oxygen isotope analyses and pleistocene ice volumes abstract the oxygen isotope record from fossil foraminifera in deep sea sediments is commonly used as a proxy for global ice volume the linkage between 18 o and ice volume however is probably nonlinear we have developed a simple numerical model of the isotopic response of the oceans to ice volume change the major features it simulates are 1 the changing mean isotopic composition of snow as a function of ice volume colder snow temperatures forced by climate change and higher elevation accumulation areas imply more negative mean 18 o 2 the nonequilibrium isotopic composition of ice sheets the past history of an ice sheet is integrated into its mean isotopic composition which introduces a lag of isotopic ice volume i e the measured 18 o record scaled to ice volume units behind true ice volume 3 selective preservation of isotopically more negative colder higher latitude ice this geographic effect can selectively amplify or dampen the isotopic response to the ice volume signal we illustrate the response of our model to simple hypothetical ice volume transitions of ice growth and ice decay sensitivity tests are illustrated for all model parameters the results suggest that oxygen isotope records reproduce the general patterns of ice volume change fairly accurately the foraminiferal isotope record however may misrepresent the true amplitude of the ice volume signal and lag true ice volume by 1000 to 3000 yr

chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

basaltic magma reservoirs factors controlling their rupture characteristics and evolution the manner in which magma enters and leaves a magma reservoir is a fundamental aspect of magmatic activity a quantitative model is developed to evaluate the variation of stress acting on the wall of a magma reservoir as a function of depth factors assessed include the size of the reservoir its depth of burial the strength of the country rocks and the variation with depth of the densities of the magma and country rocks it is shown that small magma reservoirs with halfheights less than 1 km centered at levels of neutral buoyancy can grow in all directions with nearly equal ease by injecting dikes into their surroundings at the sites of wall ruptures in larger reservoirs with half heights greater than 2 km however the stress required to cause wall failure is much less at the depth corresponding to the center of the reservoir than it is near the top or bottom thus larger reservoirs are likely to grow predominantly sideways by lateral dike injection in these cases vertical growth will not be important unless significant accumulation of low density material can occur in the upper part of the reservoir the formation of a low density layer due to gas exsolution or chemical differentiation of the magma can facilitate failure of the roof and upward dike migration from a mature reservoir as the reservoir grows and evolves through time the increasing magma pressure head at the depth corresponding to the center of the reservoir means that the amount of gas exsolution or chemical differentiation required to allow the occurrence of vertical dike emplacement also increases ultimately a vertical reservoir size is reached beyond which the excess stress required to cause lateral dike injection is so much less than that required to cause vertical injection that essentially no further vertical growth of the reservoir will occur the reservoir will thus develop a more laterally elongate shape with time the final aspect ratio will then depend mainly on the supply rate of magma to the reservoir from the mantle

degassing behaviour at basaltic volcanoes new insights from experimental investigations of different conduit geometry and magma viscosity abstract understanding gas magma dynamics in volcanic conduits and linking them with the associated geophysical signals at the surface is of fundamental importance in monitoring active volcanoes in the past a significant number of studies have been devoted to this topic addressing the onset the key factors governing each degassing regime and the related markers at the surface particularly at basaltic volcanoes here we first review such a broad and increasing literature focusing on the main approaches used in the attempt of deciphering conduit dynamics by indirect observations 1 analogue laboratory experiments and 2 seismo acoustic measurements then we combine the two techniques into a novel set of experiments addressing a crucial yet unexplored issue the irregularity i e the departure from an ideal smooth cylindrical shape of the conduit surface we built a set of epoxy conduits with various fractal dimensions dc i e irregularity of the internal surface using silicone oil as a proxy for magma different degassing patterns bubbly slug and churn annular flow were reproduced by changing systematically 1 injected gas flux 5 to 180 10 3 l s 2 analogue magma viscosity 10 to 1000 pas 3 fractal dimension dc of the conduit surface i e dc 2 dc 2 18 and dc 2 99 the experiments were monitored by means of a video camera and a set of sensor aimed to detect the seismic and acoustic signals results show that viscosity strongly influences the transition among degassing patterns and the recurrence rate of slug bursts at the surface moreover we observed an increase of the exponent of the power law equation linking squared seismic amplitude to gas flow rate with conduit roughness the opposite trend was noticed with increasing liquid viscosity these results have important implications for linking seismic tremor to eruption source parameters such as the volume discharge rate at different volcanoes or for investigating its temporal evolution at a single vent

coupled conduit flow and shape in explosive volcanic eruptions volcanic conduit flow models generally utilise one of two conduit profile modes which are referred to as parallel sided typically surface choked or lithostatically pressure balanced their limitations in application to supersonically erupted basaltic and rhyolitic explosive eruptions are investigated likely changes in conduit profile over time due to catastrophic failure resulting from high wall stresses and abrasive erosion are investigated using a semi analytical model although time independent this model nonetheless reveals likely trends in shape from a simple parallel sided geometry it is found that for rhyolites early wall failure will occur in all but unusual circumstances resulting in expansion of the conduit a reduction in wall stresses and a trend towards a lithostatically pressure balanced solution at depth in both rhyolitic and basaltic eruptions abrasive erosion results in conduit flaring near the surface allowing the choking point to descent from the surface into the vent substantially changing exit conditions and resulting in a trend towards the a lithostatically pressure balanced solution at the surface although a truly lithostatically pressure balanced system can never be attained due to the effects of compressibility at supersonic velocities it is no less valid than the more commonly utilised parallel sided system for a variety of realistic eruption scenarios and can be a useful guide for potential steady states

