did magma ascent rate control the explosive effusive transition at the inyo volcanic chain california rhyolitic eruptions often begin explosively and then shift to extrusions of lava it is widely believed that the style of eruption is dependent on magma flow rate within the conduit fast ascent leading to explosive eruption and slow ascent favoring effusive behavior currently the velocity of rhyolite magma flowing in a conduit is unknown and therefore eruption models remain untested against geologic evidence of magma ascent we show that explosively erupted rhyolite magma from the inyo volcanic chain ascended slowly centimeters per second at same rates as the dome building phases we propose that the explosive effusive transition sub plinian rhyolite eruptions is governed by the alleviation of exsolved volatile pressure in the shallow conduit rather than the magma flow rate

an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

late stage groundmass differentiation as a record of magma stagnation fragmentation and rewelding in the 2011 eruption of shinmoedake of the kirishima volcano group sub plinian eruptions were followed by lava extrusion with intermittent vulcanian explosions the interstices of microlites and relatively large nanolites 0 4 nm width in the groundmass of pyroclasts groundmass interstices were studied to reveal shallow magmatic processes that lead to different eruption styles the pumice of the sub plinian eruption has the least differentiated groundmass interstices composition the groundmass interstices of the dense juvenile fragments of the vulcanian explosions are divided into two groups the first group is the most differentiated as expected from their lava like texture whereas the second group is less differentiated and covers the range of sub plinian pumice the vulcanian pumice overlaps the dense juvenile fragments despite their high vesicularity this seemingly contradictory relationship of composition in the interstitial groundmass indicates the clastogenic lava origin of the less differentiated group of dense juvenile fragments in contrast magmas stagnated in the shallow conduit maintained elevated temperature and water content allowing groundmass interstices to differentiate by microlite growth these magmas then erupted as pumice in the vulcanian explosions and were effused as lava that was fragmented by subsequent explosions to be later sampled as a dense juvenile fragment the groundmass crystallinity increased by 9 1 vol in 5 45 days between the sub plinian and vulcanian eruptions increasing initial melt and magma viscosity from 106 1 7 4 and 107 0 8 3 pa s to 106 9 8 4 and 108 2 9 7 pa s respectively this viscosity increase by nanolite crystallization could have facilitated the stress fragmentation of conduit magma leading to vulcanian explosions post fragmentation expansion of the sub plinian pumice could have been suppressed by this viscosity increase resulting in their low vesicularity late stage groundmass differentiation can thus control shallow magmatic processes

effects of soil aggregates on debris flow mobilization results from ring shear experiments abstract rates and styles of landslide motion are sensitive to pore water pressure changes caused by changes in soil porosity accompanying shear deformation soil may either contract or dilate upon shearing depending upon whether its initial porosity is greater or less respectively than a critical state porosity attained after sufficiently high strain we observed complications in this behavior however during rate controlled 0 02 m s 1 ring shear experiments conducted on naturally aggregated dense loamy sand at low confining stresses 10 6 and 40 kpa the aggregated soil first dilated and then contracted to porosities less than initial values whereas the same soil with its aggregates destroyed monotonically dilated we infer that aggregates persisted initially during shear and caused dilation before their eventual breakdown enabled net contraction an implication of this contraction demonstrated in experiments in which initial soil porosity was varied is that the value of porosity distinguishing initially contractive from dilative behavior can be significantly larger than the critical state porosity which develops only after disaggregation ceases at high strains in addition post dilative contraction may produce excess pore pressures thereby reducing frictional strength and facilitating debris flow mobilization we infer that results of triaxial tests which generally produce strains at least a factor of 4 smaller than those we observed at the inception of post dilative contraction do not allow soil contraction to be ruled out as a mechanism for debris flow mobilization in dense soils containing aggregates

repeated fracture and healing of silicic magma generate flow banding and earthquakes textures in an exceptionally preserved effusive rhyolite conduit at torfajokull iceland indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit rfh process anastomosing tuffisite veins filled by fine grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval welding of the particulate material during subsequent deformation led to thorough healing of veins allowing repeated fracture of the same body of magma we propose that the rfh process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions the rfh process may also form the flow banding that is nearly ubiquitous in obsidian

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

triggering of liquefaction flow slides in coastal soil deposits abstract liquefaction flow slides have been observed in coastal soil deposits throughout the world the causes of many of these slides have never been well understood the materials involved in and circumstances surrounding eight reported liquefaction flow slides are reviewed and compared many of these slides have occurred in very similar materials shortly following small to moderate drawdowns of the adjacent water level liquefaction susceptibility of sands may be evaluated by steady state methods recent research indicates that liquefaction of loose sands similar to those typically involved in these slides may be triggered by very small changes in stress conditions if the sand is initially in equilibrium under high shear stresses the triggering of liquefaction may lead to flow sliding if the unbalanced driving stresses exceed the steady state strength of the liquefied soil elasticity methods are used to estimate the initial stress conditions in a gentle infinite slope and the effect of unloading due to drawdown it is shown that liquefaction flow slides in coastal soil deposits may be initiated by small to moderate drawdowns consistent with those observed in actual slides

explosive expansion of a slowly decompressed magma analogue evidence for delayed bubble nucleation 1 while ascending in the plumbing system of volcanoes magma undergoes decompression at rates spanning several orders of magnitude and set by a number of factors internal and external to the volcano slow decompression generally results in an effusive or mildly explosive expansion of the magma but counterexamples of sudden switches from effusive to explosive eruptive behavior have been documented at various volcanoes worldwide the mechanisms involved in this behavior are currently debated in particular for basaltic magmas here we explore the interplay between decompression rate and vesiculation vigor by decompressing a magma analogue obtained by dissolving pine resin into acetone in varying proportions analogue experiments allow direct observations of the processes of bubble nucleation and growth flow dynamics and fragmentation that is not currently possible with magmatic systems our mixtures contain solid particles and upon decompression nucleation of acetone bubbles is observed we find that mixtures with a high acetone content containing smaller and fewer solid particles experience strong supersaturation and fragment under very slow decompressions despite having low viscosity while mixtures with lower acetone content with more and larger solid particles degas efficiently without fragmentation we interpret our results in terms of delayed bubble nucleation due to a lack of efficient nucleation sites we discuss how a similar mechanism might induce violent explosive expansion in volatile rich and poorly crystalline low silica magmas by analogy with the behavior of rhyolitic magmas

microlite textures and volatile contents of obsidian from the inyo volcanic chain california 1 we test the hypothesis that microlite textures record progressive degassing during magma ascent we compare microlite number density nv and h2o concentrations in pyroclastic and effusive obsidian obs from the inyo volcanic chain ca comparisons reveal a weak correlation between microlite nv and h2o however most obsidians obss of a given volatile content exhibit a wide range of nv factors other than degassing must also influence final groundmass texture we postulate that obs textures record different ascent rates and residence histories prior to eruption from the conduit we conclude that microlites do not sensitively indicate the extent of magmatic degassing rather they provide qualitative indications of magma ascent and residence time

cyclic behavior of fine grained soils at different ph values the effects of ph on the liquefaction susceptibility of fine grained soils were examined by performing undrained cyclic ring shear tests on artificial mixtures and a natural soil under different ph conditions solutions of diluted sulphuric acid h2 s o4 and dissolved sodium hydroxide naoh were used to create acidic and alkaline environments respectively while distilled water was used as a reference liquid low plasticity kaolin and illite sand mixtures and a medium plasticity bentonite sand mixture were selected to investigate the influence of plasticity and clay mineralogy on the ph dependent response of soil to cyclic loading the results showed that the effects of ph were more pronounced for the medium plasticity mixture and depended greatly on the mineralogy of clay fraction for example in an acidic medium the kaolin sand mixture became slightly more resistant to liquefaction while the illite sand mixture became more susceptible to liquefaction the bentonite sand mixture was observed to b

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

magmatic volatiles in explosive rhyolitic eruptions obsidian clasts in rhyolitic tephra deposits preserve preeruption magmatic volatile contents providing a direct means for determining the volatile content of explosively erupted magmas small to moderate volume plinian eruptions 10 sup 3 to 10 sup 1 km sup 3 appear to be driven by 0 5 1 0 wt volatiles consisting dominantly of h sub 2 o with minor co sub 2 analysis of obsidian from eruptive sequences consisting of tephra and flows indicates that this hydrous magma abruptly overlies magma with only 0 1 0 2 wt h sub 2 o

explosive to effusive transition during the largest volcanic eruption of the 20th century novarupta 1912 alaska silicic volcanic eruptions commonly show abrupt shifts between powerful and dangerous plinian explosive episodes and gentle effusion of lava whether the onset of magma permeability and ensuing gas loss controls these transitions has been a subject of debate we measured porosities and permeabilities in samples from the a d 1912 eruption of novarupta volcano alaska and analyzed them within the context of a well constrained eruptive sequence that encompasses sustained explosive and effusive activity for the explosive samples we find that the degree of vesicle interconnectivity measured as the ratio of connected to total porosity decreases with phenocryst content and with increasing eruption intensity permeabilities of explosive samples show a weak dependence on porosity dome samples are not significantly different in permeability but are of lower porosity which together with abundant flattened vesicles is consistent with bubble collapse by permeable outgassing quantitative analysis indicates that outgassing alone was insufficient to affect the transition to effusive activity rather the change from explosive to effusive activity was probably a consequence of high versus low magma ascent rates

shear rate dependent strength control on the dynamics of rainfall triggered landslides tokushima prefecture japan a typhoon typhoon no 10 attacked shikoku island and the tyugoku area of japan in 2004 this typhoon produced a new daily precipitation record of 1317 mm on shikoku island and triggered hundreds of landslides in tokushima prefecture one catastrophic landslide was triggered in the shiraishi area of kisawa village and destroyed more than 10 houses while also leaving an unstable block high on the slope the unstable block kept moving after the event showing accelerating and decelerating movement during and after rainfall and reaching a displacement of several meters before countermeasures were put into place to examine the mechanism for this landsliding characteristic samples weathered serpentinite were taken from the field and their shear behaviours examined using ring shear tests the test results revealed that the residual shear strength of the samples is positively dependent on the shear rate which may provide an explanation for the continuous accelerating decelerating process of the landsliding the roughness of the shear surface and the microstructure of the shear zone were measured and observed by laser microscope and sem techniques in an attempt to clarify the mechanism of shear rate effect on the residual shear strength copyright 2010 john wiley sons ltd

