deep marine arc apron deposits and syndepositional magmatism in the alisitos group at punta cono baja california mexico rocks exposed at punta cono include very fine grained to coarse grained tuffs lapilli tuffs and tuff breccias deposited in a deep marine environment syndepositional basaltic intrusive activity was common in one locality a hyaloclastite peperite complex formed slumped sections with fluidal basalt clasts derived from intrusions that entered the sediment pile from below are present elsewhere abundant soft sediment folds in fine grained laminated subaqueous fall out tuff suggest steep gradients these are cut by shallow channels filled with coarse grained tuff lapilli tuff and rare tuff breccia the combination of marine fossils extreme textural immaturity abundant slump features and syndepositional magmatism indicates deposition upon the submarine flanks of an active volcano recognition of magma wet sediment interaction is hampered in volcaniclastic rocks because of the similarity between host and intrusive fragments products of magma water sediment interactions at punta maria include 1 jigsaw puzzle hyaloclastite formed by non explosive hydroclastic fragmentation of magma upon contact with water and water bearing sediment 2 peperites produced by mixing of magma with sediment and 3 an unusual tuff breccia unit the result of non explosive mixing of wisps of lava with sediment during remobilization of an unconsolidated section low explosivity magma water sediment interactions are favoured by relatively high hydrostatic pressures in sub wave base settings

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

subaqueous rhyolite block lavas in the miocene ushikiri formation shimane peninsula sw japan abstract a rhyolite mass of the miocene ushikiri formation in the western part of the shimane peninsula sw japan is a small subaqueous edifice about 600 m high and 4 km wide formed at water depths between 200 and 1000 m it consists mainly of three relatively flat lava flow units 50 300 m in maximum thickness each of which includes lobes and their polyhedral fragments the lava lobes are poorly to well vesiculated glassy to microcrystalline and flow banded and folded compared with mafic pillows they are large having thick quenched and brecciated glassy crusts because of their high viscosity surface tension and thermal conductivity their surfaces disintegrate into polyhedral fragments and grade into massive volcanic breccia the massive volcanic breccia composed of the lobe fragments is poorly sorted and covered with stratified volcanic breccia of the same rock type the rhyolite lavas commonly bifurcate in a manner similar to mafic pillow lavas however they are highly silicic with 1 5 vol phenocrysts and have elongated vesicles and flow folds implying that they were visco plastic during flowage their surface features are similar to those of subaerial block lava with respect to rheological and morphological features they are subaqueous equivalents of block lava

large scale rhyolite peperites jurassic southern chile development of a jurassic volcano tectonic rift basin in the southern andes created a setting in which thick rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition in the ultima esperanza district of southern chile extensive masses of peperite were produced when rhyolite magma underwent quenching disruption and commingling with wet unconsolidated sediments during intrusion at shallow levels beneath the sea floor the peperite forms discordant intrusive masses with volumes of up to several cubic kilometers in which large widely spaced coherent rhyolite feeder pods are surrounded by and grade into closely packed and dispersed peperite closely packed peperite consists of tightly fitting clasts separated by sediment filled fractures in dispersed peperite the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts evidence of in situ quench fragmentation is well preserved on both outcrop and thin section scales thin sections show that clast margins and in some cases entire small clasts underwent cooling contraction granulation releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment interaction between magma and wet sediment was non explosive and involved fluidization of the host sediments creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench fragmented rhyolite the ability of the magma to undergo complex intermixing with large volumes of sediment with widespread preservation of in situ fragmentation textures is interpreted to reflect a relatively low magma viscosity presumably caused by retention of volatiles in the magma at the ambient pressures involved beds of redeposited peperite within the rift basin fill indicate that some of the intrusive peperite masses reached the sea floor undergoing slumping and mass flow the peperites were thus an important local source of coarse grained debris during the evolution of the basin

miocene pillowed sills in the shimane peninsula sw japan abstract two miocene basaltic andesite pillowed sills in the shimane peninsula sw japan were intruded into wet marine sediments plastically deforming them the pillows are elongated constricted interconnected and relatively closely packed individual pillows have a poorly to moderately vesiculated somewhat crystalline rind thinner than a few centimeters and a moderately to well vesiculated more crystalline core contraction cracks and spreading cracks are poorly developed the pillows in the sills morphologically resemble pillow lava flows and during sill intrusion the magma bifurcated into pillow lobes in a manner similar to pillow lavas formation of pillows in sill probably occurs when the magma is intruded into wet sediments and protrudes fingers by the instability of the magma sediment interface with little turbulence of magma flow

