stratigraphy and dispersal of the mangaone subgroup pyroclastic deposits okataina volcanic centre new zealand abstract the mangaone subgroup from the okataina volcanic centre consists of silicic plinian pyroclastic units that lie between the oruanui deposit from the taupo volcanic centre corrected 14 c calendar age 26 5 ka and the rotoiti flow rotoehu fall deposits from okataina volcanic centre k ar age 64 ka in this study we present a new stratigraphy for the subgroup that is significantly revised from the earlier work and was developed after extensive field mapping and laboratory study we now recognize and describe a total of 12 rhyolitic units as opposed to the 8 that had been recognized previously with a total bulk eruptive volume of approximately 81 km 3 the vents for all the mangaone subgroup units were within the haroharo caldera and possibly migrated through time first within a roughly north south trending zone in the central part of the caldera and then toward the eastern part of the caldera we present new radiometric ages suggesting that the whole sequence was erupted in a relatively short time span possibly 10 15 ka our findings of more eruptions in a shorter time span than in previous work underscores the utility of detailed field studies particularly when volcanic hazards are being assessed all the fall units have plinian dispersals but show various degrees of magma water interaction the majority of the eruptions were large and intense enough to generate pyroclastic flows yet flows were observed in only 2 units and is volumetrically small

recent geologic history of lake atitlan a caldera lake in western guatemala abstract lake atitlan a caldera lake in western guatemala was investigated for evidence of recent volcanic and tectonic activity no vents faults or folds are apparent on high resolution seismic reflection profiles of lake sediment representing at least 17 500 years and probably more than 35 000 years of deposition three post caldera stratovolcanoes san pedro toliman and atitlan have grown in southern parts of the caldera and two deltas have grown from the north shore of the lake elsewhere the caldera is occupied by lake atitlan which is more than 300 m deep and has a relatively flat floor refraction profiles suggest that the original floor of the caldera lies ca 300 m below the current lake floor but prodigious amounts of methane gas in the lake sediment attenuated seismic signals and prevented any detailed view of the original caldera floor or faults along which the floor is presumed to have collapsed piston cores from deep basins of lake atitlan record ca 2 000 years of unusually rapid sedimentation ca 0 5 cm yr only one thin silicic ash layer was penetrated and it is probably from a distant source cores contain evidence of mayan disturbance of the environment around the lake and to a lesser degree of the lake itself they might also record episodes of increased thermal activity each lasting several decades heat flow measurements inside and just outside the caldera are high 290 and 230 mw m 2 suggesting hydrothermal convection and a shallow heat source high heat flow a geological record of post caldera silicic eruptions and unexplained fluctuations of lake level episodic tumescence ofthe lake floor suggest that magma remains beneath lake atitlan and that future eruptions are possible

mesoscale analysis of the activation of a cold front during cyclogenesis summary the archetypal mesoscale structure of an ana cold front comprises a narrow cold frontal rainband ncfr associated with line convection embedded within a wide cold frontal rainband wcfr due to the slantwise ascent associated with a strong transverse front mesoscale circulation whereas most mesoscale observational studies have concentrated on this mature stage the present paper explores by means of a detailed case study how such a mature frontal system develops from pre existing features the study is based upon the synthesis of data spanning many scales in the form of satellite imagery observations from conventional and multi parameter doppler radar and surface reports together with diagnostics from an operational mesoscale forecast model the cold frontal zone in this case is characterized by multiple cold fronts the dominant second cold front is studied in detail from the point of view of its evolution and its relationship to neighbouring fronts especially the leading front associated with the initial polar front cloud band the dominant cold front started as a shallow rope cloud in the cold sector and at a fairly early stage it produced a ncfr due to line convection then over a period of only a few hours a dry intrusion and an associated strong direct transverse front circulation led to the formation of a canopy of mid level cloud and a wcfr the wcfr was attributable not only to the rearward ascending outflow from the line convection but also to the re invigoration of ascent within the initial polar front cloud band that the line convection was undercutting although the transverse circulation was probably intensified by precipitation processes it is inferred that it was mainly a manifestation of the larger scale environmental flow that existed independently of the major precipitation features

chronology and dynamics of a large silicic magmatic system central taupo volcanic zone new zealand the central taupo volcanic zone in new zealand is a region of intense quaternary silicic volcanism accompanying rapid extension of continental crust at least 34 caldera forming ignimbrite eruptions have produced a complex sequence of relatively short lived nested and or overlapping volcanic centers over 1 6 m y silicic volcanism at taupo is similar to the yellowstone system in size longevity thermal flux and magma output rate however taupo contrasts with yellowstone in the exceptionally high frequency but small size of caldera forming eruptions this contrast reflects the thin rifted nature of the crust which precludes the development of long term magmatic cycles at taupo 11 refs 4 figs 1 tab

the last glacial maximum in central and southern north island new zealand a paleoenvironmental reconstruction using the kawakawa tephra formation as a chronostratigraphic marker abstract kawakawa tephra formation comprising oruanui ignimbrite flow member and aokautere ash airfall member represents the products of an exceptionally large and widespread volcanic eruption from taupo volcanic centre in the north island of new zealand the eruption occurred during the last glacial maximum and is radiocarbon dated at c 22 6 ka b p thermoluminescence ages are in broad agreement with the radiocarbon age the presence of aokautere ash in loess deposits in alluvial gully fills on river terraces and its absence from unstable sites permits a detailed assessment of geomorphic activity during the last glacial maximum widespread erosion of regolith aggradation of river valleys and deposition of loess particularly in the period following eruption of the kawakawa point to a cold dry variable climate a collation of pollen data for sediments containing aokautere ash and those 14c dated in the range 17 23 ka shows that tall forest was highly restricted in the central and southern parts of the north island an apparently subalpine grassland shrubland was present at sites from present sea level to over 800 m elevation suggesting that factors other than lower temperatures such as exposure to wind and frost fire and reduced rainfall were important in controlling vegetation patterns we conclude that the interval 23 13 ka b p broadly equivalent to oxygen isotope stage 2 represents the period of greatest environmental change in the north island

incipient granular mass flows at the base of sediment laden floods and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands extraordinary floods commonly produce thick deposits as a result of the entrainment and transport often at high concentrations of large volumes of clastic detritus this paper describes the sedimentology of a stratified bouldery sand deposit generated by a 20 km3 breakout flood from intracaldera lake taupo following the 1 8 ka taupo eruption the proximal deposit is characterised by a strongly bimodal grain size distribution with downstream fining boulders enclosed in a poorly sorted and weakly bedded coarse sand to gravel matrix this combination of characteristics is inferred to reflect simultaneous deposition of small and large particles by two distinct mechanisms i accretion of the sandy matrix from an incipient basal granular mass flow as the flood lost capacity and ii progressive emplacement of the boulders as the flood lost the competence to roll them along the channel floor the first mechanism is a product of a nonlinear particle concentration gradient in the sediment laden flow which caused it to segregate into a basal incipient granular mass flow that formed the transient depositional system indicated by the lithofacies the second mechanism is a function of the superjacent turbulent turbid flow that acted as the primary transporting system this pairing of transport and depositional systems is likely to characterise many sediment laden aqueous flows with clay mineral concentrations too low to develop non newtonian rheologies such as jokulhlaups and lahars that entrain unweathered pyroclastic material qualitative and quantitative differences in the way bedload is transported in the presence of this basal granular mass flow as compared to simple water make impossible accurate estimates of flow competence based on shields entrainment functions for beds of both uniform and mixed grain size distribution

