debris bed friction of hard bedded glaciers 1 field measurements of debris bed friction on a smooth rock tablet at the bed of engabreen a hard bedded temperate glacier in northern norway indicated that basal ice containing 10 debris by volume exerted local shear traction of up to 500 kpa the corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0 05 and 0 08 a model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1 a small exponent n 2 is likely because stresses in ice are small and flow is transient numerical calculations of the bed normal drag force on a sphere in contact with a flat bed using n 1 show that this force can reach values several hundred times that on a sphere isolated from the bed thus drastically increasing frictional resistance various estimates of basal friction are obtained from this model for example the shear traction at the bed of a glacier sliding at 20 m a 1 with a geothermally induced melt rate of 0 006 m a 1 and an effective pressure of 300 kpa can exceed 100 kpa debris bed friction can therefore be a major component of sliding resistance contradicting the common assumption that debris bed friction is negligible

a monogenetic surtla type surtseyan volcano from the eocene oligocene waiareka deborah volcanics otago new zealand a model the relics of a small monogenetic continental shelf surtseyan volcano are preserved on the north otago coast south island new zealand in the late eocene early oligocene waiareka deborah volcanics the succession consists of two parts i e a lower interval of bedded lapilli tuffs and lapillistones representing the eruptive aggradational cone building phase and an upper epiclastic sequence representing the post eruptive degradational phase all of the preserved succession appears to have been deposited below storm wave base the lapilli tuffs and lappillistones are subaqueous fall deposits modified contemporaneously by downslope grain flow and turbidity current redeposition and perhaps by local reworking caused by turbulent thermal eddies the absence of major discordances in the lapilli tuffs suggests that the active eruptive period was very short lived perhaps lasting only a few days the epiclastic succession consists of redeposited volcanic skeletal lime mud and glauconitic detritus transported by debris flows and other mass flows the initial epiclastic unit a debris flow appears to represent the sector collapse of a significant part of the cone the appearance of fossils and rounded clasts low in the epiclastic succession coincides with stabilisation of the top of the submarine volcanic edifice development of a wave planed top and its colonisation by a diverse fauna periodic storm activity swept material off the platform redepositing it as marginal talus ramps surtla a wholly submarine satellite volcanic centre of the 1963 1967 eruptive activity of surtsey is an excellent modern analogue for both the eruptive and post eruptive phases of the bridge point aorere point volcanic centre by analogy with surtla the 120 metres of lapilli tuffs and lapillistones exposed on bridge point and aorere point accumulated in only several days the 25 metres of reworked glauconitic and fossiliferous volcaniclastics represent thousands of years based on the time required for glauconite to form

magmatic and hydromagmatic conduit development during the 1975 tolbachik eruption kamchatka with implications for hazards assessment at yucca mountain nv abstract xenoliths in pyroclastic fall deposits from the 1975 tolbachik eruption constrain the timing and development of subsurface conduits associated with basaltic cinder cone eruptions the two largest tolbachik vents contain xenoliths derived from magmatic and hydromagmatic processes which can be correlated with observed styles of eruption activity although many basaltic eruptions progress from early hydromagmatic activity to late magmatic activity transient hydromagmatic events occurred relatively late in the 1975 eruption sequence magmatic fall deposits contain 0 01 0 3 vol xenoliths from 500 m depth into the dry out zone around the conduit disrupting and ejecting 10 5 10 6 m 3 of wall rock through hydromagmatic processes with conduits widening to 8 48 m hydromagmatic falls contain 60 75 vol of highly fragmented xenoliths with juvenile clasts displaying obvious magma water interaction features during the largest hydromagmatic event unusual breccia bombs formed containing a wide range of fresh and pyrometamorphic xenoliths suspended in a quenched basaltic matrix hydromagmatic activity during the 1975 tolbachik eruption occurred below likely fragmentation depths for a basalt containing 2 2 wt magmatic water this activity is more likely related to conduit wall collapse rather than variations in conduit flow pressure in contrast larger volume silicic eruptions may have transient hydromagmatic events in response to conduit flow dynamics above the magma fragmentation depth the 1975 tolbachik volcanoes are reasonably analogous to quaternary basaltic volcanoes in the yucca mountain region and can guide interpretations of their poorly preserved deposits the youngest basaltic volcanoes near yucca mountain have cone deposits characterized by elevated xenolith abundances and distinctive xenolith breccia bombs remarkably similar to 1975 tolbachik deposits extrapolation of 1975 tolbachik data suggests conduits for some yucca mountain basaltic volcanoes may have widened locally on the order of 50 m in response to late stage hydromagmatic events

eruption processes and deposit characteristics at the monogenetic mt gambier volcanic complex se australia implications for alternating magmatic and phreatomagmatic activity the 5 ka mt gambier volcanic complex in the newer volcanics province australia is an extremely complex monogenetic volcanic system that preserves at least 14 eruption points aligned along a fissure system the complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner the facies are 1 coherent to vesicular fragmental alkali basalt effusive hawaiian spatter and lava flows 2 massive scoriaceous fine lapilli with coarse ash strombolian fallout 3 bedded scoriaceous fine lapilli tuff violent strombolian fallout 4 thin medium bedded undulating very fine lapilli in coarse ash dry phreatomagmatic surge modified fallout 5 palagonite altered cross bedded medium lapilli to fine ash wet phreatomagmatic base surges and 6 massive palagonite altered very poorly sorted tuff breccia and lapilli tuff phreato vulcanian pyroclastic flows since most deposits are lithified to quantify the grain size distributions gsds image analysis was performed the facies are distinct based on their gsds and the fine ash to coarse fine ash ratios these provide insights into the fragmentation intensities and water magma interaction efficiencies for each facies the eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles except for a magmatic horizon of effusive activity occurring at both ends of the volcanic complex simultaneously the eruption foci are located along nw se trending lineaments indicating that the complex was fed by multiple dykes following the subsurface structures related to the tartwaup fault system possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates viscosity crystallinity degassing and magma discharge rate as well as hydrological parameters

descriptive nomenclature and classification of pyroclastic deposits and fragments recommendations of the iugs subcommission on the systematics of igneous rocks

phase equilibria of pantelleria trachytes italy constraints on pre eruptive conditions and on the metaluminous to peralkaline transition in silicic magmas the relationships between trachytes and peralkaline rhyolites i e pantellerites and comendites which occur in many continental rift systems oceanic islands and continental intraplate settings is unclear to fill this gap we have performed phase equilibrium experiments on two representative metaluminous trachytes from pantelleria to determine both their pre eruptive equilibration conditions pressure temperature h2o content and redox state and liquid lines of descent experiments were performed in the temperature range 750 950 c pressure 0 5 1 5 kbar and fluid saturation conditions with xh2o h2o h2o co2 ranging between zero and unity redox conditions were fixed below the nickel nickel oxide buffer nno the results show that at 950 c and melt water contents h2omelt close to saturation trachytes are at liquidus conditions at all pressures clinopyroxene is the liquidus phase being followed by iron rich olivine and alkali feldspar comparison of experimental and natural phases abundances and compositions yields the following pre eruptive conditions p 160 5 kbar t 925 25 c h2omelt 261 wt and fo2 between nno 0 5 and nno 2 a decrease in temperature from 950 c to 750 c as well as of h2omelt promotes a massive crystallization of alkali feldspar to over 80 wt iron bearing minerals show gradual iron enrichment when t and fo2 decrease trending towards the compositions of the phenocrysts of natural pantellerites despite the metaluminous character of the bulk rock compositions residual glasses obtained after 80 wt crystallization evolve toward comenditic compositions owing to profuse alkali feldspar crystallization which decreases the al2o3 of the melt leading to a consequent increase in the peralkalinity index pi molar na2o k2 o al2 o3 this is the first experimental demonstration that peralkaline felsic derivatives can be produced by low pressure fractional crystallization of metaluminous mafic magmas our results show that the pantelleritic magmas of basalt trachyte rhyolite igneous suites require at least 95 wt of parental basalt crystallization consistent with trace element evidence redox conditions through their effect on fe ti oxide stabilities control the final iron content of the evolving melt

joint bounded crescentic scars formed by subglacial clast bed contact forces implications for bedrock failure beneath glaciers glaciers and ice sheets are important agents of bedrock erosion yet the precise processes of bedrock failure beneath glacier ice are incompletely known subglacially formed erosional crescentic markings crescentic gouges lunate fractures on bedrock surfaces occur locally in glaciated areas and comprise a conchoidal fracture dipping down ice and a steep fracture that faces up ice here we report morphologically distinct crescentic scars that are closely associated with preexisting joints termed here joint bounded crescentic scars these hitherto unreported features are ca 50 200 mm deep and involve considerably more rock removal than previously described crescentic markings the joint bounded crescentic scars were found on abraded rhyolite surfaces recently exposed 20 years beneath a retreating glacier in iceland as well as on glacially sculpted precambrian gneisses in nw scotland and various precambrian rocks in ontario glaciated during the late pleistocene we suggest a common formation mechanism for these contemporary and relict features whereby a boulder embedded in basal ice produces a continuously migrating clast bed contact force as it is dragged over the hard bedrock bed as the ice embedded boulder approaches a preexisting joint in the bedrock stress concentrations build up in the bed that exceed the intact rock strength resulting in conchoidal fracturing and detachment of a crescentic wedge shaped rock fragment subsequent removal of the rock fragment probably involves further fracturing or crushing comminution under high contact forces formation of joint bounded crescentic scars is favoured by large boulders at the base of the ice high basal melting rates and the presence of preexisting subvertical joints in the bedrock bed we infer that the relative scarcity of crescentic markings in general on deglaciated surfaces shows that fracturing of intact bedrock below ice is difficult but that preexisting weaknesses such as joints greatly facilitate rock failure this implies that models of glacial erosion need to take fracture patterns of bedrock into account

towards the reconstruction of the shallow plumbing system of the barombi mbo maar cameroon implications for diatreme growth processes of a polygenetic maar volcano abstract understanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention while there is no direct evidence of the presence of diatremes beneath most of the young maars their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars as an alternative this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar forming eruptions based on field investigations pyroclast distribution componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the barombi mbo maar bmm a polygenetic maar volcano in cameroon the bmm consists of three diatremes that formed during distinct eruptive events and coalesced to produce an amalgamated maar diatreme two end member types of eruption styles from the dry magmatic to the wet phreatomagmatic explosions governed the formation of the maar in total a minimum of 0 0658 km 3 of magma dense rock equivalent corrected was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from srtm30m data corrected by rock textural data collected from the field the distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process which originated probably within the uppermost part of the diatreme from the distribution and morphology of juvenile clasts in the deposits it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions sometimes with lateral and vertical variations in the position of the explosion loci a prospective juvenile based conceptual model is proposed for the formation of the bmm the model suggests that during maar forming eruptions explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic dominated upper crater infill to deposit juvenile poor 90 vol

