palaeomagnetism mafic dykes and the protogine zone southern sweden the protogine zone pz forms the border between the svecofennian and sveconorwegian domains of southern sweden intruded in the zone are three generations of mafic dykes dated 1510 ma 1180 ma and 930 ma a palaeomagnetic study of these dykes in the central and southern parts of the pz yields two main directions of magnetization a d 291 i 78 n 31 and b d 129 i 40 n 13 these directions give palaeopoles on the sveconorwegian loop of the fennoscandian apparent polar wander path apwp a third poorly defined group c yields a mean declinationof 185 and inclination of 9 with the corresponding palaeopole situated on the c 1500 1600 ma part of the apwp two other sites yield palaeopoles located in the 1200 ma part of the path the data imply that the sveconorwegian orogeny caused a remagnetization of the two older dyke generations and that this remagnetization was complete in the southern part of the pz while a few dykes retained their original magnetization in the central part of the zone either the difference between the a and b directions record appreciable apw movement during differential cooling of the pz or the a and b directions may represent an asymmetric reversal of the earth s magnetic field in the latter case they may be contemporary and have implications for the size and form of the sveconorwegian loop of the fennoscandian apwp

apoyo caldera nicaragua a major quaternary silicic eruptive center apoyo caldera near granada nicaragua was formed by two phases of collapse following explosive eruptions of dacite pumice about 23 000 yr b p the caldera sits atop an older volcanic center consisting of lava flows domes and ignimbrite ash flow tuff the earliest lavas erupted were compositionally homogeneous basalt flows which were later intruded by small andesite and dacite flows along a well defined set of n s trending regional faults collapse of the roof of the magma chamber occurred along near vertical ring faults during two widely separated eruptions field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast followed by eruption column collapse which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse a period of quiescence was marked by the eruption of scoria bearing tuff from the nearby masaya caldera and the development of a soil horizon violent plinian eruptions then resumed from a vent located within the caldera a second phase of caldera collapse followed accompanied by the effusion of late stage andesitic lavas indicating the presence of an underlying zoned magma chamber detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume mapping of the apoyo airfall and ignimbrite deposits gives a volume of 17 2 km3 within the 1 mm isopach crystal concentration studies show that the true erupted volume was 30 5 km3 10 7 km3 dense rock equivalent approximately the volume necessary to fill the caldera a vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions this vent area is controlled by n s trending precaldera faults related to left lateral motion along the adjacent volcanic segment break fractional crystallization of calc alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at apoyo prior to caldera collapse highly atypical tholeiitic magmas resembling low k high ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber the injection of tholeiitic magmas may have contributed to the paroxysmal caldera forming eruptions

morphologie des constructions microbiennes en contexte de fan delta oligocene exemple du rift rhenan europe occidentale abstract at the eocene oligocene boundary a first paroxysmal phase of subsidence created the rhine rift graben system a lacustrine basin with variable salinity was formed in the graben in the southern part of this basin a large fan delta developed extending northward along the graben axis microbial ecosystems settled on the fringe of the lake in the paludal zone they formed carbonate encrustations on living plants or detrital vegetal remains littoral carbonate facies occur as algal bioherms and stromatolitic reefs in the distal lacustrine zone a discrete nodular texture within the sediment reveals the microbial origin of micrite and gives the limestone a breccial pattern all these facies illustrate the lateral zonation of the different types of constructions from fluvio deltaic to lacustrine in certain cases the microbial coating on plants permits to reconstruct seasonal rhythms and the processes of substratum colonisation

comparison of lake and land tephra records from the 2015 eruption of calbuco volcano chile tephra layers in lake sediment cores are regularly used for tephrostratigraphy as isochronous features for dating and recording eruption frequencies however their value for determining volcanic eruption size and style may be complicated by processes occurring in the lake that modify the thickness and grain size distributions of the deposit to assess the reliability of data from lake cores we compare tephra deposited on land during the 2015 eruption of calbuco volcano in chile to records in sediment cores from three lakes of different sizes that are known to have received primary fall deposits in general the thickness and granulometry of the deposit in lake cores and nearby terrestrial sections are very similar as anticipated however cores sampled close to here within 300 m of fluvial inflows were affected by sediment deposition from the lake s catchment they differed from primary deposits not only in their greater thickness and organic content but also in poor sorting and lack of grading cores 850 m away from the inlet were not affected we consider our results in the context of the particle settling regime as well as each lake s location bathymetry and catchment area we find that the particle settling regime is important in more distal settings where the ash particles are small and particle settling occurs in density plumes rather than as individual particles we conclude that lake cores can be useful for physical volcanology providing consideration is given to eruption parameters such as particle size and mass flux as well as lake features such as bathymetry and catchment area

