contrasting styles of welding observed in the proximal askja 1875 eruption deposits i regional welding abstract as an alternative to classical welding models of fall deposits due to the progressive accumulation of hot tephra which then weld we describe here welded deposits on the northern 1875 caldera rim of askja volcano that have welded due to the influence of hot discrete spatter bombs impacting into and supplying heat to a halo of surrounding tephra this style of welding we term local welding in contrast to regional welding which is described elsewhere carey r j houghton b f thordarson t 2008 contrasting styles of welding observed in the proximal askja 1875 eruption deposits i regional welding j volcanol geotherm res 171 1 19 doi 10 1016 j jvolgeores 2007 11 020 locally welded deposits are associated with the rhyolitic plinian phase of the 1875 eruption of askja volcano two distinct welding units w1 and w2 are interbedded with plinian fall on the northern caldera rim and grade outwards to weakly dispersed non welded fall spatter bombs are found in both welding units but vary in their characteristic sizes and internal features in the w1 unit simple bombs with homogeneous internal characteristics up to 60 cm in diameter are found in the w2 unit large discrete spatter bombs with complex internal features range up to 9 m in diameter we describe here two case studies showing the effects of a single small spatter bombs b multiple small spatter bombs and c large discrete spatter bombs varying in size vertical and lateral profiles through welding zones reveal that the primary controls on local welding are the availability of supplied or added heat and the loading capacity of the spatter bomb local welding grades are much higher than that of regional welding as the combined effects of heat compaction and insulation can provide suitable conditions which lead to dense welding and proximal to the spatter bomb rheomorphic flowage if heating and loading exceed the critical requirement for welding porosity loss via matrix welding and vesicle collapse occurs to a point where further strain must be accommodated as shearing and ductile flowage the spatter bombs are found only within the weakly dispersed welding units and are the final erupted products of each fountaining phase their low viscosities are evident by their deformation on impact and fluidal forms and hold some important clues to eruption dynamics in the shallow conduit and vent regions

spatter and welded air fall deposits generated by fire fountaining eruptions cooling of pyroclasts during transport and deposition abstract according to the physico mathematical model presented here juvenile ballistic fragments can travel through the atmosphere during fire fountaining eruptions preserving enough heat to agglutinate or to weld on impact with the ground s surface even a few kilometres from the source area the range is mostly related to exit velocity and ejection angle as well as to such typical properties of the transported particles as size density and shape the selective nature of the transport system can give rise to inverse lateral size density grading the most important interdependent factors that control the degree of final agglutination or welding are accumulation rate grain size and duration of deposition in the investigated grain size range an accumulation rate of 0 2 m h appears as a minimum value to promote welding this value cannot be considered as an absolute threshold because it can be significantly lowered by increasing the grain size duration of deposition appears as another critical factor because it controls the maximum temperature of the whole deposit and its final thickness and therefore the cooling rate of its main portion after the end of deposition in the case of high accumulation rates of coarse particles but during a short period of time primary agglutination may be the dominant factor in controlling the final sintering of particles conversely high accumulation rates of coarse particles during a long period of time give rise to both primary agglutination and welding

conduit vent structures and related proximal deposits in the las canadas caldera tenerife canary islands the las canadas caldera wall and the outer slopes of the caldera provide three dimensional exposures of numerous proximal welded fallout deposits and have been mapped in detail as a result some parts of the ucanca and guajara formations of the stratigraphy of marti et al 1994 have been divided into members that correspond to individual eruptions mapping has also revealed the occurrence of conduit vent structures associated with proximal welded fallout deposits conduit vent structures consist of an upper flaring area and a lower narrow conduit conduit vent geometry and dimensions include cylindrical plugs and eruptive fissures steeply dipping towards the caldera depression and elongated vents the flaring area can be rather asymmetric and is usually filled by down vent rheomorphic flow of the proximal fallout deposit the lower conduits are filled by lava plug agglutination of juveniles onto conduit walls and dyke intrusion with eventual dome extrusion the eruption dynamics of welded fallout deposits and magma fragmentation within the conduit are consistent with an evolution from explosive to effusive in this context conduit flow regimes evolve from turbulent to annular flow in which the conduit is progressively choked and laminar flow leading to the final conduit closure

pyroclastic flows from the 1991 eruption of unzen volcano japan pyroclastic flows from unzen were generated by gravitational collapse of the growing lava dome as soon as the parental lobe failed at the edge of the dome spontaneous shattering of lava occurred and induced a gravity flow of blocks and finer debris the flows had a overhanging tongue like head and cone or rollershaped vortices expanding outward and upward most of the flows traveled from 1 to 3 km but some flows reached more than 4 km burning houses and killing people in the evacuated zone of kita kamikoba on the eastern foot of the volcano the velocities of the flows ranged from 15 to 25 m s on the gentle middle flank observations of the flows and their deposits suggest that they consisted of a dense basal avalanche and an overlying turbulent ash cloud the basal avalanche swept down a topographic low and formed to tongue like lobe having well defined levees it is presumed to have moved as a non newtonian fluid the measured velocities and runout distances of the flows can be matched to a bingham model for the basal avalanche by the addition of turbulent resistance the rheologic model parameters for the 29 may flow are as follows the density is 1300 kg m3 the yield strength is 850 pa the viscosity is 90 pa s and the thickness of the avalanche is 2 m the ash cloud is interpreted as a turbulent mixing layer above the basal avalanche the buoyant portions of the cloud produced ash fall deposits whereas the dense portions moved as a surge separated from the parental avalanche the ash cloud surges formed a wide devastated zone covered by very thin debris the initial velocities of the 3 june surges when they detached from avalanches are determined by the runout distance and the angle of the energy line slope a comparison between the estimated velocities of the 3 june avalanches and the surges indicates that the surges that extended steep slopes along the avalanche path detached directly from the turbulent heads of the avalanches the over running surge that reached kita kamikoba had an estimated velocity higher than that of the avalanche this farther travelled surge is presumed to have been generated by collapse of a rising ash cloud plume

flow dynamics of a wide arctic canyon we extend and interpret acoustic doppler current profiler and conductivity temperature depth data collected in the summer of 1993 over barrow canyon in order to implement a high resolution 1 5 to 5 km model of the beaufort and chukchi seas this paper addresses physical processes relevant to the barrow canyon region using common dynamical analyses of both field data and numerical results the field data reveal the dominant physical processes that guide the design of our numerical experiments the observed velocity field shows an intense and variable down canyon flow with transports ranging from 0 5 to 1 4 sv a momentum analysis reveals that the cross canyon dynamic balance for the barotropic component is primarily geostrophic conversely the baroclinic cross canyon momentum balance is ageostrophic and secondary flow results from a local imbalance between the vertically varying coriolis acceleration and the cross canyon pressure gradient in addition to the moderate influence of stratification froude number of 0 4 and burger number of 0 06 the barotropic pressure gradient component across the canyon inferred from the large magnitude and little vertical variability of the residuals is dynamically important for both upcanyon and downcanyon flows that occur at different locations concurrently the along canyon dynamic balance is ageostrophic since the time derivative and the coriolis term are of the same order of magnitude temporal rossby number is approximately 1 an analysis of the longitudinal density and velocity fields from the model reveals that the main driving mechanism for generating the observed upcanyon flow is the nonlinear interaction of the variable barotropic flow with the steep topography stratification is maintained by the downcanyon advection of fresh and warm water from the bering and chukchi seas and the upcanyon advection of saltier and colder water from the arctic

