valles marineris dune sediment provenance and pathways abstract although low albedo sand is a prevalent component of the martian surface sources and pathways of the sands are uncertain as one of the principal present day martian sediment sinks the valles marineris vm rift system hosts a diversity of dune field populations associated with a variety of landforms that serve as potential sediment sources including spur and gully walls interior layered deposits ilds and landslides here we test the hypothesis that vm dune fields are largely derived from a variety of local and regional intra rift sediment sources results show several dune fields are superposed on ancient wall massifs and ilds that are topographically isolated from extra rift sand sources spectral analysis of dune sand reveals compositional heterogeneity at the basinal dune field and dune scales arguing for discrete relatively unmixed sediment sources in coprates and melas chasmata mapping is consistent with the principle sand source for dunes being noachian aged upper and lower wall materials composed of primary igneous minerals and glasses some of which show evidence for alteration in contrast dune fields in capri juventae and ganges chasmata show evidence for partial sediment derivation from adjacent early hesperian aged hydrated sulfate bearing ild units this finding indicates that these ilds act as secondary sand sources dunes containing soft secondary minerals e g monohydrated sulfate are unlikely to have been derived from distant sources due to the physical weathering of sand grains during transport isolated extra rift dune fields sand sheets and sand patches are located on the plateaus surrounding vm and the adjoining areas but do not form interconnected networks of sand pathways into the rift if past wind regimes with respect to directionality and seasonality were consistent with more recent regimes inferred from morphological analysis i e dune slip faces wind streaks and were sufficient in strength and duration small dune populations within aurorae chaos and north of eastern vm might have resulted from extended sand pathways into vm however we favor local and regional derivation of dune sand from a variety of intra rift lithologic sources for most cases dune sand sources and the mechanism by which the sand is liberated are discussed in the context of findings described herein but are broadly applicable to analysis of sediment production elsewhere on mars

ice moraine and landslide dams in mountainous terrain we review recent work on ice moraine and landslide dams in mountainous terrain thus complementing several comprehensive summaries on glacier dams in intracontinental and arctic areas of low relief we discuss the roles of tectonic and climatic forcing on ice moraine and landslide dam formation and sudden drainage and focus on similarities and differences between their geomorphic impacts on confined valleys drained by steep bedrock and gravel bed rivers despite numerous reported failures of natural dams in mountain belts throughout the world their relevance to long term dynamics of mountain rivers remains poorly quantified all types of dams exert local base level controls thus trapping incoming sediment and inhibiting fluvial bedrock incision pervasive geomorphic and sedimentary evidence of outburst events is preserved even in areas of high erosion rates suggesting that sudden dam failures are characterized by processes of catastrophic valley floor aggradation active channel widening and downstream dispersion of sediment during which little bedrock erosion seems to be achieved we find that in the absence of direct evidence of former dams a number of similarities among the geomorphic and sedimentologic characteristics of catastrophic outburst flows may give rise to ambiguous inferences on the dam forming process this is especially the case for tectonically active mountain belts where there is ample and comparable potential for the formation and failure of ice moraine landslide and polygenetic dams concomitant with climatic oscillations or earthquake disturbance hence the palaeoclimatic implications of erroneously inferring the cause of dam formation may be significant we recommend that future research on natural dams in mountainous terrain addresses a climate and earthquake controlled systematics in the pattern of formation and failure b quantification of response of mountain rivers to catastrophic outburst events and their concomitant process sequences c elaboration of a comprehensive classification of natural dams in mountainous terrain with special attention to polygenetic dams d physical based modelling of dam formation failure and routing of water and sediment outbursts and e quantitative controls on the contribution of natural dams to sediment budgets in mountainous terrain

episodes of fluvial and volcanic activity in mangala valles mars a new mapping based study of the 900 km long mangala valles outflow system was motivated by the availability of new high resolution images and continued debates about the roles of water and lava in outflow channels on mars this study uses photogeologic analysis geomorphic surface mapping cratering statistics and relative stratigraphy results show that mangala valles underwent at least two episodes of fluvial activity and at least three episodes of volcanic activity during the late amazonian the occurrence of scoured bedrock at the base of the mapped stratigraphy in addition to evidence provided by crater retention ages suggests that fluvial activity preceded the deposition of two of the volcanic units crater counts performed at 30 locations throughout the area have allowed us to construct the following timeline 1 formation of noachian highlands and possible initial flooding event s before 1 ga 2 emplacement of tharsis lava flows in the valley from 700 to 1000 ma 3 a megaflooding event at 700 800 ma sourced from mangala fossa 4 valley fill by a sequence of lava flows sourced from mangala fossa 400 500 ma 5 another megaflooding event from 400 ma 6 a final phase of volcanism sourced from mangala fossa 300 350 ma and 7 emplacement of eolian sedimentary deposits in the northern portion of the valley 300 ma these results are consistent with alternating episodes of aqueous flooding and volcanism in the valles this pattern of geologic activity is similar to that of other outflow systems such as kasei valles suggesting that there is a recurring and perhaps coupled nature of these processes on mars

