conduit enlargement during the precursory plinian eruption of aira caldera japan increase in magma flux as the result of conduit enlargement is one of the key processes that triggered caldera collapse and eruption of the ito ignimbrite from aira caldera at 29 ka we examine the total volume of the pumice fall deposit vertical variations in grain size of pumice and the lithic content in the osumi pumice deposit to investigate the trigger for caldera collapse wider distribution of the later stage unit and the upward coarsening of grain size throughout the osumi pumice fall deposit indicate an increase in magma discharge toward the onset of collapse the total volume of lithic fragments in the osumi pumice fall deposit is estimated as 1 6 km3 based on the lithic content in several representative outcrops and the total volume of the osumi pumice fall deposit the lithic fragments in the osumi pumice fall deposit indicate intense mechanical erosion of the conduit during the plinian eruption prior to caldera collapse caldera collapse requires decompression of the magma chamber by withdrawal of magma effective enlargement of the conduit diameter during precursory eruptive phases is one of the important processes that subsequently allow the rapid discharge of a large volume of magma which in turn facilitates decompression of the reservoir and induces caldera collapse

segmented lateral dyke growth in a rifting event at barðarbunga volcanic system iceland crust at many divergent plate boundaries forms primarily by the injection of vertical sheet like dykes some tens of kilometres long1 previous models of rifting events indicate either lateral dyke growth away from a feeding source with propagation rates decreasing as the dyke lengthens2 3 4 or magma flowing vertically into dykes from an underlying source5 6 with the role of topography on the evolution of lateral dykes not clear here we show how a recent segmented dyke intrusion in the bar arbunga volcanic system grew laterally for more than 45 kilometres at a variable rate with topography influencing the direction of propagation barriers at the ends of each segment were overcome by the build up of pressure in the dyke end then a new segment formed and dyke lengthening temporarily peaked the dyke evolution which occurred primarily over 14 days was revealed by propagating seismicity ground deformation mapped by global positioning system gps interferometric analysis of satellite radar images insar and graben formation the strike of the dyke segments varies from an initially radial direction away from the bar arbunga caldera towards alignment with that expected from regional stress at the distal end a model minimizing the combined strain and gravitational potential energy explains the propagation path dyke opening and seismicity focused at the most distal segment at any given time and were simultaneous with magma source deflation and slow collapse at the bar arbunga caldera accompanied by a series of magnitude m 5 earthquakes dyke growth was slowed down by an effusive fissure eruption near the end of the dyke lateral dyke growth with segment barrier breaking by pressure build up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries

processes of miss formation in a modern siliciclastic tidal flat patagonia argentina abstract the study focus on the description of several miss as erosional pockets and remnants flipped over edges and large microbial deformation structures as roll ups folds and gas domes in the context of sediment composition hydraulics and geomorphology the aim of the paper is to recognize the mechanisms of formation of miss by analyses on hydrodynamics under the influence of the geomorphology the study was conducted in an elongated inactive tidal channel colonized by microbial mats 2 5 0 3 km in a progradation environment to continuously record the water level fluctuations a hobo water level logger was placed 40 cm below the flat surface for two years the sea water enters several times a year during storms where the flood currents were characterized by faster velocity than ebb currents reaching a water depth up to 0 70 m over the tidal flat that coastal process creates miss over the tidal flat the most conspicuous microbial structures are the huge deformation roll ups several m scale associated to elongated rip off mats folds and flipped over mats the process of fluidization was postulated to explain the sand infilling of gas domes and folds we observed this process would result from the wave action on the water column that produces a temporal fluid behavior of the underlying sand

texture of microbial sediments revealed by cryo scanning electron microscopy abstract textures of modern lacustrine stromatolites on kiritimati line islands central pacific ocean and of buried layers in the stromatolitic carbonate sediments from french polynesian atoll lakes kopara have been studied using cryo scanning electron microscopy sem equipped with a freeze drying sample preparation device this study confirms that microscopic three dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria and from other extracellular polymer secretions are common components of microbial sediments of which they may form the framework in addition to this role in sediment cohesion and formation of microstructure the organic framework appears to be involved in carbonate precipitation with n the stromatolites through chemical nucleating steric and hydrodynamical controls the role of the dead organic constituents of the stromatolites is not only confirmed in micrometer size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments and to the formation of peloids that evolve into spherulites bacterial including nannobacterial carbonate bodies and carbonate impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied all these carbonate precipitation processes may cooperate in lithification of a given sediment

a brief overview of eruptions from ice covered and ice capped volcanic systems in iceland during the past 11 centuries frequency periodicity and implications eruptions from partly ice covered and ice capped volcanic systems constitute nearly 60 of all known historical i e in the past 11 centuries eruptions in iceland since the fourteenth century such eruptions have been reported in contemporary or near contemporary documents at least 120 historical eruptions have broken through the ice on the glaciated parts of five volcanic systems and have left tephra layers in ice and soil or been recorded at the time an unknown number of eruptions did not breach the overlying ice and left no record at all beginning as subglacial eruptions most eruptions break through the ice in minutes hours or days and can last from a few days to several months a single vent or the whole length of a fissure may then emerge to emit highly fragmented tephra in hydromagmatic explosions of varying strength the volume of airborne tephra varies by at least four orders of magnitude with dispersal range varying from near vent to transatlantic in most of the eruptions the magma was of basaltic composition eruption frequency is highest within the grimsvotn system where up to seven eruptions every 40 years have occurred during peaks of activity and at least 70 eruptions over historical time the observed pattern of temporally and spatially close eruptions separated by periods of low or no activity leaves open the question whether pre holocene deposits of several closely spaced subglacial eruptions can be securely distinguished from those formed in a single subglacial eruption

microbial mats as bioreactors populations processes and products microbial mats are dynamic and complex ecosystems exhibiting spatial and temporal heterogeneity the physical chemical environment is typified by steep gradients and distinct microenvironments these microenvironments support a great diversity of species with a wide range of metabolic processes these processes often result in coupled reactions and biogeochemical cycles and produce important end products such as trace gases and mineral precipitates the latter can impact the composition and character of the sediment imparting a biosignature these biosignatures can be preserved in the rock record and are useful in the interpretation of fossil record on earth and possibly as an indication of life on other planetary bodies the modern marine stromatolites of the exuma cays bahamas provide an ideal system for studying the populations processes and products in a microbial ecosystem using a multidisciplinary approach in order to acquire redox energy microbial populations need to carry out metabolic reactions at rates faster than the equivalent chemical abiotic reactions as such microbes can be viewed as bioreactors that preferably oxidize carbon to co2 to maximize the energy yield the study of the microbial role in carbonate sedimentation and lithification in these stromatolites provides a picture of microbial mats as bioreactors producing a biosignature

