venusian channels and valleys distribution and volcanological implications abstract nearly complete image coverage of venus by magellan enabled us to map various venusian channel and valley landforms and to examine their associations with other geological units global mapping reveals a nonrandom distribution the highest total concentration is in the equatorial regions characterized by highlands rift and fracture zones and associated volcanic features many channels associated with flow deposits are similar to typical terrestrial lava drainage channels they are observed to be associated with a wide range of volcanic edifices such as coronae shield volcanoes and rift and fracture zones one type of channel similar morphologically to lunar sinuous rilles is classified as a venusian sinuous rille based on the close associations of many venusian sinuous rilles with coronae we hypothesize that mantle plume or blob volcanism has caused high effusion and sustained lava eruptions essential for sinuous rille formation mantle derived high temperature low viscosity lava eruptions are responsible for the efficient erosional processes particularly for thermal erosion that seems to be at least partially required for some sinuous rille formation many valley networks are observed in highlands and in association with coronae fracture systems and source zones for low viscosity lavas both key to network formation were probably concentrated at highlands and coronae canali type channels which are morphologically unlike other known volcanic channels are limited to certain plains regions a wide range of low viscosity lava types is possible for the channel forming lava depending on the formation mechanism their lengths exceed the lengths of other common volcanic channel types on venus implying a large volume of lava and long duration of the eruption the close association of canali with plains regions implies that canali formation is probably related to the emplacement of plains a hypothesized global resurfacing event late in venusian history may be responsible for canali formation

volcanology of arnus vallis mars abstract arnus vallis av is a 300 km long sinuous rille located on the northeastern flank of the syrtis major volcano on mars observational evidence presented here suggests that av formed as an open lava channel that was at least partly incised into the pre existing terrain the lava source area consists of a sub circular pit at the southwestern end of a 7 km long straight section of channel av trends down slope from this source with an average bottom slope of 0 26 or 0 14 width varies from 1 km at the source to 0 6 km near the distal end with a mean of 0 76 km depth decreases from 180 m at the source to 25 m near the distal end the av terminus is obscured by a large impact crater we suggest that the material that flowed in av must have been a relatively high temperature low viscosity lava dynamically and perhaps compositionally similar to terrestrial komatiite or some lunar basalt lavas if correct this finding has implications for the mode of construction of syrtis major

venusian channel formation as a subsurface process 1 we constructed detailed geologic maps of channels within three widely separated regions using magellan synthetic aperture radar sar imagery and altimetry radar data in order to understand the operative processes involved in venusian channel formation channels in each region cut topographic highs including ridges and shield edifices with no deflection of the channel course by surface topography we argue that these relationships are difficult to reconcile with the widely held view that venusian channels represent surface processes whether constructional or erosional we conclude that the channels evolved through subsurface fluid flow which involved local stoping and transport of overlying material that is these channels were carved from below rather than from above we postulate that at least some venusian channels form because of subsurface fluid flow along a shallow crustal interface that forms the boundary between overlying low backscatter surface materials and underlying basal materials fluid movement along the interface may facilitate piecemeal stoping and erosion of the local surface materials from below eventually resulting in the formation of channel traces exposed at the surface the low backscatter material which hosts the channels in this study also hosts abundant coalescing shields and associated deposits which may represent shield terrain

a new model for the emplacement of columbia river basalts as large inflated pahoehoe lava flow fields extensive flows of the columbia river basalt crb group in washington oregon and idaho are dominantly inflated compound pahoehoe sheet lavas early studies recognized that crb lavas are compound pahoehoe flows with textures suggesting low flow velocities but it was thought that the great thickness and extent of the major flows required very rapid emplacement as turbulent floods of lava over a period of days or weeks however small volume 1 km compound pahoehoe flows on kilauea hawai i demonstrate that such flows can thicken by at least an order of magnitude through gradual inflation and the same mechanism has been proposed for larger 10 20 km3 pahoehoe flows in iceland the vertical distribution of vesicles and other morphologic features within crb lava flows indicate that they grew similarly by inflation small pahoehoe lobes at the base and top of many crb pahoehoe lava flows indicate emplacement in a gradual piecemeal manner rather than as a single flood we propose that each thick crb sheet flow was active for months to years and that each group of flows produced by a single eruption a flow field was emplaced slowly over many years

viscous heating in silicate melts an experimental and numerical comparison 1 the transition from newtonian to non newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate shear thinning may strongly influence the dynamics of magmatic flows but the details of its microscopic origins are not fully understood here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning we compare the results of laboratory uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup both the experimental and numerical results concord and indicate that the reduction of the temperature dependent viscosity in flowing silicate melts is a result of viscous heating viscous heating was quantified for glasses with viscosities ranging from 108 to 1011 pa s and strain rates from 10 5 to 100 s 1 the results of 48 compression experiments indicate that the transition from newtonian to non newtonian flow in the silicate melt occurs at a brinkmann number i e ratio of heat gained to heat lost around 1 whereas brittle behavior dominates the melt deformation when the deborah number i e ratio of viscoelastic relaxation time to characteristic deformation time is larger than around 0 01 the observed viscous heating significantly contributes to the viscosity decrease observed in high stress strain rate experiments and questions our current understanding of the non newtonian deformation behavior of silicate melts

volcanic processes as alternative mechanisms of landform development at a candidate crater lake site near tyrrhena patera mars 1 palos crater a prominent candidate crater lake site in the hesperia region of mars is located at the mouth of a sinuous inlet channel that extends from the northern flanks of tyrrhena patera previous studies of this site have concluded that channels and basin infill units likely formed through aqueous processes that partly involved karst like removal of materials in the subsurface by the flow of groundwater as with many locales on mars the aqueous processes hypothesized to have been involved in the formation and evolution of terrain features are compelling however the characteristics of this site suggest that an alternative set of mechanisms for landform development should be considered alongside aqueous hypotheses the main attributes of channels and infill of the central crater include 1 an association with a very large volcanic rise to the south 2 the existence of roofed segments of the main inlet channel and 3 infill of the central crater and outlet channel by materials that have characteristics consistent with those expected of volcanic plains these general attributes are consistent with formation through or in association with processes that involved the flow of lava although a volcanic model may account for the presence and basic nature of landforms previously considered supportive of a paleolake interpretation of the site an overprint of dendritic valley networks on parts of the inlet system suggests that aqueous processes may have also acted in the evolution of the site a complex interrelation between past igneous and aqueous processes at this site is possible

formation of venusian canali considerations of lava types and their thermal behaviors because liquid water is unstable at present venusian surface conditions the discovery of channels termed canali on venus thousands of kilometers long was not predicted low viscosity lavas that remain fluid for several thousand kilometers are considered to be the canali forming agents possible compositions of venusian canali forming lavas include komatiite and high fe ti lunar type basalts results of analytical and numerical models of these lavas reveal that total cooling is more efficient on venus than on earth suggesting that venusian lavas rapidly form insulating crusts and thus that the canali lavas were essentially tube fed the models also reveal that thermal erosion should be less efficient on venus than on earth suggesting that venusian channels are either the product of mechanical rather than thermal erosion or constructional processes

thermal erosion by komatiites at kambalda western australia and the genesis of nickel ores huppert et al 1 calculated that komatiites flow turbulently transfer heat efficiently to their substrate and thus potentially produce thermal erosion channels in sequences onto which they were erupted their suggestion that komatiite hosted kambalda type2 nickel ores reside in such channels has led to resurgence of interest in volcanic models for ore emplacement3 5 and caused controversy6 7 such models have important implications not only to ore genetic studies but also to interpretation of the geochemistry of komatiites8 and their sm nd systematics9 10 here we demonstrate that interspinifex fe ni cu sulphide ores which underlie some hanging wall massive sulphide ores at kambalda the site of one of the world s major nickel deposits formed through thermal erosion of underlying komatiite flows and possibly also sediments providing proof of thermal erosion however we caution against the general application of thermal erosion models to the eruption of komatiites it seems that significant thermal erosion occurred rarely and only where basal concentrations of highly thermally conductive sulphide liquid were present at the base of localized lava channels isotope data11 12 and theoretical considerations13 suggest that the komatiites were contaminated by crustal rocks before eruption although there is limited evidence that localized contamination including sulphide assimilation4 may also have occurred below lava channels localized in pre existing embayments or troughs

thermal erosion by laminar lava flows thermal erosion by hot lavas during their laminar flow over cold ground is investigated both theoretically and experimentally an analysis of the steady forced convective heat transfer by a laminar channel flow at large peclet numbers is presented which is used to determine the final steady state erosion velocity as well as the thicknesses and timescales of the associated thermal boundary layers in both the lava and ground the initial transient period involving the growth and remelting of a basal chill layer is also quantified laboratory experiments are described in which hot molten wax flowed over and thermally eroded an underlying bed of solidified wax the experimental observations of chill formation and thermal erosion are found to be in excellent agreement with the theoretical predictions the theory is applied to predict rates of thermal erosion consistent with those observed in recent flows of basaltic lava in hawaii and of carbonatite lava in tanzania

factors influencing the capability of lava to erode its substrate application to venus there is evidence that a uniquely venusian type of channel may be at least partly erosional in origin a number of published models discuss the erosion process on various planetary bodies however these relate to lavas of specifically different compositions here a numerical model is derived to investigate the sensitivity of the erosion process to various physical parameters of the lava and substrate placing constraints on the physical characteristics required to obtain a suitably rapid erosion rate the model allows for the effects of latent heat and also considers the presence of a liquid substrate boundary layer between the lava and solid substrate the results indicate that the thermal erosion process is particularly sensitive to the melting temperature when the melting temperature is doubled from 700 to 1400 k the erosion rate decreases by approximately an order of magnitude also the erosion rate associated with a venusian initial substrate temperature is approximately 1 5 times greater than the equivalent terrestrial one mechanical erosion is considered by altering the percentage of latent heat required to melt the substrate as this percentage is varied from 0 to 100 the erosion rate decreases by about a factor of 2