rapid topographic change measured by high resolution satellite radar at soufriere hills volcano montserrat 2008 2010 high resolution satellite radar observations of erupting volcanoes can yield valuable information on rapidly changing deposits and geomorphology using the terrasar x tsx radar with a spatial resolution of about 2 m and a repeat interval of 11 days we show how a variety of techniques were used to record some of the eruptive history of the soufriere hills volcano montserrat between july 2008 and february 2010 after a 15 month pause in lava dome growth a vulcanian explosion occurred on 28 july 2008 from a vent that was hidden by dense cloud we were able to show the civil authorities using tsx difference images of surface roughness change that this explosion had not disrupted the dome sufficiently to warrant continuation of a previous precautionary evacuation change difference images also proved to be valuable in mapping new pyroclastic flow deposits the valley occupying block and ash component tended to increase backscatter and the marginal surge deposits to reduce it with the pattern reversing after the event due to erosion and deposition by comparing east and west looking images acquired 12 h apart the deposition of some individual pyroclastic flows can be inferred from change differences some of the narrow upper sections of valleys draining the volcano received many tens of metres of rockfall and pyroclastic flow deposits over periods of a few weeks by measuring the changing radar shadows cast by these valleys in tsx images the changing depth of infill by deposits could be estimated in addition to using the amplitude data from the radar images we also used their phase information within the insar technique to calculate the topography during a period of no surface activity this enabled areas of transient topography crucial for directing future flows to be captured

a complex quaternary ignimbrite forming phonolitic eruption the poris member of the diego hernandez formation tenerife canary islands abstract the poris member 0 28 ma of the diego hernandez formation tenerife canary islands is a widespread succession of plinian fall surge and non welded ignimbrite deposits it was erupted from the northeastern sector of the multicyclic las canadas caldera the summit caldera complex of the las canadas edifice we present new stratigraphic data which allow the evolution of the eruption to be divided into six main stages 1 an initial surge producing phreatomagmatic phase 2 generation of a buoyant plinian column culminating in vent wall collapse and temporary vent blockage 3 a resumption of phreatomagmatic activity producing accretionary lapilli bearing ash surges and a phreatomagmatic ignimbrite 4 sustained column collapse resulting in the progressive aggradation of a complex compositionally layered ignimbrite sequence 5 a late plinian phase marked by sharp chemical zonation 6 final vent destruction and the generation of multiple surges and pyroclastic flows volume calculations indicate a total erupted volume in the order of 13 14 km 3 3 4 km 3 dense rock equivalent which together with facies and lithic analysis implies that a small scale caldera collapse occurred contributing to the incremental enlargement of the las canadas caldera the geochemistry and mingling inter relationships of four magmatic endmembers ranging from alkaline mafic to evolved phonolitic liquids indicate that two magma chambers fed stages 4 and 5 of the eruption sequence the eruption was triggered when mafic magma intruded the two chamber system

the microstructure of meteoric ice from vostok antarctica the 3623 m long 5g core collected at vostok station antarctica contains alternating layers of meteoric ice with two distinctly different microstructures in this paper we present the microstructure and impurity content of a number of specimens ranging in depth from 97 to 3416 m describe in detail the characteristics of the different layers and propose a mechanism for their microstructural development digital image analysis ion chromatography scanning electron micro scopy and energy dispersive x ray spectroscopy were used to measure texture and the location and type of impurities electron backscatter diffraction was used to determine crystal orientation the ice associated with interglacial periods is characterized by relatively coarse grains and a strong preferred orientation of the c axes in a plane encompassing the coring direction producing a vertical girdle fabric in contrast ice from glacial periods is characterized by a much smaller grain size and a strong single maximum fabric where the c axes are clustered around the vertical calcium is uniquely present in the grain boundaries of the fine grained glacial layers and its effect on grain boundary mobility and the misorientation dependence of mobility can explain the development of the discontinuous micro structure seen in glacial ice at vostok station

proximal record of the 273 ka poris caldera forming eruption las canadas tenerife lithofacies architecture analysis is applied to the most proximal exposures of the 273 ka poris formation in the diego hernandez caldera wall of las canadas here the succession records an opening plinian phase followed by generation of a pyroclastic density current pdc that was partly blocked by near vent topography the upper part of the current surmounted the barrier and deposited an ignimbrite veneer during a hiatus in energetic flow ash and various ash aggregates were deposited from a hybrid cloud comprising parts of both the plinian and the co ignimbrite plumes phreatomagmatic explosions then generated currents rich in fine grained lithics current waxing with local erosion was followed by pulsing unsteady waning recorded in repeated sets of graded tuff reversion to magmatic activity was marked by hybrid deposition as coarse pumice fallout and juvenile ballistics entered pyroclastic currents to form stratified pumice block tuff increasingly energetic flow is recorded in 9 m of mainly massive lapilli tuff that includes evidence of widespread scour and bypass lithic rich layers record incremental onset of edifice disruption due to magma withdrawal laterally variable stratification and load structures show that the pdc was unsteady and highly non uniform at times with locally rapid aggradation of gas rich ignimbrite climactic edifice disruption is marked by a lithic block layer in erosional contact with the underlying ignimbrite recording intense pyroclastic current activity and widespread bypass of material to distal volcano slopes pumice block breccia 10 m thick and with blocks up to 1 2 m in diameter forms the top of the succession and registers modification of the magmatic plumbing the timing of collapse to form a major summit depression is uncertain however the abrupt termination of the proximal aggradation of coarse breccia with no record there of any waning stage pyroclastic currents suggests subsidence at that time conceivably with formation of a caldera that contained late erupted material

40ar 39ar stratigraphy of pyroclastic units from the canadas volcanic edifice tenerife canary islands and their bearing on the structural evolution many felsic pyroclastic units of various types are exposed in different sectors of tenerife new 40ar 39ar determinations allow them to be placed more precisely in the general volcano stratigraphic succession according to geographic distribution stratigraphic position and isotopic ages four main pyroclastic phases may be identified the first san juan de la rambla phase 2 1 ma is only known in the north of tenerife in the tigaiga massif the second adeje phase 1 8 1 5 ma is most completely developed in the southwest of the island but occasionally occurs in the other sectors the third las americas phase 1 ma is only presently known in the southern region the fourth bandas del sur phase 0 7 0 15 ma is essentially exposed in the southeast sector the results of this work emphasise the complexity of the pre 1 ma eruptive history of tenerife and underline the fact that explosive volcanic activity has taken place for at least the last 2 ma vertical collapse structures have developed as a result of pyroclastic flow activity and these may be as old as 1 6 1 8 ma therefore much older than generally considered the precise location of calderas is difficult to ascertain as a result of the repeated lateral flank collapse during the construction of the canadas volcano

volcanic evolution of the island of tenerife canary islands in the light of new k ar data new age determinations from tenerife together with those previously published 93 in all provide a fairly comprehensive picture of the volcanic evolution of the island the oldest volcanic series with ages starting in the late miocene are formed mainly by basalts with some trachytes and phonolites which appear in anaga teno and roque del conde massifs in anaga ne three volcanic cycles occurred one older than 6 5 ma a second one between 6 5 and 4 5 ma with a possible gap between 5 4 and 4 8 ma and a late cycle around 3 6 ma in teno nw after some undated units the activity took place between 6 7 and 4 5 ma with two main series separated by a possible pause between 6 2 and 5 6 ma in the zone of roque del conde s the ages are scattered between 11 6 and 3 5 ma between 3 3 and 1 9 ma the whole island underwent a period of volcanic quiescence and erosion the large canadas volcano made up of basalts trachytes and phonolites was built essentially between 1 9 and 0 2 ma to the ne of this central volcano linking it with anaga is a chain of basaltic emission centers with a peak of activity around 0 8 ma the canadas caldera had several collapse phases associated with large ignimbrite emissions there were at least an older phase more than 1 ma old on the western part of the volcano and a younger one less than 0 6 ma old in the eastern side the two large valleys of guimar and la orotava were formed by large landslides less than 0 8 ma ago and probably before 0 6 ma ago the present canadas caldera was formed by another landslide less than 0 2 ma ago this caldera was later filled by the huge teide volcano which has been active even in historic times during the same period a series of small volcanoes erupted at scattered locations throughout the island the average eruptive rate in tenerife was 0 3 km3 ka with relatively small variations for the different eruptive periods this island and la gomera represent a model of growth by discontinuous pulses of volcanic activity separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years the neighbouring gran canaria on the other hand had an initial rapid shield building phase during which more than 90 of the island was built and a series of smaller pulses at a much later period a comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation several gaps are present on each island overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the canaries

