the mechanism of surtseyan volcanism during eruption the vent of a surtseyan volcano is occupied by a highly mobile slurry of tephra hyaloclastite and water as magma ascends rapidly through the slurry mixing occurs due to velocity shear acceleration of the fluid fluid contacts and mass incorporation consequent expansion of the admixed water causes the jetting and continuous up rush activity that characterizes surtseyan volcanism there is no evidence of the widely accepted fuel coolant interaction mechanism

nonexplosive and explosive magma wet sediment interaction during emplacement of eocene intrusions into cretaceous to eocene strata trans pecos igneous province west texas abstract eocene intrusion of alkaline basaltic to trachyandesitic magmas into unlithified upper cretaceous maastrichtian to eocene fluvial strata in part of the trans pecos igneous province in west texas produced an array of features recording both nonexplosive and explosive magma wet sediment interaction intrusive complexes with 40 ar 39 ar dates of 47 46 ma consist of coherent basalt peperite and disrupted sediment two of the complexes cutting cretaceous strata contain masses of conglomerate derived from eocene fluvial deposits that at the onset of intrusive activity would have been 400 500 m above the present level of exposure these intrusive complexes are inferred to be remnants of diatremes that fed maar volcanoes during an early stage of magmatism in this part of the trans pecos province disrupted cretaceous strata along diatreme margins record collapse of conduit walls during and after subsurface phreatomagmatic explosions eocene conglomerate slumped downward from higher levels during vent excavation coherent to pillowed basaltic intrusions emplaced at the close of explosive activity formed peperite within the conglomerate within disrupted cretaceous strata in the conduit walls and within inferred remnants of the phreatomagmatic slurry that filled the vents during explosive volcanism a younger series of intrusions with 40 ar 39 ar dates of 42 ma underwent nonexplosive interaction with upper cretaceous to paleocene mud and sand dikes and sills show fluidal billowed quenched margins against the host strata recording development of surface instabilities between magma and groundwater rich sediment accentuation of billowed margins resulted in propagation of intrusive pillows into the adjacent sediment more intense disruption and mingling of quenched magma with sediment locally produced fluidal and blocky peperite but sufficient volumes of pore fluid were not heated rapidly enough to generate phreatomagmatic explosions this work suggests that trans pecos texas may be an important locale for the study of subvolcanic phreatomagmatic processes and associated phenomena eocene intrusions in the study area underwent complex interactions with wet sediment at shallow levels beneath the surface in strata as old as maastrichtian which must have remained unlithified and rich in pore water for 20 25 ma

magma sediment mingling in an ordovician rift basin the plouezec plourivo half graben armorican massif france the plouezec plourivo basin ppb armorican massif is an ordovician half graben filled with red bed deposits lying on a cadomian metamorphic basement peperites crop out along its northeast coastal sector in connection with diverse magmatic and hydrothermal products combining field structural observations and geochemical analyses of magmatic rocks leads us to propose a model of emplacement of peperites in an extensional setting the ppb peperites result from mingling between magma and initially wet sediment the main corresponding facies consist of rounded magmatic bodies corrugated lava sediment contacts and a remarkable thin sheet of fluidized sediment extending over about 100 m parallel and c 10 cm above the base of a porphyritic lava flow a few ppb peperites formed under a dominant brittle regime are autoclastic breccias at the base of a lava flow and polymict breccias including both fluidal and blocky volcanic clasts sediment fluidization formation of vapor films magma sediment density contrasts and non explosive fragmentation are the main mechanisms invoked to generate the ppb peperites the c 100 m extended sediment derived sheet lying near the base of a lava flow likely formed by diapiric rise of fluidized sediment through molten lava we show that silicic contamination accompanied these peperitic processes