kinematics of a major fan like structure in the eastern part of the sveconorwegian orogen baltic shield south central sweden abstract the n s trending so called protogine zone in the baltic shield of south central sweden is usually considered to mark a tectonic boundary between the rocks of the transscandinavian igneous belt tib in the east and the sveconorwegian orogen in the west detailed structural mapping in the karlskoga kristinehamn area has shown that an anastomosing network of ductile deformation zones with generally n s strike extends ca 40 km east of the traditional protogine zone furthermore the western boundary of this ductile deformation and the tib is not constrained in the kristinehamn area reconnaissance studies indicate that they both extend westwards towards the so called mylonite zone it is suggested that the eastern limit of the sveconorwegian orogen is located some 40 km east of the present boundary and that the protogine zone concept is obsolete the term sveconorwegian frontal deformation zone sfdz is proposed as a more appropriate alternative in southern sweden south of lake vattern and to correspond to a younger set of oblique ductile deformation zones with reverse and right lateral components of movement in the easternmost part of the orogen farther to the north ductile deformation zones older than the sfdz in the karlskoga kristinehamn area display a fan like geometry in an e w cross section with steep westerly dips in the eastern part of the section vertical dips farther west and moderate easterly dips in the western part of the section kinematic analysis indicates that dip slip movements predominate with a consistent top to the east sense of movement across the entire fan like structure in their present orientation deformation zones are characterized by reverse movements in the eastern part and normal movements in the western part of the structure between kristinehamn and the mylonite zone the main foliation is gently dipping to subhorizontal indicating that the regional structure is strongly asymmetric and that the fan like structure occurs close to the foreland of the orogen deformation zones are spaced to semi penetrative in the eastern part of the fan like structure whereas the deformation is more or less penetrative and the tib rocks are transformed to orthogneisses west of kristinehamn this east to west increase in bulk strain is in accordance with an increase in syn deformational metamorphic grade across the structure younger ductile deformation zones belonging to the sfdz are responsible for a major change in orientation of the older deformation zones in the easternmost part of the structure the fan like structure is best explained by models involving the interference of two separate tectonic events deformation occurred after 1 57 ga and prior to deposition of neoproterozoic and younger cover sedimentary rocks it is not yet clear whether the initial phase of deformation early sveconorwegian or older was related to the build up of an imbricate thrust stack in a compressional regime as favoured here or to regional e w extension the younger deformation phase was related to rotation of these older structures into the compressional late sveconorwegian sfdz

tectonothermal evolution of high alumina rocks within the protogine zone southern sweden the protogine zone comprises a system of anastomosing deformation zones which approximately parallel the eastern boundary of the sveconorwegian 1200 900 ma province in south west sweden ages of granulite facies metamorphism in the sveconorwegian province require exhumation from c 30 to 35 km crustal depths after 920 880 ma 40ar 39 ar cooling ages are presented for muscovite from high alumina rocks formed by hydrothermal leaching associated with the protogine zone growth of fabric defining minerals was associated with a ductile deformational event muscovite from these rocks cooled below argon retention temperatures c 375 25 c at c 965 955 ma muscovite from granofels in zones of intense alteration indicates that temperatures 375 25 c were maintained until c 940 ma textural relations of al2sio5 polymorphs and chloritoid suggest that dated fabrics formed during exhumation the process of exhumation brittle overprint on ductile structures and hydrothermal activity along faults within the protogine zone tentatively are interpreted as the peripheral effects of initial neoproterozoic exhumation of the granulite region of south western sweden muscovite in phyllonites associated with the sveconorwegian thrust system cooled below argon retention temperatures at c 927 ma exhumation associated with this cooling could have been related to extension and onset of brittle ductile deformation superimposed on sveconorwegian contraction

in situ accretion mechanism of concavo convex lacustrine oncoids swallow nests from the oligocene of the mainz basin rhineland frg oligocene oncoids from the lacustrine su wasserschichten of the mainz basin exhibit a pustular surface and a characteristic cavity on their undersides which gave rise to their designation as swallow nests oncoids grew in situ under low energy conditions during a pause in sedimentation three features indicate that they were never overturned 1 oncoids can be placed in a stable position on a smooth surface 2 laminae which are mostly concentric around the entire oncoids display polar thickenings on the upper side during all growth stages of oncoids 3 the depression is always situated on the stable side of oncoids development of this cavity is favoured by unionid shells in the stable position serving as oncoid nuclei but occurs also under flat nuclei it is interpreted as a product of reduced growth of cyanobacteria due to oligophotic conditions on a fine grained substrate and is considered as diagnostic of the in situ growth of oncoids oncoid cortices are mainly formed by a scytonema like cyanobacterium whose carbonaceous organic remnants are occasionally preserved growth stages of individual oncoids were image processed and compared by means of shape parameters due to optimum growth conditions cyanobacteria try to occupy the entire available space hence oncoids tend to achieve a spherical form even during in situ growth renewed background sedimentation triggered the formation of pustular surfaces and finally caused the cessation of cyanobacterial growth

the deformation history of the eastern part of the southwest swedish gneiss belt abstract the deformation history of the eastern part of the sw swedish gneiss belt has been investigated regional events more than c 1700 ma are identified as d1 d2 more than c 1400 ma as d3 less than c 1130 ma as d4 and more than c 900 ma as d5 d7 thus the d4 d7 events are considered to be sveconorwegian a common deformation history is suggested for southwestern sweden gradational changes in lithology from west to east are apparently associated with different tectonic styles and intensity of strain the principle deformation of the protogine zone of central sweden is suggested to be d4 the mylonite zone of western sweden appears to have been initiated before d3 the smaland varmland granitoid belt c 1700 ma is geneally considered as early postorogenic in respect to the svecokarelian orogeny because it has been influenced by the d3 event it represents a link between svecokarelian rocks and the sw swedish gneisses

