fracture patterns at lava ice contacts on kokostick butte or and mazama ridge mount rainier wa implications for flow emplacement and cooling histories abstract cooling lava commonly develop polygonal joints that form equant hexagonal columns such fractures are formed by thermal contraction resulting in an isotropic tensional stress regime however certain linear cooling fracture patterns observed at some lava ice contacts do not appear to fit the model for formation of cooling fractures and columns because of their preferred orientations these fracture types include sheet like ladder like rectangular fracture pattern intermediate pseudo aligned individual column bounding fractures and pseudopillow straight to arcuate fractures with perpendicular secondary fractures caused by water infiltration fractures that form the edges of multiple columns along a single linear fracture despite the relatively common occurrence of these types of fractures at lava ice contacts their significance and mode of formation have not been fully explored this study investigates the stress regimes responsible for producing these unique fractures and their significance for interpreting cooling histories at lava ice contacts data was collected at kokostick butte dacite flow at south sister or and mazama ridge andesite flow at mount rainier wa both of these lava flows have been interpreted as being emplaced into contact with ice and linear fracture types have been observed on their ice contacted margins two different mechanisms are proposed for the formation of linear fracture networks one possible mechanism for the formation of linear fracture patterns is marginal bulging melting of confining ice walls will create voids into which flowing lava can deform resulting in margin parallel tension causing margin perpendicular fractures if viewed from the ice wall these fractures would be steeply dipping linear fractures another possible mechanism for the formation of linear fracture types is gravitational settling pure shear during compression and settling can result in a tensional environment with similar consequences as marginal inflation in addition to this horizontally propagating cooling fractures will be directly influenced by viscous strain caused by the settling of the flow this would cause preferential opening of fractures horizontally resulting in vertically oriented fractures it is important to note that the proposed model for the formation of linear fractures is dependent on contact with and confinement by glacial ice the influence of flow or movement on cooling fracture patterns has not been extensively discussed in previous modeling of cooling fractures rapid cooling of lava by the interaction with water and ice will increase the ability to the capture and preserve perturbations in the stress regime

new models for the origin of valles marineris closed depressions current models for the formation of the abundant large closed depressions of valles marineris have serious shortcomings purely tectonic mechanisms are inconsistent with the morphology of many depressions and removal of large quantities of ground ice from the canyon walls is inconsistent with the observed strength of the walls accordingly we offer some alternatives one possibility involves decay of ice rich bodies occupying partially sediment choked ancient graben that predated the overlying cratered and ridged plains other possibilities involve the removal of massive equatorial carbonate deposits storing much of the planet s co2 inventory generated during greenhouse conditions on early mars solution by carbonic acid derived from the atmosphere analogous to terrestrial karst requires extensive recycling of the available water supply solution by various groundwater acids possibly derived from the tharsis magmas requires less water especially if only the smaller closed depressions are due to carbonate decay alternatively volume loss due to decarbonation of carbonate during early high heat flow or a later tharsis related heat pulse can produce extensive collapse especially if the carbonates have high silica content

fracture and surface crust development in a holocene pahoehoe lava flow on the island of tenerife canaries abstract an almost horizontal pahoehoe surface in a holocene plagioclase basanite lava on tenerife displays three scales of fracture within the surface crust an early formed set of large scale fractures divides up the surface into an orthogonal set of rectangular slabs with dimensions of several metres and depths of 10 12 cm the shortest slab dimension is parallel to the flow emplacement direction inferred from a strong surface lineation the slabs are domed with the centre an average of 9 6 cm with range 4 19 6 cm above the edges of the slabs profiles of the slabs normal and perpendicular to the margins and through the crest indicate that they can be described by a power law in which the deflection of the slab h is related to the distance from the crest x with an exponent between 2 and 3 analysis of joints within the slabs indicates two smaller scale networks an intermediate scale joint network bounds blocks with rectilinear to polygonal shapes in plan view and has a characteristic mean spacing of 24 2 cm range 10 5 48 cm the major fractures in this set are normal and parallel to the slab margins a smaller scale joint network bounds polygonal equant blocks in plan view and has characteristic spacing of 6 4 cm range 3 7 10 5 cm a model of cooling from the pahoehoe surface is used to constrain the growth of the crust and timing of fracture development the large scale slabs are attributed to localised accumulation of gas beneath the growing crust causing buoyant forces the tensile stresses caused by uplift are sufficient to form the large scale fractures after 2 or 3 h of cooling the intermediate scale fracture network is attributed to the flexure of the slab crust the smaller scale polygonal joint network is related to the build up of isotropic tensile stresses in the cooling slab crust due to thermal contraction with fracture development being promoted by the flexure of the slabs an analysis of the slab deformation indicates that lava crust is weak the weakness is explained by division of the crust into three zones an outer zone with small scale joints that cause negligible strength a middle zone of elastic behaviour in which stress can build up and a lower zone of plastic deformation the crustal slabs display profiles similar to that expected in a bending elastic plate the deformation of the 10 cm thick crust can be explained if the elastic zone was about 2 cm thick this result agrees with an independent calculation of elastic zone thickness based on the position of the brittle ductile transition being located at the 600 c isotherm at a depth of about 2 cm when the crustal slabs were rifted apart