erosion by flowing martian lava new insights for hecates tholus from mars express and mer data 1 we have used new compositional information on martian basaltic rocks from the mars exploration rover spirit and data from new stereo imaging by the high resolution stereo camera hrsc on the mars express orbiter to constrain an existing analytical numerical computer model to assess the potential of martian lavas to form lava channels by erosion of substrate the basaltic rocks studied in gusev crater by spirit have compositions consistent with lavas of higher liquidus temperature and lower dynamic viscosity than terrestrial tholeiitic basalts suggesting that they had a greater potential for turbulent flow and erosion of substrate during flow emplacement than terrestrial basalts more similar to lunar mare basalts we modeled a specific case to determine whether these lavas could have formed part of a 66 km long channel on the martian shield volcano hecates tholus this channel formed on relatively steep slopes 2 8 based on measurements from the hrsc digital terrain model our results suggest that erosion rates were of the order of tens to hundreds of centimeters day depending on flow rate and ice content of the substrate eruption durations required to erode the hecates channel 100 30 m deep over the first 20 km depth decreasing downstream range from weeks to months which are consistent with the known eruption durations of terrestrial basaltic lavas and the previous modeling of wilson and mouginis mark 2004 any hecates lava channels formed from erosion by lava could have served as conduits for later fluvial activity as recently described by fassett and head 2004 2005

pahoehoe and aa in hawaii volumetric flow rate controls the lava structure the historical records of kilauea and mauna loa volcanoes reveal that the rough surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate 5 10 m3 s and the smooth surfaced variety called pahoehoe forms at a low volumetric rate 5 10 m3 s this relationship is well illustrated by the 1983 1990 and 1969 1974 eruptions of kilauea and the recent eruptions of mauna loa it is also illustrated by the eruptions that produced the remarkable paired flows of mauna loa in which aa formed during an initial short period of high discharge rate associated with high fountaining and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate with little or no fountaining the finest examples of paired lava flows are those of 1859 and 1880 1881 we attribute aa formation to rapid and concentrated flow in open channels there rapid heat loss causes an increase in viscosity to a threshold value that varies depending on the actual flow velocity at which when surface crust is torn by differential flow the underlying lava is unable to move sufficiently fast to heal the tear we attribute pahoehoe formation to the flowage of lava at a low volumetric rate commonly in tubes that minimize heat loss flow units of pahoehoe are small usually 1 m thick move slowly develop a chilled skin and become virtually static before the viscosity has risen to the threshold value we infer that the high discharge rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers rapid near surface vesiculation of gas rich magma leads to eruptions with high discharge rates high lava fountains and fast moving channelized flows we also infer that long periods of sustained flow at a low discharge rate which favor pahoehoe result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway

rima marius the moon formation of lunar sinuous rilles by constructional and erosional processes abstract lunar sinuous rilles are large kilometers long tens of meters wide channels commonly found on the lunar maria although there is broad consensus that sinuous rilles were generated during lava flow emplacement disagreement remains regarding the relative contributions of erosional and constructional processes during rille formation careful mapping of portions of rima marius centered at 16 37 n 49 54 w using lunar reconnaissance orbiter lro narrow angle camera nac images with resolutions as high as 0 5 m pixel reveal morphologic evidence for the origin of sinuous rilles specifically shallow

discussion of pulsed inflation of pahoehoe lava flows implications for flood basalt emplacement by s w anderson e r stofan e r smrekar j e guest and b wood earth planet sci lett 168 1999 7 18

volcanism on mars more than 15 years of planetary exploration of mars have given insight into the geologic processes that have shaped its surface the newly acquired viking data have shown that volcanism is one of the most important geologic processes operating on mars throughout its history in situ chemical analyses of martian soil by the viking lander spacecraft indicate mafic to ultramafic source rocks this is consistent both with available remote sensing data which indicate the presence of mafic minerals such as pyroxene and olivine and with petrologic modeling based on available geophysical data which suggest that martian lavas are probably iron rich and ultramafic these data strongly suggest that basaltic volcanism is widespread on mars and much of the photogeological data may be studied in this context photogeological analysis of the martian surface has shown two main types of volcanic morphologies the first type is central volcanoes which are volcanic landforms developed by continued and prolonged eruption from a point source vent this category includes 1 shields the classic low profile volcanic mountains of which olympus mons is the most spectacular example 2 domes steep sided constructs such as tharsis tholus that may represent lower rates of eruption than the shields or possibly more silicic lava compositions 3 highland patera radially textured low profile volcanoes that occur in the cratered terrain and are interpreted as ash shields 4 alba patera an apparently unique volcanic landform consisting of a vast volcanic center over 1500 km across with flank slopes of less than a tenth of a degree and 5 various small features such as cinder cones the second major category is volcanic plains which are units recognized by several criteria of which the presence of mare ridges and flow lobes are the most useful volcanic plains are subdivided into four main groups 1 simple flows broad smooth to rolling plains that contain numerous mare type ridges but no flow lobes interpreted as being composed of thick single cooling units 2 complex flows displaying multiple overlapping flow lobes interpreted to be indicative of thin multiple cooling units 3 undifierentiated flows plains that typically lack any morphologic identifying feature but are considered to be volcanic partly on the basis of their association with large volcanic centers and 4 questionable plains volcanic units heavily modified by other processes erosion tectonism etc so that their origins are uncertain when these categories of volcanic morphologies are combined with relative age data provided by crater statistics a volcanic history for mars can be derived as follows early heavy bombardment of mars was accompanied and followed by small scale fluvial channeling extensive flood volcanism the plateau plains and ash shield volcanism in the cratered terrain shortly after this time less extensive flood volcanism continued to resurface the planet during formation of the northern southern hemisphere dichotomy central volcanism became more prominent with the development of the alba patera center as well as the older shields and domes of the northern hemisphere early tharsis and elysium regions the development of the tharsis and elysium uplifts may have triggered the release of large scale catastrophic floods producing large channels continued uplift and lithospheric thinning concentrated volcanic activity in the tharsis region producing large shield volcanoes and extensive lava plains both central vent and plains volcanism have been active throughout martian history but the volumes of extrusion have gradually decreased with time this is consistent with a moonlike thermal history involving a lithosphere of increasing thickness with time gradually turning off the volcanism although many questions remain regarding martian volcanism the viking data have provided a remarkable detailed overview of the probable nature of the volcanic history of mars

the perseverance ultramafic complex western australia the product of a komatiite lava river the perseverance ultramafic complex is a body of olivine rich komatiitic rocks spatially associated with the agnew nickel deposit in the agnew wiluna greenstone belt of the archaean yilgarn block in western australia the complex consists of a central lenticular body up to 700 m thick of olivine adcumulates flanked by laterally extensive sheet like bodies of olivine orthocumulates and spinifextextured komatiite flows rocks progressively further away from the central lens have chemical compositions reflecting higher original proportions of komatiite liquid to cumulus olivine parent liquids had mgo contents between 25 and 32 mgo approximately chondritic al ti ratios and hree patterns and moderate depletion in lree olivines within the adcumulate lens show a progressive increase in forsterite content from fo93 at the bottom to fo94 5 at the top calculated original olivine compositions in ihe flanking rocks are similar to those at the base of the central lens original olivine nickel contents show a symmetrical variation from maximum values of 3500 ppm at the top of the central lens through minimum values of 1000 ppm at the base and margins of the central lens to intermediate values in the distal rocks the complex as a whole shows evidence for nickel depletion relative to other komatiite suites these observations are explained in terms of prolonged eruption and flow of komatiitic lava down a major flow channel or lava river adcumulates crystallized on the floor and sides of the central channel which was formed at an early stage by thermal erosion of floor rocks episodic overflow of the central channel produced distal flood plain rocks consisting of olivine orthocumulates and layered flows lavas became more magnesian and nickel rich with time giving rise to the observed spatial variation in primary olivine composition nickel depletion of the earliest lavas is attributed to pre eruption segregation of large volumes of immiscible fe ni sulfide which were concentrated to form the underlying agnew nickel deposit

formation of an eroded lava channel within an elysium planitia impact crater distinguishing between a mechanical and thermal origin a lava channel identified on the wall of an elysium planitia impact crater is investigated to identify the dominant erosion mechanism mechanical vs thermal acting during channel formation observations of channel morphology are used to supplement analytical models of lava channel formation in order to calculate the duration of channel formation the velocity of the lava flowing through the channel and the erosion rate in each erosion regime considered results demonstrate that the channel observed in the elysium planitia impact crater formed primarily due to mechanical erosion in a more general sense results of this study suggest that lava channels can form primarily due to thermal erosion in the presence of more gradual slopes and more consolidated substrates whereas lava channels can form primarily due to mechanical erosion in the presence of more energetic flows on steeper slopes and more poorly consolidated substrates therefore both erosion regimes must be considered when analyzing origins of eroded lava channels that cut through strata of different strengths

an overview of venus geology the magellan spacecraft is producing comprehensive image and altimetry data for the planet venus initial geologic mapping of the planet reveals a surface dominated by volcanic plains and characterized by extensive volcanism and tectonic deformation geologic and geomorphologic units include plains terrains tectonic terrains and surficial material units understanding the origin of these units and the relation between them is an ongoing task of the magellan team

development of lava tubes in the light of observations at mauna ulu kilauea volcano hawaii during the 1969 1974 mauna ulu eruption on kilauea s upper east rift zone lava tubes were observed to develop by four principal processes 1 flat rooted crusts grew across streams within confined channels 2 overflows and spatter accreted to levees to build arched roofs across streams 3 plates of solidified crust floating downstream coalesced to form a roof and 4 pahoehoe lobes progressively extended fed by networks of distributaries beneath a solidified crust still another tube forming process operated when pahoehoe entered the ocean large waves would abruptly chill a crust across the entire surface of a molten stream crossing through the surf zone these littoral lava tubes formed abruptly in contrast to subaerial tubes which formed gradually all tube forming processes were favored by low to moderate volume rates of flow for sustained periods of time tubes thereby became ubiquitous within the pahoehoe flows and distributed a very large proportionof the lava that was produced during this prolonged eruption tubes transport lava efficiently once formed the roofs of tubes insulate the active streams within allowing the lava to retain its fluidity for a longer time than if exposed directly to ambient air temperature thus the flows can travel greater distances and spread over wider areas even though supply rates during most of 1970 1974 were moderate ranging from 1 to 5 m3 s large tube systems conducted lava as far as the coast 12 13 km distant where they fed extensive pahoehoe fields on the coastal flats some flows entered the sea to build lava deltas and add new land to the island the largest and most efficient tubes developed during periods of sustained extrusion when new lava was being supplied at nearly constant rates tubes can play a major role in building volcanic edifices with gentle slopes because they can deliver a substantial fraction of lava erupted at low to moderate rates to sites far down the flank of a volcano we conclude therefore that the tendency of active pahoehoe flows to form lava tubes is a significant factor in producing the common shield morphology of basaltic volcanoes