hydrological flowpaths and nitrate removal rates within a riparian floodplain along a fourth order stream in brittany france three main reservoirs were identified that contribute to the shallow subsurface flow regime of a valley drained by a fourth order stream in brittany western france i an upland flow that supplied a wetland area mainly during the high water period it has high n no3 and average cl concentrations ii a deep confined aquifer characterized by low nitrate and low chloride concentrations that supplied the floodplain via flow upwelling iii an unconfined aquifer under the riparian zone with high cl and low n no3 concentrations where biological processes removed groundwater nitrate this aquifer collected the upland flow and supplied a relict channel that controlled drainage from the whole riparian zone patterns of n no3 and cl concentrations along riparian transects together with calculated high nitrate removal indicate that removal occurred mainly at the hillslope riparian zone interface i e first few metres of wetland whereas dilution occurred in lower parts of the transects especially during low water periods and at the beginning of recharge periods stream flow was modelled as a mixture of water from the three reservoirs an estimation of these contributions revealed that the deep aquifer contribution to stream flow averaged 37 throughout the study period while the contribution of the unconfined reservoir below the riparian zone and hillslope flow was more variable from ca 6 to 85 relative to rainfall events and the level of the riparian water table at the entire riparian zone scale no3 removal probably from denitrification appeared most effective in winter despite higher estimated upland no3 fluxes entering the riparian zone during this period copyright 2003 john wiley sons ltd

hydrovolcanic explosions the systematics of water pyroclast equilibration the initial contact of external water with erupting magma and the mass ratio of water to magma govern the development of hydrovolcanic phenomena the phase relations of water within the runout system and the separation of vapor or liquid from the pyroclasts explain gradational transitions between some pyroclastic flows base surges mud hurricanes mudflows and sheetfloods

origin and emplacement of a pyroclastic flow and surge unit at laacher see germany abstract within the pyroclastic ejecta sequence of laacher see volcano west germany are many repeated phreatomagmatic depositional cycles a complete cycle begins with a thin low aspect ratio fines depleted breccia layer followed by a sequence with several coarse to fine grained cross bedded units in turn overlain by a massive pyroclastic flow layer each sequence is interpreted in terms of water magma interaction and effects upon the eruption column the breccia represents the triggering eruption with a high eruption column where fines were winnowed out and fallback from high altitudes produced a low aspect ratio layer the cross bedded sequence containing more fine grained ash resulting from an increase in amount of water represents a fluctuating eruption column of sufficient height to produce large antidunes within 700 meters of the rim the massive pyroclastic flow layer which contains more fine grained ash than the underlying layers represents the culmination of the eruption where an increase in amount of water interacting with the magma resulted in a dense and very low eruption column producing a dense laminar moving pyroclastic flow

a shallow marine alkali basalt tuff cone in the middle miocene jinzai formation izumo sw japan abstract the jinzai basalt is an alkali basalt tephra unit erupted on the japan sea side of sw japan immediately after the opening of the japan sea ceased 15 ma the jinzai basalt comprises a large volume of glassy alkali basalt clasts of phreatomagmatic origin and is interpreted to have constituted a tuff cone the tuff cone grew to a height of 100 m above the 50 to 150 m deep submarine substrate but the summit and a part of the flank were lost due to syn and post eruptive sliding slumping and erosion the restored tuff cone has a crater 400 500 m wide and 100 m high and the cone rim reaches 1500 m in width with bedding dipping outward at a maximum angle of 20 30 the primary deposits include thinly bedded lapillistone to lapilli tuff and tuff and thickly bedded lapilli tuff and tuff which are thought to be correlated to a small volume phreatomagmatic explosion and more voluminous one respectively the shallow marine jinzai basalt tuff cone is essentially similar in constituents internal structures configuration and size to other tuff cones built in shallow surface water this tuff cone however appears dominated by phreatomagmatic products due to dynamic interaction of rapidly ascending magma and water saturated unconsolidated sediments or vent filling slurry of water and ejecta and has been entirely altered through syn and post eruptive reactions of hot basalt glass and interstitial seawater

contrasting types of surtseyan tuff cones on marion and prince edward islands southwest indian ocean ten surtseyan tuff cones on marion island 46 54 s 37 46 e and seven on prince edward island 46 38 s 37 57 e were erupted on shallow submerged coastal plains related to normal faulting they range from pleistocene to holocene in age and exhibit a variable degree of erosion by the sea fundamental differences irrespective of age exist between two types type i cones have diameters of 1 1 5 km rim heights of about 200 m and steep 27 outer slopes deposits are plastered against nearby cliffs beds are thin including layers of accretionary lapilli and less than 10 lithic clasts numerous bomb sags soft sediment deformation structures and gravity slides occur on one of these cones mudflows formed small tunnels which resemble lava tubes associated with channels sometimes having oversteepened walls these cones reflect comparatively low energy conditions and probably resulted from extremely wet surges interspersed with mudflows and ballistic falls type ii cones have smaller diameters 0 5 km but widespread fallout surge aprons rim heights are about 100 m and average slope angles are 18 bedding is massive with variable lapilli matrix ratio and more than 10 lithic clasts without bomb sags these cones formed under drier perhaps hotter and more violently explosive conditions than type i though not as energetic as the phreatomagmatic eruptions of terrestrial tuff rings the two types of surtseyan eruptions are explained by invoking not only different water magma ratios in the conduit but also different mechanisms of water magma interaction the slurry model of kokelaar is favoured for type i and a fuel coolant model for type ii the decisive factor is considered to be rate of effusion with rim closure and exclusion of sea water playing a secondary role