submarginal debris transport and till formation in active temperate glacier systems the southeast iceland type locality abstract exhaustive sedimentological analysis of freshly exposed subglacial surfaces and moraines in southern iceland provides diagnostic sedimentological signatures of a debris transport pathways through active temperate glacier snouts and b till production in subglacial traction zones dominated by deforming layers three till end members are recognised based on stratigraphic architecture 1 thin and patchy tills over eroded bedrock 2 single push moraines and complexes and 3 overridden moraines or outwash fans typical till thicknesses are 0 10 1 40 m with each till relating to a deformation event driven by the seasonally tuned processes of glacier sub marginal shearing freeze on squeezing and bulldozing clast form trends demonstrate progressive modification towards mature forms in subglacial traction zones with till being clearly differentiated from scree and glacifluvial deposits clast macrofabric strengths are variable rarely matching those of laboratory shearing experiments except where obviously lodged clasts are abundant they also consistently record former glacier flow directions but localized variability is introduced by bedrock protuberances cavity infill clast interference and freshly imported plucked clasts within tills macrofabrics strengthen from lower b horizon to upper a horizon tills but at the outer edges of sub marginally thickening till wedges or push moraines seasonally driven cycles of squeezing flowage freeze on melt out and bulldozing give rise to a range of clast macrofabric strengths as well as superimposed deformation signatures this reflects two extremes of till emplacement including the more mobile flowing and often liquefied matrixes in push squeeze moraines and in contrast the lodgement deformation and ploughing at the thin end of sub marginal till wedges

the architecture eruptive history and evolution of the table rock complex oregon from a surtseyan to an energetic maar eruption abstract the table rock complex trc pliocene pleistocene first documented and described by heiken heiken g h 1971 tuff rings examples from the fort rock christmas lake valley basin south central oregon j geophy res 76 5615 5626 is a large and well exposed mafic phreatomagmatic complex in the fort rock christmas lake valley basin south central oregon it spans an area of approximately 40 km2 and consists of a large tuff cone in the south trc1 and a large tuff ring in the northeast trc2 at least seven additional smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions the first period of activity trc1 initiated with a surtseyan style eruption through a 60 70 m deep lake the trc1 deposits are dominated by multiple 1 2 m thick fining upward sequences of massive to diffusely stratified lapilli tuff with intermittent zones of reverse grading followed by a finely laminated cap of fine grained sediment the massive deposits are interpreted as the result of eruption fed subaqueous turbidity current deposits whereas the finely laminated cap likely resulted from fallout of suspended fine grained material through a water column other common features are erosive channel scour and fill deposits massive tuff breccias and abundant soft sediment deformation due to rapid sediment loading subaerial trc1 deposits are exposed only proximal to the edifice and consist of cross stratified base surge deposits the eruption built a large tuff cone above the lake surface ending with an effusive stage which produced a lava lake in the crater 365 m above the lake floor a significant repose period occurred between the trc1 and trc2 eruptions evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences the trc2 eruption was the last and most energetic in the complex general edifice morphology and a high percentage of accidental material suggest eruption through saturated trc1 deposits and or playa lake sediments trc2 deposits are dominated by three dimensional dune features with wavelengths 200 500 m perpendicular to the flow and 20 200 m parallel to the direction of flow depending on distance from source large u shaped channels 10 32 m deep run up features over obstacles tens of meters high and a large 13 m chute and pool feature are also identified the trc2 deposits are interpreted as the products of multiple erosive highly inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions

tectonic controls on the pleistocene holocene wudalianchi volcanic field northeastern china abstract the wudalianchi volcanic field developed during the pleistocene holocene but is dormant at present its latest eruption occurred in 1719 1721 ad the volcanic rocks are high potassium alkaline basalts derived from the upper mantle c 100 120 km depth as indicated by geochemical data the field is located in an old tectonic transition zone surrounded by four regional normal faults the volcanic craters are aligned along ne nne striking fractures and faults although a nnw striking sub surface fracture zone probably controlled the eruptions beginning at 1 33 0 08 ma beneath the volcanic field the moho interface lies at a depth of 33 5 35 km eruptions evolved from fissure type to central type eruptions the field is characterized by an intraplate tectonic setting in a non orogenic compressional regime which resulted from the subduction of the west pacific plate beneath the eastern asian continental margin starting 3 2 ma ago

lateral plucking as a mechanism for elongate erosional glacial bedforms explaining megagrooves in britain and canada megagrooves are kilometre scale linear topographic lows carved in bedrock separated by ridges typically in areas of largely devoid of till they have been reported from several areas covered by pleistocene glaciations such as canadian nw territories michigan and nw scotland here we report two previously undocumented megagroove fields from ungava canada and northern england and present new analyses of the megagrooves from nw scotland this paper seeks to determine the nature of the lithological and structural controls on the occurrence and formation of megagrooves analysis of both geomorphological and bedrock properties shows that megagrooves are generally a confined to well stratified or layered bedrock such as meta sedimentary rocks with closely spaced joints and tend not to occur on massive rocks such as gneiss or granite or thick bedded sedimentary rocks b subparallel to palaeo ice flow and the strike of the strata and tend not to occur where palaeo ice flow is at high angles to the strike of strata c produced by significant glacial erosion by sustained unidirectional ice flow detailed analysis of megagrooves in nw scotland shows that neither glacio fluvial erosion nor differential abrasion was the dominant mechanism of formation a mechanism here termed lateral plucking is suggested that involves block plucking on rock steps parallel to ice flow removal of joint bounded blocks from such rock steps involves a component of rotation along a vertical axis block removal may be enhanced by a direct component of shear stress onto the vertical stoss sides the lateral plucking mechanism results in horizontal erosion at right angles to the ice flow and enhances the groove ridge topography megagrooves are potentially useful as palaeo ice flow indicators in areas devoid of till and can thus complement the palaeo ice stream datasets which are presently largely based on soft sediment landform studies

fault bounded blocks and their role in localising sedimentation and deformation adjacent to the alpine fault southern new zealand abstract continental transform boundaries in detail consist of zones of fault bounded blocks adjacent to the principal active transform fault s relative movement of these blocks in response to plate boundary motions gives rise to differential vertical displacements of the earth s crust and hence exercises a degree of control on the tectonic evolution of sedimentary basins adjacent to the transform boundary the cenozoic sedimentary basins of southwest new zealand have several features in common that may be attributed directly to the movement on the adjacent plate boundary however their location detailed sedimentary evolution and tectonic development may best be understood in the context of the relative displacements of a number of fault bounded blocks in response to the plate motions in particular movements of the fiordland block bounded by the alpine and moonlight fault zones exercised a large measure of influence on the development of the basins the tectonic model presented is consistent with plate boundary motions inferred from marine magnetic anomalies while at the same time providing an explanation for many of the more detailed features of the basins

models of maar volcanoes lunar crater nevada usa maar volcanoes are generally understood to be the result of highly energetic explosive interaction between magma and water groundwater or surface water two end member conceptual models have been proposed to explain the dimensions diameter depth of maar craters 1 an incremental growth model where a crater grows due to subsidence and ejection of debris over the course of many explosions and the final size is an integrated result of multiple explosive events 2 a model in which the dimensions of a maar crater are the result of the largest single explosion during the lifetime of the maar major explosion dominated model in the latter case the maar size can be used to estimate the energy and depth of the largest explosion which in turn allows estimation of the magma mass involved this paper describes lunar crater maar nevada usa and tests the two models as explanations for the characteristics of the volcano in particular the major explosion dominated model this model implies magma mass and supply rates that are unrealistic and the tephra at the maar do not contain key features observed in the ejecta at large single explosion craters the incremental growth model seems most suitable based upon geological evidence

descriptive nomenclature and classification of pyroclastic deposits and fragments the iugs subcommission on the systematics of igneous rocks herein presents its recommendations on the nomenclature and classification of pyroclastic and mixed pyroclastic epiclastic deposits using descriptive mainly granulometric criteria the recommendations are the result of an international inquiry by means of questionnaires during the last four years

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

sediment evacuation and glacial erosion rates at a small alpine glacier 1 glacial erosion is known to be driven by sliding which occurs in brief episodes in the melt season what is not known is how well this sliding history is reflected in the sediment output from a glacier and to what degree temporary sediment storage in the glacier smoothes the history of sediment production we document sediment evacuation rates for three seasons at bench glacier a small temperate alpine glacier in the chugach range of alaska two lines of evidence suggest that subglacial sediment storage is small strong hysteresis in the relationship between sediment and water discharge argues for sediment exhaustion from the subglacial system on seasonal and flood event timescales uplift of the glacier surface during periods of enhanced basal motion reflects intimate contact of the glacier sole with up glacier inclined stoss slopes of bedrock bumps sediment evacuation rates at bench glacier should therefore closely track glacial erosion rates over annual timescales much of the sediment emerges in a major pulse that closely follows the termination of enhanced sliding each year suggesting that the conduits and well connected cavities that allowed removal of englacial water to terminate sliding also promoted efficient transport of sediment this small glacier efficiently modifies the landscape lowering its bed by 1 2 mm yr while sliding only 1 2 m yr our sparse measurements of instantaneous bed load discharge average 41 of concurrent suspended sediment discharge assuming that half of the suspended load derives from abrasion of subglacially quarried clasts roughly two thirds of the sediment evacuated can be attributed to quarrying

phreatomagmatic eruptive and depositional processes during the 1949 eruption on la palma canary islands abstract in 1949 a 5 week long magmatic and phreatomagmatic eruption took place along the active volcanic ridge of la palma canary islands two vents duraznero and hoyo negro produced significant pyroclastic deposits the eruption began from duraznero vent which produced a series of deposits with an upward decrease in accidental fragments and increase in fluidal ash and spatter together inferred to indicate decreasing phreatomagmatic interaction hoyo negro erupted over a 2 week period producing a variety of pyroclastic density currents and ballistic blocks and bombs hoyo negro erupted within and modified an older crater having high walls on the northern to southeastern edges southwestern to western margins of the crater lay 50 to 100 m lower strongly contrasting deposits in the different sectors n se vs sw w were formed as a result of interaction between topography weak eruptive columns and stratified pyroclastic density currents tephra ring deposits are thicker and coarser grained than upper rim deposits formed along the higher edges of the crater and beyond the crater margin valley confined deposits are thicker than more thinly bedded mantling deposits on higher topography these differences indicate that the impact zone for the bulk of the collapsing tephra laden column lay within the crater and that the high crater walls inhibited escape of pyroclastic density currents to the north and east the impact zone lay outside the low sw w rims however thus allowing stratified pyroclastic density currents to move freely away from the crater in those directions depositing thin sections