the formation and failure of natural dams of the numerous kinds of dams that form by natural processes dams formed from landslides glacial ice and late neoglacial moraines present the greatest threat to people and property landslide dams form in a wide range of physiographic settings the most common types of mass movements that form landslide dams are rock and debris avalanches rock and soil slumps and slides and mud debris and earth flows the most common initiation mechanisms for dam forming landslides are excessive rainfall and snowmelt and earthquakes landslide dams can be classified into six categories based on their relation with the valley floor type i dams 11 of 184 landslide dams from around the world that we were able to classify do not reach from one valley side to the other type ii dams 44 span the entire valley floor in some cases depositing material high on opposite valley sides type iii dams 41 move considerable distances both upstream and downstream from the landslide failure type iv dams 1 are rare and involve the contemporaneous failure of material from both sides of a valley type v dams 1 also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river type vi dams 3 involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side many landslide dams fail shortly after formation in our sample of 73 documented landslide dam failures 27 of the landslide dams failed less than 1 day after formation and about 50 failed within 10 days over topping is by far the most common cause of failure the timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry material characteristics of the blockage rate of inflow to the impoundment size and depth of the impoundment bedrock control of flow and engineering controls such as artificial spill ways diversions tunnels and planned breaching by blasting or conventional excavation glacial ice dams can produce at least nine kinds of ice dammed lakes the most dangerous are lakes formed in main valleys dammed by tributary glaciers failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam cold polar ice dams generally drain supraglacially or marginally by downmelting of an outlet channel warmer temperate ice dams tend to fail by sudden englacial or subglacial breaching and drainage late neoglacial moraine dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries these late neoglacial dams pose hazards because 1 they are sufficiently young that vegetation has not stabilized their slopes 2 many dam faces are steeper than the angle of repose 3 these dams and lakes are immediately downslope from steep crevassed glaciers and near vertical rock slopes and 4 downstream from these dams are steep canyons with easily erodible materials that can be incorporated in the flow and increase flood peaks the most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake generated by icefalls rockfalls or snow or rock avalanches melting of ice cores or frozen ground and piping and seepage are other possible failure mechanisms natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam although data are few for the same potential energy at the dam site ownstream flood peaks from the failure of glacier ice dams are smaller than those from landslide moraine and structed earth fill and rock fill dam failures moraine dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than 1011 1012 joules differences in flood peaks natural dam failures appear to be controlled by dam characteristics and failure mechanisms

paleohydrology and sedimentology of a post 1 8 ka breakout flood from intracaldera lake taupo north island new zealand sudden releases of impounded water from lakes in volcanic regions constitute a major and frequently repeated hazard an outburst flood from taupo caldera new zealand released 20 km 3 of water within decades following an ignimbrite emplacing eruption ca 1 8 ka paleohydrologic reconstruction of the taupo flood provides estimates of peak discharge at the outlet in the range 17 000 35 000 m 3 s dimensionless analysis demonstrates that 1 failure of the barrier was essentially instantaneous 2 the event may be treated hydraulically as a dam break and 3 the peak discharge was a function of outlet geometry rather than lake volume or breach formation rate paleohydraulic reconstructions based on empirical relations derived from historic dam breaches yield only order of magnitude estimates of peak discharge calculations based on the physical dimensions of the outlet channel and hydraulic principles are likely to be more accurate and are in close agreement with computer implemented dynamic flow routing models the latter give peak discharges and maximum stage levels similar to constraints imposed by field evidence and estimates of flow depth and velocity the long duration of the taupo flood and the relatively narrow confined flood route resulted in minimal attenuation of the flood wave compared with modern dam breach events and flood deposits can be traced as far as 232 km downstream caldera lake breakout floods may be among the most far reaching hazards associated with volcanism

phase equilibria of a fluorine rich leucogranite from the st austell pluton cornwall highly evolved leucogranitic rocks in the st austell pluton cornwall of hercynian age contain accessory muscovite topaz and fluorite the authors have studied the h sub 2 o saturated melting behavior of one representative sample its solidus and liquidus pass through the points 663 and 725 sup 0 c respectively at 1 kbar 640 and 665 sup 0 c at 2 kbar 610 and 717 sup 0 c at 4 kbar and 608 and 700 sup 0 c at 8 kbar plagioclase is on the liquidus at low pressure and topaz is on the liquidus at 4 kbar the fluorite is consumed in the formation of the first formed liquid calcium can partition into an evolved granitic melt if complexed by fluorine the fluorite appears to be largely primary in fresh fluorite granite at st austell and not to reflect the albitization of oligoclase in the surrounding biotite granite such fluorine rich leucogranites can be expected to be of subsolvus character

liquidus equilibria in the system k2o na2o al2o3 sio2 f2o 1 h2o to 100 mpa ii differentiation paths of fluorosilicic magmas in hydrous systems we investigated phase equilibria in the six component system na2o k2o al2o3 sio2 f2o 1 h2o at 100mpa to characterize differentiation paths of natural fluorine bearing granitic and rhyolitic magmas topaz and cryolite are stable saturating solid phases in calcium poor systems at 100mpa the maximum solidus depression and fluorine solubility in evolving silicic melts are controlled by the eutectics haplogranite cryolite h2o at 6408c and 4 wt f and haplogranite topaz h2o at 6408c and 2 wt f topaz and cryolite form a binary peralkaline eutectic at 6608c 100mpa and fluid saturation the low temperature nature of this invariant point causes displacement of multiphase eutectics with quartz and alkali feldspar towards the topaz cryolite join and enables the silicate liquidus and cotectic surfaces to extend to very high fluorine concentrations more than 30 wt f for weakly peraluminous and subaluminous compositions the differentiation of fluorine bearing magmas follows two distinct paths of fluorine behavior depending on whether additional minerals buffer the alkali alumina ratio in the melt in systems with micas or aluminosilicates that buffer the activity of alumina magmatic crystallization will reach either topaz or cryolite saturation and the system solidifies at low fluorine concentration in leucogranitic suites precipitating quartz and feldspar only the liquid line of descent will reach topaz or cryolite but fluorine will continue to increase until the quaternary eutectic with two fluorine bearing solid phases is reached at 5408c 100mpa and aqueous fluid saturation the maximum water solubility in the haplogranitic melts increases with the fluorine content and reaches 12 5 0 5 wt h2oat the quartz cryolite topaz eutectic composition a continuous transition between hydrous fluorosilicate melts and solute rich aqueous fluids is not documented by this study our experimental results are applicable to leucocratic fluorosilicic magmas in multicomponent systems however the presence of calcium may severely limit enrichment of fluorine by crystallization of fluorite