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

the caldera forming eruption of the quaternary payun matru volcano andean back arc of the southern volcanic zone abstract the payun matru caldera is a quaternary andean back arc caldera 36 25 s 69 12 30 w and represents the main volcanic edifice of the homonym payun matru volcanic field located within the payenia basaltic province the payun matru volcanic field is also characterized by the payun liso volcano and 220 monogenetic basaltic cones and associated lava flows the pleistocene holocene payun matru caldera presents a long lived pre caldera stage which built a multi vent and shield like volcanic edifice mainly trachytic in composition the syn caldera stage is represented by the trachytic portezuelo ignimbrite the caldera forming eruption deposits constrained between 148 and 82 ka and the resulting caldera depression of 8 5 km in diameter afterwards the caldera event volcanism continued mostly along the caldera rim stratigraphic sections of the portezuelo ignimbrite and its correlations are presented in addition to mineral compositions and previous whole rock chemical analyses only the extra caldera deposits can be observed while the intra caldera ignimbrite is hidden by post caldera volcanic products and unconsolidated modern sediments different facies within the ignimbrite were recognized being most of them massive or eutaxitic and massive lapilli tuff facies deposited by dense pyroclastic density currents fall deposits are found only proximally and restrained to the caldera margin the absence of a widespread fall deposit suggests the lack of a sustained eruptive column at the onset of eruption massive tuff facies at the topmost deposits indicates the development of a co ignimbritic plume as the pyroclastic density currents wanned on the basis of color size and shape vesicularity and crystallinity four distinct juvenile clasts types were defined gray fiamme black juvenile dark gray and light gray pumice available chemical analysis of gray fiamme and black juvenile clasts indicate a similar trachytic composition although their mineralogical composition presents some differences several eruptive units for the northern and southern outer slopes of the caldera were defined on the basis of the juvenile types present and discontinuity surfaces the onset of caldera collapse is not marked by the presence of a lithic breccia but it is suggested by the appearance of dark gray and light gray pumice along with black juveniles in the topmost eruptive units together with an increase in lithic content relative to previous units the sequential appearance of different juvenile clasts types suggests a zoned magma chamber in terms of crystal content and composition

a reappraisal of seismicity recorded during the 1996 gjalp eruption iceland in light of the 2014 2015 barðarbunga holuhraun lateral dike intrusion this study performs a reanalysis of the seismicity recorded during the 1996 gjalp eruption that occurred at nw vatnajokull iceland the seismicity was recorded by the temporary hotspot network consisting of 30 three component broadband stations in total 301 events were identified between 29 september and 12 october and their phases were manually picked a velocity model was estimated from p phase travel times by using velest events were first located using the algorithm nonlinloc in order to obtain absolute locations precise relative locations were obtained with hypodd by utilizing catalog and cross correlation differential travel times results show that events clustered first along the sw rim of the bar arbunga caldera and later along the gjalp fissure with most hypocentral depths located between 3 and 8 km waveforms of the 10 largest events that followed the bar arbunga earthquake were inverted in order to obtain moment tensors for all events we found that the deviatoric moment tensor fits the data better than pure double couple or full moment tensor solutions events along the bar arbunga caldera exhibited reverse focal mechanisms while those at the gjalp fissure exhibited mostly strike slip faulting seismic velocity variations calculated using ambient noise interferometry point to the possibility that a small subglacial eruption occurred at bar arbunga before the main earthquake this removed melt from the magma chamber causing its roof to collapse and also resulted in the lateral migration of magma towards the gjalp fissure the 2014 2015 bar arbunga holuhraun eruption shares common characteristics with the 1996 gjalp eruption although the size of the latter was much smaller

the collapse of barðarbunga caldera iceland lying below vatnajokull ice cap in iceland bar arbunga stratovolcano began experiencing wholesale caldera collapse in 2014 august 16 one of the largest such events recorded in the modern instrumental era simultaneous with this collapse is the initiation of a plate boundary rifting episode north of the caldera observations using the international constellation of radar satellites indicate rapid 50 cm d 1 subsidence of the glacier surface overlying the collapsing caldera and metre scale crustal deformation in the active rift zone anomalous earthquakes around the rim of the caldera with highly nondouble couple focal mechanisms provide a mechanical link to the dynamics of the collapsing magma chamber a model of the collapse consistent with available geodetic and seismic observations suggests that the majority of the observed subsidence occurs aseismically via a deflating sill like magma chamber

vertical and lateral collapses on tenerife canary islands and other volcanic ocean islands recent studies demonstrate that tenerife has undergone large lateral collapses this has led to the suggestion that the las canadas caldera one of the best exposed calderas in the world is the result of lateral collapse we have tested this idea using the available structural stratigraphic volcanological and geochronological data we conclude that the las canadas caldera is the result of a complex sequence of vertical collapse events associated with a long history of phonolitic explosive activity in the central part of tenerife our results indicate however that vertical collapses may have played a major role in triggering lateral collapses we propose that the association of vertical and lateral collapse events such as inferred for tenerife can also explain similar sequences of events interpreted to have affected other large volcanic ocean islands

influence of relative sea level change and tidal inlet development on barrier spit stratigraphy sandy neck massachusetts abstract sandy neck is a 10 km long sheltered mesotidal barrier spit along the southeastern new england coast bordering cape cod bay its stratigraphy contains evidence of relative sea level rsl rise and tidal inlet development and enables reconstruction of accretion and progradation of this barrier through time core trench and ground penetrating radar gpr data show that the lower boundary of the uppermost eolian facies climbs about 9 m from the proximal to the distal part of the spit away from the mainland anchor point this change is thought to be related primarily to a comparable rise in rsl the same boundary also shows a shallowing trend in a seaward direction indicating net spit progradation through time the absence of inlet channel facies in the stratigraphy of the proxima part of the barrier spit in contrast to the presence of inlet facies in more distal parts of the spit indicates that some time elapsed after barrier initiation before an inlet thalweg developed in its earliest stages this thalweg was poorly defined wide and shallow but its characteristics changed as the barrier spit lengthened in the central part of sandy neck the assemblage of inlet channel facies becomes markedly thicker reflecting thalweg deepening in response to paleotopographic and substrate lithologic factors the stratigraphy of the distal part of the spit shows the effects of continued but more gradual inlet deepening that may be tied directly to increasing barrier length the process controlled stratigraphy presented here differs from current facies models because i accounts for temporal changes in coastal environmental processes and parameters it suggests a high preservation potential for proximal eolian facies in the spit sequence underneath a cover of fine grained and organic back barrier sediment once reliably dated the new stratigraphic data will help to improve the regional rsl curve for the southern cape cod bay area

evolution of the caldera forming eruption at crater lake oregon indicated by component analysis of lithic fragments crater lake caldera 8 10 km formed 6845 years b p 14c age during the climactic eruption of the volcanic edifice known as mount mazama is intermediate in size between small calderas associated with central vent eruptions and large calderas that have ring fracture vent systems our quantitative study of lithic fragments in the ejecta confirms the existing model of changes in vent configuration during the climactic eruption of mount mazama initial activity was from a single vent that produced a rhyodacite pumice fall from a plinian column altered preexisting volcanic rocks are the predominant lithic type in the plinian deposit and their extensive hydrothermal alteration is considered as evidence of their relatively deep origin the wineglass welded tuff lies atop the plinian deposit and contains a higher proportion of fresh volcanic rocks suggesting enlargement of the single vent by slumping of its walls this same vent enlargement caused the plinian eruption column to collapse and feed valley hugging pyroclastic flows that deposited the wineglass welded tuff when enough material was erupted from the shallow magma chamber that its roof was no longer adequately supported mount mazama collapsed to form the caldera while highly energetic pyroclastic flows produced the climactic ignimbrite a lag breccia that represents the proximal facies of the compositionally zoned climactic ignimbrite lies atop the wineglass welded tuff and contains predominantly altered volcanic rocks of deeper origin accompanied by minor granitoids from the magma chamber walls azimuthal differences in lithic component proportions in the lag breccia correlate well with the geology of the caldera walls indicating that the climactic ignimbrite was ejected by multiple vents along a ring fracture system systematic lithic component changes within the lag breccia suggest different quarrying levels that reflect waxing and waning of the discharge rate during the caldera collapse phase of the climactic eruption our lithic component analysis demonstrates that calderas that may be too small to experience structural resurgence such as crater lake nevertheless may form by syneruptive subsidence along ring fractures