variation in residual shear strength of the soil with the salinity of pore fluid slope stabilization procedures for seven landslides in the niigata prefecture of japan had provided an excellent and rare opportunity to examine the composition of ground water and residual shear strength of the sliding surface soil as well as unsheared mudstone ring shear tests were conducted on the soil samples collected from sliding surface soil revealed during the installation of 3 m diameter drainage galleries shear strength and index properties of the soil were measured by mixing the soil with distilled water and sea water and after leaching the nacl from the pore water the residual shear strength of the soil sample with distilled water was 3 5 lower than that with the sea water the residual shear strength of the intact rock powder after leaching the dissolved salt was close to that of the soil from sliding surface and the residual shear strength of the soil from sliding surface after mixing with sea water was close to that of the intact rock powder the increase in residual shear strength

geological hazards in loess terrain with particular reference to the loess regions of china abstract the considerable morphodynamic energy provided by the continuing tectonic evolution of asia is expressed in high erosional potentials and very high rates of sediment production that make this continent unequalled as a terrestrial source of primary silt many of these environments are hazardous threatening human occupation health and livelihood especially in regions of dense population such as the loess lands of north china dry loess can sustain nearly vertical slopes being perennially under saturated however when locally saturated it disaggregates instantaneously such hydrocompaction is a key process in many slope failures made worse by an underlying mountainous terrain of low porosity rocks gully erosion of loess may yield very high sediment concentrations 60 by weight characteristic vertical jointing in loess influences the hydrology enlarged joints develop into natural sub surface piping systems which on collapse produce a loess karst terrain collapsible loess up to 20 m thick is common on the western loess plateau foundation collapse and cracked walls are common many rapid events following periods of unusually heavy monsoonal rain slope failure is a major engineering problem in thick loess terrain flow slide and spread types being common the results are often devastating in both urban and rural areas an associated hazard is the damming of streams by landslides the human population increases the landslide risk in china notably through imprudent land use practices including careless water management a number of environmentally related endemic diseases arise from the geochemistry of loess and its groundwaters including fluorosis cretinism kaschin beck disease keshan disease and goitre the chinese desert margins also have a major atmospheric dust problem the effect of such dust upon human health in these extensive regions including many large cities has yet to be evaluated but pneumoconiosis is thought to affect several million people in north and west china

textural development of mixed finely porphyritic silicic volcanic rocks inyo domes eastern california magma mixing in silicic volcanic rocks of the inyo chain is manifest by bimodal mineral chemistry and distinctive banding defined by either variations in 1 glass color and chemistry or 2 microlite abundance within colorless glass bands composed of brown or microlite rich colorless glass are characterized by more mg rich mafic silicates and more calcic plagioclase than microlite poor colorless glass domains a rhyolitic and a dacitic mixing end member can be defined on the basis of this mineral distribution thermobarometry using the mineral assemblages unique to each end member indicates that prior to mixing the dacite was more oxidized and water rich than the rhyolitic magma determined mineral crystallization conditions compare favorably with experimentally derived phase equilibria for melts of broadly similar composition the spatial association of contrasting mineral assemblages with the flow banding in these rocks implies that the banding is a direct product of magma mixing it formed as either 1 volumes of the comingled magmas were immediately vented to the surface and quenched preserving domains of two distinct glass compositions or 2 bands of dacitic magma precipitated microlites when comingled with the rhyolitic magma in an attempt for the two magmas to approach equilibrium

the petrologic history of the sanganguey volcanic field nayarit mexico comparisons in a suite of crystal rich and crystal poor lavas abstract to evaluate if intermediate magmas erupting from volcan sanganguey mexico and the surrounding volcanic field are formed by mixing of basalts and rhyolites or if they initially exist as intermediate liquids a detailed petrological study is presented for eight andesite and dacite magmas six of the samples erupted from the central edifice four andesites and two dacites are crystal rich 50 vol whereas the remaining two samples one andesite and one dacite erupted from monogenetic vents in the peripheral volcanic field and are crystal poor 5 vol despite the variation in crystallinity all samples are multiply saturated in five to seven mineral phases plagioclase orthopyroxene titanomagnetite ilmenite apatite clinopyroxene hornblende in all samples plagioclase spans a 30 40 mol an range in composition and orthopyroxene spans a range in mg of 5 10 pre eruptive temperatures and oxygen fugacites relative to the nno buffer range from 853 24 to 1085 16 c and 0 1 0 1 to 0 9 0 1 nno on the basis of fe ti two oxide thermometry application of the plagioclase liquid hygrometer to the samples reveals maximum h 2 o contents that range from 1 7 6 2 wt comparison with phase equilibrium experiments demonstrates that all plagioclase and orthopyroxene compositions in the crystal poor samples could have grown from their respective whole rock compositions comparison of crystal rich samples with phase equilibrium experiments reveals the presence of sodic xenocrysts which reflect resorption textures and an estimated excess plagioclase crystal cargo of 6 vol the excess plagioclase crystal cargo is not distinguishable from phenocrystic plagioclase based on composition or texture suggesting that they were also grown in intermediate melts and are therefore described as antecrystic no calcic plagioclase xenocrysts an 79 typical of hydrous arc basalts are observed thus it is likely that the excess plagioclase in the crystal rich samples were originally formed in intermediate magmas for the crystal poor samples we propose that the mechanism producing the complex phenocryst assemblages is degassing cooling as it may shift equilibrium plagioclase compositions kinetically inhibit crystal growth and increase melt viscosity leading to complex textures notably the hypothesis of degassing cooling induced crystallization requires that the intermediate melts initially exist as liquids prior to crystallization supporting the hypothesis that intermediate melts are generated in the deep crust and arrive in the upper crust as liquids for the crystal rich samples degassing cooling may also be the mechanism generating a portion of the compositional and textural variation in the mineral assemblages and some incorporation of antecrysts or xenocrysts must occur as evidenced by an excess plagioclase crystal cargo however we find no definitive evidence supporting the incorporation of crystals initially grown in basalts or rhyolites given the similarities in phase assemblage mineral compositions mineral textures and intensive variables between the crystal poor and rich samples we conclude that the melts arriving into the upper crust beneath volcan sanganguey and the surrounding peripheral volcanic field are intermediate in composition and are initially formed as liquids in the deep crust

the loess of north central china geotechnical properties and their relation to slope stability abstract loess is an aeolian deposit consisting of predominantly silt sized quartz particles and containing variable amounts of clay sized minerals loess is generally classified as a water softening material because upon wetting the loess fabric rapidly weakens or collapses the strain hardening of malan loess and the brittle failure of lishi and wucheng loess are in strong contrast to the failure behaviour of these loess deposits in a remoulded state from tests carried out on samples with varying moisture contents using a modified bromhead ring shear apparatus it was found that the effective apparent cohesion gradually increases and the effective internal friction angle decreases with an increase in moisture content when the moisture content reaches a material specific threshold the effective cohesion decreases rapidly and the effective internal friction angle stabilises at a residual value the frequent failure of loess slopes in the western part of the chinese loess plateau is closely related to progressive weathering along zones in these slopes which causes a dramatic decrease in strength from the peak strength condition progressive weathering is common in the loess slopes in the western part of the chinese loess plateau during the process shear strength reduction along potential slip surfaces may be achieved by leaching of readily soluble salts destruction of cementation bonds and redistribution of particles localized collapse of the loess fabric causes internal deformation and consequently peak strength conditions are concentrated on a progressively smaller area of the failure plane therefore the mode of failure of loess slopes is generally determined by brittle failure of the undisturbed and unweathered central parts of the slopes it is important that both the weathered and unweathered strength of the loessmaterials in this area be established in order to analyze the stability of existing loessslopes many of which are steepand lie directly above domestic and industrial urban areas

the effect of calcium ions in pore water on the residual shear strength of kaolinite and sodium montmorillonite this paper reviews the literature concerning the effects of pore water chemistry on residual shear strength it presents the results of index limit and residual shear strength testing undertaken on kaolinite and sodium montmorillonite using concentrations of calcium chloride similar to the calcium levels measured in groundwaters obtained from landslipped calcareous mudrock of the fuller s earth formation the residual shear strength of the sodium montmorillonite increased markedly for the sample treated and tested with 80 mg 1 cat compared with that tested with deionized water greater concentra tions of ca2 resulted in only a marginal further increase in the residual shear strength these changes were also reflected in the interpreted parameters or and c the liquid limit of the montmorillonite decreased dramatically when the samples were treated with 80 mg 1 ca2 and this effect persisted and or was slightly enhanced when the concentration of ca2 was increased the residual shear strength

a nanolite record of eruption style transition microlite characteristics have been successfully used as indicators of magma decompression rate however microlites do not record the explosive effusive transitions in sub plinian eruptions when such transitions are governed by the decrease of exsolved volatile pressure in the shallow conduit rather than by the magma ascent rate to overcome this limitation we studied the nanolites in the quenched products of the 2011 shinmoedake japan eruption nanolites are nanometer scale components of the groundmass minerals and exhibit a steeper slope of crystal size distribution than that of microlites in the 2011 eruption the style of activity had undergone transformation from sub plinian eruption to vulcanian explosion and intermittent effusion of lava we found that although the products formed by different eruptive activities have similar microlite characteristics such products can be distinguished clearly by their mineral assemblage of nanolites the samples of pumice of sub plinian eruptions and vulcanian explosions and the dense juvenile fragments of lava in descending order of explosivity contained respectively nanolites of low ca pyroxene low ca pyroxene plagioclase and low ca pyroxene plagioclase fe ti oxides nanolites crystallize when undercooling of the magma due primarily to dehydration increases rapidly near the surface the water contents of the interstitial glass indicate that the quenched depths did not differ greatly between eruption styles hence the different nanolite assemblages of each eruption style are assumed to have resulted from differences in magma residence time near the surface therefore we propose that nanolites have the potential to indicate the physicochemical conditions of magma at the transition points of eruption styles

failure mechanisms in loess and the effects of moisture content changes on remoulded strength abstract investigations into the failure mechanisms of loess slopes in the loess region of north central china have shown that a majority of mass movements are associated with shallow slab slides predominantly moving under tension the effects of soil moisture content are of paramount importance when dry vertical slopes in loess may exceed 15 m in height but under conditions close to saturation the structural strength of loess rapidly decreases causing failure of the slopes mass failure of loess occurs as collapse due to hydroconsolidation and disintegration of the loess fabric as a result of liquefaction or fluidization and can be associated with progressive failure processes local destruction of the openwork fabric of the loess can lead to small plastic movements within the loess deposits this is as close as loess can come to creep