toward a comprehensive upper quaternary tephra and ignimbrite stratigraphy in new zealand using electron microprobe analysis of glass shards abstract new zealand quaternary marine and terrestrial sequences contain numerous tephras or volcanic ash horizons that are the distal correlatives of voluminous welded ignimbrite sheets erupted from central north island electron microprobe analyses of glass shards from the distal tephras demonstrate their homogeneity and are shown to identify each tephra examined by matching tephras from stratigraphically controlled sequences the first comprehensive tephra stratigraphy spanning from 50 000 to 700 000 yr ago and covering the new zealand region is advanced analyses on glass shards from the unwelded base of ignimbrite sheets are comparable to the distal tephra analyses and allow correlation between ignimbrites and to the distal tephras the better exposed tephra record constrains the number of separate eruptive events and the stratigraphy of the ignimbrites both of which were previously confused by lack of outcrop samples from pumiceous marine sediments were found to contain two or more chemically distinct populations of glass the pumice is in cross bedded sands or sand lenses within conglomerate attesting to a shallow high energy environment where reworking could occur however each glass population could be matched to older known tephras

tungsten solubility in evolved granitic melts an evaluation of magmatic wolframite abstract a variety of parameters that potentially control the solubilities of synthetic hubnerite mnwo4 and ferberite fewo4 in haplogranitic melts rich in fluxing compounds have been determined at 200 mpa in order to test the hypothesis that wolframite can occur as a magmatic mineral the melts considered representative of highly evolved pegmatites contain 1 1 1 7 and 2 02 wt of li2o p2o5 and b2o3 respectively although the molar al na k ratio is one if li is considered to be an alkali element the molar al na k li asili of the melt is 0 88 and the melt is peralkaline hubnerite and ferberite solubilities at 800 c are strongly controlled by melt composition they are much lower in subaluminous melt asili 1 05 than in peralkaline melt by contrast hubnerite and ferberite solubilities are nearly independent of the fluorine content of the melt for up to 8 wt f hubnerite and ferberite solubilities at 800 c are also nearly independent of oxygen fugacity over a log f o 2 range of approximately ni nio 3 to ni nio 2 which implies that the predominant oxidation state of tungsten in the melts is 6 even at moderately reduced conditions a series of experiments on haplogranitic melt rich in fluxing compounds with 0 and 6 wt f at 850 650 c shows that temperature strongly influences hubnerite solubility e g the solubility product of hubnerite k sp hub in the melts with 6 wt f decreases from 71 0 7 9 10 4 mol2 kg2 at 850 c to 4 4 2 3 10 4 mol2 kg2 at 650 c our experimental results show that ferberite solubilities are much higher than those of hubnerite with the same composition of melt but the mn fe and w concentrations in natural melt inclusions indicate that these melts were undersaturated with wolframite at 800 c however flux rich melts crystallize at lower temperatures in nature and the occurrence of natural magmatic wolframite was evaluated by comparing experimental solubility product values extrapolated to 500 c with wolframite activities at these temperatures magmatic wolframite is unlikely in flux rich peralkaline melts but by contrast flux rich subaluminous to peraluminous melt inclusions appear to have been saturated with wolframite at approximately 500 c because of the strong temperature dependence of wolframite solubility the dominant effect of fluxing compounds is to lower the solidus temperature of the melt

the saturation behaviour of pumice and some sedimentological implications abstract caldera forming explosive eruptions from rhyolitic volcanoes typically produce huge quantities of pyroclastic material blanketing the landscape and triggering long term voluminous and widespread sedimentary responses pumice is a major component of such eruptions but its behaviour in the sedimentary environment is little understood its most notable feature is its very low density which frequently enables pumice to float on water for long periods of time before eventually waterlogging enough to sink once fully saturated pumice behaves in a broadly similar way to normal clastic material in terms of its hydrodynamic behaviour modified by its lower density however relatively little is known about its earlier behaviour in the sedimentary environment and in particular the manner and rate by which initially dry pumice absorbs water in this paper we develop a mathematical model which predicts that the time required for a pumice clast to saturate is proportional to the square of its radius tests on samples of real pumice accord with predicted trends and match lacustrine pumiceous facies observed in the field the ability to predict the time required for large pumice clasts to sink may provide a tool for estimating the timescales over which certain rapid post eruptive sedimentary processes operate

pliocene subaqueous fans and gilbert type deltas in maar crater lakes hopi buttes navajo nation arizona usa maar craters of the mio pliocene hopi buttes volcanic field of arizona formed within a broad playa system and accumulated a variety of lacustrine sedimentary deposits many craters initially held isolated groundwater fed lakes ephemeral streams crossing the playa entered some of the lake filled craters and built coarse grained gilbert type deltas and subaqueous fans along the margins of these craters the small coarse grained fans and deltas have many features in common with much larger coarse grained deltaic and fan deltaic deposits however the local production of coarse grained volcanic sediment low gradients in the local stream catchment steep subaqueous relief and the small size of the receiving basins resulted in a unique combination of features cone shaped subaqueous fans initially formed at the mouths of incised feeder streams the fans are small accumulations of steeply dipping gravelly tephra that consist almost entirely of overlapping lobes constructed by density modified grain flows gravelly gilbert type tephra deltas formed in brimfull craters fed by a freely migrating feeder stream they are concave lakeward mimicking the underlying crater wall topography complex deltaic geometries are defined by topset strata that steeply onlap tall foreset beds they suggest that feeding stream floods caused rapid and comparatively large variations in lake level within the small crater lakes bed specific carbonate alteration is common and probably resulted from both influx of detrital carbonate across the playa and alteration of tephra beds by carbonate saturated lakewaters during between flood periods of high net evaporation

high precision analysis of hydrous rhyolitic glass inclusions in quartz phenocrysts using the electron microprobe and inaa a high precision electron microprobe emp technique has been developed that is capable of analyzing major minor and trace element abundances si ti ai fe mn mg ca na k cl in hydrous rhyolitic glasses the technique was developed to characterize the chemical compositions of rhyolitic glass inclusions in phenocrysts that occur in layers of paleozoic altered volcanic ash the compositions of these inclusions serve as excellent chemical fingerprints of the altered volcanic ash layers for use in stratigraphic correlation the precision and reproducibility of the analyses is sufficientnot only to distinguish one altered volcanic ash layer from another on the basis of inclusion compositions but also to discern differencesin the compositions of different inclusions from the same layer a high precision instrumental neutron activation analytical inaa technique was also developed that is capable of measuring an additional suite oftrace elements e g sc co rb cs sr ba la ce sm eu tb yb lu zr hf ta in rhyolitic glass inclusions in quartz phenocrysts with excellent accuracy and precision after correcting for the presence of the host quartz the abundances of elements measured by the emp technique and the corrected abundances determined using the inaa technique are identical within analytical uncertainty thus demonstrating the internal consistency of the results

life cycle of a geyser discharge apron evidence from waikite geyser whakarewarewa geothermal area north island new zealand abstract waikite a geyser located in the whakarewarewa geothermal area on the north island new zealand has a history of eruptive dormancy cycles that have been attributed to natural and anthropogenic causes the last cycle involved an active period from 1932 to 1968 69 that was followed by a period of dormancy that continues today such cycles are important because they control the temporal development of the discharge apron when the geyser is active growth of the discharge apron is dictated by the precipitation of opal a which is controlled by factors such as discharge patterns water chemistry ph temperature rate of cooling and the resident microbiota with dormancy conditions change radically because water no longer flows down the discharge apron instead the discharge apron lapses into a phase of degradation that on waikite is evident from 1 deflation of the apron surface 2 blocks splaying off the apron margins along margin parallel fractures 3 tension fractures 4 saucer shaped collapse zones 5 increasingly unstable surfaces resulting from subsurface opal a dissolution 6 fractures from which steam and other gases emanate and 7 incursion of native vegetation around the edge of the apron and on the distal parts of the discharge apron when the geyser becomes active again silica precipitation will resume and the discharge apron will once again accrete vertically and expand laterally analysis of the waikite system shows that successions that develop on geyser discharge aprons are formed of unconformity bounded packages of sinter that reflect the eruptive dormancy history of the parent geyser

