Lava Transport Research Articles

Insights into the development of small-volume long lava flows: A case study of the Coalstoun Lakes Volcanic Field, southeast Queensland, Australia

Long lava flows exceeding 50 km in length are usually produced during large-volume flood basalt eruptions (>100 to 10,000 km3) but can also occur from small to moderate-volume (<30 km3) basaltic eruptions in continental intraplate monogenetic volcanic fields. Eruptive volume, therefore, is not an a priori barrier to producing long lava flows. Key factors that promote long lava flows include efficient lava transport systems that minimise heat loss, long-lived and sustained effusion rates to maintain flow advancement, and lava flow across low topographic gradients (<1°-10°) with minimal topographic barriers.Here, we focus on an anomalously young and poorly studied basaltic monogenetic volcanic field in southeast Queensland, Australia, that formed part of the broader intraplate volcanism in eastern Australia since the Late Cretaceous. The Coalstoun Lakes Volcanic Field (CLVF) comprises three lava fields: the Barambah Basalt Flow Field, the Deep Creek Flow Field and the Hunters Hill Flow Field. Basalt from the Barambah Basalt Flow Field has been redated here by Ar40/Ar39 analysis of groundmass material, yielding a weighted mean age of 0.520 ± 0.016 Ma. The Barambah Basalt Flow Field contains most of the eruptive volume and has advanced up to 165 km from the vent. The Hunters Hill and the Deep Creek flow fields are comparatively smaller in volume and have advanced ∼30 and ∼ 20 km from the vent, respectively. Lava tubes are only known from proximal regions and do not appear to be a significant factor in promoting long run-out in the CLVF. Flow confinement and utilisation of existing drainage networks are features of both lava flow fields, and advancement down the sand-based and ephemeral Burnett River significantly promoted long run-out despite low topographic gradients.New whole-rock geochemical data on our CLVF samples indicates that all lavas are hawaiites, a common feature of other Quaternary long lava flows globally. Overall, there is some compositional variation, but a cryptic zonation is readily apparent in trace element abundances, which helps to further distinguish the flow fields as the products of separate but closely spaced eruptions. The combination of field and geochemical data indicates that the long lava flow of the Barambah Basalt Flow Field resulted from a sustained and relatively low effusion eruption, creating a pāhoehoe flow field that continuously advanced across the landscape, utilising a drainage system that guided lava flow and helped to circumvent any topographic barriers.

Lithostratigraphy, geology and geochemistry of the Tertiary volcanic rocks on Svartenhuk Halvø and adjoining areas, West Greenland

The Palaeogene volcanic succession in the northern part of the Nuussuaq Basin in West Greenland comprises three formations: the Vaigat and Svartenhuk Formations of Paleocene age (61–58 Ma) and the Naqerloq Formation of Eocene age (57–54 Ma). In this study, we formalise and describe the volcanic stratigraphy on Svartenhuk Halvø and the areas with lavas that flowed across the basin boundary onto the adjoining basement areas in the north and east. The Vaigat Formation comprises three members. The Kakilisaat and Nerutusoq Members are of minor volume and consist of, respectively, crustally contaminated basalts and chemically enriched basalts with relatively high contents of incompatible trace elements. They are overlain by the voluminous Nunavik Member of tholeiitic picrites (MgO ≥12 wt%) and subordinate magnesian basalts. The oldest volcanic deposits are commonly foreset-bedded hyaloclastites, and the overlying subaerial lavas are mainly thin, grey, crumbling flows. Eruption sites were mainly within the basin, with depocentres in the south and hyaloclastite and lava transport directions towards the north. Thicknesses vary from up to at least 2000 m in the south to ≥380 m in the northernmost exposures close to 72°N. The Svartenhuk Formation comprises four members. The lowest, Kuugaartorfik Member, is up to 100 m thick and consists partly of quartzofeldspathic and partly volcanogenic sediments; it is restricted to northern Svartenhuk Halvø and the Innerit peninsula. The overlying volcanic Tunuarsuk, Nuuit and Skalø Members are voluminous and widespread, with a combined thickness of up to 1800 m. They consist of tholeiitic basalts with similar chemical compositions but with correlatable stratigraphic variation patterns. The Tunuarsuk Member consists of interspersed flow groups of thin, grey flows and massive, brown flows; the Nuuit Member comprises mainly massive brown flows, and the Skalø Member is dominated by light grey flows. The Svartenhuk Formation oversteps the Vaigat Formation on the basement in the north and east. In these distal areas the Tunuarsuk and Nuuit Members constitute the major volumes, and preserved thicknesses are up to 1400 m. In northern and eastern Svartenhuk Halvø and also farther to the north and east, foreset-bedded hyaloclastites indicate transport directions towards the north and possibly east from eruption sites within the basin. The Naqerloq Formation comprises one member, the Arfertuarsuk Member, consisting of flows of brown basalt with relatively enriched chemistry and a single trachyte flow. The member is only found in western Svartenhuk Halvø and on Skalø, where it conformably overlies the older lavas with up to 350 m thickness preserved after erosion. Dykes of all three formations are present. The distribution of dykes of the Naqerloq Formation suggests that this originally extended much farther east. Picrites and basalts of the Vaigat and Svartenhuk Formations are geochemically related; the picritic lavas represent erupted primitive magmas, whereas the basaltic lavas represent fractionated melts formed in deep magma chambers. The melts formed from a geochemically depleted but heterogeneous mantle; in addition melts from enriched sources were occasionally incorporated. The enriched basalts of the Naqerloq Formation arose from another mantle source. Low contents of V, Cu and Ni in some crustally contaminated lavas indicate that accumulation of these elements may be present at depth.