thermal erosion of felsic ground by the laminar flow of a basaltic lava with application to the cave basalt mount st helens washington 1 the thermal erosion of cold felsic ground by the steady laminar flow of a hot basaltic lava is examined theoretically and experimentally initially a chill layer is grown and then remelted at the base of the lava flow a steady thermal erosion velocity is then established which is limited by the buoyant instability of the melted ground or by the effective freezing temperature of the basaltic lava when the theoretical analysis is applied to the longest lava tube system of the cave basalt on mount st helens it is found that about 100 days of flow is sufficient to produce the observed ground erosion in little red river ape and lake caves

depth profiles of venusian sinuous rilles and valley networks abstract more than 200 venusian channels and valleys have been mapped based on analyses of magellan sar images sinuous rilles are the most abundant channels among six types of venusian channels and they are widely distributed on venus morphological characteristics of venusian sinuous rilles include sinuous narrowing reaches source depressions and length of several 10s to a few 100s of km this type of channels is known to exist on the moon and possibly on mars valley networks on venus often occur in the vicinity of or in connection to sinuous rilles cross sectional morphologies of sinuous rilles and valley networks are of special importance in discussing their formation processes both qualitatively and quantitatively we reconstructed cross sectional profiles of 6 sinuous rilles and 2 valley networks using a new radar clinometric method reconstructed cross sections revealed that floors of the channels and valleys are clearly lower than the surrounding plains this finding implies that the sinuous rilles and the valley networks have erosional origins longitudinal depth profiles of the sinuous rilles show distinct decreasing trends toward the termini such decreasing trends of depths are qualitatively in agreement with theoretical models and laboratory experiments of thermal erosion in order to verify this assertion quantitatively we conduct simple 1 dimensional model calculations under the assumption that both channel forming lavas and ground substrate are tholeiitic basalt for initial lava thicknesses in the range 2 6 m the model calculations yield good matches to the depth profiles estimated duration of lava effusion ranges from several months to a few years these numerical results support thermal erosion of the sinuous rilles but do not necessarily exclude contributions from mechanical erosion processes valley networks seem to have formed under a strong structural control in comparison to sinuous rilles the valleys vary widely in characteristics of the depth profile and flow directions relative to surface slopes therefore valley networks appear to have originated from diverse formation mechanisms

on the cross sectional geometry of thermal erosion channels formed by turbulent lava flows a new theoretical framework for interpreting the cross sectional geometry of self formed lava channels is introduced the method may be applied to any lava channel or tube and is demonstrated by studying thermal erosion by a hot turbulently flowing lava the cross section of a thermal erosion channel is often characterized by undercutting of the channel walls when the upper surface of the lava flow falls beneath the original ground level the degree of lateral undercutting is shown here to be a function of the flow parameters which enables the shape of the channel cross section to be used to determine a lower bound upon the lava flow rate the solution for the geometry of the lava channel is then coupled to an existing two dimensional model for thermal erosion by komatiite flows to determine the effects of lateral erosion and variable flow cross section upon cooling of the lava and contamination of the lava by assimilated ground material lateral thermal erosion is shown to lead to a thinning of the lava flow with time even at a constant eruption rate which reduces its erosive power and enhances cooling of the lava finally a linear stability analysis of the evolution equation for the channel cross section suggests that inefficient lateral mixing in a thin lava flow can lead to a morphological instability and eventual splitting of the channel the instability is suppressed at short wavelengths by the variation in melting rate with local wall curvature and the fastest growing symmetric mode has a wavelength equal to half the channel width which corresponds to the formation of midstream islands

quantification of extraterrestrial lava flow effusion rates through laboratory simulations we have used carefully controlled laboratory simulations to develop a model which relates lava flow morphology to effusion rate and rheology through comparisons with measured and estimated eruption rates on earth this approach allows us to constrain eruptive styles and compositions of extraterrestrial lava flows by applying this model to lava flows on the moon mars and venus we have determined that all the common flow morphologies domes folds and levees on these planets could have been produced by basalt like or andesite like lavas through either continuous or episodic emplacement the presence of more evolved magma compositions on other planets is not required to explain any of the observed lava flow morphologies

an igneous origin for features of a candidate crater lake system in western memnonia mars the association of channels inner terraces and delta like features with martian impact craters has previously been interpreted as evidence in favor of the past existence of crater lakes on mars however examination of a candidate crater lake system in western memnonia suggests instead that its features may have formed through igneous processes involving the flow and ponding of lava accumulations of material in craters and other topographic lows throughout much of the study region have characteristics consistent with those of volcanic deposits and terraces found along the inner flanks of some of these craters are interpreted as having formed through drainage or subsidence of volcanic materials channels previously identified as inlets and outlets of the crater lake system are interpreted instead as volcanic rilles these results challenge previous interpretations of terrace and channel features in the study region and suggest that candidate crater lakes located elsewhere should be reexamined

role of mechanical erosion in controlling the effusion rate of basaltic eruptions in many basaltic eruptions observations show that the effusion rate of magma has a typical dependence on time the effusion rate curves show first a period of increasing and later a decreasing phase by a maximum value we present a model to explain this behavior by the emptying of a magma reservoir through a vertical cylindrical conduit with elliptical cross section coupled with the its widening due to mechanical erosion produced by the magma flow the model can reproduce the observed dependence on time of effusion rate in basaltic eruptions eruption duration and the maximum value of effusion rate depend on the size of magma chamber on lava viscosity and strongly on erosion rate per unit traction

erosion by flowing lava field evidence erosion of substrate materials by melting or mechanical means has been suggested in active lava flows on earth and other planets although there are many references to lava erosion on earth unambiguous evidence is rare geological relationships commonly cited as evidence of downcutting by lava can be explained without recourse to erosion in order to assess possible erosion by flowing lava we carried out field studies of tube fed basalt flows sheet flows of the columbia river basalt province crb and precambrian komatiites unequivocal evidence for thermal erosion melted dacite substrate was found at the cave basalt lava tube mount st helens for which fluid dynamic analysis indicates laminar flow although erosion was enhanced in areas of locally steep slopes possibly as a result of localized turbulence other lava tubes in our study display strong but inconclusive evidence for erosion komatiite flows display good evidence for erosion of their substrate possibly in a turbulent regime but assessment of the extent of erosion is hampered by limited and disrupted exposures no evidence for thermal erosion was found in the crb our findings suggest that an erosional origin for planetary sinuous rilles and canali would be favored by high reynolds number flows high mass flux low viscosity lava steep slopes and substrates having a lower melting temperature than the lava or low mechanical strength e g regolith

estimation of volcanic eruption conditions for a large flank event on elysium mons mars mars orbiter camera image 5 23 m pixel and topographic data from the mars orbiter laser altimeter data are used to analyze a channel on the upper northern flank of the elysium mons volcano mars we show that this channel is best explained as the product of the thermal erosion of the preexisting surface by a high speed turbulent lava flow fed from a fissure eruption we deduce the volume eruption rate 3400 13 000 m3 s dense rock equivalent the total volume of erupted lava 29 39 km dense rock equivalent and the duration of the activity 40 130 days perhaps significantly the erupted lava volume is essentially identical given the uncertainties in the analysis to the volume of the observed caldera 31 km3 which has implications for volume limited eruptions on mars and potentially for the unusual steep upper slopes of the volcano the eruption that produced the channel appears to have been typical for elysium mons and implies that the other lava channels on the upper flank of the volcano may also have taken 1 4 months to have formed

the physical volcanology of archaean komatiite sequences from forrestania southern cross province western australia abstract the archaean supracrustal sequence at forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km lithologies range from olivine cumulates which crystallised at the interface between substrate and flowing lava to spinifex textured rocks which underwent rapid crystallisation within ponded lava lobes characteristic associations of different rock types are interpreted in terms of variations in eruption rate which presumably reflect proximity to the vent or to a major lava river thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment where lava flow was continuous and eruptive velocities were high sheeted bodies of olivine cumulate formed from voluminous turbulent flow of lava over relatively refractory substrates such as basalt and oxide facies banded iron formation ribbon shaped olivine cumulate bodies formed in an equally dynamic environment within major lava channels where the substrate was less refractory and consisted of quartz rich sedimentary rocks which were susceptible to thermal erosion distributary lava channels in more distal settings are represented by thinner ribbon like accumulations of olivine mesocumulate to orthocumulate unusual poikilitic pyroxene bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks pyroxene and plagioclase bearing cumulates are present at the top of other channel sequences and reflect late stage ponding and in situ differentiation of komatiitic lava within the channel the channel facies rocks are flanked by sequences comprising multiple thin spinifex textured flow units which formed from the episodic overflow of lava from the distributary channels similar rocks also occur stratigraphically above many of the thick olivine cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle

the heating of substrates beneath basaltic lava flows as populations around volcanoes grow the potential for society to be impacted by lava flows is increasing while lava flows are known to ignite bulldoze and or bury structures little is known about potential impacts to buried infrastructure we measure temperature profiles below molten rock to constrain a heat transfer model thermomagnetic and palaeomagnetic measurements on soil samples from beneath a 2014 hawaiian lava flow are then used to verify the model finally we illustrate the model s utility in lava flow hazard assessments by modelling a hypothetical lava flow active for 4 weeks in auckland new zealand the modelling predicts the upper 1 7 m of dry soil would exceed 100 c after 1 week and the upper 3 8 m of soil would exceed 100 c after 4 weeks determining the depth profile of substrate heating has important implications for planning and preparedness e g siting buried infrastructure volcanic impact and risk assessments and decision making before and during lava flow crises e g mitigation measures to be employed

a network of lava tubes as the origin of labyrinthus noctis and valles marineris on mars abstract the role of lava tube networks and lava channels is reassessed as the primordial stage of the volcano erosional processes that formed the labyrinthus noctis valles marineris system instead of a tectonic origin the combined use of ctx crism hirise imagery and mola profiles has provided valuable insight in the evolution of pit chains into fossae first and then chasmata later due to mass wasting processes caused by the erosional effect of the lava flows that draped valles marineris and other outflow channels although a quantitative evaluation of eruption rates is difficult even with digital terrain models dtms because of the mixing between new flows and paleoflows a comparison with elysium and other tharsis outflow channels suggests that the availability of lava supply is correlated to their widths the images of ubiquitous lava flows rather than sporadic light toned deposits strengthen the role of lava over that of water in the erosional processes that formed labyrinthus noctis and carved valles marineris like many other outflow channel on mars the erosional evolution of the outflow channels shows an increasing trend of age and a decreasing trend of depth from the sources on tharsis to the mouths at chryse planitia this finding coupled with the observation of lava flows mantling chryse planitia may have profound implications for the water inventories thought to have filled the lowlands with an ocean