thermomechanical milling of accessory lithics in volcanic conduits accessory lithic clasts recovered from pyroclastic deposits commonly result from the failure of conduit wall rocks and represent an underutilized resource for constraining conduit processes during explosive volcanic eruptions the morphological features of lithic clasts provide distinctive textural fingerprints of processes that have reshaped them during transport in the conduit here we present the first study focused on accessory lithic clast morphology and show how the shapes and surfaces of these accessory pyroclasts can inform on conduit processes we use two main types of accessory lithic clasts from pyroclastic fallout deposits of the 2360 b p subplinian eruption of mount meager british columbia as a case study i rough and subangular dacite clasts and ii variably rounded and smoothed monzogranite clasts the quantitative morphological data collected on these lithics include mass volume density 2 d image analysis of convexity c and 3 d laser scans for sphericity and smoothness s shaping and comminution i e milling of clasts within the conduit are ascribed to three processes 1 disruptive fragmentation due to high energy impacts between clasts or between clasts and conduit walls 2 ash blasting of clasts suspended within the volcanic flux and 3 thermal effects we use a simplified conduit eruption model to predict ash blasting velocities and lithic residence times as a function of clast size and source depth thereby constraining the lithic milling processes the extent of shape and surface modification i e rounding and honing is directly proportional to clast residence times within the conduit prior to evacuation we postulate that the shallow seated dacite clasts remain subangular and rough due to short 2 min residence times whereas monzogranite clasts are much more rounded and smoothed due to deeper source depths and consequently longer residence times up to 1 h larger monzogranite clasts are smoother than smaller clasts due to longer residence times and to greater differential velocities within the ash laden jet lastly our model residence times and mass loss estimates for rounded clasts are used to estimate minimum attrition rates due to volcanic ash blasting within the conduit e g 12 cm3 s 1 for 25 cm clasts sourced at 2500 m depth

abrupt shifts between wet and dry phases of the 1875 eruption of askja volcano microscopic evidence for macroscopic dynamics abstract the eruption of askja in 1875 was a powerful eruption with intervals of sustained activity yet also with abrupt shifts in eruption style shifts from dry to wet fall from buoyant to collapsing column conditions and sharp fluctuations of eruption intensity occurred throughout the duration of the eruption deep ascent processes including decompression bubble nucleation and early growth of bubbles were similar among the four main phases regardless of style or column conditions uniformly high vesicle number densities 8 0 108 to 2 109 cm 3 suggest that magma degassing throughout the eruption was a disequilibrium process with very high nucleation rates power law distributions of the subpopulations of small bubbles on cumulative number density plots suggest continuous nucleation of bubbles shallow processes of bubble growth and magma ascent were at times decoupled from deep ascent processes at the onset of each phase the erupted magma appears to have a mature vesicle signature suggesting extended residence times in the shallow conduit irrespective of decompression or magma ascent rate during each phase the textural and density data suggest more uniform conditions of ascent and shallow degassing two pauses in the eruption were accompanied by shifts in vent position and we suggest that magma during this eruption was intruded in an elongated dike like geometry and vent migrations between phases led to the eruption of magma that had slightly greater residence time and hence more mature textures the major yet reversible shifts between dry or magmatic and wet or phreatomagmatic eruption styles were driven by changing external conditions accompanying the shifts in vent position the migration of the vent into and out of water sources rather than by changes of mass flux the shift between buoyant phreatoplinian wet plume conditions to the production of wet density currents is inferred to be a function of vent widening during the high intensity phreatoplinian phase thereby decreasing the exit velocity of the wet mixture in terms of the role of external water in the phreatoplinian phase our qualitative textural observations and quantitative data suggest that the magma was a foam prior to fragmentation however presently we cannot resolve the exact role and timing of the interaction with the external water

experimental study of analogue vent erosion towards nozzle shapes abstract volcanic vent shapes evolve over the course of an explosive eruption as the high pressure ejecta erode the solid conduit walls the changing shape affects the fluid behavior of the expelled gases potentially the plume rise column collapse and the type and amount of accidental debris ejected field observations of evolving vent shape are difficult or impossible while the mechanics of this process have been studied numerically almost no laboratory studies have examined these behaviors thus a series of test samples was manufactured using 3d printing techniques which were eroded using high pressure compressed air four material combinations were considered using stainless steel and alumina powders consolidated with binder concentrations of 16 and 38 material properties were evaluated using compression testing while the erosion behavior was studied through high speed visualization and pressure measurements with identical samples at constant inflow conditions in all cases the erosion showed an initial rapid erosion of the vent exit followed by quasi steady state conditions all samples produced converging diverging nozzle shapes with exit areas increasing by 1 5 to 6 times depending on the sample properties behavior correlated strongly with binder concentration as low binder materials had significant increases in nozzle area when using high density particles these weaker samples disintegrated due to particle abrasion from higher kinetic energy impacts the nozzle shapes produced supersonic exit conditions with mach numbers from 1 48 to 2 51 for the various samples however exit pressures were not balanced suggesting that the optimum is not the isentropic pressure balanced shape previously postulated

glacial and periglacial erosion of the appalachians abstract the marine 18 o record indicates that the first ice advance into the appalachians equivalent to that of the late wisconsin occurred in the pliocene at about 2 4 ma only periodic localized glacial and periglacial activity occurred in the northern appalachians from 2 33 to 0 85 ma after 0 85 ma middle to late pleistocene eight out of ten glaciations covered the northern appalachians and brought periglacial conditions to the southern appalachians during the last 850 000 years glaciation extended to the late wisconsin limit within the appalachians for 15 of the time and extended to the early wisconsin limit just north of the appalachians for 25 of the time pleistocene glacial erosion of the appalachians of 120 200 m is necessary to account for the estimated volume of pleistocene ocean sediment derived from the appalachians and the estimated volume of pliocene pleistocene glaciogenic ocean sediment derived from the entire laurentide ice sheet this amount of glacial erosion occurring over the maximum expectable duration of pliocene pleistocene ice cover of 247 000 years yields glacial erosion rates from 0 5 0 8 m ka 1 such depths of erosion by thick warm based ice streams were localized in the lowlands and estuaries of the northern appalachians in the maritimes the presence of thin cold based ice caused minimal erosion of uplands periglacial landforms and tundra boreal forest were distributed throughout the appalachian highlands and the northern coastal plain during the glacial maxima periglacial erosion of 0 15 0 30 m ka 1 near the ice in pennsylvania decline to 0 1 0 2 m ka 1 in the southern piedmont each cold period almost completely renewed the regolith by eroding slopes and filling valleys the cumulative effect of the eight to ten major cold events could have produced as much as a few ten s of meters of periglacial ridge top lowering during the pleistocene both estimated glacial and periglacial erosion rates greatly exceed estimated current fluvial erosion rates therefore glacial and periglacial processes should be considered the dominant processes in shaping the present appalachian landscape the northern appalachians may be evolving towards a form where the landscape provides just the shape necessary to permit the least work conveyance of the ice across the region during glacial maxima the southern appalachians may be evolving towards a form where the landscape provides just the hillslope form necessary to transport the debris provided by periglacial conditions