tectonic evolution of juventae chasma mars and the deformational and depositional structural attributes of the four major light toned rock exposures therein abstract juventae chasma a canyon peripheral to the valles marineris trough system is the product of a unique merger of a martian chaotic terrain depression and an extensional tectonic chasm the chaotic terrain portion is set along a nne ssw axis while a linear trough the chasm component is present along a wnw ese axis within the chasm are four discrete bodies of layered light toned rocks of sedimentary origin features a d the chaotic terrain component formed during the early hesperian as a result of expulsion of liquid water along pre existing planes of weaknesses resulting from a wnw ene compression the chasm component formed in response to ne sw extension during the middle to late amazonian the light toned features a d are layered at

catastrophic erosion of hellas basin rim on mars induced by magmatic intrusion into volatile rich rocks 1 malea and hesperia plana form large sectors of the rim of hellas basin that display partly eroded volcanic shields and plains these regions have topographic profiles that appear to be several hundred meters lower than those of adjacent rim sectors and lack prominent massifs of remnant basement that would be expected to stand above the lava plains we interpret that before the volcanic edifices were constructed these regions were denuded by an early stage of voluminous sill intrusion into friable volatile rich impact breccia magma volatile interactions may have resulted in catastrophic generation of debris flows deposited into the adjacent basin particularly if co2 were involved later lavas covered the eroded terrain in turn the lavas were eroded locally by volatile interactions across mars huge channel systems erosional features in volcanic terranes and vast layered deposits may be due to magma volatile interactions

mars outflow channels a reappraisal of the estimation of water flow velocities from water depths regional slopes and channel floor properties methods used so far to assess the flow velocities of the water commonly assumed to be responsible for forming the major outflow channel systems on mars have relied widely on various versions of the manning equation this has led to problems in allowing for the difference between the accelerations due to gravity on mars and earth and for the differences of scale between martian floods and most river systems on earth we reanalyze the problem of estimating water flow velocities in martian outflow channels using equations based on the darcy weisbach friction factor instead of the manning n factor we give simplified formulae appropriate to mars for the darcy weisbach friction coefficient as a function of bedrock size distribution for a given channel floor slope and water flood depth similar mean flow velocities are implied for a wide range of values of the ratio of bed roughness to water depth relevant to martian outflow channels using a recent rederivation of manning s equation based on turbulence theory we obtain a new value of 0 0545 s m 1 3 for the manning n coefficient appropriate to martian channels and show that previous analyses have generally overestimated though in some cases underestimated water flow velocities on mars by a factor of order two combining the consequences of this flow velocity overestimate with likely overestimates of flow depth from assuming bank full flow we show that discharges may have been overestimated by a factor of up to 25 leading to corresponding overestimates of subsurface aquifer permeabilities rates of filling of depressions with water and grain sizes of sediments on channel floors despite the availability of an improved value for the manning n coefficient for mars we strongly recommend that modified forms of the original version of the manning equation should be replaced by the modern form or preferably by the darcy weisbach equation in future work

obstacle marks produced by flow around stranded ice blocks during a glacier outburst flood jokulhlaup in west greenland the effects of glacier ice block grounding on the morphology and sedimentology of proglacial fluvial outwash were examined during a glacier outburst flood or jokulhlaup near sondre stromfjord west greenland observations made during and after the 1987 jokulhlaup both on the surface of an ice contact delta and within a confined valley sandur plain provided information about the formation of ice block obstacle marks and the significance of these bedforms for sandur morphology and sedimentology flow directions determined from obstacle mark morphology have been used successfully to chart flow direction changes on the falling limb of the jokulhlaup maximum flow depths for scour around stranded ice blocks may be given by 0 5 0 9 times the diameter of the ice block as estimated from the depth of scour the height of the obstacle shadow or the extent of ice block meltout sediments minimum flow depths can be represented by the height of the obstacle shadow above the mean bed level the internal composition of the shadow indicates the ability of the flow to transport various sizes of material into the lee of obstacles ice block obstacle marks within the distal portion of the sandur initiated waning stage channel change proximal and lateral erosion around stranded ice blocks extended downstream from the ice block forming chute channels which then captured waning stage flows resulting in significant bar incision with associated deposition of lobate or deltaic deposits it is suggested that ice block obstacle marks are important in terms of channel morphology channel morphological change and their usefulness as palaeohydrological indicators

gravity and magnetic studies of the subglacial grimsvotn volcano iceland implications for crustal and thermal structure the highly active subglacial grimsvotn volcano is associated with a 20 mgal bouguer gravity high and a broad magnetic low the magnetic low roughly coincides with the gravity high but short wavelength magnetic highs are superimposed on the low especially along caldera rims forward models of the gravity and magnetic fields show variable magnetization 0 12 a m 1 in the uppermost 1 2 km of the crust probably reflecting differences in the proportions of crystalline rocks and hyaloclastites and variations in hydrothermal alteration the models are consistent with but do not require a 10 km3 molten magma chamber in the shallow crust beneath the main caldera between 1 5 and 4 km depth a dense body 2 95 3 0 mg m 3 with a volume of 400 km3 is located probably composed of gabbro modeling suggests that this intrusive body is nonmagnetic which may imply that its temperature is at or above the curie point the distribution of geothermal activity at the glacier bed is consistent with heat extraction from this dense hot intrusive body it may be an important source for the presently observed geothermal heat output of 2000 4000 mw at grimsvotn this high heat transfer rate is difficult to reconcile with estimates of long term heat production in an icelandic rift zone volcano the heat flux may be a transient phenomenon caused by enhanced vertical permeability due to a recent caldera collapse and subsequent heat mining of the intrusive body