on the role of thermal conductivity on thermal erosion by lava lava channels are abundant on the surface of both the moon and venus there is evidence that erosion may have played a part in their formation here a study of the relationship between thermal conductivity and erosion rate is presented a numerical model is used to investigate the sensitivity of the erosion rate to the relative value of the thermal conductivity of the lava solid substrate and liquid substrate boundary layer results show that the highest rates for thermal erosion are achieved by a high liquid substrate and low solid substrate thermal conductivity and that the thermal erosion process is most sensitive to the liquid substrate s thermal conductivity

dynamics of a confined lava flow on kilauea volcano hawaii this paper presents a new method of analysing lava flow deposits which allows the velocity discharge rate and rheological properties of channelled moving lavas to be calculated the theory is applied to a lava flow which was erupted on kilauea in july 1974 this flow came from a line of fissures on the edge of the caldera and was confined to a pre existing gully within 50 m of leaving the vent the lava drained onto the floor of the caldera when the activity stopped but left wall and floor deposits which showed that the lava banked up as it flowed around each of the bends field surveys established the radius of curvature of each bend and the associated lava levels and these data together with related field and laboratory measurements are used to study the rheology of the lava the results show the flow to have been fast moving but still laminar with a mean velocity of just over 8 m s 1 the lava had a low or negligible yield strength and viscosities in the range 85 140 pa s an extension of the basic method is considered and the possibility of supercritical flow discussed

mangala valles mars a reassessment of formation processes based on a new geomorphological and stratigraphic analysis of the geological units abstract mangala valles has always been viewed as the typical outflow channel formed by catastrophic floods of water a new analysis has shown that the geomorphological traces of fluvial or lacustrine processes within mangala valles can be better explained by fluid lava flooding the channels and filling pre existing impact craters as for the circum chryse outflow channels where no clear source of water or mechanism able to replenish water at its hydraulic head is observed there is no geologic trace of a sudden removal of a volume of water ice necessary to carve mangala valles neither maars nor rootless cones typical volcanic features indicative of interaction between lava and ground ice were found past works suggested that the formation of mangala valles occurred in late amazonian age when the climate of mars was similar to that seen today that is absolutely not liquid water friendly the present work shows how the origin of mangala valles may go back to noachian or even pre noachian when other studies have concluded that the climate was not liquid water friendly even assuming limited periods of obliquity favourable to liquid water in the history of mars which is at odds with the widespread presence of unaltered olivine and jarosite it is very difficult to find plausible mechanisms of aquifer recharge or signs of catastrophic water release at the notch of mangala valles that could feed the multiple episodes or even a single episode of fluvial flooding suggested in the literature this evidence and other analysis will show that the presence of water and eventually ground ice is not incontrovertible in the equatorial regions and should not be given for granted as commonly done so far in the literature the geomorphological analysis of the mars reconnaissance orbiter mro images provided in this paper combined with themis and mola data show how mangala fossa from which mangala valles originated as a breakout is an erosional channel formed by the flow of lava in a original tube coming from daedalia planum rather than a tectonic graben or the sign of a dike rupturing to the surface

partial melting at the earth s surface implications for assimilation rates and mechanisms in subvolcanic intrusions abstract the relatively recent routine availability of isotopic and elemental analytical results from large sample suites of igneous rocks has revolutionized the study of magmatic processes large uncertainties remain however with regard to the highly variable rates of magma differentiation and ascent through the crust although the repose interval prior to an eruption may be equivalent to magma residence time at a certain level in the crust this cannot be assumed a priori even where this happens to be the case uncertainties in determining such intervals for prehistoric eruptive sequences may be large with respect to process rates consequently independent insights from geologically well constrained situations must be sought this paper first presents documentation of an example wherein fragments of dacite incorporated by a mafic lava flow on the surface were melted to the point that they viscously deformed in response to stresses in this flow the transformation from the solid state to a largely molten state is inferred to have occurred within a few weeks these results are then discussed in the context of examples from the literature that support rapid melting and digestion of xenoliths in magma it is suggested that the rate of crustal assimilation may be sufficiently rapid that it can operate within the durations of extended eruptive episodes years decades which are generally much shorter than typical repose periods at arc volcanoes decades millennia

mathematical modeling of thermomechanical erosion beneath proterozoic komatiitic basaltic sinuous rilles in the cape smith belt new quebec canada virtually all of the economic ni cu platinum group element pge mineralization in the central part of the cape smith belt of new quebec is hosted by thick olivine cumulate units in the katinniq member of the raglan formation at the base of the 1 9 ga chukotat group these units transgress underlying gabbros and pelitic metasediments forming 50 200 m deep and 300 1 000 m wide v shaped embayments and have been interpreted on the basis of surface geology deep diamond core drilling and magnetic inversion models to represent the remnants of one or more large long at least 20 km possibly 50 km sinuous komatiitic basalt lava channels that formed by thermomechanical erosion of their substrates we have used a mathematical model to test these hypotheses regarding komatiitic lava emplacement and erosion by lava our modeling predicts that an initially 10 m thick komatiitic basalt flow should have flowed turbulently near the vent and should have thermomechanically eroded unconsolidated pelitic sediment during emplacement to reach the observed degree of contamination of 10 at distances of 30 60 km downstream from the source furthermore our models predict that at these distances downstream a fully inflated 100 m thick komatiitic basalt flow would have had thermal erosion rates over consolidated gabbroic substrate of 0 7 1 5 m day requiring 70 140 days to incise a 100 m deep channel depending on the initial temperature of the lava the paleoslope and the initial temperature and solidus temperature of the gabbro these erosion rates would have been associated with volumetric flow rates of 105 106 m3 s and eruption volumes of 103 104 km3 although these flow rates are orders of magnitude larger than those of most modern terrestrial basaltic flows they are of the same order as those estimated for the largest terrestrial flood basalt flows and with those inferred for some of the largest extraterrestrial flows our predicted flow volumes are also of the same order as those of the largest terrestrial flood basalt units consistent with the great thickness and widespread distribution of the chukotat group our modeling of thermomechanical erosion of gabbro by komatiitic basalt results in negligible contamination middle ree heavy ree nb ta ti represent contamination by underlying povungnituk semipelites this result is consistent with present models for the genesis of the ni cu pge mineralization in the raglan formation that involve thermomechanical erosion of unconsolidated sulfidic semipelitic sediments and decoupling of the miscible silicate and immiscible sulfide components

lunar mare volcanism stratigraphy eruption conditions and the evolution of secondary crusts abstract lunar mare basalt deposits cover 17 of the lunar surface occur preferentially in topographic lows on the nearside and have a total volume estimated at 1 107 km3 analysis of returned samples and photogeologic and remote sensing studies shows that mare volcanism began prior to the end of heavy bombardment the period of cryptomare formation in pre nectarian times and continued until the copernican period a total duration approaching 3 5 4 ga stratigraphic analyses show that the flux was not constant but peaked in early lunar history during the imbrian period which spans the period 3 85 3 2 ga average volcanic output rate during this peak period was about 10 2 km 3 a very low relative to the present global terrestrial volcanic output rate and comparable to the present local output rates for individual volcanoes such as vesuvius and kilauea hawaii volcanic landforms indicate that many eruptions were of high volume and long duration some eruptions associated with sinuous rules may have lasted of the order of a year and emplaced 103 km3 of lava representing the equivalent in one year of about 70 000 years at the average flux shallow magma chambers were uncommon the nearsidefarside mare deposit asymmetry can be readily explained by differences in crustal thickness magma ascending from the mantle or from a buoyancy trap at the base of the crust should preferentially extrude to the surface on the nearside but should generally stall and cool in dikes in the farside crust extruding only in the deepest basins the occurrence of farside maria within craters whose diameter is generally near to or less than the thickness of the crust may be accounted for by the difference between local and regional compensation dikes that establish pathways to the surface on the nearside should have very high volumes comparable to the volumes associated with many observed flows and sinuous rilles an abundance of dikes should exist in the lower crust of the moon many more than those feeding surface eruptions the upper limit is 37 50 of the crust by volume the presence and abundance of such dike swarms have important implications for the interpretation of the average composition of the lunar crust and the composition of basin and crater ejecta the interplay between thermal contraction and differentiation leads to net cooling and ultimate contraction of the outer portions of the moon resulting in a regional horizontal compressive stress acting on the lunar crust in addition overall cooling and deepening of sources require the production of ever larger volumes of magma in order to reach the surface with time stresses became high enough so that few dikes could open to the surface causing eruptive activity to be severely diminished in the eratosthenian and perhaps to cease in the copernican period lower stress levels are required to terminate eruptive activity on the lunar farside consistent with the imbrian age of the farside maria and the nearside location of the youngest maria lunar mare deposits provide an example of the transition from primary crusts to secondary crusts and illustrate the significance of several factors in the evolution of secondary crusts such as crustal density variations in crustal thickness presence of impact basins state and magnitude of stress in the lithosphere and general thermal evolution these factors are also responsible for the extremely low volcanic flux even during periods of peak extrusion

high resolution imaging science experiment hirise images of volcanic terrains from the first 6 months of the mars reconnaissance orbiter primary science phase 1 in the first 6 months of the mars reconnaissance orbiter s primary science phase the high resolution imaging science experiment hirise camera has returned images sampling the diversity of volcanic terrains on mars while many of these features were noted in earlier imaging they are now seen with unprecedented clarity we find that some volcanic vents produced predominantly effusive products while others generated mostly pyroclastics flood lavas were emplaced in both turbulent and gentle eruptions producing roofed channels and inflation features however many areas on mars are too heavily mantled to allow meter scale volcanic features to be discerned in particular the major volcanic edifices are extensively mantled though it is possible that some of the mantle is pyroclastic material rather than atmospheric dust support imaging by the context imager ctx and topographic information derived from stereo imaging are both invaluable in interpreting the hirise data