ocean ice interaction in the marginal ice zone using synthetic aperture radar imagery ocean ice interaction processes in the marginal ice zone miz by wind waves and mesoscale features such as up downwelling and eddies are studied using earth remote sensing satellite ers 1 synthetic aperture radar sar images and an ocean ice interaction model a sequence of seven sar images of the miz in the chukchi sea with 3 or 6 days interval are investigated for ice edge advance retreat simultaneous current measurements from the northeast chukchi sea as well as the barrow wind record are used to interpret the miz dynamics sar spectra of waves in ice and ocean waves in the bering and chukchi sea are compared for the study of wave propagation and dominant sar imaging mechanism by using the sar observed ice edge configuration and wind and wave field in the chukchi sea as inputs a numerical simulation has been performed with the ocean ice interaction model after 3 days of wind and wave forcing the resulting ice edge configuration eddy formation and flow velocity field are shown to be consistent with sar observations

internal flow variations and diachronous sedimentation within extensive sustained density stratified pyroclastic density currents flowing down gentle slopes as revealed by the internal architectures of ignimbrites on tenerife during a protracted explosive eruption at least four laterally extensive and sustained pyroclastic density currents radiated across the flanks of las canadas volcano tenerife each pyroclastic current developed marked local and regional spatial variations in response to the incised gently concave substrate topography the locations of these variations shifted in space as rapid sedimentation from the current progressively buried and modified the topography this complex shifting response of the density currents to minor topographic variations has been reconstructed in high resolution over a wide area 500 km2 using the internal architecture of cryptic time surfaces entrachrons marked by compositional zoning in the deposit including variations in clast types valley side field relations reveal that the currents were density stratified yet at a single instant in time the lower levels of each current comprised a granular fluid at some locations but were fully dilute and turbulent at others moreover the locations of these variations shifted geographically as the topography changed during the eruption the variations within the current are recorded by numerous superbly exposed gradational transitions from various stratified to massive lithofacies both laterally and in the downcurrent direction individual currents were regionally widespread and travelled 15 km but deposited only in longitudinally restricted localised zones that spanned several small valleys and interfluves the currents bypassed slopes up and downcurrent of the restricted depositional zones without depositing the locations of deposition then gradually shifted with time such that the extensive deposit sheet was gradually assembled beneath the sustained current in a diachronous fashion onlap relationships of internal entrachrons reveal that the base of the ignimbrite sheet and even the bases of individual flow units are markedly diachronous deposition of a flow unit commenced and ceased at different times in different places this study suggests that in hazard assessments a models of density currents that incorporate only pre existing topography e g from dems may give misleading results in the case of sustained currents because sedimentation from these significantly modifies the topography during emplacement altering flow paths b frequencies and scales of previous pyroclastic currents determined from pyroclastic successions are likely to be significantly under estimated because currents commonly bypass without leaving a deposit record and c even where preservation appears to be complete an ignimbrite at a single exposure commonly will not record the current s entire flow history at that site

welding and rheomorphism of phonolitic fallout deposits from the las canadas caldera tenerife canary islands the las canadas caldera is a nested collapse caldera formed by the successive migration and collapse of shallow magmatic chambers among the pyroclastic products of this caldera are phonolitic fallout deposits that crop out in the caldera wall and on the extracaldera slopes these deposits exhibit an uninterrupted facies gradation from nonwelded to lava like and record continuous volcanic deposition densely welded and lava like facies result from the extreme attenuation and complete homogenization of juvenile clasts that destroy original clast outlines and any evidence of fallout deposition agglutination contributes significantly to the final degree of flattening observed in the welded facies after deposition rheomorphic flowage occurs emplacement temperatures for one of the welding sequences are calculated from magmatic temperatures and a model of tephra cooling during fallout results are 486 c for the nonwelded facies and 740 c for the moderately welded facies for the same welding sequence a cooling time between 25 and 54 days is estimated from published experimental and computational data as the possible duration of welding and rheomorphism following deposition and agglutination the lava like pyroclastic facies had the rheological properties of viscous lavas and flowed down the outer slopes away from the caldera some lava like masses detached from proximal areas to more distal regions during deposition the eruptive style evolved from plinian fallout to fountain fed spatter deposition this evolution was accompanied by a decrease in explosive power and a lower height of the eruptive column which produce higher emplacement temperatures and more effective heat retention of pyroclasts

application of aerosondes to high resolution observations of sea surface temperature over barrow canyon 1 continuous observation of sea surface temperature sst were obtained for a period of 27 hours over barrow canyon on 20 21 september 2002 using a small robotic aircraft called the aerosonde these observations demonstrated significant spatial and temporal variations in the sst at scales that would be very difficult to measure using mooring and satellite observations over a region of 25 km2 sst values varied from 1 6 c a sharp sst front with a temperature gradient of 3 c was observed over the northwestern shelf of barrow canyon which moved northward during the observation period relatively cold water was transported from the southwest into the region current speeds were estimated using the time series of observations this analysis demonstrates new technology for oceanographic observations and supports previous evidence that the flow through barrow canyon has a strongly sheared flow into the arctic ocean

emplacement and rheomorphic deformation of a large lava like rhyolitic ignimbrite grey s landing southern idaho the miocene grey9s landing ignimbrite reaches 70 m thick and covers at least 400 km 2 in the central region of the snake river plain it shows particularly intense welding and rheomorphic deformation and although parts are eutaxitic most is lava like with flow banding and no fiamme a near ubiquitous penetrative flow lamination associated with a well developed elongation lineation is folded into small intrafolial tight to isoclinal oblique and sheath folds which are refolded by larger folds in the upper parts structural and kinematic analysis reveals that welding and early deformation occurred rapidly during deposition from a very hot 1000 c high mass flux pyroclastic density current that flowed westward across a graben faulted landscape as hot particles were deposited they rapidly agglutinated and coalesced and underwent noncoaxial shear in a subhorizontal ductile shear zone close to the current deposit interface the shear zone is interpreted to have been less than 2 m thick it produced and deformed the rheomorphic fabric and it migrated upward with the rising current deposit interface during aggradation so that it transiently affected all levels of the resultant thick ignimbrite deformation was progressive and after the density current had dissipated viscous spreading and downslope flow continued and involved an increasingly thick portion of the sheet this folded the flow banding and f 1 intrafolial isoclines into larger sheath folds and into more upright periclines near the top of the ignimbrite we demonstrate that structural and kinematic analysis can elucidate the emplacement history of rheomorphic ignimbrites