tumulus development on lava flows insights from observations of active tumuli and analysis of formation models here we use observations of active flows along with detailed morphometric field measurements of more than 70 tumuli on flows at mount etna italy kilauea and hualalai us volcanoes to constrain a previously published model that estimates the pressure needed to form tumuli in an attempt to discover the nature and magnitude of pressure variations within active lava flow interiors we then consider how tumuli differ from idealized circular plates we incorporate observations of active tumuli and find that they may grow asymmetrically yet produce a symmetrical tumulus and can form where the flow path significantly changes direction bending models of clamped edges provide the most reasonable head estimates for the tumuli in our study tumulus formation requires the proper combination of cooling and effusion rate if cooling is too extensive and effusion rate too low the crust will provide too much resistance to bending if cooling is too limited and effusion rates too high crusts will not develop or have insufficient strength to resist fracture and subsequent breakouts we do not find it surprising that tumuli are rarely found over well established lava tubes that typically have rigid walls overlying crusts that exceed 2 m in thickness and provide too much resistance to bending silicic flows lack tumuli because the viscosity gradients within the flow are insufficient to concentrate stress in a localized area

crease structures indicators of emplacement rates and surface stress regimes of lava flows crease structures are features commonly found on lava flow surfaces and consist of a fracture with curved walls that extend outward from a linear valley these crease structures are found on flows of nearly all compositions and crystallinities we have mapped the distributions of crease structures on many flows in the western united states and found that 1 axial length is not dependent upon composition and crystallinity 2 adjacent crease structures are generally aligned in an en echelon pattern 3 crease structures located adjacent to flow margins are generally perpendicular to these margins and 4 at mount st helens washington large lobe bisecting crease structures are found on lobes situated on slopes of less than 20 degrees a primary surface feature found on many crease structures is striations striations are sets of long stripes on the walls of the crease structure that extend approximately parallel to the axis of the central valley and they appear to be analogous to those found on the faces of cyclically fractured basalt columns observations of developing crease structures on 6 of the nearly 20 mount st helens dome lobes show that they form throughout the extrusion of flows situated on slopes of less than 20 degrees but only at the very beginning and or end of extrusion of flows on steeper slopes these observations imply that crease structures form when the lava flow is forced to spread laterally either as the flow advances over a flat area or as the down slope movement stagnates near the end of extrusion causing the rate of spreading to exceed the downslope rate of flow this lateral spreading of lava results in the concentration of tensile stress along a line oriented perpendicular to the direction of spreading the cooled crust of the extrusion is therefore torn apart about this line of tensile stress concentration forming a central valley that exposes hot ductile material from the flow interior to the atmosphere the presence of striations on many crease structure walls implies that the emplacement mechanism is similar to that suggested for columnar basalts where each striation is produced by incremental fracturing the stress responsible for crease structure formation is due to both cooling and lateral spreading of lava calculations show that stress due to spreading is three orders of magnitude higher than that due to cooling fracture advance times were calculated using a simple one dimensional conductive cooling model adjusted for the special geometry of the crease structure the model closely approximates measured crease structure emplacement times at mount st helens and gives reasonable estimates for crease structures found on older silicic lava flows we then used this model to calculate emplacement times of entire domes that consist of a single crease structure by dividing the dome volume by the formation time we are able to calculate extrusion rates for these older domes these values which include the first flow rate estimates reported for rhyolitic lavas 0 03 106 m 3 s similar to average flow rates of 0 7 40 m 3 s calculated for mount st helens dacite lobes from detailed topographic maps