variations in eruption style during the 1931 a d eruption of aniakchak volcano alaska abstract the 1931 a d eruption of aniakchak volcano alaska progressed from subplinian to effusive eruptive style and from trachydacite to basaltic andesite composition from multiple vent locations eyewitness accounts and new studies of deposit stratigraphy provide a combined narrative of eruptive events additional field compositional grain size componentry density and grain morphology data document the influences on changing eruptive style as the eruption progressed the eruption began on 1 may 1931 a d when a large subplinian eruption column produced vesicular juvenile rich tephra subsequent activity was more intermittent as magma interacted with groundwater and phreatomagmatic ash and lithic rich tephra was dispersed up to 600 km downwind final erupted products were more mafic in composition and the eruption became more strombolian in style stratigraphic evidence suggests that two trachydacitic lava flows were erupted from separate but adjacent vents before the phreatomagmatic phase concluded and that basaltic andesite lava from a third vent began to effuse near the end of explosive activity the estimated total bulk volume of the eruption is 0 9 km 3 which corresponds to approximately 0 3 km 3 of magma eruption style changes are interpreted as follows 1 a decrease in magma supply rate caused the change from subplinian to phreatomagmatic eruption 2 a subsequent change in magma composition caused the transition from phreatomagmatic to strombolian eruption style additionally the explosion and effusion of a similar magma composition from three separate vents indicates how the pre existing caldera structure controlled the pathway of shallow magma ascent thus influencing eruption style

fragmentation phenomena in populations of magmatic crystals fragmentation of crystals is an important mechanism and a component of particle dynamics in igneous and metamorphic rocks that has received surprisingly little attention recent advances in textural analysis extraction techniques digital imaging and computer assisted measurements enable rapid accumulation of 3d data on particle shapes and size distributions this paper reviews fragment size distributions fsd that result from fragmentation lognormal fractal loggamma and weibull discusses their genesis mechanisms and presents relevant examples of fragmentation from experimental physics next the paper considers fsds of feldspars on digitized images of thin sections and on published images and quartz extracted from vesicular pumice by acid from eight well known large volume eruptive units the acid solution of pumice enables examination of volume abundance 3d shapes proportions of fragmented crystals and measurements of their csds and fsds fsds were also measured in samples of welded tuff and a granite disaggregated by electric pulse products of syneruptive shock wave fragmentation and fragmentation by an electric pulse are found to be fractal with large breakage probabilities branching ratios and fractal dimensions of 2 to 3 in contrast most quartz fragments in pumice obey a lognormal distribution and fragmentation is driven by a melt inclusion decrepitation mechanism which results in low breakage probability and small number 2 3 of fragments per breakage cycle these results are consistent with one atmosphere heating experiments of quartz phenocrysts that led to melt inclusion decrepitation and caused quartz to break up into several smaller pieces collectively having lognormal fsd measured melt inclusion size distributions suggest decrepitation of outermost melt inclusions and low survival rate for large inclusions and inclusions with large radius crystal size ratio the modeling of periodic fragmentation of crystals with melt inclusions due to overheating and or decompression which may occur many times during the lifetime of a long lived magma body may explain concave down lognormal csds abundant in igneous rocks the genesis of lognormality can be explained by the fragmentation algorithm of kolmogorov 1941 other algorithms may generate lognormal like loggamma distributions fragmentation serves as an important size limiting factor a nucleation aid and it facilitates isotopic and trace elemental exchange

magma fragmentation by rapid decompression the processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology observations of explosive eruptions1 3 as well as theoretical analyses of the underlying processes4 have raised the question of whether the rapid decompression of highly viscous vesicular magma that results from the collapse of a lava dome or volcanic edifice can in itself produce explosive magma fragmentation and pyroclast formation here we report the results of a laboratory investigation of pyroclast formation following rapid decompression samples of magma from the 1980 eruption of mount st helens when rapidly depressurized from initial pressures of up to 12 mpa and at temperatures in the range 750 825 c fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions moreover we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events

understanding which parameters control shallow ascent of silicic effusive magma the estimation of the magma ascent rate is key to predicting volcanic activity and relies on the understanding of how strongly the ascent rate is controlled by different magmatic parameters linking potential changes of such parameters to monitoring data is an essential step to be able to use these data as a predictive tool we present the results of a suite of conduit flow models soufriere that assess the influence of individual model parameters such as the magmatic water content temperature or bulk magma composition on the magma flow in the conduit during an extrusive dome eruption by systematically varying these parameters we assess their relative importance to changes in ascent rate we show that variability in the rate of low frequency seismicity assumed to correlate directly with the rate of magma movement can be used as an indicator for changes in ascent rate and therefore eruptive activity the results indicate that conduit diameter and excess pressure in the magma chamber are amongst the dominant controlling variables but the single most important parameter is the volatile content assumed as only water modeling this parameter in the range of reported values causes changes in the calculated ascent velocities of up to 800

crystallization history of obsidian dome inyo domes california samples obtained by u s department of energy research drilling at the 600 year old obsidian dome volcano provide the rare opportunity to examine the transition from volcanic dome to plutonic intrusion textures in a silicic magma system textures in the lavas from obsidian dome record multiple periods of crystallization initiated in response to changes in undercooling t related to variable degassing in the mag ma phenocr ysts formed first at low t a drastic increase in t related to loss of a vapor phase during initial stages of eruption caused nucleation of microlites all of the lavas thus contain phenocrysts and microlites extrusion and subsequent devitrification of the dry 0 1 wt h2o magma crystallized spherulites and fine grained rhyolite at high t a granophyric texture representing crystallization at a moderate t formed in the intrusions beneath obsidian dome textures in the intrusion apparently represent crystallization of hydrous 1 2 wt h2o rhyolitic magma at shallow depths

morphology effusion rates and petrology of postglacial lavas of laguna del maule volcanic field chilean andes and implications for their plumbing system we analyzed the dimensions morphology and petrography of nine postglacial lava flows ranging from andesites to rhyolites located at the laguna del maule volcanic field in the southern andes of chile our data were used in conjunction with theoretical models of magma ascent and lava flow advance to estimate time scales of lava emplacement effusion rates magma chamber volume and conduit dimensions the results indicate that the parameters of these past eruptions correspond to eruptions of months to a few years in duration with peak effusion rates 100m 3 s and were fed by magma chambers of 6 to 30km 3 and conduits of radius 20 to 70m thermobarometric results indicate crystallization pressures between 260 and 450mpa and temperatures of 780 to 1060 degrees c for the analyzed units these results taken together with recent geophysical studies suggest that the most likely scenario in case of an eruption corresponds to a medium size eruption 0 1 1km 3 initial activity would be explosive followed by the extrusion of lava flows that can last several months

eruption dynamics and explosive effusive transitions during the 1400 cal bp eruption of opala volcano kamchatka russia abstract deposits and pumice from the 1400 cal bp eruption of opala volcano record activity that occurred at the explosive effusive transition resulting in intermittent or stop start behavior where explosive activity resumed following a pause the eruption deposited distinctive biotite bearing rhyolite tephra across much of kamchatka and its stratigraphy consists of a lithic rich pumice fall overlain by pumice falls and pyroclastic density deposits with the proportion of the latter increasing with height this sequence repeats such that the middle of the total deposit is marked by a lithic rich fall with abundant obsidian clasts notably the eruptive pumice are poorly vesiculated with vesicle textures that record fragmentation of a partially collapsed magmatic foam the eruption vent baranii amphitheater is filled with obsidian lavas of the same composition as the rhyolite tephra based upon the stratigraphic and compositional relations we divide the eruption into four phases phase i initiated with eruption of a lithic rich pumice fall followed by eruption of plinian falls and pyroclastic density currents during phase ii the eruption paused for at least 5 6 h in this time microlites nucleated and began to grow in the magma phase iii essentially repeated the phase i sequence obsidian lavas were emplaced during phase iv the pumice textures suggest that the magma ascended very near the threshold decompression rate for the transition between explosive fast and effusive slow behavior the pause during phase ii likely occurred as decompression slowed enough for the magma to develop sufficient permeability for gas to escape resulting in collapse of the magmatic foam stopping the eruption and temporarily sealing the conduit after about 5 6 h eruption resumed with once again magma decompressing very near the explosive effusive transition phase iii ended when the decompression rate slowed and lava dome emplacement began distributions of pumice and lithic clasts and inclusion of data from previous workers indicate minimum deposit volumes of 0 75 and 0 75 1 15 km3 dre and eruption column heights of 18 and 20 km for phases i and iii respectively phases i iii had a likely total duration of 60 80 h including a pause in activity of 5 6 h during phase ii this study demonstrates that analysis of vesicle textures from numerous pumice combined with stratigraphic data can reveal syn eruptive changes in and links between magma permeability decompression rate and eruption style

plagioclase nucleation and growth kinetics in a hydrous basaltic melt by decompression experiments isothermal single step decompression experiments at temperature of 1075 c and pressure between 5 and 50 mpa were used to study the crystallization kinetics of plagioclase in hydrous high k basaltic melts as a function of pressure effective undercooling teff and time single step decompression causes water exsolution and a consequent increase in the plagioclase liquidus thus imposing an effective undercooling teff accompanied by increased melt viscosity here we show that the decompression process acts directly on viscosity and thermodynamic energy barriers such as interfacial free energy controlling the nucleation process and favoring the formation of homogeneous nuclei also at high pressure low effective undercoolings in fact this study shows that similar crystal number densities na can be obtained both at low and high pressure between 5 and 50 mpa whereas crystal growth processes are favored at low pressures 5 10 mpa the main evidence of this study is that the crystallization of plagioclase in decompressed high k basalts is more rapid than that in rhyolitic melts on similar timescales the onset of the crystallization process during experiments was characterized by an initial nucleation event within the first hour of the experiment which produced the largest amount of plagioclase this nucleation event at short experimental duration can produce a dramatic change in crystal number density na and crystal fraction triggering a significant textural evolution in only 1 h in natural systems this may affect the magma rheology and eruptive dynamics on very short time scales