an integrated model of magma chamber conduit and column for the analysis of sustained explosive eruptions abstract explosive volcanic eruptions comprise a complex series of processes involving withdrawal from the magma chamber magma ascent along the conduit and eruption column dynamics numerous studies have modeled the different sub domains of a volcanic system but their interplay has seldom been analyzed to this end we developed c 3 c cubed that stands for chamber conduit and column a new integrated model that describes the dynamics of an explosive eruption as a series of steady state regimes and as a function of geometry and initial conditions of the magma reservoir we used global sensitivity analysis to quantify the role of the relevant model parameters and describe the interplay between the different volcanic sub domains in particular we analyzed the evolution of a sustained explosive eruption in order to identify the conditions for buoyant super buoyant and collapsing columns input data were based on field reconstructions of quaternary explosive eruptions in the vulsini volcanic district roman province central italy model results show that 1 the column regime although affected by complex interactions among several factors mostly depends on the conduit radius the volatile content i e supersaturation concentration at the top of the chamber and length of the conduit in decreasing level of importance 2 the amount of mass erupted is independent of the conduit radius and depends mostly on volatile supersaturation the radius of the magma chamber the length of the conduit and the overpressure at the conduit inlet 3 the mass flow rate column height and duration of the eruption are largely controlled by the conduit radius 4 the flow pressure and density at the conduit exit are mostly controlled by the conduit inlet overpressure at the onset of the eruption and by the length of the conduit at the end of the eruption 5 the exit velocity from the conduit is mostly controlled by the volatile content the length of the conduit and the inlet overpressure in this model framework and with specific reference to selected plinian events of the vulsini volcanic district simulation results show that column collapse is not achieved for reasonable eruption durations order of hours and conduit widths tens of meters this is consistent with field reconstructions suggesting that column collapse did not likely occur and that pyroclastic flows were therefore generated by independent mechanisms from ring fissures and or multiple vents concomitant to caldera collapse

mechanical interaction between active volcanoes in iceland 1 we test the possibility of mechanical interaction between eight central volcanoes in the central part of the active iceland rift zone the average distance between the volcanoes is 30 km all are thought to have shallow magma chambers and many contain collapse calderas we analyzed many finite element models with the volcanoes subject to a tensile stress of 5 mpa equal to the maximum in situ tensile strength of the crust in a direction parallel to the spreading vector n105 e the results show zones between many nearby volcanoes where the tensile stresses exceed the in situ tensile strength of the crust the results indicate that mechanical interaction between volcanoes in a pair such as simultaneous dike emplacement seismogenic faulting and deformation may be common in this part of iceland in agreement with observations

formation and development of normal fault calderas and the initiation of large explosive eruptions the ring fractures that form most collapse calderas are steeply inward dipping shear fractures i e normal faults at the surface of the volcano within which the caldera fault forms the tensile and shear stresses that generate the normal fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano numerical results indicate that normal fault calderas may initiate as a result of doming of an area containing a shallow sill like magma chamber provided that the area of doming is much larger than the cross sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming this model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera when the caldera fault does not slip eruptions from calderas are normally small nearly all large explosive eruptions however are associated with slip on caldera faults during dip slip on and doming of a normal fault caldera the vertical stress on part of the underlying chamber suddenly decreases this may lead to explosive bubble growth in this part of the magma chamber provided its magma is gas rich this bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption

crustal deformation associated with the 1996 gjalp subglacial eruption iceland insar studies in affected areas adjacent to the vatnajokull ice cap abstract crustal deformation signals associated with the september 30 october 13 1996 gjalp subglacial eruption vatnajokull ice cap iceland have been identified using interferometric analysis of synthetic aperture radar images insar in areas outside the ice cap on september 29 1996 an mw 5 6 earthquake occurred at the nearby bardarbunga volcano and on september 30 seismicity propagated 20 km southwards where the gjalp eruption occurred analysis of interferograms spanning different times from 1992 to 2000 allows us to separate two distinct co eruptive deformation periods in areas outside the ice cap diking at the bardarbunga caldera rim appears to be responsible for deformation during the first week of eruption while significant deflation occurred at bardarbunga only after october 6 when most of the magma had already been erupted at gjalp a pressure connection between the bardarbunga volcano and the gjalp eruptive fissure is inferred fault slip in three areas up to 30 km from the center of the bardarbunga volcano was triggered by the deflation local deformation signals there are consistent with small fault movements

variations in the thermal output of the subglacial grimsvotn caldera iceland a 69 year record 1922 1991 of heat release has been obtained from the subglacial geothermal system of the grimsvotn caldera within the vatnajokull ice cap the data were derived from in situ measurements of the volume of meltwater accumulated in the caldera lake subtracting climatically induced melting the overall fluctuations in the heat flux are closely related to volcanic activity and are dominated by a main pulse of 11 600 mw caused by a major eruption in 1938 gradually declining to 1600 mw in 1976 1982 heat extracted from the roof of a magma chamber with the aid of hydrothermal convection may have given a basic contribution to the heat flux of 1500 to 2000 mw an upper bound the variable part of the heat flux from 0 to 10 000 mw was released from magma erupted at the base of the glacier and from shallow intrusions the total heat released over the period 1922 to 1991 was 8 1 1 6 1018j equivalent to the energy released by the solidification and cooling of 2 1 0 4 km of basaltic magma the contribution to the total heat flux was 45 max from a magma chamber 35 min from shallow intrusions and 20 from eruptions this implies that magma at the roof of a chamber solidified and cooled at the rate of 1 2 1 6 107 m a 1 or about 1 km over the last 69 years heat release at grimsvotn was probably more intense in the 19th century when volcanic eruptions were more frequent

ice volcano interaction of the 1996 gjalp subglacial eruption vatnajokull iceland volcanic eruptions under glaciers can cause dangerous floods and lahars1 2 3 and create hyaloclastite fragmented glassy rock mountains4 5 6 7 8 but processes such as the rate of heat transfer between ice and magma edifice formation and the response of the surrounding glacier are poorly understood because of the lack of data here we present observations from the fissure eruption at vatnajokull ice cap iceland in october 1996 in the 13 days of the eruption 3 km3 of ice were melted and the erupted magma fragmented into glass forming a hyaloclastite ridge 6 7 km long and 200 300 m high under 500 750 m of ice meltwater of temperatures of 15 20 c flowed along a narrow channel at the glacier bed into the grimsvotn subglacial lake for five weeks before draining in a sudden flood or jokulhlaup subsidence and crevassing of the ice cap occurred over the eruptive fissure and the meltwater path whereas elsewhere the glacier surface remained intact suggesting that subglacial eruptions do not trigger widespread basal sliding in warm based glaciers

deep crustal melt plumbing of barðarbunga volcano iceland understanding magmatic plumbing within the earth s crust is important for understanding volcanic systems and improving eruption forecasting we discuss magma plumbing under bar arbunga volcano iceland over a 4 year period encompassing the largest icelandic eruption in 230 years microseismicity extends through the usually ductile region of the earth s crust from 7 to 22 km depth in a subvertical column moment tensor solutions for an example earthquake exhibits opening tensile crack behavior this is consistent with the deep 7 km seismicity being caused by the movement of melt in the normally aseismic crust the seismically inferred melt path from the mantle source is offset laterally from the center of the bar arbunga caldera by 12 km rather than lying directly beneath it it is likely that an aseismic melt feed also exists directly beneath the caldera and is aseismic due to elevated temperatures and pervasive partial melt under the caldera

on the appearance of cyanobacterial calcification in modern stromatolites abstract in august 1991 calcified magnesian calcite filamentous cyanobacteria scytonema mirabile appeared on the surface of a modern lacustrine stromatolite lake r2 rangiroa atoll tuamotu archipelago french polynesia previously the topmost photosynthetic layer of this sediment was devoid of such microbial biomineralisers the calcified cyanobacteria are accompanied by magnesian calcite grains which are evocative of bacterial precipitates although the appearance of cyanobacterial calcification might be related to environmental changes in the lake in particular a decrease in water salinity the persistence of non calcified specimens of a filamentous cyanobacteria phormidium crossbyanum which was once a dominant species of the lake benthos shows that calcification and environmental conditions might be only indirectly related thus attempts to use the fossil record of calcareous cyanobacterial skeletons to infer changes in the chemical composition of past oceans cannot be made without due regard of biological processes like the appearance of new competitors

