dune bedforms produced by dilute pyroclastic density currents from the august 2006 eruption of tungurahua volcano ecuador a series of pyroclastic density currents were generated at tungurahua volcano ecuador during a period of heightened activity in august 2006 dense pyroclastic flows were confined to valleys of the drainage network while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross stratified ash exposed on the surface here the description measurement and classification of more than 300 dune bedforms are presented four types of dune bedforms are identified with respect to their shape internal structure and geometry length width thickness stoss and lee face angles and stoss face length 1 elongate dune bedforms have smooth shapes and are longer in the flow direction than wide or thick internal stratification consists of stoss constructional thick lensoidal layers of massive and coarse grained material alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests 2 transverse dune bedforms show linear crests perpendicular to the flow direction with equivalent lengths and widths internally these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration steep truncations of the bedsets are visible on the stoss side only 3 lunate dune bedforms display a barchanoidal shape and have stratification patterns similar to those of the transverse ones finally 4 two dimensional dune bedforms are much wider than long exhibit linear crests and are organized into trains elongate dune bedforms are found exclusively in proximal deposition zones transverse lunate and two dimensional dune bedforms are found in distal ash bodies the type of dune bedform developed varies spatially within an ash body transverse dune bedforms occurring primarily at the onset of deposition zones transitioning to lunate dune bedforms in intermediate zones and two dimensional dune bedforms exclusively on the lateral and distal edges of the deposits the latter are also found where flows moved upslope elongate dune bedforms were deposited from flows with both granular based and tractional flow boundaries that possessed high capacity and competence they may have formed in a subcritical context by the blocking of material on the stoss side we do not interpret them as antidune or chute and pool structures the dimensions and cross stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate but we rule out their interpretation as antidunes a similar conclusion holds for lunate dune bedforms whose curved shape results from a sedimentation rate dependent on the thickness of the bedform finally two dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport i e in areas where currents were able to spread laterally in low velocity zones we suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension this produced differential draping with no further reworking we propose the name regressive climbing dunes for structures produced by this process a rapid decrease in current velocity possibly triggered by hydraulic jumps affecting the entire parent flows is inferred to explain their deposition this process can in principle hold for any kind of particulate density current

linking the 8 2 ka event and its freshwater forcing in the labrador sea 1 the 8 2 ka event was the last deglacial abrupt climate event a reduction in the atlantic meridional overturning circulation amoc attributed to the drainage of glacial lake agassiz may have caused the event but the freshwater signature of lake agassiz discharge has yet to be identified in 18o of foraminiferal calcite records from the labrador sea calling into question the connection between freshwater discharge to the north atlantic and amoc strength using mg ca paleothermometry we demonstrate that 3 c of near surface ocean cooling masked an 1 0 decrease in western labrador sea 18o of seawater concurrent with lake agassiz drainage comparison with north atlantic 18o of seawater records shows that the freshwater discharge was transported to regions of deep water formation where it could perturb amoc and force the 8 2 ka event

structure and emplacement of the pliocene roque nublo debris avalanche deposit gran canaria spain abstract collapse of the flanks of the pliocene roque nublo stratocone on gran canaria lead to large debris avalanches that traveled far beyond the coastline with velocities that may have exceeded 100 m s the deposits comprised at least 14 km 3 of debris and covered an area of about 180 km 2 in the southern half of the island the debris filled deep canyons are largely exhumed at present incomplete disintegration poor fluidization and a good preservation of megablocks are thought to be mainly due to the confinement of the debris avalanche within deep canyons the rheological behavior of lithologic domains varies with their size internal strength and relative position within the deposit the deformation structures indicate that distinct components flowed whereas others slid throughout the transport in a relatively dense mode the following factors are believed to have been important in generating the roque nublo debris avalanche deposit 1 unusual abundance of pyroclastic flow and other volcaniclastic deposits making up the upper roque nublo cone 2 highly asymmetric basement beneath the cone with a very high southern ridge and a steep erosional southern slope leading to major instabilities 3 intrusion of dikes and cryptodomes as well as hydrothermal alteration in the crater area further weakening the volcano

emplacement of the 18 may 1980 lateral blast deposit ene of mount st helens washington facies variations east northeast of mount st helens preserve a record of depositional processes in the 18 may 1980 lateral blast cloud this paper reports new field grain size and component data from the ene sector of the timber blowdown zone and presents a model for blast flow and sedimentation the first erupted ejecta was rich in juvenile components and extends to the distal blowdown limit the last erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge the blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high density turbidity current the possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns depositional conditions were strongly influenced by the rate of suspended load fallout from the blast within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered settling zone able to detach gravitationally and drain into local depressions the resulting proximal facies resembles a low aspect ratio ignimbrite fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered settling zone component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers with increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions building up antidune like bedforms wavelengths of these bedforms range from 20 to 1 m and decrease away from vent there is a systematic relationship between antidune migration direction and depositional slope the transition from proximal ignimbrite like to distal surge like facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high velocity pyroclastic flows

flow characteristics of large eruption triggered debris flows at snow clad volcanoes constraints for debris flow models abstract theoretical modelling is not yet adequate to predict the behavior of debris flows which can be an extremely hazardous hydrologic process commonly associated with volcanic eruptions particularly at snow clad stratovolcanoes to provide a realistic basis for modelling the behavior of large 1000 m3 s debris flows this paper summarizes kinematic volumetric and hydraulic characteristics of ten large historic volcanic debris flows from four different volcanoes although debris flows larger than these are known to have occurred in the past the ten summarized here define a practical upper range in magnitude of more typical flows to be considered for future hazard prediction peak flow velocities of the ten debris flows studied were indirectly measured to be between about 5 and 20 m s on gradients of 0 005 to 0 25 m m but locally they were as great as 40 m s hydraulic average depths were as great as 25 m but were more commonly between 5 and 15 m in channels up to 400 m wide computed peak discharges volumetric flow rates were as high as 105 m3 s and total volumes were as much as 108 m3 total flow volumes increased by as much as four times in relatively steep channels as eroded sediment was incorporated into the debris flows flows generally achieved supercritical flow and deposited minimal volumes of sediment on gradients steeper than 0 02 m m subcritical flow and active deposition predominated on gradients less than 0 01 m m although flows travelled tens of kilometers on such low gradients while laying down deposits total distances travelled as debris flows were as far as 120 km

numerical models of caldera scale volcanic eruptions on earth venus and mars volcanic eruptions of gassy magmas on earth venus and mars produce plumes with markedly different fluid dynamics regimes in large part the differences are caused by the differing atmospheric pressures and ratios of volcanic vent pressure to atmospheric pressure for each of these planets numerical simulations of an eruption of magma containing 4 weight percent gas were run on a workstation on venus the simulated eruption of a pressure balanced plume formed a dense fountain over the vent and continuous pyroclastic flows on earth and mars simulated pressure balanced plumes produced ash columns ash falls and possible small pyroclastic flows an overpressured plume illustrated for mars exhibited a complex supersonic velocity structure and internal shocks

erosional furrows formed during the lateral blast at mount st helens may 18 1980 nearly horizontal quasi periodic erosional features of 7 m average transverse wavelength and of order 100 m length occur in scattered locations from 3 5 to 9 km from the crater at mount st helens under deposits of the lateral blast of may 18 1980 we attribute the erosional features to scouring by longitudinal vortices resulting from flow instabilities induced by complex topography namely by streamline curvature in regions of reattachment downstream of sheltered regions and by the cross flow component of flow subparallel to ridge crests the diameter of the vortices and their transverse spacing inferred from the distance between furrows are taken to be of the order of the boundary layer thickness the inferred boundary layer thickness 14 m at 9 km from the source of the blast is consistent with the running length from the mountain to the furrow locations by using knowledge of ablation patterns on bodies and lofting of dust in high speed flow we are able to infer some features of the flow field within the blast within the furrows the erosion rate was of the order of 9 kg m 2 s 1 about 4 times greater than that expected from laboratory data obtained in flow free of longitudinal vortices the orientation of furrows induced by the cross flow instability can be used to measure the upwash angle and estimate the flow mach number at the central ridge of spirit lake the mach number is inferred to have been about 2 5 and the flow velocity approximately 235 m s the similarities and differences between the furrows reported here and channels observed at other volcanoes are discussed

evidence of erosional self channelization of pyroclastic density currents revealed by ground penetrating radar imaging at mount st helens washington usa the causes and effects of erosion are among the least understood aspects of pyroclastic density current pdc dynamics evidence is especially limited for erosional self channelization a process whereby pdcs erode a channel that confines the body of the eroding flow or subsequent flows we use ground penetrating radar imaging to trace a large pdc scour and fill from outcrop to its point of inception and discover a second larger pdc scour and fill the scours are among the largest pdc erosional features on record at 200 m wide and at least 500 m long estimated eroded volumes are on the order of 106 m3 the scours are morphologically similar to incipient channels carved by turbidity currents erosion may be promoted by a moderate slope 5 15 substrate pore pressure retention and pulses of increased flow energy these findings are the first direct evidence of erosional self channelization by pdcs a phenomenon that may increase flow velocity and runout distance through confinement and substrate erosion

