planetary surface dating from crater size frequency distribution measurements partial resurfacing events and statistical age uncertainty abstract we describe the procedure to fit a cumulative production function polynomial to a partial crater size frequency distribution the technique is of particular use in deriving ages for surfaces which have undergone partial resurfacing events namely erosional or depositional events which have affected a limited diameter range of the crater population we demonstrate its use in obtaining times for both the surface formation and the resurfacing event we give a practical outline of the method for making age measurements from crater counts and how to identify resurfacing effects in the results we discuss the conversion of production function polynomials between common presentations and the statistical uncertainty of the determined ages with respect to the non linear chronology function and a minor refinement of data binning

volcano ice interactions precursory to the 2009 eruption of redoubt volcano alaska abstract in late summer of 2008 after nearly 20 years of quiescence redoubt volcano began to show signs of abnormal heat flow in its summit crater in the months that followed the excess heat triggered melting and ablation of redoubt s glaciers beginning at the summit and propagating to lower elevations as the unrest accelerated a variety of morphological changes were observed including the creation of ice cauldrons areas of wide spread subsidence punctures in the ice carved out by steam and deposition from debris flows in this paper we use visual observations satellite data and a high resolution digital elevation model of the volcanic edifice to calculate ice loss at redoubt as a function of time our aim is to establish from this time series a proxy for heat flow that can be compared to other data sets collected along the same time interval our study area consists of the drift glacier which flows from the summit crater down the volcano s north slope and makes up about one quarter of redoubt s total ice volume of 4 km 3 the upper part of the drift glacier covers the area of recent volcanism making this part of ice mass most susceptible to the effect of volcanic heating moreover melt water and other flows are channeled down the drift glacier drainage by topography leaving the remainder of redoubt s ice mantle relatively unaffected the rate of ice loss averaged around 0 1 m 3 s over the last four months of 2008 accelerated to over twenty times this value by february 2009 and peaked at greater than 22 m 3 s just prior to the first major explosion on march 22 2009 we estimate a cumulative ice loss over this period of about 35 million cubic meters m m 3

the dynamics of magma mixing in a rising magma batch the conditions under which two magmas can become mixed within a rising magma batch are investigated by scaling analyses and fluid dynamical experiments the results of scaling analyses show that the fluid behaviours in a squeezed conduit are determined mainly by the dimensionless number i mu 1 u g delta rho r 2 where 1 is the viscosity of the fluid u is the velocity g is the acceleration due to gravity is the density difference between the two fluids and r is the radius of the tube the parameter i represents a balance between the viscous effects in the uppermost magma which prevent it from being moved off the conduit walls and the buoyancy forces which tend to keep the interface horizontal the experiments are carried out using fluid pairs of various density and viscosity contrasts in a squeezed vinyl tube they show that overturning of the initial density stratification and mixing occur when i order 10 1 the two fluids remain stratified when i order 10 3 transitional states are observed when 10 3 i 10 1 these results are nearly independent of reynolds number re 1 rho 1 ur mu 1 and viscosity ratio mu 1 mu 2 in the range of 10 2 mu 1 mu 2 10 3 and re 1 300 applying these results to magmas shows that silicic to intermediate magmas overlying mafic magma will be prone to mixing in a rising magma batch this mechanism can explain some occurrences of small volume mixed lava flows

pyroclast snow interactions and thermally driven slurry formation part 2 experiments and theoretical extension to polydisperse tephra erosion of snow by pyroclastic flows and surges presumably involves mechanical scour but there may be thermally driven phenomena involved as well to investigate this possibility layers of hot up to 400 oc uniformly sized fine to medium grained sand were emplaced vertically onto finely shaved ice snow7 thus there was no relative shear motion between sand and snow and no purely mechanical scour in some cases large vapor bubbles commonly more than 10 mm across rose through the sand layer burst at the surface and caused complete convective overturn of the sand which then scoured and mixed with snow and transformed into a slurry in other cases no bubbling occurred and the sand passively melted its way downward into the snow as a wetting front moved upward into the sand a continuum of behaviors between these two cases was observed vig orous bubbling and convection were generally favored by high temperature small grain size and small layer thickness a physically based theory of heat and mass transfer at the pyroclast snow interface developed in part 1 of this paper does a good job of explaining the observations as a manifestation of unstable vapor driven fluidization the theory when extrapolated to the behavior of actual poorly sorted pyroclastic flow sediments leads to the prediction that the observed thermal scour phenomenon should also occur for many real pyroclastic flows passing over snow ther mal scour is therefore likely to be involved in the generation of lahars

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

geology of xanthe terra outflow channels and the mars pathfinder landing site the mars pathfinder landing site mpls was selected for its location at the mouths of ares and tiu valles outflow channels on mars where sediments from a variety of geologic units could have accumulated in a lacustrine environment recent geologic mapping at 1 2 m scale of the xanthe terra channels and chryse planitia near mpls reveals a progression of fluvial erosion and deposition throughout the hesperian period from the geology the watersheds of the channels were inferred and a sequence of paleogeologic maps was produced three outflow episodes are recognized 1 broad sheetwash during the early hesperian emanating from eastern valles marineris and equatorial chaos regions with initial erosion into highland plateau basement 2 early channel formation and 3 subsequent flooding which deepened the channels to their current morphologies the most recent flooding issued from simud and tiu valles during the late hesperian indicated by stratigraphic superposition striations along the floor of the channel and the orientation of streamlined islands in the flood direction this suggests that surface sediments deposited at mpls are most likely to have come from these channels and to a lesser extent from ares vallis a ranking scheme was developed to assess the potential origins for the sediments found at mpls the most abundant materials are inferred to have come from the noachian and hesperian highland plains and reworked sediments from previous floods underlying materials from earlier floods could also be found at mpls as ejecta from nearby impact craters

