characteristics of individual swash events are best represented by the behavior of a small fraction of the time series bispectral analysis indicates that there is a nonlinear transfer of energy from the incident wave frequency f to the infragravity frequency f 2 which only occurs in the bay suggesting that the local behavior is caused by interactions between successive swash cycles which are magnified by channeling caused by the beach cusp geometry the local behavior and the bispectral signatures are not present in offshore measurements and they are not present in runup time series collected when the beach was planar these results provide evidence that interactions between successive runups are a fundamental characteristic of beach cusp bays ultimately these interactions could lead to the growth of an infragravity wave with an alongshore wavelength forced by the presence of beach cusps

experiments on internal strain in lava dome cross sections simple experiments have been conducted to study the strain evolution in lava dome cross sections a viscous fluid is injected vertically from a reservoir into a feeding conduit silicone putty is used as analogue magma two dimensional experiments allow the assessment of the internal strain within the dome particle paths are symmetrical on either side of a central line passing through the feeding conduit and display parabolic trajectories the highest strain zone is located above the extrusion zone in cross sections stretch trajectories show a remarkable concentric pattern wrapping around the extrusion zone of the analogue magma to the lateral margins a triple junction of stretch trajectories defines an isotropic point in the strain field in the main central part of the dome an intermediate zone of reversed sense of shearing is caused by a change in the sign of the velocity gradient with respect to that in the upper and lower zones knowledge of this evolving strain pattern can provide a better understanding of the evolution of natural domes also it can help to unravel the kinematic history of ancient domes partly removed by erosion

insights into the evolution of the yenkahe resurgent dome siwi caldera tanna island vanuatu inferred from aerial high resolution photogrammetry the yenkahe dome tanna island vanuatu is one of the most spectacular examples of presently active post caldera resurgence exhibiting a very high uplift rate over the past 1000 years 156 mm year on average although numerous inhabited areas are scattered around the dome the dynamics of this structure and associated hazards remain poorly studied because of its remote location and dense vegetation cover a high resolution photogrammetric campaign was carried out in november 2011 over the dome georeferenced photographs were treated by structure from motion and multiple view stereophotogrammetry methods to produce a 3d digital surface model dsm of the area and its associated orthophotograph this dsm is much more accurate than previously available srtm and aster digital elevation models dems particularly at minimal coastline and maximal altitudes yasur culmination point 390 m while previous mapping relied mostly on low resolution dems and satellite images the high precision of the dsm allows for a detailed structural analysis of the yenkahe dome notably based on the quantification of fault displacements the new structural map inferred from the 3d reconstruction and morphological analysis of the dome reveals a complex pattern of faults and destabilization scars reflecting a succession of constructive and destructive events numerous landslide scars directed toward the sea highlight the probable occurrence of a tsunami event affecting the south eastern coast of tanna simulations of landslide triggered tsunamis show the short time propagation of such a wave 1 2 min which could affect coastal localities even following relatively small destabilized volumes a few million cubic meters

seismotectonic deformation of the volcanic edifice prior to the 1998 lava eruption of volcan de colima mexico the andesitic stratovolcano volcan de colima is one of the most active volcanoes in mexico the recent eruption of volcan de colima began in november 1998 and was preceded by a 12 month period of seismic activity that included five earthquake swarms about 600 events with magnitudes from 0 5 to 2 7 were located within a 50 km2 area extending northward from the crater of volcan de colima to the pleistocene volcano nevado de colima the majority of hypocenters within this area did not exceed 5 km depth below sea level we investigated earthquake focal mechanisms and seismotectonic deformations of the volcanic edifice focal mechanisms during four earthquake swarms indicated normal faulting associated with extensional processes which is in agreement with the general stress regime near the volcano revealed by field measurements of fault slips earthquakes in the fifth swarm had focal mechanisms associated with inverse faulting showing a significant change in the stress situation just before the beginning of the eruption the calculated deformations varied from 1 3 10 11 to 2 7 10 9 the first swarm of november december 1997 resulted in a n s horizontal elongation that was two times greater than the e w horizontal shortening the volume was also subject to vertical shortening the second and third swarms observed in march and may 1998 showed uniform horizontal n s and e w elongations accompanied by a vertical elongation of the volume in june july 1998 the situation of november december 1997 was repeated with n s horizontal elongation greater than the e w horizontal shortening of the volume accompanied by intensive vertical shortening during the last swarm of october november 1998 slight e w elongation of the seismic volume was accompanied by strong n s shortening and very slight vertical shortening we assume that the seismic activity prior to the 1998 eruption of volcan de colima developed along two intersecting tectonic structures the n s trending colima rift and the e w trending system of faults associated with tamazula fault during the first stage november 1997 july 1998 the passageway for magma was developed along the tamazula fault system under horizontal extension without any surface manifestation in october november 1998 the seismic events began to cluster along the colima rift structures under predominantly compressional stresses this condition culminated with the extrusion of andesitic block lava from the summit crater