freshwater oncolites created by industrial pollution onondaga lake new york abstract onondaga lake is a moderately saline eutrophic lake characterized by waters rich in calcium sodium chloride and bicarbonate large quantities of caco3 that are precipitated in the lake result from excess calcium supplied as calcium chloride wastes produced by soda ash manufacturing to lake waters that are at or near saturation with respect to caco3 from solution of carbonate rocks in the drainage basin beaches along the leeward northeastern shore of the lake are composed almost entirely of oncolites ranging from a few millimeters to several centimeters in maximum dimension offshore in 1 2 m of water the oncolites are biscuit shaped concretions as much as 15 cm in diameter the oncolites consist mainly of low magnesium calcite but dissolution of the carbonate with dilute acid results in a mass of blue green algal filaments of the same approximate size and shape as the original oncolite most oncolites have an obvious nucleus the most common nucleus is the hollow stem and cortication tubules of charophytes charophytes do not occur in onondaga lake today although they are common in other limestone bedrock lakes in central new york state charophytes probably were eliminated by the marked increases in salinity of the lake that resulted from the introduction of soda ash manufacturing on the lake shores around 1880 which means that growth of the oncolites began at least 100 years ago

three empirical methods for the calculation of distal volume of tephra fall deposits abstract three empirical methods for the calculation of distal tephra fall volumes are presented and tested the three methods allow calculation of approximate distal tephra fall volumes under different conditions and abundances of field data all methods use proximal data to extrapolate the distal situation they are based on i shape and dispersal axis of proximal isopachs method of ellipses ii break in slope distance on log t vs a diagram where one distal isopach is available method of a ip with one distal isopach and iii distal thinning rate and break in slope distance on log t vs a diagram method of k 1 vs a ip the application of the three methods to published data of 23 32 eruptions in which it is possible to define at least two segments on the log t vs a diagram yields different results the method of ellipses yields good results if the thickness of reconstructed distal isopachs is greater than about 1 cm whereas the method of a ip with one distal isopach is more accurate if the distal isopach has a thickness below the half thickness at the break in slope distance t t ip the method of distal thinning rate k 1 vs break in slope distance a ip allows calculation of distal thinning rate and break in slope distance from only proximal data in this case calculated distal volumes approximate the actual one only if the deposits have a proximal volume total volume ratio greater than 0 7 otherwise they must be considered as minimum values to be added to the proximal volumes an empirical correlation between proximal thinning rate k and column height h t 7 6 k 0 47 r 0 90 is also presented column heights calculated in this way are within 20 of the observed published values in 94 of the examined cases

minimum volume of a tephra fallout deposit estimated from a single isopach a simple method is presented to calculate the minimum volume of an exponentially thinning fall deposit for which only one isopach is available this method exploits the fact that the product of the area enclosed within an isopach a and its thickness t reaches a maximum value ta max for a certain isopach it is shown that the minimum volume is vmin 3 69ta which is equal to the actual volume when ta ta max calculation of vmin for 327 isopachs from 74 fallout deposits shows that it is often just a bit lower than the volume calculated from all the available isopachs it is therefore proposed that vmin provides a useful minimum volume estimate for fallout deposits for which only one isopach is available

the plinian eruptions of 1912 at novarupta katmai national park alaska the three day eruption at novarupta in 1912 consisted of three discrete episodes episode i began with plinian dispersal of rhyolitic fallout layer a and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers the plinian column was sustained throughout most of the interval of ash flow generation in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite accordingly plinian layer b which fell in unbroken continuity with purely rhyolitic layer a is zoned from 99 to 15 rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in mageik creek and the valley of ten thousand smokes vtts only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent indicating that ignimbrite emplacement barely outlasted the plinian phase on near vent ridges the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley filling equivalents but exhibit evidence for turbulent deposition and recurrent scour episode ii began after a break of a few hours and was dominated by plinian dispersal of dacitic layers c and d punctuated by minor proximal intraplinian flows and surges after another break dacitic layers f and g resulted from a third plinian episode iii intercalated with these proximally are thin intraplinian ignimbrites and several andesite rich fall flow layers both cd and fg were ejected from an inner vent 400 m wide nested within that of episode i into which the rhyolitic lava dome novarupta was still later extruded two finer grained ash layers settled from composite regional dust clouds layer e which accumulated during the d f hiatus includes a contribution from small contemporaneous ash flows and layer h settled after the main eruption was over both are distinct layers in and near the vtts but distally they merge with cd and fg respectively they are largely dacitic but include rhyolitic shards that erupted during episode i and were kept aloft by atmospheric turbulence published models yield column heights of 23 26 km for a 22 25 km for cd and 17 23 km for fg and peak mass eruption rates of 0 7 1x108 0 6 2x108 and 0 2 0 4x108 kg s 1 respectively fallout volumes adjusted to reflect calculated redistribution of rhyolitic glass shards are 8 8 km3 4 8 km3 and 3 4 km3 for episodes i ii and iii microprobe analyses of glass show that as much as 0 4 km3 of rhyolitic glass shards from eruptive episode i fell with cde and 1 1 km3 with fgh most of the rhyolitic ash in the dacitic fallout layers fell far downwind se of the vent near the rhyolite dominated ignimbrite however nearly all of layers e and h are dacitic showing that the downwind rhyolitic ash is of co plinian rather than co ignimbrite origin