the new worldwide collapse caldera database ccdb a tool for studying and understanding caldera processes collapse calderas are one of the most important volcanic structures not only because of their hazard implications but also because of their high geothermal energy potential and their association with mineral deposits of economic interest the objective of this work is to describe a new general worldwide collapse caldera database ccdb in order to provide a useful and accessible tool for studying and understanding caldera collapse processes the principal aim of the ccdb is to update the current field based knowledge on calderas merging together the existing databases and complementing them with new examples found in the bibliography and leaving it open for the incorporation of new data from future studies this database does not include all the calderas of the world but it tries to be representative enough to promote further studies and analyses we have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references and their information has been summarized in a database linked to a geographical information system gis application thus it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database e g age structure the information recorded in the ccdb can be grouped in seven main information classes caldera features properties of the caldera forming deposits magmatic system geodynamic setting pre caldera volcanism caldera forming eruption sequence and post caldera activity additionally we have added two extra classes the first records the references consulted for each caldera the second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin a further purpose of this work is to construct the ccdb web page in this web page where registered users can acquire the current database version as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes additionally the ccdb includes a formulary that will facilitate the incorporation of new calderas into the database

the timing of caldera collapse at mount katmai in response to magma withdrawal toward novarupta several thin layers of nonjuvenile lithic rich tephra from mount katmai are intercalated with 1912 plinian pumice fall deposits that originated at novarupta 10 km away demonstrating that caldera collapse at katmai and eruption at novarupta were synchronous emplaced by phreatic explosions and avalanche generated dustclouds each katmai derived layer is thickest and coarsest at the caldera rim and pinches out within 5 8 km in contrast thicknesses and clast sizes of plinian layers a g decrease radially away from novarupta the first katmai derived layer lies between novarupta derived layers b2 and b3 indicating that collapse began after release of 6 9 km of magma at novarupta the first major earthquake ms 6 5 at 2356 h on 6 june 11 hours after the eruption began may have marked the onset of collapse the katmai river landslide and several debris flow deposits are also sandwiched within the plinian deposits indicating that they too were emplaced during the eruption

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

thermomechanical behavior of large ash flow calderas the fundamental thermal and mechanical processes that occur within the ash flow caldera magma chamber systems remain largely enigmatic to date the only models of caldera collapse are simple mostly elastic or viscoelastic mechanical models that can predict some of the conditions preceding the collapse they cannot however predict the collapse itself because they are incapable either of reproducing the formation of faults or of accounting for the brittle ductile transitions and fault related thermal anomalies we have constructed analytical and numerical therrnomechanical that account for both elastic plastic ductile rheology and physical properties of the caldera rocks the overpressured magma is evacuated through a central vent and transforms into ash flow units deposited within the forming caldera brittle deformation faulting and subsequent collapse of the structure are reproduced the results show that in the absence of a regional stress field the collapse on both sides will occur only for aspect ratios i e caldera diameter to the depth of the magma chamber exceeding 5 and that internal embedded faults may also appear when the aspect ratio exceeds 10 thermal conductivity contrasts in ash flow calderas give rise to strong heat refraction that localizes deep seated thermal anomalies to the outer sides of the faults in the presence of regional extension the border faulting can be attenuated or disappear and the faults tend to localize around the central part of the chamber roof coupled therrnomechanical modeling suggests that the outer sides of the border faults have high trapping capabilities for hydrothermal fluids the geometry of the brittle ductile transition largely controls that of the fractured zones within and around the chamber roof thus justifying a new mechanical definition of magma chamber geometry

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

structural diversity of biogenic carbonate particles in microbial mats non skeletal carbonate particles in microbial mats were studied using thin sections and scanning electron microscopy the microbial mats form biolaminated units so called potential stromatolites in salterns this study emphasizes the coexistence of different particle forms and makes a genetic connection between the heterogeneity of the organic substrate built by bacteria and diatoms and their extracellular polymeric substances eps whereas allochthonous particles are scarce lanzarote microbial mats provide various autochthonous surfaces for the attachment of cells and eps including sheaths and capsules of cyanobacteria frustules of diatoms metabolic products such as gas bubbles liquid globules and faecal pellets as well as the carbonate precipitates themselves morphologically different carbonate precipitates are i calcified organic clumps peloids ii particles composed of concentric aragonite and biofilm laminae ooids and oncoids hi isolated particles floating in gel supported mats and coated by rims of fibrous cement cortoids iv particles bound by cryptocrystalline matrices or cement resulting in aggregate grains and v lobate cement which fills out spaces and pores and fixes the particles peloids are suggested to represent faecal pellets although microbial systems also generate cell clumps by non faecal processes ooid and oncoid constructions clearly record alternating processes of biofilm accumulation and aragonite encrustation further characteristic features of carbonate particles generated within a microbial mat are i an irregular distribution ranging from isolated particles floating within the gel like matrix to closely packed particles ii the amalgamation of different particle types e g peloids and ooids in aggregate grains iii the heterogeneous nature of nuclei comprising bacterial clumps intraclasts individual cells cell colonies and bubbles iv the enrichment of remains casts and imprints of cells within precipitates and v deformation e g truncated cortices of particles

monitoring a temporal change of seismic velocity in a volcano application to the 1992 eruption of mt merapi indonesia multiplets i e events with similar waveforms are selected from shallow earthquakes recorded on merapi volcano indonesia before the eruption of february 2nd 1992 two multiplet families are found with their sources close to the summit their seismograms are analyzed using the moving window cross spectrum technique which measures the precise time delay between seismic phases in the entire seismogram for both families of multiplets a gradual decrease in the arrival times of coda waves is observed as a function of the date prior to the eruption coda waves are becoming progressively faster up to 1 2 per cent as the time interval to the eruption shortens this observation is interpreted as the consequence of an increase in the seismic velocity inside the volcano the increase in velocity started in may 1991 and was observed until september 1991 4 months before the eruption this velocity increase may be related to an increase in pressure in the magma chamber or in the conduits and to the resulting closure of the surrounding cracks

deterministic non linear source processes of volcanic tremor signals accompanying the 1996 vatnajokull eruption central iceland summary observations and theoretical considerations have cast doubt on the suggestion that volcanic tremor source processes may be modelled by a linear oscillator that is set into resonance by a sustained disturbance volcanic tremor signals that accompanied the 1996 vatnajokull subglacial eruption central iceland have been analysed using methods from the discipline of non linear dynamics in order to investigate the possibility that they originated from a non linear source the volcano seismic phenomena associated with the eruption were recorded by a permanent network equipped with broad band seismometers hotspot using a sampling rate of 20 samples s 1 the eruption was preceded by increased seismic activity for a period of 2 days which also included a large earthquake with a moment magnitude of 5 6 the tremor during the first 2 days of the eruption has a high signal to noise ratio at the nearest station to the eruption site and starts as a continuous signal later evolving to low amplitude background tremor interrupted by high amplitude cigar shaped bursts having an average duration of 250 s the phase space which describes the evolution of the behaviour of a non linear system was reconstructed from the original tremor seismograms using the delay embedding theorem suggested by takens the delay time used for the reconstruction was selected after examining the autocorrelation function which showed a first zero crossing at a timelag of 4 samples and the average mutual information that showed no minimum indicating that the tremor process may have been undersampled based also on phase space portraits for different delay times a delay time of one sample interval 0 05 s was used the sufficient embedding dimension for phase space reconstruction was selected by applying the false nearest neighbours method which revealed complete unfolding of the tremor attractor at dimensions 7 8 implying upper bounds of its fractal dimension in the range 3 5 4 0 the phase space prediction errors of different segments of the tremor time series were compared in order to check whether the attractor dynamics change substantially with time it was found that for continuous tremor there was almost no dynamic variation in contrast to the background tremor and the superposed bursts that gave a maximum prediction error when the former was used to predict the latter this difference in dynamics also had an effect on their spectra the amplitude spectrum of a burst or continuous tremor has a much sharper decay at high frequencies than that of the background tremor a possible physical mechanism that may explain these observed characteristics involves turbulent slug flow of magma in a narrow cylindrical conduit generating the different dynamic regimes as the reynolds number varies