plate boundary tectonics global sea level changes and the development of the eastern south island continental margin new zealand southwest pacific the great south basin is a mid cretaceous continental margin rift related to the break up of the new zealand sector of gondwanaland syn rift cretaceous paleogene sediment accumulated up to a thickness of 8 km followed by post rift subsidence of the northwest and southeast basin margins to form the canterbury basin and campbell plateau respectively the sedimentary evolution of the eastern south island continental margin is described within the context of four megasequences containing eight regionally recognizable seismic sequences syn rift phases 1 2 non marine graben fill post rift phases 3 6 epeiric marine transgression paraconformity phase 7 mid oligocene maximum transgression and syn orogenic phase 8 plate boundary driven terrigenoclastic regression the successive phases of this overall transgressive regressive cycle can be equated with the natural and progressive development of the sedimentary sequence within known regional factors the eastern south island margin sequence forms an appropriate southern hemisphere test of the global sea level curves of vail et al 1977 and haq et al 1986 although eustatic sea level cycles may have played some part in the differentiation of transgressive phases 3 7 the stratigraphic sequence is adequately interpreted in terms of intrinsic basin evolution within a local tectonic context

a transition from strombolian to phreatomagmatic activity induced by a lava flow damming water in a valley abstract the golan heights is a plio pleistocene volcanic plateau cinder cones of late pleistocene age are very common in the eastern and northern golan while phreatomagmatic deposits are relatively rare and occur just in two structures the maar of birket ram and the tuff ring of mt avital the complex of mt avital includes two large cinder cones a tuff ring with an elongated central depression and several basaltic flows some of them breach the cinder cones the exposed eruptive history of the complex includes 1 an early stage of basaltic lava flows 2 strombolian activity and the buildup of the southern cinder cone 3 a second stage of basaltic flows and the buildup of the northern cinder cone and then a transition to 4 phreatomagmatic explosions the phreatomagmatic deposits include surges lapilli fallout deposits and coarse grained lithic tuff breccias which were found up to 200 m above the central depression basaltic and scoriaceous clasts are the main component of all deposits while juvenile material is usually a minor component almost absent in the lapilli deposits it is suggested that the phreatomagmatic events in mt avital were induced by the infiltration of water from a lake that existed in a nearby topographic low quneitra valley the lake was formed or significantly expanded at about 300 ka due to a lava flow that blocked the drainage of the valley to the west the interlayering of tuff and scoria at the top of the northern cinder cone and the good preservation of a lava flow top breccia under the surges imply that the phreatomagmatic activity immediately followed and even coincided with the last stages of strombolian activity it is suggested that the dry wet transition was triggered by the effusion of the second stage lavas and the buildup of the northern cinder cone which probably caused a reduction of pressure in the magmatic system and allowed the lake water an access to the magmatic system the minimum age of the phreatomagmatic events is determined by a 54 ka musterian site which lies directly on top of the tuff in the quneitra valley

depositional processes of the suwolbong tuff ring cheju island korea the suwolbong pyroclastic sequence in the western part of cheju island korea comprises partly preserved rim beds of a quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline the sequence consists of breccia lapillistone lapilli tuff and tuff eighteen sedimentary facies are established and organized into six lateral facies sequences lfs and seven vertical facies sequences vfs the lfs 1 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar bedded lfs 1 undulatory bedded lfs 4 or climbing dune bedded lfs 5 lapilli tuff units they are representative of relatively dry base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting the lfs 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent it results from relatively wet base surge in which sorting is poor due to the cohesion of damp ash the lfs 3 comprises well sorted lapilli tuff and stratified tuff further downcurrent suggestive of deposition from combined fall and surge of relatively dry hydroclastic eruption all seven vertical facies sequences generally comprise two facies units of coarse grained fines depleted lapilli tuff and an overlying fine grained tuff these sequences are suggestive of deposition from base surge that consists of a turbulent head and a low concentration tail depositional processes in the suwolbong tuff ring were dominated by a relatively dry base surge the base surge comprises turbulent and high concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport as the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air it develops large scale turbulent eddies and is segregated into coarse grained bedload and overlying fine grained suspension forming thinly stratified units further downcurrent the base surge may be either cooled and deflated or pushed up into the air depending on its temperature the suwolbong tuff ring comprises an overall wet to dry cycle with several dry to wet cycles in it suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise

glacial erosion and bedrock properties in nw scotland abrasion and plucking hardness and joint spacing subglacial erosion beneath glaciers occurs predominantly by abrasion and plucking producing distinct erosional forms the controls on the relative importance of abrasion vs plucking are poorly understood on the one hand glacial conditions that favour or suppress cavity formation ice velocity ice thickness and water pressure are thought to favour plucking or abrasion respectively conversely bedrock properties are also known to control landforms but this has rarely been analysed quantitatively in this study we compare landforms and bedrock properties of sandstone and quartzite at the bed of a palaeo ice stream near ullapool in nw scotland the boundary between the rock types is at right angles to the westward palaeo ice flow and palaeoglacial conditions on both rock types were similar we report quantitative parameters for bedrock properties schmidt hammer hardness and joint spacing and use morphometric parameters to analyse the landforms torridon sandstone is soft but thick bedded and with a wide joint spacing erosional bedforms include roche moutonnees with smoothed tops and concave stoss sides whalebacks and elongate p forms indicating a high proportion of abrasion over plucking cambrian quartzite is hard but thin bedded with narrow joint spacing erosional landforms are angular to subangular with abundant plucked lee faces suggesting a high proportion of plucking over abrasion hardness and joint spacing thus exert a strong control on subglacial erosional landforms and the mechanisms that formed them thus glacial conditions ice velocity ice thickness can only be inferred from glacial erosional landforms if the effects of bedrock properties of the substrate are considered

eruptive conditions and depositional processes of narbona pass maar volcano navajo volcanic field navajo nation new mexico usa phreatomagmatic deposits at narbona pass a mid tertiary maar in the navajo volcanic field nvf new mexico usa were characterized in order to reconstruct the evolution and dynamic conditions of the eruption our findings shed light on the temporal evolution of the eruption dominant depositional mechanisms influence of liquid water on deposit characteristics geometry and evolution of the vent efficiency of fragmentation and the relative importance of magmatic and external volatiles the basal deposits form a thick 5 20 m massive lapilli tuff to tuff breccia deposit this is overlain by alternating bedded sequences of symmetrical to antidune cross stratified tuff and lapilli tuff and diffusely stratified clast supported reversely graded lapilli tuffs that pinch and swell laterally this sequence is interpreted to reflect an initial vent clearing phase that produced concentrated pyroclastic density currents followed by a pulsating eruption that produced multiple density currents with varying particle concentrations and flow conditions to yield the well stratified deposits only minor localized soft sediment deformation was observed no accretionary lapilli were found and grain accretion occurs on the lee side of dunes this suggests that little to no liquid water existed in the density currents during deposition juvenile material is dominantly present as blocky fine ash and finely vesiculated fine to coarse lapilli pumice this indicates that phreatomagmatic fragmentation was predominant but also that the magma was volatile rich and vesiculating at the time of eruption this is the first study to document a significant magmatic volatile component in an nvf maar diatreme eruption the top of the phreatomagmatic sequence abruptly contacts the overlying minette lava flows indicating no gradual drying out period between the explosive and effusive phases the lithology of the accidental clasts is consistent throughout the vertical pyroclastic stratigraphy suggesting that the diatreme eruption did not penetrate below the base of the uppermost country rock unit a sandstone aquifer 360 m thick by comparison other nvf diatremes several tens of kilometers away were excavated to depths of 1 000 m beneath the paleosurface e g delaney pt ship rock new mexico the vent of a violent volcanic eruption in beus ss ed geological society of america centennial field guide rocky mountain section 2 411 415 1987 this can be accounted for by structurally controlled variations in aquifer thickness beneath different regions of the volcanic field variations in accidental clast composition and bedding style around the edifice are indicative of a laterally migrating or widening vent that encountered lateral variations in subsurface geology we offer reasonable evidence that this subsurface lithology controlled the availability of external water to the magma which in turn controlled characteristics of deposits and their distribution around the vent

identification of maars and similar volcanic landforms in the west eifel volcanic field through image processing of dtm data efficiency of different methods depending on preservation state the west eifel volcanic field comprises 98 maars tuff rings and scoria rings of volcanoes younger than 700 ka digital terrain models dtms allow to automatically measure morphologic parameters of volcanic edifices such as slope angles diameters elevations floor and slope surface areas based on their morphological characteristics we subdivided the west eifel volcanoes into five morphometric groups which reflect different stages of erosion group i ii and iv comprise clear ring shaped structures the difference between these groups is that a tephra ring is well preserved in group i partially preserved in group ii and absent in group iv the original shapes of group iii maars have been lost more substantially than in groups i ii or iv but they nevertheless retain a negative shape a depression and have characteristic channel systems which can be used as search criteria maar diatremes of group v are eroded down to their feeder pipes and form hills in order to locate potential volcanic depressions that are likely to be maar volcanoes we defined common search criteria such as circular negative landforms or particular drainage system patterns for all groups except the least well preserved group v these criteria were taken as the basis for further processing of the dtm data the first processing step consisted of constructing a residual relief calculated as the difference between a filtered smoothed topographic surface and the original dtm data this identifies local topographic features we propose a method for regulating the degree of smoothing which is based on filtering of local maxima according to their distance from a surface constructed from local minima the previously defined search criteria for groups i to iv such as specific ranges of curvature slope circularity density of the drainage network were then applied to the residual relief in order to extract maar shapes not all criteria work equally well for all morphological groups combinations of multiple search criteria therefore yield the best results and efficiently identify most known maars they also separate some probable new hitherto unrecognized maars from a large number of other local depressions we also compared the erosional state of maars to their absolute ages published estimates of erosion rates for maars in the french massif central suggest a general trend of erosion rates decreasing with time elapsed since eruption however this cannot explain the strongly varying ages for maars of the same morphometric group i e similar preservation state in the west eifel volcanic field the spatial distribution of the morphometric groups shows some regularity for example strongly eroded maars are concentrated in the gerolstein area where maar density is highest whereas most well preserved maars are located east of the eifel north south depression ensd most maars affected by fluvial erosion lie near the kyll and kleine kyll streams these observations suggest differential recent uplift of the west eifel volcanic field with stronger uplift occurring west of the ensd