experimental constraints on plagioclase crystallization during h2o and h2o co2 saturated magma decompression experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions interpretation of natural samples may be complicated however by both the decompression path and the composition of exsolving volatiles which affect the time evolution of crystal textures here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of mount st helens rhyodacite three families of experiments were employed to simulate varied ph2o t trajectories single step h2o saturated decompression ssd continuous h2o saturated decompression cd continuous h2o co2 saturated decompression quantitative textural data crystal abundance number density and size are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products these are the first experiments to quantify feldspar nucleation and growth rates during h2o co2 saturated decompression we find that reducing the initial melt water content through addition of co2 increases nucleation rates relative to the pure water case an effect most pronounced at low dp dt moreover these early formed textural distinctions persist at the lowest pressures examined suggesting that deep h2o co2 fluids could leave a lasting textural fingerprint on erupted magmas crystals formed prior to decompression during the annealing process also modulate sample textures and growth on pre existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions the phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption finally quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive effusive transition at mount st helens this comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the may 18 plinian eruption such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories

formation of phreatic caves in an eogenetic karst aquifer by co2 enrichment at lower water tables and subsequent flooding by sea level rise formation of extensive phreatic caves in eogenetic karst aquifers is widely believed to require mixing of fresh and saltwater extensive phreatic caves also occur however in eogenetic karst aquifers where fresh and saltwater do not mix for example in the upper floridan aquifer these caves are thought to have formed in their modern settings by dissolution from sinking streams or by convergence of groundwater flow paths on springs alternatively these caves have been hypothesized to have formed at lower water tables during sea level low stands these hypotheses have not previously been tested against one another analyzing morphological data and water chemistry from caves in the suwannee river basin in north central florida and water chemistry from wells in the central florida carbonate platform indicates that phreatic caves within the suwannee river basin most likely formed at lower water tables during lower sea levels consideration of the hydrological and geochemical constraints posed by the upper floridan aquifer leads to the conclusion that cave formation was most likely driven by dissolution of vadose co2 gas into the groundwater sea level rise and a wetter climate during the mid holocene lifted the water table above the elevation of the caves and placed the caves tens of meters below the modern water table when rising water tables reached the land surface surface streams formed incision of surface streams breached the pre existing caves to form modern springs which provide access to the phreatic caves phreatic caves in the suwannee river basin are thus relict and have no causal relationship with modern surficial drainage systems neither mixing dissolution nor sinking streams are necessary to form laterally extensive phreatic caves in eogenetic karst aquifers dissolution at water tables potentially driven by vadose co2 gas offers an underappreciated mechanism to form cavernous porosity in eogenetic carbonate rocks copyright 2012 john wiley sons ltd

experimental fragmentation of crystal and vesicle bearing silicic melts we experimentally investigate the effect of crystals on the fragmentation behavior of a three phase melt gas crystals system under rapid decompression starting materials are cylinders of hydrated haplogranite melts containing alumina crystals that are placed at 6 30 mpa and 600 800 c in a fragmentation bomb for foaming subsequently these bubble and crystal bearing melts are rapidly decompressed within 30 mpa for phenocryst and microlite bearing samples

long period seismicity during magma movement at volcan de colima during the period from february to september 2005 volcan de colima produced 30 vulcanian explosions of sufficient magnitude to produce pyroclastic flows of variable size with a total volume of at least 2 5 106 m3 swarms of long period events were associated with each event their duration ranging from about 6 h to 3 days and each swarm containing up to 886 events the characteristics of the swarms have been studied to understand the source mechanism and their relationship with the vulcanian explosions in total 12 548 long period events were analysed using various comparative and statistical methods patterns were not apparent in the data with no correlation between different properties of the swarms duration magnitude or frequency of occurrence of lp events and the magnitude of the associated vulcanian explosion whether recorded by seismicity volume of pyroclastics or altitude of the eruption column this along with other characteristics of the swarms such as the continuation of the swarm after the explosion with an increase in long period event amplitude in some cases suggests that the mechanism is not merely associated with the pressurization under an impermeable cap and resulting pressure differentials between adjacent volumes within the system it is more likely that the production of long period events is dominated by brittle fracturing on the margins of an ascending magma body a model is proposed whereby the unloading above the ascending magma column produced by a vulcanian explosion resulted in an increase in ascent rate reflected in the increasing amplitude of long period events the results reflect the complexity of non linear processes involved during magma ascent degassing crystallization and rupture of the impermeable plug during the vulcanian process at volcan de colima as at many volcanoes long period events represent a useful precursor for eruptive activity for monitoring this paper highlights some useful analyses that can be carried out which could illustrate certain characteristics of an eruptive episode a preliminary model is presented of the conduit processes at work during the cyclic extrusive and explosive activity during 2005

characterizing hydrological processes on loess slopes using electrical resistivity tomography a case study of the heifangtai terrace northwest china abstract from the perspective of engineering geology loess has long been considered as a homogeneous and porous material it is commonly believed that water penetrates loess via pores and in some cases causing mass movements however several researchers have expressed doubts about this mechanism as a cause of slope failures in loess and moreover the actual hydrological processes operating in loess deposits and their effect on slope failures have not been fully investigated here we present the results of an electrical resistivity survey of the heifangtai loess terrace in northwestern china designed to characterize the hydrological processes in loess slopes and their relationship with slope failures the heifangtai loess terrace is located on the fourth terrace of the yellow river and consists of 57 m thickness of aeolian loess 2d and 3d electrical resistivity tomography ert was used to monitor the movement of ground water before and after irrigation and rainfall events and the evolution of a sink hole in the toe of the landslide deposits our main findings are as follows 1 based on the 2d ert results the depth of infiltration into the thick unsaturated loess is not more than 5 m in the profile at the top of the landslide 2 electrical resistivity decreased as a result of water infiltration through sinkholes and this process can increase the soil water content and induce soil liquefaction which can eventually result in land sliding 3 landslide deposits block the groundwater drainage channels through the loess which results in the concentration of water in the toe of the landslide consequently groundwater together with rainfall triggers the failure of sinkholes or cracks which may induce a continuing process of new slope failures at the sites of past landslide

reconstruction of the sibinal pumice an andesitic plinian eruption at tacana volcanic complex mexico guatemala abstract the tacana volcanic complex located on the mexico guatemala border has had numerous small historical explosions the latest being a phreatic explosion in 1986 the stratigraphic record however suggests that much more voluminous eruptions have occurred in the past here we document one such eruption the sibinal pumice deposit which occurred ca 23 540 years b p the deposit consists of two pumice rich units 1 a lower stratified member sm that consists of at least seven normally graded fall layers interbedded with pyroclastic wet surge layers and 2 an upper massive member mm made up of a single fall deposit both members can reach up to 2 5 m in thickness and are separated by a single massive indurated yellowish pumice rich reworked layer sm was dispersed to the northeast n70 e with its 6 cm isopach covering an area of 275 km2 whereas mm was dispersed to the north n22 e with its 70 cm isopach covering an area of ca 330 km2 yellow vesicular pumice fragments light gray lithics and hydrothermally altered red and black dense lithics are abundant in both members although altered lithics are more abundant at the base of each reaching 15 vol pumice compositions range from basaltic to andesitic 48 61 wt sio2 anhydrous basis but are highly altered they contain plagioclase andesine labradorite augite hypersthene fe ti oxides and rare amphibole no compositional or mineralogical differences occur between the units indicating a common magma source during the same eruption the sibinal pumice eruption started with a weak pulsating column that reached at most 19 km in height ejecting 2 9 km3 of tephra 1 1 km3 dre at an average mass discharge rate of 4 7 107 kg s with repeated hydromagmatic explosions that generated wet surges down the slopes of the volcano the eruption ceased for a while during which rainfall generated a widespread lahar that eroded the top of sm the eruption began anew with a sustained and stable plinian column that ejected 4 6 km3 1 9 km3 dre of tephra at a mass discharge rate of 8 1 107 kg s the lack of any compositional differences in the magma suggests that the change in eruptive style was driven by influxes of external water provoking hydromagmatic explosions that cleared the conduit then the system changed to dry system and the eruption style changed from pulsating to stable plinian as the conduit became wider the mass discharge rate also increased

effect of syneruptive decompression path on shifting intensity in basaltic sub plinian eruption implication of microlites in yufune 2 scoria from fuji volcano japan abstract to constrain the timing and conditions of syneruptive magma ascent that are responsible for shifting eruption intensity we have investigated a basaltic sub plinian eruption that produced yufune 2 scoria in fuji volcano 2200 years ago we deduced magmatic decompression conditions from groundmass microlite textures including decompression path i e evolution in decompression rate and approximate decompression rate in order to relate them to eruption intensity the microlites revealed decompression conditions after water saturation at 700 1100 m depth the temporal change in scoria size indicates that the magma discharge rate and resultant eruption intensity increased from unit a to unit b and then declined toward ending units d and e the overall decompression rate in each eruptive unit has a positive correlation with eruption intensity the variation in decompression rate was enlarged in the final units where the maximum remained the same as the peak through the eruption 0 13 0 22 mpa s for units b and c while the minimum was 0 025 mpa s the large variation here is due to 1 variation in flow velocity across conduit and 2 part of the erupted magma in unit d experienced remarkably slow decompression 0 002 0 003 mpa s resulting from decreased overpressure in the reservoir following the major eruption of unit b furthermore crystal size distribution csd of microlites implied that the earliest erupted magma unit a had once been decompressed slowly 0 005 0 012 mpa s having been arrested by material in the conduit vent system which was followed by an increase in decompression rate due to removal of the material at the initiation of the eruption in addition the magma that had been ascending slowly before the unit d eruption may record the increase in decompression rate this increased rate resulted from being pushed up by the successive magma at the start of that eruption two factors had a major impact on eruption intensity first magma decompression rate determined the degree of gas phase separation from ascending magma judging from csd different decompression rates had been generated at least at the start of microlite crystallization the second factor is the conduit radius that in combination with magma ascent rate controlled the magma discharge rate before the major eruption of unit b the conduit radius likely increased as evidenced by xenoliths of basaltic lava and lithic fragments with the same petrography as the xenoliths in unit a in unit e the conduit radius decreased through inward development of high density magma from the conduit margin

chemo mechanical effects in kaolinite part 1 prepared samples the results of laboratory tests on chemo mechanical interactions in kaolinite samples prepared with inorganic salt acid and base solutions are presented in particular the effects of the different solutions on the liquid limit one dimensional compressibility permeability and residual shear strength are considered kaolinite proves relatively insensitive to salt solutions whereas variations in pore fluid ph cause large strength changes associated with non negligible changes in one dimensional compressibility the partly inconsistent effects induced by acids and bases on the residual shear strength and on the one dimensional compressibility do not permit any easy interpretation of the experimental evidence mineral dissolution and the associated formation of new compounds are probably involved in the behaviour observed these aspects will be investigated in part 2 of this paper