middle miocene volcaniclastic dikes at kukedo shimane peninsula sw japan fluidization of volcaniclastic beds by emplacement of syn volcanic andesitic dikes abstract at kukedo northern shimane peninsula sw japan volcaniclastic peperite dikes extend parallel to andesitic dikes in volcaniclastic turbidites of the middle miocene takashibiyama formation they mainly comprise a poorly sorted mixture of volcaniclasts derived from the host volcaniclastic turbidites andesitic dikes in direct contact with the volcaniclastic turbidites have fluidal contacts locally protruding into the volcaniclastic turbidites volcaniclastic beds are disorganized in a zone up to 1 m wide and are partly welded within 1 30 cm from the contacts one major volcaniclastic dike 0 5 1 5 m wide extends upwards from the lower segment to the upper segment of a laterally offset andesitic dike 1 5 3 m wide coarse lithic clasts are locally concentrated along the wall of the dike and juvenile clasts derived from the andesitic dike are sparsely set in the poorly sorted volcaniclastic mixture juvenile clasts are poorly vesicular and microcrystalline and mainly polyhedral in shape sparse fluidal juvenile clasts are fractured in situ into small clasts by quench cracks fluidization of host volcaniclastic turbidites occurred to form volcaniclastic dikes during emplacement of andesitic dikes driven by the excess pore fluid pressure imparted from the stress concentration at the advancing magma head and heating of pore water in the host volcaniclastic beds the volcaniclastic dike extending directly from the andesitic dike was produced by small scale dynamic mixing of magma and watery volcaniclastic beds upon offsetting of the andesitic dike welding was favored by the increased water vapor pressures and high temperatures in direct contact with andesitic dikes

quantitative experiments on phreatomagmatic explosions abstract an experimental set up for controlled generation of thermal explosions fuel coolant interactions has been built by an interdisciplinary research group of volcanologists physicists and engineering scientists the tee haus thermal explosion experiment during the last 3 years more than 500 experimental runs were performed aim of the project was not to build a mini volcano but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects such small scale mixtures however most probably also occur during phreatomagmatic events in nature mixtures of water and magma on a large scale may be modelled by combination of small elements earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively therefore a carbonate melt approx 0 3 kg of 1 1 mass ratio na2co3 k2co3 was used for initial experimental series because of easy handling and more magma similarity by injection of water into this melt explosive mixtures were produced and ignited self triggering explosive interactions occurred in a wide melt temperature range 720 to 1040 c at melt temperatures exceeding 1000 c co2 increasingly degassed presence of non condensible gas bubbles in the melt strongly reduced the explosivity the intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities because of more intensive mixing a minimum injection velocity was found to be approx 0 7 m s 1 experiments with silicate melts derived from diverse remelted volcanic rocks were carried out with a slightly modified set up explosions were ignited by an additional trigger a shock wave of approx 8 j in volcanic systems shock waves of this magnitude are abundant volcanic tremor etc strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750 c the produced explosions exerted repulsion forces of up to 24 000 n with a typical pulse duration of 1 ms carbonate melt resp 1 5 ms silicate melts the ejection velocity was found to be in a range between 200 and 400 m s 1 explosion energy values up to 500 j were calculated the maximum explosion pressure was calculated to be in the range between 10 and 100 mpa only part of the melt so called interactive melt reacts explosively with injected water most of the melt is passively ejected by and after the thermal explosion different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 m to 180 m the amount of interactive melt is proportional to the intensity of the explosion the maximum amount of water vapourized at the explosive interaction so called interactive water was estimated thus the water melt mass ratio for explosive interactions could be calculated the values range between 1 25 and 1 35 carbonate melt resp between 1 6 and 1 25 silicate melts

imaging p and s attenuation in the sacramento san joaquin delta region northern california we obtain 3d q p and q s models for the delta region of the sacramento and san joaquin rivers a large fluvial agricultural portion of the great valley located between the sierra nevada batholith and the san francisco bay coast ranges region of active faulting path attenuation t values have been obtained for p and s data from 124 distributed earthquakes with a longer variable window for s based on the energy integral we use frequency dependence with an exponent of 0 5 consistent with other studies and weakly favored by the t s data a regional initial model was obtained by solving for q as a function of seismic velocity in the final model the great valley basin has low q with very low q 50 for the shallowest portion of the delta there is an underlying strong q contrast to the ophiolite basement which is thickest with highest q under the sacramento basin and a change in structure is apparent across the suisun bay as a transition to thinner ophiolite moderately low q is found in the upper crust west of the delta region along the faults in the eastern north bay area whereas moderately high q is found south of the delta implying potentially stronger ground motion for earthquake sources to the south very low q values in the shallow crust along parts of the major fault zones may relate to sediment and abundant microfractures in the lower crust below the san andreas and calaveras hayward rodgers creek fault zones the observed low q is consistent with grain size reduction in ductile shear zones and is lowest under the san andreas which has large cumulative strain similarly moderately low q in the ductile lower crust of the bay area block between the major fault zones implies a broad distributed shear zone online material distribution of earthquake magnitude figures of t values 3d initial regional velocity and q models station terms and q p inversion with 1d initial q p model

