geomagnetic cosmogenic and climatic changes across the last geomagnetic reversal from equatorial indian ocean sediments abstract high resolution records of beryllium 10be production and relative paleointensity have been obtained across the matuyama brunhes m b reversal from the equatorial indian ocean maldives area both magnetic and geochemical analyses were performed from the same discrete samples to avoid any artificial depth offset the authigenic 10be concentrations were normalized with respect to 9be in order to correct for potential environmental effects while the relative paleointensity was derived from the remanent magnetization intensity after accounting for changes in magnetic concentration within the sediment the relative paleointensity and the 10be 9be records are both characterized by large deviations which culminate in the middle of the reversal in contrast to most previous studies and despite relative high deposition rate 4 7 cm ka we observed a perfect synchronism between the 10be 9be peak the lowest value of relative paleointensity and the switch in direction which indicates that bioturbation and post depositional processes did not affect the magnetic record this leaves no ambiguity for the stratigraphic position of the reversal located within marine isotopic stage 19 as revealed by the planktonic 18o record from the same core the magnetic data depict a two phase process with a precursory event preceding the rapid polarity switch while only the second phase is present in the 10be record similarly to other low latitude records from the indonesian area using an orbitally tuned age model we obtain an age of 772 ka 5 ka for the middle of the transition while the precursory event occurred almost 20 ka before we believe that the bimodal distribution emerging from the compilations of the ages of the m b reversal results from the succession of these two events microtektites from the australasian impact were found at 0 6 m below the transition 790 ka 5 ka b p and confirm that this large event occurred 12 ka prior to the polarity transition the distribution of tektite abundance was used to deconvolve the 10be 9be signal the results confirm that the beryllium changes are concentrated during the transitional period thus likely in presence of a multipolar geomagnetic field or in the vicinity of a geomagnetic pole that favored the penetration of cosmic rays and consequently increased the 10be production the absence of 10be during the precursor indicates that the present site and the indonesian ones were far away from a geomagnetic pole and that interlatitudinal atmospheric mixing was limited the geomagnetic pole positions above the indonesian sites during the precursor would thus be incompatible with the corresponding inclined dipolar field during this period and suggest the dominance of low degree harmonics

field dependent postdepositional grain realignment in the pdrm process experimental evidence and implications the field dependence of postdepositional remanent magnetization pdrm has been investigated by two component pdrm acquisition experiments in deviating magnetic fields of different strengths the experiments have been performed on samples consisting of crushed natural magnetite grains in pseudo single domain psd states embedded 0 1 by weight in a slowly curing epoxy araldite the increasing viscosity with time of the epoxy is thought to mimic consolidation processes in sediments which gradually inhibits magnetic grain realignment resulting pdrm directions are partly intermediate between final and initial field directions which are unresolvable by alternating field demagnetization to 60 mt the results suggest that partial pdrm acquired in initial low fields are modified by subsequent exposure to stronger fields this may be due to alignment of grains not affected by the initial field or by remobilization of grains previously aligned in the weaker initial field the suggested field dependent pdrm smoothing may prevent improvement of paleomagnetic records by deconvolution procedures and may imply that longitudinally confined paleomagnetic records in sediments of transitional polar paths during reversals may partly be artifacts of the pdrm recording process

water magma interaction some theory and experiments on peperite formation experiments using molten thermite as a magma analog produce peperite when the melt interacts with wet sand these experiments also show explosive behavior developing strombolian and surtseyan like bursts the results demonstrate that the application of fuel coolant interaction fci theory is appropriate for interpretation of peperites the theory described includes discussion of the importance of mass interaction ratios of wet sediment and magma rs which determine thermal equilibrium temperature limits and contact interface dynamics the dynamics of the interface between magma and wet sediments involves heat transfer over a wide range of rates from passive quenching to explosive fragmentation a vapor film layer develops at the interface and acts both as an insulating barrier promoting passive quenching as well as a potential energy reservoir that can cause magma fragmentation mingling of the magma with wet sediments and explosive quenching when the vapor film becomes unstable an important parameter in determining the behavior of the vapor film is the value of rs which controls whether heat can be convectively removed from the layer as more is being added from its contact with magma if rs 1 for fully saturated sediments there is enough water in the sediments to make convective heat flow effective in quenching the magma but below that value there is the potential that the vapor film will be unstable producing highly dynamic phenomena including explosive fragmentation at values of rs 0 1 there is insufficient water to allow the escalation of explosive fragmentation

a paleomagnetic and rock magnetic study of the manicouagan impact structure implications for crater formation and geodynamo effects we report rock magnetic and paleomagnetic data from the similar to 214ma manicouagan canada impact crater based on 25 widely distributed sites of impact melt and basement rocks collected at the surface as well as from boreholes drilled to depths 1 5km titanomagnetite and titanohematite carry the magnetic remanence in impact melts above 320m elevation and in most basement rocks impact melts below 320m contain solely titanomagnetite magnetic susceptibility and saturation magnetization proxies for titanomagnetite concentration increase more than tenfold toward the base of the thickest impact melt that underwent fractional crystallization the titanomagnetite enriched zone partially contributes to a 2000nt magnetic anomaly in the crater s center stepwise demagnetization reveals a single normal polarity magnetization component in all samples regardless of the magnetic phases present coeval lock in remanence times for titanomagnetite and titanohematite indicate that the titanohematite formed 570 degrees c during oxi exsolution the average paleomagnetic direction and intensity coincide well with 214ma reference values we find no evidence for an aberration of the geomagnetic field over the several thousands of years it took to cool a 481m thick portion of the impact melt body hence the energy released by the manicouagan impact that created one of the 10 largest known craters on earth provoked no measurable disturbance of the geodynamo magnetic anisotropy of clast free impact melts define magnetic lineation directions that are in places radially oriented with respect to the crater s center centrifugal flow of the melt within the evolving transient crater probably generated the fabric

rootless tephra stratigraphy and emplacement processes volcanic rootless cones are the products of thermohydraulic explosions involving rapid heat transfer from active lava fuel to external sources of water coolant rootless eruptions are attributed to molten fuel coolant interactions mfcis but previous studies have not performed systematic investigations of rootless tephrostratigraphy and grain size distributions to establish a baseline for evaluating relationships between environmental factors mfci efficiency fragmentation and patterns of tephra dispersal this study examines a 13 55 m thick vertical section through an archetypal rootless tephra sequence which includes a rhythmic succession of 28 bed pairs each bed pair is interpreted to be the result of a discrete explosion cycle with fine grained basal material emplaced dominantly as tephra fall during an energetic opening phase followed by the deposition of coarser grained material mainly as ballistic ejecta during a weaker coda phase nine additional layers are interleaved throughout the stratigraphy and are interpreted to be dilute pyroclastic density current pdc deposits overall the stratigraphy divides into four units unit 1 contains the largest number of sediment rich pdc deposits units 2 and 3 are dominated by a rhythmic succession of bed pairs and unit 4 includes welded layers this pattern is consistent with a general decrease in mfci efficiency due to the depletion of locally available coolant i e groundwater or wet sediments changing conduit vent geometries mixing conditions coolant and melt temperatures and or coolant impurities may also have affected mfci efficiency but the rhythmic nature of the bed pairs implies a periodic explosion process which can be explained by temporary increases in the water to lava mass ratio during cycles of groundwater recharge

thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from molten fuel coolant interaction experiments 1 thermohydraulic explosions were produced by molten fuel coolant interaction mfci experiments using remelted shoshonitic rocks from vulcano italy the fragmentation history and energy release were recorded the resulting products were recovered and analyzed with the scanning electron microscope fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode these clasts relate to the thermohydraulic phase of the mfci where most of the mechanical energy is released they are here called active particles the total surface area of such particles is proportional to the energy of the respective explosions other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime these fragments called passive particles form after the thermohydraulic phase during the expansion phase of the mfci in order to verify thermohydraulic explosions in volcanic eruptions we compared experimental products with samples from phreatomagmatic base surge deposits of vulcano ash particles from the experiments show features similar to those from the deposits suggesting that the experiments reproduced the same fragmentation dynamics to achieve discrimination between active and passive particles we calculated shape parameters from image analysis the mass of active particles in base surge deposits was calculated as the material properties for the natural samples are identical to the experimental ones the energy measurements and calculations of the experiments can be applied for a single phreatomagmatic eruption at vulcano a maximum mechanical energy release of 2 75 1013 j was calculated representing a tnt analogue of 6 5 kt

quantitative experiments on phreatomagmatic explosions abstract an experimental set up for controlled generation of thermal explosions fuel coolant interactions has been built by an interdisciplinary research group of volcanologists physicists and engineering scientists the tee haus thermal explosion experiment during the last 3 years more than 500 experimental runs were performed aim of the project was not to build a mini volcano but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects such small scale mixtures however most probably also occur during phreatomagmatic events in nature mixtures of water and magma on a large scale may be modelled by combination of small elements earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively therefore a carbonate melt approx 0 3 kg of 1 1 mass ratio na2co3 k2co3 was used for initial experimental series because of easy handling and more magma similarity by injection of water into this melt explosive mixtures were produced and ignited self triggering explosive interactions occurred in a wide melt temperature range 720 to 1040 c at melt temperatures exceeding 1000 c co2 increasingly degassed presence of non condensible gas bubbles in the melt strongly reduced the explosivity the intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities because of more intensive mixing a minimum injection velocity was found to be approx 0 7 m s 1 experiments with silicate melts derived from diverse remelted volcanic rocks were carried out with a slightly modified set up explosions were ignited by an additional trigger a shock wave of approx 8 j in volcanic systems shock waves of this magnitude are abundant volcanic tremor etc strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750 c the produced explosions exerted repulsion forces of up to 24 000 n with a typical pulse duration of 1 ms carbonate melt resp 1 5 ms silicate melts the ejection velocity was found to be in a range between 200 and 400 m s 1 explosion energy values up to 500 j were calculated the maximum explosion pressure was calculated to be in the range between 10 and 100 mpa only part of the melt so called interactive melt reacts explosively with injected water most of the melt is passively ejected by and after the thermal explosion different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 m to 180 m the amount of interactive melt is proportional to the intensity of the explosion the maximum amount of water vapourized at the explosive interaction so called interactive water was estimated thus the water melt mass ratio for explosive interactions could be calculated the values range between 1 25 and 1 35 carbonate melt resp between 1 6 and 1 25 silicate melts

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

subkilometer martian volcanoes properties and possible terrestrial analogs kilometer to subkilometer cones with large central craters are the most numerous volcanic structures in the northern plains of mars based on examination of more than 12 000 high resolution viking orbiter images most of these very small cones are located in parts of eastern acidalia and utopia isidis elysium planitia measurements of base and pit diameter for some 2000 well resolved features show small variations in the histogram of base diameters between and within regions but remarkable similarity in the distribution of crater cone diameter ratios this suggests that the small cones had a similar origin with only minor variations in local conditions the acidalia cones are generally larger more widely distributed and occur on a greater variety of terrains than those in utopia isidis elysium based on morphology distribution and the crater cone ratio histograms the smallest martian cones may be similar to icelandic pseudocraters somewhat larger steeper cones with smaller crater cone ratios which also occur in eastern acidalia may be analogs of terrestrial cinder cones

magnetic exploration of ocean crust for craters of impact origin model results magnetic data sets over deep ocean areas may contain clues to the existence of craters formed by the impact of an extraterrestrial body with the earth s ocean crust to aid in the magnetic exploration of the ocean crust for oceanic impact craters basic but effective computations from an impact model are studied from an aeromagnetic point of view the main assumption of the analysis is that a sufficiently large impact can excavate large volumes of magnetized basalt vaporize basalt and raise basalt to temperatures above the curie temperature approximately 500 c to alter the preimpact magnetization of the ocean floor and result in a magnetic anomaly being associated with an oceanic impact crater in the absence of an existing theory on the influence of impacts on ocean crustal magnetization the representation of a crater on the ocean floor by a simple potential provides apparently for the first time quantitative estimates of the crater s magnetic anomaly along a horizontal surface numerical results

hydrovolcanic explosions the systematics of water pyroclast equilibration the initial contact of external water with erupting magma and the mass ratio of water to magma govern the development of hydrovolcanic phenomena the phase relations of water within the runout system and the separation of vapor or liquid from the pyroclasts explain gradational transitions between some pyroclastic flows base surges mud hurricanes mudflows and sheetfloods

the origin of bore core remanences mechanical shock imposed irreversible magnetizations summary repeated laboratory induced weak mechanical shocking c 0 57 kg m s 1 of marine sandstone samples showing drilling induced remanence from commercial bore cores from the north sea and prudhoe bay causes increases in their low field susceptibility x and their ability to acquire an isothermal remanent magnetization irm these enhancements are reduced by some 20 per cent by af demagnetization in 100 mt doubling the intensity of the shock doubles the susceptibilities and irms acquired the susceptibility increase ceases after 300 to 400 shocks for the north sea samples and 20 to 30 shocks for those from prudhoe bay while the irm saturates after 800 1000 and 30 50 shocks respectively continental haematite bearing sandstones from commercial bore cores with no drilling induced remanence subjected to the same shocks do not show these evects diverences in the magnetic mineralogy of shocked and unshocked marine samples suggest that the magnetic enhancement is predominantly due to the creation of pyrrhotite by shock induced irreversible crystallographic changes in iron bearing sulphides when shocked during commercial drilling these new ferromagnetic minerals acquire strong chemical crystalline remanences associated with a wide spectrum of grain sizes in the magnetic field of the drill string and these are resistant to both thermal and af demagnetization similar processes are likely in any situation involving the shock of physically metastable iron bearing minerals particularly anoxic sediments a 5 cm non magnetic collar between the drill stem and crown should drastically reduce the magnetic intensity of this evect under commercial conditions but would not prevent its occurrence

short polarity intervals within the matuyama transitional field records from hydraulic piston cored sediments from the north atlantic abstract detailed sampling of two short magnetozones within the matuyama chronozone recorded at dsdp site 609 49 86 n 335 77 e confirms that one the cobb mountain subchronozone 1 12 ma is a very short full normal polarity interval and that the other the older interval is a record of a geomagnetic excursion which occurred at approximately 1 55 ma the cobb mountain subchron lasted approximately 25 000 years one third the duration of the jaramillo subchron the normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions we interpret the antisymmetry as reflecting a dependence upon the sense of the reversal without significant changes in the relative contributions of non dipole terms the polarity interval recorded at 1 55 ma lasted only 8 800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section this feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the gilsa subchron similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth s outer core