shallow plumbing and eruptive processes of a scoria cone built on steep terrain abstract dark peak lunar crater volcanic field central nevada usa is an eroded pliocene monogenetic basaltic volcano that exposes intrusions while preserving some pyroclastic deposits and lavas allowing reconstruction of the shallow magma feeding system and its relation to eruptive processes variably welded agglomerates record strombolian and hawaiian fountaining dikes fed degassed magma to a bocca on the lower cone slopes and fed a small lava field the cone was built on the side of a steep ridge with small side drainages had a maximum diameter of about 1 km and was 125 m high above the highest point on the paleotopography the eruption was fed by an 1 km long narrow 1 3 m feeder dike that locally flared in the upper tens of meters to form an 30 m wide conduit around which the cone was built the conduit shape and the transition depth from feeder dike to conduit are consistent with data from other exposed plumbing systems of small volume basaltic volcanoes that were dominated by magmatic volatile driven eruption styles supporting inferences that their conduits are relatively shallow features upper 150 m

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

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

global sea level stratigraphy an appraisal abstract although pleistocene sea level concepts were first developed in the mediterranean basin research has been revolutionized during the past 15 years by massive dating programs applied in other world areas reappraisal of four key sequences nonetheless shows that the chronometric framework is imperfect as a result available paleosea level traces continue to depend on deep sea stratigraphy for confirmation rather than the reverse they do not provide independent support for the milankovitch radiation cycles nor do they invite deductive hypothetical dating by means of such astronomical reconstructions inferred initial sea levels based on uplift assumptions or 18 o determinations on coral and mollusca show considerable divergence but broadly agree that sea levels assigned to isotope stages 5a and 5c were lower than today systematic studies should be expanded in environments that allow lithostratigraphic resolution of interdigitated marine and continental sediments meticulously complemented by biostratigraphic work direct information on the complexity and significance of regressional hemicycles helps place sea level stages recorded by transgressive facies into a multidimensional context essential for comprehensive interpretation

the decompression of volcanic jets in a crater during explosive volcanic eruptions during explosive volcanic eruptions fragmented silicic magma and volatiles exit the vent with pressures typically in the range 10 100 atm and at the speed of sound of the mixture we show that for magmatic volatile contents no in the range 0 03 no 0 06 this has the approximate value 0 95 0 05 nort 12 where r is the gas constant and t is the eruption temperature this speed is nearly independent of the vent pressure and vent radius by assuming there is negligible mixing with the air we have modelled the decompression of such jets following eruption from the vent for free decompression into the atmosphere the velocity of the decompressed jet has the approximate value 1 85 0 05 nort 12 this is nearly independent of the eruption rate for decompression into a crater the process is more complex at low eruption rates with low vent pressure the material becomes underpressured as it rises in the crater a shock then forms in the crater this recompresses the magma volatile mixture which then issues from the crater as a relatively slow subsonic jet at atmospheric pressure as the eruption rate increases such shocks move towards the top of the crater and ultimately cannot form in the crater instead the material issues from the crater as a high speed supersonic jet the upward thrust provided by the crater walls on this high pressure jet then increases the upward velocity above that of a freely decompressing jet the presence of a crater may therfore cause collapse in relatively small eruptions whereas it may promote formation of buoyant eruption columns at higher eruption rates if a crater grows through erosion during an eruption column collapse will typically ensue even if the mass flux remains steady

the mechanism of volcanic eruptions a steady state approach a theory of a volcanic eruption as a steady magma flow from a magma chamber to the surface is described magma is considered as a two component two phase medium and magma flow is steady one dimensional and isothermal mass and momentum exchange between condensed and gaseous phases is assumed to be in equilibrium with nucleation of bubbles as a fast process at small oversaturations three basic flow structures in a conduit are recognized and three resulting types of eruption are described 1 in the discrete gas separation dgs flow melt with discrete gas bubbles erupts this type includes lava eruptions and strombolian activity 2 in the dispersion regime there is continuous eruption of a gas pyroclastic suspension catastrophic explosive eruptions are included in this type 3 here a partly destroyed foam is erupted with permeable structure that allows gas escape near the surface this structure evolves with an increase in permeability and a decrease in the total porosity this type corresponds to lava dome extrusions a criterion dividing dgs flow from the other two regimes is obtained di u n1 3a c0 where u is the magma ascent velocity is magma viscosity n is number of bubble nuclei in a unit mass of magma a is the solubility coefficient for volatiles in magma and c0 is the volatile content the boundary between the dispersion regime and the partly destroyed foam regime depends on u c0 and particle size dp for the dispersion regime the basic governing parameters are magma chamber depth h conduit conductivity b2 b is the characteristic cross dimension of the conduit and excess pressure in the magma chamber instability of the dispersion regime is investigated numerically the dependence of magma discharge rate on the governing parameters is described as a combination of equilibrium surfaces with cusp catastrophes magma chamber depth is a splitting parameter in most cusps and controls the existence of unstable magma flow with sharp regime changes catastrophic rise of eruption intensity is possible if h hcr the critical chamber depth hcr depends approximately linearly on c0 and is about 15 km at c0 0 05 some scenarios of eruption evolution are described in a steady state approximation and the mechanism of catastrophic explosive eruptions is proposed the theory is applied to well documented eruptions tolbachik 1975 1976 and mount st helens 1980 the theory can also describe changes in a volcanic system such as destruction of the conduit or chamber walls destruction of the edifice changes in magma composition and caldera subsidence due to magma evacuation