Volume, Effusion Rate, and Lava Transport During the 2021 Fagradalsfjall Eruption: Results From Near Real‐Time Photogrammetric Monitoring

AbstractThe basaltic effusive eruption at Mt. Fagradalsfjall lasted from 19 March to 18 September 2021, ending a 781‐year repose period on Reykjanes Peninsula, Iceland. By late September 2021, 33 near real‐time photogrammetric surveys were completed using satellite and airborne images, usually processed within 3–6 hr. The results provide unprecedented temporal data sets of lava volume, thickness, and effusion rate. This enabled rapid assessment of eruption evolution and hazards to populated areas, important infrastructure, and tourist centers. The lava flow field has a mean lava thickness exceeding 30 m, covers 4.8 km2 and has a bulk volume of 150 ± 3 × 106 m3. The March–September mean bulk effusion rate is 9.5 ± 0.2 m3/s, ranging between 1 and 8 m3/s in March–April and increasing to 9–13 m3/s in May–September. This is uncommon for recent Icelandic eruptions, where the highest discharge usually occurs in the opening phase.

Linking lava morphologies to effusion rates for the 2014–2015 Holuhraun lava flow field, Iceland

AbstractDetermining the parameters that control fissure-fed lava morphologies is critical for reconstructing the complex emplacement histories of eruptions on Earth and other planetary bodies. We used a geomorphological map of the 2014–2015 Holuhraun lava flow field, in combination with new constraints on lava emplacement chronology and two independently derived time-averaged discharge rate (TADR) data sets, to analyze correlations between lava morphology and effusion rate. Results show that lava morphologies are dominantly controlled by effusion rate at the vent during the early phases of the eruption and by lava transport processes as the system evolves. Initially, TADR and its variance, which reflect pulsation in the lava supply rate from the vent, directly affect lava emplacement styles. However, as the eruption progresses, the lava transport system exerts a stronger control with channels and ponds that can either dampen variation in local effusion rate or create surges during sudden drainage events. The Holuhraun eruption predominantly produced rubbly lava in its earlier eruption phases and transitioned into the production of spiny lava toward the end of the eruption. However, a drop of TADR during the first phase of the eruption correlates with a decrease in rubbly lava formation and an increase in spiny lava production. This suggests that a change in effusion rate caused the observed transition in lava type. Our findings show that rubbly lava is formed under relatively high local effusion rates with pulsating supply conditions, whereas spiny lava is formed under lower local effusion rates and steadier supply.