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

age and provenance of the target materials for tektites and possible impactites as inferred from sm nd and rb sr systematics sm nd and rb sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects tektites have large negative e nd 0 values and are uniform within each tektite group while the e sr 0 are large positive values and show considerable variation within each group chemical trace element and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust each tektite group is characterized by a uniform nd model age t chur nd interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites 1 1 15 ae for australasian tektites 2 1 91 ae for ivory coast tektites 3 0 9 ae for moldavites 4 0 65 ae for north american tektites and 5 0 9 ae for high si irghizites sr model ages t ur sr are variable within each group reflecting rb sr fractionation and in the favorable limit of very high rb sr ratios approach the time of sedimentation of the parent material which melted to form the tektites australasian tektites are derived from 0 25 ae sediments moldavites from 0 0 ae sediments ivory coast tektites from 0 95 ae sediments possible parent sediments of other tektite groups have poorly constrained ages our data on moldavites and ivory coast tektites are consistent with derivation from the ries and bosumtwi craters respectively irghizites are isotopically distinct from australasian tektites and are probably not related sanidine spherules from a cretaceous tertiary boundary clay have initial e nd 2 e sr 5 and are not derived from old continental crust or meteoritic feldspar they may represent a mixture of basaltic oceanic crust and sediments implying an oceanic impact these isotopic results are also consistent with a volcanic origin for the spherules

40ar 39ar age constraints for the jaramillo normal subchron and the matuyama brunhes geomagnetic boundary our mid pleistocene 4oar 39ar age recalibration of the geomagnetic polarity timescale is nearly in accord with the oxygen isotope climate record calibration of the astronomical timescale proposed by johnson 1982 and shackleton et al 1990 4oar 39ar ages of a normally magnetized rhyolite dome in the valles caldera northern mexico yielded a weighted mean age of 1 004 0 019 ma a k ar age of 0 909 0 019 ma for this rock by doell and dalrymple 1966 was the linchpin for the recognition and calibration of the jaramillo normal subchron jns other 4oar 39ar ages from the vailes caldera and 4oar 39ar ages of ivory coast tektites indicate that the jns began at about 1 11 ma and ended before 0 92 ma probably near 0 97 ma the matuyama brunhes boundary occurred between 0 79 ma and 0 76 ma on the basis of 4oar 39ar sanidine ages from 1 three reversely magnetized rhyolite domes of the valles caldera 0 793 0 018 ma 0 794 0 007 ma and 0 812 0 023 ma and pumice 0 789 0 006 ma from the reversely magnetized oldest toba tuff of sumatra and 2 pumice 0 764 0 005 ma and 0 757 0 009 ma from the lower and upper units of the normally magnetized bishop tuff the age of the boundary may be close to 0 77 ma as deduced from rates of sedimentation in ancient lake bonneville utah increasingly aware that the polarity of earth s magnetic field has changed repeatedly through geologic time and isotopic dating of these polarity reversals resulted in a geomagnetic polarity time scale gpts for the late cenozoic this combined paleomagnetic and isotopic research played a pivotal role in confirming plate tectonic theory glen 19821 which revolutionized the earth sciences continual refinement of isotopic ages which define the gpts has increased the usefulness of geomagnetic polarity reversals as marker hori zons in many types of geologic studies we present 4oar 39ar data and ages for some key volcanic units that support changes in the isotopic age chronology of the gpts using volcanic rocks erupted near the time of the geologically youngest two geomagnetic polarity reversals figure 1 the older of the two reversals originally called by doell and dalrymple 1966 the jaramillo event is now named the jaramillo normal subchron jns it is recorded in middle pleistocene rocks as a brief episode of normal polar ity within the late matuyama chron the younger of the two called the matuyama brunhes boundary mbb is regis tered in middle pleistocene rocks worldwide and consists of

physical controls and resulting morphological forms of quaternary ice contact volcanoes in western canada abstract volcanism in association with large quantities of ice manifests itself in a variety of morphological forms developed under differing physical conditions these physical conditions include the location amount thickness and type of confining ice the location and quantity of trapped water and the surrounding topography ice in the form of thick continental ice sheets thinner alpine glaciers or even perennial accumulations of snow can influence the resulting morphological form of the volcano the chemical composition of the erupting magma effusion rate and total erupted volume will also affect the shape of the resulting edifice quaternary volcanoes influenced by many of these factors are found in western canada the morphological forms range from tuyas to subglacial mounds to stratovolcanoes this paper provides a review of ice contact volcanism and volcanic products formed in these environments using canadian examples for illustration

ivory coast microtektite strewn field description and relation to the jaramillo geomagnetic event abstract during the present study the ivory coast microtektite layer was found in cores from five equatorial atlantic sites bringing the total number of ivory coast microtektite bearing cores to eleven the strewn field appears to be restricted to between 9 n and 12 s latitude there is a general increase in the concentration of microtektites towards the bosumtwi crater which is generally thought to be the source of the ivory coast tektites the relationship between the onset of the jaramillo subchron and the ivory coast microtektite layer has been investigated in six cores a plot of the difference in depth between the base of the jaramillo subchron and the microtektite layer versus sediment accumulation rate was used to determine the average post depositional remanent magnetization pdrm acquisition depth and the age difference between the onset of the jaramillo subchron and the deposition of the microtektites assuming that the pdrm acquisition depth does not vary with sediment accumulation rate we find that the average pdrm acquisition depth is 7 cm and that the microtektites were deposited approximately 8 ky after the onset of the jaramillo subchron this indicates that the impact responsible for the ivory coast tektites and microtektites could not be causally related to the geomagnetic reversal at the base of the jaramillo subchron

the formation of circular littoral cones from tube fed pāhoehoe mauna loa hawai i pyroclastic cones along the southwest coast of mauna loa volcano hawai i have a common structure a an early formed circular outer rim 200 400 m in diameter composed mostly of scoria and lapilli and b one or more later formed inner rims composed almost exclusively of dense spatter the spatter activity locally fed short lava flows that ponded within the outer rims based on various lines of evidence these cones are littoral in origin relationships between the cones and associated flows the degassed nature of the pyroclasts and although not unequivocal the position of the cones relative to known eruptive vent locations on mauna loa additional support for the littoral interpretation comes from their similarity to smaller littoral cones that have been observed forming during the ongoing kilauea eruption the structure of these mauna loa cones however contrasts with that of standard hawaiian littoral cones in that there is or once was a complete circle of pyroclastic deposits furthermore they are large even though associated with tubefed p hoehoe flows instead of a the following origin is proposed an initial flow of tube fed p hoehoe into the ocean built a lava delta with a base of hyaloclastite collapse of an inland portion of the active tube into the underlying wet hyaloclastites or a water filled void allowed sufficient mixing of water and liquid lava to generate strong explosions these explosions broke through the top of the flow and built up the outer scoria lapilli rims on the solid carapace of the lava delta eventually the supply of water diminished the explosions declined in intensity to spattering and the initial rim was filled with spatter and lava