coupled fluid and solid evolution in analogue volcanic vents volcanic eruptions emit rock particulates and gases at high speed and pressure which change the shape of the surrounding rock simplified analytical solutions field studies and numerical models suggest that this process plays an important role in the behavior and hazards associated with explosive volcanic eruptions here we present results from a newly developed laboratory scale apparatus designed to study this coupled process the experiments used compressed air jets expanding into the laboratory through fabricated rock analogue material which evolves through time during the experiment the compressed air was injected at approximately 2 5 times atmospheric pressure we fabricated rock analogues from sand and steel powder samples with a three dimensional printing process we studied the fluid development using phase locked particle image velocimetry while simultaneously observing the solid development via a video camera we found that the fluid response was much more rapid than that of the solid permitting a quasi steady approximation in most cases the solid vent flared out rapidly increasing its diameter by 20 to 100 after the initial expansion the vent and flow field achieved a near steady condition for a long duration the new expanded vent shapes permitted lower vent exit pressures and larger jet radii in one experiment after an initial vent shape development and establishment of steady flow behavior rock failure occurred a second time resulting in a new exit diameter and new steady state this second failure was not precipitated by changes in the nozzle flow condition and it radically changed the downstream flow dynamics this experiment suggests that the brittle nature of volcanic host rock enables sudden vent expansion in the middle of an eruption without requiring a change in the conduit flow

oceanic mechanisms for amplification of the 23 000 year ice volume cycle situated adjacent to the largest northern hemispher ice sheets of the ice ages the mid latitude north atlantic ocean has an important role in the earth s climate history it provides a significant local source of moisture for the atmosphere and adjacent continents forms a corridor that guides moisture bearing storms northward from low latitudes and at times makes direct contact along its shorelines with continental ice masses evidence of major ice ocean air interactions involving the north atlantic during the last 250 000 years is summarized outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures summer heat storage winter sea ice extent and global sea level these oceanic responses in turn influence the winter moisture flux back to the ice sheets as well as ablation of land ice by calving spectral data indicate that the oceanic moisture and sea level feedbacks in part controlled by glacial melt products amplify milankovitch insolation forcing of the volumetrically dominant mid latitude ice sheets at the 23 000 year precessional cycle

thermal history of low metamorphic grade paleoproterozoic sedimentary rocks of the penokean orogen lake superior region evidence for a widespread 1786 ma overprint based on xenotime geochronology abstract paleoproterozoic strata in northern michigan wisconsin and minnesota were deposited between 2 3 and 1 75 ga within the rifted margin and subsequent foreland basin of the penokean orogen these strata show evidence for multiple regional metamorphic events previously attributed entirely to the penokean orogeny 1875 1835 ma metasandstones from the marquette range supergroup and the animikie mille lacs and north range groups were sampled at multiple localities across minnesota wisconsin and michigan for metamorphic xenotime suitable for in situ shrimp u pb geochronology all samples are from the northern penokean foreland basin where the metamorphic grade is greenschist to sub greenschist and the strata are virtually undeformed xenotime u pb ages in these samples have a bimodal population with means of 1786 4 ma n 32 and 1861 10 ma n 9 xenotime of both ages are contained in metasandstones from the basal chocolay group in michigan and wisconsin and the mille lacs group and north range groups in minnesota the older age records a regional low temperature thermal event that is slightly older than the overlying menominee group in michigan and the animikie group in minnesota and ontario this 1861 ma event coincides with regional uplift that led to the formation of the unconformity between the menominee group and the overlying baraga group in michigan hence xenotime growth must have occurred at shallow burial depths younger units from the menominee and baraga groups in michigan and the animikie group in minnesota record only the 1786 ma event a dominant 1800 1790 ma metamorphic monazite population that overprints penokean interval monazite has been documented within amphibolite to granulite facies rocks immediately north of the niagara fault zone within the vicinity of gneiss domes and granitic plutons in contrast the 1786 ma xenotime ages are from low grade virtually undeformed rocks 50 150 km from the high grade zones and thus do not appear to reflect a local thermal imprint rather the geographic extent of the 1786 ma xenotime growth event suggests that it reflects a basin wide subtle thermal pulse it is proposed that the xenotime ages record widespread subtle heating triggered by renewed subduction along the orogen due to yavapai interval convergence the 1800 1700 ma yavapai terrane forms an accretionary belt throughout the central and southwestern u s and truncates the southern part of the penokean orogen in central wisconsin and southeastern minnesota about 200 km south of the sample sites alternatively an 1800 1765 ma interval of gravitational collapse of overthickened crust of the penokean orogen immediately north of the niagara fault zone may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the penokean foreland and resulted in xenotime growth

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

asynchronous maximum advances of mountain and continental glaciers the last maximum glacier advance in many mountain ranges appears to have predated the last maximum advance of the wisconsinan continental ice sheets 20 000 years b p published evidence from widely spaced localities in north and south america europe asia and hawai i suggests that some mountain glaciers extended farther during one or more stades early in the last glaciation roughly 115 000 30 000 years b p than at its end when low global temperature and high precipitation presumably were most favorable for the growth of glaciers variations in the timing of glacier advances in different mountain ranges suggest that alpine glaciers are dependent on regional climate as well as global conditions their study might provide sensitive tests for theories of global climate change glacial deposits are often used as time markers in neotectonic and geomorphic studies and their ages sometimes are inferred from distant measurements or from proxy histories of global climate change undated geomorphic features such as moraines sharply incised streams and river terraces which commonly have been associated with the last maximum glacier advance 20 000 years b p or last deglaciation 13 000 years b p may actually be older by a factor of 2 to 4 or more conversely deposits assigned to the maximum advances of the penultimate global glaciation 130 000 years b p may actually date from early in the most recent glacial cycle and be younger by a comparable factor rates of faulting or folding estimated from displacements or deformation and the assumed ages of such geomorphic features might be too great or too small by corresponding factors