depositional architecture of a maar pitted playa sedimentation in the hopi buttes volcanic field northeastern arizona u s a abstract primary and reworked tephra in the mio pliocene bidahochi formation of northeastern arizona was produced by and partly preserved in maar volcanoes and related hydrovolcanic structures the temporal and geometric relationships among lithofacies of the bidahochi formation illustrate the dynamic interplay between volcanism and sedimentation in an essentially closed sedimentary system maar volcanism in the low relief basinal environments of the bidahochi appears to have resulted in little net aggradation both because the total amount of magma erupted was relatively modest and because the large maar volcano craters produced areas of lowered base level within the basin that formed sediment sinks for much of the newly erupted tephra the volcanically eroded maar craters commonly contain deposits of lakes multiple stream channels and eolian dunes they formed an integral part of the basin s overall depositional environment and affected sedimentation in the playa lake stream systems beyond the vents both during and following the major eruptive period

evolution of lumpy glacial landscapes at 10 100 m scales glaciated landscapes are lumpy sporting rocky bedforms with abraded upvalley faces and sharp quarried downvalley edges using models that operate at two time scales i address glacial landform development at these scales in order to explore the dependence of bed topography and erosion rate on glacier and rock properties sliding at the bed a prerequisite for both abrasion and quarrying is strongly modulated by water pressures short time scale models designed to capture the transient hydro sliding system beneath a temperate glacier quantify the temporal and spatial pattern of effective stress on the glacier bed the bed is subjected to repetitive stressing focused on the edges of bed steps the frequency and magnitude of stress events increasing with ice thickness the numerical model of bed evolution at longer time scales incorporates both abrasion and quarrying of fracture bound blocks the probability of quarrying of a block by a particular stress event is taken to depend inversely upon both block size and the depth of the niche in which it sits upglacier migrating rock bedforms with 10 100 m wavelengths inevitably emerge reflecting efficient quarrying of blocks from downvalley facing steps in the bed the relative importance of abrasion and quarrying and bed roughness are controlled by fracture spacing and major steps in the valley floor are attributable to transitions in fracture spacing

mixed deposits of complex magmatic and phreatomagmatic volcanism an example from crater hill auckland new zealand a series of alternating phreatomagmatic wet and magmatic dry basaltic pyroclastic deposits forming the crater hill tuff ring in new zealand contains one unit m1 which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system the principal characteristics of m1 are a rapid lateral changes in the thicknesses of and proportions in juvenile components in individual beds and b wide ranges of juvenile clast densities in every sample m1 appears to have been associated with an elongate source of highly variable and fluctuating magma water ratios and magma discharge rates this contrasts with the only other documented mixed wet and dry basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred

shallow plumbing systems for small volume basaltic volcanoes 2 evidence from crustal xenoliths at scoria cones and maars abstract monogenetic basaltic volcanoes record complex eruption processes and the relationships between those processes and shallow plumbing are poorly understood this paper explores these relationships building upon earlier studies of exposed shallow plumbing in the upper hundreds of meters of crust beneath volcanoes produced by magmatic eruption processes and those produced by phreatomagmatic processes such as scoria cones and maars respectively eruptive facies and xenolith abundances are described at three scoria cones and at tephra rings around two maars in the san francisco volcanic field arizona usa well constrained subvolcanic sedimentary stratigraphy provides constraints on the depths of origin of xenoliths of different types sedimentary xenolith contents at scoria cones are

clasts with stoss lee form in lodgement tills a discussion abstr act cla sls modifi ed b y glacial erosion a rc d escribed rrom lodge ment till s in front of the g lacier m yrd alsjokull south i cel and many cl asts show modifica ti on of their lower surfaces in th e sa me way as th e ir upper o nes h o wever the lower surfaces have a smooth ed do wn g lacier face and a trunca ted up g lacier fa ce which is th e o pposite o ri entatio n to that o f th e upper surfaces this so call ed d ouble sloss i ee form is interp re ted as a res ponse to basa l trans port over a brading ma teri als fo ll owing depos ition of th e clast a nd succeeded by g lacia l erosion it is sugges ted th a t clasts with a double stoss iee rorm are a diagnos ti c criteri on fo r subg lacial depos ition by lodgement furlh ermore the distributi o n and orienta tion of cl as ts with a stoss i ee form was in ves ti gated o n a ground mora ine surface 17 3 of2199 cl as ts with a n a ax is di a meter 30 cm had a stoss iee fo rm the proportion of clasts with th eir smoo th ed e nds facing up glacier within 22 5 of the ice fl o w directi on was 72 7 thus th e preferred stoss sid e ori enta tion is closely rela ted to th e ice mo vement a nd indica tes th e direc ti on from which th e ice came only 11 7 of bo ulders with a divergent stoss sid e ori ent a ti on a re located in conn ecti o n with annual mo rain es it is sugges ted th a t such bould ers have mainl y been re de posit ed beneath the ice a nd not at the ice rront by minor adva nces of the gl acier

stratigraphy geomorphology geochemistry and hazard implications of the nejapa volcanic field western managua nicaragua abstract the nejapa volcanic field nvf is located on the western outskirts of managua nicaragua it consists of at least 30 volcanic structures emplaced along the n s nejapa fault which represents the western active edge of the managua graben the study area covers the central and southern parts of the volcanic field we document the basic geomorphology stratigraphy chemistry and evolution of 17 monogenetic volcanic structures ticomo a b c d and e altos de ticomo nejapa san patricio nejapa norte motastepe el hormigon la embajada asososca satelite refineria and cuesta el plomo a and b stratigraphy aided by radiocarbon dating suggests that 23 eruptions have occurred in the area during the past 34 000 years fifteen of these eruptions originated in the volcanic field between 28 500 and 2 130 yr bp with recurrence intervals varying from 400 to 7 000 yr most of these eruptions were phreatomagmatic with minor strombolian and fissural lava flow events a future eruption along the fault might be of a phreatomagmatic type posing a serious threat to the more than 500 000 inhabitants in western managua

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

experimental determination of a double valued drag relationship for glacier sliding the contribution of glaciers to sea level rise and their effects on landscape evolution depend on the poorly known relationship between sliding speed and drag at the ice bed interface results from experiments with a new rotary laboratory device demonstrate empirically for the first time a double valued drag relationship like that suggested by some sliding theories steady drag on a rigid sinusoidal bed increases peaks and declines at progressively higher sliding speeds due to growth of cavities in the lee sides of bed undulations drag decreases with increased sliding speed if cavities extend beyond the inflection points of up glacier facing surfaces so that adverse bed slopes in contact with ice diminish with further cavity growth these results indicate that shear tractions on glacier beds can potentially decrease due to increases in sliding speed driven by weather or climate variability promoting even more rapid glacier motion by requiring greater strain rates to produce resistive stresses although a double valued drag relationship has not yet been demonstrated for the complicated geometries of real glacier beds both its potential major implications and the characteristically convex stoss surfaces of bumps on real glacier beds provide stimulus for exploring the effects of this relationship in ice sheet models

abrupt plant physiological changes in southern new zealand at the termination of the mi 1 event reflect shifts in hydroclimate and pco2 abstract a rise in atmospheric co2 is believed to be necessary for the termination of large scale glaciations although the antarctic ice sheet is estimated to have melted from 125 to 50 its modern size there is thus far no evidence for an increase in atmospheric co2 associated with the mi 1 glacial termination in the earliest miocene here we present evidence from a high resolution terrestrial record of leaf physiological change in southern new zealand for an abrupt increase in atmospheric co2 coincident with the termination of the mi 1 glaciation and lasting approximately 20 kyr quantitative p co 2 estimates made using a leaf gas exchange model suggest that atmospheric co2 levels may have doubled during this period from 516 111 ppm to 1144 410 ppm and subsequently returned back to 425 53 ppm the 20 kyr interval with high p co 2 estimates is also associated with a period of increased moisture supply to southern new zealand inferred from carbon and hydrogen isotopes of terrestrial leaf waxes the results provide the first high resolution record of terrestrial environmental change at the oligocene miocene boundary document a 20 kyr interval of elevated p co 2 and increased local moisture availability and provide insight into ecosystem response to a major orbitally driven climatic transition

cinder cone clustering in the transmexican volcanic belt implications for structural and petrologic models cinder cone distributions have most often been characterized using univariate statistics here a new technique to volcano distribution studies cluster analysis is applied to cinder cone distribution in the central transmexican volcanic belt tmvb a total of 1016 cinder cones are identified over an area of approximately 60 000 km2 application of cluster analysis reveals structure in cinder cone distribution using a search radius parameter of 16 km 75 of the cinder cones within the central tmvb are found within eight clusters of 45 159 cones each these clusters are each 2000 to 5000 km2 in area only 22 cones are found within clusters of three or fewer cinder cones indicating that clustering is a pervasive phenomenon some petrologic variation is evident among clusters low mg alkaline cinder cones are found within a single cluster 360 400 km from the trench application of alignment analysis techniques including the two point azimuth method the hough transform and two dimensional fourier analysis demonstrates that cinder cone alignments have common orientations on a regional scale within three clusters all located in the southernmost part of the michoacan guanajuato volcanic field in the western portion of the study area these alignments consist of tens of cinder cones are 20 50 km long and are all oriented with azimuths of 020 040 parallel to the direction of plate convergence high mg lavas which last fractionated at pressures in excess of 8 kbar are only found associated with these alignments indicating that magma transport is significantly enhanced in these areas although local alignments of three to six cinder cones occur within mapped fault zones that transect the area regional cinder cone alignment patterns and the distribution of clusters themselves do not appear to be affected by the presence of these fault zones the cluster analysis the alignment analysis and some petrologic data support the hypothesis that the tmvb is segmented near 101 w

three dimensional p and s wave velocity structure below unzen volcano abstract unzen volcano located in southwestern japan erupted on november 17 1990 after 198 years of dormancy and has been ejecting lava since may 20 1991 in this paper we present three dimensional p and s wave velocity variations below unzen volcano using 22 473 p and 14 349 s wave arrival times from 3457 local earthquakes recorded on a local network of twelve seismic stations the model was parameterized by 24 000 approximately 2 0 km cubic blocks targeting a volume of 120 100 20 km a prominent low velocity anomaly greater than 4 slowness perturbation starting from 2 5 to 5 0 km depth beneath the volcano is observed on both the p and s wave inversions below unzen volcano slightly lower amplitude low velocity anomalies are further observed to a depth of 12 5 km for p and 7 5 km for s waves shallow low velocity anomalies are observed below the chijiwa bay area northwest of unzen and deeper anomalies are seen below shimabara bay southwest of shimabara peninsula this nw se trending feature represents an elongated system of dykes which supply the shallow magma reservoirs