geochemistry of komatiites from forrestania southern cross province western australia evidence for crustal contamination the 2 9 ga old forrestania greenstone belt is intermediate in character between typical 3 0 ga and 2 7 ga belts in containing both al depleted and al undepleted komatiites the extensive nickel exploration database on the belt provides a unique opportunity to examine geochemical variations in the context of volcanological evolution within each of the six komatiitic units in the greenstone belt there is a transition from komatiites 18 28 wt mgo in the liquid to high and low magnesium komatiitic basalts 5 18 wt mgo with stratigraphic height and much of the chemical variance within the komatiitic suite can be explained in terms of the addition or fractionation of olivine there are however aspects of the incompatible element signatures which must be attributed to other factors the more primitive komatiites from the lower ultramafic units are al depleted al2o3 tio2 10 5 13 0 and depleted in both light and heavy ree lasmn 0 5 gdybn 1 5 komatiites from the uppermost ultramafic units are variably less depleted in al al2o3tio2 16 5 20 5 this is interpreted to reflect decreasing retention of garnet in the source region of the forrestania komatiites over time and possibly a decrease in the depth of melting within the al depleted suite there are also anomalous enrichments in sio2 feo tio2 and zr which correlate with regional variations in the composition of substrate metasedimentary rocks banded iron formation versus felsic tuff coupled with field evidence for trough structures at the base of lava channel sequences these chemical anomalies provide strong evidence for supracrustal thermal erosion by komatiite lavas the forrestania data indicate that 2 9 ga old komatiites were generated over a range of melting conditions spanning those typical of older and younger archaean sequences and that contamination by supracrustal thermal erosion can influence komatiite chemistry on a regional scale

emplacement and erosive effects of lava in south kasei valles mars abstract although it has generally been accepted that the martian outflow channels were carved by floods of water observations of large channels on venus and mercury demonstrate that lava flows can cause substantial erosion recent observations of large lava flows within outflow channels on mars have revived discussion of the hypothesis that the martian channels are also produced by lava an excellent example is found in south kasei valles skv where the most recent major event was emplacement of a large lava flow calculations using high resolution digital terrain models dtms demonstrate that this flow was locally turbulent similar to a previously described flood lava flow in athabasca valles the modeled peak local flux of approximately 106 m3 s 1 was approximately an order of magnitude lower than that in athabasca which may be due to distance from the vent fluxes close to 107 m3 s 1 are estimated in some reaches but these values are probably records of local surges caused by a dam breach event within the flow the skv lava was locally erosive and likely caused significant kilometer scale headwall retreat at several cataracts with tens to hundreds of meters of relief however in other places the net effect of the flow was unambiguously aggradational and these are more representative of most of the flow the larger outflow channels have lengths of thousands of kilometers and incision of a kilometer or more therefore lava flows comparable to the skv flow did not carve the major martian outflow channels although the skv flow was among the largest and highest flux lava flows known in the solar system

morphology and emplacement of a long channeled lava flow near ascraeus mons volcano mars 1 channeled lava flows hundreds of kilometers long are common on the lower flanks of the tharsis montes on mars our analysis of a 690 km long lava flow along the southwest perimeter of ascraeus mons shows that it was emplaced on low local slopes 100 km in length but the maximum effusion rates necessary on earth are essentially the minimum for martian flows on the basis of our calculated effusion rates the eruption duration was 3 to 7 earth months assuming a constant effusion rate and continuous eruption the morphology of the ascraeus mons flow shows similarities to terrestrial and simulated channeled flows downstream changes in morphology resemble those observed in the 1907 flow mauna loa volcano hawaii and channeled polyethylene glycol peg flows braided sections of the channel in the ascraeus mons flow contain islands which are hundreds of meters across and resemble features observed in the 1907 and 1984 flows on mauna loa volcano crosscutting relationships suggest islands in the proximal section were shaped by thermal and mechanical erosion whereas islands in the medial section are inferred to be material rafted by surges of lava through the channel overall understanding the morphology of long lava flows on mars is essential to the interpretation of their emplacement and constraining eruption conditions in the saddle regions of the tharsis volcanoes

quantification of submarine lava flow morphology through analog experiments we developed a technique for determining paleoeffusion rates and emplacement times for submarine lava flows using observations of flow morphology and estimates of eruption volume eruption temperature lava viscosity and preflow topography using laboratory simulations and correlating these results with sea floor observations four submarine lava flow morphologies are considered to be diagnostic of specific effusion rates jumbled folded and lineated sheets and striated pillows we applied this approach to the coaxial flow emplaced in less than 14 days on the juan de fuca ridge in early summer 1993 and we calculated an effusion rate of 100 m3 s giving an emplacement time of 10 days

meander properties of venusian channels venusian channels which probably are lava channels have meander properties that relate to their mode of formation their channel meanders probably formed in response to flow dynamics as in the case of terrestrial rivers but low sinuosity indicates less developed morphology the meanders generally follow terrestrial river trends for wavelength l to width w ratios suggesting an equilibrium adjustment of channel form slightly higher l w in comparison to terrestrial rivers occurs for venusian channel types other than sinuous rilles the unusually low l w values for some venusian and lunar sinuous rifles probably indicate modification of original meander patterns by erosional channel widening by lavas the slightly larger radius of curvature to wavelength ratios of venusian channels may be attributed to the short time during which the lava channels formed and or they may represent a quasi equilibrium state for the system multiple wavelength meanders of specific venusian channels may have resulted from changes in discharge or from structural control

venus volcanism initial analysis from magellan data magellan images confirm that volcanism is widespread and has been fimdamentally important in the formation and evolution of the crust of venus high resolution imaging data reveal evidence for intrusion dike formation and cryptodomes and extrusion a wide range of lava flows also observed are thousands of small shield volcanoes larger edifices up to several hundred kilometers in diameter massive outpourings of lavas and local pyroclastic deposits although most features are consistent with basaltic compositions a number of large pancake like domes are morphologically similar to rhyolite dacite domes on earth flows and sinuous channels with lengths of many hundreds of kilometers suggest that extremely high effusion rates or very fluid magmas perhaps komatiites may be present volcanism is evident in various tectonic settings coronae linear extensional and compressional zones mountain belts upland rises highland plateaus and tesserae volcanic resurfacing rates appear to be low less than 2 km 3 yr but the significance of dike formation and intrusions and the mode of crustal formation and loss remain to be established

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

the volcanology of komatiites as deduced from field relationships in the norseman wiluna greenstone belt western australia abstract komatiites in the western part of the norseman wiluna greenstone belt in the yilgarn block of western australia display a wide variety of volcanic facies ranging from very thin differentiated munro township type flow units through to very thick olivine rich cumulate flow units containing high proportions of adcumulate dunite cumulate flow units have been mapped in detail in the agnew wiluna segment of the norseman wiluna greenstone belt in the yakabindie and mt keith areas thick lenticular bodies of olivine adcumulate are flanked and overlain by thinner sheet like sequences of finer grained olivine orthocumulates they show fine scale internal layering broad scale cryptic layering and upper fractionated sequences containing harrisites pyroxene bearing cumulates and in some cases gabbroic derivatives low grade disseminated sulphide mineralisation occurs within the dunite lenses in the stratigraphically equivalent kathleen east area adcumulate dunite grades laterally into olivine orthocumulates and spinifex textured flows a similar relationship is also seen further south where the perseverance ultramafic complex consists of a thick central dunite lens flanked on one side by thin fine grained orthocumulate sequences and on the other by intercalated orthocumulates and spinifex textured flows these field relationships indicate an extrusive origin for the adcumulate dunite bodies the walter williams formation in the west central part of the norseman wiluna belt is a very large cumulate flow unit 150 km long and 30 km wide in presently exposed extent it consists of a central sheet like body of adcumulate dunite underlain and overlain by olivine orthocumulates the adcumulate sheet is everywhere capped by a distinctive thin layer of olivine harrisite at its northernmost extent the flow unit consists of cyclically layered olivine and pyroxene bearing cumulates capped by gabbros dolerites and pyroxene spinifex textured material interpreted as a periodically replenished lava lake sequence both the lenticular dunite bodies of the agnew wiluna belt and the sheet like dunite body of the walter williams formation are interpreted as crystallisation products of very large submarine komatiite lava flows which erupted and flowed at very high rates the adcumulate dunites are interpreted as the products of in situ crystallisation at low degrees of supercooling at the top of upward and inward accreting crystal piles at the bases of the flows flanking and overlying olivine orthocumulates reflect higher rates of heat loss at the site of crystallisation due to lower lava flow rates field evidence from perseverance and the general geometry of the dunite lenses suggest that the lenses formed within large thermal erosion channels developed by turbulent lava rivers flowing over low melting felsic volcanic substrates the textural range exhibited by komatiites can be integrated into a comprehensive model for the geometry of large komatiite flow fields formed by rapid extrusion dunite sheets form close to the eruption sites and dunite lenses as a result of channellisation further away from the vent or close to the vent in the situation where the substrate is non refractory and thermal erosion can take place readily kambalda type volcanic facies develop in more distal environments where lava emplacement is channellised and episodic and munro type flow units represent small scale lava tubes formed at low flow rates on the distal flanks of major eruptions or close to the vent of very small ones waning eruption rates leads to proximal facies being overridden by distal ones a common observation in komatiite sequences the size of thermal erosion channels requires very rapid eruption rates comparable to those in phanerozoic flood basalt terrains

structures textures and cooling histories of columbia river basalt flows flood basalt flows of the columbia river basalt group commonly exhibit well developed colonnade and entablature structures formed during cooling of individual flows the colonnade refers to the well defined columnar structure of many flows which appears in the lowermost 10 to 30 and in places the uppermost 10 to 20 entablature refers to those parts of a flow that have smaller column diameters and more irregular pattern fractures than those of colonnade entablature commonly occupies the central 60 to 70 of a flow in some cases however the entablature is interrupted by colonnade in such cases the entablature and colonnade may be repeated once or twice within a single flow in other cases flows may lack an entablature entirely petrographic examination reveals a pronounced correlation between structural and textural features thin sections of colonnade basalt have relatively coarse cruciform or octahedral fe ti oxide crystals and 20 glassy mesostasis the central entablature basalt in contrast has much more mesostasis as much as 60 and has textural features such as feathery fe ti oxide crystals indicative of more rapid cooling these features are the reverse of what might be expected for normal conductive cooling because cooling rates at the flow margins should be greater than those in the flow interior we reconciled the observed basalt textures with thermal models by taking into account rainfall and deranged drainage during crystallization of the flow downward migration of water through cracks would cause convective cooling of underlying unsolidified lava this effect and its consequences have been modeled numerically in order to clarify the physical constraints on the mechanism by which the central part of the lava can be quenched we have found that a water infiltration front can explain the presence of a thick quenched flow interior provided that the infiltration front moves downward just behind the lava solidification front despite heavy rainfall on hawaii for example 250 cm yr at kilauea iki exposed prehistoric lava flows on that island exhibit only colonnades we deduce from this that heavy rainfall alone is inadequate to quench flow interiors extensive flooding by deranged drainage or perhaps extremely heavy rainfall 250 cm yr is apparently required to produce entablatures the model also can be used to explain the occurrence of repeated entablature and colonnade by considering the effects of intermittent inundation the imposition of boundary conditions that mimic the removal and reoccurrence of flooding yields cooling histories that are consistent with the development of multiple colonnades and entablatures