the averno 2 fissure eruption a recent small size explosive event at the campi flegrei caldera italy the averno 2 eruption 3 700 50 a b p was an explosive low magnitude event characterized by magmatic and phreatomagmatic explosions generating mainly fall and surge beds respectively it occurred in the western sector of the campi flegrei caldera campanian region south italy at the intersection of two active fault systems oriented ne and nw the morphologically complex crater area largely filled by the averno lake resulted from vent activation and migration along the ne trending fault system the eruption generated a complex sequence of pyroclastic deposits including pumice fall deposits in the lower portion and prevailing surge beds in the intermediate upper portion the pyroclastic sequence has been studied through stratigraphical morphostructural and petrological investigations and subdivided into three members named a through c member a was emplaced during the first phase of the eruption mainly by magmatic explosions which generated columns reaching a maximum height of 10 km during this phase the eruption reached its climax with a mass discharge rate of 3 2 106 kg s intense fracturing and fault activation favored entry of a significant amount of water into the system which produced explosions driven by variably efficient water magma interaction these explosions generated wet to dry surge deposits that emplaced member b and c respectively isopachs and isopleths maps as well as areal distribution of ballistic fragments and facies variation of surge deposits allow definition of four vents that opened along a ne oriented 2 km long fissure the total volume of magma extruded during the eruption has been estimated at about 0 07 km3 dre the erupted products range in composition from initial weakly peralkaline alkali trachyte to last emplaced alkali trachyte isotopic data and modeling suggest that mixing occurred during the averno 2 eruption between a more evolved less radiogenic stored magma and a less evolved more radiogenic magma that entered the shallow reservoir to trigger the eruption the early phases of the eruption during which the vent migrated from sw to the center of the present lake were fed by the more evolved uppermost magma while the following phases extruded the less evolved lowermost magma integration of the geological and petrological results suggests that the averno 2 complex eruption was fed from a dyke shaped shallow reservoir intruded into the ne sw fault system bordering to the west the la starza resurgent block within the caldera floor

the hekla eruption 1980 1981 the sixteenth eruption of hekla since 1104 began on august 17th 1980 after the shortest repose period on record only ten years the eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the hekla volcanic ridge the production rate declined rapidly after the first day and the eruption stopped on august 20th a total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity in the following seven months steam emissions were observed on the volcano activity was renewed on april 9th 1981 and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope

tephrostratigraphy and petrological study of chikurachki and fuss volcanoes western paramushir island northern kurile islands evaluation of holocene eruptive activity and temporal change of magma system a tephrostratigraphic and petrological study of the chikurachki 1816 m tatarinov lomonosov volcanic chain ctl volcanic chain and fuss 1772 m located at the southern part of paramushir island in the northern kurile islands was carried out to reveal the explosive eruption history during the holocene and the temporal change of the magma systems of these active volcanoes tephra successions were described at 54 sites and more than 20 major eruptive units were identified consisting of pumice fall scoria fall and ash fall deposits each of which are separated by paleosol or peat layers the source volcano of each recognized tephra layer was confirmed by correlation with proximal deposits of each eruption center with respect to petrography and whole rock and glass chemistry the age of each layer was determined by radiocarbon dating and the stratigraphic relationship with the dated widespread tephra from kamchatka according to the thickness of paleosols bracketed between tephra layers the holocene activity in this region was initiated by eruptions from the tatarinov and lomonosov volcanoes after the eruptions the fuss and chikurachki volcanoes started their explosive activities at ca 7 5 ka bp soon after the deposition of widespread tephra from the kurile lake caldera in southern kamchatka compared with fuss located on the back arc side chikurachki has frequent repeated explosive and voluminous eruptions whole rock compositions of the rocks of the ctl volcanic chain and fuss are classified into medium k and high k groups respectively these suggest that magma systems beneath the ctl volcanic chain and fuss differ from each other and have been independently constructed the rocks of the chikurachki volcano are basalt basaltic andesite and have gradually evolved their chemical compositions when graphed on a sio2 oxide diagram these form smooth trends from mafic to more felsic this suggests that the magma system evolved mainly by fractional crystallization in contrast matrix glass chemistries for fuss pumices are distinct for each eruption and show different k2o levels on a sio2 k2o diagram this implies that the magma system of fuss has been frequently replaced both volcanoes have been active under the same subduction system however the chikurachki volcano will continue eruptive activity under a stable magma system with a higher magma discharge rate whereas fuss may continue construction with an intermittent supply of distinct small magma batches

scoria cone formation through a violent strombolian eruption irao volcano sw japan scoria cones are common volcanic features and are thought to most commonly develop through the deposition of ballistics produced by gentle strombolian eruptions and the outward sliding of talus however some historic scoria cones have been observed to form with phases of more energetic violent strombolian eruptions e g the 1943 1952 eruption of paricutin central mexico the 1975 eruption of tolbachik kamchatka maintaining volcanic plumes several kilometers in height sometimes simultaneous with active effusive lava flows geologic evidence shows that violent strombolian eruptions during cone formation may be more common than is generally perceived and therefore it is important to obtain additional insights about such eruptions to better assess volcanic hazards we studied irao volcano the largest basaltic monogenetic volcano in the abu monogenetic volcano group sw japan the geologic features of this volcano are consistent with a violent strombolian eruption including voluminous ash and fine lapilli beds on order of 10 1 km3 dre with simultaneous scoria cone formation and lava effusion from the base of the cone the characteristics of the volcanic products suggest that the rate of magma ascent decreased gradually throughout the eruption and that less explosive strombolian eruptions increased in frequency during the later stages of activity during the eruption sequence the chemical composition of the magma became more differentiated a new k ar age determination for phlogopite crystallized within basalt dates the formation of irao volcano at 0 4 0 05 ma