topography of valles marineris implications for erosional and structural history compilation of a simplified geologic geomorphic map onto digital terrain models of the valles marineris permitted an evaluation of elevations in the vicinity of the troughs and the calculation of depth of troughs below surrounding plateaus thickness of deposits inside the troughs volumes of void spaces above geologic geomorphic units and volumes of deposits the central troughs north ophir north and central candor and north melas chasmata lie as much as 11 km below the adjacent plateaus in ophir and candor chasmata interior layered deposits reach 8 km in elevation if the deposits are lacustrine and if all troughs were interconnected lake waters standing 8 km high would have spilled out of coprates chasma onto the surrounding plateaus having surface elevations of only 4 5 km in this case interior deposits above about 4 km in the central troughs would not be lacustrine they could be volcanic on the other hand the troughs may not have been interconnected at the time of interior deposit emplacement they may have formed isolated ancestral basins the existence of such basins is supported by independent structural and stratigraphic evidence the ancestral basins may have eventually merged perhaps through renewed faulting to form northern subsidiary troughs in ophir and candor chasmata and the coprates north melas ius graben system the peripheral troughs are only 2 5 km deep shallower than the central troughs they may have formed from a combination of erosional collapse and structural activity chaotic terrain is seen in the peripheral troughs near a common contour level of about 4 km on the adjacent plateaus which supports the idea of release of water under artesian pressure from confined aquifers the layered deposits in the peripheral troughs may have formed in isolated depressions that harbored lakes and predated the formation of the deep outflow channels if these layered deposits are of volcanic origin they may have been emplaced beneath ice in the manner of table mountains areal and volumetric computations show that erosion widened the troughs by about one third and that deposits occupy one sixth of the interior space even though the volume eroded is larger than the volume deposited topographic and geologic considerations imply that material eroded from trough walls was probably part of the interior layered deposits but not their sole source additional material may have come from subterranean piping from reworking of local disintegration products on the floors such as chaotic materials or from eolian influx but overall it is likely that the additional material is volcanic and that it forms mostly the upper more diversely bedded layers of the interior deposits

the light toned stratified sedimentary rock exposures in western juventae chasma mars in context abstract juventae chasma an isolated depression northeast of the valles marineris trough system hosts four mounds of light toned layered materials these are erosional remnants interpreted to consist of sedimentary rocks the geological setting of juventae chasma is a complex amalgamation of terrains and surfaces of different morphologies ages and structures most previous published studies assumed or interpreted that the light toned mound forming materials were deposited in the chasm after it opened or began to open here we use detailed observations of the geologic setting the nature of chasm wall rocks and the distribution and nature of the contacts of the light toned materials with adjacent geologic materials to argue that the light toned mounds in juventae chasma are more likely remnants of light toned materials that existed in the subsurface before the chasm opened up the rock cut by the chasm was not a monolithic unit e g lava flows and might have included both light toned and dark toned sedimentary rocks bearing a range of diagenetic properties the light toned materials were deposited as clasts precipitates or both within craters or depressions that were filled buried lithified and then exposed when juventae chasma formed because the sedimentary rock types and stratal package properties differ from one mound to the next the buried craters or depressions could have existed at different times and provide sedimentary records of different environments that existed at different times during the planet s early history noachian period upon exposure these materials as well as chasm wall rocks and chaotic terrain blocks underwent differential erosion as a function of rock physical properties to produce the geomorphic expressions observed today

catastrophic flood sediments in chryse basin mars and quincy basin washington application of sandar facies model viking visible and thermal infrared observations and terrestrial catastrophic flood deposits provide clues to identify the outflow channel sediments that went into chryse basin on mars on earth sandar outwash plains formed by coalescence of many jokulhlaup floods are described in terms of three laterally adjacent facies proximal midfan and distal the missoula flood sediments deposited in quincy basin washington comprise a miniature analog of chryse basin the terminology and general characteristics of the sandar facies model are applied to quincy basin although the depositional environment and clast sizes are somewhat different higher energy flood larger clasts subaqueous rather than subaerial deposition for example the ephrata fan a deposit of boulders cobbles and pebbles forms the midfan facies analog a downfan sandy deposit reworked into a dune field comprises the distal facies analog in chryse basin the midfan is defined by a heterogeneous rocky 0 25 intermediate albedo 0 21 0 26 intermediate thermal inertia 260 460 j m 2 s 0 5 k 1 surface while the distal facies has a low albedo 0 14 0 16 and higher thermal inertia 340 700 j m 2 s 0 5 k 1 the chryse midfan unit has rocks and windblown dust exposed at the surface the sand of the distal facies in chryse acidalia is reworked by the wind as in quincy basin the viking 1 and mars pathfinder landing sites are located on the midfan unit observations that can be made at the mars pathfinder site might help in reevaluating whether or not viking 1 landed on outflow channel sediments

compressional and shear velocity logging method apparatus shear and compressional velocities of acoustic waveforms in the earth formations surrounding a well bore are measured by analyzing the waveforms propagated through different lengths of substantially the same formation intervals phase analysis techniques applied to waveforms windowed for compressional waves are used to determine the propagation velocity of the compressional waves through the differential path lengths the waveforms are then windowed for shear waves the discrete fourier transforms of the windowed waveforms are taken and the cross spectral magnitude and phase for each frequency is computed as the product of the discrete fourier transform sample of one with the conjugate of the other several different moveouts are calculated the resultant phases from each analyzed and a band specified where the zero phase moveouts are roughly consistent this ultimately specifies a final accurate shear window moveout for determining using phase analysis techniques the shear propagation velocity characteristics of the respective earth formation intervals