constraints on the origin of zonation of the granite complexes in the fichtelgebirge germany and czech republic evidence from a gravity and geochemical study compositional zoning of granitic plutons is a common feature but the spatial variation of zoning is rarely studied in detail it is suggested that important additional information can emerge if the compositional zoning is regarded with relation to the root zones i e with the granite shape at depth a combined geochemical and gravity investigation was performed on the hercynian fichtelgebirge granites which are composed of an older intrusive complex oic and younger intrusive complex yic the oic and yic are distinct from each other with respect to magma origin differentiation pattern shape of the granite complexes at depth and the zonation pattern in relation to the root zone the oic shows normal compositional zoning whereas the yic both reverse yic 1 and normal zoning yic 2 the general zonation pattern of the oic and the yic is the result of a combination of multiple injections of single magma batches and in situ differentiation during magma emplacement in addition to the general zonation pattern local zonation in individual granite bodies of the yic was caused by sidewall crystallisation producing a less differentiated marginal facies g2 and a more differentiated core facies g3 and g4 it is suggested that the complex zonation patterns of the fichtelgebirge granite complexes reflect interactions between the rate of magma output which is partly governed by their rheological properties and the rate of deformation which provides the room for the magma to be emplaced

melt inclusions in quartz from an evolved peraluminous pegmatite geochemical evidence for strong tin enrichment in fluorine rich and phosphorus rich residual liquids abstract we have investigated the magmatic evolution of a late stage f and p rich pegmatite forming aluminosilicate liquid and the geochemical controls on magmatic mineralizing processes by remelting totally crystallized melt inclusions in quartz and analyzing the quenched glass by epma and sims the quartz phenocrysts were sampled from a pegmatite that occurs in a variscan granite genetically associated with cassiterite and wolframite mineralized greisen veins at the ehrenfriedersdorf sn w deposit central erzgebirge se germany the melt inclusion compositions imply that the pegmatite forming liquid achieved extreme levels of chemical differentiation it contained high abundances of sn f p li rb cs nb ta and be and abnormally low concentrations of ca y sr and ree for a granite and it was strongly peraluminous the molar al 2 o 3 cao na 2 o k 2 o ranged from 1 3 to 2 0 fractions of the pegmatite forming liquid were extremely enriched in p 2 o 5 f al 2 o 3 and the molar abundances of f p in the glasses correlate strongly with moles of network modifying al ions implying that the bulk liquid included f p and al bearing complexes formation of these complexes reduced the activities of f p and al in bulk liquid suppressed the crystallization of magmatic topaz and p rich minerals and allowed the liquid to become enriched in these constituents some fractions of the ehrenfriedersdorf aluminosilicate liquid contained 1000 2000 ppm sn these levels of sn enrichment were up to 2 orders of magnitude greater than that ever reported for nonmineralized metaluminous and peraluminous igneous materials and are consistent with some experimentally derived sn solubilities in cassiterite saturated granitic liquids at geologically relevant pressures and temperatures this concordance implies that cassiterite could have crystallized directly from this highly evolved p and f rich peraluminous granitic liquid without the involvement of hydrothermal fluids

microthermometric study of silicate melt inclusions in variscan granites from se germany volatile contents and entrapment conditions melt inclusions in granitic rocks are generally crystalline and must oberpfalz regions along the northern margin of the be homogenized to glass before analysis we employ a simple kinetic bohemian massif some of these granites particularly in technique with microthermometry to estimate total volatile contents the german erzgebirge are highly evolved geochemically and p t conditions of trapping of the melts from their hoand famous for the tin and uranium mineralization mogenization behaviour this information complements chemical associated with them several publications have condata from microprobe or ion probe analysis and can be obtained centrated on the geology geochemistry and ore from inclusions too small for direct analysis we present here values associations of the granites forster tischendorf 1994 for volatile contents as equivalent wt h2o trapping temperatures seltmann 1994 tischendorf forster 1994 hecht et and trapping pressures derived from studies of melt inclusions in some al 1997 siebel et al 1997 but their magmatic volatile variscan granites of the erzgebirge fichtelgebirge and oberpfalz contents and emplacement level are difficult to determine germany the volatile concentrations range from 3 to 10 equiv wt and have not often been addressed the granites from h2o microprobe analysis shows that the most volatile rich these regions contain abundant melt inclusions and these inclusions contain up to 8 wt f estimates of trapping offer the potential of constraining melt water contents pressures and temperatures suggest that crystallization began at an and conditions of trapping emplacement level of 3 6 5 km near 700 c with normal biotite all three regions contain composite plutons or plutonic and two mica granites and ended with topaz albite granite complexes which range in composition from un 650 c and pegmatitic phases down to 520 c this wide differentiated biotite granites to more highly evolved tworange of trapping temperatures and volatile contents is a general mica granites and finally to extremely evolved granites feature of melt inclusions in granites many of the granites studied containing li micas and topaz in some cases this comare highly evolved geochemically but the extreme volatile enrichment positional range is covered by different units of a single typical for these also occurs locally in the unspecialized granites pluton e g falkenberg granite oberpfalz whereas in other places individual plutons are more restricted in composition the erzgebirge region is particularly rich

oblique back arc rifting of taupo volcanic zone new zealand taupo volcanic zone tvz is a back arc rift innorth island new zealand its geometry andkinematics are investigated using shaded reliefimages field examination of faults and offset streamchannels the results show that tvz trends nne is 250 km long by hikurangi margin in the north island of new zealand 20 km wide and consists of fivesegments extension is principally manifest as steeplydipping 60 90 normal faults parallel to tvz these in the last 300 kyr have experienced acomponent of dextral shear tvz is therefore anoblique back arc rift the dextral shear is 37 of thetotal displacement which for previously estimatedspreadingrates 7mm yr correspondsto 2 6mm yr thisvalueissimilartopreviousestimatesofthedextralshear from the back arc to the forearc domains in thenorthisland distributeddextralshearacrosstvzthussuggests that strain partitioning across the plateboundary at latitudes of tvz is less significant thanpreviously thought

geomorphology and quaternary stratigraphy the roles of morpho litho and allostratigraphy abstract the subdivision and ordering of quaternary sediments and landforms is very often based on geomorphology quaternary landforms such as moraines river terraces palaeoshorelines and indeed any other landform can be arranged and ordered in time based on their relative position in the landscape morphostratigraphy is the subdivision of sedimentary units primarily on the basis of surface form whilst surface form is often used as a criterion for separating sedimentary units morphostratigraphy also usually incorporates lithological criteria to aid stratigraphical subdivision and morpho and lithostratigraphy are often interlinked in quaternary science however another approach allostratigraphy has increasingly been applied to subdivide and order quaternary sediment landforms allostratigraphy uses discontinuities to subdivide sedimentary successions allostratigraphy enables lateral variations in lithology to be included within the same stratigraphical unit furthermore it enables lithologically similar sediments stacked on top of each other to be divided into separate units based on discontinuities this paper examines the applications of morpho litho and allostratigraphy to subdivide sediment landform assemblages and critically reviews their role in quaternary stratigraphy