metamorphosed and dismembered ophiolite suites in the basement units of the eastern alps abstract the pre alpine basement of the eastern alps contains several ultramafic mafic rock complexes four of which have been investigated although polyphase deformation and metamorphism obscures much of the primary features field petrographical and geochemical evidence favours ophiolitic origins in mid ocean ridge and back arc tectonic settings the stubach complex belongs to the penninic basement of the tauern window and is interpreted as an obducted back arc ophiolite with ultramafic cumulates gabbro and minor amounts of basalt which shows the geochemistry of subduction related magma generation the rating complex belongs to a group of strongly dismembered basement slices carrying the palaeozoic austroalpine sequence of the northern greywacke zone basaltic rocks are accompanied by former metamorphic peridotite containing chromite concentrations nearly all members of an ophiolite suite are present in the speik complex in the austro alpine crystalline basement the source rocks are metamorphic peridotite harzburgite with chromite occurrences cumulate ultramafic and mafic rocks isotropic gabbro basalt and or dolerite and oceanic sediments the plankogel complex also part of the austro alpine crystalline basement represents a tectonic melange blocks of serpentinite and amphibolite derived from metamorphic peridotites and mid ocean ridge basalts together with marble and quartzite are contained in a micaschist matrix admixed is a rock assemblage with alkali basaltic amphibolites interpreted as a seamount environment the ritting plankogel and possibly parts of the speik complexes contain relict high pressure minerals and therefore went through a stage of subduction the inferred ages of all complexes is late precambrian to early palaeozoic the ophiolite suites have their distinct characteristics and contrasting tectonic settings and are therefore considered to be genetically unrelated to each other

may 18 1980 mount st helens deposits in south coldwater creek washington beginning at 0832 pacific daylight time pdt mount st helens washington produced a rockslide debris avalanche and a lateral explosion blast that generated a laterally directed pyroclastic surge called here a blast surge the resulting deposits are thickest in the east west trending valley of north fork toutle river immediately north of the volcano but are almost as thick in south coldwater creek a small tributary of the toutle river in the lee of an east west trending ridge johnston ridge just north of the north fork toutle river the avalanche surmounted johnston ridge 300 m relief and flowed down the valley of south coldwater creek closely following the avalanche and in part contemporaneously with it the blast surge left thin up to 0 5 m deposits on johnston ridge and thick up to 15 m deposits in south coldwater creek these deposits have a lower layer layer a1 which is friable massive poorer in fines particles 1 16 mm and more poorly sorted than the upper layer layer a2 layer a2 is less friable because it contains more silt and clay sized material beneath layer a1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface this zone designated as layer a0 is a poorly mixed layer containing schlieren of soil shattered volcanic fragments and considerable quantities of organic material layer a1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge layer a2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head layer a0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge it is concluded that a strongly fines depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high velocity particulate density current

a sequence of a plinian eruption preceded by dome destruction at kelud volcano indonesia on february 13 2014 revealed from tephra fallout and pyroclastic density current deposits abstract a plinian style eruption with a radially spreading umbrella cloud occurred on february 13 2014 at kelud volcano indonesia we present the sequence of this plinian event based on a geological survey of the eruption products and analysis of satellite images the eruptive deposits were divided into four major depositional units units a b c and d and used to determine the sequence of events the plinian phase was preceded by partial destruction of the existing lava dome and generation of high energy pyroclastic density currents pdcs mainly toward the ne that produced a series of depositional subunits units a 0 2 and blown down trees stage 1 in the main phase of the plinian eruption tephra fallout unit b was widely distributed over east java stage 2 the winds above the volcano significantly affected the tephra dispersal process in stage 3 the plinian column collapsed generating dense pdcs that flowed down the volcano valleys producing pumiceous lobate deposits unit c the declining phase of the eruption produced fine rich fallout tephra layers unit d 1 2 from low level eruption plumes and or ash lofted from pdcs the eruption sequence constructed from field observations is supported by geophysical observations the deposit features and componentry suggest that newly ascended magma triggered the destruction of the lava dome and the generation of high energy pdcs and during the subsequent climactic phase the dome was completely destroyed we estimate a total erupted volume of 0 25 0 50 km 3 bulk deposit volume 0 14 0 28 km 3 in dense rock equivalent and a mass eruption rate of 6 5 2 8 10 7 kg s with a volcanic explosivity index of 4 the eruption sequence and physical parameters of the 2014 eruption will help assess future volcanic activity and hazards at kelud volcano

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

evidence for a widespread climatic anomaly at around 9 2 ka before present 1 the 8 2 ka event was triggered by a meltwater pulse mwp into the north atlantic and resultant reduction of the thermohaline circulation thc this event was preceded by a series of at least 14 mwps their impact on early holocene climate has remained almost unknown a set of high quality paleoclimate records from across the northern hemisphere shows evidence for a widespread and significant climatic anomaly at 9 2 ka b p this event has climatic anomaly patterns very similar to the 8 2 ka b p event cooling occurred at high latitudes and midlatitudes and drying took place in the northern tropics and is concurrent with an mwp of considerable volume 8100 km3 as the 9 2 ka mwp occurs at a time of enhanced baseline freshwater flow into the north atlantic this mwp may have been despite its relatively small volume sufficient to weaken thc and to induce the observed climate anomaly pattern

a seismically constrained mass discharge rate for the initiation of the may 18 1980 mount st helens eruption we calculate the vertical mass discharge rate from mount st helens for the first few minutes of the may 18 1980 cataclysmic eruption using a new method based on seismic constraints the observed seismic waves indicate that the seismic source is a series of single forces we model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet the momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model only two parameters are necessary for the calculation mach number and sound velocity the calculated mass ejected in the first 100 s is 1 6 10 11 4 6 10 11 kg since the total blast deposit is 3 2 10 11 4 1 10 11 kg one possible interpretation is that the directed blast had a significant 30 vertical component

researching the earth and a few of its neighbors during my career our knowledge of erupting geysers and volcanoes in the solar system has exploded in this prefatory i tell how i became fascinated with high speed processes through studying meteorite impact dynamics and then how my initial idea of studying old faithful geyser as a volcanic analog led me to work not only on the dynamics of eruption of mount st helens in 1980 but also on geysers erupting on io a fiery satellite of jupiter triton a frigid satellite of neptune and enceladus an active satellite of saturn unforeseeably the study of these events also led to work on mineral thermodynamics and the hydraulics and geomorphic evolution of rapids in the grand canyon this is a narrative not a formal review article but the reader can find references in the related resources section to explore topics in more detail

subaqueous eruption and emplacement of ot2 in the middle miocene iizuka formation noto peninsula japan abstract ot2 okada tuff 2 is one of six rhyolitic tuff beds that occur in a middle miocene marine diatomite lower iizuka formation and are known as the okada tuff ot2 is well sorted and thins and thickens infilling channels 1 3 m across and 0 30 cm deep with a thickness up to 30 cm small channels 10 30 cm across and 1 10 cm deep are commonly superimposed within wider channels a thin diatomite layer is ripped from the substrate up to a level immediately above the small channels the basal part beneath the ripped up diatomite layer is dominated by ash components with vague wavy stratifications above the ripped up diatomite layer diatomite clasts are dispersed and pumice lapilli increase upward in amount and size with local interruption at a middle lower level by a poorly defined pumice lapilli poor thin layer many glass and pumice shards are blocky with curviplanar surfaces and jigsaw cracks presumably produced by phreatomagmatic explosions the poorly defined two layers of ot2 are interpreted to be ash turbidites individually produced by discrete phreatomagmatic explosions lenticular bedforms and channels indicate that the currents were fully turbulent and unsteady fed by the submarine eruptions upward coarsening of pumice probably represents settling lag of hot pumice ot2 perhaps provides potential diagnostic features to identify eruption fed turbidites

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

fluid dynamics of the 1997 boxing day volcanic blast on montserrat west indies 1 directed volcanic blasts are powerful explosions with a significant laterally directed component which can generate devastating high energy pyroclastic density currents pdcs such blasts are an important class of eruptive phenomena but quantified understanding of their dynamics and effects is still incomplete here we use 2 d and 3 d multiparticle thermofluid dynamic flow codes to examine a powerful volcanic blast that occurred on montserrat in december 1997 on the basis of the simulations we divide the blast into three phases an initial burst phase that lasts roughly 5 s and involves rapid expansion of the gas pyroclast mixture a gravitational collapse phase that occurs when the erupted material fails to mix with sufficient air to form a buoyant column and thus collapses asymmetrically and a pdc phase that is dominated by motion parallel to the ground surface and is influenced by topography we vary key input parameters such as total gas energy and total solid mass to understand their influence on simulations and we compare the simulations with independent field observations of damage and deposits demonstrating that the models generally capture important large scale features of the natural phenomenon we also examine the 2 d and 3 d model results to estimate the flow mach number and conclude that the range of damage sustained at villages on montserrat can be reasonably explained by the spatial and temporal distribution of the dynamic pressure associated with subsonic pdcs