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

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

discussion of field evidence and hydraulic modeling of a large holocene jokulhlaup at jokulsa a fjollum channel iceland by douglas howard sheryl luzzadder beach and timothy beach 2012 this paper discusses howard et al 2012 who reconstruct the peak discharge of a glacial outburst flood or jokulhlaup for part of the jokulsa a fjollum in north central iceland they propose that this flood was the largest on earth weconsider that the magnitude of the jokulhlaup proposed by howard et al 2012 warrants much more robust field evidence and demands more carefully parameterised hydraulic modelling for these reasons we firstly i present their study in the context of previous research ii highlight issues with attributing landforms and sediments to jokulhlaups and iii consider uncertainty regarding the timing and magnitude of jokulhlaups along the jokulsa a fjollum we argue herein that whilst a range of landforms and sediments that are attributable to jokulhlaups can be observed along the jokulsa a fjollum these are not necessarily diagnostic of jokulhlaups secondly we critically discuss iv the major underlying assumptions of their study and v their calculations and subsequent interpretations these assessments lead us to consider that the proposal by howard et al 2012 of the largest flood on earth is highly unrealistic especially when due consideration is given to a possible source area and a trigger mechanism

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

possible juventae chasma subice volcanic eruptions and maja valles ice outburst floods on mars implications of mars global surveyor crater densities geomorphology and topography 1 this article discusses image topographic and spectral data from the mars global surveyor mgs mission that provide new information concerning the surface age geomorphology and topography of the juventae chasma maja valles system our study utilizes data from two instruments on board mgs images from the mars orbiter camera moc and topography from the mars orbiter laser altimeter mola within maja valles we can now observe depositional bars with megaripples that unequivocally show catastrophic floods occurred in the channel viking impact crater densities indicated the chasma and channel floor areas were all one age late hesperian to amazonian however moc data indicate a marked difference in densities of small craters between juventae chasma maja valles and the channel debouchment area in chryse planitia basin although other processes may contribute to crater variability young resurfacing events in the chasma and episodes of recent erosion at maja valles channel head may possibly account for the disparate crater densities along the chasma channel system relatively young volcanic eruptions may have contributed to resurfacing as in juventae chasma a small possible volcanic cone of young dark material is observed moc data also indicate previously unknown interior layered deposit mounds in the chasma that indicate at least two periods of mound formation finally mola topography shows that the entire floor of the chasma lies at the same elevation as the channel debouchment area in chryse basin resulting in a 3 km high barrier to water flow out of the chasma blocked ponded water would rapidly freeze in the current and likely past climate of mars for catastrophic flow to occur in maja valles some process is required to melt ice and induce floods out of the chasma we suggest subice volcanic eruption and calculate estimates of water discharges and volumes that these eruptions might have produced

evidence age and thickness of a frozen paleolake in utopia planitia mars abstract recent mapping studies west of elysium mons mars have pinpointed subice features that suggest the existence of a frozen paleolake in utopia planitia as recently as 1 8 billion years ago the subice features are interpreted to be hyaloclastic ridges and hills table mountains associated jokulhlaup deposits and fluvial channels photoclinometric studies of these features and of a basal scarp around the northwest flank of elysium mons interpreted to have been an ice sheet boundary indicate that the maximum thickness of ice within the basin may have been about 180 m this thickness of ice during a relatively late stage of martian geologic history would have important implications concerning the atmospheric the climatic and possibly the exobiologic history of the planet

interior trough deposits on mars subice volcanoes widespread several kilometer thick successions of layered deposits occur as mounds that partly fill the troughs or chasmata that compose the valles marineris on mars like terrestrial subice volcanoes the layered deposits occur in a volcano tectonic setting within basins that may have held ponded water or ice on the basis of their dimensions morphologies and associated catastrophic floods and other geologic events as shown in viking and new mars global surveyor mgs data sets we suggest that the interior deposits are volcanic in origin and possibly generated by subice eruptions a tuya origin for the mounds can explain the lack of external sediment mound heights that can rival the plateau local flat topped mesas morphologically distinct mounds of different ages horizontal to steep dips fine grained materials indications of rare volcanic vents and lava flows and spectral composition the extremely diverse layering of west candor chasma and possible volcanic cones in melas may have formed by related subaerial eruptions consistent with the suggestion that interior deposits are eroding out of the wall rock some deposits could have been erupted from sites along the walls

