settling and compaction of olivine in basaltic magmas an experimental study on the time scales of cumulate formation a series of centrifuge assisted settling experiments of 30 vol olivine in 70 vol basaltic melt was conducted to elucidate the formation mechanisms and time scales of gravitational cumulates the settling experiments were performed in a centrifuging piston cylinder at 200 1 500g 1 270 1 280 c and 0 8 1 1 gpa on previously annealed and texturally equilibrated samples the mechanical settling of the dense olivine suspension occurs at about 1 6 the speed of simple stokes settling resulting in a sedimentation exponent n 4 1 6 in agreement with predictions from analogue systems the porosity m of the orthocumulate resulting from gravitational settling of crystals is about 54 and formation times of olivine orthocumulates result to 0 1 10 m day 1 for an initial crystal content of the melt of 1 5 and grain sizes of 2 10 mm after mechanical settling olivine grains rest on each other and further compaction occurs through pressure dissolution at grain contacts olivine reprecipitation where olivine is in contact with melt and concomitant expulsion of excess liquid from the cumulate layer with centrifugation at 400g for 50 h porosities as low as 30 3 vol were achieved the olivine content at the bottom of the gravitational cumulate is 1 m log h a t where is the density difference between crystals and melt h the crystal layer thickness a the acceleration and t the time of centrifuging compaction is hence proportional to effective stress integrated over time indicating that pressure dissolution is the dominant mechanism for chemical compaction the compaction limit that is the lowermost porosity to be reached by this mechanism is calculated by equating the lithostatic and hydraulic pressure gradients in the cumulate and results to 3 5 porosity for the experiments crystal size distribution curves and a growth exponent n of 3 1 3 indicate that diffusion controlled ostwald ripening is the dominant crystal growth mechanism the above relationship combined with a linear scaling for grain size as appropriate for reaction controlled pressure solution creep allows calculation of formation times of adcumulates if chemical compaction is dissolution reprecipitation limited then single layers of natural olivine adcumulates of m thickness with 70 75 vol olivine at the base as observed in the rhum layered intrusion would have typical formation times of 0 4 3 years for grain sizes of 2 10 mm this time scale compares favourably with characteristic cooling times of sills if a greater than 20 m thick series of cumulate layers pressurizes a base layer with the porosity still filled by a melt then compaction proceeds to the compaction limit within a few years it can thus be expected that in layered mafic intrusions where cumulates are continuously deposited from a large magma chamber and which characteristic cooling times of more than decades a compaction zone of several tens of metres forms with adcumulates only maintaining porosities in the order of 5 in conclusion gravitational settling and gravitation driven chemical compaction are feasible cumulate forming processes for dense mafic minerals in basaltic magmas and in particular in large layered intrusions

rapid precipitation of silica opal a disguises evidence of biogenicity in high temperature geothermal deposits case study from dagunguo hot spring china abstract dagunguo spring located in the tengchong geothermal area in the western part of yunnan province china is a very active spring with water temperatures of 78 to 97 c and ph of 7 7 to 8 8 the vent pool 5 6 m in diameter and up to 1 5 m deep is lined with opal a that was precipitated from the near boiling spring waters a glass suspended in the pool was coated with opal a in two months and two pvc pipes that drained water from the pool in late 2010 became lined with opal a precipitates in less than three months the opal a accumulated at rates of 0 5 to 0 75 mm month in the spring pool and 2 5 to 3 5 mm month in the pvc pipes the opal a precipitates irrespective of where they developed are formed primarily of silicified microbes and opal a spheres along with minor amounts of native sulfur detrital quartz and clay mainly kaolinite the fabrics in these opal a deposits were dictated largely by the growth patterns of the filamentous and rod shaped microbes that dominate this low diversity biota and the amount of opal a that was precipitated around them many of the microbes were preserved as rapid opal a was precipitated on and around them before the cells decayed with continued precipitation however the microbes became quickly engulfed in the opal a precipitates and morphological evidence of their presence was lost in essence the process that controls their preservation ultimately disguised them to the point where cannot be seen critically this loss of morphological identity takes places even before opal a starts its diagenetic transformation towards quartz

ice nucleation and overseeding of ice in volcanic clouds 1 water is the dominant component of volcanic gas emissions and water phase transformations including the formation of ice can be significant in the dynamics of volcanic clouds the effectiveness of volcanic ash particles as ice forming nuclei in is poorly understood and the sparse data that exist for volcanic ash in have been interpreted in the context of meteorological rather than volcanic clouds in this study single particle freezing experiments were carried out to investigate the effect of ash particle composition and surface area on water drop freezing temperature measured freezing temperatures show only weak correlations with ash in composition and surface area our measurements together with a review of previous volcanic ash in measurements suggest that fine ash particles equivalent diameters between approximately 1 and 1000 m from the majority of volcanoes will exhibit an onset of freezing between 250 260 k in the context of explosive eruptions where super micron particles are plentiful this result implies that volcanic clouds are in rich relative to meteorological clouds which typically are in limited and therefore should exhibit distinct microphysics we can expect that such overseeded volcanic clouds will exhibit enhanced ice crystal concentrations and smaller average ice crystal size relative to dynamically similar meteorological clouds and that glaciation will tend to occur over a relatively narrow altitude range

a multidisciplinary effort to assign realistic source parameters to models of volcanic ash cloud transport and dispersion during eruptions during volcanic eruptions volcanic ash transport and dispersion models vatds are used to forecast the location and movement of ash clouds over hours to days in order to define hazards to aircraft and to communities downwind those models use input parameters called eruption source parameters such as plume height h mass eruption rate duration d and the mass fraction m63 of erupted debris finer than about 4 or 63 m which can remain in the cloud for many hours or days observational constraints on the value of such parameters are frequently unavailable in the first minutes or hours after an eruption is detected moreover observed plume height may change during an eruption requiring rapid assignment of new parameters this paper reports on a group effort to improve the accuracy of source parameters used by vatds in the early hours of an eruption we do so by first compiling a list of eruptions for which these parameters are well constrained and then using these data to review and update previously studied parameter relationships we find that the existing scatter in plots of h versus yields an uncertainty within the 50 confidence interval of plus or minus a factor of four in eruption rate for a given plume height this scatter is not clearly attributable to biases in measurement techniques or to well recognized processes such as elutriation from pyroclastic flows sparse data on total grain size distribution suggest that the mass fraction of fine debris m63 could vary by nearly two orders of magnitude between small basaltic eruptions 0 01 and large silicic ones 0 5 we classify eleven eruption types four types each for different sizes of silicic and mafic eruptions submarine eruptions brief or vulcanian eruptions and eruptions that generate co ignimbrite or co pyroclastic flow plumes for each eruption type we assign source parameters we then assign a characteristic eruption type to each of the world s 1500 holocene volcanoes these eruption types and associated parameters can be used for ash cloud modeling in the event of an eruption when no observational constraints on these parameters are available