New insights into the genesis of pyroducts of The Galápagos islands, Ecuador

There has been little research on the genesis and development of pyroducts (or lava tubes) originating from Gala´pagos volcanoes. Pyroducts are responsible for the lateral, post-eruptive transport of lava because they are highly effective as thermal insulators. After eruptions terminate, these conduits often become accessible as caves. In March 2014 the 16th International Symposium on Vulcanospeleology brought a large group of vulcanospeleological specialists to the Islands. During the meeting a number of pyroducts were visited and studied in context on the island of Santa Cruz and around Isabella’s Sierra Negra volcano in the western, most active, part of the Gala´pagos. The longest of the caves, Cueva del Cascajo, about 3 km in length, was partly surveyed and nine other caves were visited. Structural features such as thickness of roof, evidence of downcutting, presence of oxbows, secondary ceilings, lavafalls, collapses and pukas were particularly studied for evidence they reveal about developmental stages of pyroducts. The resulting data show that the pyroducts were formed by "inflation" with the primary roof consisting of uninterrupted paŻhoehoe sheets. No pyroducts were identified that developed by the crusting-over of channels. The studies strongly confirm inferences drawn from other hot-spot related islands, such as Hawai’i.

Deformation of rhyolite lava crust associated with intermittent inner flow of lava: palaeomagnetic evidence

SUMMARY A palaeomagnetic study has been conducted to examine the deformation of thick crusts of rhyolite lava while its inner portions continue to flow. The Sanukayama rhyolite lava, which erupted in the Pleistocene in Kozushima Island, Japan, was chosen as the investigation site because of its well-exposed vertical lithofacies variations classified into three distinct zones (pumiceous, obsidian and crystalline). The targets of this study are the pumiceous and obsidian zones, which constitute the crust of the lava. Thermal demagnetization reveals three remanent magnetization components from the pumiceous and obsidian samples but only a single magnetization component from the inner crystalline rhyolite samples. Alternating field demagnetization is ineffective in isolating the magnetization components in the pumiceous and obsidian samples. The multiple components of remanent magnetization of the crust are interpreted to have been acquired during cooling as thermoremanent magnetizations. We suspect intermittent lava transport of the inner portions, the primary mode of rhyolite lava advancement, to be responsible for the presence of multiple components in pumice and obsidian of the lava crust. When the inner portions of the lava retain mobility to flow out of the crust, the solidified crust of the lava surface below the magnetite Curie temperature remains susceptible to deformation. Analysis of palaeomagnetic directions from the crust allows the deformation of the crust to be described in terms of rotation. Although the mode of rhyolite lava advancement is not well understood, because of its infrequent occurrence, our observations offer an important insight on how the mobile part of the lava is associated with the deformation of the crust during continued lava advance.

Monitoring the time-averaged discharge rates, volumes and emplacement style of large lava flows by using MIROVA system: the case of the 2014-2015 eruption at Holuhraun (Iceland)

The 2014-2015 eruption at Holuhraun has produced more than 1.5 km 3 of basaltic magma and can be considered one of the major effusive events of the last two centuries in the world. During this eruption the MIROVA system (a volcanic hot-spot detection system based on MODIS middle infrared – MIR - data) has been used to detect and locate the active portions of the lava flow(s), and to measure the heat radiated by the growing lava field. According to these data the eruption was characterized by a slow decay of the radiant power, accompanied by a change in the lava transport mechanism that shifted from open channels, at the beginning of the eruption, to lava tubes, during the last months of activity. Despite the evident evolution of lava transport mechanism, we found that the overall decreasing trend of the thermal flux was mainly controlled by the exponential decline of lava discharge rates, while the increasing insulation of the flow field had a strong impact in transporting efficiently the lava at the distal flow front(s). Our results suggest the apparent time averaged lava discharge rates ( TADR ), derived from satellite thermal data, may fluctuate around the real effusion rate at the vent, especially in the case of large lava flows emplacing in nearly flat conditions. The magnitude and frequency of these fluctuations are mainly controlled by the emplacement dynamic, (i.e. occurrence of distinct major flow units), while the transition from channel- to tube-fed lava transport mechanism play only a minor role (±30%) in the retrieval of TADR using MIR data . When the TADR values are integrated to calculate erupted lava volumes, the effects of pulsating emplacement dynamic become smoothed and the eruptive trend become more clear. We suggest that during the Holuhraun ’s eruption, as well as during many other effusive eruptions, the MIR-derived radiant flux essentially mimic the overall trend of lava discharge rates, with only a minor influence due to the emplacement style and evolving eruptive conditions. From a monitoring and operational perspective, MIROVA demonstrates to be a very valuable tool to follow and, possibly, forecast the evolution of on-going effusive eruption.