concepts of the evolution of the austroalpine basement complex eastern alps during the caledonian variscan cycle the austroalpine basement complex has a complicated pre alpidic history which begins with the caledonian era in the late precambrian and early paleozoic a magmatic sedimentary rock sequence is formed presumably in an island arc or active continental margin environment subduction with eclogite formation is followed by collision high grade metamorphism and anatexis in the ordovician this caledonian basement is preserved in parts of the austroalpine crystalline mass the post caledonian deposits are mainly shelf type sediments with intercalated volcanics although there is evidence for an oceanic basin to the south the variscan facies zones are arranged in sw ne direction oblique to the alpidic trend in a first stage of variscan orogeny in the carboniferous south east vergent decollement nappes syntectonic flysch deposits and granitoids are formed along with regional metamorphism this is followed by a second stage in the permian with north west vergent thrusting renewed granite formation and metamorphism the variscan nappe pile is today exposed in a deeper level in the west or northwest than in the east or southeast

paleozoic tectonostratigraphic units of the northwest and central dinarides and the adjoining south tisia in the central dinarides and south tisia different paleozoic complexes occur in four geotectonic zones 1 comparatively autochthonous units located in the cores of disrupted anticlines of the external dinarides 2 allochthonous disrupted units accompanied by more predominant triassic formations in the sava nappe which is thrust onto the northeastern margin of the external dinarides 3 allochthonous disrupted units also together with triassic formations in the pannonian and durmitor nappes of the internal dinarides and 4 polymetamorphic sequences in basement of the pannonian basin and south tisia respectively this paper presents basic geological features for the main paleozoic areas included in these four zones the tectonostratigraphic units of the first two zones were related to the gondwana passive continental margin those of the third zone to the paleotethyan oceanic realm and those of tisia to the active laurussia margin geodynamic evolution of all these paleozoic complexes was related to opening and closure of the rheic and paleotethys oceans rifting processes along north gondwana started in the silurian locally in the cambrian ordovician and were followed by the late silurian devonian opening of the paleotethys subduction processes were active by the end of the devonian and at the beginning of the carboniferous along the laurussia margin they were followed during the westphalian by main variscan deformation during collision of gondwana and laurussia associated metamorphism was very low grade in the paleozoic units of the sava nappe low grade to epidote amphibolite grade within the paleozoic complexes of the pannonian and durmitor nappes in the internal dinarides and poly metamorphic with migmatites and granitoids in south tisia these processes gave rise to a pangea stage with the variscan basement disconformably overlain by late carboniferous and permian sediments

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

aeromechanic analysis of pyroclastic density currents past a building an aeromechanic analysis of pyroclastic density currents pdcs past a building is carried out on the results of a computer simulation the analysis shows that pdcs strongly interact with buildings resulting in turbulent boundary layer separation and recirculation these results could be used to better assess the hazard of pdcs impacting urban areas and be of service to civil protection authorities and urban planners who work in active volcanic areas

pyroclastic dune bedforms macroscale structures and lateral variations examples from the 2006 pyroclastic currents at tungurahua ecuador pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions for much of their dynamics they behave as particulate density currents and share similarities with turbidity currents they occasionally deposit dune bedforms with peculiar lamination patterns from what is thought to represent the dilute low concentration and fluid turbulence supported end member of the pyroclastic currents here we present a high resolution dataset of sediment plates lacquer peels with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the august 2006 eruption of tungurahua ecuador most of the sedimentary features contain backset bedding and preferential stoss face deposition from the ripple scale few cm to the largest dune bedform scale several m length similar patterns of erosive based backset beds are evidenced recurrent trains of sub vertical truncations on the stoss side of structures reshape and steepen the bedforms in contrast sporadic coarse grained lenses and lensoidal layers flatten bedforms by filling troughs the coarsest clasts up to 10 cm least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend the stratal architecture exhibits strong lateral variations within tens of centimeters with very local truncations both in flow perpendicular and flow parallel direction we infer that the bedforms sedimentary patterns result from four formation mechanisms differential draping slope influenced saltation truncative bursts and granular based events whereas most of the literature makes a straightforward link between backset bedding and froude supercritical flows we reconsider this interpretation indeed features that would be diagnostic of subcritical dunes antidunes and chute and pools can be found on the same horizon and in a single bedform only laterally separated by short distances 10s of cm our data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as gortler vortices backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre existing morphology quantification of near bed flow velocities are made via comparison with wind tunnel experiments we estimate that shear velocities of ca 0 30 m s 1 equivalent to pure wind velocity of 6 to 8 m s 1 at 10 cm above the bed could emplace the constructive bedsets whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 40 m s 1

response of a marine terminating greenland outlet glacier to abrupt cooling 8200 and 9300 years ago greenland s largest outlet glacier using 10 be surface exposure ages and 14 c dated lake sediments our chronology of ice margin change demonstrates that jakobshavn isbrae advanced to deposit moraines in response to abrupt cooling recorded in central greenland ice cores ca 8 200 and 9 300 years ago while the rapid dynamically aided retreat of many greenland outlet glaciers in response to warming is well documented these results indicate that marine terminating outlet glaciers are also able to respond quickly to cooling we suggest that short lag times of high ice flux margins enable a greater magnitude response of marine terminating outlets to abrupt

geological implications and applications of high velocity two phase flow experiments abstract beds of solid particles under high pressure have been suddenly decompressed the resulting high speed two phase flows develop a highly inhomogeneous character with interpenetrating regions of high and low particle concentration the flow is highly turbulent with large velocity and density fluctuations the flows approach homogeneity only when highly expanded with low particle concentrations

revealing exhumation of the central alps during the early oligocene by detrital zircon u pb age and fission track double dating in the taveyannaz formation

kinematics of cretaceous nappe tectonics in the austroalpine basement of the koralpe region eastern austria krohe a 1987 kinematics of cretaceous nappe tectonics in the austroalpine basement of the koralpe region eastern austria tectonophysicz 136 171 196 the austroalpine basement of the koralpe area austria was affected by penetrative ductile deformation in the cretaceous three metamorphic events have been established ht lp metamorphism of unknown age a hercynian mt mp metamorphism and a cretaceous eo alpine mt mp metamorphism in the koralpe area cretaceous rb sr cooling ages of muscovite indicate temperatures of at least 500 o c during the eo alpine event the most intense deformation occurred along intracrustal shear zones plattengneis north to northeast directed thrusting is indicated by an inverted metamorphic gradient regional transposition of an older foliation s c fabrics and oblique quartz fabrics after development of high temperature mylonites low angle normal shear zones low temperature mylonites cut the uppermost parts of the austroalpine basement and indicate an extensional tectonic setting at about 85 to 90 ma s c and quartz fabrics indicate downdip movements of the hanging wall towards the northeast intracrystalline a slip in quartz caused strong preferred lattice orientation in high and low temperature mylonites during the e alpine deformation the basal a slip system is more pronounced at lower temperatures whereas a prism a system predominates at higher temperatures the overall kinematic history of the austroalpine nappe complex in the study area suggests a model in which an intracrustal south over north imbrication took place in an early stage of eo alpine collision these movements were outlasted by sinistral strike slip movement possibly due to oblique collision along irregular margins of the austroalpine and brianpnnais complexes the collision imbrication process resulted in crustal thickening isostatic uplift and thus extension of the higher parts of the crust low angle normal faulting initiated sedimentation in the intramontane c tic gosau basins of late cretaceous age

surface morphology of caldera forming eruption deposits revealed by lidar mapping of crater lake national park oregon implications for deposition and surface modification abstract large explosive eruptions of silicic magma can produce widespread pumice fall extensive ignimbrite sheets and collapse calderas the surfaces of voluminous ignimbrites are rarely preserved or documented because most terrestrial examples are heavily vegetated or severely modified by post depositional processes much research addresses the internal sedimentary characteristics flow processes and depositional mechanisms of ignimbrites however surface features of ignimbrites are less well documented and understood except for comparatively small volume deposits of historical eruptions the 7700 calendar year b p climactic eruption of mount manama usa vented 50 km 3 of magma deposited first as rhyodacite pumice fall and then as a zoned rhyodacite to andesite ignimbrite as crater lake caldera collapsed lidar collected during summer 2010 reveals the remarkably well preserved surface of the manama ignimbrite and related deposits surrounding crater lake caldera in unprecedented detail despite forest cover the 1 m lateral and 4 cm vertical resolution lidar allows surface morphologies to be classified surface morphologies are created by internal depositional processes and can point to the processes at work when pyroclastic flows come to rest we describe nine surface features including furrow ridge sets and wedge shaped mounds in pumice fall eroded by high energy pyroclastic surges flow parallel ridges that record the passage of multiple pyroclastic flows perched benches of marginal deposits stranded by more mobile pyroclastic flow cores hummocks of dense clasts interpreted as lag deposit transverse ridges that mark the compression and imbrication of flows as they came to rest scarps indicating ignimbrite remobilization fields of closely spaced pits caused by phreatic explosions fractures and cracks due to extensional processes resulting from ignimbrite volume loss and stream channels eroded in the newly formed surface the nine morphologies presented here illustrate a dynamic depositional environment that varied spatially and with time during the eruption and show that multiple processes modified the ignimbrite after deposition both during and after the eruption