evolving infrasound detections from bogoslof volcano alaska insights from atmospheric propagation modeling bogoslof volcano a back arc volcano in alaska s aleutian arc began an eruptive sequence in mid december 2016 that ended in late august 2017 with 70 individual eruptive episodes because there were no local seismic or infrasound stations on the island the alaska volcano observatory avo relied on distant geophysical networks and remote sensing techniques to assess activity during the eruption avo maintains six infrasound arrays to monitor activity along the aleutian arc adak the island of four mountains okmok akutan sand point and dillingham eruption detection at infrasound arrays is subject to local as well as mesoscale meteorological conditions that vary greatly over both short and long timescales infrasound detections from the array nearest to bogoslof okmok with a latency of about 3 min played a crucial role in monitoring activity during the eruption despite the relative proximity of the okmok array to bogoslof 60 km infrasound detections were not uniformly observed with only about two thirds of the events successfully detected the farthest array at dillingham 816 km detected approximately half of the explosive events with all other arrays detecting less than half of the events we compare observations with infrasound propagation model predictions using both normal mode and parabolic equation forward models to interpret the variation in detections of the 70 explosive events across the avo infrasound network the forward models utilize the newly created publicly available avo g2s atmospheric reconstruction using numerical weather predictions data for the lower atmosphere coupled with upper atmosphere empirical models of wind speeds and temperature we find that long range detections 100 km of bogoslof events are largely aligned with seasonal variability in favorable propagation conditions while regional detections 100 km are less consistent with propagation modeling understanding the output of numerical models in comparison to past observations will facilitate their use in future operational settings for avo and other observatories

mechanisms for ballistic block ejection during the 2016 2017 shallow submarine eruption of bogoslof volcano alaska ejection of ballistic blocks was a characteristic feature of the 2016 2017 bogoslof eruption high resolution satellite images acquired throughout the duration of the 9 month long eruptive period permitted the recognition and mapping of ballistic blocks on the surface of bogoslof island many of the satellite images recorded the accumulation of ballistic material over several individual eruptive events but a few images recorded the effects of a single event the nonuniform spatial distribution of blocks suggests that some of the eruption columns were inclined ballistic trajectories were estimated using the eject model and indicate that accumulation of blocks on bogoslof island required launch angles of 45 80 and initial velocities of 50 100 ms 1 to reproduce observed travel distances the amount of ballistic fallout observed in satellite data indicates that there must have been a shallow submarine source of rock within the conduit upper edifice system dense accidental cryptodome trachyandesite and juvenile basalt to trachybasalt scoria make up the bulk of the surface ejecta abundant accidental fragments and inclined eruption columns point to periodic vent wall collapse and jetting around edges of temporarily blocked vents as the likely cause of ballistic ejection

young cumulate complex beneath veniaminof caldera aleutian arc dated by zircon in erupted plutonic blocks mount veniaminof volcano alaska peninsula provides an opportunity to relate quaternary volcanic rocks to a coeval intrusive complex veniaminof erupted tholeiitic basalt through dacite in the past 260 k y gabbro diorite and miarolitic granodiorite blocks ejected 3700 14 c yr b p in the most recent caldera forming eruption are fragments of a shallow intrusive complex of cumulate mush and segregated vapor saturated residual melts sensitive high resolution ion microprobe shrimp analyses define 238 u 230 th isochron ages of 17 6 2 7 ka 5 11 10 ka and 10 2 4 0 ka 2 for zircon in two granodiorites and a diorite respectively sparse zircons from two gabbros give 238 u 230 th model ages of 36 8 ka and 26 7 ka zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid although historic eruptions have been weakly explosive strombolian fountaining and small lava effusions the young ages of plutonic blocks as well as late holocene dacite pumice are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions

geology and eruptive history of bogoslof volcano bogoslof volcano is a shallow submarine subaerial volcano in the southern bering sea about 100 km west of the community of dutch harbor alaska the subaerial parts of the volcano consist of two small islands bogoslof island and fire island that together have a total area of about 1 6 km2 bogoslof was first depicted on a russian map in 1772 and since then has been observed and visited occasionally the volcano has had at least nine periods of eruptive activity since 1796 and all of its historical eruptions have been similar in style historical bogoslof eruptions involved the effusion of basalt trachybasalt basaltic trachyandesite and trachyandesite lava domes with above sea level relief of 100 200 m many of the eruptions are accompanied by the formation of tuff rings and ejection of ballistic particles historical observations suggest that eruption clouds are relatively ash poor minor ash fallout has typically occurred within about 100 km of the volcano many of the historical eruptions began at vents that were below sea level and thus seawater has played an important role in the style of eruptive activity exhibited by the volcano at times eruptive activity has been characterized by surtseyan style eruptions and magma interaction with wet vent fill deposits at other times the eruptive style has been more magmatically driven and has resulted in the formation of pyroclastic flows and small ash clouds preliminary studies of the deposits produced during the 2016 2017 eruption indicate vertical sequences of coarse grained horizontally bedded pyroclastic flow and fall deposits with numerous blocks bombs and lapilli of dense juvenile and accidental lithic material these deposits were emplaced by near vent pyroclastic flows surges and explosions some of which originated from shallow highly crystalline cryptodomes