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

sensitivity and response time of natural systems to climatic change in the late quaternary abstract although the leading theory for the cause of climatic change the milankovitch perturbations in insolation indicates variable rates of change at different latitudes any discussion of the sensitivity and response time of different natural systems is most practical when the climatic change is considered to be relatively abrupt and global rather than gradual and regional the oxygen isotope record in glacial ice should have the swiftest response time but the stratigraphic record in ice sheets at particular localities may be obscured or confused if the glacier flow is irregular the oxygen isotope stratigraphy of ocean sediment cores is a sensitive reflection primarily of contemporaneous ice volume and to a lesser extent ocean temperature ocean temperature however is recorded more specifically by microfossils in ocean sediments and may reflect global insolation directly except that in the north atlantic the influence of glacial meltwater and various feedbacks may override the insolation factor the differential lags in the response of isotopes and faunas to insolational change are apparent in the ocean sediment stratigraphy ice sheets themselves at their terminus may respond to climatic change only slowly if a change in snow accumulation is the factor because of the time involved in building a thickness sufficient for flow to great distances but if wastage near the margin is the factor then the response may be more rapid some glaciers however may advance rapidly without regard to climatic change as in the case of surging or they may retreat just as rapidly if they terminate in deep water through iceberg formation world sea level is depressed with glacial growth and thus reflects the volume of ice sheets but isostatic changes in the crust complicate the sea level response not only locally beneath the ice load but elsewhere as well the pluvial lakes of the american southwest correlated in their high levels with intervals of glaciation show a direct response to climatic patterns resulting from the expansion of the north american ice sheets and thus may lag with respect to the primary climatic change that causes ice sheet growth north african lakes on the other hand respond to the intensity of monsoonal circulation which is controlled directly by insolational shifts the response of plants and insects to deglaciation can be immediate in the case of pioneers on newly exposed terrain although the deglaciation itself may lag behind climatic change succession can follow rapidly but then more slowly as other plants immigrate from great distances it is heare where the more realistic assumption of continuing climatic change may be necessary to explain the dynamics of long range vegetational change following the last deglaciation the distribution and abundance of vertebrate populations reflect primarily their food resources and the disharmonious faunas of the last glacial period in north america with components that no longer have overlapping ranges perhaps may be explained by vegetational associations that likewise may have no modern analogues the cause may be a different climatic seasonality wastage of the ice sheet itself lagging behind the climatic change may have changed the seasonal climatic patterns sufficiently to bring about the precipitous extinction of many vertebrate species

effects of the crater on eruption column dynamics 1 during explosive eruptions a mixture of pyroclasts and volcanic gas forms a buoyant eruption column or a pyroclastic flow we systematically investigate how the condition that separates these two eruption styles column collapse condition depends on crater shape and magma chamber conditions by integrating the theoretical models for conduit flow flow inside a crater and eruption column dynamics the results show that previous model predictions of column collapse condition based on the relationship between magma discharge rate and water content nf strongly depend on crater shape depth d and opening angle when a crater is present the decompression and or compression of gas pyroclast mixture inside and just above the crater result in two distinct types of column collapse collapse with increasing hm side collapse and that with decreasing lm side collapse hm side collapse is caused by an increase in conduit radius during the waxing stage of an eruption lm side collapse is associated with a decrease in magma chamber pressure during the waning stage the value of for hm side collapse varies by two orders of magnitude depending on crater shape for fixed nf this estimate also depends on assumed models for decompression into the atmosphere the value of for lm side collapse is 108 kg s 1 when d tan 102 m these results are consistent with the field observations from the st helens 1980 and pinatubo 1991 eruptions

explosive volcanic eruptions on mars tephra and accretionary lapilli formation dispersal and recognition in the geologic record explosive volcanic eruptions potentially involving large amounts of magmatic and entrained water are thought to have been very important in the noachian and hesperian periods earlier in mars history and basaltic plinian eruptions are likely to have occurred throughout martian history previous treatments of explosive volcanic plumes on mars have simply extrapolated plume models for the earth s atmosphere to mars taking account of known limitations to the applicability of this approach which suggest that convective plumes may be capable of reaching only 20 km height on mars we introduce the concept of inertial plumes capable of carrying clasts to much greater heights and dropping them into the lower atmosphere over a wide area atmospheric circulation patterns guarantee wide dispersal and thick deposits of tephra resulting from both types of eruption the presence of large amounts of water in convecting explosive eruption plumes can also lead to condensation of water on small particles and the consequent accretion of other particles as the smaller particles fall through the plume producing accretionary lapilli formation of accretionary lapilli significantly alters the spatial distribution and grain sizes of pyroclastic fall deposits from those involving discrete juvenile clasts with negligible clast interactions we model the eruption and dispersal of tephra and the formation of accretionary lapilli on mars under current atmospheric conditions and explore the consequences of this for the geometry and grain size of deposits formed from explosive eruption plumes we show that explosive eruptions can produce thick widespread deposits of ash and lapilli similar to those thought to have produced mantling deposits in several regions of mars we develop a detailed example that shows that local hydrovolcanic explosive eruptions and solely magmatic eruptions originating from the nearby apollinaris patera could have emplaced tephra and accretionary lapilli in the columbia hills region of the mars exploration rover gusev site if the atmospheric pressure was higher early in mars history than now eruptions would have led to somewhat more extensive pyroclast dispersal than under current conditions

the last interglacial ocean the final effort of the climap project was a study of the last interglaciation a time of minimum ice volume some 122 000 yr ago coincident with the substage 5e oxygen isotopic minimum based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean last interglacial sea surface temperatures sst were compared with those today there are small sst departures in the mid latitude north atlantic warmer and the gulf of mexico cooler the eastern boundary currents of the south atlantic and pacific oceans are marked by large sst anomalies in individual cores but their interpretations are precluded by no analog problems and by discordancies among estimates from different biotic groups in general the last interglacial ocean was not significantly different from the modern ocean the relative sequencing of ice decay versus oceanic warming on the stage 6 5 oxygen isotopic transition and of ice growth versus oceanic cooling on the stage 5e 5d transition was also studied in most of the southern hemisphere the oceanic response marked by the biotic census counts preceded led the global ice volume response marked by the oxygen isotope signal by several thousand years the reverse pattern is evident in the north atlantic ocean and the gulf of mexico where the oceanic response lagged that of global ice volume by several thousand years as a result the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years these regional lead lag relationships agree with those observed on other transitions and in long term phase relationships they cannot be explained simply as artifacts of bioturbational translations of the original signals