residual strength of kaolin and bentonite the influence of their constituent pore fluid the residual strength of clays is affected by their mineral composition and by the nature of their constituent pore fluid in principle both factors may influence either strength parameters or stresses but it is not yet completely clear how this influence works previous publications have elucidated some aspects of clay residual strength behaviour this paper investigates further the roles of minerals and pore fluid composition on the residual strength of clayey soils it describes and analyses tests on kaolin bentonite and their mixtures exposed to distilled water and sodium choride solutions with given concentrations the results show that the shear strength of kaolin is not affected by the solutions used whereas the residual strength of bentonite varies greatly because of the inward salt diffusion towards the clay further for the clay mixtures with any of the pore fluids considered the weaker component has the greater influence on the behaviour of the mixture la resistance residuelle des argile

eruption dynamics inferred from microlite crystallization experiments application to plinian and dome forming eruptions of mt pelee martinique lesser antilles decompression experiments have been conducted to simulate syn eruptive crystallization in volcanic conduits to infer magma ascent rates and conditions during dome forming and plinian eruptions of silicic arc volcanoes the experiments were carried out on starting material with the composition of mt pele e rhyolitic interstitial melt 76 wt sio2 and consisted of three consecutive steps hydration decompression and annealing hydration saturated and undersaturated was performed at 8508c and 200mpa and was followed by isothermal decompression either linearly or stepwise to a final pressure pf of 30 or 5 10mpa decompression rates range from 0 003 to 25mpamin 1 decompression durations of 15 min to 40 days two samples were cooled by 258c and 508c during a 3 day step at pf subsequent to decompression the samples were held for up to 15 days at pf the experiments generated three types of crystals as products of pre syn and post decompression crystallization the experiments basically differ from previous studies in that they are specifically designed to discriminate crystal nucleation from growth and to evaluate the influence of pre decompression crystals on the decompression induced crystallization the effects of pre decompression crystals decompression rate undercooling pf and terminal cooling have been determined on plagioclase nucleation growth morphology and composition the main results suggest a positive correlation between decompression rate and the number density of plagioclase crystals nucleated at pf and highlight the effect of pre decompression crystals in further decompression induced crystallization the relations between the decompression conditions and the plagioclase characteristics have been used to infer mt pele e eruption dynamics suggesting that 1 plinian magmas ascend from the reservoir within less than 1h 1 10 m s 2 dome and block and ash flow magmas ascend within more than 2 5 days giving time for syn decompression crystallization around pre existent microlites 3 dome magmas provide evidence for long stagnation and cooling at low pressure and 4 surge magmas ascend without significant crystallization within less than 4 days and massively nucleate plagioclase at very low pressure the extent and violence of dome destruction may depend on the size or age of the dome with large or old domes favouring mildly explosive block and ash flows whereas small or young protodomes may generate highly explosive surges

using microlites to gain insights into ascent conditions of differing styles of volcanism at soufriere hills volcano abstract microlite textural characteristics are analyzed in the products of vulcanian explosions ash venting occurring synchronously with lava effusion syn extrusive ash venting a precursory explosion and lava dome effusion associated with phases 3 and 5 of the current soufriere hills volcano shv eruption 2d microlite population statistics and crystal size distributions are used to infer decompression pathways for each style of volcanism in addition magma ascent rates for each eruptive style were calculated using microlite number density toramaru et al 2008 strong differences between pyroclastic and effusive microlites are observed in 2d population statistics and crystal size distributions indicating higher amounts of smaller microlites 0 0084 to 0 012 mm however syn extrusive ash venting samples have comparatively greater amounts of large microlites the similarity between lava dome and vulcanian explosions at larger microlite sizes suggests comparable crystallization conditions in the deeper conduit system 2 km depth the contrast in eruption style despite similarity at depth suggests a shallow control to eruption processes at shv the greater amounts of large microlites in syn extrusive ash venting compared to the lava dome indicates two distinct decompression pathways within a single conduit inferred to result from differences in ascent rate sluggish conduit margin ascent leading to lower decompression rates is inferred to produce the increased degree of crystal growth observed in syn extrusive ash venting compared with the lava dome calculated magma ascent rates for vulcanian explosions and syn extrusive ash venting have similar ranges while lava effusion is up to a factor of 10 faster differences in magma ascent rate are inferred to result from variations in the depth at which ascent rate is calculated for each eruptive style ascent rate is calculated at microlite nucleation depth using the method of toramaru et al 2008 however the average microlite nucleation depth will vary because of differences in microlite nucleation rate during ascent more small microlites in the lava dome suggest greater nucleation rates in the shallow conduit recording an on average shallower magma ascent rate 2 km depth

experimental constraints on parameters controlling the difference in the eruptive dynamics of phonolitic magmas the case of tenerife canary islands we have performed phase equilibrium experiments to determine the pre eruptive conditions of the explosive eruption of montana blanca 2020 bp that occurred from a satellite vent located in the east flank of teide volcano tenerife island crystallization experiments were performed using a phonolitic obsidian from the fall out deposit that contains 5 wt anorthoclase diopside and magnetite with minor amounts of biotite and ilmenite set in a glassy matrix that contains microlites of ca rich alkali feldspars temperature was varied between 850oc and 800oc and pressure between 200mpa to 50 mpa the oxygen fugacity fo2 was varied between nno 0 2 0 2 log units above to the ni nio solid buffer to nno 2 whilst dissolved water contents varied from 7 to 1 5 wt comparison between natural and experimental phase proportions and compositions indicates that the main body of phonolite magma was stored at 850 15oc 50 20mpa 2 5 0 5wt h2o at an fo2 around nno 0 5 prior to eruption equivalent to depths between 1 to 2 km below the surface some clinopyroxene hosting h2o rich melt inclusions possibly originates from intermittent supply of phonolitic magma stored at somewhat deeper levels 100 mpa the ca and fe rich composition of alkali feldspar phenocrysts rims and microlites attests to the intrusion of a mafic magma in the reservoir just prior to eruption as borne out by banded pumices appearing in the later products of the eruptive sequence the comparison with other phonolitic magmas from tenerife and elsewhere e g vesuvius laacher see shows that differences in the eruptive dynamic of phonolitic magmas can be correlated to differences in storage depths along with variation in pre eruptive volatile contents

conduit processes during the climactic phase of the shinmoe dake 2011 eruption japan insights into intermittent explosive activity and transition in eruption style of andesitic magma abstract the climactic phase of the shinmoe dake 2011 eruption was characterized by sub plinian events january 26 p m 27 a m and 27 p m and lava accumulation in the crater late on january 27 to january 29 all of which were accompanied by vulcanian events the magma discharge rate for lava accumulation was close to that of the sub plinian events we discuss evolution of syneruptive magma ascent through the climactic phase on the basis of decompression induced crystallization of groundmass microlites in order to reveal how the final eruption style was determined at the explosive effusive transition we examined pumice and lava like pyroclasts from three sub plinian events the january 28 vulcanian event and a lava block ballistically ejected during the february 1 vulcanian event the samples exhibit variable groundmass textures due to variable syneruptive ascent conditions not due to pre eruptive inhomogeneity of the andesitic magma and different degrees of syneruptive volatile exsolution preceding quenching eruptive units were first characterized according to bulk density assuming that slower ascent resulted in higher density a decrease in the magma ascent rate led to a gradual shift to an effusive eruption style there was little variation in ascent rate among magmas erupted simultaneously during the first and second sub plinian events bulk density 0 8 2 1 g cm3 but differences increased in the third sub plinian and january 28 vulcanian eruptions with minor appearance of slow magma 0 9 2 8 g cm3 only slow magma occupied the shallow conduit at the completion of lava accumulation february 1 lava 2 1 g cm3 among the first and second sub plinian units the early stage of the second sub plinian event had the greatest variation in ascent rate with extension to a slightly higher bulk density 1 0 2 1 g cm3 than the other events 0 8 1 7 g cm3 ascent rate variations in the early stage of the second sub plinian event the third sub plinian event and the january 28 vulcanian event are all due to the preceding calm phase in the intermittent explosive activity the moving magma column which showed a vertical zonation in the degree of degassing formed in the shallow conduit during the calm phase due to a decrease in the discharge rate and this resulted in the co ejection of magmas with variable degassing upon the start of explosive events the crystal size distribution csd of plagioclase microlites constrains the syneruptive branching i e a point of divergence in ascent rate and eruption style which occurred after the start of microlite crystallization the csds of the different samples show differences only in small crystal sizes slope at these size fractions is smallest for low density 2 0 g cm3 of the january 28 and february 1 events and greatest for high density pyroclasts 2 0 g cm3 of the third sub plinian event we propose based on the similarity in csd that the shallow magma feeding system was common for the two craters active during the january 28 explosive effusive hybrid activity where the vulcanian event and emplacement of lava of the later february 1 event occurred in different parts of the original crater the samples have variable plagioclase microlite number densities 1 4 105 3 5 106 mm 3 based on variations in small crystal sizes among different csds this variability resulted not only from the usual increasing trend in number density with increasing degree of undercooling but also the decreasing trend associated with the strong undercooling during explosive eruptions

transitions between explosive and effusive phases during the cataclysmic 2010 eruption of merapi volcano java indonesia transitions between explosive and effusive activity are commonly observed during dome forming eruptions and may be linked to factors such as magma influx ascent rate and degassing however the interplay between these factors is complex and the resulting eruptive behaviour often unpredictable this paper focuses on the driving forces behind the explosive and effusive activity during the well documented 2010 eruption of merapi the volcano s largest eruption since 1872 time controlled samples were collected from the 2010 deposits linked to eruption stage and style of activity these include scoria and pumice from the initial explosions dense and scoriaceous dome samples formed via effusive activity as well as scoria and pumice samples deposited during subplinian column collapse quantitative textural analysis of groundmass feldspar microlites including measurements of areal number density mean microlite size crystal aspect ratio groundmass crystallinity and crystal size distribution analysis reveal that shallow pre and syn eruptive magmatic processes acted to govern the changing behaviour during the eruption high an up to 80 mol an microlites from early erupted samples reveal that the eruption was likely preceded by an influx of hotter or more mafic magma transitions between explosive and effusive activity in 2010 were driven primarily by the dynamics of magma ascent in the conduit with degassing and crystallisation acting via feedback mechanisms resulting in cycles of effusive and explosive activity explosivity during the 2010 eruption was enhanced by the presence of a plug of cooled magma within the shallow magma plumbing system which acted to hinder degassing leading to overpressure prior to initial explosive activity