lower permian oncolites from south china implications for equatorial sea level responses to late palaeozoic gondwanan glaciation the oncoid bearing chuanshan formation is a regionally extensive carbonate deposit of predominantly asselian to early sakmarian early permian age in south china occupying an area of some 500 000 km2 throughout south china the oncoid bearing horizons are generally stable and broadly comparable in lithology fossil content and the morphology of the oncoid grains four types of microfacies are recognized from the oncolite succession and overall they suggest a moderate to high energy wave agitated shallow marine carbonate platform environment an analysis of the stratigraphic distribution of oncoid grain size density thickness and the bedding structures of the oncolite beds and the number of coating laminae indicate the presence of metre scale cyclothems suggestive of possible high frequency cycles of sea level fluctuation compared to carbonate successions above and below that lack oncolites and in conjunction with evidence from sequence stratigraphic and isotopic geochemical analyses of coeval carbonate deposits in south china and elsewhere the origin of the chuanshan oncolites is linked to a drastic drop in global sea level at the pennsylvanian permian boundary that can be correlated closely in timing with the zenith of the late palaeozoic gondwanan glaciation it is further suggested that the eustatic changes apparent from the deposition of the chuanshan oncolites and similar coeval deposits in lower palaeolatitudes were coupled with and influenced by the contemporaneous high latitude gondwanan glaciation the largest and longest known such event in phanerozoic earth history

chronology and dispersal characteristics of recently last 5000 years erupted tephra of cotopaxi ecuador implications for long term eruptive forecasting abstract cotopaxi the highest active volcano on earth and one of the most dangerous of ecuador is constituted by a composite cone made up of lava and tephra erupted from the summit crater the activity of the present volcano begun with large volume plinian eruptions followed by a succession of small volume lava emissions and pyroclastic episodes which led to the edification of a symmetrical cone the growth of the cone was broken by an episode of slope failure the scar of which is now obliterated by recent and historical products volcanic history eruptive frequency and characteristics of the activity were investigated by studying the stratigraphy of tephra and carrying out fifteen new 14 c dating on paleosols and charcoals the investigated period is comprised between the slope failure and the present the deposit of the volcanic landside dry debris avalanche of rio pita previously believed to be between 13 000 and 25 000 yr b p is now considered to have an age slightly older than 5000 yr b p the stratigraphy of tephra of the last 2000 years reveals the existence of 22 fallout layers seven of them were dated with 14 c whereas three were ascribed to the eruptions of 1534 1768 and 1877 on the basis of comparison with historical information maximum clast size distribution isopleths of 9 tephra layers points out that the sustained explosive eruptions of cotopaxi during the last 2000 years are characterized by very high dispersive power plinian plumes with column heights between 28 and 39 km and high intensity peak mass discharges from 1 1 to 4 1 10 8 kg s the magnitude mass of tephra fallout deposits calculated from distribution of thickness isopaches are however moderate from 0 8 to 7 2 10 11 kg the limited volume of magma erupted during each explosive episode is consistent with the lack of caldera collapses small volume pyroclastic flows and surges virtually accompanied all identified tephra fallouts during such an activity large scale snow ice melting of the summit glacier produced devastating mudflows comparable in scale to those of 1877 eruption by assuming a 1 1 correspondence between fallout episodes and generation of large scale lahar we have estimated an average recurrence of one explosive lahartriggering event every 117 years over the last two millennia this value compares well with that calculated by considering the period since spanish conquest the probability of having an eruption like this in 100 or 200 years is respectively of 0 57 and 0 82 such an high probability underscores the need for quick actions aimed at the mitigation of cotopaxi lahar hazard along all the main valleys which originate from the volcano

plinian airfall deposits of basaltic composition two widespread scoria fall deposits locally known as the fontana lapilli and san judas formation are demonstrated to have issued from masaya caldera complex nicaragua and to be the first documented plinian airfall deposit of basaltic composition data from maps of median grain size average maximum pyroclast diameter and average maximum diameter of lithic fragments provide conclusive evidence of the plinian nature of both deposits the volume within the 1 mm isopach of the fontana lapilli is 12 km3 3 4 km3 dense rock equivalent and within the san judas formation it is 1 2 km3 0 35 km3 dre theoretical eruption parameters calculated for the fontana lapilli and san judas formation respectively include muzzle velocity 170 m s and 150 m s column height 50 km and 18 km volume eruption rate 2 105 m3 s and 1 104 m3 s and duration 2 h and 10 h these are all similar to the calculated parameters for silicic plinian eruptions basaltic magmas are generally considered incapable of erupting in the manner characteristic of plinian eruptions recognition of them argues that factors other than high viscosity may be of fundamental importance