δd and δ18o evidence for inputs to groundwater at a wetland coastal boundary in the southern great lakes region of canada abstract the stable isotope compositions 18o and d of water have been used to determine the relative contributions of different water sources to the groundwater in a barrier sand bar that separates a coastal freshwater marsh from lake erie canada an extensive groundwater study was initiated by environment canada at point pelee national park after elevated nutrient concentrations were measured in the park s marsh located on a spit of land that extends 15 km south into lake erie as part of this larger study which includes groundwater and nutrient modelling the stable isotope compositions 18o and d of water have been used to independently determine the nature and extent of groundwater flow within sand deposits that separate the marsh from lake erie one of the two study sites chosen for this investigation is located near a large septic system tile bed that receives human waste from a public toilet facility within the park and could potentially release nutrients to the marsh via the groundwater both transects studied are well suited to isotopic investigation because they are located between two potentially recharging surface water bodies with temporally variable isotopic compositions and because the transects are subject to recharge by local precipitation which exhibits large seasonal isotopic variations these differing isotopic compositions when considered spatially and temporally make it possible to establish the source and movement of groundwater within the transects the oxygen and hydrogen isotopic compositions of surface waters from the point pelee marsh and lake erie lie on an evaporation line with a slope of 5 7 the isotopic composition of lake erie remained relatively stable for over 21 months 18o 7 5 to 6 7 whereas the marsh exhibited considerable spatial and temporal variability 18o 8 4 to 0 1 the d and 18o values of precipitation samples vary seasonally with local surface temperatures and constitute a local meteoric water line of d 8 0 18o 9 8 while influx of precipitation is evident and similar in the upper two metres of both transects the relative contribution to groundwater from lake erie and the point pelee marsh appears to differ the average 18o and d groundwater values measured for the narrower transect were 7 0 and 54 respectively and reflect significant groundwater recharge from both lake erie 67 and the marsh 28 precipitation constitutes the primary source of groundwater recharge to the wider transect located near the large septic system tile bed with average groundwater values of 18o 9 1 and d 65 the isotopic profile of groundwater in this area is very stable both with depth and over time and contrasts sharply with the narrower northern transect which is much more dynamic these findings suggest that the septic system tile bed is well situated in the wider portion of the barrier sand bar where a larger head difference between the lake and the marsh would be required to produce groundwater flow equivalent to that observed in the narrower transect the differences in groundwater recharge between the two sites arise primarily from the difference in width and account for the 2 and 10 differences in 18o and d values respectively measured in the base flow groundwater of the two transects

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

the generation of overpressure in felsic magma chambers by replenishment abstract evidence of magma mixing is common in the products of explosive felsic eruptions and it is generally accepted as a common mechanism for triggering such events in order to quantify the potential for magma mixing to trigger explosive eruptions we have developed a simple analytical model based on previous models of magma chamber replenishment which considers the injection of volatile rich mafic magma into a chamber occupied by a homogeneous volatile rich felsic magma we assume that the overpressure caused by the injection of new magma is not sufficient to trigger an eruption for which additional overpressure is required two mixing related mechanisms have traditionally been considered to have the potential to generate the additional overpressure 1 exsolution of volatiles from the felsic magma during forced convection and 2 exsolution of volatiles from the mafic magma by oversaturation during cooling and subsequent crystallization our calculations suggest that exsolution of volatiles from the felsic magma is not an effective mechanism to generate additional overpressure however significant overpressure can be achieved by volatile exsolution from the mafic magma during its cooling and crystallization the time scale between intrusion and eruption considered in our model is of the order of a few days to a few months which coincides with petrological and geophysical evidence obtained from magma mixing related eruptions we also suggest that replenishment of shallow felsic magma chambers by mafic magma in most cases does not lead to large scale mixing as eruption will occur before thermal equilibrium between the two magmas is reached so that density and viscosity contrasts between the two magmas will remain significant

on the influence of magma chambers in controlling the evolution of explosive volcanic eruptions abstract the influence of magma chambers on the evolution of explosive volcanic eruptions is examined by coupling a model of an elastic chamber containing compressible magma bower s m woods a w 1997 the control of magma volatile content and chamber depth on the mass erupted during explosive volcanic eruptions j geophys res 102 10273 10290 with a model of conduit flow wilson l sparks r s j walker g p l 1980 explosive volcanic eruptions iv the control of magma properties and conduit geometry on eruption column behaviour geophys j r astr soc 63 117 148 it is found that for a given magma volatile content and conduit geometry variations in the chamber volume depth and aspect ratio have a significant impact on the mass erupted and the eruption duration shallow volatile saturated chambers of large cross sectional area tend to erupt a much larger fraction of the chamber volume 10 than deep volatile unsaturated chambers of the same magma 0 1 1 0 however in either case the steady decrease in chamber pressure over the course of an eruption only leads to a decrease in eruption rate of 10 50 therefore for a given chamber volume shallow chambers of large cross sectional area produce much more long lived and voluminous explosive eruptions in contrast we show that for a given chamber geometry variations in magma volatile content can lead to significant changes in both the eruption rate and the erupted volume in turn this also affects the eruption duration finally we note that as described by wilson et al increases in conduit radius lead to very large increases in eruption rate and hence a decrease in eruption duration

origin and physical and chemical characteristics of glacial overburden in essex and kent counties southwestern ontario the overburden of essex and kent counties southwestern ontario has been described as consisting of a clayey silt to silty clay till overlying a gravelly unit resting on bedrock recent quaternary geology mapping has identified additional materials and redefined the origin of others by determining the stratigraphic position and physical and geochemical properties of materials encountered in a sonic drilling program and field mapping catfish creek till was deposited on the bedrock surface during the nissouri stadial as ice advanced south over the area as ice retreated during the erie interstade fine grained glaciolacustrine material was deposited in glacial lake leverett and overlay catfish creek till tavistock till was deposited over glacial lake leverett material as the huron lobe readvanced south during the port bruce stadial as the huron lobe retreated north coarse grained glaciolacustrine materials were deposited in the leamington area ice from the erie lobe deposited the port stanley till alo

erosion processes in volcanic conduits and application to the ad 79 eruption of vesuvius abstract the flow of gas magma and pyroclasts through a volcanic conduit produces erosion of the conduit wall erosion may be produced by the impact of pyroclasts on the conduit wall fluid shear stress at the wall conduit wall collapse and volcanic tremor using a two phase flow non equilibrium model of magma ascent along the volcanic conduits demonstrated that the erosion due to the impact of particles on the wall can occur only above the magma fragmentation level of the conduit where the particles or pyroclasts remove the wall material by an abrasion process this abrasion process was found to be the largest near the conduit exit where the gas magma velocities are the largest the erosion due to the fluid shear stress at the wall can be produced along the entire length of a conduit depending on the wall roughness and yield strength of wall rocks this shear stress is the largest near the magma fragmentation level where the gas magma viscosity and velocity gradients are very large the collapse of conduit wall due to the difference between the gas magma and lithostatic pressures can occur below and above the magma fragmentation level causing the production of lithics directly when the wall collapses inward and indirectly when the wall collapses outward the effectiveness of different erosion mechanisms was tested with the magma characteristics conduit geometry and wall rock properties pertaining to the ad 79 eruption of vesuvius it was found that during the white and gray magma plinian eruption phases the lithics should had come from the deep as well as from the shallow regions of the conduit the conclusions from erosion modeling are also consistent with the limited field data whereby the gray magma phase deposits are associated with larger lithic content and larger proportion of deep limestone fragments