rock jointing and abrasion forms on roches moutonnees sw finland the influence of bedrock structures especially joints on the formation of roches moutonnees and different glacial abrasion forms on roches moutonnees were studied in the southwest archipelago of finland the size and form of roches moutonnees are closely connected with the frequency and state of joints in the bedrock the fewer and smaller the joints are the larger are the roches moutonnees that can be formed vertical joints in the direction of the ice movement form the weakest zones and determine the width of the roches moutonnees joints dipping against glacial movement determine the dip of the eroded rock surface horizontal joints provide a suitable base for large and gently dipping roches moutonnees which are normally lower than others with less frequent horizontal joints striae friction cracks facets fracture lines and plucked surfaces and their positions on the surfaces of roches moutonnees were observed on the basis of these observations a stereographic model of the distribution of different abrasion features on the surfaces of roches moutonnees was produced

migration of volcanism in the san francisco volcanic field arizona the remanent magnetization of volcanic rocks has been determined at 650 sites in the san francisco volcanic field in the southern part of the colorado plateau the polarity of remanent magnetization combined with k ar age determinations spatial and petrographic associations stratigraphic relations and state of preservation of the cinder cones provides a basis for assignment to known magnetic polarity epochs of 610 mafic vents and 100 intermediate to silicic flows flow sequences and vents the age assignments for basaltic rocks include 243 brunhes 0 73 ma vents 220 matuyama 0 73 to 2 48 ma vents and 147 pre matuyama 2 48 to about 5 0 ma vents basaltic volcanism migrated northeastward before matuyama time at a rate of 1 2 cm yr and eastward s87 5 e over the past 2 5 m y at a rate of 2 9 0 3 cm yr concomitant acceleration in total magma production from 75 to 1 400 10 6 km3 yr and frequency of basaltic eruptions from 1 per 17 000 yr to 1 per 3 000 yr occurred between 5 and 0 25 ma for the past 0 25 m y magma production 180 10 6 km3 yr and perhaps eruption frequency have decreased this evolutionary sequence coupled with the lead and strontium isotopic composition of the rocks can be explained by magmatism caused by shear heating at the base of the lithosphere we propose that this eastward drift of volcanic activity represents absolute westward motion of the north american plate our model is in agreement with a model in which the african plate is fixed to the deep mantle

numerical modelling of subglacial erosion and sediment transport and its application to the north american ice sheets over the last glacial cycle abstract present day sediment distribution offers a potentially strong constraint on past ice sheet evolution glacial system models gsms however cannot address this constraint while lacking appropriate representations of subglacial sediment production and transport incorporating these elements in gsms is also required in order to quantify the impact of a changing sediment cover on glacial cycle dynamics towards these goals we present a subglacial process model hereafter referred to as the sediment model that incorporates mechanisms for sediment production entrainment transport and deposition bedrock erosion is calculated by both hallet s and boulton s abrasion laws separately and by a novel quarrying law parametrized as a function of subglacial cavity extent these process oriented erosion laws are compared against a simple empirical relationship between erosion rate and the work done by basal stress sediment entrainment is represented by philip s law for regelation intrusion and soft bed deformation is included as a subglacial sediment transport mechanism the model is driven by the data calibrated mun 3d gsm and a newly developed subglacial hydrology module the sediment model is applied to the last north american glacial cycle and predicts sediment thickness and cumulative erosion patterns results are obtained in the context of a sensitivity analysis and are compared against the present day distribution of glacigenic sediment and geological estimates of laurentide ice sheet erosion given plausible ranges for the sensitivity parameters chosen a priori based on available literature or on heuristic arguments the calculated erosion depths overlap with the geological estimates of laurentide erosion most of the runs in the sensitivity set produce unrealistically thick and continuous moraines along the eastern southern and western margins of the north american ice complex which suggests that the model overestimates sediment entrainment and thus englacial transport a realistic sediment distribution is only obtained when the entrainment rate is capped at the average basal melting rate which suggests that modelled entrainment and basal melting rates should be of the same order of magnitude

glacial valley cross profile development the influence of in situ rock stress and rock mass strength with examples from the southern alps new zealand abstract the evolution of the glacial valley cross profile form is commonly attributed primarily to the glaciological variables that control the erosion of the channel however studies in the new zealand southern alps suggest that the rock mass strength rms of the eroded rock mass is a major control on slope stability and hence on the final form of the trough rms of the slope rock will alter with time and erosional excavation and oversteepening of the valley slopes the in situ rock stress field induced by 1 the extreme topography in the axial ranges of the new zealand southern alps and 2 tectonically by collision of the australian and pacific crustal plates may play a role in valley slope development by controlling the location of rock failure and reducing rms this provides weakened rock that may provide sites for selective glacial erosion of the rock mass hence valley form evolution models should also take into account the rms and the in situ stress field acting in the eroded rock mass this study has implications for the development and modification of alpine glacial troughs in similar tectonic settings elsewhere

low temperature emplacement of phreatomagmatic pyroclastic flow deposits at the monogenetic mt gambier volcanic complex south australia and their relevance for understanding some deposits in diatremes the occurrence of pyroclastic flow deposits is not restricted to polygenetic and or felsic volcanic systems but can also occur at mafic monogenetic volcanic centres at the c 5 ka mt gambier volcanic complex australia two small volume pyroclastic flow deposits resulted from phreatomagmatic eruptive phases of the blue lake east and the valley lake maar craters the facies descriptions of the two massive poorly sorted lapilli tuff and tuff breccia deposits are given low grade carbonized wood fragments in the deposits mark low emplacement temperatures 180 270 c for these deposits with only c 12 23 of the total thermal energy of the original magma preserved at the time of deposition the blue lake east vent erupted by forming a short lived vertical eruption column which could not be sustained and collapsed the valley lake pyroclastic flow deposit has a highly asymmetric dispersal to the west indicating that this deposit was formed during an eruptive phase with a large westward directed lateral component such massive poorly sorted deposits outside the maar crater indicate that some similar deposits in diatremes may have originated from surface eruptions and not just from subsurface debris jets

maar diatreme phreatomagmatism at hopi buttes navajo nation arizona usa remnants of an extensive maar diatreme volcanic field are magnificently exposed at various depths of erosion in the hopi buttes volcanic field of northeastern arizona field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that 1 lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre eruptive mio pliocene bidahochi formation and the muddy bidahochi sediment was soft and wet at the time of maar eruptions 2 beds higher within the rims contain generally increased proportions of sandstone clasts from the triassic wingate formation in the diatremes late emplaced breccia has deeper seated lithics than more marginal breccia emplaced earlier and 3 many vents are topped by megacryst enriched scoria and spatter and deep seated xenoliths are known only from upper diatreme and craterfilling tephra these observations show that 1 eruptions at hopi buttes involved interaction of magma with unconsolidated mudrock at shallow levels and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment 2 the locus of explosive activity migrated downward as eruptions progressed and 3 the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water

role of transient water pressure in quarrying a subglacial experiment using acoustic emissions 1 probably the most important mechanism of glacial erosion is quarrying the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments although evidence indicates that erosion rates depend on sliding speed rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice bed separation to study quarrying in real time a granite step 12 cm high with a crack in its stoss surface was installed at the bed of engabreen norway acoustic emission sensors monitored crack growth events in the step as ice slid over it vertical stresses water pressure and cavity height in the lee of the step were also measured water was pumped to the lee of the step several times over 8 days pumping initially caused opening of a leeward cavity which then closed after pumping was stopped and water pressure decreased during cavity closure acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically with repeated pump tests this crack grew with time until the step s lee surface was quarried our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded natural basal water pressure fluctuations should also concentrate stresses on rock steps increasing rates of crack growth stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size which may help explain the effect of sliding speed on erosion rates

streamlined hard beds formed by palaeo ice streams a review fast flowing ice streams occur within modern ice sheets and also operated in pleistocene ice sheets the reconstruction of palaeo ice streams normally relies on the mapping of mega scale glacial lineations msgls and drumlins composed of soft sediment mainly till analysis of new satellite imagery and digital terrain models demonstrates the presence of large fields of kilometre scale glacial lineations comprising rock drumlins megagrooves and megaridges in this paper we describe and analyse a number of such hard bed landform systems from the former laurentide and british irish ice sheets occurring in a variety of palaeo ice stream settings these are attributed to erosion of crystalline and sedimentary rock below fast flowing ice streams bedrock properties such as hardness fracture spacing and bedding and their orientation with respect to ice flow have a profound effect on the occurrence and character of elongate rock bedforms elongate streamlined forms on hard crystalline rock as on the canadian shield only form under special circumstances in contrast sedimentary strata are highly susceptible to form streamlined hard beds specifically if bedrock strike is parallel to ice flow large scale elongate rock bedforms are erosional in origin formed by preferentially focused abrasion or by lateral plucking depending on bedrock type many palaeo ice stream footprints previously mapped in the laurentide ice sheet on the basis of soft bed bedforms are shown to be significantly larger extending up ice across sedimentary strata and onto precambrian crystalline rocks hard bed streamlined forms further show that ice streaming does not necessitate a deformable bed but can equally occur on smooth hard beds

post depositional intrusion and extrusion through a scoria and spatter cone of fountain fed nephelinite lavas las herrerias volcano calatrava spain the effusion of lava flows from the base of volcanic cones is a common process in continental monogenetic basaltic fields however there are no descriptions in the literature of the structures that channel the lava output through the cone the pliocene quaternary las herrerias volcano calatrava spain was constructed from the superposition of a nephelinite spatter and scoria cone and related lavas over a maar different eruptive styles contributed to the construction of the current volcanic cone phreatomagmatic hawaiian strombolian and violent strombolian quarrying has exposed a lava pond inside the volcanic cone the intrusion of lava through cone forming pyroclastic deposits and the ponding of associated lava flows within the maar crater low fire fountaining activity formed a lava pond that stagnated within the crater protected by the cone s highly welded spatter deposits at its base and overlying scoria deposits once the pressure exerted by ponding lava and overlying pyroclastic deposits exceeded the yield strength of the damming rock walls the melt oozed out through fractures to form an intricate network of dikes and sills and extruded through the lower part of the volcanic cone forming lava flows we propose that lava intrusions similar to those here described may represent the final stage of feeding systems for lava flows associated with scoria and spatter cones