mineralogy of juventae chasma sulfates in the light toned mounds mafic minerals in the bedrock and hydrated silica and hydroxylated ferric sulfate on the plateau 1 juventae chasma contains four light toned sulfate bearing mounds denoted here as a d from west to east inside the trough mafic outcrops at the base of the mounds and in the wall rock and light toned layered deposits of opal and ferric sulfates on the plateau hyperspectral visible near infrared compact reconnaissance imaging spectrometer for mars crism spectra were used to identify monohydrated and polyhydrated sulfate phs outcrops of layered material on the bright mounds most of the monohydrated sulfate signatures closely resemble those of szomolnokite feso4 h2o characterized by a water band near 2 08 m while some areas exhibit spectral features more similar to those of kieserite mgso4 h2o with a band centered closer to 2 13 m the largest phs outcrops occur on the top of mound b and their spectral features are most consistent with ferricopiapite melanterite and starkeyite but a specific mineral cannot be uniquely identified at this time coordinated analyses of crism maps mars orbiter laser altimeter elevations and high resolution imaging science experiment images suggest that mounds a and b may have formed together and then eroded into separate mounds while mounds c and d likely formed separately mafic minerals low ca pyroxene high ca pyroxene and olivine are observed in large 2 10 km wide outcrops in the wall rock and in smaller outcrops 50 500 m across at the floor of the canyon most of the wall rock is covered by at least a thin layer of dust and does not exhibit strong features characteristic of these minerals the plateau region northwest of juventae chasma is characterized by an abundance of light toned layered deposits one region contains two spectrally unique phases exhibiting a highly stratified terraced pattern crism spectra of one unit eroded into swirling patterns with arc like ridges exhibit a narrow 2 23 m band assigned to hydroxylated ferric sulfate a thin layer of a fractured material bearing an opaline silica phase is observed at the contact between the older plateau unit and the younger hydroxylated ferric sulfate bearing light toned layered deposits hydrothermal processes may have produced an acidic environment that fostered formation of the hydrated silica and hydroxylated ferric sulfate units

grjota valles and implications for flood sediment deposition on mars 1 grjota valles is one of mars four amazonian aged fissure headed catastrophic flood channels it originates at the northwestern most cerberus fossa and stretches several hundred kilometers eastward and southward moats around in channel knobs and various small scale mounds similar to mounds seen in athabasca valles are both attributed to flood related processes the floodwater sinks are ambiguous and the lack of recognizable flood deposits is attributed to two causes the first cause is the channel s high width to depth ratio which may have facilitated floodwater recession during flooding the second cause is the tendency of floodwater under lower gravity to move a higher percentage of material as washload as flood deposits are comprised of bedload and suspended load this tendency would result in fewer depositional bedforms our analysis of grjota valles supports the idea that formation of terrestrial style flood deposits is energetically unlikely on mars and that their presence indicates special paleohydraulic conditions

characteristics of the numerous and widespread recurring slope lineae rsl in valles marineris mars abstract recurring slope lineae rsl are narrow 0 5 5 m dark features on mars that incrementally lengthen down steep slopes fade in colder seasons and recur annually these traits suggest that liquid water is flowing in the shallow subsurface of mars today here we describe high resolution imaging science experiment hirise observations of rsl within valles marineris vm we have identified 239 candidate and confirmed rsl sites within all the major canyons of vm with the exception of echus chasma about half of all the globally known rsl locations occur within vm and the areal density of rsl on coprates montes appears to be the greatest on the planet vm rsl are heterogeneously distributed as they are primarily clustered in certain areas while being conspicuously absent in other locations that appear otherwise favorable rsl have been found on many of the interior layered deposits ilds within vm such ild rsl appear to traverse bedrock instead of regolith like all other rsl forty six of the vm rsl sites show incremental lengthening and exhibit similar behavior in most of the canyons of vm but the rsl duration at one site in juventae chasma is significantly reduced furthermore the lengthening seasonality depends solely on slope orientation with typical vm rsl on a given slope lengthening for 42 74 of a mars year there are always rsl lengthening within vm regardless of the season if rsl are caused by water such a long active season at hundreds of vm rsl sites suggests that an appreciable source of water must be recharging these rsl thermophysical modeling indicates that a melting temperature range of 246 264 k is needed to reproduce the seasonal phenomenology of the vm rsl suggesting the involvement of a brine consisting of tens of wt salt the mechanism s by which rsl are recharged annually remain uncertain overall gaining a better understanding of how rsl form and recur can benefit the search for extant life on mars and could provide details about an in situ water resource

generation of martian chaos and channels by debris flows abstract a debris flow mechanism is proposed to account for the formation of chaos and the large channels debouching into chryse planitia from the adjacent southern uplands of mars the debris is thought to have originated through a mechanism of collapse in the chaotic terrains which exist at the head of these channels as well as locally along the channels this proposition is based on the detailed morphologic similarities between martian channel source areas and the heads of both subaerial and subaqueous terrestrial debris flows the downslope movement of the debris produced the channels through a modification of earlier collapse areas b active bed erosion and c loading induced collapse the large scale channel geometry and the assemblage of related morphologic features on mars correspond tto that observed in subaqueous debris flow chutes on the mississippi delta front through various mechanisms of strain dependent viscosity decrease the debris flow gained mobility downstream turned into a debris avalanche and moved onto chryse planitia at very high velocities this high velocity avalanche eroded a series of streamlined remnants near the channel mouths and deposited its load as a thin blanket over a large area of the basin creating virtually no depositional relief