structural and petrophysical characterization of the upper basement crustal section at odp iodp site 1256 east pacific ocean 1 new ocean crust is constantly being formed from mid ocean ridge axis voluminous flows of lava are emplaced also away from the ridge axis adding off axis crustal layers to the crustal pile research on deep sea is of great importance to better understand the mechanisms and the nature of the crust forming and evolution in this work we decipher the first stages of the postmagmatic evolution of an intact volcanic section from the upper oceanic crust at ocean drilling program integrated ocean drilling program odp iodp site 1256 eastern pacific ocean using for the first time an innovative core log integration technique to match direct core related and indirect borehole related data by depth shifting and reorienting individual core pieces recovered by drilling we are able to identify the clusterization of structures and physical properties within distinct downdeep strong and weak lava zones reflecting the cooling and tectonic evolution of lavas rather than lithological variations we define the evolution of the structural zones that typically affect lava flows colonnades and entablature zones studying an off axis lava flow encountered in present day upper ocean crust for the same off axis flow we are also able to suggest the lava flow direction nw se and its relationships with the paleoridge axis despite the environmental difficulties in the study of the subseafloor under deep water and using only one dimensional data deriving from ocean drilling this work shows how an array of diverse data can be integrated into a coherent interpretation of lava flow history obtaining detailed information on the mechanism of submarine lava emplacement and flow

evoluton of polygonal fracture patterns in lava flows cooling induced fractures also known as columnar joints divide basaltic lava flows into prismatic columns with polygonal cross sections the regularity and symmetry of the fracture patterns have long fascinated naturalists in view of the recent selection of two candidate nuclear waste sites in areas where polygonally fractured volcanic rocks are located a better understanding of the fracture patterns is required field data indicate that the tetragonal networks at flow surfaces evolve systematically to hexagonal networks as the joints grow inward during solidification of lava this evolution occurs by the gradual change of most orthogonal intersections to nonorthogonal intersections of about 120 degrees the surface features and intersection geometries of columnar joints show that joint segments at any given level form sequentially yet harmoniously

conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone a conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone was developed based on the results of lithological studies and a series of ponded infiltration tests conducted at the box canyon site near the idaho national engineering and environmental laboratory the infiltration tests included one 2 week test in 1996 three 2 day tests in 1997 and one 4 day test in 1997 for the various tests initial infiltration rates ranged from 4 1 cm d 4 75 10 7 m s to 17 7 cm d 2 05 10 7 m s and then decreased with time presumably because of mechanical or microbiological clogging of fractures and esicular basalt in the near surface zone as well as the effect of entrapped air the subsurface moisture redistribution was monitored with tensiometers neutron logging time domain reflectrometry and ground penetrating radar a conservative tracer potassium bromide was added to the pond water at a concentration of 3 g l to monitor water flow with electrical resistivity probes and water sampling analysis of the data shows evidence of preferential flow rather than the propagation of a uniform wetting front we propose a conceptual model describing the saturation desaturation behavior of the basalt in which rapid preferential flow occurs through the largest vertical fractures followed by a gradual wetting of other fractures and the basalt matrix fractures that are saturated early in the tests may become desaturated thereafter which we attribute to the redistribution of water between fractures and matrix lateral movement of water takes place within horizontal fracture and rubble zones enabling development of perched water bodies

near n s paleo extension in the western deccan region india does it link strike slip tectonics with india seychelles rifting this is the first detailed report and analyses of deformation from the w part of the deccan large igneous province dlip maharashtra india this deformation related to the india seychelles rifting during late cretaceous early paleocene was studied and the paleostress tensors were deduced near n s trending shear zones lineaments and faults were already reported without significant detail an e w extension was envisaged by the previous workers to explain the india seychelles rift at 64 ma the direction of extension however does not match with their n s brittle shear zones and also those faults sub vertical ne sw nw se and few n s we report and emphasize in this work slickenside bearing fault planes brittle shear zones and extension fractures in meso scale enabled us to estimate the paleostress tensors directions and relative magnitudes the field study was complemented by remote sensing lineament analyses to map dykes and shear zones dykes emplaced along pre existing n s to ne sw nw se shears fractures this information was used to derive regional paleostress trends a nw se ne sw minimum compressive stress in the oldest kalsubai subgroup and a n s direction for the younger lonavala wai and salsette subgroups were deciphered thus a nw ne to n s extension is put forward that refutes the popular view of e w india seychelles extension paleostress analyses indicate that this is an oblique rifted margin field criteria suggest only ne sw and nw se with some n s strike slip faults brittle shear zones we refer this deformation zone as the western deccan strike slip zone wdsz the observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies these geophysical findings too indicate oblique rifting in this part of the w indian passive margin we argue that the seychelles microcontinent separated from india only after much of the dlip erupted further studies of magma rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required