granitoid magmatism of the nw bohemian massif revealed gravity data composition age relations and phase concept the late variscan granitoids of the nw bohemian massif northeast bavaria west bohemia constitute four partly contiguous granitoid complexes fichtelgebirge northern oberpfalz waidhaus rozvadov and bor incorporating more than 20 intrusive units based on gravity data the granites can be modeled as steeply inclined slab and wedge like bodies with thicknesses between 2 and 8 km a rough estimate of the total volume of the granites is approximately 18 000 km3 within the four areas named above composition ranges from less evolved dioritic rocks known as the redwitzite suite to highly evolved granites the redwitzites comprise metaluminous rocks with dominant i type features these rocks yield aberrantly old rb sr ages 545 415 ma low initial sr ratios 0 706 0 708 and high and variable nd t values 1 to 4 sr nd isotopes of the redwitzites show contamination trends towards the granites suggesting mixing between mantle magma and crustal granitic melts an older plutonic association granites of bor leuchtenberg weissenstadt marktleuthen zainhammer is mildly peraluminous displaying features of both i and s type granitoids these granites are characterized by lower carboniferous ages rb sr k ar u pb low to intermediate initial sr ratios 0 707 0 708 and high nd t values 2 to 4 which overlap with those of paragneisses from the zone of erbendorf vohenstrauss zev and from the western part of the tepla barrandian it is postulated that the older granites were formed either by partial melting of zev or tepla barrandian crust or alternatively of preexisting mature crust contaminated by mantle material the younger granites are strongly peraluminous and of s type they yield upper carboniferous rb sr and k ar ages and exhibit a range towards high initial sr ratios 0 710 0 720 and low nd t values 4 to 8 similar values are found in moldanubian paragneisses and in saxothuringian metasediments both of which provide potential source rock lithologies for these granites

shallow seismic stratigraphy and pockmarks of a hydrothermally influenced lake lake rotoiti new zealand lake rotoiti formed 11 850 20 000 years ago when lava flows dammed the drainage system through the okataina caldera one of the major rhyolitic centres of the intracontinental back arc taupo volcanic zone of north island new zealand the lake morphology reflects this complex origin with remnants of the old caldera rim the lava dam and relict river system being preserved high resolution seismic reflection profiles 7 khz show the drowned river system channelled inflowing water into the lake basin scouring and depositing sediments and maintaining an active sublacustrine channel in the former river valley airfall tephras are preserved in lake sediments the tephrachronology established from the surrounding catchments can be correlated with the siesmic stratigraphy for eight major eruptions over the last 11 850 years hydrothermal fields underlying the lake generate gaseous sediments which mask seismic penetration this is indistinguishable from seismic masking caused by biogenic gas accumulation within the sediments venting of hydrothermal gases at the surface creates pockmarks from the seismic stratigraphy beneath pockmarks seven stages of pockmark genesis have been identified which show pockmarks grow decay migrate and persist over several thousand years

sequence stratigraphy of lacustrine deposits a quaternary example from the bonneville basin utah the late quaternary lacustrine sedimentary record in the bonneville lake basin in the eastern great basin provides an excellent example of sequence stratigraphy two sequences referred to as the little valley and bonneville alloformations are exposed in the bluffs of the sevier river where it has entrenched its pleistocene delta between leamington and delta utah both alloformations contain offshore marl units and fine grained deltaic or underflow fan deposits they can be identified and mapped by tracing the unconformity separating them and employing a number of geochronometric tools including amino acid epimerization in fossil gastropods radiocarbon and thorium 230 ages and basaltic tephrochronology thin transgressive sand of the little valley alloformation is overlain by deeper water marl and down lapping regressive phase deltaic silt the bonneville alloformation lies unconformably above the little valley deposits fine grained deltaic sediments deposited during the transgressive phase of lake bonneville fill the entrenched sevier river valley that was eroded subsequent to the little valley lake cycle marl deposited during the deep water phase is overlain by down lapping deposits of the regressive phase below the provo shoreline but is the uppermost unit in the altitudinal range where the lake was lowered catastrophically during the bonneville flood the sequence stratigraphic interpretation leads to the conclusion that the sevier river delta as a whole is probably made up of a number of sediment sequences each composed of several facies recognition of this complexity could be important in potential applications of the stratigraphic model

insights into caldera and regional structures and magma body distribution from lava domes at rotorua caldera new zealand abstract lava domes emplaced after caldera forming eruptions are often the only clue to the structure of the caldera and to the regional structure at the time of caldera formation this is because the deposits from the subsequent eruptions commonly bury subsidence controlling caldera faults and regional cross faults within calderas other calderas and lava domes in the taupo volcanic zone tvz new zealand have been previously reported to show strong regional control at rotorua caldera there is a general lack of surface expression of faults but rhyolite lava domes are well exposed and aligned with regional structures that have been mapped outside of the caldera our summary of published data and new observations allows these lava domes to be subdivided into three groups based upon their petrology geochemistry relative age and spatial distribution 1 the kaikait huna group of pre collapse 451 ka crystal rich domes at the caldera boundary 2 the utuhina group of crystal poor high alkali post collapse domes c 200 ka surrounding the locus of collapse and directly related to the earlier 240 ka mamaku ignimbrite eruption and 3 the ruamata group of crystal rich more calcic less devitrified post collapse 60 36 ka domes further out from the collapse locus mapping of the morphology and internal structures of the utuhina group domes shows an ene wsw elongation direction and vent alignment which is parallel to and down strike of a the extensional rift orientation independently mapped either side of the caldera b the local caldera morphology and c structures mapped at nearby okataina caldera complex a second nw se trending orientation in the caldera morphology is parallel to a pre collapse basement fabric and has partially controlled the locations of the domes a third orientation is hinted at through consistent n s striking flow bands within ngongotaha dome which are parallel with a gravity anomaly we suggest that this orientation is a n s section of a caldera ring fault that links regional structures and was utilised during caldera collapse a subsequent dike intrusion also used this structure erupting a fissure style dome that contains radiating flow bands which mimic the orientation of the feeder dike however the structure morphology and location of post collapse lava domes generally echo the unburied regional structures outside the caldera indicating that dike intrusion also occurred along regional structures we suggest that the regional structures at rotorua caldera which are similar to those at okataina caldera complex show that the kink in the orientation of the rift across the tvz to the se of rotorua was active to at least 240 ka and possibly earlier this kink is significant due to its proximity and influence on both rotorua and okataina caldera systems and their associated geothermal fields we conclude that internal structures such as flow bands and morphology of lava domes are direct consequences of the shape and location of the feeder dike which in turn is controlled by the structure of the caldera and region we also suggest that lava dome eruptions at rotorua were from a series of small related nestled magma bodies a key outcome of this study is the reconstruction of the wider structure of the caldera and surrounding structure based upon the localised internal features distribution and morphology of related lava domes