Equilibrium Viscosity and Disequilibrium Rheology of a high Magnesium Basalt from Piton De La Fournaise volcano, La Reunion, Indian Ocean, France

Lava flows are a common hazard at basaltic to intermediate volcanoes in general and have posed a significant threat to La Reunion Island specifically over the past centuries. In sustained flow units, the efficiency of lava transport away from the vent is dominated by cooling. For basaltic to intermediate lavas it is the ability of the lava to solidify during cooling which exerts a first-order control on the extent of flow. As a consequence, understanding the sub-liquidus rheology of lavas has become a key focus in lava flow research in the past decade. To date, a systematic understanding of lava rheology during emplacement conditions has been largely hampered by a lack of experimental data. Here we present new data on the rheological evolution of a crystalizing high Mg basalt from Piton de la Fournaise. Sub-liquidus experiments were performed at constant cooling rates of 0.5 to 5 K/min, to mimic conditions similar to those experienced by lava during flow on the surface and magma during dike and sill emplacement. The data show that the effective viscosity of the crystallizing suspension increases until reaching a specific sub-liquidus temperature, the so-called “rheological cut-off temperature” (T cutoff ), at which the lava effectively solidifies. This departure to high viscosity is a consequence of rapid crystallization and is found to be primarily controlled by the imposed cooling-rate. Based on our experimental data, we adapt the failure forecasting method (FFM), which is commonly used to describe the self-accelerating nature of seismic signals to forecast material failure, to predict the rheological cut-off temperature (T cutoff ). The data presented serve to expand the modest existing experimental database on non-equilibrium rheology of lavas as a step towards understanding the underlying process dynamics.

Shear Rate‐Dependent Disequilibrium Rheology and Dynamics of Basalt Solidification

AbstractMagmas and lavas undergo a range of shear rates during transport and emplacement. Further, transport of magma and lava occurs at subliquidus conditions where the melt crystallizes at varying temperature, pressure, and oxygen fugacity. Transport efficiency and eruption style are governed by magma rheology, which evolves during cooling, crystallization and degassing. Quantification of magma rheology rests almost exclusively on experimentation at constant temperature and shear rate. We present the first study on the effect of shear rate on subliquidus basalt rheology at conditions relevant to lava flows and shallow magmatic systems. The results reveal that basalts reach their rheologic death or cutoff temperature (Tcutoff; i.e., the point at which the sample rheologically solidifies and flow stops) at higher temperatures when flowing faster, whereas crystallization is suppressed when the shear rate is low. We explore the implications of shear‐enhanced crystallization for modeling and forecasting of lava flow hazards and our understanding of magma and lava transport/storage systems.

Dynamics and viscosity of ‘a’a and pahoehoe lava flows of the 2012–2013 eruption of Tolbachik volcano, Kamchatka (Russia)

The 2012–2013 flank eruption of Tolbachik volcano (Kamchatka) lasted 9 months and produced 0.54 km3 of basaltic trachyandesite lava, thus becoming one of the most voluminous historical lava effusions of basic composition in subduction-related environments globally. From March to July 2013, the volcano monotonously erupted lava of constant composition (SiO2 = 52 wt%) with a nearly stable effusion rate of 18 m3/s. Despite the uniform eruptive and emplacement conditions, the dominant style of lava propagation throughout that time gradually changed from ‘a’a to pahoehoe. We report results of instrumental field measurements of the ‘a’a and pahoehoe flow dynamics (documented with time-lapse cameras) as well as the lava viscosity determined by flow rate and shear stress (using penetrometer) methods. Maximal propagation velocities of the ‘a’a fronts ranged from 2 to 25 mm/s, and those of the pahoehoe from 0.5 to 6 mm/s. The flow front velocities of both lava types experienced short-period fluctuations that were caused by complex flow mechanics of the advancing flow lobes. Minimal viscosities of lava of the ‘a’a lobes ranged from 1.3 × 105 to 3.3 × 107 Pa s (flow rate method), and those of the pahoehoe from to 5 × 103 to 5 × 104 Pa s (shear stress method). Our data include the first ever measured profiles of viscosity through the entire thickness of actively advancing pahoehoe lava lobes. We have found that both the ‘a’a and pahoehoe flows were fed by identical parental lava, which then developed contrasting rheological properties, owing to differences in the process of lava transport over the ground surface. The observed transition from the dominant ‘a’a to the dominant pahoehoe propagation styles occurred due to gradual elongation and branching of the lava tube system throughout the course of the eruption. Such evolution became possible because the growing lava field, composed of semisolidified flows, provided an environment for shallow subsurface intrusions and internal migrations of lava that, with time, developed into branches of the lava tube system. Based on our data, we propose phenomenological models of the ‘a’a and pahoehoe flow mechanics.