mesoscale eddies of the arctic ocean subsurface baroclinic eddies in the arctic ocean have a diameter of 10 20 km and are confined in depth between 50 and 300m during the arctic ice dynamics joint experiment aidjex in the central beaufort sea between march 1975 and may 1976 extensive information on eddy characteristics statistics and behavior was collected from four manned drifting ice camps during this 14 month period 146 eddies were encountered of which 19 were found to be repeated crossings making a total of 127 separate eddies observed during this period on the basis of the 1975 1976 aidjex data set it is concluded that eddies are 1 prevalent in the amerasia basin of the arctic ocean and in particular the beaufort sea where they may occupy up to a quarter of the available surface area 2 located in the depth range of 50 300 m although a few deeper eddies are also present 3 the second largest producers 32 of kinetic energy found within the upper 200 m of the beaufort sea and 4 predominantly anticyclonic in rotational tendency these eddies apparently originate north of point barrow alaska as a result of instability in the eastward flowing alaskan coastal current and transfer seasonally varying water properties from the chukchi sea and alaskan shelf into the arctic ocean they appear to translate in response to barotropic forcing over short time scales and over longer time periods move with the mean geostrophic field the season of formation for an eddy may be identified by the anomalous thermal properties still residing in its core

quaternary ring plain and valley confined pyroclastic deposits of aragats stratovolcano lesser caucasus lithofacies geochronology and eruption history abstract aragats stratovolcano located on the south armenian block a micro plate in the arabian eurasian collision zone has a complex and long lasting quaternary eruption history cataclysmic eruptions formed widespread pyroclastic deposits which are interrupted by numerous basaltic to trachytic lava flows as well as lacustrine and lacustrine alluvial sediments contrary to previous concepts detailed mapping of lithofacies variation combined with stratigraphic documentation and new high precision 40ar 39ar ages revealed four eruption cycles each started with mafic lavas and generally finalized with pyroclastic deposits these data contributed to a fundamental reinterpretation of the development of the aragats volcanic province avp within an estimated activity of one million years aragats itself experienced three eruption stages two of which culminated in pyroclastic deposits of remarkable lithofacies variability with ages of 0 97 to 0 90 ma and 0 75 to 0 65 ma respectively vertical and lateral changes in proximal and distal facies distribution of pyroclastic flow deposits indicate temporal variations in eruption intensity and periods of relative volcanic quiescence aragats explosive activities were dominated by plinian eruptions with columns as high as 25 km while others are characterized by low eruption columns followed by instantaneous collapse in one case associated with vent widening possible caldera collapse and breccia formation furthermore one avp unit features juvenile lapilli and blocks including banded and armored pumice indicating magma mingling and their involvement in the erupting mixture before magma fragmentation

particle velocity fields and depositional processes in laboratory ash flows with implications for the sedimentation of dense pyroclastic flows we conducted laboratory experiments on dam break flows of sub 250 m volcanic ash generated by the release of gas fluidized and variably non expanded to expanded up to 35 beds in order to gain insights into the internal kinematics of pyroclastic flows the flows were typically several cm thick and had frontal speeds of up to 2 m s 1 high speed videos taken through the transparent sidewall of the 3 m long channel were analyzed with a particle tracking algorithm providing a spatial and temporal description of transport and sedimentation the flows deposited progressively as they traveled down the flume being consumed by sedimentation until they ran out of volume deposition commenced 5 20 cm rearward of the flow front and for a given expansion proceeded at a rate independent of distance from the lock gate deposit aggradation velocities were equal to those inferred beneath quasi static bed collapse tests of the same ash at the same initial expansions implying that shear rates of up to 300 s 1 have no measurable effect on aggradation rate the initially non expanded and just fluidized flow deposited progressively at a rate indicative of an expansion of a few percent perhaps due to shear induced reynolds dilation during initial slumping the fronts of the flows slid across the flume floor on very thin basal shear layers but once deposition commenced a no slip condition was established at the depositional interface within the flows the trajectory of the constituent particles was linear and sub horizontal the velocities of the particles increased with height above the depositional interface reached a maximum then declined slightly towards the flow surface perhaps due to air drag at a given location the velocity profiles were translated upwards as the deposit aggraded the results show that even cm thin poorly expanded flows of ash deposit progressively as inferred for many pyroclastic flows the change from frontal slip to rearward no slip conditions at the bases of the laboratory flows are qualitatively consistent with some textural features of pyroclastic flow deposits

a new approach to kimberlite facies terminology using a revised general approach to the nomenclature of all volcanic rocks and deposits descriptive to genetic although kimberlite pipes bodies are usually the remains of volcanic vents in vent deposits and subvolcanic intrusions the terminology used for kimberlite rocks has largely developed independently of that used in mainstream volcanology existing kimberlite terminology is not descriptive and includes terms that are rarely used used differently and even not used at all in mainstream volcanology in addition kimberlite bodies are altered to varying degrees making application of genetic terminology difficult because original components and depositional textures are commonly masked by alteration this paper recommends an approach to the terminology for kimberlite rocks that is consistent with usage for other volcanic successions in modern terrains the eruption and emplacement origins of deposits can often be readily deduced but this is often not the case for old variably altered and deformed rock successions a staged approach is required whereby descriptive terminology is developed first followed by application of genetic terminology once all features including the effects of alteration on original texture and depositional features together with contact relationships and setting have been evaluated because many volcanic successions consist of both primary volcanic deposits as well as volcanic sediments terminology must account for both possibilities

a record minimum arctic sea ice extent and area in 2002 1 arctic sea ice extent and area in september 2002 reached their lowest levels recorded since 1978 these conditions likely resulted from 1 anomalous warm southerly winds in spring advecting ice poleward from the siberian coast 2 persistent low pressure and high temperatures over the arctic ocean in summer promoting ice divergence and rapid melt

varieties of shallow temperature maximum waters in the western canadian basin of the arctic ocean the properties and spreading pathways of shallow temperature maximum waters stms in the western canadian basin are investigated using ctd and mooring data obtained in 1997 98 as part of the sheba surface heat budget of the arctic ocean drift experiment and available historical data three distinct varieties of stm are recognized on the basis of salinity range 1 surface mixed layer water smlw with s 32 psu these stms carry sufficient heat within the upper layers of the ocean to significantly affect the rates of ice cover and decay for example during the winter of 1997 98 anomalously warm stm 0 c originating from ecsw was observed to spread northwards along the northwind ridge and chukchi plateau where the maximum reduction of the ice covers was subsequently observed in late summer 1998 maslanik et al 1999 regional climate variability and ice cover in the western canadian basin are thus affected not only by anomalous atmospheric circulation patterns but also by the circulation of upper ocean water masses

event stratigraphy of a caldera forming ignimbrite eruption on tenerife the 273 ka poris formation the 273 ka poris formation in the bandas del sur group records a complex compositionally zoned explosive eruption at las canadas caldera on tenerife canary islands the eruption produced widespread pyroclastic density currents that devastated much of the se of tenerife and deposited one of the most extensive ignimbrite sheets on the island the sheet reaches 40 m thick and includes plinian pumice fall layers massive and diffuse stratified pumiceous ignimbrite widespread lithic breccias and co ignimbrite ashfall deposits several facies are fines rich and contain ash pellets and accretionary lapilli eight brief eruptive phases are represented within its lithostratigraphy phase 1 comprised a fluctuating plinian eruption in which column height increased and then stabilized with time and dispersed tephra over much of the southeastern part of the island phase 2 emplaced three geographically restricted ignimbrite flow units and associated extensive thin co ignimbrite ashfall layers which contain abundant accretionary lapilli from moist co ignimbrite ash plumes a brief plinian phase phase 3 again dispersing pumice lapilli over southeastern tenerife marked the onset of a large sustained pyroclastic density current phase 4 which then waxed phase 5 covering increasingly larger areas of the island as vents widened and or migrated along opening caldera faults the climax of the poris eruption phase 6 was marked by widespread emplacement of coarse lithic breccias thought to record caldera subsidence this is inferred to have disturbed the magma chamber causing mingling and eruption of tephriphonolite magma and it changed the proximal topography diverting the pyroclastic density current s down the guimar valley phase 7 phase 8 involved post eruption erosion and sedimentary reworking accompanied by minor down slope sliding of ignimbrite this was followed by slope stabilization and pedogenesis the fines rich lithofacies with abundant ash pellets and accretionary lapilli record agglomeration of ash in moist ash plumes they resemble phreatomagmatic deposits but a phreatomagmatic origin is difficult to establish because shards are of bubble wall type and the moisture may have arisen by condensation within ascending thermal co ignimbrite ash plumes that contained atmospheric moisture enhanced by that derived from the evaporation of seawater where the hot pyroclastic currents crossed the coast ash pellets formed in co ignimbrite ash clouds and then fell through turbulent pyroclastic density currents where they accreted rims and evolved into accretionary lapilli