low wavenumber structure and evolution of the hurricane inner core observed by airborne dual doppler radar abstract the asymmetric dynamics of the hurricane inner core region is examined through a novel analysis of high temporal resolution three dimensional wind fields derived from airborne dual doppler radar seven consecutive composites of hurricane olivia s 1994 wind field with 30 min time resolution depict a weakening storm undergoing substantial structural changes the symmetric and asymmetric mechanisms involved in this transformation are considered separately to zeroth order the weakening of the primary circulation is consistent with the axisymmetric vortex spindown theory of eliassen and lystad for a neutrally stratified atmosphere vertical shear however increased dramatically during the observation period leading to a strong projection of the convection onto an azimuthal wavenumber 1 pattern oriented along the maximum vertical shear vector recent theoretical ideas elucidating the dynamics of vortices in vertical shear are used to help explain this asymmetry the role of asymmetric vorticity d

a miocene island arc volcanic seamount the takashibiyama formation shimane peninsula sw japan abstract the miocene volcanic complex of the takashibiyama formation consists largely of subalkali subaqueous basalt to andesite lavas and andesite to dacite subaqueous volcaniclastic flow deposits most of subaqueous lavas are moderately to intensely brecciated with rugged rough surfaces and ramp structures similar to subaerial block lava volcaniclastic flow deposits commonly include basalt to andesite lava fragments and or pyroclastic materials and are similar in internal features to debris flow and turbidite deposits subaqueous block lavas and minor pillow lavas form volcanic piles above the vents being flanked or covered with volcaniclastic flow deposits each volcanic pile is several kilometers wide and several hundred meters thick and overlaps one after another as the major eruptive center migrates from time to time thus the volcanic complex comprises three or more superimposed volcanic piles which are intruded by basalt to andesite dikes and sills and are underlain by mafic to intermediate large hypabyssal to plutonic bodies hydrothermal alteration occurs in places but especially concentrates in the lower part of the volcanic complex after the cessation of opening of the japan sea the volcanic complex started to erupt in a middle bathyal environment on the japan sea side and became emergent as it grew to a thickness over 1500 m during 4 m y under a n s trending horizontal compressional stress it is a volcanic seamount probably in a typical island arc setting

variation in peperite textures associated with differing host sediment properties peperites formed by mixing of magma and wet sediment are well exposed along punta china baja california mexico where two sills intrude a section of lava flows limestones and volcaniclastic rocks irregular lobes and dikes extend from the sills several meters into host sediments including highly comminuted flow top breccias lithic lapilli tuff breccias and shelly micrites whereas intrusive contacts with lava flows are sharp and planar where one sill intruded both coarse grained volcaniclastic rock and fine grained limestone textural differences between the hosts produced strikingly different styles of peperite blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias the blocks consequently underwent in situ fragmentation as they were rapidly quenched the high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse grained permeable host elutriation of fine grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter scale fluidization of the host the contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous immiscible fluids fluidal peperite intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off grain by grain as magma advanced into the host thin section scale elutriation pipes formed microglobular peperite represents a frozen example of a fuel coolant interaction fci between basaltic magma and fluidized micrite host the intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma vapor host interface such intimate intermixing of magma and water bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism

secondary welding of submarine pumice lithic breccia at mount chalmers queensland australia very thick units of massive pumice and lithic clast rich breccia in the early permian berserker beds at mount chalmers queensland are deposits from cold water supported volcaniclastic mass flows emplaced in a below wave base submarine setting adjacent to syn volcanic andesitic and rhyolitic sills and dykes the pumice lithic breccia shows a well developed eutaxitic texture the eutaxitic foliation is parallel to intrusive contacts and extends as far as a few metres away from the contact at these sites pumice clasts are strongly flattened and tube vesicles within the pumice clasts are compacted and aligned parallel to the direction of flattening some lenticular pumice clasts contain small 2 mm round quartz filled amygdales and spherulites further away from the sills and dykes the pumice clasts have randomly oriented delicate tube vesicle structure and are blocky or lensoid in shape round amygdales were generated by re vesiculation of the glass and the spherulites indicate devitrification of the glass at relatively high temperatures the eutaxitic texture is therefore attributed to re heating and welding compaction of glassy pumice lithic breccia close to contacts with intrusions in cases involving sills secondary welding along the contacts formed extensive conformable eutaxitic zones in the pumice lithic breccia that could be mistaken for primary welding compaction in a hot primary pyroclastic deposit