lahars initiated by the 13 november 1985 eruption of nevado del ruiz colombia the eruption of nevado del ruiz in colombia on 13 november 1985 was accompanied by the formation of four lahars mud flows triggered by the melting of glacial ice near the summit of the volcano the lahars began as flows of water sand and gravel but they incorporated clay by eroding the soil along the steep valleys through which they passed the largest flow was a cohesive debris flow more than 45 m deep and moving at 12 ms 1 when it debouched from the canyon of the rio lagunilla 2 5 km from armero where it killed 25 000 people the continuing volcanic activity and abundant remaining glacial ice create an extremely high risk of future destructive flows

arrested diatreme development standing rocks east hopi buttes navajo nation usa abstract maar diatreme volcanoes defined by their relatively large pyroclastic debris filled subsurface structures and craters that cut into the pre eruptive land surface are typically found in small volume mafic to ultramafic monogenetic volcanic fields diatremes are associated with strong explosions throughout most of their development focused along feeder dikes and generally attributed to magma water interaction or high magmatic volatiles detailed mapping of the magnificently exposed standing rocks east sre diatreme shows evidence of additional eruptive complexity and offers new insights into how the plumbing and vent structures of small volume volcanoes evolve during an eruption sre is part of a larger basanitic volcanic complex that includes several diatremes formed along a series of irregular offset nw se trending dikes exposed 300 m below the pre eruptive land surface its similarly oriented elliptical shaped diatreme structure comprises predominantly country rock lithic rich breccia of coarse inhomogeneously mixed wall rock blocks sourced from above and below the current surface plus sparse juvenile material domains of pyroclastic deposits crosscut the country rock breccia deposits and the best exposed is the nw massif rising 35 m above the current erosional surface it represents a cross section of an evolving crater floor and comprises matrix rich lapilli tuff and spatter deposits cut by irregularly distributed dikes some with very complex textures the most significant deposit in terms of volume is an unbedded lapilli tuff that is poorly sorted and has a well mixed population of wall rock and juvenile clast varieties thus resembling deposits typical of diatremes it is overlain by and locally intercalated with spatter deposits and this irregular contact demarcates the base of what was during eruption an uneven evolving crater floor the generally massive variably welded spatter deposits constitute mostly lapilli sized juvenile clasts with fluidal folded over shapes and ropy surfaces subordinate thermally altered wall rock and variegated domains of lapilli tuff sre shows a progressive transition from fissure to diatreme and overall evolution from more explosive to weakly explosive eruption styles recorded at the conduit crater transition diatreme development was initiated by deep quarrying explosive eruptions along a fissure to form the country rock rich breccia only parts of the fissure remained active as magma feeding the highly explosive eruptions along the fissure localized into discrete point sources forming the matrix rich lapilli tuff deposits these superimposed deposits record the passage of multiple debris jets and subvertical fallback from shallow cratering arising from explosions triggered by magma water interaction at numerous discrete sites however instead of continuing to build a well formed diatreme the system switched to weak spattering with intermittent explosive activity and near surface dike emplacement into the unconsolidated anisotropic pyroclastic debris of the crater floor dominant spatter from strombolian style bursts accumulated on the topographically varied evolving unstable syn eruptive crater floor and led to local failure and remobilization this study demonstrates how the combination of fissure behavior and sensitivity of the shallow plumbing system to local conditions during an eruption can lead to a decrease in eruptive footprint within the diatreme structure and an overall decrease in explosivity resulting in the arrested development of an immature diatreme