subglacial erosion and englacial sediment transport modelled for north american ice sheets abstract reconstructions of north american glacial history have either been based on interpretation of the geologic record or on numerical models of continental ice dynamics because such models make no explicit reference to the geological products of glaciation the full potential of a combined approach has not been realized we present the results of a process based model of sediment production entrainment transport and deposition that represents a first step toward uniting these two lines of investigation in this model basal ice processes are dependent on ice sheet geometry hydrology and dynamics a three dimensional thermomechanical ice sheet model yields the ice geometry flow and thermal conditions that are used by a model of subglacial hydrology together these drive the basal process model comparisons of simulation results with mapped debris distributions of the hudson bay paleozoic carbonates and of the distinctive dubawnt group are used to test the validity of the transport model dispersion of debris radially out of hudson bay is greatly overestimated by the model and dispersion toward the bay equally underestimated this is because the modelled ice sheet has a single dome over hudson bay at glacial maxima including last glacial maximum rather than a saddle separating ice domes over quebec labrador and keewatin estimated rates of glacial erosion for the cordilleran and laurentide ice sheets are compared against those predicted by the model the model underestimates these rates because it underestimates the vigour of the quarrying process

experiments on the dynamics and sedimentary products of glacier slip abstract experimental work in glacial geomorphology is focused almost entirely on processes in the thin shear zone at the beds of sliding glaciers where ice at its pressure melting temperature moves over either rigid rock or deformable till experiments with rotary shear devices illuminate constitutive behavior there central to the dynamics of fast moving glaciers and provide a foundation for interpreting the sedimentary record of glacier slip and associated sediment transport results from experiments designed to study ice sliding over a rigid wavy bed shear deformation of till and plowing of clasts across the surface of a till bed point to a common conclusion drag at the bed can decrease with increasing slip velocity thereby concentrating driving stress elsewhere and promoting rapid glacier flow this rate weakening behavior at glacier beds is in contrast to the viscous slip resistance assumed in ice sheet models and most efforts to determine distributions of basal drag from glacier surface velocities ring shear experiments in which various basal tills and more idealized materials are sheared to high strains provide quantitative insight into grain size evolution mixing at contacts between basal tills microstructure development particle fabric development and development of anisotropy of magnetic susceptibility preferred orientations of principal magnetic susceptibilities provide the most dependable and complete description of till shear patterns when applied to basal tills of the geologic record magnetic till fabrics measured along thick till sections and calibrated experimentally indicate that deformation of the bed by two lobes of the laurentide ice sheet was shallow

the distribution and magnitude of subglacial erosion on millennial timescales at engabreen norway we quantify the magnitude of millennial scale glacial erosion at engabreen a temperate glacier in coastal northern norway using the in situ cosmogenic nuclides carbon 14 14 c and beryllium 10 10 be in bedrock exposed recently by glacial retreat nuclide concentrations show no dependence on distance down or across the valley as such resulting holocene erosion depths along two transects perpendicular to glacier flow are highly variable with no systematic distribution ranging from 0 10 to 2 95 m we observed 14 c 10 be ratios elevated above the production ratio in samples of abraded bedrock which is counter to the expectation for surfaces covered during the holocene and exposed only recently muon reactions produce nuclides at greater depths than do spallation reactions and 14 c at production rates at higher than those of 10 be resulting in 14 c 10 be ratios that increase with depth therefore elevated 14 c 10 be ratios indicate that sampled sites were deeply plucked during recent cover the little ice age in this case and then rapidly abraded prior to retreat our results suggest that while glacial erosion can generate a u shaped valley profile over long periods of time e g 10 5 10 7 years the discontinuous nature of glacial plucking produces spatially variable patterns of erosion over shorter millennial timescales

the contribution of discontinuous rock mass failure to glacier erosion geomechanical rock mass properties control the response of bedrock to applied stresses and can be summarized in a linear mohr coulomb equation which defines the principal parameters determining failure nevertheless in studying the erosion of bedrock by glacier ice little attention has been paid to failure criteria though a coincidence of erosion all and forms with fracture systems at regional and local scales has been demonstrated few studies have analysed the precise nature of the fracture geometry or proposed its mechanical impact in association with glacier ice this investigation proposes that since almost all bedrock possesses identifiable fracture systems the properties of discontinuous rock mass drm be regarded as defining primary conditions of stress and stability which are subsequently modified directly and indirectly by glacier ice consequent rock mass failure modes are prescribed by discontinuity geometry and applied stresses and evidence from north wales confirms the validity of the theoretical treatment of rock mass properties and explains the accordance of landforms with structure

rate weakening drag during glacier sliding accurately specifying the relationship between basal drag on a hard rough glacier bed and sliding speed is a long standing and central challenge in glaciology drag on a rigid bed consisting of steps with linear treads inclined upglacier a good idealization for the bedrock morphology of some hard bedded glaciers has been considered in sliding theories but never studied empirically balancing forces parallel to step treads indicates that drag should be independent of sliding speed and cavity size and set by the limit equilibrium condition sometimes called iken s bound in this study we used a large ring shear device to slide ice at its pressure melting temperature across a stepped bed over a range of steady sliding speeds 29 348 m yr 1 and under a steady effective pressure 500 kpa contrary to expectation drag decreased 42 with increasing sliding speed and cavity size experimental deviations from theory cannot explain this decrease in drag with increasing sliding speed i e rate weakening we suggest that stress bridging in ice between ice bed contact zones and cavities causes stress gradients that require viscous deformation of ice to sustain stress equilibrium so that contact zones can be at shear stresses below limit equilibrium values a parameter linearly dependent on sliding speed that scales the extent of ice deformation to areas of ice bed contact allows the experimental drag relationship to be fitted with a simple sliding model rate weakening drag has now been observed for two contrasting bed morphologies stepped and sinusoidal highlighting the need to consider such behavior in glacier flow models

the influence of rock mass strength on glacial valley cross profile morphometry a case study from the southern alps new zealand the erosional morphology in the vicinity of the main divide of the southern alps and fiordland new zealand appears to be a product of the interaction between alpine fault induced tectonic processes rock mass strength of the uplifted and eroded bedrock and the processes acting to denude the developing mountain landscape the magnitude of the effects of glacial erosion on the landscape is directly controlled by the size and physical properties of the glaciers whilst the form of the trough is a direct consequence of the rock mass strength rms properties of the slope rock realistic models of development of the cross profile shape of glacial valleys must take into consideration the rms properties of the eroded substrate

time and space variations in magmatic and phreatomagmatic eruptive processes at easy chair lunar crater volcanic field nevada usa the products of monogenetic volcanoes often record complex sequences of eruptive processes easy chair volcano lunar crater volcanic field nevada usa was formed by a monogenetic eruption along a 2 5 km long series of en echelon fissure vents hawaiian to strombolian fountains along the fissures dominated initial activity producing a series of agglomerate ramparts focusing of eruptive activity to two central vents and the formation of two overlapping scoria cones followed the early phase fountain fed lavas from those cones merged to form a channel that fed lava onto a flow field at the foot of the cones focusing of subsurface magma flow toward the central conduits may have reduced magma flux in the remaining fissures and the southern segment s entered a phase of phreatomagmatic explosions that destroyed the early agglomerate rampart and formed a maar and tephra ring composed of lapilli tuff rich in clasts derived from pre easy chair lavas and early agglomerates the eruption closed with a minor phase of magmatic activity that deposited scoria lapilli and bombs on top of the phreatomagmatic deposits the eruptive sequence indicates that relatively low hazard strombolian to hawaiian activity can be replaced by more hazardous phreatomagmatic explosions well into a monogenetic eruption

the significance of rock structure lithology and pre glacial deep weathering for the shape of intermediate scale glacial erosional landforms the main landforms within the glacially scoured precambrian rocks of the swedish west coast are closely connected to the principal structural pattern and have lately been explained as mainly a result of etch processes probably during the mesozoic and with a possible second period of etching during the neogene to explore the effect of multiple glacial erosion on the rock surfaces an island with two different lithologies and with striae from different directions was selected for a detailed study focusing on the shape of roches moutonnees air photo interpretation of bedrock lineaments and roches moutonnees combined with detailed field mapping and striae measurements are used to interpret the structural and lithological control on the resulting shape the study reveals a significant difference in shape between roches moutonnees in augen granite and orthogneiss low elongated and streamlined roches moutonnees occur in the gneiss area striated by a late weichselian ice flow from the ne this ice flow is subparallel with both the local dominant trend of topographically well expressed joints and the schistosity of the gneiss frequently there are no signs of quarrying on the lee sides of the gneiss roches moutonnees and hence they resemble the shape of whalebacks or ruwares as typically associated with the exposed basal weathering surface found in tropical areas the granite roches moutonnees were formed by an older ice flow from the ese which closely followed the etched wnw ese joint system of the granite late weichselian ice flow from the ne caused only minor changes of the landforms on the contrary marks of the early ese ice flow are poorly preserved in the gneiss area where it probably never had any large effect as the flow was perpendicular to both schistosity and structures and accordingly also to the pre glacial relief the study demonstrates that coincidence between ice flow direction and pre glacially etched structures is most likely to determine the effects of glacial erosion copyright 2002 john wiley sons ltd

boron concentrations of volcanic fields in different geotectonic settings abstract whole rock boron and other mobile and immobile element concentrations are reported for the alkaline maar volcanic rocks from the bakony balaton highland volcanic field bbhvf hungary and for three other geographically distinct maar volcanic fields from diverse tectonic settings spain canary islands tenerife new zealand waipiata otago and mexico pinacate sonora and ceboruco cone field boron concentrations along with other fluid immobile and incompatible element concentrations are used to study the fluid enrichment of the above mentioned intraplate volcanic materials the fluid addition was the highest in the trans mexican volcanic belt ceboruco cone field which is associated with recent subduction the bbhvf also shows high fluid enrichment the average b content of the tenerife canary islands samples 6 4 g g is similar to that of the bbhvf 6 9 g g the fluid enrichment is higher in the bbhvf than in the waipiata volcanic field wvf the magmatic source regions for all the investigated volcanic rocks were affected by fluid components to different degrees and despite the distant relationship to subduction zones all show evidence of a subduction derived fluid component