character mass distribution and origin of tephra fall deposits from the 2009 eruption of redoubt volcano alaska highlighting the significance of particle aggregation abstract the 2009 eruption of redoubt volcano included 20 tephra producing explosions between march 15 2009 and april 4 2009 utc next generation radar nexrad data show that plumes reached heights between 4 6 km and 19 km asl and were distributed downwind along nearly all azimuths of the volcano explosions lasted between 0 8 mm thick including communities along the kenai peninsula 80 100 km and the city of anchorage 170 km trace ash on march 15 a small 4 6 km asl phreatic explosion containing minor non juvenile ash erupted through the summit ice cap the first five magmatic explosions events 1 5 occurred within a 6 hour period on march 23 plumes rose to heights between 5 5 km and 14 9 km asl during 2 to 20 minute duration explosions and were dispersed mainly along a nne trajectory trace ash fall was reported as far as fairbanks owing to a shift in wind direction and heavy snowfall during these events field discrimination among many of these layers was possible all deposits comprise a volumetrically significant amount of particle aggregates yet only event 5 deposits contain coarse clasts including glacier ice the most voluminous tephra fall was deposited on march 24 event 6 from a 15 minute explosion that sent a plume to 18 3 km asl and dispersed tephra to the wnw within 10 km of the vent this deposit contains 1 11 cm pumice clasts in a matrix of 1 2 mm aggregate lapilli a small dome was presumably emplaced between march 23 and march 26 and was subsequently destroyed during 1 14 minute magmatic explosions of events 7 8 march 26 that sent plumes between 8 2 km and 19 km asl ash fell along a broad swath to the ese covering communities along the kenai peninsula with up to 1 mm of ash proximal deposits are largely composed of aggregate lapilli of 1 2 mm with very little coarse juvenile material events 9 18 march 27 sent plumes between 5 2 km and 15 5 km asl during particle size data showing a preponderance of fine ash even in the most proximal locations along with the abundance of aggregate lapilli documented in most samples confirms that particle aggregation played a significant role in the 2009 eruption and induced premature fallout of fine ash

thermal disequilibrium at the top of volcanic clouds and its effect on estimates of the column height satellite images of large volcanic explosions reveal that the tops of volcanic eruption columns are much colder than the surrounding atmosphere1 we propose that this effect occurs whenever a mixture of hot volcanic ash and entrained air ascends sufficiently high into a stably stratified atmosphere although the mixture is initially very hot it expands and cools as the ambient pressure decreases we show that cloud top undercoolings in excess of 20 c may develop in clouds that penetrate the stratosphere this is consistent with observations of the 4 april 1982 eruption of el chichon1 and the 18 may 1980 eruption of mount st helens2 furthermore from our model results we predict that for a given cloud top temperature variations in the initial temperature of 100 200 c may correspond to variations in the column height of 5 10 km we deduce that the present practice of converting satellite based measurements of the temperature at the top of volcanic eruption columns to estimates of the column height will produce rather inaccurate results and should therefore be discontinued

overview chronology and impacts of the 2016 2017 eruption of bogoslof volcano alaska the 2016 2017 eruption of bogoslof volcano a back arc shallow submarine volcano in alaska s aleutian arc began in december 2016 and included 70 explosive events and at least two episodes of subaerial dome building because the volcano had no local monitoring stations during the eruption a combination of distant seismic stations regional infrasound sensors lightning detection a variety of satellite data and observer reports and a field visit in 2018 were used to recreate the events that occurred during the nine month eruption following precursory seismicity that started in september 2016 the eruption began in december 2016 with a series of explosive events that persisted through mid march 2017 after a 6 week hiatus activity resumed on may 17 and lasted through the end of august 2017 and consisted of additional explosions and two short lived subaerial lava domes that formed in june and august for most of the eruption bogoslof s vent was submerged in shallow seawater though during several of the longer events a subaerial edifice grew and the vent migrated above sea level resulting in more ash rich volcanic clouds eruptive products geophysical signals and eruptive style are all broadly consistent with vulcanian activity where slow magma ascent led to repetitive dome or plug formation overpressurization in the upper conduit and sudden release during short lived explosions infiltration of seawater may have prohibited large domes from forming especially in the first half of the eruption when explosions were closely spaced in time the largest four explosions in the sequence occurred after inter event times of 10 days or more three events produced ashfall on nearby communities and mariners east and south of bogoslof and the eruption resulted in dozens of flight cancelations and flight diversions around the volcano and its ash clouds

did ice charging generate volcanic lightning during the 2016 2017 eruption of bogoslof volcano alaska the 2016 2017 shallow submarine eruption of bogoslof volcano in alaska injected plumes of ash and seawater to maximum heights of 12 km more than 4550 volcanic lightning strokes were detected by the world wide lightning location network wwlln and vaisala s global lightning dataset gld360 over 9 months lightning assisted monitoring efforts by confirming ash producing explosions in near real time but only 32 out of the 70 explosive events produced detectable lightning what led to electrical activity within some of the volcanic plumes but not others and why did the lightning intensity wax and wane over the lifetime of individual explosions we address these questions using multiparametric observations from ground based lightning sensors satellite imagery photographs acoustic signals and 1d plume modeling detailed time series of monitoring data show that the plumes did not produce detectable lightning until they rose higher than the atmospheric freezing level approximated by 20 c temperatures for example on 28 may 2017 event 40 the delayed onset of lightning coincides with modeled ice formation in upper levels of the plume model results suggest that microphysical conditions inside the plume rivaled those of severe thunderstorms with liquid water contents 5 g m 3 and vigorous updrafts 40 m s 1 in the mixed phase region where liquid water and ice coexist based on these findings we infer that thunderstorm style collisional ice charging catalyzed the volcanic lightning however charge mechanisms likely operated on a continuum with silicate collisions dominating electrification in the near vent region and ice charging taking over in the upper level plumes a key implication of this study is that lightning during the bogoslof eruption provided a reliable indicator of sustained ash rich plumes and associated hazards above the atmospheric freezing level