rift related jurassic basaltic phreatomagmatic volcanism in the central transantarctic mountains precursory stage to flood basalt effusion the middle jurassic kirkpatrick flood basalts and comagmatic ferrar intrusions in the transantarctic mountains represent a major pulse of tholeiitic magmatism related to early stages in the breakup of gondwana a record of the volcano tectonic events leading to formation of this continental flood basalt province is provided by strata underlying and only slightly predating the kirkpatrick lavas in the central transantarctic mountains the lavas rest on widespread 7500 km2 tholeiitic pyroclastic deposits of the prebble formation the prebble formation is dominated by lahar deposits and is an unusual example of a regionally developed basaltic lahar field related partly fault controlled pyroclastic intrusions cut underlying strata and vents are represented by the preserved flanks of two small tephra cones associated with a volcanic neck lahar and air fall deposits typically contain 50 60 accidental lithic fragments and sand grains derived from underlying triassic lower jurassic strata in the upper part of the beacon supergroup juvenile basaltic ash and fine lapilli consist of nonvesicular to scoriaceous tachylite sideromelane and palagonite and have characteristics indicating derivation from hydrovolcanic eruptions the abundance of accidental debris from underlying beacon strata points to explosive phreatomagmatic interaction of basaltic magma with wet sediment and groundwater which appears to have occurred in particular where rising magma intersected upper beacon sand aquifers composite clasts in the lahar deposits exhibit complex peperitic textures formed during fine scale intermixing of basaltic magma with wet sand and record steps in subsurface fuel coolant interactions leading to explosive eruption the widespread sustained phreatomagmatic activity is inferred to have occurred in a groundwater rich topographic basin linked to an evolving jurassic rift zone in the transantarctic mountains coeval basaltic phreatomagmatic deposits of the mawson and exposure hill formations which underlie exposures of the kirkpatrick basalt up to 1500 km to the north along strike in victoria land appear to represent other parts of a regional extension related middle jurassic phreatomagmatic province which developed immediately prior to rapid outpouring of the flood basalts this is consistent with models which assign an important role to lithospheric stretching in the generation of flood basalt provinces

evolution of the fossa cone vulcano abstract in decreasing order of abundance the principal emplacement mechanisms for deposits of the fossa edifice of vulcano include dry surge wet surge pyroclastic fall lahar lava flow pyroclastic flow and epiclastic processes most samples from pyroclastic beds on fossa contain two grain size populations the coarser mode at 0 5 1 5 resulted from modified ballistic transport whereas the finer mode at 2 5 3 5 is related to surge transport reconstruction of the entire stratigraphy of the cone permits the beds to be grouped into five principal eruptive cycles all of which postdate the lower pilato ash of lipari dated at 11 000 to 8 500 y b p pte nere cycle palizzi cycle commenda cycle pietre cotte cycle and the modern cycle additional cycles may be present but exposures on the cone are insufficient for their adequate definition all cycles follow a similar stochastic pattern starting with surge eruptions and ending with effusion of lava from the crater rim the frequency of pyroclastic fall events increases with time throughout each cycle this progressive decrease in the efficiency of water melt interaction through the duration of each cycle may be due to a gradual rise of each new magma column coupled with an increase in magmatic volatiles tectonism and magma mixing both play a role in triggering eruptions on fossa the present crater lies at the intersection of two prominent tectonic trends a nnw fracture system parallels the main eruptive centers and an ene set aligns with the migration of minor vents the silicic lavas of fossa contain a mixture of two components xenocrysts of plagioclase augite and olivine surrounded by a dark glass are intimately dispersed into an evolved rhyolitic matrix

types of phreatomagmatic volcanoes in the western snake river plain idaho usa abstract the western snake river plain graben in southwestern idaho includes a large hydrovolcanic field which was produced in late miocene to pleistocene time by the interaction of rising basaltic magmas with the waters and water saturated deposits of an enormous freshwater lake lake idaho the phreatomagmatic volcanoes in this field may be grouped into three types emergent subaqueous and subaerial emergent volcanoes which began erupting under water and built up above the lake level are relatively large and symmetrical are dominated by bedded tuffs and late magmatic deposits and are excellent indicators of water depth at the time of the eruption subaqueous volcanoes which never built up above the lake level are relatively small and asymmetrical are dominated by basal massive deposits and are potentially useful in discriminating between deep and very deep water settings subaerial volcanoes which were formed when magmas intercepted buried aquifers and interacted explosively with water are small tuff rings and maars with variable shapes are composed of subequal although variable proportions of basal massive deposits bedded tuffs and late magmatic deposits and are useful in determining the stratigraphic successions underlying them