geomagnetic reversals from impacts on the earth the impact of a large extraterrestrial object on the earth can produce a geomagnetic reversal through the following mechanism dust from the impact crater and soot from fires trigger a climate change and the beginning of a little ice age the redistribution of water near the equator to ice at high latitudes alters the rotation rate of the crust and mantle of the earth if the sea level change is sufficiently large 10 meters and rapid in a few hundred years then the velocity shear in the liquid core disrupts the convective cells that drive the dynamo the new convective cells that subsequently form distort and tangle the previous field reducing the dipole component near to zero while increasing the energy in multipole components eventually a dipole is rebuilt by dynamo action and the event is seen either as a geomagnetic reversal or as an excursion sudden climate changes from other causes such as volcanic eruptions could also trigger reversals this mechanism may not be the sole cause of geomagnetic reversals but it can account for the rapid drop of the dipole component preceding a reversal the predominance of multipole components during a transition the associations of microtektites temperature drops and extinctions with reversals and the possible correlation between peaks in the geomagnetic reversal rate and the times of mass extinctions the model may also account for the long term changes in the average rate of reversals we make several testable predictions

mars a review and synthesis of general environments and geological settings of magma h2o interactions the advent of a global cryosphere likely occurred very early in the history of mars and much of the available water and related volatiles co 2 clathrates etc were sequestered within and below the cryosphere this means that magmatism plutonism and volcanism as a geological process throughout the history of mars cannot be fully understood without accounting for the interaction of magma and water and related species in both solid and liquid form we review and outline the probable configuration of water and ice deposits in the history of mars describe environments and modes of magma h 2o interaction and provide specific examples from the geological record of mars magma and water ice interactions have been interpreted to have formed 1 massive pyroclastic deposits 2 large scale ground collapse and chaotic terrain 3 major outflow channels 4 mega lahars dwarfing terrestrial examples 5 sub ice sheet eruptions and edifices 6 pseudocraters 7 landslides on volcanic edifice flanks and 8 hydrothermal sites the global nature of the cryosphere its longevity and the diversity of environments means that mars is an excellent laboratory for the study of magma h 2o interactions and the role of related volatile species

using the characteristics of rootless cone deposits to estimate the energetics of explosive lava water interactions during volcanic eruptions the interaction of magma and groundwater can produce thermohydraulic explosions capable of significantly increasing the eruption energy the most well known mechanism by which explosive magma water interactions occur molten fuel coolant interaction mfci is a complex series of macro and microscale processes which have been simulated using laboratory scale experiments as a natural analog for mfci experiments we utilize rootless cone beds formed by lava water explosions to estimate explosion energy the specific mechanical energy of the lava water explosions studied here occurs over a broader range 4 to 178 kj kg than mfci experiments and includes estimates for the highest energy lava water explosions studied to date explosion energy is partitioned similarly over the two systems with kinetic transport and fragmentation energy making up 25 40 and 42 80 of the mechanical energy respectively which overlap the ranges estimated for mfci experiments our study of lava water explosions therefore provides a field scale analog of mfci laboratory experiments for understanding the energetics and therefore hazards of mfci in natural systems

icelandic pseudocraters as analogs to some volcanic cones on mars pseudocraters are rootless vents formed by the interaction of lava flows with surface or near surface water this interaction can produce mild explosions and the accumulation of scoria and spatter into small constructs pseudocraters in several localities in iceland were examined in the field and compared to similar appearing features observed on mars the icelandic pseudocrater cones in this study range in size from 6 to 70 m in diameter have summit craters which range from 2 to 28 m in diameter many cones lack craters entirely and have flanks that are either concave up or convex up the size and spacing of icelandic pseudocraters might be a function of the availability of water in which larger closely spaced features result from efficient lava water interaction as suggested by the environments in which the features formed possible martian pseudocrater cones in amazonis planitia range in diameter from 30 to 180 m and have craters 12 to 80 m in diameter a numerical model for volcanic explosions was adapted to study the formation of pseudocraters under terrestrial and martian conditions the results suggest that explosions forming martian cones require significantly less water calculated masses are less by a factor of 4 to 16 than those forming icelandic pseudocraters despite their larger sizes this is attributed to the low gravity and atmospheric pressure in the mars environment and is consistent with the likely lower abundance of water which might be present as interstitial ice at shallow depths in the regolith locations of potential pseudocraters on mars at latitudes as low as 8 n imply the presence of crustal ice stores at the time of their formation

geochemistry of cenozoic microtektites and clinopyroxene bearing spherules abstract we have determined the major and trace element compositions of 176 individual microtektites spherules from the australasian ivory coast and north american microtektite and clinopyroxene bearing cpx spherule layers trace element contents for up to 30 trace elements were determined by instrumental neutron activation analysis inaa and major element compositions were determined using energy dispersive x ray edx analysis in combination with a scanning electron microscope sem in addition petrographic data were obtained for the cpx spherules using the sem and edx this is the first trace element study of individual australasian microtektites and the data revealed the presence of a previously unrecognized group of australasian microtektites with high contents of ni up to 471 ppm in previous studies the high mg hmg australasian microtektites were thought to be related to the hmg australasian tektites but our trace element data suggest that the high ni hni australasian microtektites rather than the high mg microtektites are related to the high mg australasian tektites we find that cenozoic microtektites spherules from a given layer can be distinguished from microtektites spherules from other layers as a group but it is not always possible to determine which layer an individual microtektite spherule came from based only on trace element compositions the cpx spherules and most of the microtektites have cr co and ni contents that are higher than the average contents of these elements in the upper continental crust suggesting the presence of a meteoritic component the highest cr co and ni contents are found in the cpx spherules and low si cpx related microtektites unetched to slightly etched cpx spherules have ni cr and ni co ratios that generally lie along mixing curves between the average upper continental crust and chondrites the best fit appears to be with an ll chondrite the moderately to heavily etched cpx spherules have values that lie off the mixing curves in a direction that suggests ni loss probably as a result of solution of a ni rich phase olivine the ni rich australasian microtektites also have ni values that lie close to mixing curves between the average upper continental crust and chondrites however both the cpx spherules and hni australasian microtektites appear to have ir and to a lesser extent au contents that are much too low to have ni ir ratios similar to chondritic values we have no explanation for the low ir and au contents except to speculate that they may be the result of a complex fractionation process the ivory coast and north american microtektites do not have high enough siderophile element contents to reach any firm conclusions regarding the presence of or nature of a meteoritic component in them trace element compositions are consistent with derivation of the cenozoic microtektite spherule layers from upper continental crust the normal australasian microtektites appear to have been derived from a graywacke or lithic arenite with a range in clay and quartz content the source rock for the high mg australasian microtektites is not known but the hmg microtektites do not appear to be normal australasian microtektites that were simply contaminated by meteorites or ultramafic rocks the average ivory coast microtektite composition can be matched with a mixture of target rocks at the bosumtwi crater the average composition of the north american microtektites suggests an arkosic source rock but with graywacke and quartz rich end members however we could not match the composition of the north american microtektites with lithologies in impact breccias recovered from the chesapeake bay impact structure that is believed to be the source crater likewise we could not match the composition of the cpx spherules with mixtures of basement rocks and overlying sedimentary deposits for which compositional data are available at the popigai impact crater that may be the source crater for the cpx spherules this may be because the cpx spherules were derived in large part from clastic surface rocks sandstones and shales for which no compositional data are available