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

light toned layered deposits in juventae chasma mars abstract we examine hypotheses for the formation of light toned layered deposits in juventae chasma using a combination of data from mars global surveyor s mars orbiter camera moc mars orbiter laser altimeter mola and thermal emission spectrometer tes as well as mars odyssey s thermal emission imaging system themis we divide juventae chasma into geomorphic units of i chasm wall rock ii heavily cratered hummocky terrain iii a mobile and largely crater free sand sheet on the chasm floor iv light toned layered outcrop llo material and v chaotic terrain using surface temperatures derived from themis infrared data and slopes from mola we derive maps of thermal inertia which are consistent with the geomorphic units that we identify llo thermal inertias range from 400 to 850 j m 2 k 1 s 1 2 light toned layered outcrops are distributed over a remarkably wide elevation range 5 8 km from the chasm floor to the adjacent plateau surface geomorphic features the absence of small craters and high thermal inertia show that the llos are composed of sedimentary rock that is eroding relatively rapidly in the present epoch we also present evidence for exhumation of llo material from the west wall of the chasm within chaotic and hummocky terrains and within a small depression in the adjacent plateau the data imply that at least some of the llo material was deposited long before the adjacent hesperian plateau basalts and that juventae chasma underwent and may still be undergoing enlargement along its west wall due to wall rock collapse chaotic terrain evolution and exposure and removal of llo material the new data allow us to reassess possible origins of the llos gypsum one of the minerals reported elsewhere as found in juventae chasma llo material forms only at low temperatures 60 c and thus excludes a volcanic origin instead the data are consistent with either multiple occurrences of lacustrine or airfall deposition over an extended period of time prior to emplacement of hesperian lava flows on the plateau above the chasm

new light on caldera evolution askja iceland the large multiple caldera volcanic system of askja central iceland is composed principally of subglacial basaltic hyaloclastite pillow lava formations and postglacial basaltic scoria and flows traditionally such calderas are believed to be formed by downfaulting and ring fracture collapse whereas this certainly applies to the smaller a d 1875 caldera the older main caldera may have developed positive relief during subglacial construction of laterally confined hyaloclastite ridges above erupting fractures this is supported by the evidence of a large negative gravity anomaly that reaches minima over the marginal low density ridges but which is less negative within the caldera where relatively dense postglacial lavas are believed to cover a more limited hyaloclastite succession beneath the caldera floor

origin of martian outflow channels the eolian hypothesis the ancient outflow channels on mars exhibit fossil flow patterns similar to current flow patterns in terrestrial desert regions eroded by wind we have modeled outflow channel formation by a cascade process in which sand sized particles produced by rock weathering in the chaotic terrain source areas of the outflow channels were transported by the wind across the adjacent plateau surfaces and sand blasting and deflating the surface comparisons of model predictions with photogeologic data suggest that rates of rock breakdown by physical and chemical action along the channels during the channel forming era must have been much higher than they have been on similar terrains formed after the channel era ended a tenuous atmosphere comparable to the present one could have transported materials sufficiently rapidly to excavate outflow channels in about 106 years however there may be difficulties in explaining how abrading particles could survive high velocity impacts in a thin atmosphere over the many hundreds or even thousands of kilometers that they were transported in outflow channels it is difficult to avoid the conclusion that a climate change affecting rock breakdown rates atmospheric density or both occurred early in martian history to end the era of outflow channel formation particulate materials generated during this era may be important components of the north circumpolar dune field and the polar layered deposits

formation and evolution of the chaotic terrains by subsidence and magmatism hydraotes chaos mars the origin of the martian chaotic terrains is still uncertain and a variety of geologic scenarios have been proposed we provide topographic profiles of different chaos landscapes notably aureum and hydraotes chaos showing that an initial shallow ground subsidence occurred at the first step of the chaos formation we infer that the subsidence was caused by intrusion of a volcanic sill which could have produced consequent melting as well as release of ground water from disrupted aquifer signs of a volcanic activity are observed on the floor of hydraotes chaos a complex and deep depression located at the junction of three channels the volcanic activity is represented by small 0 5 to 1 5 km diameter rounded cones with summit pits the cone s size and morphology as well as the presence of possible surrounding lava flows suggest that they are primary volcanic cones similar to terrestrial cinder cones the identification of volcanic activity on the deepest chaos where the lower crustal thickness and the faults fractures system contributed to the magma rising reveals that magmatic activity proved by the cones and possibly help by structural activity has been a major factor in the formation of chaotic terrains

ice sculpture in the martian outflow channels many landforms in martian outflow channels have characteristics that suggest sculpture by glaciers ice streams or ice sheets viking orbiter and terrestrial satellite images were examined at similar resolution to compare features of the martian outflow channels to features produced by the movement of ice on earth many resemblances were found they include the anastomoses sinuosities and u shaped cross profiles of valleys hanging valleys and linear scour marks on valley walls grooves and ridges on valley floors and streamlining of bedrock highs the question of whether ice could have moved in the martian environment is investigated since gradients on martian channel floors are extremely low or even reversed a certain thickness of ice is required to initiate glacial motion if ice filled the martian channels to the level of interior plateaus flow could have occurred even under present climatic conditions although warmer climates in the past would have been more favorable additionally if the channel fluids were brines ice might have been wet based and would have moved readily by slipping over the ground it is envisioned that springs or small catastrophic outbursts discharged fluids from structural outlets or chaotic terrains the fluids built icings that may have grown into substantial masses and eventually flowed like glaciers down preexisting valleys alternatively the fluids may have formed rivers or floods that formed ice jams and consolidated into icy masses in places where obstacles blocked their flow these masses of ice or slush were probably episodic and temporary but where they occurred within the channels they may have produced landforms that are characteristic of glacial sculpture on earth