the 1989 submarine eruption off eastern izu peninsula japan ejecta and eruption mechanisms the submarine eruption of a new small knoll which was named teishi knoll off eastern izu peninsula behind the izu mariana arc occurred in the evening of 13 july 1989 this is the first historic eruption of the higashi izu monogenetic volcano group the eruption of 13 july followed an earthquake swarm near ito city starting on 30 june there were subsequent volcanic tremors on 11 and 12 july and the formation of the teishi knoll on the 100 m deep insular shelf 4 km northeast of ito city there were five submarine explosions which were characterized by intermittent domelike bulges of water and black tephra jets which occurred within 10 min on 13 july ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria highly vesiculated pumice and lithic material petrographical features suggest that the pumice was produced by vesiculation of reheated wet felsic tuff of an older formation the teishi knoll before the eruption was a circular dome 450 m across and 25 m high with steep sides and a flat summit considerations of submarine topographic change indicate the knoll was raised by sill like intrusion of 106 m3 of magma beneath a 30 m thick sediment blanket this shallow intrusion is assumed to have started on 11 july when volcanic tremors were observed for the first time but there was no indications of violent interaction between wet host sediments and intruding magma the submarine eruption of 13 july appears to have been friggered by a major lowering of the magma column the basalt scoria having crystal contents of more than 60 is assumed to be derived from the cooled plastic margin of the shallow intrusive body however glassy scoria which would indicate the interaction between hot fluidal magma and external water was not observed a scenario for the 1989 submarine eruption is as follows when rapid subsidence of the hot interior of the intrusive magma occurred reduced pressure caused the implosion of cooled plastic magma adjacent pressurized hot host material and wet sediment the mixing of these materials triggered the vigorous vapor explosions

endogenous growth of a miocene submarine dacite cryptodome rebun island hokkaido japan momo iwa rebun island hokkaido japan is a dacite cryptodome 200 300 m across and 190 m high the dome is inferred to have intruded wet poorly consolidated sediment in a shallow marine environment the internal structure of the dome is concentric with a massive core banded rim and narrow brecciated border all of which are composed of compositionally uniform feldspar phyric dacite boundaries between each of the zones are distinct but gradational the massive core consists of homogeneous coherent unfractured dacite and is characterized by radial columnar joints 60 200 cm across the banded rim encircles the massive core and is 40 m wide it is characterized by large scale flow banding parallel to the dome surface the flow banding comprises alternating partly crystalline and more glassy bands 80 150 cm thick the outermost brecciated border is up to 80 cm thick and consists of in situ breccia and blocky peperite the in situ breccia comprises polyhedral dacite clasts 5 20 cm across and a cogenetic granular matrix the blocky peperite consists of polyhedral dacite clasts 0 5 2 cm across separated by the host sediment mudstone the internal structures of the dome suggest endogenous growth involving a continuous magma supply during a single intrusive phase and simple expansion from the interior although much larger the internal structures of momo iwa closely resemble those of lobes in subaqueous felsic lobe hyaloclastite lavas

a hot miocene subaqueous scoria flow deposit in the shimane peninsula sw japan abstract a subaqueous scoria flow deposit occurs in the miocene takashibiyama formation in sw japan faunal assemblages in associated sediments suggest an outer shelf to upper bathyal environment with water depth likely between 200 and 1500 m the deposit 5 7 m thick is similar to subaerial scoria flow deposits containing mainly non to moderately vesiculated hyalopilitic basaltic andesite lapilli and coarse ash the essential basaltic andesite clasts are variably vesiculated even within single clasts and are commonly cracked with curviplanar surfaces whereas the inner parts are cracked in a pattern similar to perlitic cracks the clasts are blocky with curviplanar surfaces if non to poorly vesiculated and are fibrous with irregular shapes and flattened by compaction if more vesiculated at one proximal location fines depleted rootless pipes 5 90 cm wide and 1 3 m long occur and spherical to amoebic vesicles 0 01 0 3 mm in diameter and filled with stilbite quartz or greenish brown clay are common in the fine ash matrix and in siltstone clasts contained in the deposit the vesicles represent open spaces once supported by gas and the rootless pipes are interpreted as gas escape pipes magnetization vectors of essential clasts are dispersed not at random but around a single direction suggesting that the clasts gained thermoremanent magnetism mainly during cooling in the flow or during post emplacement cooling and then rotated to some extent by differential movement of the flow or by gas escape in the deposit the vesicularity shape and texture of essential clasts suggest a phreatomagmatic eruption involving magmatic vesiculation and dynamic interaction of magma and ambient water the perlitic cracks in the inside of the essential clasts and the paleomagnetic vectors of the essential clasts indicate that the flow was initially hot and that the deposit cooled slowly internal depositional features change rapidly over only 500 m rootless pipes and vesicles in the fine ash matrix and siltstone clasts are therefore absent at distal locations indicating that an initially cohesionless debris flow quickly became sufficiently turbulent to mix with ambient seawater