athabasca valles mars a lava draped channel system athabasca valles is a young outflow channel system on mars that may have been carved by catastrophic water floods however images acquired by the high resolution imaging science experiment camera onboard the mars reconnaissance orbiter spacecraft reveal that athabasca valles is now entirely draped by a thin layer of solidified lava the remnant of a once swollen river of molten rock the lava erupted from a fissure inundated the channels and drained downstream in geologically recent times purported ice features in athabasca valles and its distal basin cerberus palus are actually composed of this lava similar volcanic processes may have operated in other ostensibly fluvial channels which could explain in part why the landers sent to investigate sites of ancient flooding on mars have predominantly found lava at the surface instead

ivory coast microtektites and geomagnetic reversals the reported association of the ivory coast microtektite occurrence with the onset of the jaramillo normal polarity subchron has given support to the notion that impact events can give rise to geomagnetic reversals we re evaluate the paleomagnetic stratigraphy of two critical deep sea sediment cores bearing ivory coast microtektites k9 57 and v27 239 and show that this event likely occurred during the jaramillo subchron approximately 30 ky after its onset and 40 ky before its termination this new result thus argues against any causal relationship between the ivory coast tektite producing event and a geomagnetic polarity reversal

lava water interaction and hydrothermal activity within the 2014 2015 holuhraun lava flow field iceland abstract lava that erupted during the 2014 2015 holuhraun eruption in iceland flowed into a proglacial river system resulting in aqueous cooling of the lava and an ephemeral hydrothermal system we carried out a monitoring study of this system from 2015 to 2018 to document the cooling of the lava over this time using thermocouple measurements and data logging sensors the heat loss rate from advection through this hydrothermal system in august 2015 was 5 5 108 w since eruption aqueous cooling likely accounted for 1 of the total heat loss from the lava this estimate excludes steam losses from fumaroles as well as any groundwater that was not released to the surface and thus is a lower bound near the terminus of the flow advection of heat by flowing water may have locally accounted for tens of percent of the total cooling of that part of the flow our data quantify the importance of water cooling for this lava flow and can be compared with models to better understand lava water interactions more generally we also provide detailed methods for simple low cost monitoring of similar instances in the future

topographic and stochastic influences on pahoehoe lava lobe emplacement a detailed understanding of p hoehoe emplacement is necessary for developing accurate models of flow field development assessing hazards and interpreting the significance of lava morphology on earth and other planetary surfaces active p hoehoe lobes on k lauea volcano hawai i were examined on 21 26 february 2006 using oblique time series stereo photogrammetry and differential global positioning system measurements during this time the local discharge rate for peripheral lava lobes was generally constant at 0 0061 0 0019 m3 s but the areal coverage rate of the lobes exhibited a periodic increase every 4 13 0 64 min this periodicity is attributed to the time required for the pressure within the liquid lava core to exceed the cooling induced strength of its margins the p hoehoe flow advanced through a series of down slope and cross slope breakouts which began as 0 2 m thick units i e toes that coalesced and inflated to become approximately meter thick lobes the lobes were thickest above the lowest points of the initial topography and above shallow to reverse facing slopes defined relative to the local flow direction the flow path was typically controlled by high standing topography with the zone directly adjacent to the final lobe margin having an average relief that was a few centimeters higher than the lava inundated region this suggests that toe scale topography can at least temporarily exert strong controls on p hoehoe flow paths by impeding the lateral spreading of the lobe observed cycles of enhanced areal spreading and inflated lobe morphology are also explored using a model that considers the statistical likelihood of sequential breakouts from active flow margins and the effects of topographic barriers

fragmentation mechanisms associated with explosive lava water interactions in a lacustrine environment rootless cones form when partially outgassed lava interacts explosively with external water the explosions represent an end member system that can elucidate mechanisms of explosive magma water interactions in the absence of magmatic fragmentation induced by outgassing the proportion of finely fragmented ejecta i e ash generated in rootless explosions may contribute significantly to the energy of the explosion even if the ash volume is small relative to coarser ejecta laboratory experiments indicate that the degree of melt water mixing and energy release are proportional to the abundance of blocky grains fragmented by brittle disintegration which effectively contribute thermal energy to the system to constrain the mechanisms and dynamics of rootless explosive activity we assess the nature and modes of fragmentation and ejecta characteristics through morphological textural and density analysis of rootless tephra associated with a p hoehoe lava flow in a lacustrine lake basin environment we observe strong correlations between the mean grain size and the mass percentage of both blocky negative power law trend and fluidal positive logarithmic tephra clasts of all sizes we interpret these trends as scale dependent fragmentation behavior due to the decreasing efficacy of hydrodynamic fragmentation as it occurs over finer scales especially over the ash size range additionally all analyzed beds contain fine ash sized blocky and mossy clasts which are thought to be diagnostic of a high transfer rate of thermal to mechanical energy characteristic of molten fuel coolant interactions these results agree with a recent model of rootless cone formation prior fragmentation theory and scaled laboratory experiments and therefore provide a field based analog for future experimental and modeling efforts