The complex rheology of megacryst-rich magmas: The case of the mugearitic “cicirara” lavas of Mt. Etna volcano

Multiphase magma rheological properties play a fundamental role on lava flow transport, emplacement and morphology. To date, however, the three-phase (melt+crystals+bubbles) rheology of natural magma remains relatively understudied. We present here a series of high-temperature experiments designed to investigate the multiphase rheology of a mugearitic megacryst-bearing lava from Mt. Etna. A peculiar textural feature of this magma is the abundance of cm-size plagioclase crystals (megacrysts) together with smaller size crystals (phenocrysts and microlites), yielding a very wide crystal size distribution. We combined different experimental techniques (rotational and compressional rheometry) to investigate the rheology of this natural lava under different degrees of partial melting at subliquidus conditions. Results indicate that natural megacryst-bearing mugearite magmas from Mt. Etna display a wide range of behaviors as a function of temperature (T=1000–1200°C) and crystal content (ϕX=0.2–0.7). In the investigated T range, the deformation mechanism of these magmas varies from mainly brittle (T<1050°C) to mainly ductile (T>1085°C). At T=1075°C, both ductile and brittle behavior have been observed. In the ductile regime, these magmas behave as non-Newtonian fluids (at least up to T=1100°C) showing marked apparent shear thinning behavior. The observed rheological behavior is due to a complex response related to a non-homogenous deformation of the natural sample (e.g. viscous and/or brittle shear localization), favored by the presence of bubbles. Consequently, the obtained flow parameters can be considered as representative of the bulk rheology of natural magmas, commonly characterized by similar non-homogeneous deformation styles. We applied the obtained data to discuss the flow and emplacement conditions of these peculiar lava flows. We demonstrated that at eruptive temperatures, the presence of a pre-eruptive crystal cargo and bubbles facilitates the achievement of critical crystal content during flow and cooling, ultimately controlling lava transport and emplacement. Flow conditions can be maintained in the presence of an efficient insulation and, importantly, of deformable vesicles promoting and enhancing shear localization.

Lava field evolution and emplacement dynamics of the 2014–2015 basaltic fissure eruption at Holuhraun, Iceland

The 6-month long eruption at Holuhraun (August 2014–February 2015) in the Bárðarbunga-Veiðivötn volcanic system was the largest effusive eruption in Iceland since the 1783–1784 CE Laki eruption. The lava flow field covered ~84km2 and has an estimated bulk (i.e., including vesicles) volume of ~1.44km3. The eruption had an average discharge rate of ~90m3/s making it the longest effusive eruption in modern times to sustain such high average flux. The first phase of the eruption (August 31, 2014 to mid-October 2014) had a discharge rate of ~350 to 100m3/s and was typified by lava transport via open channels and the formation of four lava flows, no. 1–4, which were emplaced side by side. The eruption began on a 1.8km long fissure, feeding partly incandescent sheets of slabby pāhoehoe up to 500m wide. By the following day the lava transport got confined to open channels and the dominant lava morphology changed to rubbly pāhoehoe and ‘a’ā. The latter became the dominating morphology of lava flows no. 1–8. The second phase of the eruption (Mid-October to end November) had a discharge of ~100–50m3/s. During this time the lava transport system changed, via the formation of a <1km2 lava pond ~1km east of the vent. The pond most likely formed in a topographical low created by a the pre-existing Holuhraun and the new Holuhraun lava flow fields. This pond became the main point of lava distribution, controlling the emplacement of subsequent flows (i.e. no. 5–8). Towards the end of this phase inflation plateaus developed in lava flow no. 1. These inflation plateaus were the surface manifestation of a growing lava tube system, which formed as lava ponded in the open lava channels creating sufficient lavastatic pressure in the fluid lava to lift the roof of the lava channels. This allowed new lava into the previously active lava channel lifting the channel roof via inflation. The final (third) phase, lasting from December to end-February 2015 had a mean discharge rate of ~50m3/s. In this phase the lava transport was mainly confined to lava tubes within lava flows no. 1–2, which fed breakouts that resurfaced >19km2 of the flow field. The primary lava morphology from this phase was spiny pāhoehoe, which superimposed on the ‘a’ā lava flows no. 1–3 and extended the entire length of the flow field (i.e. 17km). This made the 2014–2015 Holuhraun a paired flow field, where both lava morphologies had similar length. We suggest that the similar length is a consequence of the pāhoehoe is fed from the tube system utilizing the existing ‘a’ā lava channels, and thereby are controlled by the initial length of the ‘a’ā flows.