an expanded model and application of the combined effect of crystal size distribution and crystal shape on the relative viscosity of magmas abstract this study examines the combined effect of crystal size distributions csd and crystal shape on the rheology of vesicle free magmatic suspensions and provides the first practical application of an empirical model to estimate the relative effect of crystal content and csd s on the viscosity of magma directly from textural image analysis of natural rock samples in the form of a user friendly texture rheology spreadsheet calculator we extend and apply established relationships between the maximum packing fraction m of a crystal bearing suspension and both its rheological properties and the polydispersity of a csd by using analogue rotational rheometric experiments with glass fibres and glass flakes in silicone oil acting as magma equivalent this study also provides new insights in the relationship between m and the aspect ratio rp of suspended particles

study of the 1920 haiyuan earthquake induced landslides in loess china many large scale landslides induced by earthquakes have been reported in loess soils they often cause catastrophes because they travel long distances at high velocity to clarify the mechanism of these landslides field survey was performed on the landslides triggered by the haiyuan earthquake china 1920 and a series of ring shear tests was conducted on the loess soils collected from a landslide the field surveys revealed that most of the loess landslides triggered by the haiyuan earthquake occurred on concave slopes gentler than 15 with long runout distance showing very small equivalent friction angle cyclic ring shear tests on saturated specimens in the undrained condition showed that when cyclic shear stress was applied to a loose loess specimen pore pressure was built up gradually before the failure and after that large pore pressure was quickly generated due to the failure of loess soil structure meanwhile to investigate the mechanism that produces the high mobility of loess landslides ring shear tests were conducted on loess specimens with large shear displacement of up to several meters under the undrained condition and it was found that the initial normal stress has no effects and overconsolidation ratio has little if any effects on the apparent friction angle in the steady state for loess soils results of tests on soils at different saturation degrees revealed that the reduction in shear strength results more likely from the generation of pore water pressure not of pore air pressure

eruption mechanisms and short duration of large rhyolitic lava flows of yellowstone abstract large volume effusive rhyolite lava flows are a common but poorly understood occurrence from silicic volcanic centers we integrate characterization of lava flow topographic morphology and petrographic textures and zoning of crystals with physical models of viscous fluid flow in order to interpret the eruption durations and discharge rates for the most recent effusive volcanic eruptions from yellowstone these large volume 10 70 km3 crystal poor rhyolite lavas erupted within the yellowstone caldera as 100 200 m thick flows and have a cumulative erupted volume of 650 km3 that is similar to less frequent caldera forming events but occur as individual eruptions spread over 100 ka most of this work is focused on the axisymmetric 124 ka 50 km3 summit lake flow we examined crystallinity major and trace element concentrations oxygen and hydrogen isotopic values and quartz morphology and zoning in samples from the center to margin of this flow water contents down to 0 1 wt and d values of 110 are low and require closed system degassing until near surface lithostatic pressure while major elements are consistent with water undersaturated pre eruptive storage and crystallization at 4 8 km depth we found some evidence for subtle km scale zoning within the lavas but describe significant microscopic scale compositional diversity including sharp boundaries between high ti cores and 200 m thick rims on quartz phenocrysts embayed quartz external morphology and rim growth may be the result of undercooling during coalescence of magma bodies during shallow transport between dikes and sills modeling the emplacement of the lava flow as a simple viscous fluid suggests that emplacement of rhyolite lava at 800 c occurred over 2 to 5 yrs with high discharge rates 100 m3 s such high magma discharge rates are accommodated through 6 km long fissures that allow for slower magma ascent velocities of 10 yr flow durations and significant cooling of the flow front that should result in a more complex compound flow morphology than observed higher temperatures require unrealistically wide 50 m dike widths to accommodate large discharge rates petrographic and isotopic evidence from crystals suggests recharge and merger of individual magma batches occurs on a similar timescale to the eruption duration and may directly cause overpressure and emplacement of these rhyolite lava flows from a shallow ephemeral magma chamber large volume rhyolitic lavas are able to erupt effusively through elongate fissures that utilize preexisting zones of crustal weaknesses such as ring fractures less common explosive eruptions at yellowstone may result when ascending magmas are forced through narrower conduits or when recharge rates are especially high the results of this study provide a unique top down constraint on effusive eruption rates make new interpretations of common petrographic textures and presents a comprehensive model for eruption control

post failure mobility of saturated sands in undrained load controlled ring shear tests the undrained shear behavior of soils with progress of shear displacement is essential to the understanding of liquefied slope failures with long travel distance in this paper using a newly devel

field testing of irrigation effects on the stability of a cliff edge in loess north west china as it is not uncommon that irrigation has triggered landslides in the chinese loess plateau a full scale field testing in a typical cracked plateau in heifangtai gansu province of china to simulate such phenomenon was conducted in the study piezometers were installed to measure the groundwater and crackmeters as well as inclinometers were used to measure the slope movement water was pumped from the nearby source daily to simulate the irrigation process there was a sharp increase of pore pressure in the undisturbed loess below the crack when the inclinometer registered considerable movement due to the inflow of water via the crack such increase in pore pressure also dissipated rapidly eventually the irrigation water daylighted at the major cracks from the face of the plateau leading to local failures the results show that cracks have significant effects on the flow of the irrigation water into the ground the rapid rise in the local pore water pressure had reduced the stability of the slope

h2o content and temperature limit the explosive potential of rhyolite magma during plinian eruptions abstract recent rhyolite eruptions on earth have demonstrated their capacity to produce a multitude of hazards including ash formation lasting months and impacting the large reaches of the southern hemisphere nevertheless the underlying mechanisms driving these eruptions are not yet fully understood magmatic volatiles especially h2o dictate whether volcanic eruptions proceed explosively or effusively experimental evidence for the role played by h2o in driving explosive fragmentation is rare in particular in the eruption of rhyolitic magma here we show that when hydrous rhyolitic obsidians from chaiten volcano chile are experimentally heated above their glass transition temperatures at ambient 1 atm conditions two different behaviors result depending on starting h2o concentration and temperature obsidians vesiculate to stable or quasi steady state foams when h2o is 1 wt for a wide range of temperatures 728 1032 c but will explode within just tens of seconds 874 c explosive activity occurs above chaiten s estimated eruption temperature but for lower temperatures only foaming occurs whether a foaming sample remains coherent or explodes depends on two interrelated factors the peclet number pe a dimensionless ratio of diffusive and viscous timescales and the timescale or rate of decompression which is dictated in part by the h2o vapor pressure gradient between bubbles and atmosphere at or above 1 4 wt h2o and for a range of permissible chaiten eruption temperatures 780 825 c pe is large 10 meaning viscous deformation aiding vapor expansion is the dominant mode of growing bubbles consequently vesiculation can proceed rapidly due to high initial overpressure and low melt viscosity until the point is reached that the melt deforms at a rate greater than its relaxation rate resulting in fragmentation below 1 4 wt h2o and for temperatures equal to or lower than chaiten s eruption decompression and deformation rates inferred from experimental clast expansion and bubble growth behavior are at least an order of magnitude smaller than the explosive cases which is insufficient for critical melt rupturing pe estimations highlight the first order influence of temperature and h2o content on physical degassing behavior since the starting h2o content and temperature dictate many the parameters that govern bubble growth pressure gradient melt viscosity and chemical diffusivity our experiments shed light on what limits fragmentation in natural eruptions and offer an alternative explanation for h2o contents measured in plinian fall deposits from chaiten and other rhyolite centers in essence pyroclastic obsidian h2o contents could reflect temperature and p h2o limitations on fragmentation furthermore since foam expansion under low p high t h2o conditions can foster strain rates in excess of the melt s relaxation rate and thus drive fragmentation we contend that in addition to rapid magma ascent and high magma supply rates rapid clast expansion under low pressure conditions may also be important for explosive magma fragmentation

bubble nucleation in magmas a dominantly heterogeneous process abstract bubble nucleation is at the heart of magma ascent and degassing nucleation kinetics control how easily a magma exsolves its volatiles and thereby its explosivity close to the surface homogeneous nucleation i e without assistance from crystal substrates or other phases often requires attainment of large supersaturation pressures p n 100 mpa the change in pressure required for onset of nucleation while heterogeneous nucleation on pre existing crystals can occur at significantly lower values p n p n given experimental constraints on pressure conditions at the onset of nucleation decompression rate meters e g toramaru 2006 also relate the number density of bubbles in pyroclasts a readily obtained textural characteristic to rates of pressure change dp dt during ascent all these treatments require an explicit or implicit decision as to whether magma degassing was dominated by homogeneous or heterogeneous nucleation which is guided by the choice of surface tension values this contribution exploits the current textural datasets available for eruptions involving a variety of magma compositions to test the assumptions that homogeneous and or heterogeneous nucleation dominate bubble vesiculation in natural melts the comparison between pyroclast textures and those obtained via decompression experiments in the laboratory indicates that supersaturation pressures and decompression rates required for homogeneous nucleation are unrealistically high for natural magmas i review the arguments for and against homogenous nucleation and defend the alternate view that heterogeneous nucleation may dominate in all magma types including rhyolites in order to solve this central debate our focus needs to be shifted towards understanding the pre decompression phase equilibria and textural characteristics of magnetite in addition to the state of the melt structure