dark materials in valles marineris indications of the style of volcanism and magmatism on mars rifting on the equatorial canyon system of valles marineris provides a unique view of the interior of the martian crust to depths reaching 7 km exposing several in situ bedrock units which testify to past volcanic and magmatic processes on mars a thick regionally extensive deposit observed in coprates and juventae chasmata is interpreted on the basis of spectral reflectance erosional morphology and tendency for eolian mobilization to be composed of mafic glass possibly an ancient martian analogue of the lunar terra mantling deposits spectral mapping suggests that the dark floor covering materials in the lower canyons are derived from this unit a series of cliffs in the ophir chasma wall rock is interpreted to be exposures of resistant bedrock the spectral signature of this massive and uniform unit most closely resembles that of terrestrial mafic rocks altered to or coated by crystalline hematite application of computer mapping techniques to probable young volcanic materials in the central troughs yields an inferred distribution of volcanic activity consistent with the interpretation of extrusion along faults near the margins of the canyon floors this result supports the hypothesis that the valles originated through tectonic extension

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

stratigraphy and erosional landforms of layered deposits in valles marineris mars the complex stratigraphy of layered deposits suggests a diversity of origins ages and post depositional modification histories the complexities within some layered deposits indicate changes in the dominant source materials in space and time the stratigraphy of layered deposits in the isolated martian chasmata hebes juventae and gangis is not well correlated this indicates that at least these chasmata had isolated depositional environments resulting in different stratigraphic sequences separated layered deposits in ophir candor and melas chasmata might have been a single continuous deposit in each chasma chaotic terrains are found in conjunction with layered deposits in juventae gangis and capri eos chasmata in these chasmata layered deposits unconformably overlie chaotic terrains chaotic terrain formation may have provided water to form paleolakes and lacustrine deposition of thick layered deposits may have occurred if the canyons were closed a very thick sequence of the layered deposits has been exposed by erosion a combination of gradual processes such as evaporation of ice and eolian and fluvial transport in addition to structural processes may be responsible for this erosion another alternative is that catastrophic water release under the layered deposits disrupted and initiated erosion of the layered deposits newly identified units of anomalous color are confined to the depressions or reentrants in western candor chasma the difference in color between these units and the surrounding terrain is most consistent with a somewhat greater content of bulk crystalline hematite in these anomalous units the presence of the candor units is a result of original and or secondary deposition which is different from the primary and dominant formation of the layered deposits

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

the geologic evolution of mars episodicity of resurfacing events and ages from cratering analysis of image data and correlation with radiometric ages of martian meteorites we have investigated 12 prominent regions on mars in detail with respect to their geologic evolution through time on the basis of detailed geologic mapping exercises and determining age relationships through crater counting techniques using imagery new data in combination with previously obtained data have been analyzed by way of a refined method of cratering age extraction that also gives fine details of periods of resurfacing we have found that there has been volcanic and fluvial glacial geologic activity on the martian surface at all times from 4 ga ago until today this activity shows episodic pulses in intensity of both volcanic and fluvial glacial processes at 3 8 3 3 ga 2 0 1 8 ga 1 6 to 1 2 ga 800 to 300 m y 200 m y and 100 m y and a possible weaker phase around 2 5 2 2 ga ago in between these episodes there was relative quiescence of volcanic and or fluvial glacial activity the episodes we find on the martian surface in the crater frequency analyses of hrsc moc and themis data coincide with some age groups of the martian meteorites 1 3 ga 600 300 m y 170 m y it appears that the surface activity expressions and their episodicity are related to the interior evolution of the planet when convection in the asymptotic stationary state changes from the so called stagnant lid regime to an episodic behavior similarities in episodic behavior are found for the other terrestrial planets venus the earth s moon and the earth itself suggesting a common general relationship in the evolutionary tracks

young volcanic deposits in the valles marineris mars abstract a study of the interior deposits of the central valles marineris has led to the discovery of a sequence of deposits that cover the chasma floors and range in thickness from that of thin dust to several kilometers the emplacement of the deposits was the last major event in the history of the valles marineris following deposition of older layered interior beds warping faulting erosion and landslide emplacement the young deposits are of three major types 1 dark patches typically occurring along faults 2 light colored deposits locally associated with craters and 3 rugged mottled deposits with in places light colored lobate fronts these young materials may be of volcanic origin as suggested by the low albedo and spectrally gray colors of some their association with faults and possible volcanic craters and calderas and their embayment relations and lobate margins no other mechanism explains all the observed features and relations as well as volcanism if these deposits are volcanic implications are that 1 volcanism was associated with rifting in the valles marineris 2 the volcanism was explosive in places and 3 the volcanism may be as young as the late tharsis volcanism and locally may well be recent