Morphological evolution of a new volcanic islet sustained by compound lava flows

We investigated the creation of a volcanic islet and emplacement of lava flows in the sea by analyzing data from the island-forming eruption at Nishinoshima, Japan, that has been continuing since November 2013. Aerial observations and satellite images were used to perform a quantitative analysis of the eruption processes. The most intriguing characteristic of the lava flows is the development of lobes and tubes from breakouts and bifurcations of andesitic ‘a’ā-type lava flows. Internal pathways that fed lava to the active flow front were eventually developed by crust solidification and dominated the lava transport. The average discharge was ~2 × 10 5 m 3 /day, and the total volume of erupted material reached ~0.1 km 3 at the end of February 2015. Fractal analysis of the lava-flow margins suggests that the growth pattern is self-similar, with a fractal dimension (D) of ~1.08–1.18, which is within the range of subaerial basaltic lava flows. The morphological evolution of Nishinoshima is controlled primarily by effusion of lava with an apparent viscosity of 10 4 –10 6 Pa∙s, average discharge of ~2.3 m 3 /s, and eruption duration lasting ~2 yr. Our data and analyses suggest that the effect of lava coming in contact with seawater, as well as the variations in the lava discharge rate on local and overall scales, are important factors affecting the development of crust and the lava transport system.

How lava flows: New insights from applications of lidar technologies to lava flow studies

Mafic lava flows are common; for this reason, they have long been a focus of volcanological studies. However, field studies of both older and active flows have been hampered by difficulties in field access; active flows are hot, whereas older flows have rough and jagged surfaces that are difficult to traverse. As a result, morphometric studies of lava flows have generally lagged behind theoretical studies of flow behavior. The advent of laser scanning (LS) (i.e., lidar, light detection and ranging) technologies, both airborne mapping (ALSM) and terrestrial (TLS), is promoting detailed studies of lava flows by generating data suitable for production of high-resolution digital elevation models (DEMs). These data are revolutionizing both the visual and quantitative analysis of lava flows. First and foremost, this technology allows accurate mapping of flow boundaries, particularly in vegetated areas where bare earth imaging dramatically improves mapping capabilities. Detailed imaging of flow surfaces permits mapping and measurement of flow components, such as channels, surface folds, cracks, blocks, and surface roughness. Differencing of preeruptive and posteruptive DEMs allows analysis of flow thickness variations, which can be related to the dynamics of lava emplacement. Multitemporal imaging of active flows provides information not only on the rates and locations of individual flow lobes, but also measurement of pulsed lava transport. Together these new measurement capabilities can be used to test proposed models of channel formation, lava tube formation, rates of flow advance, and flow conditions within lava channels; they also provide new ways to assess the hazard and risk posed by lava flow inundation. Early published studies illustrate the potential of applying lidar to volcanic terrain; it is clear, however, that the availability of high-resolution digital topography is poised to revolutionize the study of mafic lava flows.

Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile

The emplacement mechanisms of rhyolitic lava flows are enigmatic and, despite high lava viscosities and low inferred effusion rates, can result in remarkably, laterally extensive (>30 km) flow fields. Here we present the first observations of an active, extensive rhyolitic lava flow field from the 2011-2012 eruption at Cordón Caulle, Chile. We combine high-resolution four-dimensional flow front models, created using automated photo reconstruction techniques, with sequential satellite imagery. Late-stage evolution greatly extended the compound lava flow field, with localized extrusion from stalled, ~35 m-thick flow margins creating >80 breakout lobes. In January 2013, flow front advance continued ~3.6 km from the vent, despite detectable lava supply ceasing 6-8 months earlier. This illustrates how efficient thermal insulation by the lava carapace promotes prolonged within-flow horizontal lava transport, boosting the extent of the flow. The unexpected similarities with compound basaltic lava flow fields point towards a unifying model of lava emplacement.