stress fields generating ring faults in volcanoes ring faults in volcanoes have been recognized for a long time but their mechanics of formation is still poorly understood while the subsidence on a ring fault during a large eruption from the associated chamber is easily understood the initiation of the fault itself has been difficult to explain for a ring fault to form the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface point above the center of the chamber empirical evidence however shows that during most periods of unrest in active volcanoes the stress field is not favorable for the initiation of ring faults we made several tens of boundary element models of magma chambers of various geometries and subject to different loading conditions the results indicate that overpressure or underpressure in the chamber as the only loading is unlikely to initiate ring faults for a spherical chamber subject to horizontal extension or doming and a sill like chamber subject to horizontal extension the tensile and shear stresses at the surface of the volcano peak at a certain radial distance from the surface point above the center of the chamber then however the maximum stresses occur at the boundary of the chamber itself which would normally lead to sheet injection rather than to ring fault formation by contrast a sill like magma chamber subject to doming gives rise to a stress field suitable for the initiation of a ring fault

caldera formation on santorini and the physiography of the islands in the late bronze age the caldera of santorini is a composite structure with a subsidence history extending over 100 ka or more geomorphological mapping shows that the present day caldera wall is a complex assemblage of cliff surfaces of different ages and that collapse at santorini has repeatedly exhumed earlier caldera cliffs and unconformities cliffs bounding the southern southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the minoan eruption in the late bronze age the well scalloped shape of these cliffs is attributed to large scale rotational landslip around the margins of the minoan caldera the deposit from one landslip is preserved subaerially minoan landslips in southeast santorini detached along the basement unconformity exposing a cliff of the prevolcanic island the caldera wall in the north northeast and east preserves evidence for three generations of cliff those of minoan age and two earlier generations of caldera wall the two early calderas can be dated relative to a well established statigraphy of lavas and tuffs the presence of in situ minoan tephra plastered onto the present day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the minoan eruption field relationships permit reconstruction of the physiography of bronze age santorini immediately before the minoan eruption the reconstruction differs from some previously published versions and is believed to be the most accurate to date bronze age sa ntorini had a large flooded caldera formed 21 ka ago this caldera must have acted as an excellent harbour for the bronze age inhabitants of the island the 3 6 ka minoan eruption deepened and widened the extant caldera the volume of minoan collapse 25 km3 is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of minoan ignimbrite ponded as intracaldera tuff

caldera development during the minoan eruption thira cyclades greece the well known caldera of thira santorini greece was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a ne trending line of vents before the minoan eruption of 1400 b c thira consisted of three lava shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half andesitic lavas formed the overlapping shields of the north and were contemporaneous with and in many places interbedded with the southern tuff deposits although there appears to be little difference between the composition of magmas erupted differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial producing lava flows whereas in the south eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs many of these tuffs are plastered onto the walls of what appears to have been an older caldera most probably associated with an eruption of rhyodiacitic tephra 100 000 years ago the minoan eruption of about 1400 b c had four distinct phases each reflecting a different vent geometry and eruption mechanism the minoan activity was preceded by minormore eruptions of fine ash 1 the eruption began with a plinian phase from subaerial vent s located on the easternmost of the lave shields 2 vent s grew toward the sw into the flooded depression subsequent activity deposited large scale base surge deposits during vent widening by phreatomagnetic activity 3 the third eruptive phase was also phreatomagmatic and produced 60 of the volume of the minoan tuff this activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding less

microbialite formation in seawater of increased alkalinity satonda crater lake indonesia abstract the crater lake of the small volcanic island satonda indonesia is unique for its red algal microbial reefs thriving in marine derived water of increased alkalinity the lake is a potential analogue for ancient oceans sustaining microbialites under open marine conditions current reef surfaces are dominated by living red algae covered by non calcified biofilms with scattered cyanobacteria and diatoms minor caco3 precipitates are restricted to the seasonally flooded reef tops which develop biofilms up to 500 m thick dominated by the cyanobacteria pleurocapsa calothrix phormidium and hyella microcrystalline aragonite patches form within the biofilm mucilage and fibrous aragonite cements grow in exopolymer poor spaces such as the inside of dead lysed green algal cells and reef framework voids cementation of lysed hadromerid sponge resting bodies results in the formation of wetheredella like structures hydrochemistry data and model calculations indicate that co2 degassing after seasonal mixis can shift the carbonate equilibrium to cause caco3 precipitation increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium therefore photosynthesis induced cyanobacterial calcification does not occur instead passive diffusion controlled eps mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place in contrast to extreme soda lakes the release of ca2 from aerobic degradation of extracellular polymeric substances does not support caco3 precipitation in satonda because the simultaneously released co2 is insufficiently buffered subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous unidentified mg si phase the microstromatolites probably formed when lake satonda evolved from seawater to ca2 depleted raised alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong co2 degassing the latter effect caused rapid growth of fibrous aragonite while mg si layers replaced the initially mg calcite impregnated biofilms this could be explained by dissolution of siliceous diatoms and sponge spicules at high ph followed by mg calcite dissolution and mg silica precipitation at low ph due to heterotrophic activity within the entombed biofilms

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

a numerical model for temporal variations during explosive central vent eruptions an axisymmetrical numerical model has been developed in order to find the temporal evolution of pressure the position of the exsolution level the velocity field the eruption rate and the amount of erupted material of a shallow volatile rich felsic magma chamber during a plinian central vent eruption the overpressure necessary to trigger the eruption is assumed to result from crystallization driven volatile oversaturation we solve the resulting set of equations using a finite element method the results obtained show that the pressure at the conduit entrance decreases exponentially as the eruption proceeds this produces a shifting of the exsolution level so that deeper parts of the chamber become progressively volatile oversaturated during the eruption we assess the influence of chamber geometry and the physical properties of the magma on the computed parameters using several numerical examples the results are also compared with those predicted by previous models from the literature and are found to be in good agreement with documented eruptions the model constitutes a first attempt to numerically model the dynamics and the temporal evolution of the most relevant physical parameters during withdrawal from a closed magma chamber

formation of caldera periphery faults an experimental study changing stresses in multi stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism s associated with caldera formation and the corresponding zones of structural weakness we evaluate characteristic structures resulting from doming chamber inflation evacuation collapse chamber deflation and cyclic resurgence inflation and deflation and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation the structural dome so produced is characterised bysteepening in the periphery whereas the broadening apex subsides pure evacuation causes the chamber roof to collapse along adjacent bell shaped reverse faults the distribution of concentric faults is influenced by the initial edifice morphology steep and irregular initial flanks result in a tilted or chaotic caldera floor the third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation following doming caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome the flanks of the edifice relax resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults the latter form locally uplifted and tiltedwedges half grabens that grade into horst and graben structures this superimposed fault pattern also extends inside the caldera we suggest that major pressure deviations in magma chamber s are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first order indication of the processes and mechanisms involved in caldera formation

volcanism and the dynamics of open magma chambers the repeated eruption of many volcanoes is now accounted for by events taking place in a subterranean magma chamber which is open in the sense that the inflow of material to the chamber from below leads to a magma pressure p which exceeds the lithostatic pressure pl by an amount greater than the tensile strength of the chamber walls before the contents of the magma chamber have reached thermodynamic and geochemical equilibrium this is the condition which must be satisfied when an eruption occurs we have now derived a relationship giving the critical volume of added material v required to cause an eruption as a function of magma compressibility and the volume by which the chamber expands