column on column structures as indicators of lava ice interaction ruapehu andesite volcano new zealand abstract lava flows of the mangawhero formation ca 15 60 ka on ruapehu volcano erupted during the last glaciation in a distal flow lobe at tukino on the east side of the mountain small secondary columns 10 20 cm thick have formed on the sides of large rectangular primary 0 5 3 m thick cooling columns thick 10 m zones of such small columns form a lateral and basal outer rind of the lobe as they do not mark glassy zones of quenching these secondary columns are interpreted as being formed by a second cooling event at temperatures below the boundary between the low creep and elastic regimes 600 c by rapid influx of copious amounts of water temperature drops deduced from extensional strains of the two sets of columns were used to gauge the viability of such a two stage process absence of reliable data on andesite contraction coefficients was overcome by using a sliding scale to assess a large range of values the estimates indicate that two stage chilling is feasible after flowing across relatively ice poor terrain the lava flow must have interacted with a valley glacier that provided water for further chilling the already formed primary columns and formation of the outer rind small columns given this evidence for lava ice interaction it is likely that prominent thick flows elsewhere in the mangawhero formation may have been constrained to their ridge top locations by ice conditions similar to those described by lescinsky and sisson lescinsky d t sisson t w 1998 ridge forming ice bounded lava flows at mount rainier washington geology 26 351 354

yttrium zoning in metamorphic garnets abstract yttrium zoning has been measured in garnets from the straits schist connecticut usa using the synchrotron x ray fluorescence sxrf microprobe sxrf compositional maps for mn and fe show that the major elements are concentrically zoned about the gannet core in the classic bell shaped profiles considered indicative of growth zoning yttrium however displays oscillatory zoning which varies three orders of magnitude in abundance from about 3 to 4000 ppm and is correlated with textural zoning in garnet the major element abundances by contrast display no visible inflection related to textural zoning at this time it is not possible to unequivocally identify the mechanism which has produced this oscillatory zoning most likely this reflects either the breakdown of y enriched phases changes in the rate of garnet growth with time or changes in the effective y concentration of matrix phases due to breakdown and reprecipitation of garnet as the equilibrium reactions involving gannet change this study demonstrates that the sxrf microprobe provides excellent spatially resolved compositional maps for trace elements in porphyroblasts something other analytical techniques have been unable to do it also suggests that the diffusion rates for y in garnet are probably significantly slower than those for major cations such as mn fe mg and ca as a result trace element mapping potentially provides a clearer record of the reaction history of the rock provides details of how chemical zoning in garnet is produced and allows evaluation of assumptions about chemical equilibrium between minerals

contrasting magma types and steady state volume predictable basaltic volcanism along the great rift idaho the great rift is an 85 km long 2 to 8 km wide volcanic rift zone in the snake river plain idaho three latest pleistocene to holocene basaltic lava fields craters of the moon kings bowl and wapi are located along the great rift the craters of the moon lava field is a composite of more than 60 lava flows 25 cinder cones and at least 8 eruptive fissure systems it covers 1 600 km2 and contains 30 km3 of lava flows and associated pyroclastic deposits field radiocarbon and paleomagnetic data show that the craters of the moon lava field formed in eight eruptive periods each of which was typically several hundred years or less in duration and was separated from others before and after by intervals of several hundred to several thousand years the first eruptive period began 15 000 yr b p and the latest eruptive period ended 2100 yr b p the small kings bowl lava field 3 3 km2 0 01 km3 and the larger wapi lava field 330 km2 6 km3 both formed 2250 yr b p three magma types have fed flows along the great rift the types are 1 a contaminated type that has a sio2 range of 49 64 and commonly shows petrographic evidence of contamination 2 a fractionated type that has a sio2 range of 44 54 and shows no evidence of contamination and whose chemical and mineralogical variation can be accounted for mainly by crystal fractionation and 3 a snake river plain type that has a sio2 range of 45 48 shows little evidence of fractionation and is represented by kings bowl wapi flows and olivine basalts of the snake river plain the contaminated and fractionated magma types were erupted at the craters of the moon lava field and the snake river plain magma type was erupted at the kings bowl and wapi lava fields these relations imply that the magma reservoirs are spatially isolated the magma output rate for the craters of the moon segment of the great rift was constant at 1 5 km3 1 000 yr for the period from 15 000 to 7000 yr b p the rate increased to 2 8 km3 1 000 yr from 7000 to 2000 yr b p mainly as a result of the addition of contaminated magma to the nearly constant output rate of fractionated magma the craters of the moon segment of the great rift has experienced quasi steady state volume predictable volcanism for the last 15 000 yr the recurrence interval of eruptive activity for the craters of the moon lava field ranges from several hundred to 3 000 yr because the present interval has lasted 2 100 yr another eruptive period seems likely to occur within the next 1 000 yr the steady state volume predictable relationship suggests that 5 6 km3 of lava will be erupted in the next eruptive period