seismic character and progression of explosive activity during the 2016 2017 eruption of bogoslof volcano alaska bogoslof volcano in the central aleutian arc experienced a major eruption between december 2016 and august 2017 that was characterized by explosive activity volcanic explosivity index 2 to 3 and the extrusion of lava domes the alaska volcano observatory tracked the activity in real time using seismicity observed on distant stations as well as infrasound lightning satellite data and occasional visual observations in this study we measure the duration of seismic signals associated with individual explosive events to track their progression during the two explosive phases of the eruption seismic recordings of bogoslof explosions show complex waveforms that suggest both individual explosive events and sequences of several explosions separated by lower amplitude tremor the lack of local seismic monitoring stations at distances of 1 km to 15 km distance unfortunately limits our ability to closely observe seismicity and to interpret changing conditions at the vent such as position the presence of a lava dome or plug and the role of seawater associated with the eruption we use the rate of explosive activity seismic waveform character and repose time between explosions to infer the conditions within the conduit

petrology of the 2016 2017 eruption of bogoslof island alaska the 2016 2017 eruption of bogoslof primarily produced crystal rich amphibole basalts the dominant juvenile tephra were highly microlitic with diktytaxitic vesicles and amphiboles had large reaction rims both observations support a magma history of slow ascent and or shallow stalling prior to eruption plagioclase amphibole clinopyroxene mineralogy are also suggestive of shallow magma crystallization lavas were emplaced as shallow submarine lava domes and cryptodomes that produced 70 relatively short lived and water rich explosions over the course of the 9 month long eruption the explosions ejected older trachyandesite lavas that were likely uplifted by cryptodome emplacement that began in december 2016 and continued for many months trachyte pumice similar in composition to a 1796 lava dome was entrained in basalts by the end of the eruption the pumice appears to be a largely crystalline magma that was rejuvenated entrained in the basalt and heated to 1000 c the composition of trachytes require differentiation through stronger amphibole control than the apparent shallow crustal evolution implied for the basalt this suggests that they are magmas derived from a mid crustal zone of amphibole crystallization nearby arc front volcanoes that notably lack amphibole have strikingly similar compositional trends trace element signatures of the bogoslof basalts however suggest derivation from a mantle source with residual garnet and lower degree melting than basalts from nearby arc front volcanoes the diversity of magmas erupted at bogoslof thus provides an opportunity not only to probe rare backarc compositions from the aleutian arc but also to examine the apparent role of amphibole in generating evolved compositions more broadly in arc environments

evolution of the submarine subaerial edifice of bogoslof volcano alaska during its 2016 2017 eruption based on analysis of satellite imagery the 2016 2017 eruption of bogoslof volcano involved at least 70 detected eruptive events between mid december 2016 and august 30 2017 acquisition of high resolution satellite imagery throughout the duration of the eruptive period allowed us to document and map the various morphologic changes that occurred on the subaerial part of bogoslof island the emplacement of pyroclastic flow and surge deposits caused the island to increase in area by about 1 5 km2 the dominant volcanic landforms of the eruption were a series of tuff rings emplaced around various submarine vents many of the tuff rings were mantled with surface dunes and impressive amounts of ballistic ejecta likely derived from erupting magma bodies or previously emplaced submarine lava domes debris flow deposits and surface channels extending over tuff ring surfaces apparent in multiple satellite images are evidence for explosive ejection of seawater in most cases erupting vents were initially submarine or began at subaerial lava domes and were largely flooded by seawater suggesting that water magma ratios were likely high under such conditions where water is abundant eruptive products typically reflect a high degree of water involvement and are dominated by the formation of wet tephra jets and flows and associated deposits typically consist of fine ash and lapilli contain accretionary lapilli and ash aggregates and usually form tuff cones and mounds we observed none of these features in our analysis of satellite data or during our examination of eruptive deposits on bogoslof island in 2018 on the contrary the dominant landform associated with the bogoslof eruption was tuff rings the development of tuff rings and surface dunes are commonly associated with the formation of pyroclastic base surges that are by comparison emplaced relatively dry dry base surge deposits can be generated from phreatomagmatic explosions involving superheated steam it is possible that shallow submarine magma wet sediment interactions were a characteristic and possibly a dominant eruptive process of the 2016 2017 bogoslof eruption

impacts of the 1945 and 1995 1996 ruapehu eruptions new zealand an example of increasing societal vulnerability over the past 50 yr the risk to society from volcanic eruptions has increased sharply due to an increased population more developed and diversified economies and a more technologically advanced infrastructure this fact is demonstrated vividly by the impacts from the two largest eruptions of the twentieth century from the cone volcano of ruapehu which suggest that the vulnerability of key sectors in new zealand society has increased by one to two orders of magnitude over this period both the 1945 and 1995 1996 eruptions included explosive phases that dispersed ash over a wide area of the north island for a period of several months individual ash falls were only a few millimeters thick in communities within 150 km of the volcano and only trace amounts were found in communities farther away the 1995 1996 eruption caused similar physical effects to the 1945 eruption but had considerably greater social and economic impacts the greatest contribution to the cost of the 1995 1996 eruption estimated in excess of 130 million new zealand dollars was the impact on the alpine tourist industry in the central north island essentially nonexistent in 1945 other significant impacts were felt by the rapidly growing aviation and electricity generation sectors the cost of any future eruption of the same magnitude is likely to grow as the vulnerability of our society increases at a rapid rate

crystal mush compaction and the origin of pegmatitic segregation sheets in a thick flood basalt flow in the mesozoic hartford basin connecticut

the 1989 1990 eruption of redoubt volcano alaska impacts on aircraft operations abstract the december 1989 june 1990 eruption of redoubt volcano affected commercial and military air operations in the vicinity of anchorage alaska these effects were due to the direct impact of volcanic ash on jet aircraft as well as to the rerouting and cancellations of flight operations owing to eruptive activity between december and february five commercial jetliners were damaged from ash encounters the most serious incident took place on december 15 1989 when a boeing 747 400 aircraft temporarily lost power of all four engines after encountering an ash cloud as the airplane descended for a landing in anchorage while there were no injuries to passengers the damage to engines avionics and aircraft structure from this encounter is estimated at 80 million four additional encounters between jet aircraft and redoubt ash clouds occurred in the anchorage area on december 15 and 16 1989 and february 21 1990 none resulted in engine failure two additional encounters took place on december 17 1989 when jet airliners encountered the redoubt cloud over west texas at the time of these encounters the cloud was up to 55 hours old and had traveled in excess of 2 900 nautical miles 5 300 km following the december 15 encounters anchorage international airport remained open however most airline companies canceled operations for up to several days as communications between federal agencies and airlines improved and as a better understanding of the nature and behavior of ash rich eruption clouds was achieved most airlines resumed normal service by early january 1990 the resulting loss of revenue at anchorage international airport during several months following the eruption is estimated to total 2 6 million the impact on general aviation and military operations consisted mostly of cancellation and rerouting of flights