the interaction between concentrated pyroclastic density currents and snow a case study from the 2008 mixed avalanche from volcan llaima chile the incorporation of snow and ice by pyroclastic density currents pdcs can generate mixed avalanches and pose significant hazards at snow clad volcanoes commonly the poor preservation of these thin deposits combined with the subtle characteristics of pdc snow interaction has limited their recognition in the geological record a small volume 2 5 105 m3 basaltic andesite mixed avalanche deposit formed during the 2008 eruption of volcan llaima chile provides insight into pdc and snow interactions pyroclasts accumulated on the crater rim and collapsed to form a flow that swept up to 2 8 km from source and spread across 6 09 105 m2 of the snow clad slopes the pdc snow interaction at the crater rim or during flow propagation resulted in distinct deposit and pyroclast textures these included abundant blocky non vesicular cauliflower clasts and blocky poorly vesicular scoria the thin and sheet like mixed avalanche deposit had a lumpy surface lacked marginal levees was very loose and fine ash depleted although the flow likely incorporated snow and or ice mechanically the overall coarseness of the mixture precluded effective fluidization related to vaporization many of the features described herein are distinctive features of other mixed avalanche deposits worldwide and should be considered key indicators of pdc snow interaction

pyroclast snow interactions and thermally driven slurry formation part 1 theory for monodisperse grain beds lahars are often produced as pyroclastic flows move over snow this phenomenon involves a complicated interplay of mechanical and thermal pro cesses that need to be separated to get at the funda mental physics the thermal physics of pyroclast snow interactions form the focus of this paper a theoretical model is developed of heat and mass transfer at the interface between a layer of uniformly sized pyroclasts and an underlying bed of snow for the case in which there is no relative shear motion between pyroclasts and snow a microscale view of the interface is required to properly specify boundary conditions the physical model leads to the prediction that the upward flux of water vapor which depends upon emplace ment temperature pyroclast grain size pyroclast layer thickness and snow permeability is sometimes suffi cient to fluidize the pyroclasts uniform fluidization is usually unstable to bubble formation which leads to vigorous convection of the pyroclasts themselves thus predicted threshold conditions for fluidization are tantamount to predicted thresholds for particle convection such predictions are quantitatively in good agreement with results of experiments described in part 2 of this paper because particle convection commonly causes scour of the snow bed and transfor mation of the pyroclast layer to a slurry there exists a thermal scour process for generating lahars from pyroclastic flows moving over snow regardless of the possible role of mechanical scour

pyroclastic deposits of the november 13 1985 eruption of nevado del ruiz volcano colombia abstract the november 13 1985 eruption of nevado del ruiz produced a series of pyroclastic flows and surges that eroded channels on the surface of the summit glacier and generated lahars which descended down most of the rivers that drain the volcano the stratigraphy of the proximal pyroclastic deposits indicates that there were at least four episodes to the eruption episode i deposited an unusual surge consisting of small pieces of ice mixed with ash and exhibiting planar stratification ballistically emplaced fragments are also intercalated with this unit during episode ii at least two pyroclastic flows were erupted their deposits contain the most evolved pumice of the entire eruption sio2 content of matrix glass ranges between 74 5 and 74 9 episode iii is marked by the emplacement of a welded tuff with an average sio2 content of about 66 in the matrix glass the final episode iv was characterized by the development of a high altitude eruption column and the emplacement of several nonwelded pyroclastic flows banded pumice are common in the pyroclastic flow as well as in the pumice fall deposits co existing dark and light pumice bands differ in sio2 content by 3 5 and in general are similar to the composition of the welded pumice from episode iii the compositional zonation of the pyroclastic deposits from episode i to iv suggests that a nearsurface compositionally stratified portion of the magma body was tapped during episode ii during episodes iii and iv the main body of magma was involved although the coexistence of the compositionally distinct pumice clasts at similar stratigraphic levels argues for mixing of magma from different levels in the chamber during the eruptive process

effects of the november 13 1985 eruption on the snow pack and ice cap of nevado del ruiz volcano colombia abstract about twenty five to thirty percent of the ice on the ice cap of nevado del ruiz was fractured and destabilized by earthquakes and explosive activity on november 13 1985 comparison of aerial and ground photographs taken before and after the eruption allows calculation of the volume of snow and ice removed from each glacial basin during the eruption about 16 4 2 km 2 of the surface area of the ice and snow was lost the corresponding volume is estimated to be close to 0 06 km 3 or 9 of the total volume of ice and snow at least four distinct processes including explosions ice avalanches surficial erosion of snow and slush avalanches were responsible for transporting snow firn and ice downslope heat from pyroclastic flows and surges and their deposits was responsible for melting snow and ice both at the surface of the ice cap in situ and during its downslope transport voluminous lahars were triggered in 3 of the 4 main drainages on the volcano by the approximately 43 million m 3 of liquid water about 62 million m 3 of snow firn and ice produced by these combined processes thermal and mechanical effects of the pyroclastic flows and surges included scouring direct melting and entraining snow firn and ice which left several erosional features on the ice cap and deposits both on the ice and on the edges of the ice cap subsurface and englacial drainage of water through crevasses was deduced from depositional and morphological features at the ice margins some meltwater might have been trapped in reservoirs but some water was also released shortly after the eruption the estimated volume of released meltwater is about 43 million m 3 i e 2 4 times more than the approximate 11 12 million m 3 of water incorporated into the lahars thus roughly 21 million m 3 of water were released but did not contribute to lahar generation this non contributing water was 1 included in snow avalanches and sediment laden slush avalanches outside the lahars channels or bound up in the wet pyroclastic surge deposits 2 incorporated in the phreatic explosive products or saturated firn by meltwater which issued from the thermal effect of the tephra fall 3 sublimated into steam during the passage of the pyroclastic flows or 4 stored within the ice meltwater may still be be trapped within the ice cap and may be a potential source of water for future lahars