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

assessment of resolution and accuracy of the moving window cross spectral technique for monitoring crustal temporal variations using ambient seismic noise summary temporal variations in the elastic behaviour of the earth s crust can be monitored through the analysis of the earth s seismic response and its evolution with time this kind of analysis is particularly interesting when combined with the reconstruction of seismic green s functions from the cross correlation of ambient seismic noise which circumvents the limitations imposed by a dependence on the occurrence of seismic events in fact because seismic noise is recorded continuously and does not depend on earthquake sources these cross correlation functions can be considered analogously to records from continuously repeating doublet sources placed at each station and can be used to extract observations of variations in seismic velocities these variations however are typically very small of the order of 0 1 per cent such accuracy can be only achieved through the analysis of the full reconstructed waveforms including later scattered arrivals we focus on the method known as moving window cross spectral analysis that has the advantage of operating in the frequency domain where the bandwidth of coherent signal in the correlation function can be clearly defined we investigate the sensitivity of this method by applying it to microseismic noise cross correlations which have been perturbed by small synthetic velocity variations and which have been randomly contaminated we propose threshold signal to noise ratios above which these perturbations can be reliably observed such values are a proxy for cross correlation convergence and so can be used as a guideline when determining the length of microseismic noise records that are required before they can be used for monitoring with the moving window cross spectral technique

the fault nature of the ms 5 4 volcanic earthquake preceding the 1996 subglacial eruption of grimsvotn volcano iceland abstract the seismic activity preceding and accompanying the september 30 1996 subglacial eruption of the grimsvotn volcano iceland developed in three stages 1 the first stage first 19 h included the main shock of magnitude m s 5 4 and its aftershocks along the northern slope of the bardarbunga volcano situated about 20 km nw from grimsvotn 2 seismic foci during the next 17 h marked a line connecting two volcanoes bardarbunga and grimsvotn this stage culminated in the opening of a fissure and the beginning of eruption 3 the third stage was observed during the eruption midnight of september 30 october 7 and included continuous seismic activity along the northern slope of the bardarbunga volcano as well as more western distributed epicenters which together had clustered the ring fault epicentral zone a finite fault broadband teleseismic p waveform inversion was applied to the main shock of september 29 the inversion showed that the rupture had developed downdip from the hypocenter in about 3 s and the main asperity was ruptured at depths between 4 and 6 km a comparison with the deep distribution of aftershocks showed that their hypocenters were situated above the destroyed asperities at the depths between 0 and 4 km these space time seismic patterns and the results of the finite fault inversion may be interpreted in the following way there was a magma chamber under bardarbunga volcano covered by the strong rocks or asperities the main shock of september 29 broke the asperity and formed a path for magma transport then a fissure was formed connecting the opened magma chamber with the feeding structures of grimsvotn volcano and magma was injected laterally to the site of eruption the continuous seismic activity during the third period was related to the collapse of rocks around the released parts of the magma chamber the role of the m s 5 4 earthquake in this sequence was decisive

strike slip faulting during the 2014 barðarbunga holuhraun dike intrusion central iceland over a 13 day period magma propagated laterally from the subglacial bar arbunga volcano in the northern rift zone iceland it created 30 000 earthquakes at 5 7 km depth along a 48 km path before erupting on 29 august 2014 the seismicity which tracked the dike propagation advanced in short bursts at 0 3 4 7 km h separated by pauses of up to 81 h during each surge forward seismicity behind the dike tip dropped moment tensor solutions from the leading edge show exclusively left lateral strike slip faulting subparallel to the advancing dike tip releasing accumulated strain deficit in the brittle layer of the rift zone behind the leading edge both left and right lateral strike slip earthquakes are observed the lack of non double couple earthquakes implies that the dike opening was aseismic

monitoring volcanic and geothermal areas by full seismic moment tensor inversion are non double couple components always artefacts of modelling non double couple mechanisms ndc in volcanic environments are good indicators of local modifications of stress induced by dike injection high fluid pressure movements or by thermal cooling nevertheless the possible biasing effects of wave propagation in structurally complicated regions the presence of noise or a not appropriate station coverage can make the identification of ndc as true source phenomena uncertain in this paper we illustrate possible sources of fake ndc and we demonstrate that by the error analysis that we apply it is possible to discriminate spurious artifacts and physically meaningful solutions tests with synthetic seismograms and the analysis of observed ones in volcanic and geothermal areas are thoroughly discussed

the impact of low ice cover on shoreline recession a case study from western point pelee canada abstract the western shore of point pelee national park ppnp in ontario canada exhibited high recession rates in the winter of 2005 2006 during the period 1959 2004 the western side of the park generally exhibited accretion attributed to longer duration of ice coverage during winter and favorable topography and sedimentary conditions in comparison to the eastern side compared to the winter of 2003 2004 changes in lake erie water levels winter wind regime and human made structures and interferences were negligible during 2004 2006 within the colchester to southeast shoal littoral cell in western lake erie where ppnp is located however the winter of 2005 2006 exhibited overall low ice cover conditions unlike the typical freeze up conditions in western lake erie the 2006 high recession rates on the western shore of point pelee are attributed to the low ice cover conditions in the winter of 2005 2006 while less than average ice cover does not in its own cause erosion in certain areas it increases the opportunity for sediment removal by waves by removing a significant source of protection during winter the episode illustrates the potential impacts of ice free and low ice cover winters on the western shore of point pelee and shores under similar conditions future climate change patterns are expected to result in shorter ice durations and more frequent ice free winters in the western basin of lake erie exposing western ppnp to wave action during the winter season and resulting in increased erosion the increased opportunity for shore erosion will negatively impact ecologically and economically viable spaces in ppnp

dolomite formation within microbial mats in the coastal sabkha of abu dhabi united arab emirates microbial mediation is the only demonstrated mechanism to precipitate dolomite under earth surface conditions a link between microbial activity and dolomite formation in the sabkha of abu dhabi has until now not been evaluated even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences such an evaluation is the purpose of this study which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of abu dhabi along a transect from the intertidal to the supratidal zone this investigation revealed a close association between microbial mats and dolomite suggesting that microbes are involved in the mineralization process observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats in current models microbial dolomite precipitation is linked to an active microbial activity that sustains high ph and alkalinity and decreased sulphate concentrations in pore waters such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone by contrast these models cannot be applied to the supratidal zone where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity as no abiotic mechanism is known to form dolomite at earth surface conditions two different hypotheses can reconcile this result in a first scenario all of the dolomite present in the supratidal zone formed in the past when the mats were active at the surface in a second scenario dolomite formation continues within the buried and inactive mats in order to explain dolomite formation in the absence of active microbial metabolisms a revised microbial model is proposed in which the mineral template properties of exopolymeric substances play a crucial role

structure of the grimsvotn central volcano under the vatnajokull icecap iceland the subglacial grimsvotn central volcano lying within a volcanic zone directly above the core of the iceland mantle plume is one of the most active in iceland local regional and teleseismic earthquake data recorded on a temporary seismometer array across western vatnajokull icecap during the summer of 1998 have provided a three dimensional image of the shallow crustal structure of the volcano microearthquake activity at depths of 1 4 km along the grimsvotn caldera rim coincided with inflation of a shallow magma chamber beneath the caldera which culminated in a 0 1 km3 eruption in december 1998 tomographic inversion of these earthquakes define the extent of a low velocity body beneath grimsvotn with a volume of 20 km3 extending to 3 km below the surface this low velocity body is flanked by high velocities under the caldera rim delays in the p wave arrival times through the grimsvotn caldera from regional and teleseismic earthquakes and from two detonations 150 km east of grimsvotn are 0 10 0 15 s greater than the delays through the uppermost 3 4 km of crust shown by local earthquake arrivals this suggests the presence of a further low velocity body at depths greater than 3 4 km beneath grimsvotn presumed to be due to the presence of melt using the distribution of local seismicity and shear wave attenuation we estimate the maximum lateral extent of the region containing partial melt to be 7 8 km e w and 4 5 km n s p wave delays require a thickness of less than 1 km of pure high percentage partial melt assuming a sill like magma chamber