a miocene basanite peperitic dyke at stanley northwestern tasmania australia abstract a miocene basanite dyke at stanley northwestern tasmania australia displays well preserved peperite texture the dyke is 2 m wide and has intruded basaltic breccia host sediment one contact of the dyke is fluidally shaped and amoeboid apophyses 10 25 cm long extend into the host sediment whereas the other contact is characterized by blocky peperite texture comprising tabular to wedge shaped clasts up to 30 cm across separated by host sediment the clasts have internal spherical fractures and some show splinter texture vesicles are common in the clasts and those intersected by clast margins have been filled with sediment the interior of the dyke comprises close packed blocky peperite consisting of tabular wedge shaped and polyhedral clasts tens of centimetres across separated by host sediment these clasts show well developed jigsaw fit texture the textures and structures in the basanite dyke are inferred to have formed in two stages an earlier hotter apophysis forming stage and a later cooler angular clast forming stage both of which occurred during the intrusion of magma into wet poorly consolidated sediment in a shallow marine environment during the apophysis forming stage the magma had relatively low viscosity and progressively displaced wet sediment the wet sediment around the dyke was partly fluidized by vaporization of pore water the angular clast forming stage reflects a change in the rheological behaviour of the magma from ductile to brittle most likely in response to decreasing temperature the chilled parts of the dyke were subject to stress arising from cooling contraction and also from continued pulsatory movement of hotter still ductile magma in the interior of the dyke resulting in brittle fragmentation brittle fragmentation was accompanied by movement of host sediment into the newly created open spaces forming blocky peperite spherical fractures splinter texture and sediment filled vesicles formed during the angular clast forming stage because the host sediment is texturally identical either side of the dyke increasing magma viscosity as temperature decreased combined with pulsatory intrusion were evidently important in the production of the blocky peperite

peperite a review of magma sediment mingling the study of peperite is important for understanding magma water interaction and explosive hydrovolcanic hazards this paper reviews the processes and products of peperite genesis peperite is common in arc related and other volcano sedimentary sequences where it can be voluminous and dispersed widely from the parent intrusions it also occurs in phreatomagmatic vent filling deposits and along contacts between sediment and intrusions lavas and hot volcaniclastic deposits in many environments peperite can often be described on the basis of juvenile clast morphology as blocky or fluidal but other shapes occur and mixtures of different clast shapes are also found magma is dominantly fragmented by quenching hydromagmatic explosions magma sediment density contrasts and mechanical stress as a consequence of inflation or movement of magma or lava magma fragmentation by fluid fluid shearing and surface tension effects is probably also important in fluidal peperite fluidisation of host sediment hydromagmatic explosions forceful intrusion of magma and sediment liquefaction and shear liquification are probably the most important mechanisms by which juvenile clasts and host sediment are mingled and dispersed factors which could influence fragmentation and mingling processes include magma host sediment and peperite rheologies magma injection velocity volatile content of magma total volumes of magma and sediment involved total volume of pore water heated presence or absence of shock waves confining pressure and the nature of local and regional stress fields sediment rheology may be affected by dewatering compaction cementation vesiculation fracturing fragmentation fluidisation liquefaction shear liquification and melting during magma intrusion and peperite formation the presence of peperite intraclasts within peperite and single juvenile clasts with both sub planar and fluidal margins imply that peperite formation can be a multi stage process that varies both spatially and temporally mingling of juvenile clast populations formed under different thermal and mechanical conditions complicates the interpretation of magma fragmentation and mingling mechanisms