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

rootless cones on mars indicating the presence of shallow equatorial ground ice in recent times high resolution mars orbiter camera moc images have revealed the existence of clusters of small cones in the cerberus plains marte valles and amazonis planitia mars these cones are similar in both morphology and planar dimensions to the larger of icelandic rootless cones which form due to explosive interactions between surficial lavas and near surface groundwater impact crater size frequency relationships indicate that surfaces upon which the cones sit are no older than 10 ma if martian cones form in the same manner as terrestrial rootless cones then equatorial ground ice or ground water must have been present near the surface in geologically recent times

emplacement and inflation of pahoehoe sheet flows observations and measurements of active lava flows on kilauea volcano hawaii inflated pahoehoe sheet flows have a distinctive horizontal upper surface which can be several hundred meters across and are bounded by steep monoclinal uplifts the inflated sheet flows we studied ranged from 1 to 5 m in thickness but initially propagated as thin sheets of fluid pahoehoe lava generally 20 30 cm thick individual lobes originated at outbreaks from the inflated front of a prior sheet flow lobe and initially moved rapidly away from their source velocities slowed greatly within hours due to radial spreading and to depletion of lava stored within the source flow as the outward flow velocity decreases cooling promotes rapid crustal growth at first the crust behaves plastically as pahoehoe toes form after the crust attains a thickness of 2 5 cm it behaves more rigidly and develops enough strength to retain incoming lava thus increasing the hydrostatic head at the flow front the increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow resulting in uniform uplift of the entire sheet flow lobe initial uplift rates are rapid flows thicken to 1 m in 1 2 hours but rates decline sharply as crustal thickness increases and as outbreaks occur from the margins of the inflating lobe one flow reached a final thickness of nearly 4 m after 350 hr inflation data define power law curves whereas crustal cooling follows square root of time relationships the combination of data can be used to construct simple models of inflated sheet flows as the flow advances preferred pathways develop in the older portions of the liquid cored flow these pathways can evolve into lavatube systems within a few weeks formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1 2 orders of magnitude slower if advance of the sheet flow is terminated the tube remains filled with lava that crystallizes in situ rather than draining to form the cave like lava tubes commonly associated with pahoehoe flows inflated sheet flows from kilauea and mauna loa are morphologically similar to some thick icelandic and submarine sheet flows suggesting a similar mechanism of emplacement the planar sheet like geometry of flood basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly instantaneous emplacement as literal floods of lava

characteristics of rootless cone tephra emplaced by high energy lava water explosions the effects of external water during hydrovolcanic eruptions are difficult to quantify however research investigating the explosive interactions between surface lava flows and water has provided new insights into explosion dynamics previous work has focused on relatively low energy lava water explosions from a 100 m diameter cone in the rau holar cone group in iceland for comparison we study ejecta derived from higher energy explosions at the 400 m diameter pu u k holo rootless cone in hawai i one of the largest rootless cones on earth this comparative approach elucidates the effect of lava water mixing conditions and range of explosivity on ejecta characteristics utilizing grain size analysis we found that pu u k holo beds typically have median grain sizes at the cone rim that are smaller than 3 8 mm and are on average finer grained than those of the rau holar cone ash sized grains the most energetically significant ejecta size fraction typically make up 20 of bed ejecta with a maximum of 90 both greater proportions than found in beds of the rau holar cone utilizing grain morphology crystal texture and surface feature analysis we found that the abundance of ash sized grains associated with high heat transfer rates i e active particles increases at pu u k holo as explosivity increases which is an important consideration for energy modeling this dataset also includes ejecta from the finest grained beds and therefore highest energy lava water explosions studied to date

structure and origin by injection of lava under surface crust of tumuli lava rises lava rise pits and lava inflation clefts in hawaii tumuli are positive topographic features that are common on hawaiian pahoehoe lava flow fields particularly on shallow slopes and 75 measured examples are presented here to document the size range tumuli form by up tilting of crustal plates without any crustal shortening and are thus distinguished from pressure ridges which are up buckled by laterally directed pressure the axial or star like systems of deep clefts that characterize tumuli are defined here as lava inflation clefts their tips advanced into red hot lava and they widened as uplift proceeded and while the lava crust was thickening flat surfaced uplifts formed like tumuli by injection of lava under a surface crust were previously called pressure plateaus but lava rise is proposed instead the pits that abound among lava rises previously attributed to collapse or subsidence are generally formed because the lava around them rose and the name lava rise pit is proposed unique examples of tumuli and lava rises from which lava drained out under a surface crust 1 5 to 2 5 m thick are described from kilauea caldera these examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava and is able to do so because of its higher vesicle content the fluid lava loses many of its gas bubbles during residence beneath the crust the bulk densities of samples from tumuli show a general downward increase the form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust suggesting that the lava was stably density stratified it is inferred that stable stratification was regulated by out flows of the more vesicular lava fractions loss of bubbles through the lava inflation clefts and entry of injected lava at its level of neutral buoyancy below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava

explosive lava water interactions ii self organization processes among volcanic rootless eruption sites in the 1783 1784 laki lava flow iceland we have applied quantitative geospatial analyses to rootless eruption sites in the hnuta and hrossatungur groups of the 1783 1784 laki lava flow to establish how patterns of spatial distribution can be used to obtain information about rootless cone emplacement processes and paleo environments this study utilizes sample size dependent nearest neighbor nn statistics and voronoi tessellations to quantify the spatial distribution of rootless eruption sites and validate the use of statistical nn analysis as a remote sensing tool our results show that rootless eruption sites cluster in environments with abundant lava and water resources but competition for limited groundwater in these clusters can cause rootless eruption sites to develop repelled distributions this pattern of self organization can be interpreted within the context of resource availability and depletion topography tends to concentrate lava fuel and water coolant within topographic lows thereby promoting explosive lava water interactions in these regions given an excess supply of lava within broad sheet lobes rootless eruption sites withdraw groundwater from their surroundings until there is insufficient water to maintain analogs to explosive molten fuel coolant interactions rootless eruption sites may be modeled as a network of water extraction wells that draw down the water table in their vicinity rootless eruptions at locations with insufficient groundwater may either fail to initiate or terminate before explosive activity has ceased at nearby locations with a greater supply of water thus imparting a repelled distribution to observed rootless eruption sites

explosive lava water interactions in elysium planitia mars geologic and thermodynamic constraints on the formation of the tartarus colles cone groups 1 volcanic rootless constructs vrcs are the products of explosive lava water interactions vrcs are significant because they imply the presence of active lava and an underlying aqueous phase e g groundwater or ice at the time of their formation combined mapping of vrc locations age dating of their host lava surfaces and thermodynamic modeling of lava substrate interactions can therefore constrain where and when water has been present in volcanic regions this information is valuable for identifying fossil hydrothermal systems and determining relationships between climate near surface water abundance and the potential development of habitable niches on mars we examined the western tartarus colles region 25 27 n 170 171 e in northeastern elysium planitia mars and identified 167 vrc groups with a total area of 2000 km2 these vrcs preferentially occur where lava is 60 m thick crater size frequency relationships suggest the vrcs formed during the late to middle amazonian modeling results suggest that at the time of vrc formation near surface substrate was partially desiccated but that the depth to the midlatitude ice table was 42 m this ground ice stability zone is consistent with climate models that predict intermediate obliquity 35 between 75 and 250 ma with obliquity excursions descending to 25 32 for lava thicknesses ranging from 30 to 60 m and ground ice fractions ranging from 0 1 to 0 3 an ice volume of 4 23 km3 could have been melted and or vaporized by the time the lava solidified and the associated hydrothermal systems could have retained temperatures 273 k for up to 1300 years