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

the transition from endogenous to exogenous growth of lava domes with the development of shear bands the transition from an endogenous to an exogenous regime of lava dome growth must be achieved by the formation of discontinuities within the dome such transitions and vice versa are an important characteristic of most long lived lava domes and often coincide with significant changes in the dynamics of magma supply and lava dome collapse events for the purpose of this paper following recent experimental and observational evidence we assume that such a transition occurs when shear bands are generated a model for the formation of shear bands and therefore the growth transition within a dome and coupled conduit domain is presented shear bands are most likely to initiate at the junction of the conduit and base of the dome where the shear stress experienced between new lava entering the dome and existing lava is greatest stress accumulation within the shear bands is likely to lead to brittle shear resulting in the formation of fractures finite element modelling of lava flow shows that such shear bands only develop for certain extrusion rates and lava viscosities similarly the growth regime of the lava dome will depend upon the extrusion rate and viscosity within the conduit which is largely controlled by volatile loss and the growth of crystals in the upper part of the conduit we consider a simplified rheology during lava dome growth considering isothermal conditions with crystal growth the development of shear bands in the conduit is explored with a numerical model parameterized with values appropriate for soufriere hills volcano montserrat during october to december 1996 this lava dome forming eruption experienced a transition from endogenous to exogenous growth as it grew in height by about 90 m modelling indicates that the observed fall in magma extrusion rate from about 2 0 m3s 1 to 0 5 m3s 1 as a result of the increased pressure head from the dome and the evolution in viscosity could have subsequently changed the dome growth regime due to the development of shear bands our models provide insight into the shear stress fields possible within the conduit and the shear stresses required for shear band development

evolution of the mechanics of the 2004 2008 mt st helens lava dome with time and temperature the 2004 2008 eruption of mount st helens washington usa formed a typical example of a pelean spiny lava dome with solid spines of crystalline silicic lava extruded along shear zones bounded by fault gouge cashman et al 2008 iverson et al 2006 creation of and movement along shear zones in this erupting lava recorded as shallow micro earthquakes were thus key controls on eruption style and rate in addition to dome stability as eruption style and lava dome stability often change within individual eruptions it is important to identify how the strength of dome rocks changes with temperature time texture and extrusion rate however critical values of the shear strength of dome lava at eruptive temperatures have never been measured we have found in controlled laboratory experiments simulating eruption conditions that dome lava strength increased during the course of the 2004 2008 eruption of mount st helens as the textures became more crystalline and less porous increasing lava strength during the eruption would lead to deformation becoming increasingly localised to shear zones explaining why the extrusion style and centre were so consistent acoustic emissions and microstructure analysis indicate that fracturing was more localised at higher temperatures resulting in higher strengths at eruptive temperature compared to ambient temperatures for later erupted samples the strength of these later erupted samples at eruptive temperatures increased markedly with strain rate this would damp any increases in effusion rate that may result from changes in magma pressure explaining the steady effusion rate at mount st helens from 2004 to 2008