on the wind driven variability of the flow through bering strait analysis of time series from 1981 to 1982 near bering strait shows the meridional wind component the current and the oceanic pressure gradient to be well correlated we argue that because of the constraining coastal geometry there are wind driven flow divergences which perturb the oceanic pressure field and account for most of the current variability for example northerly winds cause a pronounced set down of sea level southeast of bering strait and if the winds are strong enough they will reverse the mean slope downward to the north through the strait winds during 1976 1977 were also well correlated with the bering strait transport estimates of coachman and aagaard 1981 and we therefore use the calculated winds from 1946 to 1982 to examine the low frequency transport variability there is a marked seasonal cycle with summer transport about 50 greater than during winter the long term mean transport is calculated as less than 0 6 sv which is much lower than earlier estimates abnormally large transports apparently occurred during a number of years from 1948 to 1967 but during the past 15 years transports have tended to be considerably less corresponding to the stronger northerly winds during these years the interannual wind variability near bering strait together with the corresponding variability in the transport through the strait is in fact part of the large scale variability of the atmospheric circulation over the north pacific and its oceanic effects

friction melting catastrophic dilation and breccia formation along caldera superfaults at glencoe caldera volcano friction melts and magmas transformed explosively to froth or spray where they encountered rapid decompression in dilatant sections of superfaults the friction melts were blasted upwards plastering free surfaces and were rapidly followed by fragmented magma and then liquid magma that formed intrusions irregular contacts of fault intrusions record explosive disruption of hydrothermal systems cut by the dilational faults with lithic breccias removed from the rapidly formed cavities before the arrival of melt spray layers of lithic breccia are common in ignimbrites around calderas and usually show incorporation of hot and hydrothermally altered rocks such layers may specifically reflect initial superfault dilation during caldera collapse

the quaternary pyroclastic succession of southeast tenerife canary islands explosive eruptions related caldera subsidence and sector collapse a much revised quaternary stratigraphy is presented for ignimbrites and pumice fall deposits of the bandas del sur in southern tenerife new ar 41 ar 39 data obtained for the arico granadilla fasnia poris la caleta and abrigo formations are presented allowing correlation with previously dated offshore marine ashfall layers and volcaniclastic sediments we also provide a minimum age of 287 7 ka for a major sector collapse event at the gaimar valley the bandas del sur succession includes more than seven widespread ignimbrite sheets that have similar characteristics including widespread basal plinian layers predominantly phonolite composition ignimbrites with similar extensive geographic distributions thin condensed veneers with abundant diffuse bedding and complex lateral and vertical grading patterns lateral gradations into localized massive facies within palaeo wadis and widespread lithic breccia layers that probably record caldera forming eruptions each ignimbrite sheet records substantial bypassing of pyroclastic material into the ocean the succession indicates that las canadas volcano underwent a series of major explosive eruptions each starting with a plinian phase followed by emplacement of ignimbrites and thin ash layers some of coignimbrite origin several of the ignimbrite sheets are compositionally zoned and contain subordinate mafic pumices and banded pumices indicative of magma mingling immediately prior to eruption because passage of each pyroclastic density current was characterized by phases of non deposition and erosion the entire course of each eruption is incompletely recorded at any one location accounting for some previously perceived differences between the units because each current passed into the ocean estimating eruption volumes is virtually impossible nevertheless the consistent widespread distributions and the presence of lithic breccias within most of the ignimbrite sheets suggest that at least seven caldera collapse eruptions are recorded in the bandas del sur succession and probably formed a complex nested collapse structure detailed field relationships show that extensive ignimbrite sheets e g the arico poris and la caleta formations relate to previously unrecognized caldera collapse events we envisage that the evolution of the nested las cahadas caldera is more complex than previously thought and involved a protracted history of successive ignimbrite related caldera collapse events and large sector collapse events interspersed with edifice building phases

current response to wind in the chukchi sea a regional coastal upwelling event a cruise was conducted on the noaa ship oceanographer in august and september 1986 in the chukchi sea four short term moorings were instrumented with sediment traps and aanderaa rcm4 current meters the currents at the three moorings near the alaskan coast indicate a reversal of the normal northeastward flow to southwestward this reversal was produced by wind which had begun to blow from the east northeast at up to 4 2 m s 1 the temperature time series indicated that upwelling colder water was moving upward near the coast the nearshore mooring had the largest amplitude variation of currents and the largest temperature variation the amplitude of the reversal decreased offshore the station farthest from the coast was near the ice edge and on the other side of barrow canyon and a subsea bank the flow there was consistently toward the east and is only poorly related to the wind water mass analysis was conducted using two techniques the first was a traditional ts diagram method and the second was a cluster analysis on ts pairs for the surface and near bottom waters based on the shapes of the ts curves and their positions on the ts diagrams a map of the water masses was constructed a cluster analysis was performed on the surface ts pairs from each station and separately for the bottom ts pairs for both of these techniques the line separating the groupings follows the temperature contours 5 at the surface and 4 at the bottom and the bottom salinity contours 32 5 psu

eruption and deposition of the fisher tuff alaska evidence for the evolution of pyroclastic flows abstract recognition that the fisher tuff unimak island alaska was deposited on the leeside of an 500 700 m high mountain range tugamak range more than 10 km away from its source played a major role in defining pyroclastic flows as momentum driven currents we reexamined the fisher tuff to evaluate whether deposition from expanded turbulent clouds can better explain its depositional features we studied the tuff at 89 sites and sieved bulk samples from 27 of those sites we find that the tuff consists of a complex sequence of deposits that record the evolution of the eruption from a buoyant plume 22 km that deposited 0 2 km3 of dacite magma as a pyroclastic fall layer to erupting 10 100 km3 of andesitic magma as scoria rich pyroclastic falls and flows that were mainly deposited to the north and northwest of the caldera including those in valleys within the tugamak range the distribution of the flow deposits and their welding internal stratification and the occurrence of lithic breccia all su

stratigraphy of the bandas del sur formation an extracaldera record of quaternary phonolitic explosive eruptions from the las canadas edifice tenerife canary islands explosive volcanism has dominated the large phonolitic shield volcano of tenerife the las canadas edifice for the last 1 5 m y pyroclastic deposits of the bandas del sur formation are exposed along the southern flanks and record the last two of at least three long term cycles of caldera forming explosive eruptions each cycle began with flank fissure eruptions of alkali basalt lava followed by minor eruptions of basanite to phonotephrite lavas minor phonotephritic to phonolitic lava effusions also occurred on the flanks of the edifice during the latter stages of the second explosive cycle non welded plinian fall deposits and ignimbrites are the dominant explosive products preserved on the southern flanks of these a significant volume has been dispersed offshore many pyroclastic units of the second explosive cycle exhibit compositional zonation banded pumice occurs in most units of the third youngest explosive cycle and ignimbrites typically contain mixed phenocryst assemblages indicating the role of magma mixing mingling prior to eruption at least four major eruptions of the third cycle began with phreatomagmatic activity producing lithic poor accretionary lapilli bearing fallout and or surge deposits the repeated brief phase of phreatomagmatism at the onset of these eruptions is interpreted as reflecting an exhaustive water supply probably a small caldera lake that was periodically established during the third cycle accidental syenite becomes an increasingly important lithic clast type in ignimbrites up sequence and is interpreted as recording the progressive development of a plutonic complex beneath the summit caldera successive eruptions during each explosive cycle increased in volume with the largest eruption occurring at the end of the cycle more than ten major explosive eruptions vented moderately large volumes 1 ges 10 km 3 of phonolitic magma during the last two cycles culminating each explosive cycle was the emplacement of relatively large volume 5 10 km 3 ignimbrites with coarse vent derived lithic breccias interpreted to record a major phase of caldera collapse in the extracaldera record explosive cycles are separated by 0 2 m y periods of non explosive activity repose periods were characterized by erosion remobilization of pyroclastic deposits by discharge events and pedogenesis the current period of non explosive activity is characterized by the construction of the teide pico viejo stratovolcanic complex within the summit caldera this suggests that eruptive hiatuses in the extracaldera record may reflect effusive activity and stratovolcano or shield building phases within the summit caldera alternating effusive and explosive cycles have thus been important in the volcanic evolution of the las canadas edifice