unusual snow slurry lahars from ruapehu volcano new zealand september 1995 the first lahars of the 1995 ruapehu eruptive sequence were generated by explosively ejected crater lake water sediment and juvenile material which incorporated snow and ice to form snow slurry lahars on september 18 small amounts of crater lake water were ejected which mobilized large amounts of snow this was because unstable seasonal snow pack was present in the lahar flow path five days later a larger volume of water was ejected but proportionally less snow was mobilized beyond the volcanic cone the initially homogenized snow slurries segregated into watery flows with a floating snow slurry cap frozen deposits were emplaced as far as 45 km from the source and 1840 m lower in altitude more than 50 of the sediment within the flow was derived from the initial eruption the remainder was entrained before it reached 9 5 km beyond 9 5 km from the source the flows progressively lost their gravel and sand

swift snowmelt and floods lahars caused by great pyroclastic surge at mount st helens volcano washington 18 may 1980 the initial explosions at mount st helens washington on the moring of 18 may 1980 developed into a huge pyroclastic surge that generated catastrophic floods off the east and west flanks of the volcano near source surge deposits on the east and west were lithic sorted lacking in accretionary lapilli and vesiculated ash not plastered against upright obstacles and hot enough to char wood all attributes of dry pyroclastic surge material deposited at the surge base on steep slopes near the volcano transformed into high concentration lithic pyroclastic flows whose deposits contain charred wood and other features indicating that these flows were hot and dry stratigraphy shows that even the tail of the surge had passed the east and west volcano flanks before the geomorphically distinct floods lahars arrived this field evidence undermines hypotheses that the turbulent surge was itself wet and that its heavy components segregated out to transform directly into lahars nor is there evidence that meters thick snow slab avalanches intimately mixed with the surge to form the floods the floods must have instead originated by swift snowmelt at the base of a hot and relatively dry turbulent surge impacting hot pyroclasts probably transferred downslope momentum to the snow surface and churned snow grains into the surge base melting snow and accumulating hot surge debris may have moved initially as thousands of small thin slushflows as these flows removed the surface snow and pyroclasts newly uncovered snow was partly melted by the turbulent surge base this and accumulating hot surge debris in turn began flowing a self sustaining process feeding the initial flows the flows thus grew swiftly over tens of seconds and united downslope into great slushy ejecta laden sheetfloods gravity accelerated the floods to more than 100 km h as they swept down and off the volcano flanks while the snow component melted to form great debris rich floods lahars channeled into valleys

related magma ice interactions possible origins of chasmata chaos and surface materials in xanthe margaritifer and meridiani terrae mars abstract we examine here the close spatial and temporal associations among several unique features of xanthe and margaritifer terrae specifically the valles marineris troughs or chasmata and their interior deposits chaotic terrain the circum chryse outflow channels and the subdued cratered material that covers xanthe margaritifer and meridiani terrae though previous hypotheses have attempted to explain the origin of individual features or subsets of these we suggest that they may all be related all of these features taken together present a consistent scenario that includes the processes of sub ice volcanism and other magma ice interactions results of intrusive events during late noachian to early amazonian times

the formation of floor fractured craters in xanthe terra floor fractured craters ffc are a peculiar form of degradation of impact craters defined by the presence of crevice networks and mesas affecting crater floors they are preferentially distributed near chaotic terrains and outflow channels the scope of this paper is to present a detailed systematic analysis of ffc at xanthe terra ffc morphologies in this region are classified into five types making a picture of different stages of the same degradation process ffc are geographically intermixed with un fractured normal craters non ffc young craters are less prone to show this type of degradation as suggested by fresh ejecta layer with preserved crater floor size distributions of ffc and non ffc indicate that larger craters are preferentially fractured careful examinations of the crater floor elevations reveal that the crevices often extend deeper than the original crater cavity furthermore an onset depth for the formation of ffc is evidenced from the difference of spatial distributions between ffc and non ffc roof collapsed depressions observed in the same region have been also documented and their characteristics suggest the removal of subsurface material at depth from about 1200 to 4000 m these observations taken together suggest a subsurface zone of volume deficit at depth from 1 to 2 km down to several kilometers responsible for ffc formation then a scenario of ffc formations is presented in the context of groundwater discharge events at the late hesperian this scenario involves two key processes earth fissuring and piping erosion known to occur with rapid groundwater migrations on earth

formation and failure of volcanic debris dams in the chakachatna river valley associated with eruptions of the spurr volcanic complex alaska abstract the formation of lahars and a debris avalanche during holocene eruptions of the spurr volcanic complex in south central alaska have led to the development of volcanic debris dams in the chakachatna river valley debris dams composed of lahar and debris avalanche deposits formed at least five times in the last 8000 10 000 years and most recently during eruptions of crater peak vent in 1953 and 1992 water impounded by a large debris avalanche of early holocene age may have destabilized an upstream glacier dammed lake causing a catastrophic flood on the chakachatna river a large alluvial fan just downstream of the debris avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood application of a physically based dam break model yields estimates of peak discharge q p attained during failure of the debris avalanche dam in the range 10 4 q p 6 m 3 s 1 for plausible breach erosion rates of 10 100 m h 1 smaller short lived lahar dams that formed during historical eruptions in 1953 and 1992 impounded smaller lakes in the upper chakachatna river valley and peak flows attained during failure of these volcanic debris dams were in the range 10 3 q p 4 m 3 s 1 for plausible breach erosion rates volcanic debris dams have formed at other volcanoes in the cook inlet region aleutian arc and wrangell mountains but apparently did not fail rapidly or result in large or catastrophic outflows steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams

volcanic mixed avalanches a distinct eruption triggered mass flow process at snow clad volcanoes a generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at mount st helens nevado del ruiz and redoubt volcano between 1982 and 1989 these mixed avalanches traveled as far as 14 km at velocities up to 27 m s involved as much as 10 7 m 3 of rock and ice and left unmelted deposits of single flow units as thick as 5 m during flow downslope heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures apparently giving these mixed avalanches mobilities equal to or greater than those of dry debris avalanches of similar volume the avalanches studied for this report began as snow avalanches triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris erosion by the avalanches incorporated additional snow fragments of glacier ice and other rock debris mixed avalanche deposits were massive very poorly sorted and ungraded to inversely graded prior to melting although rock debris composed as much as 70 wt of the frozen samples the bulk volumetric content of rock debris was only 9 to 36 snow and ice composed between 36 and 72 of the frozen samples with void space making up the remainder of the volume deformation and consolidation after melting of the ice content reduced the deposits to loose porous layers that were only a fraction of their original thickness and that later gave little hint of their origin after melting and desiccation the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record