comparison of short term seismic precursors and explosion parameters during the 2016 2017 bogoslof eruption in december 2016 the unmonitored bogoslof volcano unexpectedly began erupting producing at least 70 explosions over the next 8 5 months roughly half of the explosions were preceded by observable short term precursory activity defined here as seismic activity ending within 30 min of the explosion onset or an earthquake swarm ending within a few hours before the explosion onset five different types of precursors were noted during the bogoslof eruption non swarm earthquakes earthquake swarms rapid rate events tremor and chromatic tremor most of the precursors that were detectable by the nearby seismic networks 45 km away occurred during the first 3 months of the eruption with nearly all precursory earthquake swarms occurring during this period a hydrophone deployed 7 km northeast of the summit for the last 3 months of the eruption detected weaker activity that was not recorded by the more distant seismic networks we describe the observed precursors and discuss their inter relations overall the precursors seem indicative of processes occurring throughout the volcanic system and suggest possible changes in the system during the eruption we then compare the short term precursors with explosion parameters such as plume height duration and occurrence of lightning to look for relations that may provide more insight into the eruption dynamics and subsurface processes and help to further improve forecasting we find no obvious relations between any of the precursor or explosion parameters suggesting that their use in forecasting eruption characteristics at bogoslof is limited

rapid remobilization of magmatic crystals kept in cold storage we lack thermal histories for magma reservoirs but here the magma under mount hood oregon usa is shown to have been too cold to mobilize for most of the time it has been stored which implies that magma mobilizes at which point it can be imaged geophysically very quickly prior to eruption this paper presents a novel approach to constraining the thermal history of the subsurface magma bodies that feed eruptions by combining temporal information from a variety of sources kari cooper and adam kent combine timescales derived from uranium series disequilibria textural information and trace element zoning in crystals they show that at mount hood oregon only a small fraction of the total storage duration has been spent at temperatures above the critical crystallinity the point at which magma is easily mobilized they conclude that largely liquid bodies that could be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption the processes involved in the formation and storage of magma within the earth s upper crust are of fundamental importance to volcanology many volcanic eruptions including some of the largest result from the eruption of components stored for tens to hundreds of thousands of years before eruption1 2 3 although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes they remain relatively poorly known4 5 eruptions of crystal rich magma are often suggested to require the mobilization of magma stored at near solidus conditions6 7 8 however accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures7 8 9 what has been lacking in this debate is clear observational evidence linking the thermal and therefore physical conditions within a magma reservoir to timescales of storage that is thermal histories here we present a method of constraining such thermal histories by combining timescales derived from uranium series disequilibria crystal sizes and trace element zoning in crystals at mount hood oregon usa only a small fraction of the total magma storage duration at most 12 per cent and probably much less than 1 per cent has been spent at temperatures above the critical crystallinity 40 50 per cent at which magma is easily mobilized partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near solidus temperatures is common magma storage at low temperatures indicates that although thermobarometry calculations based on mineral compositions may record the conditions of crystallization they are unlikely to reflect the conditions of most of the time that the magma is stored our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption

co eruptive tremor from bogoslof volcano seismic wavefield composition at regional distances we analyze seismic tremor recorded during eruptive activity over the course of the 2016 2017 eruption of bogoslof volcano alaska only regional recordings of the tremor wavefield exist for bogoslof making it a challenge to place the recordings in context with other eruptions that are normally captured by local seismic data we apply a technique of time frequency polarization analysis to three component seismic data to reveal the wavefield composition of bogoslof eruption tremor we find that at regional distances the tremor is dominated by p waves in the band from 1 5 to 10 hz using this information along with an enriched bogoslof earthquake catalog we obtain estimates of average reduced displacement dr for eruption tremor during 25 of the 70 bogoslof events dr reaches as high as approximately 40 cm2 for two of the major events similar to other vei 3 eruptions in alaska overall average reduced displacement displays a weak correlation to plume height during the first half of the 9 month long eruption sequence with a few notable exceptions the two events with the highest dr values also generated measurable eruption tremor at very long periods vlp between 0 05 and 0 15 hz

infrasound generated by the 2016 2017 shallow submarine eruption of bogoslof volcano alaska the 2016 2017 shallow submarine eruption of bogoslof volcano produced numerous infrasound signals over 9 months that were recorded on six alaska volcano observatory avo arrays at ranges of 59 to over 800 km from the volcano the lack of geophysical monitoring near bogoslof and the repeated production of volcanic clouds to flight levels made monitoring by remote infrasound critical during the eruption for the first time avo relied extensively on automated infrasound detections from regional arrays to dispatch timely notifications of the ongoing activity most of the 70 eruptive events were detected on at least one array but no array detected all of the events mainly because atmospheric conditions were highly variable during the eruption acoustic propagation modeling helps explain some of the variation in array detections but also highlights limitations in regional propagation models to our knowledge this is the first example of well recorded infrasound from an explosive eruption occurring in shallow seawater providing extensive insights into eruption dynamics in this unique environment the dominance of low frequency infrasound 0 1 1 hz is attributed to eruptions occurring beneath tens of meters of seawater higher frequency infrasound signals were mostly limited to eruptions where the vent was isolated from major interaction with seawater or in several cases where a lava dome grew above sea level