investigation of a self organization model for beach cusp formation and development a recent numerical investigation of self organization werner and fink 1993 suggests that the feedback process between currents and sediment response can result in self organized patterns and can be used to predict beach cusp formation and spacing a similar model based on self organization is tested here in order to understand the processes occurring during beach cusp formation and development to evaluate the sensitivity toward the parameters used and to examine how the model might relate to field observations results obtained confirm the validity of the self organization approach and its capacity to predict beach cusp spacing with values in fair agreement with the available field measurements with most of the input parameters primarily affecting the rate of the process rather than the final spacing however changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior cusp spacing tends to change with time and cusp regularity shows large long term variations cusps are found to be accretionary in the swash zone and in agreement with most observations mean flows are horn divergent over developed topography simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed results indicate that preexisting large amplitude cusps are destroyed if their spacing is substantially different from that expected under self organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach these findings support the hypothesis that self organization is a robust mechanism for beach cusp formation

petrological architecture of a magmatic shear zone a multidisciplinary investigation of strain localisation during magma ascent at unzen volcano japan shearing of magma during ascent can promote strain localisation near the conduit margins anymechanical and thermal discontinuities associated with such events may alter the chemical physicaland rheological stability of the magma and thus its propensity to erupt lava spines can record suchprocesses preserving a range of macroscopic and microscopic deformation textures attributed toshearing and friction as magma ascends through the viscous brittle transition here we use a multi disciplinary approach combining petrology microstructures crystallography magnetics and experi mentation to assess the evidence role and extent of shearing across a marginal shear zone of the1994 1995 lava spine at unzen volcano japan our results show that crystals can effectively moni tor stress conditions during magma ascent with viscous remobilisation crystal plasticity and com minution all systematically increasing towards the spine margin accompanying this we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex tural variations across the shear zone from symplectitic to granular rims towards the spine margin in addition the compaction of pores chemical and textural alteration of interstitial glass and mag netic variations all change systematically with shear intensity the strong correlation between thedegree of shearing crystal deformation and disequilibrium features together with distinct magneticproperties implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent this was accompanied by late stage or post emplacement fluid andgas induced alteration of the gouge as well as oxidation and glass devitrification understandingand recognising evidence for strain localisation during magma ascent may therefore be vital whenassessing factors that regulate the style of volcanic eruptions which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava domes

reconstructing magma failure and the degassing network of dome building eruptions volcanic eruptions are regulated by the rheology of magmas and their ability to degas both detail the evolution of stresses within ascending subvolcanic magma but as magma is forced through the ductile brittle transition new pathways emerge as cracks nucleate propagate and coalesce constructing a permeable network current analyses of magma dynamics center on models of the glass transition neglecting important aspects such as incremental strain accommodation and the key monitoring tool of seismicity here in a combined methods study we report the first high resolution 20 m neutron computed tomography and microseismic monitoring of magma failure under controlled experimental conditions the data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain to failure and importantly the geometry and efficiency of the permeable fracture network that regulates degassing events extrapolation of our findings yields magma ascent via strain localization along conduit margins thereby providing an explanation for gas and ash explosions along arcuate fractures at active lava domes we conclude that a coupled deformation seismicity analysis holds a derivation of fracture mechanisms and network and thus holds potential application in forecasting technologies

morphometric and sediment analysis of beach cusp in correlation to rip currents a case study from tropical coast west coast of india the presence of rhythmic beach cusps in foreshore regions are of persistent research interest to coastal researchers engineers with respect to their formation in beaches morphometric and sediment analysis were used to study beach cusp morphology at vengurla beach a gentle slope beach characterized by medium sand the cusp dimensions such as spacing elevation amplitude and depth were measured for detailed analysis the distinctive sediment characteristics between the horn and the bays of the cusp are different and cusp measurements show a high level of irregularity horns and bays have medium sand but sorting is varying sorting variations clearly demonstrated the difference in energy condition between horns and bay negative skewness of few horn samples has been correlated with erosional trend based on the grain size analysis result it was observed that vengurla beach cusps follow the regular grain size distribution pattern additionally the low intensity rip currents were also noticed in the study area the rate of occurrence and strength of rip currents are decided by the nearshore bathymetry and play an imperative role in rip current formation the present study is based upon the field measurements along vengurla coast and contributes to morphometric analysis of beach cusps regarding beach cusps there are no documented databases along the indian coast understanding the beach cusp formation and its seasonal changes along the beaches will be useful for effective coastal management