the role of superheating in the formation of glass mountain obsidians long valley ca inferred through crystallization of sanidine the glass mountain obsidians long valley ca are crystal poor 8 vol and highly evolved high sio2 low sr and therefore their formation required extremely efficient separation of melts from a crystal rich source a petrologic and experimental investigation of the mineral phases in glass mountain lavas identifies conditions under which phenocrysts grew and the driving mechanism for crystallization which places constraints on the possible processes that generated the obsidians the obsidian in this study gm 11 is saturated in nine phases sanidine quartz plagioclase titanomagnetite ilmenite zircon apatite allanite biotite and results of high resolution sem compositional mapping and electron microprobe analysis reveal that individual sanidine crystals are normally zoned and span a range of compositions or40 78 sanidines have a granophyric texture characterized by intergrowths of quartz and sanidine mineral phases in the natural sample are compared to h2o saturated phase equilibrium experiments conducted in cold seal pressure vessels over a range of conditions 700 850 c 75 225 mpa and all are found to be plausible phenocrysts comparison of sanidine compositions from the natural sample with those grown in phase equilibrium experiments demonstrates that sanidine in the natural sample occurs in a reduced abundance further comparison with phase equilibrium experiments suggests that sanidine compositions track progressive loss of dissolved melt water cooling suggesting that crystallization in the natural obsidian was driven predominantly by degassing resulting from decompression it is paradoxical that an effusively slowly erupted lava should contain multiple phenocryst phases including sanidine crystals that span a range of compositions with granophyric textures and yet remain so crystal poor to resolve this paradox it is necessary that the solidification mechanism degassing or cooling that produced the sanidine crystals and other mineral phases must have an associated kinetic effect s that efficiently hinders crystal nucleation and growth decompression experiments conducted in this study and from the literature collectively demonstrate that the simplest way to inhibit nucleation during degassing induced crystallization is to initiate degassing cooling from superliquidus conditions and therefore the glass mountain obsidians were superheated prior to crystallization

degassing processes during lava dome growth insights from santiaguito lava dome guatemala abstract eruptions of intermediate magma may be explosive or effusive the development of open system degassing has been proposed as a pre requisite for effusion of intermediate magma however processes leading to open system degassing are poorly understood to better understand degassing processes during lava dome extrusion we report high temporal resolution so2 emission rate measurements collected with an ultra violet imaging camera at santiaguito guatemala santiaguito is an ideal case study as the dome lava is compositionally very similar to products of the 1902 plinian eruption of the parental santa maria volcano we find that degassing is weak 0 4 1 kg s 1 but continuous and explosions are associated with small increases in emission rates up to 2 3 kg s 1 continuous repose degassing occurs through a shallow cap rock which likely represents a proto crust on the block lava flow which is extruded from the same vent the continual permeability of the upper conduit argues against a mechanism of explosion triggering in which gas pressure builds beneath a viscous cap rock or plug rather we consider degassing data better consistent with a model of shear fracturing at the conduit margins using field constraints we model the viscosity of santiaguito magma as a function of depth and show that conditions for shear fracturing are met from 150 600 m to the surface this is in line with independent estimates of explosion initiation depth we show that repose timescales are orders of magnitude longer than the timescale for shear fracture and suggest that explosions are triggered when a continuous network of smaller scale fractures develops at which point decompression occurs and an explosion is triggered fracture healing occurs by viscous relaxation however near to the surface where viscosity is highest an unconsolidated gouge layer may develop our model implies that the observed explosions are a by product of extrusion shear fracturing can drive open system degassing of crystal rich intermediate magma at shallow levels in the conduit as high magma viscosity is able to overcome the low strain rates associated with slow ascent of magma

decompression rate of magma at fragmentation inference from broken crystals in pumice of vulcanian eruption abstract decompression rate of magma at fragmentation immediately before or after fragmentation was determined using broken crystals found in the pumices ejected during the vulcanian explosion of sakurajima volcano japan the combined analysis of textural data obtained from the natural pumice samples and a simple model for the crystal fracturing in vesiculating viscous magma indicated a decompression rate 7 0 10 3 7 8 10 4 pa s for the ejecta produced during the later phase of vulcanian explosion this result suggests that the short duration of the vulcanian explosion is controlled by the rapid decrease of the magma ascent rate to a vent an understanding of the control mechanism of the termination of an eruption by precise prediction of the eruption process is essential for both volcanology and hazard mitigation

rapid crystallization of plagioclase phenocrysts in silicic melts during fluid saturated ascent phase equilibrium and decompression experiments three crystal poor obsidian samples one dacite 67 wt sio2 two rhyolites 73 and 75 wt sio2 which erupted effusively from monogenetic vents contain sparse 2 plagioclase phenocrysts that span a remarkably wide and continuous range in composition 30 mol an many but not all of the plagioclase crystals display diffusion limited growth textures e g swallow tails skeletal vermiform hypotheses to explain the paradox of a wide compositional range despite a low abundance of plagioclase include 1 incorporation of xenocrysts and or magma mingling 2 slow crystallization of plagioclase driven by slow cooling in a magma chamber 3 slow crystallization of plagioclase followed by a resorption e g heating event and 4 crystallization driven by rapid degassing i e loss of melt h2o 6 rapid cooling during ascent to test these hypotheses a series of phase equilibrium experiments were conducted under pure h2o fluid saturated conditions in a cold seal pressure vessel between 30 and 300 mpa and 750 and 950 c the results show that the plagioclase population in each obsidian sample could have grown from their respective melts with the exception of a single calcic core an60 63 in one sample the results additionally rule out slow cooling in a magma chamber because this would lead to equilibrium abundances of plagioclase 20 which are far higher than what is observed in the samples 2 nor can resorption i e heating explain the low abundance of plagioclase because this would eliminate the more sodic plagioclase crystals and hence the wide compositional range of plagioclase that is observed the most viable hypothesis is that the sparse plagioclase phenocrysts grew relatively rapidly during magma ascent to the surface this is consistent with the results of isothermal 850 c continuous decompression experiments 2 9 1 0 0 8 and 0 1 mpa h under pure h2o fluid saturated conditions which were performed on one of the rhyolites mlv 36 73 wt sio2 and quenched at ph2o1 489 58 and 40 mpa the four decompression rates correspond to degassing rates of 1 6 0 56 0 45 and 0 06 wt h2o per day decompressions 1 0 mpa ph2o h initiated above the liquidus quenched to 100 glass at all final ph2o decompressions at 0 8 mpa ph2o h also initiated above the liquidus led to plagioclase crystals nearly five times larger than those grown in runs decompressed at the same rate but initiated just below the plagioclase in curve it is the kinetic hindrance to nucleation that permits crystal growth to be concentrated on relatively few crystals leading to larger crystals plagioclase growth rates from these experiments show that the largest phenocrysts 1 mm in the mlv 36 obsidian could have grown in 42 h a cooling rate of 1 2 c h closely matches both the increase in melt viscosity with time and the effective undercooling with time that occurs during the 0 8 mpa ph2o h decompression over the first 50 h the combined results point to crystallization of sparse plagioclase driven by relatively rapid rates of degassing6 cooling vc the author 2015 published by oxford university press all rights reserved for permissions please e mail journals permissions oup com 981 j o u r n a l o f p e t r o l o g y journal of petrology 2015 vol 56 no 5 981 1006 doi 10 1093 petrology egv025

experimental study on the shearing behavior of saturated silty soils based on ring shear tests the shearing behavior of saturated silty soils has been examined extensively by performing undrained and partially drained the upper drainage valve of the shear box was open during shearing ring shear tests on mixtures of a sandy silt with different loess contents by performing tests at different initial void ratios the shear behavior of these silty soils at different initial void ratios is presented and discussed undrained shear test results showed that the liquefaction phenomena in ring shear tests were limited within the shear zone for a given void ratio or interfine void ratio both the peak and steady state shear strengths decreased with increase of loess content the partially drained shear tests revealed that a great reduction in the shear strength could result after the shear failure due to the buildup of excess pore water pressure within the shear zone the magnitude of reduction in shear strength after failure was affected by the initial void ratio the shear speed after failure as well as the loess content in the sample for a given void ratio or interfine void ratio with increase of loess content the drained peak shear strength became smaller while the brittleness index became greater it was also found that due to localized shearing the permeability of the soil within the shear box after drained shearing could be three orders of magnitude smaller than before shearing

storage and eruption of near liquidus rhyolite magma at cordon caulle chile the last three eruptions at the cordon caulle volcanic complex chile have been strikingly similar in that they have started with relatively short pre eruptive warning and produced chemically homogeneous rhyolite to rhyodacite magma with glassy to aphyric texture these characteristics collectively call for an understanding of the storage conditions leading to the rise and extraction of crystal poor silicic magma from volcanoes we have analyzed and experimentally reproduced the mineral assemblage and glass chemistry in rhyolite magma produced in the most recent eruption of cordon caulle and we use these to infer magma storage and ascent conditions fe ti oxide mineral geothermometry suggests that the rhyolite was stored at 870 920 c at these temperatures the phenocryst assemblage plag an37 cpx opx mag ilm can be reproduced under h2o saturated conditions of between 100 and 50 mpa corresponding to crustal depths between about 2 5 and 5 0 km the shallow and relatively hot magma storage conditions have implications for the rapid onset degassing efficiency and progression from explosive to mixed pyroclastic effusive eruption style at cordon caulle

the chemical and mineralogical controls upon the residual strength of pure and natural clays recent work has shown that the residual strength of clays is to a large extent determined by the type of clay minerals and the chemistry of the constituent pore water changes in either of these factors cause corresponding alterations in residual shear strength the findings have important implications for the seasonal reactivation of landslides and may also provide an explanation for the long term weathering and shear strength reduction of clay slopes results are presented here to demonstrate the separate effects of clay mineralogy and pore water chemistry upon the residual strength of pure and natural clays explanations are sought to identify the controls of these factors upon residual strength based on brief reviews of the shear strength and physico chemical behaviour of clays the findings strongly suggest that residual strength should be considered a dynamic property with increases or decreases in strength occurring in response to environmental change des etudes recentes ont montre que la resis

irrigation induced loess flow failure in heifangtai platform north west china because the construction of dams and hydro electric power stations of liujiaxia and yanguoxia across the yellow river in west gansu province north west china inhabitants in the reservoir areas were resettled in heifangtai loess platform irrigation thus started in 1968 perched water table was significantly raised and resulted in many loess flow failures in the past 30 years this type of landslide was often featured by long runout distance high speed and recurrence laboratory tests along icu and csd stress paths were performed it is shown that the loess has a strong strain softening behavior and instability can occur in drained condition before undrained liquefaction the failure mechanism of loess flow can be interpreted as the following process with increasing perched water table collapse in loess was first triggered in drained condition and led to undrained loading to adjacent soil in the lower saturated part of the loess platform under gravity loading applied by upper dry loess undrained flow failure mobilized eventually