was the 18 may 1980 lateral blast at mt st helens the product of two explosions the 18 may 1980 lateral blast at mt st helens has been interpreted as the product of a single explosion by some stratigraphers and as two closely spaced explosions by others the stratigraphic evidence that bears on this question is inconclusive strata change dramatically over short distances and this complexity provides wide latitude for interpretation some independent non stratigraphic evidence however suggests that the blast was the product of two explosions or clusters of explosions the independent evidence comes from eyewitness accounts and photographs from satellite sensors and from seismic records this paper reviews the pertinent evidence offers a new interpretation and concludes that the blast was indeed the product of two explosions or clusters of explosions

eclogites from the silvretta nappe switzerland geochemical constraints on the nature and geotectonic setting of their protoliths abstract in the austroalpine silvretta nappe of central europe eclogitic rocks of presumed paleozoic metamorphic age have recently been found their chemical composition suggests that they were derived from evolved tholeiitic magmas which have undergone variable degrees of fractionation at shallow depths involving crystallization of plagioclase plus olivine and or pyroxene the igneous protoliths probably originated at a constructive plate margin although some chemical features closely resemble those of mid ocean ridge basalts the composition of the metasedimentary country rocks suggests a tectonic setting close to a continental block perhaps in a back arc basin situation or incipient oceanic ridge

liftoff of the 18 may 1980 surge of mount st helens usa and the deposits left behind the distance that ground hugging pyroclastic density currents travel is limited partly by when they reverse buoyancy and liftoff into the atmosphere it is not clear however what deposits are left behind by lofting flows one current that was seen to liftoff was the surge erupted from mount st helens on the morning of 18 may 1980 before lofting it had leveled a large area of thick forest the blowdown zone the outer edge of the devastated area where trees were scorched but left standing the scorched zone is where the surge is thought to have lifted off deposits in the outer parts of the blowdown and in the scorched zone were examined at 32 sites the important finding is that the laterally moving surge traveled through the scorched zone and hence the change in tree damage does not mark the runout distance of the surge buoyancy reversal and liftoff are thus not preserved in the deposits where the surge lofted upwards we propose based on interpretation of eyewitness accounts and the impacts of the surge on trees and vehicles that the surge consisted of a faster dilute overcurrent and a slower undercurrent where most of the mass and heat was retained reasonable estimates for flow density and velocity show that dynamic pressure of the surge i e its ability to topple trees peaked near the base of the overcurrent we propose that where the overcurrent began to liftoff the height of peak dynamic pressure rose above the trees and stopped toppling them the slower undercurrent continued forward however scorching trees but lacked the dynamic pressure needed to topple them grain size variations argue that it slowed from 30 m s 1 when it entered the scorched zone to 3 m s 1 at the far end

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

post glacial vegetation reconstruction and a possible 8200 cal yr bp event from the low arctic of continental nunavut canada climate models suggest that the global warming during the early to mid holocene may have partly resulted from the northward advance of the northern treeline and subsequent reduction of the planetary albedo we investigated the holocene vegetation history of low arctic continental nunavut canada from a radiocarbon dated sediment core from tk 2 lake a small lake ca 200 km north of the limit of the forest tundra the pollen and loss on ignition data indicate the presence of dwarf shrub tundra in the region since the beginning of organic sedimentation at ca 9000 cal yr bp with dominance of betula especially since 8700 cal yr bp at 8100 7900 cal yr bp the dominance of the shrub tundra was punctuated by a transient decline of betula and coincident increases of ericaceae undiff vaccinium type and gramineae this suggests an abrupt disturbance of the betula glandulosa population approximately simultaneously with the sudden 8200 cal yr bp event in the north atlantic however in the absence of other sites studied in the area linkage to the 8200 cal yr bp event remains tentative the lack of any evidence of forest tundra in the region constrains the northern limit of the mid holocene advance of the forest tundra boundary in central northern canada consequently our results show that the climate models imposing a mid holocene advance of the limit of the forest tundra to the arctic coast of canada may have overestimated the positive climatic feedback effects that can result from the replacement of tundra by the boreal forest copyright 2003 john wiley sons ltd

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

experimental study of sand streaks formed in turbulent boundary layers fine to medium grained sand transported as bedload moves in lanes parallel to the flow that are thought to be preserved as parting lineation a series of six flume experiments was designed to discover the morphology and spacing of these lanes here called sand streaks as functions of local shear velocity u 9 10 3 to 4 8 10 2 m s 1 depth 5 10 2 and 9 5 10 2 m mean grain diameter 150 200 290 1380 m and sediment bedload concentration 0 0 0 39 low u flows produce predominantly straight non intersecting sand streaks moderate u flows produce sub parallel and en echelon sand streaks and moderate to high u flows produce wavy sand streaks and secondary streaks with a spacing an order of magnitude larger the wavy sand streaks are thought to be composed of sand grains in suspension close to the bed an upper grain size limit for the sand streak structure occurs at a grain size between 290 and 1380 m the spacings of the fine and medium grained sand streaks at low to moderate u 0 9 10 2 to 3 10 2m s 1 are similar to those predicted for low speed fluid streaks although the fine grained sand forms more closely spaced streaks than the medium grained sand the spacings of sand streaks formed at moderate to high u and at bedload concentrations greater than 0 15 are wider than those predicted for the low speed fluid streaks the wider spacing is thought to reflect a new type of flow immediately above the moving bed layer in which the formation of low speed streaks is inhibited this results from an increase in either grain concentration or grain size the spacing of parting lineation also wider than that predicted for low speed streaks may reflect this

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

timing of tectonic events in the menderes massif western turkey implications for tectonic evolution and evidence for pan african basement in turkey the menderes massif forms the western of the two large metamorphic culminations within the turkish alpide orogen it has three major lithologic units with a gneissic core at the base and a schist and a marble envelope overlying it successively although relations between them have been largely obscured by the polymetamorphic and structurally complex history of the massif we present a review of the available stratigraphic evidence and combine it with new isotopic ages from the central and southern parts of the massif to constrain the timing of major tectonic events that shaped the massif since the late proterozoic pt3 in the southern part of the massif sensu stricto three episodes of deformation and metamorphism are distinguished whereas in its northern part there are four the first episode occurred at 500 10 ma with intense deformation and metamorphism at high grade amphibolite facies with local anatexis in the central part of the massif the 470 9 ma old now highly deformed tonalitic and granitic intrusions mark the end of this episode on a cambrian reconstruction of continents around the eastern mediterranean the menderes massif forms the southern end of the pan african orogenic collage of northeastern africa and arabia the area of earliest palaeozoic deformation in the menderes may connect with the northwest african orogenic collage along the strike via the bozburun and saricicek diabases arkoses and schists in the center of the karacahisar dome interpreted herein as fragments of a pan african suture the earliest palaeozoic deformations to affect the rocks were probably related to the last pan african collisions and associated postcollisional convergence the southern part of the massif was undeformed from early ordovician to the early later eocene whereas the northern part was deformed metamorphosed and intruded possibly during the late triassic related to the closure of the karakaya marginal basin of palaeo tethys the next major event affecting the whole of the massif was the intense deformation and widespread metamorphism that reached high amphibolite grade in the structurally lower parts and only greenschist grade in the outermost envelope this metamorphism here called the main menderes metamorphism mmm is biostratigraphically constrained between early eocene and early oligocene time rb sr isotopic data show a spread of ages between 60 ma and 25 ma with the greatest number of determinations around 35 5 ma this number is in excellent agreement with the stratigraphic evidence and shows that mmm took place during latest eocene time or at the transition from eocene to oligocene time along the northern border of the massif deformation and metamorphism had already taken place during the late cretaceous in a high pressure low temperature hp lt metamorphic belt northernmost part of menderes massif sensu lato which was then covered by palaeocene molasse the evolution of the hp lt belt was probably related to the obduction of the bozkir ophiolites from the izmir ankara branch of the neo tethys ocean and preceded terminal collision in the evolution of the massif the mmm was a product of the latest palaeocene collision across neo tethys and the consequent internal imbrication of the menderes taurus block that resulted in the burial of the menderes massif area beneath the lycian nappe complex a low grade metamorphic event dated at 10 to 5 ma is interpreted to be related to the extensional deformation in the aegean and western turkey which has been dated stratigraphically to have begun during the tortonian 11 5 ma

volcanic geomorphology an overview abstract the review examines the role of geomorphology in analyzing the volcanoes on earth five objectives are stressed first classifications of volcanic landforms should be improved to take care of the complexity in volcanic landform generation as magmatic systems style of eruption and the erupted material all influence the morphology second geomorphology should contribute to the science of volcanology through its capability in reconstructing growth stages in complex volcanoes and also in analyzing the structural factors which contribute to the catastrophic collapse of volcanoes third geomorphology can contribute to physical volcanology by assessing the effects of topography on transport erosion and deposition of volcanogenic flows and identifying the sources and climatic tectonic conditions which govern the emplacement of volcaniclastic deposits fourth volcanic geomorphology a identifies sedimentary facies associations b constructs facies models for dynamic volcano delivery systems and c analyzes the characteristics of sediment gravity flows in order to determine relevant parameters for modelling their behaviour fifth process oriented geomorphology is critical in developing accurate methods for measuring rates of geomorphic processes that shape ephemeral volcanic constructs and for evaluating and comparing geomorphic impacts on disturbed catchments and the related hydrologic response before during and after eruptions this should help to refine parameters for the exponential decay model finally volcanic geomorphology is essential for risk assessment through geomorphic hazard zonation and composite risk zonation such treatments are necessary in order to face the enhanced challenge posed by the combination of natural hazards and the increasing number of people who are at risk around volcanoes