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

control of glacial quarrying by bedrock joints abstract a principal assumption of models that describe bedrock quarrying by glaciers is that cracks in the bed are small and isolated these cracks are assumed to grow slowly in response to glacial loading to eventually delimit the areas of quarried surfaces if this assumption is correct then quarried surface orientations will be controlled by orientations of principal stresses in the bed associated with sliding and resultant ice bed separation downglacier from rock ledges and roches moutonnees rather than by orientations of joints that predate glacial loading to test this hypothesis we compared orientations of quarried surfaces and preglacial joints in nine recently deglaciated forefields of glaciers in canada and switzerland with different bedrock lithologies and bedforms in all but one forefield quarried surfaces and major joint sets are coincident with orientations of quarried surfaces bearing no systematic relationship to sliding direction other than their obvious tendency to not face upglacier a difference in sliding direction of 64 between two areas of one glacier forefield had no effect on the orientation of quarried surfaces with the same joint set exploited in both areas these observations motivate a new characterization of subglacial bedrock that differs from that of existing quarrying models the bed is idealized as a group of rock blocks separated by preglacial joints slow crack growth necessary for quarrying would be confined to small bodies of rock bridges that connect the faces of joints this model of the bed does not allow it to be treated as an intact elastic solid with hydraulically isolated cracks as is usually assumed which would have a profound influence on characterization of quarrying mechanics

recycling of magmatic clasts during explosive eruptions estimating the true juvenile content of phreatomagmatic volcanic deposits the juvenile content of phreatomagmatic deposits contains both first cycle juvenile clasts derived from magma at the instant of eruption and recycled juvenile clasts which were fragmented and first ejected by earlier explosions during the eruption but fell back or collapsed into the vent recycled juvenile clasts are similar to accessory and accidental lithics in that they contribute no heat to further magma water interaction but previously no effective criteria have been defined to separate them from first cycle juvenile clasts we have investigated componentry parameters vesicularity clast morphology and extent of mud coating which in specific circumstances can distinguish between first cycle juvenile clasts involved in only one explosion and such recycled juvenile clasts phreatomagmatic fall deposits commonly show gross grainsize and sorting characteristics identical to deposits of purely dry or magmatic eruptions however the abundance of non juvenile clasts in pyroclastic deposits is a sensitive indicator of the involvement of external water if this component is calculated including recycled juvenile clasts with accidental and accessory clasts the contrast is even more striking data from a holocene maar deposit in taupo volcanic zone new zealand suggest that the first cycle juvenile component of the deposits is less than one third of that determined by simple juvenile lithic crystal componentry

volcanology of the aries micaceous kimberlite central kimberley basin western australia abstract the neoproterozoic 815 4 4 3 ma aries kimberlite intrudes the king leopold sandstone and the carson volcanics in the central kimberley basin northern western australia aries is comprised of a n nne trending series of three diatremes and associated hypabyssal kimberlite dykes and plugs the diatremes are volumetrically dominated by massive clast supported accidental lithic rich kimberlite breccias that were intruded by hypabyssal macrocrystic phlogopite kimberlite dykes and plugs with variably uniform to globular segregationary textured groundmasses lower diatreme facies accidental lithic rich breccias probably formed through fall back of debris into the vent with a major contribution from the collapse of the vent walls these massive breccias are overlain by a sequence of bedded volcaniclastic breccias in the upper part of the north lobe diatreme abundant poorly vesicular to nonvesicular juvenile kimberlite ash and lapilli with morphologies that are indicative of phreatomagmatic fragmentation processes occur in a reversely graded volcaniclastic kimberlite breccia unit at the base of this sequence this unit and overlying bedded accidental lithic rich breccias are interpreted to be sediment gravity flow deposits including possible debris flows derived from the collapse of the crater walls and or tephra ring deposits that surrounded the crater diatreme forming eruptions may have been initiated by magma water interactions along fracture and joint controlled aquifers within the king leopold sandstone the current level of exposure of the diatremes probably extends from the lower diatreme facies up into the base of a bedded upper diatreme sequence

the role of phreatomagmatism in a plio pleistocene high density scoria cone field llancanelo volcanic field mendoza argentina abstract the plio pleistocene llancanelo volcanic field together with the nearby payun matru field comprises at least 800 scoria cones and voluminous lava fields that cover an extensive area behind the andean volcanic arc beside the scoria cones in the llancanelo field at least six volcanoes show evidence for explosive eruptions involving magma water interaction these are unusual in the context of the semi arid climate of the eastern andean ranges the volcanic structures consist of phreatomagmatic derived tuff rings and tuff cones of olivine basalt composition malacara and jarilloso tuff cones were produced by fallout of a range of dry to wet tephra the malacara cone shows more evidence for a predominance of wet emplaced units with a steep slump slope characterized by many soft sediment deformation structures such as undulating bedding planes truncated beds and water escape features the piedras blancas and carapacho tuff rings resulted from explosive eruptions with deeper explosion loci these cones are hence dominated by lapilli tuff and tuff units emplaced mainly by wet and or dry pyroclastic surges carapacho is the only centre that appears to have started with phreatomagmatic eruptions with lowermost tephra being rich in non volcanic country rocks the presence of deformed beds with impact sags slumping textures asymmetrical ripples dunes cross and planar lamination syn volcanic faulting and accretionary lapilli beds indicate an eruption scenario dominated by excessive water in the transportational and depositional regime this subordinate phreatomagmatism in the llancanelo volcanic field suggests presence of ground and or shallow surface water during some of the eruptions each of the tuff rings and cones are underlain by thick fractured multiple older lava units these broken basalts are inferred to be the horizons where rising magma interacted with groundwater the strong palagonitization at each of the phreatomagmatic cones formed hard beds resistant to erosion and therefore the volcanic landforms are well preserved

controls on bedrock bedform development beneath the uummannaq ice stream onset zone west greenland this paper investigates the controls on the formation of subglacially eroded bedrock bedforms beneath the topographically confined region upstream of the uummannaq ice stream uis during the last glacial cycle palaeoglaciological conditions are believed to have been similar for all sites in the study characterised by thick fast flowing ice moving over a rigid bedrock bed classic bedrock bedforms indicative of glacially eroded terrain were mapped including p forms roches moutonnees and whalebacks bedform long axes and plucked face orientations display close correlation parallel and perpendicular to palaeo ice flow directions inferred from striae measurements across all sites elongation ratios length to width varied by an order of magnitude between 0 8 1 and 8 4 1 bedform properties length height width and long axis orientation from four subsample areas form morphometrically distinct populations despite their close proximity and hypothesised similarity in palaeoglaciological conditions variations in lithology and geological structures e g joint frequency joint dip joint orientation bedding plane thickness and bedding plane dip provide lines of geological weakness which focus the glacial erosion in turn controlling bedform geometries determining the relationship s between bedding plane dip relative to palaeo ice flow and bedform shape relative length amplitude and wavelength has important ramifications for understanding subglacial bed roughness cavity formation and likely erosion style quarrying and or abrasion at the ice bed interface this paper demonstrates a direct link between bedrock bedform geometries and geological structure and emphasises the need to understand bedrock bedform characteristics when reconstructing palaeoglaciological conditions

entrainment of country rock during basaltic eruptions of the lucero volcanic field new mexico as magma rises through the lithosphere it may entrain wall rock debris the entrainment process depends on the local hydrodynamic regime of the magma e g velocity temperature bulk density the extent of interaction of magma with groundwater and the mechanical properties of the wall rocks wall rock entrainment results in local flaring of dikes and conduits which in turn affects the hydrodynamics of magma ascent and eruption we studied upper crustal xenoliths erupted from small volume basaltic volcanoes of the lucero volcanic field west central new mexico in order to assess the relative importance of various entrainment mechanisms during a range of eruptive styles including strongly hydrovolcanic strombolian and effusive processes total xenolith volume fractions ranged from 0 3 0 9 in hydrovolcanic facies to 10 4 10 2 in strombolian facies the volcanoes erupted through a thick well characterized sequence of paleozoic and lower mesozoic sedimentary rocks so that erupted xenolith

reconstructing eruption processes of a miocene monogenetic volcanic field from vent remnants waipiata volcanic field south island new zealand the miocene waipiata volcanic field new zealand is an eroded phreatomagmatic intracontinental volcanic field formed during a period of weak lithospheric extension the field includes remnants of at least 55 volcanoes in an area of 5000 km2 vent filling deposits comprising predominantly lava e g plugs necks lava flows or dykes often associated with thin basal phreatomagmatic pyroclastic deposits were classified as type 1 vents and are inferred to be the remnants of scoria cones vents represented by predominantly pyroclastic infill are classified as type 2 vents and are inferred to have been the substructures of phreatomagmatic tuff ring and or maar volcanoes type 3 vent complexes are groups of closely spaced or overlapping vents with voluminous preserved lava flows they are inferred to be the remnants of volcanoes comprising adjoining to coalescing maars and tuff rings with magmatic explosive and effusive products pyroclastic rocks of most of the waipiata vents record initial phreatomagmatic explosive activity fuelled by groundwater followed by strombolian style eruptions aligned and clustered vents are accommodated to structural features of the regional basement rock otago schist

volcano stratigraphy of the extension related silicic volcanism of the cubukludag graben western turkey an example of generation of pyroclastic density currents western turkey9s extension related cumaovasi volcanic rocks lower miocene 17 ma are excellent examples of silicic eruptions the sub aerial silicic volcanism at cubukludag graben between izmir and kusadasi in west central anatolia is mainly in the form of rhyolite domes lava flows and pyroclastic deposits the initial features of volcanism derived from phreatomagmatic explosive eruptions from silicic magma that came into contact with lake waters during neogene times most of the volcanic succession represents pyroclastic density currents pdcs known as the kuner ignimbrite the deposits are fine grained and laminated at the base and pass laterally and vertically into deposits displaying well developed traction structures soft sediment deformation and or erosion channels in the ne part of the region alternate deposits of massive diffusely stratified lapilli and ash are the main products of the later explosive stage massive lithic breccias forming the top of the sequences are the proximal facies of the pdcs the lava phase mainly consists of rhyolite extruded as dome and fissure eruptions of lavas aligned along ne sw trending faults as well as from extensional cracks that are nearly perpendicular to the main graben faults considering the tectono stratigraphical aspects and geochemical nature of the study area we propose that the cumaovasi silicic volcanism was produced by extension related crustal melting during the late early miocene period 17 ma

subglacial extensional fracture development and implications for alpine valley evolution 1 bedrock stresses induced through exhumation and tectonic processes play a key role in the topographic evolution of alpine valleys using a finite difference model combining the effects of tectonics erosion and long term bedrock strength we assess the development of near surface in situ stresses and predict bedrock behavior in response to glacial erosion in an alpine valley the matter valley southern switzerland initial stresses are derived from the regional tectonic history which is characterized by ongoing transtensional or extensional strain throughout exhumation of the brittle crust we find that bedrock stresses beneath glacial ice in an initial v shaped topography are sufficient to induce localized extensional fracturing in a zone extending laterally 600 m from the valley axis the limit of this zone is reflected in the landscape today by a valley shoulder separating linear upper mountain slopes from the deep u shaped inner valley we propose that this extensional fracture development enhanced glacial quarrying between the valley shoulder and axis and identify a positive feedback where enhanced quarrying promoted valley incision which in turn increased in situ stress concentrations near the valley floor assisting erosion and further driving rapid u shaped valley development during interglacial periods these stresses were relieved through brittle strain or topographic modification and without significant erosion to reach more highly stressed bedrock subsequent glaciation caused a reduction in differential stress and suppressed extensional fracturing a combination of stress relief during interglacial periods and increased ice accumulation rates in highly incised valleys will reduce the likelihood of repeat enhanced erosion events