influence of lixiviation by irrigation water on residual shear strength of weathered red mudstone in northwest china implication for its role in landslides reactivation abstract irrigation in loess tableland areas of northwest china has induced a large number of landslides around their edges of which loess red mudstone landslide is one of the most frequent types salt sinters commonly observed at the toes of these landslides where the weathered red mudstone wrm outcrops have raised the possibility that removal of soluble salts in the wrm by irrigation water could be crucial in the reduction of its shear strength and thus in the occurrence of these landslides in order to investigate the effects of lixiviation by irrigation water on residual shear strength of the wrm samples of the wrm close to two landslides in lanzhou city one of the largest cities in northwest china were tested after being treated under different conditions these included 1 saturation with the yellow river water the only source of irrigation 2 saturation with groundwater and 3 saturation and leaching with deionized water based on the results of these tests specific mechanisms of residual shear strength reduction of the wrm due to lixiviation by irrigation water were discussed in terms of physico chemical interactions between the wrm and water it was found that lixiviation reduced residual shear strength and residual friction angle r of the wrm by up to 65 and 62 respectively after being leached 6 times this coincided with a dramatic dissolution and leaching of soluble salts from the wrm particle size distribution showed that the more the wrm was leached the more the clay fraction and the lesser coarser fractions this suggested that reduction in residual shear strength of the wrm could be attributed to mainly disaggregation of particles coarser than clay and to a lesser degree to weakening of interparticle forces among individual clays it was further postulated that variation in clay mineralogy may influence performance of particle disaggregation

conduction phenomena from theory to geotechnical practice virtually all geotechnical problems involve the flow of fluids chemicals and energy a current review of the types of direct and coupled flow through soil is given and assessment of their significance in geotechnical practice is made darcy s fourier s ohm s and fick s laws provide a suitable practical basis for the analysis of water heat electrical and chemical flows due to gradients of the same type prolonged application of gradients of the different types can cause significant changes in the state and properties of a soil and flows through it the quantification of coupled flows is most easily done by direct measurement of coupling coefficients when only two types of flow are important the application of irreversible thermodynamics is useful for quantification of flows in more complex systems relationships are provided for the expression of coupling coefficients in terms of directly measurable soil parameters such as hydraulic conductivity coupled fluid flows due to electrical and chemical gr

dynamics of obsidian flows inferred from microstructures insights from microlite preferred orientations the flow of obsidian lava leads to crystal alignments that reflect both the accumulated strain and the type of flow across the surface microlite preferred orientations are used to investigate the emplacement dynamics strain history and structural evolution of obsidian dome eastern california measurements of three dimensional microlite trend and plunge in samples from the near vent region distal flow and flow front show 1 the flow directions along the dome margins 2 the deformation style e g pure versus simple shear at the dome margins and 3 the variation in strain as a function of position within the flow microlites form well developed lineations in the plane of flow banding in all samples stereographic projections indicate that lineations trend normal to the western flow front and plunge shallowly away from the margin the radial flow pattern indicated by measurements made along the western margin suggests that extrusion was from a roughly elliptical vent these results highlight a strong correlation between microlite trend and the bulk flow direction inferred from the geometry of the flow along most of the eastern periphery lineations trend parallel to the margin and likely reflect the local flow direction as influenced by compression against the thickening flow crust marginal talus piles and topography orientation distributions imply that radial spreading accompanied by flattening was the dominant mechanism for flow emplacement comparisons of measured orientation distributions with theoretical predictions suggest that microlite fabrics in flow front and nearvent samples developed in a pure shear flow microstructures in a sample from near the distal flow base records a component of simple shear variance in microlite trend provides a measure of the amount of strain acquired during flow standard deviation in trend decreases from the near vent region to the flow margins reflecting progressive alignment of microlites during transport pure shear strain inferred from orientation distributions increases from approximately 0 3 near the vent to about 1 1 at the flow front the difference between these strains 0 8 is an estimate of the strain associated with flow emplacement such strain is similar in size to that estimated from mesoscopic structures v1 and for horizontal spreading of a fluid whose volume is equal to that of obsidian dome v1 6 this suggests that flow on the surface was sufficient to produce observed microlite preferred orientations in the flow front these techniques can be applied to interpret older dissected lavas where erosion has erased much of the original flow front or where larger scale structures indicative of flow directions are poorly preserved fl 2002 elsevier science b v all rights reserved

volcanic dilemma flow or blow volcanic eruptions can be either slow outpourings of lava or explosive ejections of fragmented rock in his perspective dingwell discusses the physical and chemical properties of magma that lead to these two styles of eruption new experiments and computer simulations are pointing to the involvement of solidlike behavior of liquid magma when it makes a transition to a glass

decompression induced crystallization in hydrated silica rich melts empirical models of experimental plagioclase nucleation and growth kinetics isothermal and isobaric crystallization of plagioclase in a water saturated synthetic rhyolitic melt is investigated through time series of decompression experiments the experimental variables are the rate at which samples are initially decompressed 30 150 and 1200 mpa h from 200 mpa and 875 c final pressure 25 to 160 mpa and holding time at final pressure up to 17 days through textural measurements of the crystals plagioclase crystallization kinetics has been characterized in terms of nucleation lag and rates of nucleation and growth plagioclase crystallization is markedly dependent on effective undercooling teff and holding time at crystallization pressure with teff increasing from 55 to 110 c i nucleation lag decreases from 1 2 days to 15 min ii maximum nucleation rates increase from 10 3 to 10 2 mm 2 s 1 and iii maximum growth rates decrease from 10 6 to 5x10 7 mm s 1 the initial decompression rate 30 150 and 1200 mpa h has no systematic control on crystallization at final pressure except for the 1200 mpa h series in which samples show nucleation difficulties from the experimental data of teff constrained plagioclase number density proportion and morphology we provide means to assess conditions of nucleation and growth of natural plagioclase microlites from rapidly ascended rhyolitic melts through the determination of plagioclase liquidus curve and teff prevailing during crystallization

undrained shear behavior of loess saturated with different concentrations of sodium chloride solution abstract a series of ring shear tests was conducted on saturated loess to investigate the effects of nacl concentration in pore water and desalinization on the shear behavior under undrained conditions the loess samples were taken from the ground surface of a frequently active landslide in china were saturated by de aired distilled water with different concentrations of nacl and then were sheared undrained after that the samples were retrieved remolded re set into the shear box and re saturated by passing through de aired distilled water such that the samples were desalinized and then were sheared undrained again through comparing the undrained shear behavior the effects of nacl in the pore water and desalinization on the undrained shear behavior of loess were examined the results showed that the variation of nacl concentration in pore water can strongly affect the shear behavior of saturated loess both the peak shear strength and steady state strength increased with increase of nacl concentration until a certain value after which they decreased with further increase of nacl concentration meanwhile the peak shear strength and steady state strength of the desalinized samples recovered to those of the original sample hence the effects of salinization are reversible these findings may be of practical importance to better understand the repeated occurrence of some irrigation induced loess landslides in china

pre eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts abstract the ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for h2o f and incompatible trace elements eleven melt inclusions from phenocrysts in air fall tephra from obsidian dome near long valley usa gave an average of 4 1 1 2 wt h2o and showed large variations in f and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types eight inclusions in phenocrysts from the taupo ultraplinian event new zealand gave 4 5 0 8 wt h2o with small variations in the other elements analyzed measured water contents are similar to earlier estimates of the pre eruptive water contents of these and similar rhyolites and major and trace element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment

seismic loading impacts on excess pore water pressure maintain landslide triggered flowslides during the 2003 sanriku minami earthquake japan a flowslide was triggered on a slope of about 13 5o the displaced landslide mass developed into a flowslide and deposited on a horizontal rice paddy after traveling approximately 130 m to study the trigger and movement mechanisms of this landslide field investigation and laboratory ring shear tests were performed field investigation revealed that the landslide originated from a fill slope where a gully was buried for cultivation some decades ago and shallow ground water was present undrained monotonic and cyclic ring shear tests on a sample pyroclastic deposits taken from the source area revealed that the soil is highly liquefiable and its steady state shear strength can be little affected by overconsolidation using the seismic records of the earthquake probable seismic loadings on the sliding surface were synthesized and applied to the samples in ring shear tests which were performed under undrained or partially drained conditions the undrained and partially drained tests revealed that shear failure can be triggered by the introduction of seismic loading and formation of excess pore water pressure the generation of excess pore water pressure along with increase of shear displacement and the inhibited dissipation of excess pore water pressure due to the thickness of the saturated soil layer above the sliding surface probably enabled the continued post failure landsliding copyright 2008 john wiley sons ltd

crystallization kinetics in continuous decompression experiments implications for interpreting natural magma ascent processes decompression experiments were performed to study the kinetics of plagioclase crystallization during ascent of hydrous rhyodacite magma these experiments differ from previous studies in that they employ a continuous rather than stepwise pressure time trajectory four series of experiments were performed at rates of 0 5 10mpa h corresponding to ascent rates of 0 007 0 14 m s experiments quenched along each decompression path allow snapshot style monitoring of progressive crystallization as expected rates of plagioclase nucleation and growth depend strongly on decompression rate regardless of decompression rate approximately equal volumes of feldspar crystallize during a given decompression interval even when the observed crystallinity is lower than the equilibrium crystallinity apparently a constant degree of solidification is maintained by a crystallization mechanism striking a shifting balance between nuclei formation and growth of existing crystals other key results pertain to the efficacy with which experimental results allow interpretation of the decompression histories of natural rocks feldspar microlites do not maintain chemical equilibrium with melt they are more anorthite rich than equilibrium plagioclase suggesting that interpretations of magma ascent processes in nature require comparisons with dynamic rather than static phase equilibrium experiments although decompression rate is an important element controlling final textures in volcanic rocks no single compositional or textural parameter uniquely records this rate of the criteria examined microlite number density and morphology are the best indicators of magma decompression rate based on a small number of comparisons between continuous and multi step decompression style at the same time integrated decompression rate 1mpa h decompression path evidently influences crystal texture with stepwise decompression yielding textures correlative with faster decompression than indicated by the time integrated decompression rate thus consideration of magma ascent style in addition to rate will undoubtedly strengthen the interpretive power of experimental studies for constraining natural magma ascent processes

liquefaction and flow failure during earthquakes two aspects of seismically induced liquefaction are discussed which are of vital engineering significance the triggering condition and the consequences of liquefaction the triggering condition is examined with respect to liquefaction analysis note being taken of the onset condition which is governed by cyclic strength consequences of liquefaction are discussed with respect to post seismic stability analysis in which the residual strength plays a major role procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced and the applicability of this method for estimating associated ground settlements is discussed the evaluation of residual strength requires a better understanding of undrained sand behaviour results of extensive laboratory rests on japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better the results of laboratory tests on silty sands are examined in the same way ah the results are