abrupt changes in north american climate during early holocene times recent studies of the greenland ice cores have offered many insights into holocene climatic dynamics at decadal to century timescales1 2 3 despite the abundance of continental records of holocene climate few have sufficient chronological control and sampling resolution to compare with the greenland findings4 butannually laminated sediments varves from lakes can provide high resolution continental palaeoclimate data with secure chronologies here we present analyses of varved sediments from deep lake in minnesota usa trends in the stable oxygen isotope composition of the sedimentary carbonate indicate a pronounced climate cooling from 8 9 to 8 3 kyr before present probably characterized by increased outbreaks of polar air decreased precipitation temperatures and a higher fraction of the annual precipitation falling as snow the abrupt onset of this climate reversal over several decades was probably caused by a reorganization of atmospheric circulation and cooling of the arctic airmass in summer that resulted from the final collapse of the laurentide ice near hudson bay and the discharge of icebergs from the quebec and keewatin centres into the tyrell sea the timing and duration of this climate reversal suggest that it is distinct from the prominent widespread cold snap that occurred 8 200 years ago in greenland and other regions1 5 6 no shifts in the oxygen isotope composition of sediment carbonate occurred at 8 2 kyr before present at deep lake but varve thickness increased dramatically probably as a result of increased deposition of aeolian dust taken together our data suggest that two separate regional scale climate reversals occurred between 9 000 and 8 000 years ago and that they were driven by different mechanisms

north atlantic holocene climate evolution recorded by high resolution terrestrial and marine biomarker records abstract holocene climatic change is driven by a plethora of forcing mechanisms acting on different time scales including insolation internal ocean e g atlantic meridional overturning circulation amoc and atmospheric e g north atlantic oscillation nao variability however it is unclear how these driving mechanisms interact with each other here we present five biomarker based paleoclimate records air sea surface temperature and precipitation from a fjordic sediment core revealing north atlantic terrestrial and marine climate in unprecedented detail the early holocene 10 7 7 8 kyrs bp is characterised by relatively high air temperatures while ssts are dampened by melt water events and relatively low precipitation the middle holocene 7 8 3 2 kyrs bp is characterised by peak ssts declining air temperatures and high precipitation a pronounced marine thermal maximum occurs between 7 5 5 kyrs bp 3000 years after the terrestrial thermal maximum driven by melt water cessation and an accelerating amoc the neoglacial cooling between 5 8 and 3 2 kyrs bp leads into the late holocene we demonstrate that an observed modern link between icelandic precipitation variability during different nao phases may have existed from 7 5 kyrs bp a simultaneous decoupling of both air and sea surface temperature records from declining insolation at 3 2 kyrs bp may indicate a threshold after which internal feedback mechanisms namely the nao evolved to be the primary drivers of icelandic climate on centennial time scales

evidence of abrupt climate change at 9 3 ka and 8 2 ka in the central canadian arctic connection to the north atlantic and atlantic meridional overturning circulation abstract this study presents the results of a multi proxy analysis of a holocene lacustrine sediment sequence recovered from a small tundra lake in the central canadian arctic and links climatic variations there to the north atlantic chironomid and pollen based reconstructions of mean july air temperature mjat document notable deviations at 9 3 ka and 8 2 ka chironomid inferred july air temperature is depressed by 1 4 c at 9 3 ka and 1 7 c at 8 2 ka relative to the long term chironomid inferred holocene average of 9 4 c these cooling events also correspond to an increase in gramineae poaceae and a decrease in betula pollen reflecting a decrease in the density of dwarf birch cover concurrent abrupt increases in several diatom taxa including aerophilic epiphytes and sphagnum indicate an expansion of moss habitats as a result of wetter conditions the expression and timing of the cooling evidenced at tk2 during the early holocene is consistent with abrupt climate events identified in the north atlantic and coincides with intervals of north atlantic cooling and weakened atlantic meridional overturning circulation amoc this research which improves our understanding of the spatial extent and magnitude of the 9 3 ka and 8 2 ka events in the central canadian arctic serves as a benchmark for coupled ocean atmosphere climate models simulating the response of the climate system to abrupt climate events and provides insight in marine atmosphere teleconnections in the circum north atlantic region

settling behaviour of concentrated dispersions and some volcanological applications abstract experiments have been carried out on the settling of non aggregating poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied plastic beads were added as a light component experiments were carried out at solids concentrations between 10 and 60 vol dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence above a critical concentration 45 to 60 depending on the grain size distribution particle segregation was suppressed and the dispersion settled to form a virtually ungraded bed hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes floating rafts of plastic and capping layers of fines elutriated by escaping pore fluid buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations two mechanisms of elutriation pipe growth were observed at intermediate concentrations pipes grew progressively upwards in the deposit as the latter aggraded whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows reverse grading of pumice pumice rafts elutriation pipes and layers of elutriated fines should characterise deposits from pumice rich submarine mass flows despite differences in the fluid phase the experiments also have implications for fluidization mechanisms in pyroclastic flows hindered settling of a concentrated poorly sorted dispersion is a type of self fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias

surge deposit misidentification at spor mountain utah and elsewhere a cautionary message for mars abstract before base surges were described in association with nuclear blasts and explosive volcanic eruptions especially the 1980 eruption of mount st helens washington laminar and cross bedded volcanogenic surge deposits were commonly misinterpreted as being of fluvial or aeolian origin one well documented example involves the water laid tuffs in and near the spor mountain beryllium mine utah other examples abound in light of how frequently volcanogenic surge deposits have been misinterpreted on earth extreme caution is urged for mars studies contrary to what has been claimed the markedly cross bedded salty deposits at meridiani planum on mars need not have been formed by a combination of aeolian and aqueous processes and their contained hematitic spherules need not have formed as aqueous concretions given the lack of indications of volcanism in the vicinity and the planet wide abundance of impact craters deposition by surges associated with distant impact targets consisting of brine soaked locally sulfidic regolith is a reasonable explanation for all features observed especially if diagenesis and weathering are considered the uniformly sized and shaped ni enriched blue gray hematitic spherules would then be some type of vapor condensation spherules including accretionary lapilli a similar interpretation is possible for deposits in the home plate area gusev crater unlike on the dry and atmosphereless moon salty impact surge deposits containing spherules should be common and well preserved on mars

the iapetus model a plate tectonic concept for the variscan belt of europe abstract a plate tectonic model is presented explaining the evolution of the variscan belt of europe it regards the iapetus ocean and its branch the tornquist sea as the only ocean involved in the caledonian variscan and alleghenian mega orogen the iapetus and its marginal basins disappeared through oblique subduction and collision in the following sequence 1 1 northeastern iapetus including the tornquist sea caledonian orogeny 2 2 central iapetus acadian orogeny 3 3 adjacent marginal basins variscan orogeny and 4 4 southwestern iapetus alleghenian orogeny the area of the variscides was part of the continental margin of gondwana made up of ridges and ensialic basins no ocean lay within the variscan area during the palaeozoic although patches of oceanic crust probably existed in rift basins e g the lizard ophiolite after the silurian early devonian closure of northwestern and central iapetus the variscan area was surrounded by cratons on three sides in a manner similar to that around the modern mediterranean sea the subsequent variscan orogeny was essentially an ensialic thrusting and folding induced by a further approach of gondwana toward laurentia after the variscan collision gondwana rotated clockwise to the west by moving along a strike slip zone this rotation during the carboniferous opened the palaeo tethys in the east and caused the alleghenian collision in the west the middle ordovician to permian palaeogeography is presented on a series of maps modelling tectonic displacement and plate motions avalonia is interpreted as the ordovician volcanic island arc of gondwana separated from mainland gondwana by an oceanic gulf or a back arc basin the iberian arc may have originated by a continental escape of iberia from the center of collision into the oceanic gulf the variscan austro alpine and southern alpine units are today located near their earliest early carboniferous position

strain partitioning across a fold and thrust belt the rhenish massif mid european variscides abstract strain data across the entire western rhenish massif illustrate the pattern of deformation partitioning within a fold and thrust belt from the foreland on the northern front of the european variscides to the suture with the internal zones of higher metamorphic grade shortening perpendicular to cleavage increases from 16 to 27 constant volume assumed in the northern rhenish massif to 51 at the southern border these values correspond to a weak general layer parallel shortening of the basin filling in cross section on the order of about 5 15 and more in the case of volume loss the geometry of apparent finite strain is nearly plane strain except in the venn anticlinorium where strain is apparent prolate and near the southern margin of the massif where it is apparent oblate net orogenic shortening in the upper crust of the fold and thrust belt is approximately 42 it is mainly achieved by folding and tectonic stacking of the deformed basin filling which was apparently detached from the lower crust distribution of strain in the orogenic wedge is controlled by different factors at different scales heterogeneity is caused by changes in lithology and average grain size by the structural position of samples in folds and their distance from thrusts in the hangingwall at deeper crustal levels and by the regional distribution of the metamorphic grade i e by the dominant deformation mechanism this last factor causes an exponential increase of strain with approach to the ductile lower crust the superposition of strain partitioning mechanisms on different scales creates a complex regional strain pattern the southern part of the thrust belt apparently suffered late westward motion in gently dipping imbricates or strike parallel motion in steeply dipping imbricates which is probably due to oblique convergence between the rhenohercynian and saxothuringian microplates