glacial erosion status and outlook glacier erosion rates range across orders of magnitude and much of this variation cannot be attributed to basal sliding rates subglacial till acts as lubricating fault gouge or sawdust and must be removed for rapid subglacial bedrock erosion such erosion occurs especially where and when moulin fed streams access the bed and are unconstrained by supercooling or other processes streams also may directly erode bedrock likely with strong time evolution erosion is primarily by quarrying aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes debris bed friction significantly affects abrasion quarrying and general glacier flow frost heave drives cirque headwall erosion as winter cold air enters bergschrunds creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock and the glacier removes the resulting blocks recent subglacial bedrock erosion and sediment flux are in many cases much higher than long term averages over glacial cycles evolution of glacial valley form feeds back strongly on erosion and deposition most of this is poorly quantified with parts open to argument glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes motivating vigorous research

shallow plumbing systems for small volume basaltic volcanoes eruptive dynamics in basaltic volcanoes are controlled in part by the conduit geometry however uncertainties in conduit shape and dike to conduit transition geometry have limited our predictive capability for hazards assessments we characterize the subvolcanic geometry of small volume basaltic volcanoes magmatic volatile driven eruptions 0 1 to 0 5 km3 based on a synthesis of field studies of five basaltic volcanoes exposed to varying degrees by erosion and exhibiting feeder dikes conduits and vent areas 250 m depth study areas include east grants ridge new mexico usa basalt ridge east basalt ridge paiute ridge and southeast crater flat nevada usa basaltic feeder dikes 250 to 100 m deep have typical widths of 4 12 m with smooth host rock contacts rhyolite tuff at depths less than 100 m heterogeneities in the host rock form preferential pathways for small dike splays and sills resulting in a 30 m effective width at 50 m depth the development of a complex conduit at depths less than 70 m is reflected in bifurcating dikes and brecciation and incorporation of the country rock the overall zone of effect at depths less than 50 m is 110 m wide 220 m elongated along the feeder dike based on comparisons with theoretical conduit flow models the width of the feeder dike at depths from 250 to 500 m is expected to range from 1 to 10 m and is expected to decrease to about 1 2 m at depths greater than 500 m the flaring shape of the observed feeder systems is similar to results of theoretical modeling using lithostatic pressure balanced flow conditions sizes of observed conduits differ from modeled dimensions by up to a factor of 10 in the shallow subsurface 50 m depth but at depths greater than 100 m the difference is a factor of 2 to 4 this difference is primarily due to the fact that observed eroded conduits record the superimposed effects of multiple eruptive events while theoretical model results define dimensions necessary for a single steady eruption phase the complex details of magma host rock interactions observed at the study areas contact welding brecciation bifurcating dikes and sills and stoping represent the mechanisms by which the lithostatic pressure balanced geometry is attained the similarity in the normalized shapes of theoretical and observed conduits demonstrates the appropriateness of the pressure balanced modeling approach consistent with the conclusions of wilson and head j geophys res 86 2971 3001 1981 for this type of volcano

basaltic ignimbrites in monogenetic volcanism the example of la garrotxa volcanic field ignimbrites are pyroclastic density current deposits common in explosive volcanism involving intermediate and silicic magmas and in less abundance in eruptions of basaltic central and shield volcanoes however they are not widely described in association with monogenetic volcanism where typical products include lava flows scoria and lapilli fall deposits as well as various kinds of pyroclastic density current deposits and explosion breccias in la garrotxa basaltic monogenetic volcanic field part of the neogene quaternary european rift system located in the northeast of the iberian peninsula we have identified a particular group of pyroclastic density current deposits that show similar textural characteristics to silicic ignimbrites indicating an overlap in transport and depositional processes these deposits can be clearly distinguished from other pyroclastic density current deposits generated during phreatomagmatic phases that typically correspond to thinly laminated units with planar to cross bedded stratification the monogenetic ignimbrite deposits correspond to a few meters to several tens of meters thick units rich in lithic and lapilli scoria fragments with an abundant ash matrix and internally massive structure emplaced along valleys and gullies with run out distances up to 6 km and individual volumes ranging from 106 to 1 5 107 m3 the presence of flattened scoria and columnar jointing in some of these deposits suggests relatively high emplacement temperatures coinciding with available paleomagnetic data that suggests an emplacement temperature around 450 500 c in this work we describe the main characteristics of these pyroclastic deposits that were generated by a number of phreatomagmatic episodes comparison with similar deposits from silicic eruptions and previous examples of ignimbrites associated with basaltic volcanism allows us to classify them as basaltic ignimbrites the recognition in monogenetic volcanism of such pyroclastic products which may extend several kilometres from source has an important consequence for hazard assessment in these volcanic fields which previously have been considered to present only minor hazards and risks

bedrock fracture control of glacial erosion processes and rates the importance of rock type and tectonic history on rates of glacial erosion and the relative roles of glacial quarrying and abrasion are poorly understood we use concentrations of cosmogenic 10be in glacial polish and measurements of bedrock fracture spacing to explore the relationship between erosion rates and rock fracturing at 23 sites along tuolumne river valley and five sites in tenaya canyon in yosemite national park california usa most sites yield 10be concentrations that can be best explained as reflecting solely postglacial nuclide accumulation six sites however display anomalously high concentrations implying incomplete removal of the pre glacial nuclide inventory during the last glaciation these require that erosion in the last glacial cycle was 2 3 m these low erosion sites occur preferentially in massive units of the cathedral peak granodiorite and the el capitan granite fractures in high erosion sites are more closely spaced than in low erosion sites with spacings that average 1 1 0 03 m and 3 3 0 1 m respectively our data suggest that the distance between fractures in the rock dictated in part by the original spacing in a particular pluton and in part by the specific tectonic history of the sierra nevada governs the pace of glacial erosion in yosemite

potential effects of subglacial water pressure fluctuations on quarrying water pressure fluctuations beneath glaciers may accelerate rock fracture by redistributing stresses on subglacial bedrock and changing the pressure of water in bedrock cracks to study the potential influence of water pressure fluctuations on the fracture of subglacial bedrock ice flow over a small bedrock step with a water filled cavity in its lee is numerically modeled and stresses on the bedrock surface are calculated as a function of transient water pressures in the cavity stresses on the bed are then used to calculate principal stress differences within the step rapid reductions in cavity water pressure increase principal stress differences in the bed increasing the likelihood of crack growth in the step and the formation of predominantly vertical fractures relatively impermeable bedrock may be most susceptible to fracturing during water pressure reductions because high water pressure in cracks within the rock can be maintained as water pressure decreases in cavities these results when considered in conjunction with the strong likelihood that increases in water pressure accelerate the removal of rock fragments loosened from the bed suggest that in zones of ice bed separation where water pressure fluctuations typically are large rates of quarrying may be higher than along other parts of glacier beds

shallow seated controls on styles of explosive basaltic volcanism a case study from new zealand abstract the pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles e g hawaiian versus strombolian or dry magmatic versus wet phreatomagmatic these transitions are controlled dominantly by variations in degassing patterns magma ascent rates and degrees of interaction with external water we use crater hill a 29 ka explosive effusive monogenetic centre in the auckland volcanic field new zealand as a case study of the transitions between these end member eruptive styles the crater hill eruption took place from at least 4 vents spaced along a nne trending 600 m long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases early explosive phases at crater hill were characterised by eruption from multiple unstable and short lived vents later dominantly extrusive volcanism took place from a more stable point source most of the crater hill pyroclastic deposits were formed in 3 phreatomagmatic p and 4 dry magmatic m episodes forming in turn the outer tuff ring and maar crater p1 m1 p2 and scoria cone 1 m2 m4 this activity was followed by formation of a lava shield and scoria cone 2 purely wet activity is represented by the bulk of p1 and p2 and purely dry activity by much of m2 m4 however m1 and parts of m2 and m4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma water interaction at each vent the nature of the wall rock lithics and these rapid variations in inferred water magma ratios imply interaction was occurring mostly at depths of 80 m and the vesicularity patterns in juvenile clasts from these and the p beds imply that rapid degassing occurred at these shallow levels we suggest that abrupt transitions between eruptive styles in time and space at crater hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent

high frequency paleoclimate signals from foulden maar waipiata volcanic field southern new zealand an early miocene varved lacustrine diatomite deposit abstract a freshwater diatomite deposit near dunedin south island new zealand provides an exceptional archive of mid latitude seasonal climate variation during latest oligocene early early miocene time the diatomite accumulated in a small 1 5 km diameter maar floored lake that formed during basaltic volcanism except for exposures of basanite dated at 23 2 ma by 40 ar 39 ar and remnants of a paleogene sandstone and conglomerate cover the diatomite body is surrounded by otago schist geophysical profiles indicate that the total diatomaceous sediment thickness may exceed 100 m two depositional facies are described from 15 5 m of weakly consolidated fresh diatomite accessible in two pits a thinly laminated facies comprising 60 of the section consists of dark brown and white couplets of average thickness 0 5 mm both brown and white laminae are composed essentially of frustules of a single pennate diatom along with 1 2 of siliceous sponge spicules dark laminae also contain abundant 5 7 5 m diameter siliceous chrysophycean stomatocysts and organic matter decalcified complete skeletons of galaxias a southern hemisphere group of freshwater or diadromous fish are found throughout diverse well preserved leaves some with attached scale insects are dominated by lauraceae rare flowers are also present trace fossils preserved in the laminated comprise 1 1 5 mm wide pale diatomaceous strings of probable faecal origin and isolated 20 40 mm diameter sand lenses interpreted as faecal remains of swimming waterfowl the second diatomite facies group comprises dark brown speckled beds interpreted as sediment gravity flows up to 14 cm thick and interspersed throughout the study section they incorporate abundant laminated diatomite flecks and leaves and minor woody plant matter and terrigenous silt almost all are capped by 1 8 mm of white diatomite that is interpreted as post flow fall out of resuspended diatom frustules associated breccia and swirly beds up to 8 cm thick are composed of variably contorted clasts of laminated diatomite in a speckled matrix nodules spheroids and films of authigenic framboidal pyrite are commonly associated with plant and fish remains as in modern lakes white laminae likely formed from diatom blooms when light flux temperature or nutrients were optimal during spring early summer dark laminae accumulating during autumn winter months laterally continuous pinstripe lamination absence of bioturbation and excellent fish and plant preservation indicate that the profundal lake and floor were anoxic periodic variations in couplet thickness of 3 10 years compare with quaternary records of el nino southern oscillation variability and indicate that the early early miocene low altitude new zealand climate was seasonal and strongly ocean influenced as it is today