thermo mechanical model for incremental fracturing in cooling lava flows abstract this study presents an idealized quantitative model of stress development and incremental fracturing in cooling lavas it appears that columnar jointing forms in lavas by a process of incremental fracturing driven by thermal stresses the fractures progress inward from the cooling surfaces as time increases the cooling of a horizontal layer of lava has been modeled analytically for three cooling regimes conductive cooling with liquid magma present conductive cooling after total solidification and hydrothermal cooling horizontal thermal stresses develop and cause vertical fractures at the upper and lower surfaces of the lava as the lava cools layer after layer becomes elastic thermal stresses develop in these thin layers between previously fractured lava where it is assumed stresses have been released and the hot interior lava where temperatures exceed the elastic temperature limit stresses are analyzed by treating each unfractured elastic layer as a slab restrained in bending unrestrained from contraction the stress level at which incremental fracturing occurs extending previous fractures and the effect of stress and temperature conditions on fracture penetration are discussed our analysis yields values for the length of incremental fracture as a function of time and material strength for both a thin lava flow cooling conductively incremental fracture lengths increase with time and for a thicker lava flow in which hydrothermal convection is important incremental fracture lengths are constant with time our results are compared to observed striae widths measured from the base of basaltic lava columns striae represent successive incremental fractures the comparison shows agreement in magnitude and trend after some amount of cooling i e striae and incremental fracture lengths increase with time at an accelerating rate

dating aboriginal stone walled fishtraps at lake condah southeast australia abstract direct dating of stone walled fishtraps has been a methodological challenge in archaeology and is generally considered insurmountable dating is usually associative linking traps to local archaeological sites and geomorphological features of known age limited excavation of sediments burying the lower sections of stone walled fishtrap features has been previously undertaken with limited success recent fine grained excavation and comprehensive ams dating and analysis of channel in fill sediments associated with an elaborate freshwater fishtrap complex at lake condah western victoria yields reliable insights into the phased construction and use of the feature an early phase of basalt bedrock removal to create a bifurcated channel was subsequently in filled with flood sediments incorporating stone artefacts and charcoal dated to c 6600 cal bp after a hiatus basalt blocks were added to the sides of the channel to create multi tiered walls within the past 600 800 years this site provides the first direct insights into the antiquity of the elaborate fishtrapping and aquaculture system developed by aboriginal people in the lake condah region and may represent one of the world s oldest known fishtraps

use of joint growth directions and rock textures to infer thermal regimes during solidification of basaltic lava flows abstract thermal contraction joints form in the upper and lower solidifying crusts of basaltic lava flows and grow toward the interior as the crusts thicken lava flows are thus divided by vertical joints that by changes in joint spacing and form define horizontal intraflow layers known as tiers entablatures are tiers with joint spacings less than about 40 cm whereas colonnades have larger joint spacings we use structural and petrographic methods to infer heat transfer processes and to constrain environmental conditions that produce these contrasting tiers joint surface morphology indicates overall joint growth direction and thus identifies the level in a flow where the upper and lower crusts met rock texture provides information on relative cooling rates in the tiers of a flow lava flows without entablature have textures that develop by relatively slow cooling and two joint sets that usually meet near their middles which indicate mostly conductive cooling entablature bearing flows have two main joint sets that meet well below their middles and textures that indicate fast cooling of entablatures and slow cooling of colonnades entablatures always occur in the upper joint sets and sometimes alternate several times with colonnades solidification times of entablature bearing flows constrained by lower joint set thicknesses are much less than those predicted by a purely conductive cooling model these results are best explained by a cooling model based on conductive heat transfer near a flow base and water steam convection in the upper part of an entablature bearing flow calculated solidification rates in the upper parts of such flows exceed that of the upper crust of kilauea iki lava lake where water steam convection is documented use of the solidification rates in an available model of water steam convection yields permeability values that agree with measured values for fractured crystalline rock we conclude therefore that an entablature forms when part of a flow cools very rapidly by water steam convection flooding of the flow top by surface drainage most likely induces the convection colonnades form under conditions of slower cooling by conductive heat transfer in the absence of water

conductive cooling of lava columnar joint diameter and stria width as functions of cooling rate and thermal gradient abstract much evidence exists that column diameter and stria width of columnar joints in lava flows vary inversely with cooling rate t t and thermal gradient t x at the point of fracture as the flow cools some evidence suggests that lava flows cool conductively a conductive cooling model yields cooling rate and thermal gradient through space and time for cooling igneous bodies both for extrusive and intrusive settings thermal gradient and cooling rate are then linked to stria width and column diameter with faster cooling leading to narrower columns and steeper gradients to narrower striae this model shows large regions within a body where the ratio of thermal gradient to cooling rate is nearly constant and predicts that the ratio of stria width to column diameter is also nearly constant this prediction is consistent with field observations both in this and other works the model also suggests that if conductive cooling dominates then joint spacing i e column diameter should increase inward from the margins of the body as should stria width