hydrogeologic processes of large scale tectonomagmatic complexes in mongolia southern siberia and on mars large scale tectonomagmatic complexes are common on earth and mars many of these complexes are created or at least influenced by mantle processes including a wide array of plume types ranging from superplumes to mantle plumes among the most prominent complexes the mongolian plateau on earth and the tharsis bulge on mars share remarkable similarities in terms of large domal uplifted areas great rift canyon systems and widespread volcanism on their surfaces water has also played an important role in the development of the two complexes in general atmospheric and surface water play a bigger role in the development of the present day mongolian plateau than for the tharsis bulge as evidenced by highly developed drainages and thick accumulation of sediments in the basins of the baikal rift system on the tharsis bulge however water appears to have remained as ground ice except during periods of elevated magmatic activity glacial and periglacial processes are well documented for the mongolian plateau and are also reported for parts of the tharsis bulge ice magma interactions which are represented by the formation of subice volcanoes in parts of the mongolian plateau region have been reported for the valles marineris region of mars the complexes are also characterized by cataclysmic floods but their triggering mechanism may differ mainly ice dam failures for the mongolian plateau and outburst of groundwater for the tharsis bulge probably by magma ice interactions although ice dam failures within the valles marineris region cannot be ruled out as a possible contributor comparative studies of the mongolian plateau and tharsis bulge provide excellent opportunities for understanding surface manifestations of plume driven processes on terrestrial planets and how they interact with hydro cryospheres

constraints on the origin and evolution of iani chaos mars 1 the origin mechanisms and geologic evolution of chaotic terrain on mars are poorly constrained iani chaos located at the head ares vallis is among the most geomorphologically complex of the chaotic terrains its morphology is defined by 1 multiple 1 to 2 km deep basins 2 flat topped fractured plateaus that are remnants of highland terrain 3 knobby fractured remnants of highland terrain 4 plateaus with a knobby surface morphology 5 interchaos grooved terrain 6 interior layered deposits ilds and 7 mantling material topography the observed geomorphology and measured fracture patterns suggest that the interchaos basins formed as a result of subsurface volume loss and collapse of the crust likely owing to effusion of groundwater to the surface regional patterns in fracture orientation indicate that the basins developed along linear zones of preexisting weakness in the highland crust multiple overlapping basins and fracture systems point to multiple stages of collapse at iani chaos furthermore the total estimated volume loss from the basins 104 km3 is insufficient to explain erosion of 104 105 km3 of material from ares vallis by a single flood comparisons with the chronology of ares vallis indicate multiple water effusion events from iani chaos that span the hesperian with termination of activity in the early amazonian recharge of groundwater through preexisting fracture systems may explain this long lived but likely episodic fluvial activity late stage early to middle amazonian aqueous processes may have deposited the ilds however the topography data indicate that the ilds did not form within lacustrine environments

tephra snow and water complex sedimentary responses at an active snow capped stratovolcano ruapehu new zealand a feature of small scale explosive volcanism at stratovolcanoes is the rapid destruction of primary near vent pyroclastic deposits by sedimentary processes a protracted series of explosive eruptions of moderate volume from september 1995 until july 1996 at mount ruapehu in new zealand its largest eruptive episode this century afforded an opportunity to study these remobilisation processes in detail all significant sub plinian eruptions occurred in mid winter forming metre thick tephra accumulations on steep slopes covered with perennial ice and seasonal snow subsequent events demonstrated the variety and complexity of the erosion processes that remobilise primary pyroclasts in such a setting these processes arose from the complex interactions of tephra with snow and ice and liquid water in varying proportions and were very diverse in nature and scale their effectiveness can be gauged from the fact that there is almost no stratigraphic record of any of the 40 eruption episodes recorded in the past 100 years at ruapehu syn eruptive remobilisation processes included the generation of eruption triggered lahars by the ejection of hot water from the crater lake post eruptive interactions mainly remobilised fall deposits from proximal areas and included rain triggered lahars which were among the largest and most hazardous events with the greatest distal impacts

a refined chronology of catastrophic outflow events in ares vallis mars abstract we investigate the geomorphology and chronology of catastrophic flooding in a major martian outflow channel ares vallis we use recently acquired stereo and colour images and derived topographic data with grid spacing of 50 m from the high resolution stereo camera hrsc onboard mars express to constrain the detailed flood geomorphology of proximal ares vallis we identify morphologic evidence for 6 distinct erosion surfaces that were likely formed by multiple episodes of catastrophic flooding these surfaces occur at distinct topographic levels exhibit multiple sets of flood grooves and contain distinct post flood crater populations crater count statistics on the flood eroded surfaces from mro context camera ctx and thermal emission imaging system themis visible images indicate that flooding was initiated during the late noachian early hesperian at 3 6 ga the morphology data confirm that the oldest flooding events occurred on the topographically highest erosion surfaces while subsequent floods abandoned the upper surfaces and continued to modify craters in the topographically lowest main channel of ares vallis into the late hesperian early amazonian 2 5 ga our results suggest that release of water to ares vallis likely occurred at repeated intervals over a long period of geological time thus supporting recent numerical modelling studies that suggest a recharging water source