brunhes matuyama magnetic reversal dated at 790 000 yr b p by marine astronomical correlations abstract the presence of the brunhes matuyama magnetic reversal in deep sea core sediments makes possible an alternative to the usual k ar radioistope method of dating the reversal as found in rocks the alternative method uses correlations of northern hemisphere summer insolation with oxygen isotope ratios from tropical cores the latitude dependent insolation variations are calculated from planetary mechanics and thus provide a highly accurate astronomical time scale the insolation variations strongly influence glacial ice volume fluctuations that dominate the oxygenisotope ratio changes recorded in core sediments the summer half year insolation variations are identified with corresponding isotope ratio changes in cores from the present through glacial stage 20 misleading effects of discontinuities or major nonuniformities of sediment deposition are avoided by an analysis of the uniformity of v28 238 and v28 239 the principal cores studied and by comparisons with other cores the top section stages 1 to 10 of v28 238 is uniformly deposited and for this section an isotope ratio time scale is chosen that agrees with the thorium uranium date for the high sea stand of the last interglacial extreme over this interval major glacial extremes strong isotope ratio minima coincide consistently with major insolation minima at times of low orbital eccentricity in addition interstadials are directly associated with precessional insolation peaks and the envelope of isotope ratio peaks resembles the envelope of precessionally dominated insolation peaks the assumption that the glacial extremes depended similarly on insolation minima during stages 10 to 20 permits minor age shifts of strong isotope ratio minima in the two cores relative to ages based on uniform overal deposition to match the ages of low eccentricity insolation minima the age shifts reflect residual nonuniformities of deposition the validity of this matching procedure is supported by a resulting consistent identification of principal isotope ratio peaks with high and low latitude coincident insolation maxima the brunhes matuyama reversal is found intinterglacial stage 19 and is dated on the astronomical time scale at 790 000 5000 yr b p

u series and amino acid racemization geochronology of bermuda implications for eustatic sea level fluctuation over the past 250 000 years abstract bermuda is a stable mid oceanic carbonate platform for which a particularly complete record of late pleistocene eustatic sea level fluctuation has been reconstructed from a detailed study of geological field relationships combined with an extensive programme of u series and amino acid racemization geochronology only twice in the past 250 000 yr has sea level in bermuda been above its present level once at approximately 200 k y when it stood at about 2 m and most recently at 125 4 k y when it stood at 5 1 m these times of interglacial high sea level are characterized by the development of patch reefs and marine calcarenites at elevations above present sea level episodes of lower sea stand onto the bermuda platform at elevations higher than 20 m are observed within the two interglacial periods and are characterized by the deposition of eolianites by contrast glacial periods are times of residual soil formation and deposition of speleothems in caves at elevations below present sea level excellent correlation is observed between the bermuda glacio eustatic sea level record and other marine and terrestrial paleoclimate records

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

volcanic ash generation effects of componentry particle size and conduit geometry on size reduction processes abstract grain size distributions gsd of pyroclastic materials are the product of processes ranging from primary fragmentation efficiency to tephra transport as such a detailed description of their physical and chemical state can provide pivotal information regarding such processes by constraining the gsd of volcanic deposits one can thereby deliver powerful constraints on the energetics and dynamics of eruptions in order to do so we must distinguish between two primary controls 1 fragmentation of magma to tephra and 2 secondary transport related processes such as abrasion and comminution within the conduit and until deposition of the particles variations in particle interactions in the conduit together with conduit geometry may be major factors in modifying the gsd as in all physicochemical processes for eruptive dynamics an experimental basis is an essential element of calibration and quantification here we have conducted the first experimental investigation linking fragmentation and ash production via the influence of 1 particle componentry and particle size and 2 the conduit geometry constricted versus unconstricted on the gsd rapid decompression experiments with loose tephra material from the fall deposit of pomici principali eruption 10 3 ka campi flegrei have been conducted in an optically transparent setup that enables the optical monitoring of particle dynamics with a high speed camera the samples employed can be classified into two main groups 1 pumices and 2 dense clasts including crystals lava clasts and wall rock fragments our results indicate that 1 a conduit diameter constriction simulating obstacles in the conduit walls is likely to reduce the average diameter of individual clasts and increase the generation of ash

late wisconsinan deglaciation of alberta processes and paleogeography abstract the scarcity of lake basins in alberta dating earlier than 11 000 bp has been interpreted as indicating the continued presence of active glacial ice because of the related implication that the presence of ice precludes the existence of an ice free corridor it is useful to examine this issue more closely due to the effects of deglaciation many areas of alberta were dominated by chaotic ice stagnation conditions with continual reversal of topography and rapid transformation of the surface the dynamic nature of the environment affects arguments regarding the existence of an ice free corridor as well as having implications for archeological site formation preservation and discovery deglaciation models utilizing paradigms of active ice retreat may result in dichotomous depictions of the land surface as either ice covered or deglaciated due to the insulating properties of supraglacial debris stagnant ice results in a situation that is simultaneously neither and both of the above conditions while the landscape is ice covered and thus not ice free an accessible landscape characterized by glacial karst topography exists on top of the stagnating ice surface general reconstructions of the impact of stagnant ice and glacial karst topography on the paleogeography of alberta are presented for discussion