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

tectonically controlled piecemeal caldera collapse a case study of glencoe volcano scotland caldera collapse at glencoe scotland was incremental and involved complex movements of numerous fault blocks before formation of the ring fault for which the volcano is renowned intracaldera depocenters had the form of grabens or half grabens locally with downsag and bounding monoclinal flexures and these changed form and location for each of the five major ignimbrite eruptions that represent the early volcanic history this piecemeal caldera collapse is in contrast to previous interpretations in which all subsidence was thought to have involved a coherent crustal block moving on the ring fault collapse and magmatic plumbing were profoundly influenced by preexisting tectonic faults trending northwest and northeast a dominant northwest trending graben controlled the general location and form of major caldera depocenters and repeatedly channeled a major river through glencoe the main graben was transected orthogonally by two cross grabens each of the first three caldera eruption cycles involved initial phreatomagmatic explosivity which built tuff cones followed by magmatic fountaining that produced lava like silicic ignimbrites the three lava like ignimbrites total more than 300 m thick two later eruptions produced eutaxitic silicic ignimbrites together more than 300 m thick and contemporaneous progressive downsag was associated with the formation of extensional fractures crevasses hundreds of meters deep sills up to 100 m thick of mingled andesite and rhyolite between the intracaldera ignimbrites were accommodated by increments of subsidence unconformities and sedimentary layers within the stratigraphic succession record fluvial erosion and abrupt switches to alluvial and or lacustrine deposition between each ignimbrite eruption the changes in drainage and sedimentation and the development of coarse debris avalanche breccias reflect tectonic faulting during the periods between major eruptions the duration of the period including the five caldera forming eruptions was probably 0 5 m y and the magnitude of the tectonic faulting that occurred prior to and during the caldera developments 0 5 km m y normal displacement is similar to that of the most actively subsiding sedimentary basins glencoe shows that the piecemeal nature of calderas may be plainly evident only in deeply dissected systems which with knowledge that caldera volcanoes commonly overlie faults suggests that piecemeal calderas may be more common than previously recognized the research also shows that the emplacement of granites s l was episodic rather than synchronous throughout the magmatic province and it suggests that numerous plutons in the province e g in donegal ireland may have formed at sites of central volcanoes that are no longer preserved

single pyroclastic beds deposited by simultaneous fallout and surge processes roccamonfina volcano italy abstract a sequence of fine grained hydrovolcanically generated pyroclastic surge deposits in the white trachytic tuff roccamonfina volcano italy contains two layers of coarse pumice clasts pumice fragments in these layers show no evidence for abrasion during lateral transport unlike pumice in the enclosing deposits and are well sorted in most locations they have textural and grain size characteristics typical of plinian fall deposits bedding of these pumice layers ranges from planar with gradual thickness variation over a few meters distance to highly variable with strong thickening in depressions the layers are interpreted to record deposition of pumice falling from a plinian eruption column through laterally moving pyroclastic surges depending on the local vigor of the surges during the fallout event bedding characteristics range from those approaching pure fallout to those approaching pure surge the simultaneous plinian and hydrovolcanic pyroclastic surge producing activity may have come from two or more separate vents or from a single vent that experienced rapid variations in magma water interaction

insights into eruption dynamics from the 2014 pyroclastic deposits of kelut volcano java indonesia and implications for future hazards abstract on 13 to 14 february 2014 a 4 h long vei 4 eruption occurred at kelut volcano java indonesia pyroclastic density currents pdcs and extensive ash fall led to 7 fatalities and disruption to flights across the asia pacific region new sedimentological descriptions of the pyroclastic deposits from the 2014 eruption were compared with eyewitness and satellite reports to elucidate temporal variations in eruptive dynamics the stratigraphy of the deposits is presented in 3 stages associated with two eruptions that occurred approximately 15 30 min apart stage 1 pdc deposits originate from the smaller onset eruption the pdc deposits from stage 2 and tephra fall deposits from stage 3 originate from the second plinian eruption during the onset eruption stage 1 low energy pdcs were produced that ran out to 70 by vol pdc deposits ran out to 4 7 km from the vent the deposits reflect an increased output of fresh fragmented magma and some conduit widening evidenced by dense lithic fragments vent instabilities and incorporation of dense material into basal margins of the plume led to the marginal collapse and formation of these pdcs stage 3 occurred in the final hour at the peak of the plinian eruption around 01 00 to 02 00 and produced reversely graded lapilli fall deposits with 90 vol pumice from a 26 km high plume this indicates that there was a sustained flux of juvenile magma to the now open vent system and expansion and fragmentation of the gas rich magma was at its most efficient our study of the eruptive sequence of kelut provides some constraints on predicted patterns for future explosive activity critical for further hazard assessment of the volcano since 1901 kelut has erupted on intervals of 1 to 23 years and the 2014 event characterises a typical explosive style of eruption that alternates regularly with effusive dome formation and collapse events this pattern depends on the dynamics of magma renewal to the system and degassing conditions in the shallow magma reservoir and upper conduit if this pattern holds and the next eruption occurs within the next two decades a return of prolonged dome growth could be anticipated

complex proximal deposition during the plinian eruptions of 1912 at novarupta alaska proximal 3 km deposits from episodes ii and iii of the 60 h long novarupta 1912 eruption exhibit a very complex stratigraphy the result of at least four transport regimes and diverse depositional mechanisms they contrast with the relatively simple stratigraphy and inferred emplacement mechanisms for the previously documented better known medial distal fall deposits and the valley of ten thousand smokes ignimbrite the proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta and numerous thin complex deposits of pyroclastic density currents pdcs with no regional analogs the locally dispersed component of the fall deposits forms sector confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100 300 m cf several kilometers for the medial distal fall deposits this locally dispersed fall material and many of the associated pdc deposits is rich in andesitic and banded pumices and richer in shallow derived wall rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition there are no marked contrasts in grain size in the near vent deposits however between locally and widely dispersed beds and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial distal samples associated pdc deposits form a spectrum of facies from fines poor avalanched beds through thin bedded landscape mantling beds to channelized lobes of pumice block rich ignimbrite the origins of the novarupta near vent deposits are considered within a spectrum of four transport regimes 1 sustained buoyant plume 2 fountaining with co current flow 3 fountaining with counter current flow and 4 direct lateral ejection the novarupta deposits suggest a model where buoyant stable regime 1 plumes characterized most of episodes ii and iii but were accompanied by transient and variable partitioning of clasts into the other three regimes only one short period of vent blockage and cessation of the plinian plume occurred separating episodes ii and iii which was followed by a single pdc interpreted as an overpressured blast involving direct lateral ejection in contrast regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume which were being strongly affected by short lived instabilities these instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent of the parameters that control explosive eruptive behavior only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products

accretionary lapilli bearing pyroclastic rocks at odp leg 192 site 1184 a record of subaerial phreatomagmatic eruptions on the ontong java plateau