miocene hydrovolcanism in nw colorado usa fuelled by explosive mixing of basic magma and wet unconsolidated sediment the yampa and elkhead mountains volcanic fields were erupted into sediment filled fault basins during miocene crustal extension in nw colorado post miocene uplift and erosion has exposed alkali basalt lavas pyroclastic deposits volcanic necks and dykes which record hydrovolcanic and strombolian phenomena at different erosion depths the occurrence of these different phenomena was related to the degree of lithification of the rocks through which the magmas rose hydrovolcanic interactions only occurred where rising basaltic magma encountered wet porous non lithified sediments of the 600 m thick miocene brown s park formation the interactions were fuelled by groundwater in these sediments there was probably no standing surface water dykes intruded into the sediments have pillowed sides and local swirled inclusions of sediment that were injected while fluidized in steam from heated pore water volcanic necks in the sediments consist of basaltic tuff sediment blocks and separated grains derived from the sediments lithic blocks mostly derived from a conglomerate forming the local base of the brown s park formation and dykes composed of disaggregated sediment the necks are cut by contemporaneous basalt dykes hydrovolcanic pyroclastic deposits formed tuff cones up to 100 m thick consisting of bedded air fall pyroclastic surge and massive poorly sorted deposits mpsds all these contain sub equal volumes of basaltic tuff and disaggregated sediment grains from the brown s park formation possible explosive and effusive modes of formation for the mpsds are discussed contemporaneous strombolian scoria deposits overlie lithified cretaceous sedimentary rocks or thick basalt lavas volcanic necks intruded into the cretaceous rocks consist of basalt clasts some with spindle shape lithic clasts and megacrysts derived from the magma and are cut by basalt dykes rarely strombolian deposits are interbedded with hydrovolcanic pyroclastic deposits recording changes in eruption behaviour during one eruption the hydrovolcanic eruptions occurred by interaction of magma with groundwater in the brown s park sediments the explosive interactions disaggregated the sediment such direct digestion of sediment by the magma in the vents would probably not have released enough water to maintain a water magma mass ratio sufficient for hydrovolcanic explosions to produce the tuff cones probably additional water perhaps 76 of the total was derived by flow through the permeable sediments especially the basal conglomerate to the formation and into the vents

granulation of weak rock as a precursor to peperite formation coal peperite coombs hills antarctica abstract peperite formed by mingling of magma with coal and with fragmented coal plus other country rock is exposed at coombs hills antarctica in rocks of the mawson formation where ferrar supergroup basalt encountered the beacon supergroup continental sedimentary succession an internally laminated 0 5 m thick coal bed passes gradationally through a coal fragment dominated peperite into a glass rich basalt dominated coal fragment matrix peperite and then into coherent basalt initial interaction of magma with water saturated coal and host sediments locally brecciated the coal subsequent mingling of basalt with a slurry of coal fragments and water driven by viscosity and density contrasts between the basalt and the slurry and by flow inhomogeneities in intruding magma led to increasingly complex mingling of the two fluids via bifurcation of fingering viscous flows this mingling was complicated by the participation of phases with time and space dependent thermal properties and viscosity coal fragments coal water slurry and basaltic magma and the generation of multiple gas phases through devolatilisation of heated coal the initial elastic response of the coal fragments to stress resulted in a fluidal mingling of coal and basalt facilitated by softening of coal during heating associated with intrusion and b localised formation of blocky coal clasts during high stress events associated with passage of a liquid and or a gas phase s through the coal the presence of fluidal and blocky shaped coal fragments in the coal peperite with both curved and planar surfaces bounding single coal clasts suggests that clast morphology in the magma sediment dispersion was controlled at very small scales by the time temperature and stress dependent qualities of the coal fluidal and blocky clast shapes in the coal peperite coupled with vesicles in the coal clasts suggest that transient properties of either the host or intrusion during non explosive magma sediment interaction may exert a strong control on clast morphology and mingling characteristics