satellite observations of the 2016 2017 eruption of bogoslof volcano aviation and ash fallout hazard implications from a water rich eruption satellite data goes viirs modis avhrr were used in near real time during the 2016 2017 eruption of bogoslof volcano alaska to detect explosive eruptive activity and to characterize the resulting volcanic clouds this study examines satellite data to estimate volcanic cloud heights and mass eruption rates for 47 of the 70 explosive events eighteen of the volcanic clouds reached an altitude in excess of 8 5 km asl where they posed a potential hazard to aviation we estimate eruption rates were in the range of 104 107 kg s eruption durations were available for 28 events and the total mass of these events was 5 7 1010 kg most of the explosions occurred from submarine vents producing volcanic clouds with water rich characteristics in satellite data we infer that these water rich phreatomagmatic events contained ice coated ash particles which changed their visible and multispectral characteristics only two of the explosions produced clouds with satellite characteristics that would be considered ash rich we conclude that these events were relatively dry eruptions with limited access to ocean water although some of the explosive events transitioned from submarine to subaerial vents we observed no change in the character of the volcanic clouds during these transitions we speculate that enhanced removal of fine grained volcanic ash likely occurred due to aggregation with implications for modeling ash transport and fallout we conclude that the majority of bogoslof s water rich volcanic clouds did contain volcanic ash despite the lack of conventional ash signature in satellite data this has implications for satellite monitoring of future water rich shallow submarine eruptions

seismo acoustic evidence for vent drying during shallow submarine eruptions at bogoslof volcano alaska characterizing the state of the volcanic vent is key for interpreting observational datasets and accurately assessing volcanic hazards this is particularly true for remote complex eruptions such as the 2016 2017 bogoslof volcano alaska eruption sequence bogoslof s eruptions in this period were either shallow submarine or subaerial or some combination of both our results demonstrate how low frequency sound waves infrasound integrated with seismic and satellite data can provide unique insight into shallow vent processes otherwise not available we use simple metrics such as the infrasound frequency index fi event duration and acoustic seismic amplitude ratio to look at changes in the elastic energy radiation and infer changes in seawater access to the vent satellite imagery before and after selected eruptions is used to ground truth inferences on vent conditions high fi and gradual increases in infrasound frequency content at bogoslof correspond with transitions from submarine to subaerial vent conditions and a diminished or absent role of water likely resulting in a drying out of the vent region event durations generally correlate with high fi and the range of fi values for each event suggesting long duration events were more effective at drying out the vent region a trend from low to high acoustic seismic amplitude ratios for some long duration events also suggests an increase in acoustic efficiency as the vent dried out we demonstrate that infrasound can serve as a robust indicator of seawater involvement for bogoslof and other shallow submarine eruptions that may not be inferable from other datasets particularly in near real time

origin of quartz clusters in vinalhaven granite and porphyry coastal maine we use the crystallographic orientations of quartz crystals as determined with ebsd to provide new evidence for the formation of clustered quartz crystals during magma crystallization vinalhaven is dominated by granite with minor porphyry that formed when granite remelted during input of coeval basalt cl zoning suggests that most quartz clusters in granite and porphyry formed by synneusis the swimming together of preformed crystals in granite most quartz pairs in clusters have random orientations only about 10 have parallel or esterel twin orientations porphyry has fewer quartz clusters and all pairs have approximately parallel or esterel twin orientations cl zoning of quartz pairs in porphyry indicates that they attached prior to a major remelting event interpretation of the vinalhaven quartz clusters leads us to propose that oriented synneusis occurs during crystal accumulation on a magma chamber floor during hindered settling some quartz crystals should have come into contact along their dipyramidal faces once in contact continued settling and loss of interstitial melt may have rotated some quartz crystals such that lattices on their dipyramidal faces matched producing parallel and esterel twin orientations and creating strong bonds between pairs only a small proportion of pairs with matched dipyramidal faces formed in the granite and during rejuvenation to produce porphyry only these oriented pairs survived hence the presence of oriented synneusis in a plutonic rock may demonstrate a history of crystal accumulation

ascent dynamics of large phreatomagmatic eruption clouds the role of microphysics 1 we examine the impact of abundant surface water interaction on the development of volcanic clouds from large scale 108 kg s 1 magma phreatomagmatic eruptions presenting the first 2 d large eddy simulations of wet volcanic plumes that incorporate the effects of microphysics the active tracer high resolution atmospheric model was forced with field derived inputs from an exceptionally large phreatomagmatic eruption the 27 ka oruanui supereruption taupo volcano new zealand surface water contents were varied from 0 to 40 wt for eruptions with equivalent magma eruption rates of 1 3 108 and 1 1 109 kg s 1 our findings confirm that increased surface water has a pronounced impact on column stability leading to transitional column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents pdcs contrary to the suggestion of previous workers however abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds owing to vigorous microphysics assisted lofting of pdcs the simulated behavior and ash cloud dimensions provide a close match to field evidence from the oruanui case study cloud heights from the collapsing eruption columns also show a notable sensitivity to changes in the altitude of the tropopause while ambient humidity primarily impacts the abundance of airborne hydrometeors particularly ice associated with the volcanic clouds general relationships between eruption style meteorological conditions and the resulting vertical profiles of volcanic emissions outlined here could also be adapted for use in operational volcanic ash forecasting and deposit reconstruction techniques already in existence

doppler weather radar observations of the 2009 eruption of redoubt volcano alaska abstract the u s geological survey usgs deployed a transportable doppler c band radar during the precursory stage of the 2009 eruption of redoubt volcano alaska that provided valuable information during subsequent explosive events we describe the capabilities of this new monitoring tool and present data captured during the redoubt eruption the minimax 250 c mm 250c radar detected seventeen of the nineteen largest explosive events between march 23 and april 4 2009 sixteen of these events reached the stratosphere above 10 km within 2 5 min of explosion onset high column and proximal cloud reflectivity values 50 to 60 dbz were observed from many of these events and were likely due to the formation of mm sized accretionary tephra ice pellets reflectivity data suggest that these pellets formed within the first few minutes of explosion onset rapid sedimentation of the mm sized pellets was observed as a decrease in maximum detection cloud height the volcanic cloud from the april 4 explosive event showed lower reflectivity values due to finer particle sizes related to dome collapse and related pyroclastic flows and lack of significant pellet formation eruption durations determined by the radar were within a factor of two compared to seismic and pressure sensor derived estimates and were not well correlated ash dispersion observed by the radar was primarily in the upper troposphere below 10 km but satellite observations indicate the presence of volcanogenic clouds in the stratosphere this study suggests that radar is a valuable complement to traditional seismic and satellite monitoring of explosive eruptions