high outflow channels on mars indicate hesperian recharge at low latitudes and the presence of canyon lakes abstract a number of martian outflow channels were carved by discharges from large dilational fault zones these channels were sourced by groundwater not surface water and when observed on high standing plateaus they provide indicators of elevated paleo groundwater levels we identify three outflow channels of hesperian age that issued from a 750 km long fault zone extending from candor chasma to ganges chasma two of these channels allegheny vallis and walla walla vallis have sources 2500 m above the topographic datum too high to be explained by discharge from a global aquifer that was recharged solely in the south polar region the indicated groundwater levels likely required regional sources of recharge at low latitudes the floodwaters that erupted from ophir cavus to form allegheny vallis encountered two ridges that restricted the flow forming temporary lakes the flow probably breached or overtopped these obstructions quickly catastrophically draining the lakes and carving several scablands after the last obstacle had been breached a single main channel formed that captured all subsequent flow we performed hydrologic analyses of this intermediate phase of the flooding prior to incision of the channel to its present depth using floodwater depths of 30 60 m we calculated flow velocities of 6 15 m s 1 and discharges in the range of 0 7 3 10 6 m 3 s 1 locally higher flow velocities and discharges likely occurred when the transient lakes were drained variable erosion at the channel and scabland crossing of mola pass 10644 suggests that the upper 25 30 m may consist of poorly consolidated surface materials underlain by more cohesive bedrock we infer that an ice covered lake with a surface elevation 2500 m probably existed in eastern candor chasma because this canyon is intersected by the ophir catenae fault system from which allegheny vallis and walla walla vallis originated we introduce a new hydrology concept for mars in which the groundwater system was augmented by recharge from canyon lakes that were formed when water was released by catastrophic melting of former ice sheets in tharsis by effusions of flood basalts this model could help to reconcile the expected presence of a thick cryosphere during the hesperian with the abundant evidence for groundwater as a source for some of the circum chryse outflow channels

late quaternary glacial stades in the cordillera central colombia based on glacial geomorphology tephra soil stratigraphy palynology and radiocarbon dating using data from glacial geomorphology tephra soil stratigraphy and mineralogy palynology and radiocarbon dating a sequence of glacial and bioclimatic stades and interstades has been identified for the last ca 50000 yr in the ruiz tolima massif cordillera central colombia six pleistocene cold stades separated by warmer interstades occurred before 48000 between 48000 and 33000 between 28000 and 21000 from 16000 to ca 14000 ca 13000 12400 and ca 11000 10000 yr bp although these radiocarbon ages are minimum limiting ages obtained from tephra layers on top of tills the tills are not significantly older because most are bracketed by dated tephra sets in measured stratigraphic sections two minor moraine stages likely reflect glacier standstill during cold intervals ca 7400 yr bp and slightly earlier finally glaciers readvanced between the seventeenth and nineteenth centuries in contrast to the ice clad volcanoes of the massif ca 34 km2 in area above an altitude of ca 4800 m the ice cover expanded to 1200 km2 during the last glacial maximum lgm and was still 800 km2 during late glacial time lgt glacier reconstructions based on the moraines suggest depression of the equilibrium line altitude ela by ca 1100 m during the lgm and 500 600 m during lgt relative to the modern ela which lies at ca 5100 m in the cordillera central glaciers in this region apparently reached their greatest extent when the climate was cold and wet e g during stades corresponding to oxygen isotope stage 3 glaciers were still expanding during the lgm ca 28000 21000 yr bp but they shrank considerably after 21000 yr bp because of greatly reduced precipitation 1997 john wiley sons ltd

emplacement and cooling of komatiite lavas we propose that komatiite lavas were emplaced as turbulent flows accompanied by vigorous forced convection with cooling rates often in excess of hundreds of c h 1 they melted and assimilated up to 10 of the ground over which they flowed forming deep channels nickel sulphide mineralization may have resulted from incorporation of sulphur rich sediment after emplacement high cooling rates persisted resulting in spinifex textures due to compositional convection

evidences for a noachian hesperian orogeny in mars a number of tectonic structures have been located at the thaumasia plateau daedalia planum and aonia terra mars they include isolated folds with axial traces up to 200 km long trains of tightly folded structures tens of km long and thrusts their size and geometry are similar to those on earth and the direction of compression seems to have varied amply with time suggesting a complex tectonic evolution crater counts on the deformed terrain point to noachian to early hesperian ages on the basis of the geometry and geological relations of these structures we propose that they form part of an old martian orogen the thaumasia aonia orogen which embraced not only the thaumasia plateau but areas of daedalia planum aonia terra and nereidum montes as well a regional coherent layering is previous to the deformation and could represent the trace of even older stresses on the martian lithosphere

quaternary eruptive history of nevado del ruiz colombia abstract nevado del ruiz has a 1 8 m y long eruptive record that includes alternate construction and destruction of three edifices during three main eruptive periods termed ancestral ruiz older ruiz and ruiz nevado del ruiz is located on a complex intersection of four groups of faults the most significant being the n20 e palestina strike slip fault and the n50 w villamaria termales normal fault ancestral ruiz was a broad stratovolcano built by two eruptive stages of lava flows starting about 1 8 ma ago and ending 1 0 ma ago a partial collapse and formation of a caldera are thought to have occurred between 1 0 and 0 8 ma ago older ruiz was a stratovolcano constructed by lava flows in three stages starting about 0 8 ma ago and ending about 0 2 ma ago extensive and voluminous welded and nonwelded pyroclastic flow deposits that partly fill preexisting valleys record the formation of a young summit caldera between 0 2 and 0 15 ma ago present ruiz is formed by a cluster of composite lava domes that probably filled the summit caldera of older ruiz present ruiz eruptive activity is mostly explosive but also includes dome growth and parasitic dome activity of la olleta and alto la piramide twelve eruptive stages occurred during the last 11 000 years accompanied by rockslide debris avalanches pyroclastic flows or surges and their subsequent interactions with the ice cap as well as by glacial erosion and mass wasting diverse processes within these twelve stages have led to a partial destruction of the summit domes this long and complex pleistocene and holocene eruptive sequence helps to put the november 13 1985 eruption into a broader perspective

eruption triggered avalanche flood and lahar at mount st helens effects of winter snowpack an explosive eruption of mount st helens on 19 march 1982 had substantial impact beyond the vent because hot eruption products interacted with a thick snowpack a blast of hot pumice dome rocks and gas dislodged crater wall snow that avalanched through the crater and down the north flank snow in the crater swiftly melted to form a transient lake from which a destructive flood and lahar swept down the north flank and the north fork toutle river