origin and evolution of the layered deposits in the valles marineris mars abstract thick sequences of layered deposits are found in the martian valles marineris they exhibit fine nearly horizontal layering and are present as isolated plateaus of what may have once been more extensive deposits individual sequences of layered deposits are as thick as 5 km the greatest total thicknesses of deposits are found in candor ophir and melas chasmata individual layer thicknesses range from about 70 to 300 m some tilting of sequences is observed but at the best image resolutions no angular unconformities are detectable in the layers the sequences of events in the canyons as deduced from morphologic and stratigraphic evidence was 1 graben formation in response to the tharsis uplift 2 canyon wall retreat and canyon enlargement roughly contemporaneous with formation of the layered deposits 3 deep erosion of the layered deposits 4 landsliding of the canyon walls and 5 eolian erosion of the layered deposits perhaps continuing up to the present we consider four hypotheses for the origin of the layered deposits they are eolian deposits they are remnants of the material that makes up the canyon walls they are deposits of explosive volcanic eruptions or they were deposited in standing bodies of water the rhythmic nature of the layers and their lateral continuity horizontality great thickness and stratigraphic relationships with other units in the canyons all appear most consistent with deposition in an aqueous environment if standing bodies of water existed in the valles marineris they were almost certainly ice covered there are three ways in which sediment could have entered an ice covered lake down through the ice cover up from the lake bottom or in from the lake margins layers of sediment could have been transported downward through an ice cover by foundering or rayleigh taylor instabilities but it is not clear whether there was a viable mechanism for repeatedly accumulating thick sediment layers on top of the ice cover subaqueous volcanic eruption on the lake bottom does not suffer from many of the morphologic arguments that make origin by subaerial volcanism seem improbable while this mechanism is attractive there are no eruptive centers observed and there is no other direct evidence to support it because canyon enlargement took place at roughly the same time as layer deposition debris from the canyon walls is an obvious and likely source for some of the material in the layered deposits however the volume of material removed from the canyon walls may be insufficient to account for all of the presently observed material we conclude that there are several geologically feasible but as yet unproven mechanisms that could have led to formation of thick deposits in ice covered paleolakes in the valles marineris present data are insufficient to choose conclusively among the various possibilities several types of data from the mars observer mission will be useful in further characterizing the deposits and clarifying the process of their origin the deposits should be considered important targets for a future mars sample return mission

subglacial floods beneath ice sheets subglacial floods jokulhlaups are well documented as occurring beneath present day glaciers and ice caps in addition it is known that massive floods have occurred from ice dammed lakes proximal to the laurentide ice sheet during the last ice age and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago we propose that drainage of lakes from beneath ice sheets will generally occur in a time periodic fashion and that such floods can be of severe magnitude such hydraulic eruptions are likely to have caused severe climatic disturbances in the past and may well do so in the future

amphiboles in the igneous environment the time is ripe for a fresh inventory of occurrences of amphibole group minerals in igneous rocks here i build on the available resources for example on the review of wones and gilbert 1982 in volume 9b of the rimg series they entitled their chapter amphiboles in the igneous environment i adopt the same title but intend to make the scope of my review somewhat broader by including non igneous rocks formed in the igneous environments much has been written about occurrences of amphibole group minerals over the last quarter of a century much of it ably summarized by deer et al 1997 and it is quite impossible to do more here than to summarize representative findings concerning the textural development and broad compositional features of minerals of the amphibole group that are important in igneous contexts amphibole group minerals are widespread in igneous rocks produced in zones undergoing extension as well as in rocks associated with subduction systems each setting may be associated with specific late stage metasomatic phenomena in which amphibole group minerals play a key role metasomatic overprints involving an amphibole also are widespread in the mantle and act as an essential step in the fertilization of sterile mantle if it is to produce alkali basaltic magmas by partial melting finally selected findings are summarized here on amphibole group minerals in igneous environments in other parts of the solar system amphibole group minerals have generally been underutilized in petrological studies reasons are not hard to find the full characterization of an amphibole presents serious challenges so much so that most investigators have been constrained to examine only those major elements that can be assessed in routine electron microprobe analyses as hawthorne and oberti 2006 pointed out in their essay on the future of schemes of classification it has become abundantly clear that progress is within

thermodynamics and fluid dynamics of effusive subglacial eruptions we consider the thermodynamic and fluid dynamic processes that occur during subglacial effusive eruptions subglacial eruptions typically generate catastrophic floods jokulhlaups due to melting of ice by lava and generation of a large water cavity we consider the heat transfer from basaltic and rhyolitic lava eruptions to the ice for typical ranges of magma discharge and geometry of subglacial lavas in iceland our analysis shows that the heat flux out of cooling lava is large enough to sustain vigorous natural convection in the surrounding meltwater in subglacial eruptions the temperature difference driving convection is in the range 10 100 c average temperature of the meltwater must exceed 4 c and is usually substantially greater we calculate melting rates of the walls of the ice cavity in the range 1 40 m day indicating that large subglacial lakes can form rapidly as observed in the 1918 eruption of katla and the 1996 eruption of gjalp fissure in vatnajokull the volume changes associated with subglacial eruptions can cause large pressure changes in the developing ice cavity these pressure changes can be much larger than those associated with variation of bedrock and glacier surface topography previous models of water cavity stability based on hydrostatic and equilibrium conditions may not be applicable to water cavities produced rapidly in volcanic eruptions energy released by cooling of basaltic lava at the temperature of 1200 c results in a volume deficiency due to volume difference between ice and water provided that heat exchange efficiency is greater than approximately 80 a negative pressure change inhibits escape of water allowing large cavities to build up rhyolitic eruptions and basaltic eruptions with less than approximately 80 heat exchange efficiency cause positive pressure changes promoting continual escape of meltwater the pressure changes in the water cavity can cause surface deformation of the ice laboratory experiments were carried out to investigate the development of a water cavity by melting ice from a finite source area at its base the results confirm that the water cavity develops by convective heat transfer