Three-dimensional lithofacies variations in hyaloclastite deposits

Analysis of the spatial lithofacies variability within lava-fed delta formations in southern Iceland has revealed complex three-dimensional volcanic architectures in hyaloclastite deposits in non-glacial settings. Two depositional environments are studied, (a) lava entering a marine embayment (Stóri-Nupur) and; (b) lava advancing into a body of water of the flanks of a Surtseyan cone (Hjörleifshöfði). Interaction between environmental factors such as shoreline geomorphology, water depth, wave energy levels, the nature of the lava transport system, lava supply rate all affect the resulting lava deltas creating complex lithofacies arrangements and stacking patterns. Recognised here are two types of hyaloclastite deltas. One of syn-sedimentary origin (Hjörleifshöfði) and one derived from primary fragmentation processes (Stóri-Nupur). Syn-sedimentary systems are dominated by destabilisation of the hyaloclastite pile leading to reworking downslope and share similarities to alluvial delta systems. Conversely, primary fragmented systems are controlled by waxing and waning cycles in volcanic effusivity whereby the hyaloclastite unit recorded is not the product of one lava flow rather than one eruptive event.

Role of Syn-eruptive Cooling and Degassing on Textures of Lavas from the AD 1783-1784 Laki Eruption, South Iceland

The Laki eruption involved 10 fissure-opening episodes that produced 15·1 km3 of homogeneous quartz-tholeiite magma. This study focuses on the texture and chemistry of samples from the first five episodes, the most productive period of the eruption. The samples comprise pumiceous tephra clasts from early fallout deposits and lava surface samples from fire-fountaining and cone-building activity. The fluid lava core was periodically exposed at the surface upon lobe breakout, and its characteristics are preserved in glassy selvages from the lava surface. In all samples, plagioclase is the dominant mineral phase, followed by clinopyroxene and then olivine. Samples contain 100 µm) with primitive cores [An* = 100 x Ca/(Ca + Na) >70; Fo > 75; En* = 100 x Mg/(Mg + Fe) >78] and more evolved rims, and >10 vol. % of skeletal, densely distributed groundmass crystals ( 40 vol. %; lava: <40 vol. %), groundmass crystal content (tephra: <10 vol. %; lava: 20–30 vol. %), and matrix glass composition (tephra: 5·4–5·6 wt % MgO; lava: 4·3–5·0 wt % MgO). Whole-rock and matrix glass compositions define a trend consistent with liquid evolution during in situ crystallization of groundmass phases. Plagioclase–glass and olivine–glass thermometers place the formation of phenocryst cores at ~10 km depth in a melt with ~1 wt % H2O, at near-liquidus temperatures (~1150°C). Phenocryst rims and groundmass crystals formed close to the surface, at 10–40°C melt undercooling and in an ~10–20°C cooler drier magma (0–0·1 wt % H2O), causing an ~10 mol % drop in An content in plagioclase. The shape, internal zoning and number density of groundmass crystals indicate that they formed under supersaturated conditions. Based on this information, we propose that degassing during ascent had a major role in rapidly undercooling the melt, prompting intensive shallow groundmass crystallization that affected the magma and lava rheology. Petrological and textural differences between tephra and lava reflect variations in the rates of magma ascent and the timing of surface quenching during each eruptive episode. That in turn affected the time available for crystallization and subsequent re-equilibration of the melt to surface (degassed) conditions. During the explosive phases, the rates of magma ascent were high enough to inhibit crystallization, yielding crystal-poor tephra. In contrast, pervasive groundmass crystallization occurred in the lava, increasing its yield strength and causing a thick rubbly layer to form during flow emplacement. Lava selvages collected across the flow-field have strikingly homogeneous glass compositions, demonstrating the high thermal efficiency of fluid lava transport. Cooling is estimated as 0·3°C/km, showing that rubbly surfaced flows can be as thermally efficient as tube-fed phoehoe lavas.

A morphometric analysis of the submarine volcanic ridge south-east of Pico Island, Azores