proximal lava drainage controls on basaltic fissure eruption dynamics hawaiian basaltic eruptions commonly initiate as a fissure producing fountains spattering and clastogenic lava flows most fissures rapidly localize to form a small number of eruptive vents the location of which may influence the subsequent distribution of lava flows and associated hazards we present results from a detailed field investigation of the proximal deposits of episode 1 of the 1969 fissure eruption of mauna ulu k lauea hawai i exceptional preservation of the deposits allows us to reconstruct vent proximal lava drainage patterns and to assess the role that drainage played in constraining vent localization through detailed field mapping including measurements of the height and internal depth of lava tree moulds we reconstruct high resolution topographic maps of the pre eruption ground surface the lava high stand surface and the post eruption ground surface we calculate the difference in elevation between pairs of maps to estimate the lava inundation depth and lava drainage depth over the field area and along different segments of fissure aerial photographs collected during episode 1 of the eruption allow us to locate those parts of the fissure that are no longer exposed at the surface by comparing with the inundation and drainage maps we find that fissure segments that were inundated with lava to greater depths typically 1 6 m during the eruption later became foci of lava drainage back into the fissure internal drain back we infer that in these areas lava ponding over the fissure suppressed discharge of magma thereby favouring drain back and stagnation by contrast segments with relatively shallow inundation typically less than 1 m such as where the fissure intersects pre eruptive topographic highs or where flow away from the vent outflow was efficient are often associated with sub circular vent geometries in the post eruption ground surface we infer that these parts of the fissure became localization points for ongoing magma ascent and discharge we conclude that lava inundation and drainage processes in basaltic fissure eruptions can play an important role in controlling their localization and longevity

nature of local magma storage zones and geometry of conduit systems below basaltic eruption sites pu u o o kilauea east rift hawaii example the fluid dynamics of the well documented eruptive episodes at pu u o o kilauea wolfe et at 1987 are used to investigate quantitativeely the size and shape of the shallow conduit system beneath the vent cooling calculations are employed to study the long term survival of conduits and the consequences of multiple dike injection events we find that the subvent conduit must have a planar geometry at depths greater than a few tens to at most a few hundreds of meters with a width of less than a few meters a length of the order of 100 m and a height of the order of 1 kin this structure is clearly the residue of the preeruption dikes although such a feature can be widened somewhat by repeated dike emplacement events there is no evidence to suggest that a larger and much more equant magma reservoir should develop at shallow depth the extensive degassing often occurring after single eruptions or between repeated eruptions previously thought to imply the presence of a large equidimensional shallow magma chamber can be readily explained by the volume of magma in a dikelike subvent planar magma storage zone on the basis of these observations which we infer to apply commonly to basaltic eruption sites we suggest that the terms chamber or reservoir be used with caution because they most often connote a relatively large equant magma body distinct from the feeder dike lying just below the vent we find that the subvent region has the same basic geometric characteristics as the parental dike and we propose that it be referred to as a planar magma storage zone

origin of small pits in martian impact craters we propose a numerical model for the formation of the closely spaced pits found in the thin ejecta related deposits superposed on the floors interior terrace blocks and near rim ejecta blankets of well preserved martian impact craters our model predicts the explosive degassing of water from this pitted material which is assumed to originally be water bearing impact melt rich breccia at the time of deposition this process is analogous to what occurred in the fall out suevite deposits at the ries impact structure in germany at ries impact heating of water bearing target material resulted in the rapid degassing of its water and other volatiles the martian environment plays an important role in enhancing the effects of this degassing by increasing the flow speed of the escaping gas the high flow rate of gas through particulate materials such as suevite tends to quickly form segregation channels or vent pipes similar to those found in the ries deposits these pipes act as conduits for the efficient high speed escape of the gas and small clasts that it entrains escaping gas and entrained clasts abraded and eroded the conduit walls flaring them to form pits above a network of pipes

sustained eruptions on enceladus explained by turbulent dissipation in tiger stripes spacecraft observations suggest that the plumes of saturn s moon enceladus draw water from a subsurface ocean but the sustainability of conduits linking ocean and surface is not understood observations show eruptions from tiger stripe fissures that are sustained although tidally modulated throughout each orbit and since the 2005 discovery of the plumes peak plume flux lags peak tidal extension by 1 rad suggestive of resonance here we show that a model of the tiger stripes as tidally flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle the phase lag and the total power output of the tiger stripe terrain while suggesting that eruptions are maintained over geological timescales the delay associated with flushing and refilling of o 1 m wide slots with ocean water causes erupted flux to lag tidal forcing and helps to buttress slots against closure while tidally pumped in slot flow leads to heating and mechanical disruption that staves off slot freezeout much narrower and much wider slots cannot be sustained in the presence of long lived slots the 10 6 y average power output of the tiger stripes is buffered by a feedback between ice melt back and subsidence to o 10 10 w which is similar to observed power output suggesting long term stability turbulent dissipation makes testable predictions for the final flybys of enceladus by cassini our model shows how open connections to an ocean can be reconciled with and sustain long lived eruptions turbulent dissipation in long lived slots helps maintain the ocean against freezing maintains access by future enceladus missions to ocean materials and is plausibly the major energy source for tiger stripe activity

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

sequence of sediments at locality 1 in zhoukoudian and correlation with loess stratigraphy in northern china and with the chronology of deep sea cores abstract according to recent dating by several methods the impressive cave sediments at locality 1 at zhoukoudian about 40 m thick have not only allowed the age of peking man to be established but have calibrated the general course of cave development permitted correlation with events both within china and farther afield and placed the sequence in the global frameword of late cenozoic climatic change the cave deposits can be divided into 17 layers which are correlated with the great loess sequence l9 l4 in china and with deep sea core oxygen isotope stages 16 6 the 14th layer upward 730 000 230 000 yr b p represents at least four glacial cycles

expansion and quenching of vesicular magma fragments in plinian eruptions the conditions of pumice generation in plinian eruptions are studied a physical model describes the behavior of gas bubbles in a magma fragment which is carried upward in a volcanic conduit and an atmospheric eruption column the effects of pressure release and cooling are calculated for a range of eruption conditions the magma fragment expands in the conduit and stops expanding soon after leaving the vent when a thin viscous rind forms against the cold mixture of magmatic gas and air this rind prevents further volume changes pumice vesicularity is a function of the decompression rate in the conduit which depends on ascent velocity and fragmentation depth it is also sensitive to the cooling rate in the atmospheric column which depends on vent radius and mass discharge rate different fragments follow different trajectories in the column and are subjected to different cooling rates this generates a range of vesicularities which reflects the eruptive conditions all else being equal pumice vesicularity increases as magma viscosity decreases these predictions are consistent with observations pumices provide quantitative constraints on conduit flow conditions and mass discharge rate these concepts are applied to two plinian eruptions vesicularity values for the bishop tuff long valley caldera require a mass discharge rate between 108 and 109 kg s 1 vesicularity variations during plinian phase 1 of the minoan eruption santorini are explained by the conduit radius increasing from about 30 m to 120 m both cases require large decompression rates in the eruption conduit suggesting that flow pressures were close to lithostatic values