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

early miocene continental extension related basaltic magmatism at walton peak northwest colorado further evidence on continental basalt genesis the walton peak lavas erupted directly onto c 1 8 ga basement and were interbedded with subaerial sediments four new k ar dates for the lavas average 22 8 0 3 ma associating them unambiguously with the earliest large scale extension related magmatism in nw colorado the lavas are basalts trachybasalts and shoshonites there is also one 100 m thick composite flow of trachydacite containing pillow like basic masses up to tens of metres in size with chilled margins elemental data and sr and nd isotopic ratios suggest that the trachydacite evolved from the basalt type forming most of the pillows by a combination of fractional crystallization and crustal assimilation nevertheless post evolution magma mixing during and immediately before extrusion of the flow has obscured the details of this process a proportion of the pillow like basic masses in the composite flow are shoshonites that did not take part in the magma mixing they are relatively rich in incompatible minor and trace elements e g nb 41 46 ppm in the shoshonites and 20 25 ppm in all other walton peak basic lavas the sr and nd isotopic ratios of the nb rich and nb poor compositions overlap open system processes in a long lived pre existing magma chamber are considered unlikely to be the main cause of the diverse basic magmas because the walton peak lavas directly overlie precambrian basement likewise crustal contamination models using either upper or lower crustal rock types of this region do not give satisfactory mechanisms to relate the nb rich and nb poor mafic liquids a genetic model invoking variable degrees of partial melting of lithospheric mantle containing hydrous minerals does not explain why the nb rich and nb poor compositions are bimodal and not a range a variety of sparse ultrapotassic magmas minettes to lamproites accompanied the basalt dominated neogene volcanism of nw colorado and surrounding area their elemental and isotopic compositions support the view that the ultrapotassic liquids originated by fusion of hydrous and halogen rich metasomatized zones within the subcontinental lithospheric mantle addition of about 15 of lamproitic melt to the low nb walton peak basic magmas reproduces the main geochemical and isotopic characteristics of the high nb magmas such a process seems to have been widespread throughout nw colorado neogene igneous activity although wholly lithospheric mantle sources for all the mafic magmas cannot be ruled out there is evidence that both subduction related calcalkaline and ocean island basanitic melts of asthenospheric origin formed the dominant components of the nw colorado volcanics addition of as little as 10 of lamproitic melt during their uprise through the subcontinental lithospheric mantle was sufficient to imprint on them continental incompatible element and radiogenic isotope characteristics

entrainment of country rock during basaltic eruptions of the lucero volcanic field new mexico as magma rises through the lithosphere it may entrain wall rock debris the entrainment process depends on the local hydrodynamic regime of the magma e g velocity temperature bulk density the extent of interaction of magma with groundwater and the mechanical properties of the wall rocks wall rock entrainment results in local flaring of dikes and conduits which in turn affects the hydrodynamics of magma ascent and eruption we studied upper crustal xenoliths erupted from small volume basaltic volcanoes of the lucero volcanic field west central new mexico in order to assess the relative importance of various entrainment mechanisms during a range of eruptive styles including strongly hydrovolcanic strombolian and effusive processes total xenolith volume fractions ranged from 0 3 0 9 in hydrovolcanic facies to 10 4 10 2 in strombolian facies the volcanoes erupted through a thick well characterized sequence of paleozoic and lower mesozoic sedimentary rocks so that erupted xenolith

anomalous chemical changes in well waters and possible relation to earthquakes water level temperature salinity electric conductivity and ph have been measured periodically for several years at three water wells located along a 17 km segment of the san andreas fault between san jaun bautista and cienega winery in central california water samples were collected at the same time for subsequent chemical analyses in the laboratory some sudden large changes in salinity and conductivity were recorded in early march 1980 at the two wells near san juan bautista these changes coincided approximately with the beginning of an episode of increased local seismicity including a magnitude 4 8 earthquake on april 13 analyses of water samples revealed corresponding changes in ion concentrations especially of na sup ca sup mg sup so sup sub 4 sup hco sup sub 3 f sup and cl sup the observed changes may be the result of mixing of waters from different aquifers through cracks developed in the water barriers by a possible crustal strain episode that may have occurred in the study area

tarawera 1886 new zealand a basaltic plinian fissure eruption abstract new zealand s biggest and most destructive volcanic eruption of historical times was that of tarawera in 1886 the resulting scoria fall has a dispersal very similar in extent to that of the vesuvius a d 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type a new estimate of the volume 2 km 3 is significantly greater than previous estimates the basalt came mainly from a 7 km length of fissure and emission and exit velocity were fairly uniform along at least 4 km of it this is one of the few documented examples of a plinian eruption from a fissure vent primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min 1 a model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1 5 3 violent phreatic explosions occurred in the southwestern extension of the fissure across the rotomahana geothermal field and it is thought that some of the water responsible for the power of the plinian eruption came from this source though its amount was not sufficient to turn the eruption into a phreatoplinian one

characteristics of dune bedded pyroclastic surge bedsets abstract the grainsize characteristics of dune bedded pyroclastic surge bedsets are surveyed the variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets and it increases as the grainsize of the coarsest bed increases reflecting an increasing velocity of emplacement deposits of wet surges identified as those which contain accretionary and ash coated lapilli tend to be finer and show less variance this partly because wet ash is cohesive but mainly because wet surges tend to be weaker dry surge bedsets are strongly fines depleted wet ones less so the lack of erosion of underlying ash layers shows that the environment is a strongly depositional one individual bedsets are demarcated by thin intervening fine ash fall layers which are the complementary ash cloud deposits settled or flushed out after the passage and decay of each turbulent surge surge deposits are generally less coarse than the coarsest associated airfall deposits which shows that they are formed by generally weaker events this study helps interpret the dune bedded parts of the landscape mantling may 18th 1980 blast deposit of mount st helens the blast was a very violent event but the variance and the grainsize of the coarsest bed are those of a relatively weak surge this suggests that the dune bedding was produced by a weak effect such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs

olivine basalt and trachyandesite peperites formed at the subsurface surface interface of a semi arid lake an example from the early miocene bigadic basin western turkey abstract miocene successions in western turkey are dominated by lacustrine fluvial and evaporitic sedimentary deposits these deposits include considerable amounts of volcaniclastic detritus derived from numerous ne trending volcanic centres in western turkey as well as in the bigadic region early miocene syn depositional ne trending olivine basalt and trachyandesite bodies that formed as intrusions and lava flows occur within the bigadic borate basin olivine basalts occur as partly emergent intrusions and trachyandesite dykes fed extensive lava flows emplaced in a semi arid lacustrine environment peperites associated with the olivine basalt and trachyandesites appear to display contrasting textural features although all the localities include a large variety of clast morphologies from blocky to fluidal fluidal clasts mainly globular ameboidal and pillow like varieties are widespread in the peperite domains associated with olivine basalts apparently due to large volume sediment fluidisation in contrast fluidal clasts related to trachyandesites are restricted to narrow zones near the margins of the intrusions and have commonly elongate and polyhedral shapes with digitate margins rather than globular and equant varieties blocky and fluidal clasts in the olivine basalt peperite display progressive disintegration suggesting decreasing temperature and increasing viscosity during fragmentation abundance of blocky clasts with respect to fluidal clasts in the trachyandesite peperite indicates that the fluidal emplacement and low volume sediment fluidisation in the early stages were immediately followed by quench fragmentation due to the high viscosity of the magma size texture and abundance of the blocky and fluidal clasts in the olivine basalt and trachyandesite peperites were mainly controlled by sediment fluidisation pulsatory magma injection and magma properties such as composition viscosity vesicularity and size abundance and orientation of phenocrysts variously combining these contrasting features to varying degrees may form diverse juvenile clast shapes in peperitic domains

origin of the joya honda maar san luis potosi mexico abstract joya honda is a quaternary maar of unusual type from the mexican basin and range province its 300 m deep crater is excavated in cretaceous limestones the surrounding tephra deposit which in places is 100 m thick begins with a series of weakly indurated pyroclastic surge and fall layers that we interpret as dry surge deposits these are overlain by the main sequence of strongly indurated massive tuff breccias that we interpret as wet surge deposits joya honda formed subaerially from the interaction of groundwater with rapidly ascending intraplate type basanitic magma carrying peridotitic mantle xenoliths local aquifer characteristics controlled the style of eruption and the nature of the deposits groundwater in the limestone hosted aquifer beneath joya honda was apparently contained within solution enhanced fractures at the onset of the eruption magma began to interact with a moderate amount of groundwater producing the dry surge deposits which are typical of deposits found at many maars and tuff rings as the eruption continued the crater grew and the hydromagmatic blasts fractured the limestones around the explosion foci a marked increase in the water magma ratio of the system followed when a large fracture or a portion of the limestone with enhanced secondary permeability was intersected by the expanding crater subsequent phreatomagmatic explosions occurred in a system with groundwater flow rates several orders of magnitude larger than in the initial dry surge stage at the maar rim these wet eruptions led to the emplacement of massive tuff breccias through a combination of fallout surges and mudflows these steeply dipping tuff breccias are similar to deposits found at many tuff cones juvenile clasts in the near vent deposits show marked upward increases in both hydration palagonitization and vesicularity the increased palagonitization with height in the section appears to be a consequence of the overall increased wetness of the eruption with time correlating with greater carbonate cementation and lithification in the upper part of the deposit the transition toward higher vesicularity is interpreted as evidence of a gradual reduction in the confining pressure for the ascending magma prior to explosive fragmentation perhaps related to unroofing during progressive excavation of the overlying maar crater thus joya honda does not support maar formation models that invoke downward displacement of explosion foci caused by formation of a cone of depression in the aquifer in order to maintain the confining pressure for the hydromagmatic blasts