asymmetric multiple block collapse at rotorua caldera taupo volcanic zone new zealand calderas worldwide have been classified according to their dominant collapse styles although there is a good deal of speculation about the processes involved recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials however many other factors also contribute to final caldera morphology rotorua caldera formed during the eruption of the mamaku ignimbrite collapse structure and evolution of rotorua caldera is interpreted based its geophysical response geology and geomorphology and the stratigraphy of the mamaku ignimbrite rotorua caldera is situated at the edge of the extensional taupo volcanic zone in which major faults strike ne sw a second less dominant fault set strikes nw se these two fault sets have a strong influence on the morphology of rotorua caldera no one style of collapse can be applied to rotorua caldera it was formed during a single eruption but subsided as many blocks and shows features of trapdoor piecemeal and downsag types of collapse here rotorua caldera is described according to its composition activity and geometry as a rhyolitic single event asymmetric multiple block single locus collapse structure the mamaku ignimbrite is the only ignimbrite to have erupted from rotorua caldera extracaldera thickness of the mamaku ignimbrite is up to 145 m whereas inside the caldera it may be greater than 1 km thick the mamaku ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence the main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower middle and upper parts lmi mmi umi respectively based on crystal content welding jointing devitrification and vapour phase alteration juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments only the most evolved pumice type occurs in the basal tephra sequence all three pumice types occur together throughout the main ignimbrite sequence whereas the andesitic fragments are only present in umi lithic lag breccias in umi indicate a late stage of caldera collapse concentration of lithic fragments increases towards the middle of the ignimbrite and may also reflect increased subsidence rate during an earlier stage collapse of rotorua caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the mamaku ignimbrite allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence

a catastrophic flood caused by drainage of a caldera lake at aniakchak volcano alaska and implications for volcanic hazards assessment aniakchak caldera located on the alaska peninsula of southwest alaska formerly contained a large lake estimated volume 3 7 10 9 m 3 that rapidly drained as a result of failure of the caldera rim sometime after ca 3400 yr b p the peak discharge of the resulting flood was estimated using three methods 1 flow competence equations 2 step backwater modeling and 3 a dam break model the results of the dam break model indicate that the peak discharge at the breach in the caldera rim was at least 7 7 10 4 m 3 s 1 and the maximum possible discharge was 1 1 10 6 m 3 s 1 flow competence estimates of discharge based on the largest boulders transported by the flood indicate that the peak discharge values which were a few kilometers downstream of the breach ranged from 6 4 10 5 to 4 8 10 6 m 3 s 1 similar but less variable results were obtained by step backwater modeling finally discharge estimates based on regression equations relating peak discharge to the volume and depth of the impounded water although limited by constraining assumptions provide results within the range of values determined by the other methods the discovery and documentation of a flood caused by the failure of the caldera rim at aniakchak caldera underscore the significance and associated hydrologic hazards of potential large floods at other lake filled calderas

the cretaceous uhangri formation sw korea lacustrine margin facies the uhangri formation forms part of the cretaceous sedimentary sequence deposited in a series of inland basins in the south western korean peninsula it comprises an approximately 400 m thick epiclastic sequence of conglomerate gravelly sandstone cherty mudstone and black shale the entire sequence can be represented by 16 distinctive sedimentary facies organized into four facies associations facies association i is characterized by thick homogeneous brownish siltstone wedge shaped disorganized conglomerate and thinly interlayered gravelly sandstone units the siltstone units were formed by large floods submerging the alluvial fan fringe floodplain whereas the conglomerate and gravelly sandstone units were deposited by sheetfloods and debris flows facies association ii consists of stratified conglomerate gravelly sandstone laminated sandstone and sandstone siltstone couplets which form fining upward cycles some facies units are low angle trough cross bedded and show broad channel geometries this association represents subaqueous delta lobes fed by high and low concentration turbidity currents in the distal delta realm facies association iii is characterized by wedged conglomerate and gravelly sandstone facies with interfingered massive sandstone bounded by scoured bases it represents a delta front where distributary channels and mouth bars are dominant facies association iv consists of laterally continuous sequence of laminated black shale crudely stratified sandstone and convoluted sandstone cherty mudstone this facies association is suggestive of depositional processes controlled by chemical equilibrium resulting from an interaction between density inflows and lake water the cherty mudstone resulted from inorganic precipitation from siliceous solution provided by acidic volcanism the uhangri sequence generally shows a fining upward trend with a transition from alluvial fan fringe coarse grained subaqueous delta to shallow lake the retrogradation was probably due to continuous subsidence related to continental rifting in the oblique slip mobile zone

sedimentology and allostratigraphy of post 240 ka to pre 26 5 ka lacustrine terraces at intracaldera lake rotorua taupo volcanic zone new zealand abstract lake rotorua partially occupies a nearly circular 20 km diameter volcano tectonic depression formed at c 240 ka by eruption of the voluminous mamaku ignimbrite three distinct lacustrine littoral terraces defined on the basis of contrasting geomorphology and field relations and separated by tephrostratigraphically dateable unconformities and basin floor disconformities fringe much of the lake basin they are here correlated with former high stands of the lake which resulted from the blockage and re establishment of a number of alternative outlets due to tectonic activity and volcanism at both the host and adjacent volcanic centres the unconformities allow division of the deposits into three allostratigraphic units each of which is then characterised by elevation and sediment provenance the

delayed sedimentary response to the a d 1886 eruption of tarawera new zealand sedimentary response to tephra deposition from substantial volcanic eruptions is generally immediate and dramatic an important exception is the a d 1886 eruption of mount tarawera new zealand which blanketed 200 km 2 with 50 cm of basaltic scoria and ash fall erupted from a fissure through the tarawera dome complex this fall deposit has suffered very little rilling or other surface erosion and there was no immediate downstream redistribution of tephra by lahars or dilute floods we attribute this lack of early response to the high permeability of the tephra deposit and gentle relief in the areas of substantial accumulation numerous small lakes occur among the dome complexes of okataina caldera and one direct result of the eruption was constriction of lake tarawera9s outflow which caused the lake to rise substantially eighteen years after the eruption collapse of a tephra bank that had controlled lake level triggered a breakout flood from this raised intracaldera lake triggering a decades long period of intense tributary erosion in the upper catchment and damaging stream bed aggradation outside the caldera this abrupt increase in sediment yield from an initially stable posteruptive landscape has no documented precedents yet may be a common posteruptive sedimentary response in caldera complexes of temperate to humid regions

hydrodynamic behaviour of taupo 1800a pumice implications for the sedimentology of remobilized pyroclasts large scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments on land erosion remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run off conversely in the submarine realm both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium despite this and the number of studies devoted to volcaniclastic sedimentation relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves which ought to underpin any assessment of transport or depositional process this is crucial as many volcanic particles exhibit variable density according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and or gaseous phases during transport and deposition investigation of the physical and hydrodynamic properties of taupo 1800a pumice with reference to sedimentary facies developed during the eruption aftermath shows that although buoyant when dry when sufficiently waterlogged cool pumice clasts will sink and behave more like quartzo feldspathic material saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air water exchange in smaller pores low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent sized quartzo feldspathic or crystal lithic particles fine grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles these unusual properties of temporary buoyancy variable saturation low density and size dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies these findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments but also for primary depositional processes during subaqueous explosive volcanism