pulsed inflation of pahoehoe lava flows implications for flood basalt emplacement abstract dilated fractures in hawaiian pahoehoe lava flows contain three zones that show the kinematics of inflation the upper columnar zone forms through thermal contraction prior to inflation the middle planar zone reflects inflation induced tension and the lower banded zone contains evidence of brittle and ductile deformation the formation of the lower banded zone requires varying strain rates during fracture propagation and is best explained by a model where small pulses of lava inject beneath the cooled flow crust through a network of preferred pathways we demonstrate via simple models of pipe flow that this inflation mechanism is incapable of producing areally extensive continental flood basalts on earth although it may explain related features on large martian volcanoes

physical constraints on geologic co2 sequestration in low volume basalt formations deep basalt formations within large igneous provinces have been proposed as target reservoirs for carbon capture and sequestration on the basis of favorable co 2 water rock reaction kinetics that suggest carbonate mineralization rates on the order of 10 2 10 3 d although these results are encouraging there exists much uncertainty surrounding the influence of fracture controlled reservoir heterogeneity on commercial scale co 2 injections in basalt formations this work investigates the physical response of a low volume basalt reservoir to commercial scale co 2 injections using a monte carlo numerical modeling experiment such that model variability is solely a function of spatially distributed reservoir heterogeneity fifty equally probable reservoirs are simulated using properties inferred from the deep eastern snake river plain aquifer in southeast idaho and co 2 injections are modeled within each reservoir for 20 yr at a constant mass rate of 21 6 kg s 1 results from this work suggest that 1 formation injectivity is generally favorable although injection pressures in excess of the fracture gradient were observed in 4 of the simulations 2 for an extensional stress regime as exists within the eastern snake river plain shear failure is theoretically possible for optimally oriented fractures if s h 0 70s v and 3 low volume basalt reservoirs exhibit sufficient co 2 confinement potential over a 20 yr injection program to accommodate mineral trapping rates suggested in the literature

morphology of rubbly pahoehoe simple flows from the deccan volcanic province implications for style of emplacement abstract lava flows with preserved bases and brecciated upper crusts constitute a morphological type that differs in character from typical pahoehoe and a a such flows have been reported from many provinces around the world previous studies had referred to these flows informally as pahoehoe flows with rubbly tops broken top pahoehoe and rubbly pahoehoe recent studies have formally applied the latter term to describe parts of the well studied laki flow in iceland as well as flows from the columbia river basalt province rubbly pahoehoe flows are abundant in the upper stratigraphic formations of the deccan volcanic province dvp and are more commonly known as simple flows this study presents detailed observations of such flows from various parts of the dvp and discusses their implications for understanding flow emplacement these flows which appear to be single units at the outcrop scale are generally much thicker and significantly more extensive than individual pahoehoe lobes that dominate the lower formations of the deccan stratigraphy they are characterised by preserved gently undulating tachylitic bases but variably disrupted crustal zones that grade into flow top breccias the breccias are constituted of highly vesicular and oxidised fragments of varying sizes that appear to have been derived from previously formed pahoehoe crusts previous work has indicated that the morphology of these flows might be related to initial inflation accompanied by rapid volatile exsolution and an increase in effusion rate and or viscosity with time this agrees reasonably well with the qualitative and quantitative models of emplacement developed for the laki flow the abundance of such flows in the upper formations of the deccan stratigraphy clearly hints at a significant shift in the nature of the deccan eruptions this could be indicative of higher eruption rates during this period this in turn raises the possibility of hazardous impact on the climate during the eruption of these flows which is also discussed in the paper

modelling the evolution of columnar joints abstract cooling cracks developing from the margins of solidifying lava flows or intrusive dykes or sills often propagate inward and divide the rock into prismatic columns it has been shown that 1 the initial crack pattern that forms at the surface of the body is not as regular as the pseudo hexagonal pattern which evolves later 2 columnar structures develop by repeated step wise crack advances forming cm to dm transverse bands on the column faces 3 even within well developed colonnades the polygonal outlines continue to shift slightly from one crack advance to the next we propose that each new crack should propagate parallel to the highest thermal gradient ahead of the current crack tip when adjacent columns are of unequal size the local asymmetry of the isotherms drives the new crack toward the biggest and hottest column we present a geometrical model and algorithm that follows this rule and mimics the successive crack advances as the cooling front migrates into the lava sheet or dyke the model polygonal pattern obtained consists of convex irregular polygons with a variable number of sides but in which pentagons and hexagons predominate the algorithm successfully reproduces the rapid evolution from an initial immature crack pattern to a pseudo hexagonal mature one in which the average number of sides approaches six it predicts also the predominance of y type crack junctions the rapid evolution toward columns of approximately equal sizes and the persistent changes in length and diverging orientation of growth steps observed along the sides of columns the model polygonal patterns exhibit geometrical properties which are very similar to those observed in well developed columnar jointed flows such as the giant s causeway