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

miocene rhyolitic welded tuff infilling a funnel shaped eruption conduit shiotani southeast of matsue sw japan at shiotani sw japan rhyolitic welded tuff forms a steep sided funnel shaped body confined by paleogene granitic rocks to an elliptical area 1 1 5 km across the shiotani welded tuff is pervasively welded and foliated concordantly with the contact that dips inward at angles of 70 90 in contrast nearby contemporary volcaniclastic deposits are non welded and gently inclined near the contact with the granite the tuff is plastically deformed and shows lineations that plunge inward at angles of 40 65 lithic and crystal clasts in the rheomorphic outer part are rotated in a plane normal to the foliations and parallel to the lineations indicating downward flow of the welded tuff the geometry and internal structures suggest that the shiotani welded tuff was emplaced and welded in a funnel shaped eruption conduit upon collapse of a plinian or phreatoplinian eruption column the majority of the conduit filling pyroclasts probably fell back en masse into the conduit heat and steam from underlying magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated welding in the conduit especially at deep levels the hot welded pyroclasts then flowed down the conduit wall during welding compaction and retreat of the magma these processes resulted in increased welding toward the contacts and welding foliations concordant with the steep wall emplacement of nearby correlative volcaniclastic mass flow deposits in a shelf to upper bathyal environment suggests a possibility that when active the shiotani conduit was under the sea welding compaction would occur even under the sea provided that the steam generated in the upper part of the conduit fill prevented water access

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

quenching and hydroclastic disruption of andesitic to rhyolitic intrusions in a submarine island arc sequence northern sierra nevada california devonian submarine island arc deposits of the sierra buttes formation in the northern sierra nevada are penetrated by numerous penecontemporaneous hypabyssal intrusions but related extrusive lavas are absent significant parts of the arc succession are occupied by andesitic to rhyolitic intrusive complexes that record the quenching and disruption of large volumes of magma injected into wet sediments at shallow levels beneath the sea floor extensive masses of intrusive hydroclastic breccia are characterized by in situ fragmentation textures and developed by progressive brittle disintegration of quenched magma pillows dispersed within the breccia formed as a series of interconnected tubes that supplied magma to the growing intrusive masses beds of redeposited hydroclastic breccia indicate that the intrusions locally reached the sea floor to undergo slumping and debris flow large scale steam explosions did not occur during magma wet sediment interaction probably due at least in part to the lithostatic and hydrostatic pressure acting on the intrusions low viscosity rhyolitic magma developed intrusive pillows and in situ hydroclastic breccia in the same fashion as quenched andesitic magma this fluid behavior was favored by retention of volatiles in the rhyolitic magma under relatively deep marine conditions quenched and disrupted hypabyssal complexes of the type described herein may be common elements of the submarine parts of island arcs and other volcanic environments where uprise of magma through thick sequences of wet sediment is impeded due to buoyancy constraints and rapid chilling

basaltic phreatomagmatic fissure at 71 gulch part 2 unusual pyroclasts from sediment magma mingling and melting the behavior of magma as it encounters unconsolidated sediment can be studied in fortuitous exposures of incised volcanic systems to help determine the conditions that control the transport arrest or mingling of that magma in the sediment the pliocene subaqueous basaltic fissure at 71 gulch idaho usa contains unusual light colored glassy mingled clasts that were produced through the incorporation of domains of siliciclastic sands and silts into basalt these clasts contain textures reflective of melting and mechanical mixing of these sediments with the rising basalt along intrusions at depths 30 m beneath the pre eruptive surface and transport into and through a shallow subsurface debris filled vent mingled clasts comprised colorless glass with locally high silica contents and visible swirls of sideromelane along with crystals derived from both the host sediment and the 71 gulch basalt the unique combination of textures minerals plus major and trace element geochemistry within the geologic context of 71 gulch points to a two phase mixing that did not contribute directly to nor impede the phreatomagmatic explosions that erupted the mingled clasts