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

highly explosive eruption of the monogenetic 8 6 ka bp la vache et lassolas scoria cone complex chaine des puys france abstract the eruption of the trachy basaltic la vache and lassolas cone complex was the youngest eruption ca 8 6 ka bp and one of the most violent in the chaine des puys france here we present field data and results of grain size componentry and clast density measurements of different layers of the widespread tephra deposit that is associated with this cone forming eruption our data indicates five main eruption phases comprising a vent opening phase a second sustained highly explosive phase a third and fourth violent strombolian phase and a fifth dominantly effusive phase the layer formed by the opening phase is rich in lithic material which was previously considered to be the result of phreatomagmatic activity the data presented here on the componentry and textures of the pyroclastic material contradict this hypothesis we propose instead that the material of the basal layer results from fragmentation caused by the explosion of a first arriving gas dominated phase the variations in eruption intensity during the main eruption phases are interpreted here to be the result of gas segregation within the plumbing system and fluxes in the magma ascent rate during the eruption significant amount of gas segregation is indicated by the deposition of both gas poor and gas rich material and by the presence of plate tephra this is also supported by the simultaneous ejection of tephra and lava from both cones during most of the explosive activity we suggest that gas segregation occurred within shallow intrusions and that fresh ascending material in the main conduit mixed with degassed material that flow back into the conduit from the intrusion before fragmentation the interaction of the ascending magma and the opening of intrusions may have controlled the evolution and explosivity of the eruption the high explosivity at the beginning of the eruption and the wide dispersal area demonstrate that scoria cone eruptions in monogenetic fields can impose a major threat to the population and infrastructures nearby as these events may occur with little warning and therefore research on this kind of eruptions is of a major importance to better understand the processes driving these events

multidisciplinary investigation on a lava fountain preceding a flank eruption the 10 may 2008 etna case a multidisciplinary approach integrating a wide data set ranging from bulk rock compositions of the erupted products to volcanic tremor long period events and tilt and gravity signals is used to investigate the source depth and magma dynamics of the 10 may 2008 lava fountain at southeast crater sec of mount etna the investigation was undertaken in the framework of the previous 2007 explosive activity as well as the subsequent effusive eruption beginning 13 may 2008 and lasting up to july 2009 all the data concur in indicating that the 10 may lava fountain was generated by the fragmentation of a foam layer trapped at the top of a shallow reservoir about 1500 1700 m below the summit of sec the shift from the episodic strombolian lava fountain activity occurring in 2007 at sec to the more powerful 10 may 2008 lava fountain is explained by the intrusion of a new more primitive magma into the shallow reservoir data also indicate that an attempted magma intrusion east of the summit area occurred during the 10 may fire fountain this event caused the fracturing and weakening of the surrounding rocks and created a preferential pathway for the penetration of the magma that only 3 days later started to feed the 2008 2009 effusive eruption

fissure eruptions in tharsis mars implications for eruption conditions and magma sources the tharsis region has been the focus of many studies of volcanism on mars not only are the largest volcanoes on the planet located here but also many of the youngest volcanic features are found within tharsis comparatively little attention has been given to understanding the origin of the smaller volcanoes within tharsis and these smaller structures may provide important information on both the regional tectonics of the area and the availability of magma from the shallow or deep crust we have identified six separate but physically close vent systems in eastern tharsis just to the east of the volcano jovis tholus these vents are typically linear fissures a few to 20 km in length that have built small shields rising to 50 85 m above the level of the surrounding topography the maximum length of individual lava flows from these fissures is 30 km but more typical lengths are 15 20 km mapping of the vent complexes reveals that smooth material interpreted to be spatter from fire fountaining is located on the rims of some of the fissures this spatter then formed either short 5 km flows or merged close to the fissure to form the longer flows there appears to have been some temporal evolution of the flow field as the oldest parts of the basement at each center are built from a series of compound flows that cannot be subdivided into individual flows we find evidence of frozen lava ponds and channelized flows from central vents photoclinometric profiles are constructed across one of the longer flows to confirm an unusual attribute originally identified by mouginis mark and christensen mouginis mark p j christensen p r 2005 new observations of volcanic features on mars from the themis instrument j geophys res 110 e8 doi 10 1029 2005je002421 of the flows from the central vent complex they are all 5 m thick which is a factor of 8 15 thinner than flows elsewhere on mars to investigate the unusual eruption conditions limited total volume of the construct and of individual flows we model the lava rheology the duration of emplacement the subsurface conditions that may have led to these eruptions and the volatile content of the magma we find lava viscosities and yield strengths to be 100 pa s and 100 pa respectively eruption rates to be 5000 m3 s 1 flow speeds to be 1 2 m s 1 durations of emplacement of individual flow units to be 5 h and the equivalent magma water content to be 0 1 0 2 mass these eruption conditions are consistent with a wide range of possible depths of the magma reservoirs feeding the eruptions small vents such as the ones studied here have also been identified in other parts of eastern tharsis so that the eruptions described here may characterize a common style of volcanism on mars that can only be identified now that image spatial resolution in the range 1 20 m pixel is available it is therefore possible that additional searches of other volcanic areas syrtis planum elysium planitia and hesperia planum may also show greater diversity of activity than is currently accepted