the intensity and magnitude of holocene plinian eruptions from mount st helens volcano dispersal characteristics of the t we wn pu ps ye yn and yb plinian fall deposits of mount st helens have been measured at 80 sites downwind of the volcano in order to model eruption dynamics and atmospheric transport isopleth contours for the sizes of maximum pumice and lithic clasts are used to calculate peak eruption column heights and intensities magma discharge based on a theoretical model of tephra dispersal new proximal thickness measurements are combined with an empirical distal extrapolation based on studies of 53 plinian deposits to calculate the magnitude erupted mass of each eruption layer yn 3510 y r b p represents the highest intensity and largest magnitude eruption at mount st helens in post glacial times modeling suggests column height grew to about 31 km before gradually declining at the end of the plinian phase 26 hours several intraplinian surge deposits are present in the upper part of the fall layer close to the volcano and up to 15 km to the northeast of mount st helens peak intensity of the plinian phase was 108 kg s and the total erupted volume was 4 km3 dre of magma small plinian style eruptions are represented by layers such as ps and pu of the pine creek eruptive period 3000 2500 yr b p and have intensities of only 106 kg s when compared with plinian eruptions from other volcanoes the holocene eruptions of mount st helens span from the lower to the middle part of the known range in intensity and magnitude and are typical of events derived from intermediate sized stratovolcanoes there is also a general correlation between the intensity of plinian eruptions within eruptive cycles and the repose period prior to each cycle this relationship may be related to a time dependent process for the accumulation of differentiated and volatile rich magma within the chamber beneath mount st helens

eruption source parameters for forecasting ash dispersion and deposition from vulcanian eruptions at tungurahua volcano insights from field data from the july 2013 eruption tungurahua volcano located in the central area of the ecuadorian sierra is erupting intermittently since 1999 alternating between periods of quiescence and explosive activity volcanic ash has been the most frequent and widespread hazard provoking air contamination episodes and impacts on human health animals and crops in the surrounding area after two months of quiescence tungurahua erupted violently on 14th july 2013 generating short lived eruptive columns rising up to 9 km above the vent characterized as a vulcanian eruption the resulting fallout deposits were sampled daily during and after the eruptions to determine grain size distributions and perform morphological and componentry analyses dispersion and sedimentation of ash were simulated numerically coupling the meteorological weather research forecasting wrf with the volcanic ash dispersion fall3d models the combination of field and numerical studies allowed constraining the eruption source parameters esp for this event which could be used to forecast ash dispersion and deposition from future vulcanian eruptions at tungurahua this set of pre defined esp was further validated using two different eruptions as blind test occurring on 16th december 2012 and 1st february 2014

impact des ecosystemes microbiens sur la sedimentation impact of microbial ecosystems on sedimentation abstract the microbial communities which often give rise to the fine lamination in sediments are very diverse both in species composition e g phototrophic and chemotrophic bacteria fungi algae and metabolism e g oxygenic and anoxygenic photosynthesis organic and inorganic chemosynthesis anaerobic and aerobic respiration organic and inorganic fermentation these microbial ecosystems have separated for more than 3 5 ga 3500 m y they intervene in the biostructuration of sediments by active mineral deposition particle trapping and sediment stabilisation they build bioconstructions stromatolites oncolites oolites and microbial bioherms further they are active in a number of biogeochemical biomineralisation processes including phosphatogenesis carbonate genesis and the formation of iron ore and sulfur deposits and petroleum and kerogen they are also active agents in the biocorrosion and bioerosion of sedimentary rocks under marine and terrestrial conditions since they have an enormous capacity for proliferation microbial laminated communities invade new habitats practically instantaneously where suitable energy sources and gradients for electron flow are available in addition such microbial systems are excellent and sensitive paleoecological indicators in order to study their impact on sedimentation and fossilisation a close collaboration between geologists and microbiologists is needed only in this way can the nature of fossil microbial ecosystems and the metabolisms that were at work be recognized and reconstructed

review of methods of estimating rhyolitic tephra volumes applications to the taupo volcanic zone new zealand there have been at least 35 rhyolitic plinian eruptions in the last 50 kyr from 2 volcanic centres in taupo volcanic zone tvz new zealand to determine the total volume of erupted material calculation techniques are reviewed and further developed all involve extrapolation from isopach maps three methods assume an exponential decay of thickness with distance from source and a further two are based on the area enclosed by each isopach comparison of methods shows those based on area to be more accurate except the log log plot of area and thickness which requires excessive extrapolation a log volume vs thickness plot is considered superior because minimal extrapolation is needed for most tephras volumes calculated by the different methods are in reasonable agreement volumes previously calculated by the crystal concentration method are 2 3 times greater than those from the six methods used here total pumice fragmentation is probably not required for the liberation of crystals and this would decrease calculated volumes during the last 50 kyr magma has been erupted at a constant mean rate in the taupo volcanic zone a simple step function shows a strong relationship between deviations from the mean rate and either the next erupted volume or the time to the next eruption extrapolation of these step functions suggests future eruptions in less than 400 years for okataina and about 8000 years for taupo the volume of rhyolitic material erupted from tvz within the last 50 kyr is estimated at 360 km3 of tephra 350 km3 of ignimbrite and 55 km3 of extrusive equivalent to about 350 km3 of magma at an average rate of eruption of 7 km3 year at least 4200 km3 of rhyolitic magma has been erupted since the inception of tvz about 0 6 m y ago if back are spreading of tvz at 7 mm yr is compensated for by dyke intrusion into the upper 10 km of crust 8400 km3 of magma is required the total volume of magma generated by partial melting is estimated at 12 600 km3 removal of this melt would cause subsidence of 2 1 km the present depth to subvolcanic basement in tvz