from rock to magma and back again the evolution of temperature and deformation mechanism in conduit margin zones abstract explosive silicic volcanism is driven by gas overpressure in systems that are inefficient at outgassing the zone at the margin of a volcanic conduit thought to play an important role in the outgassing of magma and therefore pore pressure changes and explosivity is the boundary through which heat is exchanged from the hot magma to the colder country rock using a simple heat transfer model we first show that the isotherm for the glass transition temperature whereat the glass within the groundmass transitions from a glass to an undercooled liquid moves into the country rock when the magma within the conduit can stay hot or into the conduit when the magma is quasi stagnant and cools on the centimetric scale over days to months we then explore the influence of a migrating viscous boundary on compactive deformation micromechanisms in the conduit margin zone using high pressure effective pressure of 40 mpa high temperature up to 800 c triaxial deformation experiments on porous andesite our experiments show that the micromechanism facilitating compaction in andesite is localised cataclastic pore collapse at all temperatures below the glass transition of the amorphous groundmass glass t g i e rock in this regime porosity is only reduced within the bands of crushed pores the porosity outside the bands remains unchanged further the strength of andesite is a positive function of temperature below the threshold t g due to thermal expansion driven microcrack closure the micromechanism driving compaction above t g i e magma is the distributed viscous flow of the melt phase in this regime porosity loss is distributed and is accommodated by the widespread flattening and closure of pores we find that viscous flow is much more efficient at reducing porosity than cataclastic pore collapse and that it requires stresses much lower than those required to form bands of crushed pores our study therefore highlights that temperature excursions can result in a change in deformation micromechanism that drastically alters the mechanical and hydraulic properties of the material within the conduit margin zone with possible implications for pore pressure augmentation and explosive behaviour

hot pressing in conduit faults during lava dome extrusion insights from mount st helens 2004 2008 abstract rhyodacitic volcanoes such as mount st helens msh soufriere hills mount unzen and mount pelee erupt spines mantled by layers of magma derived cataclasite and fault gouge msh produced seven lava spines from 2004 2008 composed of low porosity compositionally uniform crystalline dacite dome extrusion was attended by continuous drumbeat seismicity derived from faulting along the conduit margin at 0 5 1 km depth and evidenced by the enveloping gouge layers we describe the properties of the gouge derived fault rocks including laboratory measurements of porosity and permeability the gouge varies from unconsolidated powder to lithified low porosity low permeability fault rocks we reconstruct the subsurface ascent of the msh magma using published field observations and create a model that reconciles the diverse properties of the gouge with conditions in the conduit during ascent i e velocity temperature we show lithification of the gouge to be driven by hot pressing processes wherein the combination of elevated temperature confining pressure and dwell time cause densification and solid state sintering of the comminuted crystal rich glass poor gouge the degree of gouge lithification corresponds with residence time in the conduit such that well lithified materials reflect extended times in the subsurface due to slower ascent rates with this insight we suggest that gouge competence can be used as a first order estimate of lava ascent rates furthermore we posit gouge lithification which reduces porosity and permeability inhibits volcanic outgassing thereby increasing the potential for explosive events at spine producing volcanoes