martian craters viewed by the thermal emission imaging system instrument double layered ejecta craters 1 the thermal emission imaging system themis visible vis images provide new insight into the nature and development of the unique ejecta deposits of martian craters this study focuses on double layered ejecta dle craters to date over 100 dle craters have been examined using mainly themis vis data our observations suggest that emplacement of dle crater ejecta occurred in two stages with the inner ejecta layer emplaced similar to single layered ejecta sle crater ejecta this may have involved both ballistic and flow processes in contrast the outer ejecta layer of dle craters appears to have been emplaced through the high velocity outflow of materials from tornadic winds generated by the advancing ejecta curtain or base surge remarkably dle craters lack secondary craters which suggests that the large ejecta blocks that normally produce such craters may have either been entrained and or crushed by these winds or fragmented as a result of the presence of water in the target materials these observations suggest that volatiles either trapped in the subsurface or in the atmosphere have played a key role in the emplacement of the ejecta of dle craters and leaves open the question as to what role volatiles play in the emplacement of ejecta of other types of fluidized ejecta craters i e sle and mle craters because dle craters are found in many different regions of mars often in close proximity to other types of craters conditions e g atmospheric density that produce dle craters must fluctuate or the martian crust must be unexpectedly heterogeneous laterally and vertically while the degree of heterogeneity has yet to be recognized recent suggestions about possible martian climate change raises the possibility of impact into target materials that are periodically wet or that a significantly higher atmospheric pressure may be periodically present

the controls and consequences of substrate entrainment by pyroclastic density currents at mount st helens washington usa evidence in the deposits from the may 18 1980 eruption at mount st helens demonstrates that pyroclastic density currents pdcs produced during the afternoon of the eruption became intermittently erosive using detailed componentry and granulometry we constrain the sources for lithic blocks in the deposits and identify deposits from pdcs that became locally erosive the componentry of the lithics in the fall deposits is used as a proxy for vent erosion and assumed to represent the starting componentry for pdcs prior to entrainment from any other source we find little evidence in the pdc deposits nearest to the base of the volcano for entrainment from the steep flanks however significant evidence indicates that pdcs eroded into the debris avalanche hummocks suggesting that entrainment is favored as pdcs interact with highly irregular topography evidence for locally entrained material downstream from debris avalanche hummocks decreases with height in the outcrop suggesting that less entrainment occurs as local relief decreases and upstream topography is buried the prevalence of lithofacies containing locally entrained material at the base of unit contacts and only 10s of meters downstream from debris avalanche hummocks suggests that the majority of entrainment occurs at or near the head of the current occasionally entrained material is located high above unit contacts and deposited well after the initial head of the current is inferred to have passed indicating that entrainment can occur during periods of non deposition either from the semi sustained body of the current or from a pulsating current additionally self channelization of pdcs either by levee deposition or scouring into earlier pdc deposits occurs independently of interaction with topographic obstacles and can affect carrying capacity and runout distance while we begin to explore the mechanisms and effects of erosion on current dynamics additional laboratory and numerical studies are necessary to fully understand these processes

towards a southern european tethyan palaeomargin provenance signature sandstone detrital modes and detrital zircon u pb age distribution of the upper cretaceous paleocene monte bignone sandstones ligurian alps nw italy constraining the source terranes of alpine siliciclastic flysch sequences is crucial for building a clearer picture of the palaeogeography and geodynamic evolution of the western tethys in the framework of impending continental collision this paper presents an integrated study that involves palaeocurrent dispersal analysis sandstone petrography and detrital zircon geochronology of the upper cretaceous paleocene monte bignone sandstones a siliciclastic turbidite system deposited during the pre collisional evolution of the ligurian alps palaeocurrent analysis illustrates an overall eastward transport of the proximal sediments in the present configuration considering the ca 45 50 counter clockwise rotation of the tertiary piedmont basin and of the corsica sardinia block in the late paleogene this indicates the derivation of the sediments from the northern margin of the piedmont ligurian ocean sandstone petrography records a stratigraphic evolution from quartzose sandstones towards lithic and then to lithic sub arkosic composition this trend is interpreted to reflect the gradual unroofing of the provenance terrane the lithotypes of the recycled sedimentary rock fragments and the up section increase in dolostone and carbonate clast proportions suggest the erosion of the sedimentary cover of the southern european palaeo margin new geochronological data u pb detrital zircon ages correspond to the pre alpine stages of crustal growth recorded in the variscan maures tanneron massif and therewith confirm the derivation of the sediments from the passive palaeo european margin this conclusion highlights the importance of the lower plate in providing the source of coarse clastic deep water successions during pre collisional convergent steps results from this multi proxy provenance analysis contribute to better defining the detrital signatures associated to the continental micro fragments that constituted the palaeo european plate as it supplied deep sea siliciclastic sediments into the piedmont ligurian ocean prior to continental collision

space and time in the tectonic evolution of the northwestern iberian massif implications for the variscan belt recent advances in geochemical studies of igneous rocks isotopic age data for magmatism and metamorphism quantitative pressure temperature p t estimates of metamorphic evolution and structural geology in the northwestern iberian massif are integrated into a synthesis of the tectonic evolution that places the autochthonous

erosional characteristics and behavior of large pyroclastic density currents factors influencing the erosive behavior of large pyroclastic density currents pdcs both mainly massive and thinly stratified are not well understood to investigate the parameters influencing the erosive behavior of pdcs produced during the flowing phase of large caldera forming plinian campanian ignimbrite and phreatoplinian neapolitan yellow tuff eruptions we use scoured fall deposits at the base of or interstratified with pdc deposits from the campanian region of italy at several localities we calculated the depth of pdc erosion by comparing the measured thickness of eroded remnants to reconstructed thickness at each site estimated by isopach mapping and recorded the 1 distance from vent 2 elevation of the locality and 3 paleoslopes furthermore we have considered how these factors can be influenced by outcrop exposure depth of erosion correlates with distance from the vent in low relief landscape while across very rugged topography the only related parameter is elevation the different erosive patterns appear to show how pyroclastic currents interact with the topography in the surrounding terrain when a pdc crosses relatively flat surfaces it decelerates away from the vent decreasing its erosive capacity but when moving through steep terrain a pdc accelerates down the valley increasing its erosive capacity

a numerical investigation of choked flow dynamics and its application to the triggering mechanism of long period events at redoubt volcano alaska we use numerical simulations of transonic flow through a crack to study the dynamics of the formation of shock waves downstream from a nozzle like constriction inside the crack the model solves the full set of navier stokes equations in two dimensions via an explicit multifield finite difference representation the crack walls are assumed to be perfectly rigid and elastic coupling to the solid is not considered the simulations demonstrate how the behavior of unsteady shock waves near the walls can produce recurring step like pressure transients in the flow which in turn induce resonance of the fluid filled crack the motion of the shock waves is governed primarily by smooth low amplitude pressure fluctuations at the outlet of the crack the force induced on the walls scales with the amplitude of the shock which is a function of the magnitude of the inlet pressure aperture of the constriction and thickness of the boundary layer the applied force also scales in proportion to the spatial extent of the shock excursion which depends on the fluctuation rate of outlet pressure using the source parameters of long period lp events at redoubt volcano alaska as a guide for our simulations we infer that coupling of the shock to the walls occurs for crack inlet to outlet pressure ratios pi po 2 31 and that the position of the shock front becomes most sensitive to outlet pressure fluctuations for flow regimes with pi po 2 48 for such regimes fluctuations of outlet pressure of up to 0 5 mpa at rates up to 3 mpa s are sufficient to induce pressure transients with magnitudes up to 12 5 mpa over 0 1 2 5 m of the walls within 0 5 s these flow parameters may be adequate for triggering the lp events in the precursory swarm to the december 14 1989 eruption of redoubt according to the flow model the recurrence rate and amplitudes of l p events are inferred to be a manifestation of the response of a shallow hydrothermal reservoir to the sustained injection of superheated steam from a magma column roofing below this reservoir

a non cylindrical model for the northwest iberian allochthonous terranes and their equivalents in the hercynian belt of western europe abstract a case is made for a non cylindrical development of the european hercynian belt far travelled allochthonous terranes in the belt consist of a mixture of units including catazonal units ophiolites and units derived from the continental margin of gondwana they often depict a metamorphic history of intermediate to high pressure type and caledonian to eo hercynian age and are thought to have been put together in the early stages of the orogeny through a process partly obductive directed onto the continental margin of gondwana considerations upon the disposition of the terranes their relationships with the paleogeography of the gondwana realm and the structures in them suggest that their emplacement took place in an oblique fashion at a small angle to the margin of the continent the proposed allochthon could be related to the development of an island arc or a wedge like subplate in the southern part of the baltica plate later convergence between gondwana and laurussia involving transcurrent mostly dextral movements combined with tectonic escape phenomena would account for the present complexity of the belt and for the narrow strip the allochthonous terranes delineate in westernmost europe