seismic and hydroacoustic observations of the 2016 17 bogoslof eruption in mid december 2016 bogoslof volcano alaska began an 8 5 month long eruption that produced at least 70 explosions and three lava domes bogoslof is an emergent submarine volcano with only the very top forming a small island so the eruptive activity mostly occurred from a vent submerged beneath a bay or lagoon the bogoslof eruption was recorded on regional seismic and infrasound arrays as well as by a hydrophone that was deployed locally during the second half of the eruption since few emergent volcanic eruptions have seismo acoustic recordings these observations taken in context with satellite and other observations provide an opportunity to greatly improve our understanding of the seismo acoustic signals produced by these eruptions the instruments detected a range of activity including earthquakes tremor and mass flow events earthquakes occurred before during after and unassociated with explosions and also often occurred in swarms the 47 seismically detected swarms and 27 additional hydroacoustically detected swarms can be further broken into four types precursory post eruptive general and tremor dominated most seismic swarms were less than 10 h long had fewer than 50 earthquakes and had average earthquake rates below 20 h for the explosions we calculate a tremor magnitude frequency index and average frequency the tremor magnitudes determined from the hydrophone data show a roughly linear relation to explosion plume height though this relation was not clear in the more limited seismic data lastly we categorize the activity into five eruption phases based on seismo acoustic character precursory opening explosive i pause and explosive ii

aviation response to a widely dispersed volcanic ash and gas cloud from the august 2008 eruption of kasatochi alaska usa 1 the extensive volcanic cloud from kasatochi s 2008 eruption caused widespread disruptions to aviation operations along pacific oceanic canadian and u s air routes based on aviation hazard warnings issued by the national oceanic and atmospheric administration u s geological survey the federal aviation administration and meteorological service of canada air carriers largely avoided the volcanic cloud over a 5 day period by route modifications and flight cancellations comparison of time coincident goes thermal infrared tir data for ash detection with ozone monitoring instrument omi ultraviolet data for so2 detection shows congruent areas of ash and gas in the volcanic cloud in the 2 days following onset of ash production after about 2 5 days the area of so2 detected by omi was more extensive than the area of ash indicated by tir data indicating significant ash depletion by fall out had occurred pilot reports of visible haze at cruise altitudes over canada and the northern united states suggested that so2 gas had converted to sulfate aerosols uncertain about the hazard potential of the aging cloud airlines coped by flying over under or around the observed haze layer samples from a nondamaging aircraft encounter with kasatochi s nearly 3 day old cloud contained volcanic silicate particles confirming that some fine ash is present in predominantly gas clouds the aircraft s exposure to ash was insufficient to cause engine damage however slightly damaging encounters with volcanic clouds from eruptions of reventador in 2002 and hekla in 2000 indicate the possibility of lingering hazards associated with old and or diffuse volcanic clouds

volcano monitoring using short wavelength infrared data from satellites data from remote sensing satellites operating in the short wavelength infrared can be used to measure temperatures of about 150 c and above the gain settings of the landsat thematic mapper tm sensors are such that to record pixel integrated temperatures from 150 c to over 1000 c requires the use of several spectral bands if the radiant source occupies less than a whole pixel as is the case for fumaroles or a crusted lava surface with incandescent cracks then the pixel integrated temperature is less than the actual surface temperature of the hot areas and appears to be different at each wavelength by determining pixel integrated temperature in two spectral bands we can estimate both the temperature and size of hot areas which occupy less than a complete pixel in some cases our observations provide the only available data on the state of activity of a particular volcano in others they support and refine field observations our landsat tm data reveal otherwise unobserved precursor and subsequent activity related to the september 1986 eruption of lascar volcano chile and supplement other data on the march 1986 eruption of augustine volcano alaska they also indicate the continued presence of lava lakes at erta ale ethiopia in the absence of any ground based observations and document minor eruptive activity at mount erebus antarctica landsat multispectral scanner data have been used to measure the variation in temperature along a lava flow on sierra negra galapagos islands which occurred in 1979

a user friendly one dimensional model for wet volcanic plumes 1 this paper presents a user friendly graphically based numerical model of one dimensional steady state homogeneous volcanic plumes that calculates and plots profiles of upward velocity plume density radius temperature and other parameters as a function of height the model considers effects of water condensation and ice formation on plume dynamics as well as the effect of water added to the plume at the vent atmospheric conditions may be specified through input parameters of constant lapse rates and relative humidity or by loading profiles of actual atmospheric soundings to illustrate the utility of the model we compare calculations with field based estimates of plume height 9 km and eruption rate 4 105 kg s during a brief tephra eruption at mount st helens on 8 march 2005 results show that the atmospheric conditions on that day boosted plume height by 1 3 km over that in a standard dry atmosphere although the eruption temperature was unknown model calculations most closely match the observations for a temperature that is below magmatic but above 100 c

polymineralic orientation analysis of magmatic rocks using electron back scatter diffraction implications for igneous fabric origin and evolution abstract electron back scatter diffraction ebsd which provides an easy way of acquiring large numbers of individual crystallographic orientation data from different phases has been applied to the study of magmatic fabrics using this technique the crystallographic preferred orientation cpo of plagioclase biotite orthopyroxene hornblende and quartz in natural tonalites and quartzdiorites from the santa olalla igneous complex sw iberia deformed during the magmatic stage have been determined plagioclase is the coarser phase defining the main fabric in each sample whereas biotite can display either the same fabric as plagioclase or a completely different one the differences between these two phases occur because 1 smaller phases interact with the larger ones to produce more random orientations 2 under simple shear finer phases can completely rotate giving a girdle included in the xz plane 3 finer phases can more easily preserve relict fabrics while the coarser phases are completely reoriented by the last stress tensor the last phases to crystallize show weak to completely random cpos hornblende or completely random distributions quartz the study was completed with a shape preferred orientation analysis using the intercept method in order to detect weak magmatic lineations and numerical modelling simulations of theoretically equivalent simple shear situations for each sample