a vector based method for bank material tracking in coupled models of meandering and landscape evolution sinuous channels commonly migrate laterally and interact with banks of different strengths an interplay that links geomorphology and life and shapes diverse landscapes from the seafloor to planetary surfaces to investigate feedbacks between meandering rivers and landscapes over geomorphic timescales numerical models typically represent bank properties using grids however this approach produces results inherently dependent on grid resolution herein we assess existing techniques for tracking landscape and bank strength evolution in numerical models of meandering channels and show that grid based models implicitly include unintended thresholds for bank migration that can control simulated landscape evolution building on stratigraphic modeling techniques we develop a vector based method for land surface and subsurface material tracking that overcomes the resolution dependence inherent in grid based techniques by allowing high fidelity representation of bank material properties for curvilinear banks and low channel lateral migration rates we illustrate four specific applications of the new technique 1 the effect of resistant mud rich deposits in abandoned meander cutoff loops on meander belt evolution 2 the stratigraphic architecture of aggrading alluvial meandering channels that interact with cohesive bank and floodplain material 3 the evolution of an incising meandering river with mixed bedrock and alluvial banks within a confined bedrock valley and 4 the effect of a bank height dependent lateral erosion rate for a meandering river in an aggrading floodplain in all cases the vector based approach overcomes numerical artifacts with the grid based model because of its geometric flexibility the vector based material tracking approach provides new opportunities for exploring the coevolution of meandering rivers and surrounding landscapes over geologic timescales

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

komatiites i eruption and flow because of their high eruption temperatures and ultrabasic composition komatiite lavas had low viscosities which typically ranged from 0 1 to 10 pa s a major consequence of this low viscosity is that most lavas erupted as turbulent flows an analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms 1 and reynolds numbers much greater than the critical value of 2000 the lavas would have remained turbulent for most or all of their subsequent flow and emplacement typical horizontal flow rates are estimated to range from 0 5 to 100 m2 s per unit width of flow such turbulent lava flows would have lost their heat by convection to the surroundings at rates which are orders of magnitude greater than the rates for laminar flows which cool by conduction a quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 c hr to a few c hr 1 while the flows remained turbulent these rates are in an appropriate range to cause phenomena such as high nucleation rates strong supersaturation of the lava delayed nucleation of olivine and skeletal or dendritic crystal morphologies komatiites often flowed over ground composed of rocks with lower melting temperatures it is proposed that the turbulent lavas melted the ground to form deep thermal erosion channels melting rates at the lava source are calculated at several metres per day and deep troughs with depths of several metres to hundreds of metres and lengths of several kilometres probably formed laboratory experiments performed to simulate thermal erosion show qualitative agreement with the theory with channel depth decreasing downstream the experiments also revealed that the channel margins become undercut during thermal erosion to form overhanging sides of the channel some sinuous rilles observed in the mare regions of the moon are thought to have formed by thermal erosion hulme 1973 they provide analogues of the channels postulated to form in komatiite eruptions where conditions were in fact more favourable for thermal erosion an assessment of the role of olivine crystals precipitated in the cooling turbulent flows indicates that they will remain in suspension until the lava has come to rest contamination of komatiite lava by underlying rock can be as much as 10 per cent some illustrative calculations show how the major element and trace element compositions of residual melts can be significantly modified by combined assimilation and fractional crystallization in a moving flow assimilation of tholeiitic basalt into a komatiite can cause incompatible trace element ratios such as ti zr and y zr and the rare earth patterns of derivative lavas to vary substantially some of the variations in such geochemical parameters which are often ascribed to mantle heterogeneity also could have resulted from assimilation of the ground assimilation could have modified the isotope geochemistry of lava suites and led to apparent ages which differ from the true eruption age the thermal erosion model also provides an explanation of the formation of some nickel sulphide ores found at the bottom of thick komatiite flows it is proposed that ores can form by assimilation of sulphur rich sediment which combines with ni from the komatiite to form an immiscible liquid

lava tubes on earth moon and mars a review on their size and morphology revealed by comparative planetology abstract sinuous collapse chains and skylights in lunar and martian volcanic regions have often been interpreted as collapsed lava tubes also known as pyroducts this hypothesis has fostered a forty years debate among planetary geologists trying to define if analogue volcano speleogenetic processes acting on earth could have created similar subsurface linear voids in extra terrestrial volcanoes on earth lava tubes are well known thanks to speleological exploration and mapping in several shield volcanoes with examples showing different genetic processes inflation and overcrusting and morphometric characters on the moon subsurface cavities have been inferred from several skylights in maria smooth plains and corroborated using gravimetry and radar sounder while on mars several deep skylights have been identified on lava flows with striking similarities with terrestrial cases nonetheless the literature on this topic is scattered and often presents inaccuracies in terminology and interpretation a clear understanding of the potential morphologies and dimensions of martian and lunar lava tubes remains elusive although it is still impossible to gather direct information on the interior of martian and lunar lava tube candidates scientists have the possibility to investigate their surface expression through the analysis of collapses and skylight morphology morphometry and their arrangement and compare these findings with terrestrial analogues in this review the state of the art on terrestrial lava tubes is outlined in order to perform a morphological and morphometric comparison with lava tube candidate collapse chains on mars and the moon by comparing literature and speleological data from terrestrial analogues and measuring lunar and martian collapse chains on satellite images and digital terrain models dtms this review sheds light on tube size depth from surface eccentricity and several other morphometric parameters among the three different planetary bodies the dataset here presented indicates that martian and lunar tubes are 1 to 3 orders of magnitude more voluminous than on earth and suggests that the same processes of inflation and overcrusting were active on mars while deep inflation and thermal entrenchment was the predominant mechanism of emplacement on the moon even with these outstanding dimensions with total volumes exceeding 1 billion of m3 lunar tubes remain well within the roof stability threshold the analysis shows that aside of collapses triggered by impacts tectonics most of the lunar tubes could be intact making the moon an extraordinary target for subsurface exploration and potential settlement in the wide protected and stable environments of lava tubes

modelling the interplate domain in thermo mechanical simulations of subduction critical effects of resolution and rheology and consequences on wet mantle melting abstract the present study aims at better deciphering the different mechanisms involved in the functioning of the subduction interplate a 2d thermo mechanical model is used to simulate a subduction channel made of oceanic crust free to evolve convergence at constant rate is imposed under a 100 km thick upper plate pseudo brittle and non newtonian behaviours are modelled the influence of the subduction channel strength parameterized by the difference in activation energy between crust and mantle e a is investigated to examine in detail the variations in depth of the subduction plane down dip extent z coup first simulations show that numerical resolution may be responsible for an artificial and significant shallowing of z coup if the weak crustal layer is not correctly resolved second if the age of the subducting plate is 100 myr subduction occurs for any e a the stiffer the crust is that is the lower e a is the shallower z coup is 60 km depth if e a 20 kj mol and the hotter the fore arc base is conversely imposing a very weak subduction channel e a 135 j mol leads there to an extreme mantle wedge cooling and inhibits mantle melting in wet conditions partial kinematic coupling at the fore arc base occurs if e a 145 kj mol if the incoming plate is 20 myr old subduction can occur under the conditions that the crust is either stiff and denser than the mantle or weak and buoyant in the latter condition cold crust plumes rise from the subduction channel and ascend through the upper lithosphere triggering 1 partial kinematic coupling under the fore arc 2 fore arc lithosphere cooling and 3 partial or complete hindrance of wet mantle melting z coup then ranges from 50 to more than 250 km depth and is time dependent if crust plumes form finally subduction plane dynamics is intimately linked to the regime of subduction induced corner flow two different intervals of e a are underlined 80 120 kj mol to reproduce the range of slab surface temperature inferred from geothermometry and 10 40 kj mol to reproduce the shallow hot mantle wedge core inferred from conditions of last equilibration of near primary arc magmas and seismic tomographies therefore an extra process controlling mantle wedge dynamics is needed to satisfy simultaneously the aforementioned observations a mantle viscosity reduction by a factor 4 20 caused by metasomatism in the mantle wedge is proposed from these results i conclude that the subduction channel down dip extent z coup should depend on the subduction setting to be consistent with the observed variability of sub arc depths of the subducting plate surface

limited role for thermal erosion by turbulent lava in proximal athabasca valles mars the athabasca valles flood lava is among the most recent 50 ma and best preserved effusive lava flows on mars and was probably emplaced turbulently the williams et al 2005 model of thermal erosion by lava has been applied to what we term proximal athabasca the 75 km long upstream portion of athabasca valles for emplacement volumes of 5000 and 7500 km3 and average flow thicknesses of 20 and 30 m the duration of the eruption varies between 11 and 37 days the erosion of the lava flow substrate is investigated for three eruption temperatures 1270 c 1260 c and 1250 c and volatile contents equivalent to 0 65 vol bubbles the largest erosion depths of 3 8 7 5 m are at the lava source for 20 m thick and bubble free flows that erupted at their liquidus temperature 1270 c a substrate containing 25 vol ice leads to maximum erosion a lava temperature 20 c below liquidus reduces erosion depths by a factor of 2 2 if flow viscosity increases with increasing bubble content in the lava the presence of 30 50 vol bubbles leads to erosion depths lower than those relative to bubble free lava by a factor of 2 4 the presence of 25 vol ice in the substrate increases erosion depths by a factor of 1 3 nevertheless modeled erosion depths consistent with the emplacement volume and flow duration constraints are far less than the depth of the channel 35 100 m we conclude that thermal erosion does not appear to have had a major role in excavating athabasca valles

development of the 1990 kalapana flow field kilauea volcano hawaii the 1990 kalapana flow field is a complex patchwork of tube fed pahoehoe flows erupted from the kupaianaha vent at a low effusion rate approximately 3 5 m3 s these flows accumulated over an 11 month period on the coastal plain of kilauea volcano where the pre eruption slope angle was less than 2 the composite field thickened by the addition of new flows to its surface as well as by inflation of these flows and flows emplaced earlier two major flow types were identified during the development of the flow field large primary flows and smaller breakouts that extruded from inflated primary flows primary flows advanced more quickly and covered new land at a much higher rate than breakouts the cumulative area covered by breakouts exceeded that of primary flows although breakouts frequently covered areas already buried by recent flows lava tubes established within primary flows were longer lived than those formed within breakouts and were often reoccupied by lava after a brief hiatus in supply tubes within breakouts were never reoccupied once the supply was interrupted during intervals of steady supply from the vent the daily areal coverage by lava in kalapana was constant whereas the forward advance of the flows was sporadic this implies that planimetric area rather than flow length provides the best indicator of effusion rate for pahoehoe flow fields that form on lowangle slopes