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

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

tectonic analysis of fracturing associated with occator crater abstract occator crater a ceres crater that hosts multiple bright spots on its floor has several sets of fractures associated with it a tectonic analysis of each of these fracture sets suggests that the concentric and radial fractures around the central pit and the concentric fractures high on the crater wall near the rim most likely formed due to well known impact cratering processes uplift of the crater floor due to magmatic injection is suggested to have resulted in the concentric floor fractures at the base of the crater wall as well as the cross cutting fractures in the lower part of the southwestern wall a mathematical analysis shows that a cryomagmatic plume diapir could have both reached the height necessary to cause fracturing within occator and that the stresses required are reasonable expansion a lobate flow from beneath is proposed to have formed those linear fractures associated with the vinalia faculae circumferential fractures surrounding occator appear to have formed due to the volumetric compaction of the ejecta blanket over the buried pre existing topography

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

lava rise plateaus and inflation pits in the mccartys lava flow field new mexico an analog for pāhoehoe like lava flows on planetary surfaces

characterization and evolution of fractures in low volume pahoehoe lava flows eastern snake river plain idaho we characterize fracture evolution in pahoehoe lava flows of the eastern snake river plain idaho and highlight significant differences to flood basalt sheet flows and implications for hydrologic models there are four distinct fracture types in east ern snake river plain flows 1 column bounding 2 column normal 3 entablature and 4 inflation fractures types 1 3 are driven by thermal stress whereas type 4 is induced by lava pressure from within the flow thermal stress distribution in a flow is dictated by its aspect ratio width height which controls the shape of isotherms isotherms control column bounding fracture orientations resulting in increasingly radial fracture patterns as the aspect ratio approaches unity column normal fractures form in response to thermal stress and fracture induced stress within basalt columns overlap in the timing of column bounding and column normal fracture growth has resulted in complex fracture relationships column normal fracture growth is strongly influenced by vesicular layers which act as mechanical heterogeneities creating preferential pathways for fracture growth as well as causing jogs or terminations along column bounding fractures eastern snake river plain entablatures which preserve the shape of the central lava core during the final stages of cooling have distinctly different origins and fracture styles compared to sheet flows entablatures formed by penetration of the edges of pressurized lava cores by inflation fractures causing rapid convective cooling in addition inflation fractures significantly perturb isotherm shapes in lava flows affecting flow scale fracture patterns and densities the overall effect of all these processes is a complex pattern of fracturing that attests to a strong impact by each fracture type on the growth behavior of all other fracture types

tectonic analysis and paleo stress determination of the upper lava section at odp iodp site 1256 east pacific ocean abstract research on the deep sea is of great importance for a better understanding of the mechanism of magma emplacement and the tectonic evolution of oceanic crust however details of the internal structure in the upper levels of the oceanic crust are much less complete than that of the more fully studied sub aerial areas for the first time this study proposes a dynamic analysis using the inversion method on core data derived from the drilled basement of the present day intact oceanic crust at odp iodp site 1256 in the cocos plate the research is based on an innovative core reorientation process and combines different stress hypothesis approaches for the analysis of heterogeneous failure slip data via exploitation of two distinct techniques from the analysis of the failure slip data both techniques produce 5 distinct subsystem datasets all calculated subsystems are mechanically and geometrically admissible interpretation of the results allows the researchers to note a complex local and regional tectonic evolution deriving from the interplay of 1 the ridge push and rotation of both the east pacific rise and the cocos nazca spreading center 2 the effect of the slab pull of the middle america trench 3 the influence of lava emplacement mechanisms and 4 intra plate deformation

surface morphology of columnar joints and its significance to mechanics and direction of joint growth columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes new observations show that each band contains a single plumose structure and thus represents an individual crack or joint segment formed during a discrete growth event analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies columnar joints form by nucleation and growth of new cracks on the edges of older cracks each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face where thermal stress is concentrated inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle ductile transition of lava outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava cracks often diverge slightly from the planes of previous cracks probably because of spatial and temporal changes in directions of local principal stresses mechanical interaction causes a diverging crack to overlap curve toward and usually intersect the previous crack behind its edge leaving a blind tip that points in the overall growth direction of the columnar joints this and other directional criteria are applied to determine joint growth patterns in several lava flows of the western united states in two tiered and multi tiered flows downward growing columnar joints usually meet upward growing joints well below the middle of the flows which indicates very rapid cooling of upper portions relative to lower portions this supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows whereas conduction is probably the dominant cooling mechanism at the bases