testing the accuracy of a 1 d volcanic plume model in estimating mass eruption rate during volcanic eruptions empirical relationships are used to estimate mass eruption rate from plume height although simple such relationships can be inaccurate and can underestimate rates in windy conditions one dimensional plume models can incorporate atmospheric conditions and give potentially more accurate estimates here i present a 1 d model for plumes in crosswind and simulate 25 historical eruptions where plume height hobs was well observed and mass eruption rate mobs could be calculated from mapped deposit mass and observed duration the simulations considered wind temperature and phase changes of water atmospheric conditions were obtained from the national center for atmospheric research reanalysis 2 5 model simulations calculate the minimum maximum and average values mmin mmax and mavg that fit the plume height eruption rates were also estimated from the empirical formula mempir 140hobs4 14 mempir is in kilogram per second hobs is in kilometer for these eruptions the standard error of the residual in log space is about 0 53 for mavg and 0 50 for mempir thus for this data set the model is slightly less accurate at predicting mobs than the empirical curve the inability of this model to improve eruption rate estimates may lie in the limited accuracy of even well observed plume heights inaccurate model formulation or the fact that most eruptions examined were not highly influenced by wind for the low wind blown plume of 14 18 april 2010 at eyjafjallajokull where an accurate plume height time series is available modeled rates do agree better with mobs than mempir

moist convection and the injection of volcanic ash into the atmosphere if unsaturated water vapor is carried upward by a volcanic eruption column it may eventually become saturated owing to the decrease in temperature of the column as it expands through decompression and transfers heat to entrained air heat released as a result of the subsequent condensation of water vapor causes the air within the column to expand we show that this increases the buoyancy and therefore the total height of rise of the column the increase in height is significant in relatively small sub plinian and strombolian eruptions in which mass eruption rates lie in the range 103 to 106 kg s in such eruptions the latent heat released as the entrained water vapor condenses may provide the main source of heat which drives the ash and clasts upward the height of rise then becomes relatively insensitive to the mass flux erupted at the vent and depends primarily upon the vapor loading of the atmosphere in a moist atmosphere ash may rise several kilomtres higher than in an eruption of comparable strength in a dry environment moist convection leads to much wider ash dispersal particularly from very small eruptions subsequently rain flushing of ash from umbrella clouds may result from the water which forms through the condensation of entrained vapor the ash provides natural condensation nuclei for some of this entrained vapor whose mass may be much greater than that of the ash in small columns 107 kg s the latent heat released by condensation of vapor is relatively small in comparison with the thermal energy provided by the hot clasts and therefore moisture has no significant effect upon the eruption column dynamics furthermore the mass of water vapor originating from the erupted volatiles is usually comparable to or greater than that entrained from the ambient air our model also shows that if the erupting mixture becomes buoyant then the eruption columns associated with phreatomagmatic eruptions ascend nearly as high as plinian columns with the same mass eruption rate this is because the water which is vaporized by the hot ash at the source condenses higher in the column and thereby restores its latent heat to the ascending ash

grain scale processes in actively deforming magma mushes new insights from electron backscatter diffraction ebsd analysis of biotite schlieren in the jizera granite bohemian massif abstract in the porphyritic jizera granite bohemian massif three distinct types of lattice preferred orientations of biotite grains were revealed in schlieren delineated magmatic structures using the electron backscatter diffraction ebsd method 1 biotite basal planes 001 reorient from schlieren subparallel near the schlieren base to schlieren perpendicular in the upper part of the schlieren both orientations share subhorizontal n s to nne ssw trending a axes 2 in some domains the a axes are steep and at a high angle to the schlieren plane while the c axes plunge shallowly and rotate around an ill defined a axis 3 in other domains the ebsd coincides with background magnetic fabric of the host granite revealed using the anisotropy of magnetic susceptibility ams method that is the a axes plunge shallowly to the se or nw while the c axes are subhorizontal and cluster around the ne sw trend these multiple biotite orientations in the schlieren are interpreted to reflect 1 velocity gradient in laminar magma flow along channel like conduits localized within the high strength host phenocryst framework 2 grain scale gravity driven constrictional deformation of the magma mush and 3 overprinting background tectonic deformation transmitted across large parts of the magma chamber prior to its final crystallization the grain scale mechanisms of biotite fabric acquisition in the schlieren presumably involved rotation of biotite crystals during flow with the biotite alignment reflecting the flow geometry and kinematics replaced after flow cessation by melt aided grain boundary sliding of those biotite crystals still enclosed in melt pockets within otherwise static highly crystallized magma mush the latter process was sufficient to reorient biotite grains but not to cause destruction of the schlieren using the jizera granite as a case example we argue that the lattice preferred orientation of mineral grains in mafic schlieren is highly sensitive to reorient in response to processes both associated with the schlieren formation e g localized magma flow and those that occur later and are superimposed onto the effectively solid high strength magma mush

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

the 7 8 august 2008 eruption of kasatochi volcano central aleutian islands alaska 1 kasatochi volcano in the central aleutian islands erupted unexpectedly on 7 8 august 2008 kasatochi has received little study by volcanologists and has had no confirmed historical eruptions the island is an important nesting area for seabirds and a long term biological study site of the u s fish and wildlife service after a notably energetic preeruptive earthquake swarm the volcano erupted violently in a series of explosive events beginning in the early afternoon of 7 august each event produced ash gas plumes that reached 14 18 km above sea level the volcanic plume contained large amounts of so2 and was tracked around the globe by satellite observations the cumulative volcanic cloud interfered with air travel across the north pacific causing many flight cancelations that affected thousands of travelers visits to the volcano in 2008 2009 indicated that the eruption generated pyroclastic flows and surges that swept all flanks of the island accumulated several tens of meters of pyroclastic debris and increased the diameter of the island by about 800 m pyroclastic flow deposits contain abundant accidental lithic debris derived from the inner walls of the kasatochi crater juvenile material is crystal rich silicic andesite that ranges from slightly pumiceous to frothy pumice fine grained pyroclastic surge and fall deposits with accretionary lapilli cover the lithic rich pyroclastic flow deposits and mark a change in eruptive style from episodic explosive activity to more continuous ash emission with smaller intermittent explosions pyroclastic deposits completely cover the island but wave erosion and gully development on the flanks have begun to modify the surface mantle of volcanic deposits