observations on basaltic lava streams in tubes from kilauea volcano island of hawai i from 1986 to 1997 the pu u o o kupaianaha eruption of kilauea produced a vast pahoehoe flow field fed by lava tubes that extended 10 12 km from vents on the volcano s east rift zone to the ocean within a kilometer of the vent tubes were as much as 20 m high and 10 25 m wide on steep slopes 4 10 a little farther away from the vent some tubes formed by roofing over of lava channels lava streams were typically 1 2 m deep flowing within a tube that here was typically 5 m high and 3 m wide on the coastal plain 1 tubes within inflated sheet flows were completely filled typically 1 2 m high and several tens of meters wide tubes develop as a flow s crust grows on the top bottom and sides of the tubes restricting the size of the fluid core the tubes start out with nearly elliptical cross sectional shapes many times wider than high broad flat sheet flows evolve into elongate tumuli with an axial crack as the flanks of the original flow were progressively buried by breakouts temperature measurements and the presence of stalactites in active tubes confirmed that the tube walls were above the solidus and subject to melting sometimes the tubes began downcutting progressive downcutting was frequently observed through skylights a rate of 10 cm d was measured at one skylight for nearly 2 months

geochemistry of mineralised and barren komatiites from the perseverance nickel deposit western australia the perseverance formerly known as agnew and the neighbouring rocky s reward nickel deposits are associated with metamorphosed komatiite flows erupted onto a substrate of felsic crystal tuffs the deposits are overlain by the perseverance ultramafic complex which consists of a central dunite lens 700 m thick by 3 km wide flanked by olivine orthocumulates and spinifex textured komatiites a suite of samples of a zones of komatiite flows from the area has been analysed for major and trace elements rare earth elements and platinum group elements the samples fall into two distinct geochemical groups on the basis of major and rare earth elements 1 1 samples from a zones of spinifex textured flows flanking the central dunite show well constrained linear trends of major and trace elements typical of komatiite suites related by simple olivine fractionation 2 2 samples from the perseverance mine mineralised and basal flows from rocky s reward and from unmineralised flows at the base of the perseverance ultramafic complex all show evidence for lree enrichment relative to the first group they are also generally higher in sio2 and lower in feo for a given mgo content geochemical features of the second group are consistent with the effects of crustal assimilation this is in line with current models for nickel sulphide genesis based largely on kambalda which emphasise the role of thermal erosion of sulphidic footwall sediments the chalcophile element ni ir ru pt pd au and cu concentrations in a zone samples from the study area are typical of komatiites komatiites from the perseverance mineralised units and from rocky s reward show evidence of depletion of the more strongly chalcophile pge relative to ni and cu this feature is also evident in massive and matrix sulphide ore samples although less evident in the main perseverance cloud sulphides quantitative geochemical computer modelling indicates that the major and rare earth element chemistry of the samples analysed is consistent with derivation by combined assimilation of felsic material and crystallisation of olivine from a primitive komatiite magma the parental composition is represented by the spinifex textured units flanking the dunite lens compositions of komatiite liquids sulphides and cumulus olivines can be modelled successfully by varying degrees of batch equilibration between komatiite magma olivine and assimilated sulphidic sediment models involving combined assimilation and fractional crystallisation in which olivine and sulphide are fractionally segregated as they form are much less successful at matching the data in physical terms this suggests that the mineralisation is due to wholesale assimilation of floor rocks close to the site of sulphide accumulation comparison with quantitative computer model trends suggests an average ratio of assimilation to olivine fractionation in the order of 0 5 to 1 in order to generate the observed degrees of lree enrichment in flowtops ni depletion in olivine and pge depletion in sulphides and flow tops

a reassessment of the emplacement and erosional potential of turbulent low viscosity lavas on the moon we have reevaluated the role of thermal erosion by low viscosity lunar lavas as a mechanism for the formation of the lunar sinuous rules we have adapted the model of williams et al 1998 and used the compositions of an apollo 12 basalt and a terrestrial komatiitic basalt to investigate the compositional and environmental effects on the flow behavior of low viscosity lavas on the moon and the earth our model predicts that lunar lavas could have erupted as turbulent flows that were capable of flowing hundreds of kilometers on a sufficiently flat unobstructed substrate these results are consistent with previous studies modeling of lavas over a substrate of the same composition shows that thermal erosion rates would have been low 10 cm d 1 as a result long duration eruptions approximately months to years would have been required to incise deep tens to hundreds of meters channels partial melting and mechanical removal of the substrate a mechanism suggested by hulme 1973 to enhance erosion only slightly increases thermal erosion rates other factors such as higher flow rates or lava superheating could have produced deep rules by thermal erosion during shorter duration eruptions a superheated lunar lava not only would have had a higher erosion rate 40 cm d 1 but also would have remained uncrusted for tens of kilometers which is consistent with the open channel morphology of most sinuous rules for lunar lavas with large volatile i e vesicle contents the presence of vesicles would have tended to increase viscosity at low strain rates resulting in shorter turbulent flow distances lower thermal erosion rates and thus shallower erosion channel depths for given eruption durations

emplacement and erosion by archean komatiite lava flows at kambalda revisited we have developed a mathematical model to evaluate the flow and erosional potential of submarine channelized komatiite lavas at kambalda western australia field data from kambalda were used to constrain the choice of important input parameters and model results were compared with data from field studies and geochemical analyses our results suggest that thermal erosion is strongly dependent upon the nature and behavior of the substrate if the substrate is treated as an unconsolidated hydrous sediment that can be fluidized by vaporized seawater then our model predicts that an initially 10 m thick basal kambalda komatiite lava could have produced very high thermo mechanical erosion rates 23 10 m day crustal thicknesses of 5 20 cm at distances of 5 35 km from the source and a high degree of lava contamination 3 12 in contrast if the substrate is treated as a more consolidated anhydrous sediment that could not be fluidized then our model predicts that a kambalda komatiite flow would have had much lower thermal erosion rates 1 2 0 4 m day and degrees of contamination 1 3 and would have had crustal thicknesses of 5 20 cm at longer flow distances of 30 165 km field constraints are generally consistent with our predictions for a non fluidized substrate the reentrant embayments at kambalda are thought to form from either erosion of deep channels in a flat basaltic seafioor huppert et al 1984 or erosion of a thin 5 m sediment with minor undercutting of basalt in pre existing topography lesher et al 1984 our modeling indicates that the former was possible for long eruption durations months whereas the latter was possible for short eruption durations 2 weeks as the latter hypothesis is more consistent with the existing field evidence for thermal erosion at kambalda we believe it is the preferred interpretation

ground melting and ocellar komatiites a lead isotopic study at kambalda western australia stratigraphically and geographically restricted ocellar komatiite flows at kambalda western australia appear to represent the products of ground melting of sulphidic sediments by komatiites in lava channels that localized the fe ni cu sulphide ores an immiscible sulphide liquid formed and gravitationally separated from the melted sediment xenomelt the resultant buoyant silicate liquid being partly or wholly assimilated by the turbulently convecting komatiite magma rarely the xenomelt gravitationally migrated to the top of flows and overflowed into the less turbulent lava levees where it collected to form a separate layer overlying a komatiitic layer within a single flow there was selective preservation of the hybrid felsic layer as an upper ocellar unit within an ocellar komatiite flow in lava levees flanking lava channels the ocellar unit is enriched in elements previously concentrated in the sediments and shows u th pb isotopic systematics akin to the underlying sediments moreover the partitioning relationships of u and pb between the immiscible xenomelt and sulphide liquid enhances the range of u pb ratios for components of the ocellar unit thus allowing sufficient spread of modern uranogenic pb isotopic ratios to form isochrons albeit imprecise ones the range and similarity of model th u data from these flows 2 8 3 9 and adjacent sulphidic sediments 2 3 4 4 mostly 2 8 3 9 contrasts with the generally invariable th u within kambalda ultrabasic basic flows 3 6 3 9 and further supports the ground melting hypothesis

nature and age of pre variscan eclogite protoliths from the low to medium grade metamorphic complex of north central sardinia italy and comparisons with coeval sardinian eclogites in the northern gondwana context u pb zircon data on retrogressed eclogites sampled in the giuncana locality from the sardinian medium grade metamorphic complex yielded a weighted age of 454 6 ma this is in agreement with u pb zircon ages of 453 460 ma obtained from eclogites from the high grade metamorphic complex the giuncana eclogites are very similar to the other well known sardinian eclogites all of the sardinian eclogites show positive k rb ba u and pb anomalies and negative nb la ce and sr anomalies th is depleted in the giuncana eclogites and enriched in those from punta de li tulchi and punta tittinosu all these data reveal clear crustal contamination of the sardinian ordovician mantle ree patterns typical of normal mid ocean ridge basalt n morb characterize all of the sardinian eclogites the supply of crustal and calc alkaline materials to the sardinian mantle during the ordovician is further confirmed by the fact that most sardinian eclogites plot on the left side and well above the mantle array in a th yb v nb yb diagram in the general variscan framework of northern gondwana the sardinian eclogites are witness to the most recent back arc basins generated by the northward opening of the rheic ocean

origin of lunar sinuous rilles modeling effects of gravity surface slope and lava composition on erosion rates during the formation of rima prinz 1 lunar sinuous rilles have long been interpreted as features that formed as the result of surficial lava flow though the precise mechanism responsible for channel formation constructed versus eroded origins is still debated in assessing the origin of rima prinz a channel interpreted to have formed by erosion two erosion regimes mechanical and thermal are considered measurements of channel dimensions are used as inputs to analytical models to constrain the origin of rima prinz including lava compositions mechanical and thermal erosion rates eruption durations and lava volumes required to form the feature key results indicate that rima prinz and other large sinuous rilles could have formed as the result of thermal erosion under the weak gravity and low slope conditions characteristic of these lunar features further analysis indicates that lava composition has significant effects on channel formation model results of four considered lava compositions show that komatiite like lava will erode a similarly composed substrate most efficiently whereas a high ti basalt will erode a similarly composed substrate least efficiently ocean island basalt and low ti basalt erode similarly composed substrates at intermediate rates results indicate that rima prinz may have formed over 0 4 2 2 earth years depositing 50 250 km3 of lava over a plausible deposit area of 2450 km2 resulting deposit thicknesses suggest that the lava that incised rima prinz was most likely similar in composition to a terrestrial komatiite ocean island basalt or lunar low ti basalt further constraints on sinuous rille formation will serve as a window into the nature of volcanic activity of the moon s past