stratigraphy chronology styles and dynamics of late quaternary eruptions from taupo volcano new zealand taupo volcano is the southerly of two dormant caldera volcanoes in the rhyolite dominated central portion of the taupo volcanic zone in the north island of new zealand taupo has an average magma output rate of 0 2 m 3 s 1 over the past 65 000 years and is one of the most frequently active and productive rhyolite volcanoes known the structure of the modern inverse volcano was formed largely by caldera collapse associated with the voluminous 22 600 14 c years bp oruanui eruption and has been little modified since except for collapse following the 1850 14 c years bp eruption the products of 28 eruptions labelled psi omega a ldots z all of which post date the oruanui eruption are defined and described here twenty seven of these eruptions are represented by pyroclastic deposits of which three were accompanied by a mappable lava extrusion and one eruption z solely by evidence for a lava extrusion the deposits of seven eruptions b c e s v x and y largely correspond to previously defined tephra formations karapiti poronui opepe waimihia whakaipo mapara and taupo respectively the previously defined motutere and hinemaiaia tephras are reinterpreted to represent the products of 12 eruptions g to r while the remaining eight deposits and one eruption are newly recognized eruption psi occurred at ca 17 200 14 c or 20 500 calibrated years bp and eruption z about 1740 calibrated years bp eruption volumes vary by more than three orders of magnitude between 0 01 and more than 44 km 3 and repose periods by more than two orders of magnitude from ca 20 to 6000 years the eruption deposits reflect great variations in parameters such as volume the dispersal characteristics of the fall deposits the presence or absence of intraeruptive time breaks the formation of pyroclastic flows the degree of magma water interaction the vesiculation state of the magma on fragmentation and the relative proportions of juvenile obsidian versus foreign lithologies in the lithic fractions all but seven fall deposits are plinian in dispersal two y1 and probably w are sub plinian one y5 has been termed ultraplinian while psi and a are too poorly preserved for their dispersal to be assessed the lengths of repose periods in the post oruanui sequence range are not randomly distributed but show self similar properties fractal dimensionality repose intervals r in years of not more than 350 years follow n 53 5r 0 21 and those of not less than 350 years follow n 2096r 0 83 where n is the number of eruptions the shorter repose periods may reflect triggering processes such as regional extension affecting magma bodies during their viable lifetimes while longer repose intervals i e not less than 350 years may reflect an episodicity of major rifting events or the production of magma bodies below the volcano bulk volumes v in km 3 of the eruption products also show self similar properties fractal dimensionality with n 6 17v 0 46 however there are then apparently random relationships between eruption volumes and the preceding or succeeding repose period such that prediction of the time and size of the next eruption is impossible the post oruanui activity at taupo represents noise superimposed on the more uniform longer term activity in the central taupo volcanic zone where large greater than 100 km 3 eruptions such as the oruanui occur at more evenly spaced intervals of one per 40 60 000 years

erosion calderas origins processes structural and climatic control the origin and development of erosion modified erosion transformed and erosion induced depressions in volcanic terrains are reviewed and systematized a proposed classification addressing terminology issues considers structural geomorphic and climatic factors that contribute to the topographic modification of summit or flank depressions on volcanoes breaching of a closed crater or caldera generated by volcanic or non volcanic processes results in an outlet valley under climates with up to 2000 2500 mm annual rainfall craters and calderas are commonly drained by a single outlet the outlet valley can maintain its dominant downcutting position because it quickly enlarges its drainage basin by capturing the area of the primary depression multi drained volcanic depressions can form if special factors e g high rate geological processes such as faulting or glaciation suppress fluvial erosion normal fluvial erosion modified volcanic depressions the circular rim of which is derived from the original rim are termed erosion craters or erosion calderas depending on the pre existing depression the resulting landform should be classed as an erosion induced volcanic depression if the degradation of a cluster of craters produces a single drained irregular shaped basin or if flank erosion results in a quasi closed depression under humid climates craters and calderas degrade at a faster rate mostly at subtropical and tropical ocean island and island arc volcanoes their erosion results in so called amphitheater valleys that develop under heavy rainfall 2500 mm year rainstorms and high elevation differences structural and lithological control and groundwater in ocean islands may in turn preform and guide development of high energy valleys through rockfalls landsliding mudflows and mass wasting given the intense erosion amphitheater valleys are able to breach a primary depression from several directions and degrade the summit region at a high rate occasionally amphitheater valleys may create summit depressions without a pre existing crater or caldera the resulting negative landforms which may drain in several directions and the primary origin of which is commonly unrecognizable should be included in erosion transformed volcanic depressions

environmental response to a large explosive rhyolite eruption sedimentology of post 1 8 ka pumice rich taupo volcaniclastics in the hawke s bay region new zealand abstract the 1 8 ka eruption of the taupo ignimbrite from the taupo volcanic centre in the central north island of new zealand produced a 30 km3 ignimbrite covering an area ca 160 km in diameter centred on lake taupo remobilisation of its pyroclastic detritus triggered major alluviation in the years to decades after its emplacement in the hawke s bay region in the eastern north island three large rivers rise in the ignimbrite impacted mountainous headwaters in areas that were buried by up to 40 m of loose unconsolidated pumiceous material analysis of preserved volcaniclastic sediments along valleys shows that resedimentation occurred in two main stages characterised by contrasting sediment water ratios the first comprised erosion and redeposition of ignimbrite debris by lahars e g debris flows and hyperconcentrated flows the second was marked by decreasing sediment water ratios and the re establishment of braided fluvial systems in the former river valleys as recorded by such deposits as trough planar low angle and antidune cross bedded planar bedded and ripple cross laminated sands and gravels however there is no systematic chronological trend towards lower pumice contents in these fluvial facies four characteristic sets of co occurring lithofacies were produced during the sedimentary response 1 laharic remobilisation association 2 upper flow regime dominated stream flow association 3 lower flow regime dominated stream flow association and 4 lacustrine association individual lithofacies were strongly controlled by the unusual hydrodynamic properties of pumice particularly its low and variable density automated settling tube analysis of water saturated pumiceous sediment confirms that sieve analysis is misleading with respect to the hydrodynamic behaviour of such sediments recasting granulometric data in terms of effective hydraulic grain size shows better sorting of mixed component sediments but statistical sorting and mean grain size parameters do not discriminate between the deposits of tractional and overcapacity parent flows therefore particle behaviour is addressed by using both lithofacies and petrofacies in the description and classification of the remobilisation deposits

depth of intrusion and thermal modeling of the falkenberg granite oberpfalz germany the late carboniferous falkenberg granite exposed 2 km to the east of the german continental deep drilling ktb drill site has solidified at a depth of approximately 9 12 km the initial temperature of the intrusion was 780 800 c the shape of the pluton is approximately that of a horizontal plate with an assumed original thickness of approximately 9 km 3 km of which having now been removed by erosion the results of simple one dimensional thermal modeling based on conductive heat transfer suggest cooling to 400 and 350 c over approximately 6 and 15 m y respectively with respect to the cooling ages of micas this suggests that the intrusion is somewhat older than previously assumed the lack of thermal influence in the nearby crustal section recovered by ktb compared with the width of the contact aureole inferred from the model is explained by considerable convergence between contact and drilling site the initial synemplacement distance between the granite wall rock contact and the ktb location was at least twice the recent value this is consistent with mesozoic crustal shortening which has resulted in the antiformal stack geometry of the supracrustal slices drilled by ktb