the flow dynamics of an extremely large volume pyroclastic flow the 2 08 ma cerro galan ignimbrite nw argentina and comparison with other flow types the 2 08 ma cerro galan ignimbrite cgi represents a 630 km3 dense rock equivalent vei 8 eruption from the long lived cerro galan magma system 6 ma it is a crystal rich 35 60 pumice 10 generally and lithic poor 5 generally rhyodacitic ignimbrite lacking a preceding plinian fallout deposit the cgi is preserved up to 80 km from the structural margins of the caldera but almost certainly was deposited up to 100 km from the caldera in some places only one emplacement unit is preserved in proximal to medial settings and in most distal settings suggesting constant flow conditions but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges it interacted with valley walls resulting in flow instabilities that generated multiple depositional units often separated by pyroclastic surge deposits the cgi preserves a widespread sub horizontal fabric defined by aligned elongate pumice and lithic clasts and minerals e g biotite a sub horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed the cgi is poor in both vent derived accessory lithics and locally derived lithics from the ground surface accidental lithics locally derived lithics are small 20 cm and were not transported far from source points all data suggest that the pyroclastic flow system producing the cgi was characterised throughout by high sedimentation rates resulting from high particle concentration and suppressed turbulence at the depositional boundary layer despite being a low aspect ratio ignimbrite based on these features we question whether high velocity and momentum are necessary to account for extensive flow mobility it is proposed that the cgi was deposited by a pyroclastic flow system that developed a substantial high particle concentration granular under flow which flowed with suppressed turbulence high particle concentration and fine ash content hindered gas loss and maintained flow mobility in order to explain the contemporaneous maintenance of high particle concentration high sedimentation rate at the depositional boundary layer and a high level of mobility it is also proposed that the flow s was continuously supplied at a high mass feeding rate it is also proposed that internal gas pressure within the flow directed downwards onto the substrate over which the flow was passing reduced the friction between the flow and the substrate and also enhanced its mobility the pervasive sub horizontal fabric of aligned pumice lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow consistent with the existence of a laminar shearing granular flow regime during the final stages of transport and deposition

erosion transport and segregation of pumice and lithic clasts in pyroclastic flows inferred from ignimbrite at lascar volcano chile abstract investigations have been made on the distribution of pumice and lithic clasts in the lithic rich soncor ignimbrite 26 5 ka and the 1993 pumice flow deposits of lascar volcano chile the soncor ignimbrite shows three main lithofacies which grade into one another coarse lithic breccias range from matrix poor stratified varieties irregular shaped sheets and elongate hummocks in proximal environments to breccia lenses with pumiceous ignimbrite matrix massive lithic rich facies comprise the bulk of the ignimbrite pumice rich facies are bimodal with abundant large pumice clasts often with reverse grading rare lithic clasts and occur distally and on high ground adjacent to deep proximal valleys in the 1993 pyroclastic flow deposits lithic rich facies are deposited on slopes up to 14 whereas pumice rich facies are deposited only on slopes

eclogites of paleozoic or early alpine age in the basement of the penninic siviez mischabel nappe wallis switzerland outcrops of eclogites and retroeclogites occur in the normal limb of the siviez mischabel fold nappe penninic alps wallis switzerland the ecologites are contained in the gneisses of the ergischhorn and barneuza units turtmanntal and val de moiry where they usually associate with banded amphibolites high pressure mineral assemblages in the eclogites pyrope rich almandine and omphacite are overprinted by later amphibolite and greenschist facies metamorphic minerals according to coleman s classification the eclogites belong to the b type with pt values around 650 c for a minimum pressure of 15 kb geochemical investigations suggest a tholeiitic basalt affinity some problems still remain to solve these are 1 the geodynamic contexts and 2 the age of the eclogite facies overprint whether alpine or pre alpine in the absence of radiometric ages the following arguments favour a pre alpine lower paleozoic or older age of the high pressure metamorphism 1 1 the presence of polymetamorphic paragenesis the eclogitic mineral assemblages being the oldest 2 2 the b type of the eclogites which are associated tectonically with polymetamorphic gneisses 3 3 the presence in the same tectonic units of permian gabbros which show no signs of eclogite facies overprint 4 4 the presence of autochthonous sedimentary cover rocks that have been dated triassic to eocene this makes it difficult to assume an eo alpine cretaceous subduction of the underlying basement 5 5 the existence of striking similarity with other pre alpine eclogites related to a lower paleozoic 500 460 ma high pressure event well documented in some alpine and extra alpine basement units it could have associated with a caledonian subduction of oceanic crust

the ghomarides nappes rif coastal range morocco a variscan chip in the alpine belt the ghomarides nappes are the uppermost basement nappes in the rifan alpine belt paleozoic rocks constitute the prominent part of these nappes together with triassic and younger cover sediments four nappes are distinguished each one with a distinct stratigraphical sequence especially for the devonian and carboniferous periods sedimentological and structural arguments plead for an initial location on the northern margin of the african continent the main folding and metamorphic events are shown to be eo variscan late acadian tectonic vergence was to the north present coordinates late variscan hercynian shortening gave upright nw se trending folds alpine ductile overprints are scarce except at the base of the lowest nappe where k ar phengite ages are about 25 ma the ghomarides nappes correlate with the malaguides nappes betic cordilleras southern spain the kabylian northern algeria and calabrian southern italy paleozoic nappes the tuscan south alpine basement and the upper austro alpine nappes the proposed correlations allow us to restaure the southern margin of the hercynian belt along northwestern africa

sedimentological response to late mesozoic subduction in the penninic windows of the eastern alps late mesozoic subduction of penninic oceanic lithosphere finds its response in the sedimentary record the corresponding sediments are deposited in a deep sea trench environment and are developed as distal partly proximal flysches containing breccias and olistolites which are up to kilometer sized wildflysch in the tauern window this facies is represented by the nordrahmen zone which is the equivalent to the matrei zone it is proposed to apply the term matrei zone to the entire zone it forms the high parts of the bundner schiefer and tauernflysch formation the olistolites derive from the unstable austroalpine continental margin lower austroalpine

saltation threshold for pyroclasts at various bedslopes wind tunnel measurements abstract pyroclastic density currents represent one of the most destructive hazards associated with explosive volcanism this destructive nature does not only urge the need for but also prevents the obtainment of in situ measurements of their physical characteristics the resulting deposits offer however evidence of the physics of their sedimentation phase deposits of dilute pyroclastic density currents frequently exhibit repeated cycles of deposition and erosion yielding insights into the turbulent shearing along the ground the utilization of such field observations can be greatly enhanced by the calibration of physical properties of such flows under well constrained laboratory conditions here wind tunnel measurements were performed using pyroclastic particles the saltation threshold and surface roughness length were calculated for wind above a pyroclastic bed the results serve as an aid in linking field observations to quantitative values of turbulent shear at the base of a flow scoria and pumice particles were investigated as a function of grain size 1 fractions between 0 125 and 4 mm as well as the influence of bedslope 20 to 25 in 10 steps the results point to the dominant control of density grain size and contrary to previous assumptions differ moderately from results obtained for round beads properly utilized the dataset enables the establishment of a link between the grain size of natural deposits and the shearing extant during their emplacement depending on the type of sedimentary structure observed in the field the saltation threshold can be used as a minimal or a maximal shearing limit during emplacement of dilute pyroclastic density current deposits stoss aggrading laminations likely involve the saltation threshold as an upper limit whereas for truncation events it must have been overcome the effect of particle concentration within the flow a critical parameter for pyroclastic density currents and the extent of validity of the data are discussed

impact generated winds on mars abstract bright and dark wind streaks across mars record wind patterns related to atmospheric circulation in some cases these streaks represent erosion of a surface veneer in others they indicate sand grains mobilized by strong vortices shed off of positive relief such as crater rims while many streaks change length or orientation over time others not only remain unchanged but also may indicate a completely different wind direction these permanent streaks could reflect past circulation patterns in response to conditions related to orbital forcing e g thomas and veverka 1979 veverka et al 1981 here however we focus on a subset of permanent wind streaks unrelated to global circulation rather to impact generated winds that can extend more than 500 km away from the crater nighttime images from the mars odyssey s thermal emission imaging system themis reveal certain large craters 15 km in diameter having sets of thermally bright streaks that radiate from certain fresh impact craters these streaks extend from pre existing topographic highs crater rims wrinkle ridges beyond the continuous ejecta deposits to more than 6 crater radii unrelated to secondary craters for illustration this contribution primarily focuses on the 20 km diameter santa fe crater in chryse planitia context camera ctx images reveal that these streaks correspond to zones of erosion the thermally bright rays in nighttime images correlate with regions where coarser materials have been exposed not always resolved even in hirise images models of the impact process indicate impact generated vapor most likely generated intense winds that scoured the region well before arrival of secondary craters and later ejecta run out flows pre existing relief such as crater rims disturbed this flow and generated intense cross flow instabilities resulting in long parallel streaks