syn and posteruptive hazards of maar diatreme volcanoes abstract maar diatreme volcanoes represent the second most common volcano type on continents and islands this study presents a first review of syn and posteruptive volcanic and related hazards and intends to stimulate future research in this field maar diatreme volcanoes are phreatomagmatic monogenetic volcanoes they may erupt explosively for days to 15 years above the preeruptive surface a relatively flat tephra ring forms below the preeruptive surface the maar crater is incised because of formation and downward penetration of a cone shaped diatreme and its root zone during activity both the maar crater and the diatreme grow in depth and diameter inside the diatreme which may penetrate downwards for up to 2 5 km fragmented country rocks and juvenile pyroclasts accumulate in primary pyroclastic deposits but to a large extent also as reworked deposits ejection of large volumes of country rocks results in a mass deficiency in the root zone of the diatreme and causes the diatreme fill to subside thus the diatreme represents a kind of growing sinkhole due to the subsidence of the diatreme underneath the maar crater is a subsidence crater and also grows in depth and diameter with ongoing activity as long as phreatomagmatic eruptions continue the tephra ring grows in thickness and outer slope angle syneruptive hazards of maar diatreme volcanoes are earthquakes eruption clouds tephra fall base surges ballistic blocks and bombs lahars volcanic gases cutting of the growing maar crater into the preeruptive ground formation of a tephra ring fragmentation of country rocks thus destruction of area and ground changes in groundwater table and potential renewal of eruptions the main hazards mostly affect an area 3 to possibly 5 km in radius distal effects are comparable to those of small eruption clouds from polygenetic volcanoes syneruptive effects on infrastructure people animals vegetation agricultural land and drainage are pointed out posteruptive hazards concern erosion and formation of lahars inside the crater a lake usually forms and diverse types of sediments accumulate in the crater volcanic gases may be released in the crater compaction and other diagenetic processes within the diatreme fill result in its subsidence this posteruptive subsidence of the diatreme fill and thus crater floor is relatively large initially but will decrease with time it may last millions of years various studies and monitoring are suggested for syn and posteruptive activities of maar diatreme volcanoes erupting in the future the recently formed maar diatreme volcanoes should be investigated repeatedly to understand more about their syneruptive behaviour and hazards and also their posteruptive topographic limnic and biologic evolution and potential posteruptive hazards for future maar diatreme eruptions a hazard map with four principal hazard zones is suggested with the two innermost ones having a joint radius of up to 5 km areas that are potentially endangered by maar diatreme eruptions in the future are pointed out

generation of pyroclastic flows by explosive interaction of lava flows with ice water saturated substrate abstract we describe a new type of secondary rootless phreatomagmatic explosions observed at active lava flows at volcanoes klyuchevskoy russia and etna italy the explosions occurred at considerable up to 5 km distances from primary volcanic vents generally at steep 15 35 slopes and in places where incandescent basaltic or basaltic andesitic lava propagated over ice water saturated substrate the explosions produced high up to 7 km vertical ash steam laden clouds as well as pyroclastic flows that traveled up to 2 km downslope individual lobes of the pyroclastic flow deposits were up to 2 m thick had steep lateral margins and were composed of angular to subrounded bomb size clasts in a poorly sorted ash lapilli matrix character of the juvenile rock clasts in the pyroclastic flows poorly vesiculated with chilled and fractured cauliflower outer surfaces indicated their origin by explosive fragmentation of lava due to contact with external water non juvenile rocks derived from the substrate of the lava flows comprised up to 75 in some of the pyroclastic flow deposits we suggest a model where gradual heating of a water saturated substrate under the advancing lava flow elevates pore pressure and thus reduces basal friction in the case of frozen substrate water is initially formed by thawing of the substrate along the contact with lava on steep slope this leads to gravitational instability and sliding of a part of the active lava flow and water saturated substrate the sliding lava and substrate disintegrate and intermix triggering explosive fuel coolant type interaction that produces large volume of fine grained clastic material relatively cold steam laden cloud of the phreatomagmatic explosion has limited capacity to transport upward the produced clastic material thus part of it descends downslope in the form of pyroclastic flow similar explosive events were described for active lava flows of llaima chile pavlof alaska and hekla iceland indicating that this type of explosions and related hazard is common at snow ice clad volcanoes and sometimes happens also on fluid saturated hydrothermally altered slopes

discrete blasts in granular material yield two stage process of cavitation and granular fountaining a discrete blast within granular material such as a single subterranean explosion within a debris filled diatreme structure is typically considered to produce a single uprush of material our experiments demonstrate that apparent debris jet deposits can be formed by a two stage process of cavitation and subsequent granular fountaining bench scale experiments reported here demonstrate that for a range of overpressures and depths individual discrete buried gas blasts open space and expel particles from the blast site in two largely decoupled stages expanding gas initially pierces material nearest the blast source to open a cavity above it then a fountain of grains rises from the source into the cavity this staged motion dynamically segregates source grains from host material grains and the rates of cavity opening versus fountain rise show a power law decay relationship with initial pressure our experimental analysis has implications for maar diatreme systems field scale detonation experiments and underground nuclear testing

lava and ice interaction at stratovolcanoes use of characteristic features to determine past glacial extents and future volcanic hazards structures resulting from lava and ice interaction are common at glaciated stratovolcanoes during summit eruptions at stratovolcanoes meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers eroding the ice and enlarging preexisting glacial drainages as a result eruptions in this environment have produced few catastrophic floods lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker filling the available space and producing steep sided bodies with smooth bulbous contact surfaces quenching of lava against ice or by water forms small scale features such as tensional fractures and glass as the amount of meltwater in contact with the lava increases the type and abundance of smaller scale features become similar to those produced during subglacial eruptions into meltwater lakes identification of large and small scale lava ice contact features in the field can be used to reconstruct paleoglacial extent and combined with geochronology of lavas to determine past paleoclimate an understanding of lava ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes

geomagnetic field intensity recorded after impact in the ries meteorite crater germany summary we carried out palaeointensity experiments on 45 samples collected throughout the ca 15 ma ries crater germany to test whether meteorite impacts can influence the geodynamo a thellier thellier procedure with alteration tail and additivity checks yields a weighted average palaeointensity of 19 1 1 1 t from 15 samples that pass stringent data selection criteria the corresponding virtual dipole moment of 3 2 0 2 1022 am2 is relatively low but indistinguishable from the global palaeointensity database between 20 and 10 ma our results suggest that either the energy released during the ries meteorite impact event was too low to affect the geodynamo the ries impact lithologies cooled too fast to record any effect or the time averaged by the impact lithologies was long enough to record secular variation but that the geodynamo was remarkably stable over that time

rootless cones on mars a consequence of lava ground ice interaction fields of small cratered cones on mars are interpreted to have formed by rootless eruptions due to explosive interaction of lava with ground ice contained within the regolith beneath the flow melting and vaporization of the ice and subsequent explosive expansion of the vapour act to excavate the lava and construct a rootless cone around the explosion site similar features are found in iceland where flowing lava encountered water saturated substrates the martian cones have basal diameters of c 30 1000 m and are located predominantly in the northern volcanic plains high resolution mars orbiter camera images offer significant improvements over viking data for interpretation of cone origins a new model of the dynamics of cone formation indicates that very modest amounts of water ice are required to initiate and sustain the explosive interactions that produced the observed features this is consistent with the likely low availability of water ice in the martian regolith the scarcity of impact craters on many of the host lava flows indicates very young ages suggesting that ground ice was present as recently as less than 10 l00 ma and may persist today rootless cones therefore act as a spatial and temporal probe of the distribution of ground ice on mars which is of key significance in understanding the evolution of the martian climate the location of water in liquid or solid form is of great importance to future robotic and human exploration strategies and to the search for extraterrestrial life