volumetric observations during paroxysmal eruptions at mount etna pressurized drainage of a shallow chamber or pulsed supply abstract the october 17 to november 5 1999 eruption of mount etna s bocca nuova crater emplaced a 15 10 6 m 3 flow field the eruption was characterized by 11 paroxysmal events during which intense strombolian and lava fountain activity fed vigorous channelized a a flows at eruption rates of up to 120 m 3 s 1 each paroxysm lasted between 75 and 450 min and was separated by periods of less intense strombolian activity and less vigorous 3 s 1 effusion ground based satellite and model derived volumetric data show that the eruption was characterized by two periods during which eruption rates and cumulative volume showed exponential decay this is consistent with a scenario whereby the system was depressurized during the first eruptive period october 17 23 repressurized during an october 24 pause and then depressurized again during the second period october 25 28 the imbalance between the erupted and supplied volumes mean that the two periods involved the collection of 1 5 5 7 10 6 m 3 and 1 2 3 6 10 6 m 3 respectively or an increase in the time averaged supply to 11 6 13 6 m 3 s 1 and 12 5 14 9 m 3 s 1 two models are consistent with the observed episodic fountaining derived volumetric trends and calculated volume imbalance a magma collection model and a pulsed supply model in the former case depressurization of a shallow reservoir cause the observed volumetric trends and foam collapse at the reservoir roof powers fountaining in the pulsing case variations in magma flux account for pressurization depressurization and supply the excess volume increases in rise rate and volatile flux coupled with rapid exsolution during ascent trigger fountaining limiting equations that define critical foam layer volumes and magma rise rates necessary for hawaiian style fountaining favor the latter model

volcanic flow development at alba patera mars the hesperian amazonian age volcanic shield alba patera has associated with it some of the largest volcanic flows on mars an early hesperian flood lava phase was followed by the extrusion of high volume sheet and tube fed lavas many of which emerged from fissures on the flanks of a rising dome subsequently a plethora of sheet and tube fed lavas were erupted from linear vents located either at or within 200 km of the present summit these built complex flow fields which were erupted at rates several orders of magnitude greater than the maximum hawaiian rates later amazonian volcanism at alba saw extrusions of long narrow flows from two complex summit calderas whose formation is inferred to have been a direct result of magma supply to the volcano s flanks the outflow of thermal energy related to emplacement of some single flows 6 1028 ergs was substantially greater than earth s annual heat loss through volcanism and implies that patera building that have been a significant source of geologically rapid heat loss from mars during the early stages of volcanic centralization on the planet

the geology of the kambalda nickel field western australia the kambalda nickel field contains the major concentration of volcanic peridotitc associated nickel ores in the archean of australia it occupies an area of 400 km 2 underlain by two sequences of ultramafic mafic and felsic volcanics and sedimentary rocks which can be broadly correlated with previously erected regional stratigraphic sections most ores occur in the lower kambalda sequence at the base of the kambalda ultramafic rocks the ultramafic rocks of the upper bluebush sequence contain only minor mineralization four phases of deformation occurred over an interval of more than 300 m y resulting in a dominant north northwest structural trend faulting was active during original volcanism and continued intermittently until after mafic felsic dike intrusion the ultramafic rocks have been serpentinized and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism the kambalda ultramafic rocks are of variable thickness and in places exceed 1 000 m the lower member of the ultramafic rocks occupying approximately one third of the sequence comprises thicker high magnesium flows i e generally 36 volatile free mgo commonly separated by sulfidic quartz albite sediments thinner lower magnesium flow units characterize much of the upper member the ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few grouped ribbonlike original orebodies eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt ultramafic contact these troughs are thought to be original volcanic depressions which focused later deformation the remainder is hanging wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow hanging wall orebodies occasionally grade into stratigraphically equivalent interflow sediments the ores contain varying proportions of massive matrix and disseminated sulfides the hypogene mineral assemblage is pyrrhotite pentlandite pyrite chalcopyrite but millerite is locally significant the massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position metamorphism has caused recrystallization of the ores and some redistribution of elements there is commonly marked variation of mineralogy and s ni ratios between contact and overlying hanging wall ores and within and between original ore surfaces within shoots but the relative contributions of primary composition and metamorphism to this variation are uncertain in ore environments the footwall basalt ultramafic contact is more deformed than in non ore environments the basal ultramafic flows are thicker and richer in magnesium the trend toward lower magnesium flows upward through the ultramafic formation is less well developed and there is better textural development and compositional differentiation within flows non ore environments usually contain interflow sediments whereas there is generally a sediment free prism of ultramafic rocks above ore environments most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor volcanic exhalations probably contributed to minor ore in sulfidic sediments

contemporaneous ultramafic and felsic intrusive and extrusive magmatism in the archaean boorara domain eastern goldfields superterrane western australia and its implications abstract two komatiite emplacement styles are recognized in the boorara domain of the kalgoorlie terrane eastern goldfields superterrane yilgarn craton western australia 1 large volume dominantly cumulate textured komatiite bodies interpreted as high level intrusions based on the presence of symmetrical spinifex cumulate textural zonation together with apophysy injection and xenolith incorporation along the top and basal margins of the komatiite 2 extrusive komatiite lavas exhibit asymmetrical textural zonation with the top chilled flow margin preserving a fine grained fractured crust that coarsens downward into a supercooled former olivine spinifex zone and then into pseudomorphed olivine now serpentine orthocumulate texture above a thin basal chilled margin the intrusive komatiite unit is observed to be intercalated with quartz and feldspar phyric dacite the two lithologies exhibit primary irregular contacts against each other komatiite apophyses have been injected into the dacite and magma mingling has occurred these features are indicative of liquid liquid interaction between the two units and komatiite magmatism is interpreted to have been contemporaneous with dacitic magmatism the dacite was emplaced first and the komatiite intruded the dacite before it solidified the reconstructed stratigraphy of the boorara domain comprises a basal unit of quartzofeldspathic turbidite beds conformably overlain by high mg tholeiite basalt the intercalated dacite and komatiite units overlie the lower tholeiitic basalt with an unobserved contact an upper komatiitic basalt unit overlies the komatiite and at the stratigraphic top of the sequence are felsic volcaniclastic turbidite units with felsic porphyry intrusions these greenstone lithologies were emplaced within a deep submarine basin as indicated by carbonaceous metapelite units intercalated throughout the sequence the tectonic scale processes operating in and around the boorara domain depocentre include derivation of the komatiite unit from a likely mantle plume source the contemporaneous dacite magmatism is interpreted to be the result of melting of pre existing archaean crust during plume ascension although the exact composition of the primitive crust is yet to be determined

a model for the formation of lava tubes by roofing over a channel the formation of lava tubes is a common phenomenon on some basaltic volcanoes such as etna a model for tube formation by roofing of a channel is proposed and involves first describing lava as a bingham liquid flowing down a slope it is further assumed that lava flows in a channel with rectangular cross section as a result of heat loss into the atmosphere a crust is gradually formed on the upper surface of the flow and this crust eventually welds to the channel levees we assume that a lava tube is formed when such a crust is sufficiently thick to resist the drag of the underlying flow and to sustain itself under its own weight the minimum thickness of the crust satisfying such conditions depends on the tensile strength and shear strength of the crust itself assuming that the growth of the crust produces a downflow linear increase of the shear stress at the interface between flowing lava and the crust the distance is evaluated between the eruption vent and the point where the tube is formed the model predicts that if the flow rate is constant the thickness of the flow increases as the crust fragments grow and weld to each other and the velocity of the crust decreases to zero once the lava tube is formed the initial flow rate can be achieved by a flow thickness smaller than the vertical size of the tube with the same viscous dissipation this may explain why under steady state conditions the lava level inside a tube is frequently lower than the roof of the tube itself

the langmuir volcanic peridotite associated nickel deposits canadian equivalents of the western australian occurrences the 2 7 b y old abitibi greenstone belt is a crudely s shaped zone of metamorphosed volcanic subvolcanic and sedimentary rocks and associated intrusions through it are scattered a dozen small nickel deposits in volcanic peridotites their hosts are volcanic and subvolcanic ultramafic rocks typically containing 40 to 45 percent anhydrous mgo with spinifex tops and overlain by thinner peridotitic to pyroxenitic flows at least one deposit dumont is hosted by a dunitic intrusion others e g dundonald langmuir 2 are partially hosted in sulfidic iron formations mining of the langmuir 2 deposit permitted extensive underground mapping core logging and sample collecting the other deposits are known mainly from poor surface exposures and drill core the langmuir 1 and 2 deposits provided sufficient data to evaluate models for their origin and to compare them to nickel sulfide deposits associated with volcanic peridotites in western australia a magmatic origin is supported by the localization of the main ore zone in a paleotrough by the presence of a possible volcanic feeder beneath the ore and by the regular sequence through the basal ultramafic flow of massive ore overlain by net textured ore then by barren peridotite and lastly by a spinilex textured cap to the flow an unusual spinilex textured and sulfide bearing rock and skeletal chromite also support a magmatic origin as do the base and precious metal contents of the ores the high s se ratios averge 23 000 and slightly lower sulfur isotopic ratios average 0 5 per mil delta 34 s compared to most magmatic nickel sulfide deposits are similar to the ratios for windarra western australia and may be explained by assimilation of sulfide rich cherty iron formation other features incompatible with a purely maginatic origin include the presence of sheared and breccia ore and the high sulfur content of some ores these features result from deformation and greenschist facies metamorphism but surviving features of an earlier magmatic history argue against the ores being formed metamorphically evidence of contemporaneous fumarolic activity was found at the south end of the mine but its importance as an ore forming process was not supported by detailed petrological and geochemical studies limited data from the other deposits in the abitibi belt suggest a common origin for the langmuir deposits with minor variations attributable to the volume of ultramafic magma emplaced the volume of contained nickel sulfides the presence of sulfidic iron formations and the severity of alteration and metamorphism the deposits in western australia probably were formed by similar processes in particular the langmuir deposits display strikingly similar sulfide chemistries to the windarra deposits which are probably dae to assimilation of sulfide rich cherty iron formation