A region of crustal extension, the Azores Plateau contains excellent examples of submarine volcanic edifices constructed over a wide range of ocean depths along the Pico Ridge. Using bathymetric data and Towed Ocean Bottom Instrument (TOBI) side-scan sonar imagery, we measured the dimensions (diameter, height, slopes), shape, and texture of these volcanic edifices to further understanding of the geometric development of a submarine ridge. Our analysis and interpretation of the measurement and texture data suggest the following: (1) the various edifice types do not correlate with depth ranges, suggesting that ambient water pressure is not a controlling factor in configuration of the edifice formed; (2) the cones have a mean diameter of 948 m, height of 152 m, and slope of 15.6°, and are peaked rather than displaying flat-topped summits; and (3) while hummocky-textured cones also occur, smooth-textured cones predominate. The cones seem to develop preferentially outwards, then upwards, with only a weak correlation between diameter and height, suggesting that the cone population does not evolve self-similarly. Although smooth and hummocky cone populations are not statistically different in mean slope angle and eruption depth, the hummocky cones have a significantly greater mean diameter than the smooth cones. We suggest that hummocky-textured cones (probably involving eruption of pillow lavas) are formed after voluminous smooth textured flows. Nearest-neighbour analysis suggests that submarine cones are distributed randomly whereas subaerial cones are not. We interpret this finding to suggest subaerial cones being masked by over-covering flows, which tend to flow further than submarine lava flows. Furthermore, the spatial distribution of cones away from the ridge centre is not easily explained by a magma supply fed via lava tubes from central eruptions, because of a lack of consistent pathway down gradient. Fissure vents seem to be important in determining construction of the ridge and result in linear arrangements of edifices and cone elongation, consistent with the regional tectonic trend of this part of the Azores Plateau. Cones form as each feeding dyke intrusion cools and the eruption becomes localized along point-source vents along the fissure. The ridge seems to be predominantly formed from fissure eruptions along the ridge axis, with subordinate transport of lava down its flanks.

Channelized lava flows at the East Pacific Rise crest 9°–10°N: The importance of off‐axis lava transport in developing the architecture of young oceanic crust

Submarine lava flows are the building blocks of young oceanic crust. Lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava, the characteristics of the eruption, and the topography it encounters. The resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50–500 m of the oceanic crust. We have mapped and measured numerous submarine channelized lava flows at the East Pacific Rise (EPR) crest 9°–10°N that reflect the high‐effusion‐rate and high‐flow‐velocity end‐member of lava eruption and transport at mid‐ocean ridges. Channel systems composed of identifiable segments 50–1000 m in length extend up to 3 km from the axial summit trough (AST) and have widths of 10–50 m and depths of 2–3 m. Samples collected within the channels are N‐MORB with Mg# indicating eruption from the AST. We produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins. Modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0.4 to 4.7 × 103 m3/s, and modeled shear rates help explain the surface morphology variation. We suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off‐axis region (1–3 km) and produces the layer 2A thickening that is observed at fast and superfast spreading ridges. In addition, the rapid, high‐volume‐flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the EPR. We find that high concentrations of channelized lava flows correlate with local, across‐axis ridge morphology indicative of an elevated magma budget. Additionally, in locations where channelized flows are located dominantly to the east or west of the AST, the ridge crest is asymmetric, and layer 2A appears to thicken over a greater distance from the AST toward the side of the ridge crest where the channels are located.

Late Pliocene volcanic island growth and flood basalt‐like lava emplacement in the Hyblean Mountains (SE Sicily)

Since ∼200 Ma, the Hyblean Mountains (southeastern Sicily) have been the site of recurrent, mainly submarine, mafic volcanism. In the upper Pliocene, large volumes of tholeiitic products were erupted in a shallow marine environment, leading to the growth of volcanic islands. The deposits of this tholeiitic episode can be grouped into two main facies associations: (1) pillow lavas and hyaloclastite tuffs erupted and deposited entirely under water, and (2) submarine pillow breccias, fed from subaerial lava flows entering the ocean and subaerial lavas, which are distinguished into a coastal lava platform facies and an inland facies. Lava flows into the ocean led to the growth of lava deltas consisting of a basal flow foot breccia and an overlying sequence of pāhoehoe flow units, the latter rarely exceeding 0.5 m in thickness. In outcrop, this facies association shows evidence for significant instability of the lava deltas during growth, indicating very similar processes to those observed during the current eruption of Kīlauea volcano, Hawaii. Away from the coast of the growing island, the inland facies is characterized by thicker (up to several meters) pāhoehoe flow units showing evidence for sheet flow inflation and limited outcrops of scoria and/or bomb deposits. Evidence for lava transport through tubes exists in the form of tumuli and multiple vesicle layers; drained tubes are absent. All tholeiitic products were emitted in a single, instantaneous, voluminous event (≥120 km3), consistent with very high effusion rates similar to those of flood basalts or the 1783 Skaftár Fires (Iceland) eruption.