viscoelastic thermal stress in cooling basalt flows polygonal fractures commonly form in basaltic lava flows during cooling of the molten lava a transient thermomechanical model is constructed to apply the temperature history in a cooling basalt flow as a boundary condition in a mechanical model of viscoelastic stress in cooling basalt nearly all past modeling of stress in cooling basalt has treated the material as purely elastic below a well defined glass transition temperature an important distinction of this model is that the viscosity of the material is allowed to be temperature dependent and the effects of viscoelastic relaxation are included on the basis of this research faster cooling and higher strain rates lead to markedly higher effective glass transition temperatures consistent with previous observations by researchers the total thermal strain in the system is constant yet the percentage of strain that is not relaxed by viscous flow varies with position relative to the cooling surface this elastic strain partition is the portion of the total strain that creates stress in the material and it is directly dependent on cooling rate thus a genetic link between cooling rate and magnitude of stress is identified

the snake river plain idaho representative of a new category of volcanism studies of the volcanic geology of the snake river plain idaho and comparison with other basaltic regions suggest a new category of volcanic activity termed basaltic plains volcanism typified by the snake river plain this style of volcanism is intermediate between basaltic flood or plateau eruptions and hawaiian volcanism characteristics that are common to both hawaiian and plains volcanism are multiple lava flow units which erupt primarily from point sources formation of low shields and frequent emplacement through lava tubes or channels characteristics that are common to both flood basalts and plains volcanism are high volume flows vents aligned along rift zones and planar surfaces the recognition of plains volcanism in other areas provides a means to interpret the style of eruption and volcanic history

structural evolution of a world class epithermal orebody the martha hill deposit waihi new zealand we describe the complex structure in the highest now mined out levels of the world class martha hill gold silver deposit coromandel peninsula which is hosted in late miocene andesitic rocks a northeast striking block of rock 360 m long and up to 100 m wide was occupied by a complex vein network dominated by northeast striking gold silver lodes up to 30 m thick gold production was entirely from quartz veins the mineralization is embedded in the following geological history 1 deposition of the host volcanic rocks 2 initial tectonic fracturing 3 intrusion of clastic dikes and sills 4 start of hydrothermal alteration 5 main phase of faulting 6 main phase of veining 7 erosion and deep oxidation 8 11 deposition of further volcanic material including prominent rhyolites and formation of present topography veins occupied fractures but also followed a network of preexisting mostly normal faults with a small to medium amount of displacement faulting involved northwest southeast north south and east west extension implying complex 3 d strain clastic dikes of carbonaceous sandstone intruded during the faulting phase fault rocks were mostly brittle cataclasites or breccias but some clay rich zones displayed ductile structures many veins overprinted faults with a change from shear mode to opening mode deformation stockwork veins formed a mesh composed of two orthogonal sets one with northeast southwest and northwest southeast striking veins including the dominant mineralized lodes and the other with north south and east west striking veins although northwest striking veins were earliest during the main vein phase opening oscillated between all the vein directions with the two main lodes in the open pit area the martha and the welcome capturing most of the strain a variety of vein and breccia textures indicate repeated structurally controlled vein opening events that were associated with changes in the physical and chemical conditions of the mineralizing fluid a special concentration of structural features facilitated focusing of fluid flow to produce this world class deposit which contrasts with the less well endowed vein systems around it the overall tectonic control was dominated by northwest southeast extension and dip slip deformation the 3 d strain in this area was not only due to local interference of differently oriented structural features but also to the superposition of regional tectonic north northwest and northeast trends associated with migration of a subduction zone past the coromandel peninsula this demonstrates that structural control in an epithermal mineral deposit may originate from a number of different tectonic controls at different scales

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

a deterministic methodology for prediction of fracture distribution in basaltic multiflows the fracture distribution in basalt flows is a direct result of thermal processes thus basalt flows present a unique opportunity to characterize a nearly perfect deterministic system with its fundamental physical parameters fracture distribution data collected on cliff exposures of basalt flows near the idaho national engineering and environmental laboratory ineel are combined with calculations of cooling rate and temperature distribution from a finite element model to construct a predictive methodology for fracture spacing the methodology is based on an empirical power law relationship between inverse cooling rate and fracture spacing the methodology may be applied to unexposed basalt flows of approximately elliptical cross section whose thickness and width are constrained only by geophysical or borehole data if sufficient fracture data on nearby exposed flows are available the methodology aids waste remediation efforts at sites involving contaminant transport through fractured basalt such as the ineel and the hanford site in washington as well as involving transport and fluid flow through volcanic or intrusive rocks where thermal processes are responsible for fracturing