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

sedimentation and welding processes of dilute pyroclastic density currents and fallout during a large scale silicic eruption kikai caldera japan abstract sedimentation and welding processes of the high temperature dilute pyroclastic density currents and fallout erupted at 7 3 ka from the kikai caldera are discussed based on the stratigraphy texture lithofacies characteristics and components of the resulting deposits the welded eruptive deposits unit b were produced during the column collapse phase following a large plinian eruption and preceding an ignimbrite eruption and can be divided into two subunits units b l and b u unit b l is primarily deposited in topographic depressions on proximal islands and consists of multiple thin u shows densely welded stratified facies composed of alternating lithic rich and pumice rich layers the layers mantle lower units and are sometimes viscously deformed by ballistics the sedimentary characteristics of unit b l such as welded stratified or cross stratified facies indicate that high temperature dilute pyroclastic density currents were repeatedly generated from limited magma water interactions it is thought that dense brittle particles were segregated in a turbulent current and were immediately buried by deposition of hot lighter pumice rich particles and that this process repeated many times it is also suggested that the depositional temperature of eruptive materials was high and the eruptive style changed from a normal plinian eruption through surge generating explosions unit b l into an agglutinate dominated fallout eruption unit b u on the basis of field data welded pyroclastic surge deposits could be produced only under specific conditions such as 1 rapid accumulation of pyroclastic particles sufficiently hot to weld instantaneously upon deposition and 2 elastic particles interactions with substrate deformation these physical conditions may be achieved within high temperature and highly energetic pyroclastic density currents produced by large scale explosive eruptions

northward flow in the bering and chukchi seas the ocean circulation on the bering and chukchi sea shelves is investigated using a barotropic numerical model in the presence of the winter seasonal wind stress from the northeast observed northward transport through bering strait of 0 6 sv for the 8 month winter season 1981 1982 can be driven by a sea level difference of 0 4 m between the pacific and arctic oceans this sea level difference between basins is in agreement with hydrographic estimates in the absence of wind stress a 0 4 m sea level difference drives a northward transport through bering strait of 1 1 sv maximum transport through bering strait is geostrophically limited not frictionally or inertially limited major qualitative circulation features of the region are evident in the simulations westward intensification of the northward transport across the bering sea shelf is confirmed with the major transport occurring in the relatively deep gulf of anadyr in the absence of wind anadyr strait contributes 72 of the northward transport through bering strait with northeasterly winds there is a net clockwise circulation around saint lawrence island within the 20 m isobath norton and kotzebue sounds are little influenced by the general circulation northward transport in the chukchi sea bifurcates in the vicinity of point hope the predominant branch is directed toward the northwest in a broad canyon feature south of herald shoal and the balance becomes the alaskan coastal current between cape lisburne and point barrow

stratigraphy and k ar ages of the diego hernandez wall and their significance on the las canadas caldera formation tenerife canary islands the las canadas caldera corresponds to a volcanic caldera formed after a long period of explosive activity the structural and lithological variations which can be observed in the caldera wall make clear the complexity of the original las canadas edifice in which different eruptive centres existed the stratigraphic and structural reconstruction of the las canadas caldera indicates that the diego hernandez wall located at the eastern side of the caldera wall comprises the youngest pre caldera deposits determination of their k ar ages has provided the maximum age for the formation of the las canadas caldera the results are internally consistent with stratigraphic relationships and allow two pre caldera volcanic cycles in the diego hernandez wall to be differentiated in accordance with geological evidence the first cycle shows imprecise age limits the second cycle ranges from 0 35 to 0 17 ma this upper pre caldera age suggests that the las canadas caldera and the teide pico viejo were formed more recently than was previously assumed

origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife aggregation of airborne particles is an important way in which the atmosphere is cleansed of fine dust particles such as following explosive eruptions and meteorite impacts we identify successive stages in the growth history of particle aggregates based upon well preserved ash aggregate bearing pyroclastic layers on tenerife the layers are persistently organized into couplets made up of a lower ignimbrite layer and an upper widespread coignimbrite ash fall layer the upper part of each ignimbrite contains whole and fragmented concentric laminated accretionary lapilli whereas the overlying coignimbrite ash fall layer lacks accretionary lapilli and is composed of framework supported smaller and nonlaminated ash pellets sometimes slightly deformed or partly disaggregated the pellets resemble the cores of the larger accretionary lapilli in the underlying ignimbrite layer these field relations are repeated numerous times in several different successions and they indicate that ash pellets not accretionary lapilli form within the coignimbrite ash plumes some pellets fell directly to the ground producing coignimbrite ash fall layers but others settled into pyroclastic density currents where they accreted successive concentric laminations of fine ash as they circulated through the variously turbulent levels of the stratified current and heat of the lower part of the current dried and partly lithified them into brittle accretionary lapilli the fully formed whole and broken accretionary lapilli were then deposited from the current along with ash and pumice lapilli numerous ignimbrite veneers on tenerife have the form of ash layers a few centimeters thick that drape topography and locally contain matrix supported accretionary lapilli most volcanoes lack laterally continuous field exposure and such accretionary lapilli bearing layers might be mistaken for ash fall deposits we highlight the value of careful distinction between different types of ash aggregate facies when interpreting the origin of pyroclastic deposits for example during hazard assessments

granular disruption during explosive volcanic eruptions volcanic eruptions can inject hazardous ash clouds into the atmosphere numerical simulations and experiments on volcanic rock samples show that clasts initially formed deep in the volcanic conduit break up during collisions in the conduit thus generating fine grained clouds of ash

the late quaternary diego hernandez formation tenerife volcanology of a complex cycle of voluminous explosive phonolitic eruptions abstract the diego hernandez formation dhf 600 ca 180 ka represents the products of the most recent complete cycle of phonolitic explosive volcanism on tenerife canary islands spain we provide a revised and detailed stratigraphy new 40ar 39ar and u th he age determinations for major eruptive units a summary of new chemical data and an overview of the key characteristics of the cycle including volume estimates dispersal patterns eruption styles phreatomagmatic influences and caldera collapse episodes the complex stratigraphy of the dhf is divided into 20 named members each representing a major eruption as well as numerous unnamed members of limited present day exposure the major eruptions are represented by the fortaleza 370 ka roque 347 ka 3 km3 aldea 319 ka 3 km3 fasnia 309 ka 13 km3 poris 268 ka 3 5 km3 arafo 4 km3 caleta 223 ka 3 5 km3 and abrigo between 196 and 171 ka 20 km3 members the aldea fasnia and poris members consist of highly complex successions of plinian fall surge and flow deposits and several of the eruptions produced widespread and internally complex ignimbrite sheets phreatomagmatism occurred most frequently in the opening phase of the eruptions but also recurred repeatedly throughout many of the sequences inferred sources of water include a shallow caldera lake and groundwater and intermittent phreatomagmatic activity was an important influence on eruption style another important factor was conduit and vent instability which frequently loaded the eruption column with dense lithic debris and occasionally triggered column collapse and ignimbrite formation most of the major dhf eruptions were triggered by injection of mafic magma into existing phonolitic magma bodies two phonolitic magma types were available for eruption during the lifetime of the dhf but each was dominant at different times the results presented here support a caldera collapse rather than a landslide model for the origin of the las canadas caldera although the evolution of the caldera is evidently more complex and incremental than first thought