the 1996 eruption at gjalp vatnajokull ice cap iceland efficiency of heat transfer ice deformation and subglacial water pressure the 13 day long gjlp eruption within the vatnajkull ice cap in october 1996 provided important data on ice volcano interaction in a thick temperate glacier the eruption produced 0 8 km 3 of mainly volcanic glass with a basaltic icelandite composition equivalent to 0 45 km 3 of magma ice thickness above the 6 km long volcanic fissure was initially 550 750 m the eruption was mainly subglacial forming a 150 500 m high ridge only 2 4 of the volcanic material was erupted subaerially monitoring of the formation of ice cauldrons above the vents provided data on ice melting heat flux and indirectly on eruption rate the heat flux was 5 6 10 5 wm 2 in the first 4 days this high heat flux can only be explained by fragmentation of magma into volcanic glass the pattern of ice melting during and after the eruption indicates that the efficiency of instantaneous heat exchange between magma and ice at the eruption site was 50 60 if this is characteristic for magma frag mentation in subglacial eruptions volcanic material and meltwater will in most cases take up more space than the ice melted in the eruption water accumulation would therefore cause buildup of basal water pressure and lead to rapid release of the meltwater continuous drainage of meltwater is therefore the most likely scenario in subglacial eruptions under temperate glaciers deforma tion and fracturing of ice played a significant role in the eruption and modified the subglacial water pressure it is found that water pressure at a vent under a subsiding cauldron is substantially less than it would be during static loading by the overlying ice since the load is partly compensated for by shear forces in the rapidly deforming ice in addition to intensive crevassing due to subsidence at gjlp a long and straight crevasse formed over the southernmost part of the volcanic fissure on the first day of the eruption it is suggested that the feeder dyke may have overshot the bedrock ice interface caused high deformation rates and fractured the ice up to the surface the crevasse later modified the flow of meltwater explaining surface flow of water past the highest part of the edifice the dominance of magma fragmentation in the gjlp eruption suggests that initial ice thickness greater than 600 700 m is required if effusive eruption of pillow lava is to be the main style of activity at least in similar eruptions of high initial magma discharge

initiation and flow behavior of the 1980 pine creek and muddy river lahars mount st helens washington two large high velocity lahars volcanic debris flows were triggered by a pyroclastic surge during the first few minutes of the may 18 1980 eruption of mount st helens the initial surge cloud evolved progressively by gravity segregation from a gas mobilized highly inflated density flow to a dense water mobilized basal debris flow lahar and accompanying ash cloud as it flowed down the east flank of the volcano the main source of the water for the lahars was probably from eroded snow and ice incorporated into the flow by turbulent mixing but ground water expelled together with the rock debris by the initiating volcanic explosions also may have contributed peak lahar discharge from the pine muddy fan upper smith creek and ape canyon probably exceeded 250 000 m 3 s initially but decreased exponentially in the downstream direction total volume of the lahars was in excess of 1 4 10 7 m 3 initial peak flow velocities in excess of 30 m s also decreased markedly downstream where flow was not impeded velocity was strongly related to the depth slope term r 2 3 s 1 2 from the manning uniform flow equation as a power law function during much of the route traveled lahar flow appears to have been supercritical deposits left in channels were generally thin relative to flow depth 0 to 2 5 m particles up to small boulder size were randomly distributed in the poorly sorted nonstratified matrix indicating complete suspension in a fully developed debris flow slurry however much larger clasts were transported as bed load computed sediment concentrations of matrix slurry samples ranged from 84 to 91 solids by weight and were similar for the two lahars two indirect methods for computing peak flow velocity previously only tentatively applied to debris flows were tested for accuracy by comparing computed lahar arrival times with recorded arrival times at swift reservoir the computed velocities appear to be 15 slower than the recorded velocities which is consistent with the restriction that the velocity formulas produce only minimum values

sedimentary history and mass flow structures of chryse and acidalia planitiae mars geologic mapping and crater counting in chryse and acidalia planitiae cap reveal five major sedimentary deposits of hesperian to early amazonian age including 1 a mass flow deposited during the early hesperian near deuteronilus mensae northeast of the map region that may have resulted from the carving of kasei valles 3000 km southwest of the exposed part of the deposit 2 knobby plains material consisting of channel likely from simud and tiu valles and possibly ares and shalbatana valles and mass wasting deposits in central and eastern cap 3 material largely from maja and ares valles emplaced in at least western and southern cap outcrops in southern chryse planitia developed thermokarst 4 a thin mass flow covering much of southern chryse planitia that emanated from simud and tiu valles and 5 a thick extensive perhaps 3500 km across mass flow deposit in central and northern cap derived from accumulation and backflow of the preceding thin mass flow or perhaps melting of polar deposits other possible deposits may not be recognizable owing to burial by younger materials or a lack of morphologic signature various associated landforms appear to be consistent with the mass flow interpretations including lobate and linear scarps along deposit edges fractures related to desiccation of thick sediments troughs and ridges near the edges of the deposit indicative of secondary mass movement and deformation pitted domes and fissure fed flows possibly formed by sedimentary mud eruptions and longitudinal channel grooves perhaps formed by roller vortices no convincing evidence for paleoshorelines or stagnant ice sheets is found in cap these findings suggest that mass flow and hyperconcentrated flooding may have been the predominant processes of outflow channel dissection in cap elsewhere in the northern plains similar landforms are prevalent the mass flow interpretation does not require either multiple episodes of extraordinarily high water discharge rates achieved by freeing huge volumes of water from the crust repetitive recycling of immense volumes of water into highland aquifers at the heads of chryse channels or profound climate change mars pathfinder will most likely land on and inspect the surface of the thin mass flow that originated from the canyons of simud and tiu valles

primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of mars five main stages of radial and concentric structures formed around tharsis from the noachian through the amazonian as determined by geologic mapping of 24 452 structures within the stratigraphic framework of mars and by testing their radial and concentric orientations tectonic activity peaked in the noachian stage 1 around the largest center claritas an elongate center extending more than 20 in latitude and defined by about half of the total grabens which are concentrated in the syria planum thaumasia and tempe terra regions during the late noachian and early hesperian stage 2 extensional structures formed along the length of present day valles marineris and in thaumasia with a secondary concentration near warrego vallis radial to a region just to the south of the central margin of valles marineris early hesperian stage 3 radial grabens in pavonis syria ulysses and tempe terra and somewhat concentric wrinkle ridges in lunae and solis plana and in thaumasia sirenum memnonia and amazonis are centered northwest of syria with secondary centers at thaumasia tempe terra ulysses fossae and western valles marineris late hesperian early amazonian stage 4 structures around alba patera the northeast trending alignment of tharsis montes and olympus mons appears centered on alba patera stage 5 structures middle late amazonian represent the last pulse of tharsis related activity and are found around the large shield volcanoes and are centered near pavonis mons tectonic activity around tharsis began in the noachian and generally decreased through geologic time to the amazonian statistically significant radial distributions of structures formed during each stage centered at different locations within the higher elevations of tharsis secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout tharsis

debris flow origin for the simud tiu deposit on mars a late hesperian smooth plains deposit on mars interpreted as a debris flow extends more than 2000 km from hydraotes chaos through simud and tiu valles and into chryse planitia the simud tiu deposit widens out to 1000 km and embays streamlined landforms and knobs made up of sedimentary and perhaps volcanic deposits that were carved by earlier channeling activity morphologic features of the simud tiu deposit observed in viking and pathfinder images are generally consistent with a debris flow origin but some of the deposit s salient features are not readily explained by catastrophic flooding or ice flow internal depressions appear to be bounded by linear scarps along flow margins where differential shearing may have occurred and in areas where flow spreading may have produced zones of extensional breakup and thinning within the flow possible flow lobes within the deposit may have formed by successive flow surges within the flow unit the pathfinder landing site is on the simud tiu deposit and the observations there are consistent with debris flow the low longitudinal ridges at the site may have formed by clast interactions as the flow ground to a halt imbricated planar rocks on the ridges such as in the rock garden also may have been emplaced by debris or ice flow however stream energy calculations at ares vallis and channel geology indicate that flooding probably was incapable of emplacing the meter size boulders observed at the pathfinder site dewatering of pressurized zones in the debris flow or underlying material may be responsible for mud eruptions that formed a couple of patches of low pancakelike shields up to 5 km in diameter and for probable water flows that formed two small rille channels a few kilometers long local irregular grooves may be cracks that resulted from later desiccation and contraction of the flow material the debris flow unit apparently coalesced from outflows of water fluidized debris originating from beneath chaotic and hummocky terrains within and along the margins of simud and tiu valles the deposit is onlapped from the north by another flow deposit originating from acidalia planitia if the simud tiu debris flow had entered a standing body of water a turbidity current may have arisen from the debris flow and then backflowed over the debris flow to account for the acidalia deposit

retreat of a giant cataract in a long lived 3 7 2 6 ga martian outflow channel we describe the evolution of an similar to 600 m deep tributary outflow channel to ares vallis mars high resolution topography image analysis and crater statistics indicate that this tributary canyon developed by the upstream migration of a large similar to 300 m tall cataract during multiple flood events that span similar to 1 b y of mars history 3 7 2 6 ga issuing from hydapsis chaos these floods were initiated at a similar time and occurred over a similar time range to flooding in ares vallis suggesting a potential regional control on flood initiation and chaos formation in addition we provide evidence that cataract retreat and significant incision within the tributary canyon occurred only after a series of downcutting events within ares vallis topography data and crater statistics taken from the floor of ares vallis indicate an similar to 300 m base level drop that coincides temporally with an early amazonian ca 2 6 ga flood event and cataract formation within the tributary canyon the results both confirm the hypothesis of long term multiple flood events within martian outflow channels and demonstrate the influence of base level change on their incision

eruption and mass wasting induced processes during the late holocene destructive phase of nevado del ruiz volcano colombia abstract the november 13 1985 eruption was characteristic of the arenas eruptive stage of nevado del ruiz the most recent of a series of twelve eruptive stages that have occurred in the past 11 000 years eruptive sequences deposits and processes similar to that of 1985 have characterized the behavior of nevado del ruiz during three major prehistorical and historical eruptive stages the approximately 3300 3100 yr b p hedionda the 16th century azufrado and the mid 1800 s lagunillas eruptive stages that partly destroyed the present ruiz summit according to the interpretation of the stratigraphic record of prehistorical eruptions and historical accounts almost every recent magmatic event was small or short lived nevertheless rockslide debris avalanches and catastrophic debris flows were triggered in all the eruptions owing to slope failures related to specific tectonic features of ruiz volcano and or to significant interactions between pyroclastic debris and the ice cap evidence for headward retreat of avalanche scarps during multiple eruptions reinforce the case that large slope failures can occur repeatedly at a large volume volcano like ruiz without reconstruction of the edifice the latest ruiz eruptions that involved rockslide debris avalanches resemble in part the shiveluch 1964 event for which evidence of lateral blast deposits are lacking but differ in part from this type because non eruptive and mass wasting processes also triggered rockslide debris avalanches many factors render the cluster of domes of the ruiz summit unstable including 1 deeply dissected troughs opened toward the north northeast azufrado east lagunillas and south recio flanks 2 strongly hydrothermally altered north and east flanks of the summit 3 currently glaciated or recently deglaciated high cliffs 4 steep unstable margins of the ice cap on the north and east thus in light of its past behavior a small eruption or an earthquake might trigger catastrophic rockslide debris avalanches furthermore such avalanches as well as glacial outburst floods and ice avalanches could induce debris flows by mobilizing weathered water saturated and unconsolidated rocks or deposits