the morphology and formation of flow lobe tumuli on icelandic shield volcanoes three types of tumuli are recognized on monogenetic shield volcanoes in iceland namely 1 lava coated tumuli 2 upper slope tumuli and 3 flow lobe tumuli flow lobe tumuli grade into lava rises which are larger inflation structures but both structures have a similar mode of emplacement the areal distribution of flow lobe tumuli in the flow field differs from that of lava coated tumuli and upper slope tumuli flow lobe tumuli are frequent in the medial and distal parts of pahoehoe flow fields whereas the other two tumulus types are more frequent in the proximal parts of the flow fields the flow lobe tumuli also differ from the two other tumulus types as regards their morphology they are significantly larger have shallower flanks and do not have extensive outflows from the cracks the vesicle distribution within the flow lobe tumuli is similar to that of p type flow lobes a fairly dense zone is situated between the upper and lower vesicular zones each flow lobe tumulus is an individual lobe that inflates and forms tension cracks in the lava crust major lava tubes are mostly absent from the areas where flow lobe tumuli form and therefore the emplacement of flow lobe tumuli must be related to secondary short lived pathways of lava the flow lobe tumuli inside the flow fields of icelandic shield volcanoes are morphologically identical to those flow lobe tumuli that are found in the margins of flow fields of other icelandic volcanoes these observations allow us to model the flow lobe tumuli as morphological variants of p type flow lobes forming along terminating secondary lava paths flow lobe tumuli form at low lava supply rates of the order of 10 2 10 5 m3 s and are generated by inflation of the lava crust as a result of magmatic overpressure in the associated lava core we model the tumulus as a circular elastic plate that becomes uplifted inflated and fractured by a uniform magmatic overpressure at its bottom we conclude that 1 the effective thickness of the lava crust that is subject to bending is 0 2 0 4 m and remains essentially constant during the formation of the tumulus and 2 for flow lobe tumuli that are supplied with magma from tubes that originate in overflow from the lava lake of a shield volcano the elevation difference between the tumuli and the surface of the source lava lake is commonly around 100 m this elevation difference can give rise to a static magmatic overpressure of 2 5 mpa in the tumulus whereas the overpressure needed to form a typical flow lobe tumulus is only 0 2 1 mpa

rootless cone eruption processes informed by dissected tephra deposits and conduits rootless cones result from the explosive interaction between lava flows and underlying water saturated sediment or volcaniclastic deposits rootless explosions can represent a significant far field hazard during basaltic eruptions but there are few detailed studies of their deposits a rootless cone field in the 8 5 ma ice harbor flow field of the columbia river basalt province nw usa is revealed by sections through rootless conduit and cone structures the ice harbor lava flow hosting the rootless cones was emplaced across a floodplain or lacustrine environment that had recently been mantled by a layer of silicic volcanic ash from a major explosive eruption our observations indicate a two stage growth model for the rootless cones 1 initial explosions generated sediment rich tephra emplaced by fallout and pyroclastic density currents and 2 later weaker explosions that generated spatter rich fountains variable explosive activity resulted in a wide range of pyroclast morphologies and vesicularities cross sections through funnel shaped conduits also show how the conduits were constructed and stabilised the growth model is consistent with decreasing water availability with time as inferred for rootless cones described in iceland the ice harbor rootless cones provide further lithological data to help distinguish between rootless cone derived tephra and tephra generated above an erupting dyke

explosive lava water interactions i architecture and emplacement chronology of volcanic rootless cone groups in the 1783 1784 laki lava flow iceland to determine the relationships between rootless cone emplacement mechanisms morphology and spatial distribution we mapped the hnuta and hrossatungur groups of the 1783 1784 laki lava flow in iceland we based our facies maps on differential global positioning system dgps measurements photogeological interpretations and supporting field observations the study area covers 2 77 km2 and includes 2216 explosion sites to establish the timing of rootless cone formation we incorporated tephrochronological constraints from eighty eight stratigraphic sections and determined that the hnuta and hrossatungur groups are composite structures formed by the emplacement of six geographically and chronologically discrete domains rootless eruptions initiated in domain 1 on the first day of the laki eruption june 8 1783 and lasted 1 2 days the second episode of rootless activity began in domain 2 on june 11 and lasted 1 3 days the four domains of the hrossatungur group dominantly formed after june 14 and exhibit a complex emplacement sequence that reflects interactions between the laki lava contemporaneously emplaced rootless cones and an existing topographic ridge in the study area we identify three distinct rootless cone archetypes i e recurring morphological forms that are related to tube channel and broad sheet lobe fed eruptions we assert that emplacement of lava above compressible substrates e g unconsolidated sediments may trigger rootless eruptions by causing subsidence induced flexure and failure of the basal crust thereby allowing molten lava fuel to come into direct contact with groundwater coolant and initiating analogs to explosive molten fuel coolant interactions mfcis

a terrestrial shallow marine transition in the archean opemisca group east of chapais quebec abstract the stella and hauy formations of the archean opemisca group in the eastern chapais syncline feature numerous fluvial to shallow marine transitions throughout the stratigraphic sequence during the first depositional phase stella formation relatively high gradient alluvial fans prograde into a shallow marine environment the coarse clastic sediments reflect a scott type depositional environment the initial depositional environment is envisaged to be a high energy wave and storm flood dominated coastline bordered by coastal alluvial fans shallow marine deposits consist of coarse to very coarse grained sandstones and intraclast and pebble to cobble conglomerates the prevalent shallow marine sediments are related to storm and or flood deposition terrigenous sediments of the donjek type depositional environment prevail during the second depositional phase hauy formation the shallow marine sediments although still wave and storm flood dominated are finergrained and when correlated with the terrigenous sediments imply a lower gradient and a maturing of the coastline furthermore contemporaneous volcanism of andesitic shoshonitic to basaltic composition occurs during this evolutionary stage the sedimentary basin gradually evolved during the two depositional phases the repetition of terrigenous and shallow marine sediments throughout the stratigraphic sequence coarsening and fining sequences in the conglomerates and the alluvial fan setting strongly suggest a fault bounded basin margin tectonic uplift of the hinterland initiates alluvial fan progradation onto the shelf whereas quiescent periods are documented by marine transgression the clast composition of the conglomerates and the alluvial fan environment suggest local derivation of clasts the synvolcanic chibougamau pluton and the lake dore complex acted as a stable basement during deposition of the opemisca group the small landmass served as a stable platform for shallow marine deposition even when tectonically influenced a late phase intraplutonic cratonic development of an initial backarc basin system internal zone of the abitibi greenstone belt is proposed as the overall depositional setting of the chapais basin