les ecosystemes microbiens marqueurs des variations des niveaux lacustres en contexte de fan deltas exemple du rift rhenan meridional europe occidentale the rhine graben begins to develop in the upper eocene during the first paroxysmal stage a belt of sandstones and conglomerates the so called conglome rats cotiers is deposited on the rift border in the graben an euryhaline lacustrine basin regulates the extension of fan deltas which are developing along the boundary faults of the rift

sedimentation of tephra by volcanic plumes i theory and its comparison with a study of the fogo a plinian deposit sao miguel azores sedimentation of ejecta from volcanic plumes has been studied as a function of distance from the source in the fogo a plinian deposit sao miguel azores the fogo a trachytic pumice deposit is reversely graded and can be divided into two parts on the basis of pumice colour abundance of syenite accessory lithic clasts and distribution the lower syenite poor part was dispersed to the south and was clearly influenced by wind the upper syenite rich part is coarsegrained and has a nearly symmetrical distribution around the vent elongation of isopachs to the east indicate a weak wind influence the grain size variations of lithic and crystal components in the upper coarse part were studied total accumulation and accumulation per unit area expressed in kg m2 show good fits to a gaussian function at distances greater than 7 km for grain diameters less than 2 cm these results agree with a theoretical model for a radially spreading turbulent current moving over a quiescent fluid the gaussian coefficient is shown to be a function of grain size and the flow rate of material into the umbrella region of the eruption column the coefficient is therefore also a function of column height the column height deduced from these data is 21 km which is in broad agrrement with the column height of 27 km deduced from maximum clast dispersal using the method of carey and sparks 1986 the accumulation of clasts larger than 2 cm agrees with a theory for the fallout of clasts from the margins of the ascending eruption column which treats the plume as a succession of large eddies that decrease their mass of particles as an exponential function of time calculations are also presented for the influence of the radial inflow of surrounding air into the column on the deposition of clasts these calculations constrain the wind speed during the later part of the fogo a eruption to be at most a few metres per second the study has allowed four different dynamic categories of clast behaviour to be recognised in eruption columns

pyroclastic geology of the rhyolitic volcano of la primavera mexico explosive eruptions of this large quaternary rhyolitic volcano have produced voluminous pumice fall deposits and ignimbrites a stratigraphie study reveals more than 30 fall deposits totalling more than 90 km3 which have come from many different vents some are of plinian type and one is among the largest known in the world the main ignimbrite rio caliente is of intra plinian type in all 140 km3 of rhyolitic material were erupted during the past 105 years of which 50 km5 are dispersed well outside the volcano the dense rock equivalent volume is 60 km3 of which fall deposits ignimbrites and lava bodies comprise 45 25 and 30 respectively the average output rate of 0 06 km3 per century is nearly an order of magnitude less than for the most productive rhyolitic volcanoes known a circular area 10 km across in the centre of la primavera contains updomed lacustrine ashes and associated sediments including a remarkable giant pumice bed which probably accumulated in a caldera lake la primavera has no record of historical eruptions and hot springs are the only signs of present activity but the updoming is thought to be due to the uprise of a new acid pluton beneath the volcano and future eruptions are probable

tephrostratigraphy and provenance from iodp expedition 352 izu bonin arc tracing tephra sources and volumes from the oligocene to recent provenance studies of widely distributed tephras integrated within a well defined temporal framework are important to deduce systematic changes in the source scale distribution and changes in regional explosive volcanism here we establish a robust tephro chronostratigraphy for a total of 157 marine tephra layers collected during iodp expedition 352 we infer at least three major phases of highly explosive volcanism during oligocene to pleistocene time provenance analysis based on glass composition assigns 56 of the tephras to a japan source including correlations with 12 major and widespread tephra layers resulting from individual eruptions in kyushu central japan and north japan between 115 ka and 3 5 ma the remaining 101 tephras are assigned to four source regions along the izu bonin arc one of exclusively oligocene age is proximal to the bonin ridge islands two reflect eruptions within the volcanic front and back arc of the central izu bonin arc and a fourth region corresponds to the northern izu bonin arc source first order volume estimates imply eruptive magnitudes ranging from 6 3 to 7 6 for japan related eruptions and between 5 5 and 6 5 for ibm eruptions our results suggest tephras between 30 and 22 ma that show a subtly different izu bonin chemical signature compared to the recent arc after a 11 m y gap in eruption tephra supply from the izu bonin arc predominates from 15 to 5 ma and finally a subequal mixture of tephra sources from the palaeo honshu and izu bonin arcs occurs within the last 5 ma