unusual ice diamicts emplaced during the december 15 1989 eruption of redoubt volcano alaska abstract ice diamict comprising clasts of glacier ice and subordinate rock debris in a matrix of ice snow grains coarse ash and frozen pore water was deposited during the eruption of redoubt volcano on december 15 1989 rounded clasts of glacier ice and snowpack are as large as 2 5 m clasts of redoubt andesite and basement crystalline rocks reach 1 m and tabular clasts of entrained snowpack are as long as 10 m ice diamict was deposited on both the north and south volcano flanks on redoubt s north flank along the east side of drift piedmont glacier and outwash valley ice diamict accumulated as at least 3 units each 1 5 m thick two ice diamict layers underlie a pumice lithic fall tephra that accumulated on december 15 from 10 15 to 11 45 ast a third ice diamict overlies the pumiceous tephra some of the ice diamicts have a basal ice sandstone layer the north side icy flows reached as far as 14 km laterally over an altitude drop of 2 3 km and covered an area of about 5 7 km 2 on crescent glacier on the south volcano flank a composite ice diamict is locally as thick as 20 m it travelled 4 3 km over an altitude drop of 1 7 km covering about 1 km 2 the much higher mobility of the northside flows was influenced by their much higher water contents than the southside flow s erupting hot juvenile andesite triggered and turbulently mixed with snow avalanches at snow covered glacier heads these flows rapidly entrained more snow firn and ice blocks from the crevassed glacier on the north flank a trailing watery phase of each ice diamict flow swept over and terraced the new icy deposits the last and perhaps each flood reworked valley floor snowpack and swept 35 km downvalley to the sea ice diamict did not form during eruptions after december 15 despite intervening snowfalls these later pyroclastic flows swept mainly over glacier ice rather than snowpack and generated laharic floods rather than snowflows similar flows of mixed ice grains and pyroclastic debris resulted from the november 13 1985 eruption of nevado del ruiz volcano and from eruptions of snowclad mount st helens in 1982 1984 such deposits at snowclad volcanoes are initially broad and geomorphically distinct but they soon become extensively reworked and hard to recognize in the geologic record

resurfacing history of the northern plains of mars based on geologic mapping of mars global surveyor data 1 geologic mapping of the northern plains of mars based on mars orbiter laser altimeter topography and viking and mars orbiter camera images reveals new insights into geologic processes and events in this region during the hesperian and amazonian periods we propose four successive stages of lowland resurfacing likely related to the activity of near surface volatiles commencing at the highland lowland boundary hlb and progressing to lower topographic levels as follows highest elevations indicated stage 1 upper boundary plains early hesperian 2 0 to 2 9 km stage 2 lower boundary plains and outflow channel dissection late hesperian 2 7 to 4 0 km stage 3 vastitas borealis formation vbf surface late hesperian to early amazonian 3 1 to 4 1 km and stage 4 local chaos zones early amazonian 3 8 to 5 0 km at acidalia mensa stage 2 and 3 levels may be lower 4 4 and 4 8 km respectively contractional ridges form the dominant structure in the plains and developed from near the end of the early hesperian to the early amazonian geomorphic evidence for a northern plains filling ocean during stage 2 is absent because one did not form or its evidence was destroyed by stage 3 resurfacing remnants of possible amazonian dust mantles occur on top of the vbf the north polar layered deposits appear to be made up of an up to kilometer thick lower sequence of sandy layers early to middle amazonian in age overlain by late amazonian ice rich dust layers both units appear to have outliers suggesting that they once were more extensive

the ice thickness distribution of unteraargletscher switzerland results from 15 years of work on glacier bed mapping by radio echo soundings on finsteraar lauteraar and unteraargletscher switzerland are summarized and a new and greatly improved map of the ice thickness distribution presented in contrast to the tongue of unteraargletscher its two main tributaries lauteraar and finsteraar are both deep and narrow and ice thickness determination depends on the detection of more than just the primary reflection migrating the data led to considerably improved bed determination wherever possible additional information on ice thicknesses gained from numerous hot water drillings to the glacier bed is used as an independent verification of the results of the radar measurements and a fair agreement is found

lunar swirls examining crustal magnetic anomalies and space weathering trends 1 we have used multispectral images from clementine and data from lunar prospector s magnetometer to conduct a survey of lunar crustal magnetic anomalies prominent lunar swirls and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies we find that all swirls and swirl like albedo patterns are associated with areas of magnetized crust but not all areas of magnetized crust are colocated with swirl like albedo anomalies all observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower feo values compared with their surroundings as determined with a multispectral iron mapping method we discuss these results in relation to the various hypotheses for swirl formation the comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined feo content between swirls and nearby ordinary surfaces the compositional difference could be explained as a consequence of 1 magnetic shielding of the surface from the solar wind which could produce anomalous space weathering little darkening with limited reddening and potentially alter the predictions of the multispectral iron mapping algorithm while the compositional contrast could be enhanced by delivery of lower feo ejecta from outside the swirl and 2 accumulation of fine plagioclase rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly therefore we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation we describe future measurements that could contribute to solution of the puzzle of swirl origin

volcano glacier interactions on composite cones and lahar generation nevado del ruiz colombia case study the catastrophic lahars triggered by the 13 november 1985 eruption of the ice clad nevado del ruiz volcano colombia demonstrate that the interaction of hot pyroclasts with snow and ice can release 30 50 million m 3 of meltwater in 30 90 minutes the 1985 eruption caused a 16 loss in area and a 9 loss in volume of snow firn and ice turbulent pyroclastic density currents mechanically mixed with snow and produced meltwater at a rate of 0 5 1 6 mm s 1 laboratory experiments suggest that turbulent fluidized pyroclastic density currents exert mechanical and thermal scour thereby efficiently transferring heat from hot pyroclasts to snow ice cap loss at nevado del ruiz continued between 1985 and 2000 representing a 52 decline in area and a 30 fall in volume ice 60 190 m thick caps the east and southeast summit plateau whereas an ice field 30 m thick and devoid of snow is retreating on the north northeast and west edges this asymmetrical distribution of ice reflects combined long term effects of the 1985 eruption and of the post 1985 ice cap retreat should volcanic activity resume steep sided glaciers can fail and pyroclastic flows and surges can sweep the snowpack and generate mixed avalanches and lahars although the potential source of meltwater has decreased since 1985 extensive debris at the ice cap margins can be incorporated to future lahars