impact and volcanism a momentum scaling law for erosion the controversy that raged through the 1950s 1970s over similarities and differences between meteorite impact and volcanic processes is revisited in this paper we propose that there are quantitative similarities in erosion caused by high speed ejecta produced by either impacts or volcanic processes field and petrographic data from the manicouagan impact crater canada are used to demonstrate that during the emplacement of the impact melt sheet erosion occurred at a rate of 2562 kg m 2 s 1 field data for the mount st helens lateral blast of may 18 1980 suggest an erosion rate of 2 kg m 2 s 1 and field data for a small pyroclastic flow on august 7 1980 suggest an erosion rate of 14 kg m s 1 it is proposed that these three rates were determined dominantly by the momentum of the ejecta and a quantitative formulation based on lofting theory is given a new application of the monte carlo approach to analysis provides minimum most likely and maximum estimates for both the field and the theoretical analyses the substantial erosion that occurs by fast moving flows results in mixing of stratigraphic components over large distances and to very fine scales

lower palaeozoic volcanism of the eastern alps and its geodynamic implications a geodynamic model for the lower palaeozoic evolution of the eastern alps upper ordovician to middle devonian gurktal nappe styrian greywacke zone palaeozoic of graz is discussed based on new field work and numerous geochemical analyses of volcanic rocks it is suggested that in the upper ordovician there existed on the one hand a passive continental margin or continental rift flooded by the sea where mildly alkaline basalts erupted magdalensberg series carinthia kaser series gurktaler alpen on the other hand a convergent plate boundary in the form of an active continental margin or island arc or in the form of an orogen formed by continent continent collision which can be recognized by the eruption of calc alkaline volcanic rocks probably also existed in the upper ordovician blasseneck porphyroid styrian greywacke zone nockseries gurktaler alpen in the lower silurian a reorganization of the plate boundaries took place resulting in the disintegration of the upper ordovician collision zone along deep reaching faults further extensional movements during the silurian and devonian led to the disintegration of a broad shelf platform and to the eruption of alkaline within plate basalts the extensional movements were responsible for the thinning of the continental crust and a general subsidence which probably lasted until lower carboniferous time

solid inclusions in chrome spinels and platinum group element concentrations from the hochgrossen and kraubath ultramafic massifs austria a great variety of platinum group mineral sulfide and silicate inclusions in chrome spinel from hochgrossen and kraubath ultramafic massifs and platinum group element contents of three different rock types have been investigated both ultramafic massifs are tectonically isolated bodies variably serpentinized and metamorphosed greenschist to lower amphibolite facies and show ophiolitic geochemical affinities the chromite from massive chromitites and disseminated in serpentinized dunites and serpentinites exhibits compositional zonation as the result of alteration during serpentinization and metamorphism three distinctive alteration stages are indicated in the chrome spinels from the hochgrossen whereas alteration is less significant in chromites from kraubath the core of chrome spinel represents the least altered part surrounded by an inner rim characterized by slight compositional differences in cr mn fe2 and al with respect to the core the outer rim is formed by ferritchromite with a sharp boundary to the inner rim and shows a significant decrease of al mg cr and increase of fe2 fe3 and ni compared to the core two different groups of inclusions in chrome spinel are present the first group occurs within the chromite core and comprises olivine orthopyroxene amphibole sulfides and platinum group minerals i e dominated by ru os ir sulfides the second group is formed by chlorite serpentine galena pyrite arsenopyrite pt pd rh dominated sulfarsenides and sperrylite in particular the abundance of pt pd rh sulfarsenides and arsenides is typical of both ultramafic massifs and is very unusual for chromitites from ophiolites morphology paragenesis and chemical composition indicate a different origin for these two groups of inclusions the first group is intimately related to the crystallisation of the chromite host the second group of inclusions clearly displays a secondary formation during serpentinization and metamorphism closely related to the alteration of chrome spinel and the development of ferritchromite the distribution patterns of the platinum group elements from massive chromitites disseminated chrome spinel bearing serpentinites and serpentinites exhibit variable enrichment of rh pt and pd rh pt for the hochgrossen and kraubath massifs respectively these results are in accordance with the occurrence and distribution of platinum group mineral phases a remobilisation of pt pd and rh together with ni cu and possibly fe as bisulfide and or hydroxide complexes and deposition of metals by the reaction of the metal bearing hydrothermal fluid with chromite is proposed

u pb dating of an early paleozoic bimodal magmatism in the french massif central and of its further metamorphic evolution in the marvejols area southern french massif central the gneissic marvejols supergroup is overthrust on the metasedimentary serie du lot deposited in part prior to 540 ma the allochtonous terranes are characterized by the occurrence of a leptyno amphibolitic group a complex association of mafic and felsic rocks of igneous and sedimentary derivation a 480 10 ma age has been obtained by u pb dating of zircons for the crystallization of both mafic and felsic meta igneous rocks these rocks were emplaced during an important extensional tectonics relics of eclogites pyrigarnites coronite gabbros and hp trondhjemites are clear evidence for a further hp ht event dated at 415 6 ma on zircons from a hp trondhjemite subsequently the marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345 10 ma

numerical analysis of the effect of topography on deposition from dilute pyroclastic density currents abstract pyroclastic density currents pdcs vary between two end members concentrated and dilute when a pdc interacts with an uneven topography the flow field variables velocity pressure bulk density particle concentration may drastically change near the flow substrate boundary these changes may significantly affect the sediment flux and the resulting deposits can record the effects in their facies architecture here we show by means of numerical simulations how a dilute pyroclastic density current interacts with four different types of simple topography namely flat one hill one valley and two hills our numerical scheme treats the very fine particles as being in full thermo mechanical equilibrium with the volcanic gas i e a dusty gas a dusty gas air mixture is defined as a mixture of dusty gas and atmospheric air the trajectories of the coarser particles or discrete phase three grain size classes of 1 mm 5 mm and 10 mm and density of 1500 kg m 3 are tracked as lagrangian particles that interact with the dusty gas air mixture through two way momentum and energy coupling numerical results are used to analyze the local effects of topography on the deposition of the lagrangian particles by monitoring with time and space the local changes at the boundary between the current and the substrate the results show that the sediment flux in the flow boundary zone increases near the stoss sides of hills and in the valleys relative to the flat reference case whereas it decreases along the lee flanks and on top of the hills we use the sediment flux in the flow boundary zone and the grain size distribution of the lagrangian particles as proxies of the deposit features and by these parameters we qualitatively compare simulations with deposits of known eruptions

neoproterozoic to early palaeozoic magmatic evolution in the gondwana derived austroalpine basement to the south of the tauern window eastern alps in the austroalpine basement to the south of the tauern window distinct suites of metabasites occur with orthogneisses in pre early ordovician units tholeiitic and alkaline within plate basalt type metabasites are associated with acid meta porphyroids in the post early ordovician thurntaler phyllite group according to their correlated trace element abundances metabasite zircons crystallized with their host rocks protolith pb pb zircon ages whole rock ta yb th yb and oxygen sr nd isotope data define two principal evolution lines an older evolution at elevated th yb typical of subduction related magmatism started by 590 ma n morb type and 550 530 ma volcanic arc basalt type basic suites which mainly involved depleted mantle sources it finished with mainly crustal source 470 450 ma acid magmatites an other evolution line by tholeiitic and 430 ma alkaline within plate basalt type suites in both pre and post early ordovician units is characterized by an intraplate mantle metasomatism and enrichment trend along multicomponent sources these magmatic evolution lines can be related to a plate tectonic scenario that involved terranes in a progressively mature neoproterozoic to ordovician active margin and a subsequent palaeo tethys passive margin along the north gondwanan periphery

superlakes megafloods and abrupt climate change about 8200 years ago the climate of much of the northern hemisphere cooled abruptly for a period of about 200 years in their perspective clarke et al examine the most likely culprit for this cooling an outburst of fresh water from a vast ice dammed glacial lake in north america the superlake had formed when the kilometers thick ice sheet covering much of north america disintegrated when the ice dam became unstable fresh water flooded from the lake into the north atlantic it remains unclear how this fresh water affected ocean circulation or whether the outburst occurred in more than one stage but the timing points strongly to the outburst flood as the trigger of the 8200 year climate event

40ar 39ar dating of the inner carpathians variscan basement and alpine mylonitic overprinting abstract the 40ar 39ar dating method has been applied to mylonitic rocks from the carpathian mountains these are variscan metamorphic and magmatic rocks that were overprinted during the alpine orogeny the inner western carpathians are conventionally described as a nappe pile formed by northward movement and then subjected to intense strike slip and normal faulting resulting in the formation of mylonitic rocks precise stratigraphie ages provide clear constraints for the geodynamic interpretation related to the succession of alpine events such as the pre gosauian phase during the turonian 90 ma the 40ar 39ar method allows to define the age of the ancient variscan basement and the ages of the alpine overprinting in the high and low tatras mineral ages of around 340 350 ma are yielded by gneisses micaschists and amphibolites providing a younger limit for the variscan metamorphism the emplacement cooling of the variscan granites is defined between 305 and 335 ma these rocks are overprinted by several alpine tectonothermal events the major events are dated at 65 and 86 88 ma during which the mylonites were generated an older event around 140 ma earlier defined by the rb sr method is now confirmed by the 40ar 39ar method dating of phengites extracted from high pressure metamorphic triassic rocks from the kosice area yields new data for the age of this event at 155 ma