dike emplacement at bardarbunga iceland induces unusual stress changes caldera deformation and earthquakes a 45 km long regional dike was emplaced over a period of 2 weeks in august 2014 at the boundary between the east and north volcanic zones in iceland this is the first regional dike emplacement in iceland monitored with modern geophysical networks the importance of which relates to regional dikes feeding most of the large fissure e g eldgja 934 and laki 1783 and lava shield e g early holocene skjaldbreidur and trolladyngja eruptions during this time the dike generated some 17 000 earthquakes more than produced in iceland as a whole over a normal year the dike initiated close to the bardarbunga volcano but gradually extended to the northeast until it crossed the boundary between the east volcanic zone evz and the north volcanic zone nvz we infer that the strike of the dike changes abruptly at a point from about n45 e coinciding with the trend of the evz to n15 e coinciding with the trend of the nvz this change in strike occurs at latitude 64 7 exactly the same latitude at which about 10 ma dikes in east iceland change strike in a similar way this suggests that the change in the regional stress field from the southern to the northern part of iceland has been maintained at this latitude for 10 million years analytical and numerical models indicate that the dike induced stress field results in stress concentration around faults and particularly shallow magma chambers and calderas in its vicinity such as tungnafellsjokull kverkfjoll and askja in particular the dike has induced high compressive shear and tensile stresses at the location of the bardarbunga shallow chamber and caldera ring fault where numerous earthquakes occurred during the dike emplacement many of which have exceeded m5 the largest m5 7 the first segment of the dike induced high tensile stresses in the nearby part of the bardarbunga magma chamber ring fault resulting in radially outward injection of a dike from the chamber at a high angle to the strike of the regional dike the location of maximum stress at bardarbunga fluctuates along the chamber ring fault boundary in harmony with dike size and or pressure changes and encourages ring dike formation and associated magma flow within the chamber caldera collapse and or eruption in some of these volcanoes is possible most likely in bardarbunga but depends largely on the future development of the regional dike

subsurface sediment profiles below point pelee indicators of postglacial evolution in western lake erie an extensive drilling program undertaken along the western barrier bar at point pelee national park ontario canada yielded considerable subsurface sediment data relevant to the nature and lateral geometry of sedimentary units below the point pelee foreland four major sedimentary units were identified a basal clay rich till a fine grained glaciolacustrine sand a medium grained sand unit subdivided into a poorly sorted shoreface sand and an aeolian dune sand derived from the shoreface sand and an organic marsh gyttja deposit the present study confirms the existence of a planar wave eroded till surface below the southern portion of point pelee at an elevation of approximately 164 m asl following this low water period in the basin lake levels rose abruptly to an elevation several metres above 172 m asl this resulted in erosion of the upper part of the glaciolacustrine sand during a later period of stable higher lake levels perhaps coinciding with the nipissing flood event about 4000 bp this resulted in a planar surface at approximately 169 5 m asl several radiocarbon dates on basal gyttja from the marsh averaging 3200 bp reflect a subsequent drop in levels to about 2n3 m below present levels though undated the initiation of shoreface and dune sand deposition is roughly coeval with the basal marsh deposits

compositional layers in the zoned magma chamber of the grizzly peak tuff the 34 ma grizzly peak tuff in west central colorado preserves evidence for distinct compositional layers in a high level magma chamber intracaldera tuff consists of a single cooling unit at least 2 7 km thick zoned from a high silica rhyolite base to an eroded low silica rhyolite top two horizons of heterogeneous tuff beneath wedges of caldera collapse breccia contain fiamme of dacite to mafic latite along with the rhyolitic fiamme that make up the rest of the tuff compositional zoning defined by fiamme is a step function rather than a continuous gradient seven petrographic groups of fiamme each have distinct compositions separated by compositional gaps as the same clusters and gaps are found in collections of fiamme from widely separated stratigraphic levels step function zoning must be intrinsic to the chamber rather than a consequence of the tapping process we interpret the seven fiamme groups as seven separately converting layers in a density stratified magma reservoir tapped by eruption of the grizzly peak tuff excluding the thin heterogeneous tuff horizons 3 km of caldera fill is drawn from the three most silicic layers suggesting that the three rhyolitic layers were each at least 1 km thick in the magma chamber

insights into the kinematics of a volcanic caldera drop probabilistic finite source inversion of the 1996 bardarbunga iceland earthquake article i nfo the accurate modelling and prediction of volcanic eruptions depends critically on information concerning the interaction between the caldera and the underlying magma chamber knowledge concerning the kinematics of a caldera in the course of an eruption is therefore essential here we provide detailed seismological constraints on the kinematics of a volcanic caldera drop and the geometry of a caldera ring fault for this we performed a finite source inversion of the september 29 1996 bardarbunga iceland earthquake that was caused by caldera subsidence our methodology is based on spectral element simulations of seismic wave propagation through a realistic model of the icelandic crust and upper mantle a particularly robust feature is the initiation of the rupture in the north western part of the ring fault that is about 10 km in diameter from there it spread to the other fault segments within about 3 s without invoking super shear propagation sensu stricto we can explain this unusually fast rupture propagation by the triggering of fault segments through p waves that propagated across the caldera our results favour outward dipping fault segments in the western half of the ring fault while the eastern half is preferentially inward dipping this variability may reflect structural heterogeneities or an irregular magma chamber geometry the individual segments of the caldera ring fault radiated approximately equal amounts of energy this indicates that the caldera dropped coherently as one single block the work presented here is intended to aid in the design of realistic models of magma chamber and caldera dynamics

carbonate laminae recorded in a siliciclastic tidal flat colonized by microbial mats abstract microbial mats in siliciclastic coastal environments are considered as non lithifying systems that lack the potential for calcification this work documents precipitation and preservation of well defined laterally continuous carbonate laminae in low relief microbial mats from a siliciclastic supratidal flat in northern patagonia paso seco argentina petrographic epifluorescence and sem eds studies of surficial and buried microbial mats show that they are composed of repeated sediment sequences comprised of four different types of laminae which are from base to top a a sand and silt lamina b a lamina largely composed of organic matter displaying moulds of subvertical cyanobacteria filaments c a lamina composed of dense micritic carbonate which is cut by moulds of vertical cyanobacteria filaments and d a lamina composed of organic matter containing abundant horizontal cyanobacteria filaments the formation of each different lamina is strongly controlled by the environmental conditions characterized by episodic seawater flooding followed by several days to weeks in which water remains covering the sediment and gradually evaporates producing a salinity increase and the precipitation of calcite gypsum and halite thus the basal sand and silt lamina forms as the result of the transport of siliciclastic grains and particles during seawater flooding the overlying lamina b composed of organic matter with moulds of subvertical cyanobacteria filaments is formed when calm water conditions are recovered and organic material is produced by photosynthetic microbial activity the following lamina c composed of dense micritic carbonate precipitates when the salinity of stagnant water reaches caco3 supersaturation finally the uppermost lamina d composed of organic matter with horizontal cyanobacteria filaments is developed while the sediment surface is drying and gypsum and halite precipitate although these minerals are not preserved in the sediment because they dissolve during subsequent inundations all these observations show that well developed laterally continuous carbonate laminae may be formed and preserved in a siliciclastic tidal environment if biotic and abiotic sedimentary processes closely interact to create the required conditions the studied microbial mats increase sediment impermeability which favours water retention in the flat and thus subsequent evaporation of the retained seawater moreover microbial cells and eps extracellular polymeric substance suspended in seawater might act as nuclei for caco3 precipitation which will later settle down on the microbial mat in addition eps of the microbial mats may also serve as nucleus for in situ carbonate precipitation