large eruption triggered ocean island landslide at tenerife onshore record and long term effects on hazardous pyroclastic dispersal an extensive debris avalanche deposit has been discovered on canadas volcano tenerife canary islands the onshore component of the 733 3 ka abona landslide deposit exposes classic block facies and mixed facies across 90 km 2 three lines of evidence together show that the avalanche was triggered by an ignimbrite forming explosive eruption 1 the deposit is enclosed by phonolitic ignimbrites and is draped by a plinian fallout layer all within a single eruption unit 2 it contains prismatic jointed pumice blocks that were hot during landslide emplacement indicated by chilled rims and breadcrust surfaces 3 these blocks yield the same 40 ar 39 ar date as the associated ignimbrite and fall deposit landslide hummocks dammed surface water forming ephemeral lakes perched on the volcano flank phonolite dome growth destabilized the southeast sector of a mid pleistocene canadas caldera wall and created a major breach that affected the passage of destructive pyroclastic density currents on tenerife for 0 5 m y showing that landslides can have enduring consequences for pyroclastic dispersal and hazards

synoptic flow and density observations near an arctic shelf break analyses of data from three shipborne surveys describe the quasi synoptic density and velocity fields near barrow canyon alaska the canyon parallels the northwestern coast of alaska and contains three different water masses these are 1 warm and fresh alaskan coastal waters that originate from the bering strait 2 cold and moderately salty waters that originate from the chukchi shelf and 3 warm and salty waters that originate from the atlantic layer of the arctic ocean a halocline separates the chukchi shelf and atlantic layer waters the halocline slopes upward into the canyon where it is then twisted to slope across the wide canyon an intensification of the beaufort gyre near the shelf break just seaward of barrow canyon raises the halocline more than 100 m toward the surface locally upwelling favorable winds raise the arctic halocline which thus is ventilated within barrow canyon adjacent to the coast in the absence of winds the halocline slopes acrosscanyon in the thermal wind sense due to a northward flowing coastal current velocity measurements from a towed acoustic doppler current profiler reveal a northward flowing jet that transports about 0 3 sv sv 106 kg m23 of bering sea summer water into the arctic ocean at speeds that exceed 0 7 m s21 total northward transports through the canyon exceed 1 0 sv the warm waters of this coastal current supply more than 100 w m22 of heat to the atmosphere the jet separates both from the bottom and from the coast hence a laterally and vertically sheared jet forms which breaks into three branches at about 71 88n latitude

the neapolitan yellow tuff a large magnitude trachytic phreatoplinian eruption eruptive dynamics magma withdrawal and caldera collapse the results of a detailed stratigraphical and volcanological study has allowed the definition of the neapolitan yellow tuff nyt as the largest known trachytic phreatoplinian eruption with an estimated volume of not less than 30 km3 dre the nyt sequence can be subdivided in a lower and an upper member sedimentological and textural variations in time and space indicate that different eruptive and depositional mechanisms operated during the eruption the lower member eruptive phase was characterized by alternating phreatoplinian and magmatic explosions while the upper member eruptive phase was characterized by a random sequence of phreatomagmatic and magmatic explosions particles produced by the same explosion were deposited contemporaneously by fallout and surge mechanisms either at different or at the same distance from the vent compositional variations show that the nyt sequence cannot be modeled as an inverted compositionally zoned magma body with magma becoming more basic in the course of the eruption the chamber composed of three geochemically distinct magma layers was tapped at different depths according to the eruption dynamics each phreatoplinian explosion tapped concurrently all the magma layers while magmatic explosions tapped only one of them the onset of the nyt caldera collapse is well constrained between lower and upper member eruptive phases the collapse which is only gravimetrically detectable occurred inside the larger phlegrean fields caldera which therefore can be defined as a nested caldera

stratigraphy structure and geochronology of the las canadas caldera tenerife canary islands after a long period of subaerial fissure fed extrusions of basaltic magmas 12 to 3 ma volcanic activity was then concentrated in the central part of tenerife phonolitic magma chambers formed and a central volcanic complex was constructed the las canadas edifice the formation of a large depression the las canadas caldera truncated the top of the edifice the active twin strato cones teide pico viejo are sited in this depression the history of the las canadas caldera and edifice are established from stratigraphy geochronology k ar dates and volcanological studies two different groups are recognized separated by a major unconformity the lower group is dated at 2 to 3 ma and includes the products of several volcanic centres which together represent several cycles the upper group ranges from 1 56 to 0 17 ma and includes three different formations representing three long term 100 to 300 ka volcanic cycles the periods of dormancy between each formation were of 120 to 250 ka duration the las canadas caldera is a multicyclic caldera which formed over the period 1 18 0 17 ma each cycle of activity represented by a formation culminated in caldera collapse which affected different sectors of the las canadas edifice geological observation and geochronology support an origin by collapse into a magma chamber the minimum volume of pyroclastic ejecta is substantially greater than the present caldera depression volume 45 km3 but approaches the inferred volume of the original caldera depression 140 km3 after the formation of the caldera sector collapses could also occur at the northern flank of the volcano causing the disappearance of the northern side of the caldera wall

circulation dense water formation and outflow on the northeast chukchi shelf we investigated circulation and water mass modification processes in the chukchi sea using 1 temperature salinity and velocity data collected between september 1991 and september 1992 from moorings in bering strait and the northeast shelf and 2 meteorological data and ice concentrations derived from special sensor microwave imager ssm i imagery in october 1991 and from february to august 1992 the mean monthly circulation was steady and northward from november to january strong northeast winds diverted the low salinity bering inflow onto the western shelf and weakened and reversed flow over the northeast shelf the winds also opened extensive polynyas wherein cold hypersaline salinity 34 waters formed the fall winter circulation enhanced dense water formation by diverting low salinity waters away from and prolonging the residence time of water within the polynyas alongshore convergence of the coastal flow swept some of the cold hypersaline water offshore across isobaths but the coastal current carried most of it into the arctic ocean through barrow canyon the dense outflow mixed little during its descent through the upper canyon where bottom friction and rotation but not entrainment were important in the plume momentum balance if the outflow is not diluted by mixing in the lower canyon where theory suggests this will occur then the densest shelf waters can ventilate layers deeper than the halocline salt rejection estimates using ssm i data and surface heat budget calculations agree within a factor of 2 with independent estimates from the mooring hence remote sensing techniques can monitor winter salt rejection rates in the chukchi sea

generation of submesoscale vortices a new mechanism small scale intense vortices composed of water with anomalous values of quasi conservative tracers have been observed in many oceans they are particularly numerous in the beaufort gyre of the arctic ocean possible mechanisms responsible for the generation of these features are examined conservation of temperature salinity and nutrients requires that beaufort sea vortices be created from water that flows into the arctic ocean from the pacific ocean across the broad shallow chukchi sea much of this water passes through barrow canyon near point barrow alaska observations in the canyon show the generation of vortices of approximately the same size as those observed in the beaufort sea barrow canyon is therefore a likely site for the generation of beaufort sea vortices most observed beaufort sea vortices have a central potential vorticity near zero because of a relative vorticity close to their chukchi sea source water is typically stratified with little relative vorticity so that its potential vorticity is nonzero a key step in the generation of the vortices is therefore the reduction of the potential vorticity of chukchi sea water to nearly zero it is proposed that this occurs through the action of frictional torques occurring as the chukchi sea water flows through barrow canyon rather than througji the action of vertical mixing because this flow is driven by a downstream pressure gradient it is concentrated on the right hand wall of the canyon by the coriolis force the frictional torques thus preferentially reduce the potential vorticity of the fluid anticyclonic vortices are therefore preferentially generated it is suggested that a similar scenario may play a role in the generation of submesoscale vortices containing water from the mediterranean and labrador seas