a new scenario for the last magmatic eruption of la soufriere of guadeloupe lesser antilles in 1530 a d evidence from stratigraphy radiocarbon dating and magmatic evolution of erupted products la soufriere of guadeloupe is a dangerous volcano characterized over the last decade by moderate seismic and fumarolic unrest in the last 15 000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank collapse events sometimes associated with a magmatic eruption we propose a new age of 1530 a d and a new eruptive scenario for the last magmatic eruption on the basis of a novel statistical analysis of radiocarbon age dates and new field and geochemical data this eruption is the only magmatic eruption likely to have occurred in guadeloupe during the last 1400 years the eruption mainly involved an andesitic magma which in the first phase of the eruption partially mixed with a slightly more differentiated magma stored in a small and shallow magma chamber ascent of magma to the surface generated a partial collapse of the hydrothermally altered edifice that increased the magma discharge and led to a sub plinian phase with scoria fallout and column collapse pyroclastic flows followed by near vent pyroclastic scoria fountains the eruption ended with growth of a lava dome our revised interpretation of the last magmatic eruption of la soufriere constitutes the most likely key to a future magmatic eruption scenario for this volcano which displays strong evidence of unrest since 1992

effects of lithospheric flow on the formation and evolution of a transform margin a simple model is developed for the processes occurring at a transform continental margin and its predictions are compared with observation the model emphasises the importance of ductile flow in the lithosphere near the transform at initiation within continental lithosphere the upper part of the crust is assumed to be brittle while the lower crust and mantle behave as a newtonian fluid shearing along the transform induces a flow parallel to the direction of motion with the approach and passage of the abutting spreading centre flow beneath the continental side of the transform slows and stops this results in thinning of the lower crust and lithosphere as material is dragged away by the moving oceanic plate the thinning produces an initial thermal uplift combined with isostatic subsidence followed by thermal relaxation and subsidence at and near the margin the model is developed for uniform viscosity and for viscosity that decays exponentially with depth the results are found to be generally consistent with the observed structure at transform margins in particular at the southwest newfoundland transform margin it is suggested that the newfoundland ridge and analogous features may be formed partly from continental crust emplaced by this mechanism

test of self organization in beach cusp formation 1 field observations of swash flow patterns and morphology change are consistent with the hypothesis that beach cusps form by self organization wherein positive feedback between swash flow and developing morphology causes initial development of the pattern and negative feedback owing to circulation of flow within beach cusp bays causes pattern stabilization the self organization hypothesis is tested using measurements from three experiments on a barrier island beach in north carolina beach cusps developed after the beach was smoothed by a storm and after existing beach cusps were smoothed by a bulldozer swash front motions were recorded on video during daylight hours and morphology was measured by surveying at 3 4 hour intervals three signatures of self organization were observed in all experiments first time lags between swash front motions in beach cusp bays and horns increase with increasing relief representing the effect of morphology on flow second differential erosion between bays and horns initially increases with increasing time lag representing the effect of flow on morphology change because positive feedback causes growth of beach cusps third after initial growth differential erosion decreases with increasing time lag representing the onset of negative feedback that stabilizes beach cusps a numerical model based on self organization initialized with measured morphology and alongshore uniform distributions of initial velocities and positions of the swash front at the beginning of a swash cycle reproduces the measurements except for parts of one experiment where limited surveys and a significant low frequency component to swash motions might have caused errors in model initialization

rapid dome growth at montagne pelee during the early stages of the 1902 1905 eruption a reconstruction from lacroix s data information obtained from various parts of the two books on montagne pelee by lacroix enables an estimate to be made of the size of l etang sec summit crater the volume of the 1902 1905 lava dome and its growth rate at various stages of development during the week preceding the 8 may nuee ardente dome growth was between 28 and 38 m3 s 1 leading to a volume of 17 23 106 m3 on the morning of the catastrophe considering that significant parts of the dome 1 3 were removed by the 8 and 20 may climactic eruptions a high magmatic flux could have continued until at least 27 may when the total remaining volume was estimated to 53 106 m3 after moderate activity in june july of order 10 m3 s 1 vigorous dome growth resumed dramatically leading to the third climactic eruption of 30 august a true calculation for this period being not feasible because of poor quality of the data from november 1902 to july 1903 most of the effusive activity was concentrated in the great spine erupted volume 15 106 m3 magma flux 1 2 m3 s 1 which was eventually destroyed by collapse and minor nuees ardentes the end of the eruption was characterized by a very low effusion rate 1 m3 s 1 in average from august 1903 to october 1905