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

a case of no wind plinian fallout at pululagua caldera ecuador implications for models of clast dispersal the caldera of pululagua is an eruptive centre of the northern volcanic zone of the south american volcanic arc located about 15 km north of quito ecuador activity leading to formation of the caldera occurred about 2450 b p as a series of volcanic episodes during which an estimated 5 6 km3 dre of hornblende bearing dacitic magma was erupted a basal pumice fall deposit covers more than 2 2x104 km2 with a volume of about 1 1 km3 and represents the principal and best preserved plinian layer circular patterns of isopachs and pumice lithic and md isopleths of the basal fallout bf around the caldera indicate emplacement in wind free conditions absence of wind is confirmed by an ubiquitous normally graded thin ash bed at the top of the lapilli layer which originated from slow settling of fines after cessation of the plinian column co plinian ash the unusual atmospheric conditions during deposition make the bf deposit particularly suitable for the application and evaluation of pyroclast dispersal models application of the carey and sparks 1986 model shows that whereas the 3 2 1 6 and 0 8 cm lithic isopleths predict a model column height of about 36 km the 6 4 cm isopleth yields and estimate of only 21 km the 4 9 and 6 4 cm isopleths yield a column height of 28 km using the model of wilson and walker 1987 the two models give the same mass discharge rate of 2x108 kg s 1 a simple exponential decrease of thickness with distance as proposed by pyle 1989 for plinian falls fits well with the bf exponential decrease of size with distance is followed by clasts less than about 3 cm suggesting in agreement with wilson and walker 1987 that only a small proportion of large clasts reach the top of the column variations with distance in clast distribution patterns imply that in order to obtain column heights by clast dispersal models the distribution should be known from both proximal and distal zones knowledge of only a few isopleths irrespective of their distance from the vent is not sufficient as seemed justified by the method of pyle 1989

eruption of the nevado del ruiz volcano colombia on 13 november 1985 tephra fall and lahars a small plinian eruption of the nevado del ruiz volcano in colombia ejected 3 5 x 1010 kilograms of mixed dacite and andesite tephra on 13 november 1985 with a maximum column height of 31 kilometers above sea level small pyroclastic flows and surges generated during the initial stage of the eruption caused surface melting of approximately 10 of the volcano9s ice cap leading to meltwater floods the erosive floods incorporated soils and loose sediments from the volcano9s flanks and developed into lahars which claimed at least 25 000 lives

seasonal variability in phosphorus speciation and deposition in a calcareous eutrophic lake abstract settling particulate matter was collected in sediment traps from the major depositional basin of onondaga lake a calcareous eutrophic lake in syracuse new york u s a sediment traps were deployed at three depths from july to november and for a brief period under ice cover in february to investigate both vertical and seasonal variations in sedimentation a sequential chemical extraction fractionation method was applied to the collected material to quantify the following phosphorus p fractions or forms loosely bound p sorbed p caco3 associated p and fe and al bound p extractable biogenic p easily degradable organic p and biogenic inorganic poly phosphates calcium mineral p e g apatite and refractory organic p extractable biogenic p and refractory organic p were strongly correlated suggesting a common autochthonous origin in plankton calcium mineral p appears to be of terrigenous origin a dramatic increase in the loosely bound p content of the particulate matter was observed during fall turnover when dissolved fe2 which had accumulated in the anoxic hypolimnion mixed with the surface waters and oxidized with attendant adsorption of p we estimate that the labile or exchangeable p i e loosely bound and extractable biogenic p forms content of the settling particulates constitutes 50 of the total p influx to the sediments over the period of study the labile p flux over the july november study period was 11 mg p m 2 day 1 and the labile p flux estimated from sediment traps deployed under ice cover was 6 mg p m 2 day 1 from these values the annual average labile p flux to the sediments was estimated to be 9 mg p m 2 day 1 only slightly less than the estimated annual average sediment p release rate of 10 mg p m 2 day 1 this indicates that the sediments are approximately at steady state with respect to current external p loadings to the lake

the october 1902 plinian eruption of santa maria volcano guatemala abstract the october 1902 eruption of santa maria volcano guatemala was one of the largest this century it was preceded by a great earthquake on april 19 centered at the volcano as well as numerous other major earthquakes the 18 20 hour long plinian eruption on october 25 produced a column at least 28 km high reaching well into the stratosphere the airfall pumice deposit covered more than 1 2 million km2 with a trace of ash and was only two meters thick at the vent white dacitic pumice dark gray scoriaceous basalt with physically and chemically mixed intermediate pumice and loose crystals of plagioclase hornblende hypersthene biotite and magnetite make up the juvenile components of the deposit lithic fragments are of volcanic plutonic and metamorphic origin the plinian deposit is a fine grained crystal rich single pumice fall unit and shows inverse grading mapping of the deposit gives a volume of 8 3 km3 within the one mm isopach crystal concentration studies show that the true volume erupted was at least 20 km3 equivalent to 8 5 km3 of dense dacite and that 90 of the ejecta was less than 2 mm in diameter the plinian volume eruption rate averaged 1 2 105 m3s 1 and the average gas muzzle velocity of the column exceeded 270 ms 1 a total of 8 3 1018 j of energy were released by the eruption a knowledge of both theoretically derived eruption parameters and contemporary information allows a detailed analysis of eruption mechanisms this eruption was the major stratospheric aerosol injection in the 1902 1903 period however mid to low latitude northern hemisphere temperature deviation data for the years following the eruption show no significant temperature decrease this may be explained by the sulfur poor nature of dacite magmas suggesting that volatile composition rather than mass of volatiles is the controlling parameter in climatic response to explosive eruptions