the 26 5 ka oruanui eruption new zealand an introduction and overview abstract the 26 5 ka oruanui eruption from taupo volcano in the central north island of new zealand is the largest known wet eruption generating 430 km3 of fall deposits 320 km3 of pyroclastic density current pdc deposits mostly ignimbrite and 420 km3 of primary intracaldera material equivalent to 530 km3 of magma erupted magma is 99 rhyolite and 90 pdc deposits range from mm to cm thick ultra thin veneers enclosed within fall material to 200 m thick ignimbrite in proximal areas the farthest travelled 90 km most energetic pdcs velocities 100 m s 1 occurred during phase 8 but the most voluminous pdc deposits were emplaced during phase 10 grain size variations in the pdc deposits are complex with changes seen vertically in thick proximal accumulations being greater than those seen laterally from near source to most distal deposits modern lake taupo partly infills the caldera generated during this eruption a 140 km2 structural collapse area is concealed beneath the lake while the lake outline reflects coeval peripheral and volcano tectonic collapse early eruption phases saw shifting vent positions development of the caldera to its maximum extent indicated by lithic lag breccias occurred during phase 10 the oruanui eruption shows many unusual features its episodic nature wide range of depositional conditions in fall deposits of very wide dispersal and complex interplay of fall and pdc activity

the interplay of evolved seawater and magmatic hydrothermal fluids in the 3 24 ga panorama volcanic hosted massive sulfide hydrothermal system north pilbara craton western australia the 3240 ma panorama volcanic hosted massive sulfide vhms district is unusual for its high degree of exposure and low degree of postdepositional modification in addition to typical seafloor vhms deposits this district contains greisen and vein hosted mo cu zn sn mineral occurrences that are contemporaneous with vhms orebodies and are hosted by the strelley granite complex which also drove vhms circulation hence the panorama district is a natural laboratory to investigate the role of magmatic hydrothermal fluids in vhms hydrothermal systems regional and proximal high temperature alteration zones in volcanic rocks underlying the vhms deposits are dominated by chlorite quartz albite assemblages with lesser low temperature sericite quartz k feldspar assemblages these assemblages are typical of vhms hydrothermal systems in contrast the alteration assemblages associated with granite hosted greisens and veins include quartz topaz muscovite fluorite and quartz muscovite sericite chlorite ankerite these vein systems generally do not extend into the overlying volcanic pile fluid inclusion and stable isotope studies suggest that the greisens were produced by high temperature 590 c high salinity 38 56 wt nacl equiv fluids with high densities 1 3 g cm3 and high 18o 9 3 0 6 these fluids are compatible with the measured characteristics of magmatic fluids evolved from the strelley granite complex in contrast fluids in the volcanic pile including the vhms ore forming fluids were of lower temperature 90 270 c lower salinity 5 0 11 2 wt nacl equiv with lower densities 0 88 1 01 g cm3 and lower 18o 0 8 2 6 these fluids are compatible with evolved paleoarchean seawater fluids that formed the quartz chalcopyrite sphalerite cassiterite veins which are present within the granite complex near the contact with the volcanic pile were intermediate in temperature and isotopic composition between the greisen and volcanic pile fluids t 240 315 c 18o 4 3 1 5 and are interpreted to indicate mixing between the two end member fluids evidence of mixing between evolved seawater and magmatic hydrothermal fluid within the granite complex together with the lack of evidence for a magmatic component in fluids from the volcanic pile suggest partitioning of magmatic hydrothermal from evolved seawater hydrothermal systems in the panorama vhms system this separation is interpreted to result from either the swamping of a relatively small magmatic hydro thermal system by evolved seawater or density contrasts precluding movement of magmatic hydrothermal fluids into the volcanic pile variability in the salinity of fluids in the volcanic pile combined with evidence for mixing of low and high salinity fluids in the massive sulfide lens is interpreted to indicate that phase separation occurred within the panorama hydrothermal system although we consider this phase separation to have most likely occurred at depth within the system as has been documented in modern vhms systems the data do not allow the location of the inferred phase separation to be determined

the taupo eruption new zealand i general aspects the ca a d 186 taupo eruption was the latest eruption at the taupo volcanic centre occurring from a vent at horomatangi reefs now submerged beneath lake taupo in the central north island of new zealand minor initial phreatomagmatic activity was followed by the dry vent 6 km 3 hatepe plinian outburst large amounts of water then entered the vent during the 2 5 km 3 hatepe phreatoplinian ash phase eventually stopping the eruption though large amounts of water continued to be ejected from the vent area causing gullying of the ash deposits after a break of several hours to weeks phreatoplinian activity resumed generating the 1 3 km 3 rotongaio ash notable for its fine grainsize and for containing significant quantities of non or poorly vesicular juvenile material the vent area then became dry again and eruption rates and power markedly increased into the 23 km 3 taupo ultraplinian phase which is the most powerful plinian outburst yet documented synchronous with this ultraplinian activity lesser volumes of non to partially welded ignimbrite were generated by diversion of ejecta from or partial collapse of the eruption column the rapid rate of magma withdraw al during this phase removed support from the vent area to trigger local vent collapse and initiate the catastrophic eruption of the ca 30 km 3 taupo ignimbrite finally after some years lava was extruded on to the floor of the reformed lake taupo and floating fragments derived from the lava carapace were driven ashore the known eruption volume is more than 65 km 3 while additional volumes are represented by primary material now beneath lake taupo and layer 3 to the ignim brite phases a total volume of more than 105 km 3 is likely equivalent to more than 35 km 3 of magma plus more than 3 km 3 of lithic debris airfall deposits more than 10 cm thick blanketed 30000 km 2 of land east of the vent while ignimbrite covers a near circular area of 20000 km 2 widespread and locally severe ground shaking occurred during but mostly after the eruption associated with subsidence in the lake taupo basin secondary deposits are abundant above and extending beyond the taupo ignimbrite consisting of the products of surface water interacting with the still hot ignimbrite and subsequent water reworking of the light pumiceous materials the complexity and size of this eruption preclude accurate forecasting of the size nature and return period of the inevitable next eruption from the taupo volcanic centre

facies architecture in depositional systems resulting from the interaction of acidic springs alkaline springs and acidic lakes case study of lake roto a tamaheke rotorua new zealand the facies architecture in hot spring systems tends to become more varied and complicated as the degrees of freedom in the system increase discharge aprons fed by waters from a single vent will for example be characterized by predictable downslope facies changes that reflect downslope changes in water chemistry and temperature the facies architecture however becomes exponentially more intricate when more factors start to impact the system this phenomenon is readily apparent in the geothermal area around lake roto a tamaheke located in the whakarewarewa thermal village rotorua new zealand where the facies architecture developed in response to the interactions between acid lake acid hot spring and alkaline hot spring depositional regimes with additional extraneous sediment being brought into the area by volcanic ash clouds wind blown pollen and surface run off from the surrounding drainage basin much of the complexity in the facies architecture of this system stems from the temporal varianc