Low Effusion Rate Research Articles

SO2 emission rates and incorporation into the air pollution dispersion forecast during the 2021 eruption of Fagradalsfjall, Iceland

During the low-effusion rate Fagradalsfjall eruption (19 March – 18 September 2021), the emission of sulfur dioxide (SO2) was frequently measured using ground-based UV spectrometers. The total SO2 emitted during the entire eruption was 970 ± 540 kt, which is only about 6% of the SO2 emitted during the similar length Holuhraun eruption (2014–2015). The eruption was divided into five phases based on visual observations, including the number of active vents and the occurrence of lava fountaining. The SO2 emission rate ranged from 44 ± 19 kg/s in Phase 2 to 85 ± 29 kg/s in Phase 5, with an average of 64 ± 34 kg/s for the entire eruption. There was notable variability in SO2 on short timescales, with measurements on 11 August 2021 ranging from 17 to 78 kg/s.SO2 flux measurements were made using scanning DOAS instruments located at different distances from and orientations relative to the eruption site augmented by traverses. Four hundred and forty-four scan and traverse measurements met quality criteria and were used, along with plume height and meteorological data, to calculate SO2 fluxes while accounting for wind-related uncertainties.A tendency for stronger SO2 flux concurrent with higher amplitude seismic tremor and the occurrence of lava fountaining was observed during Phases 4 and 5 which were characterized by intermittent crater activity including observable effusion of lava and gas release interspersed with long repose times. This tendency was used to refine the calculation of the amount of SO2 emitted during variably vigorous activity. The continuous seismic tremor time series was used to quantify how long during these eruption phases strong/weak activity was exhibited to improve the calculated SO2 flux during these Phases.The total SO2 emissions derived from field measurements align closely with results obtained by combining melt inclusion and groundmass glass analyses with lava effusion rate measurements (910 ± 230 kt SO2). Specifically, utilizing the maximum S content found in evolved melt inclusions and the least remaining S content in accompanying quenched groundmasses provides an identical result between field measurements and the petrological calculations. This suggests that the maximum SO2 release calculated from petrological estimates should be preferentially used to initialize gas dispersion models for basaltic eruptions when other measurements are lacking.During the eruption, the CALPUFF dispersion model was used to forecast ground-level exposure to SO2. The SO2 emission rates measured by DOAS were used as input for the dispersion model, with updates made when a significant change was measured. A detailed analysis of one mid-distance station over the entire eruption shows that the model performed very well at predicting the presence of volcanic SO2 when it was measured. However, it frequently predicted the presence of SO2 that was not measured and the concentrations forecasted had no correlation with the concentrations measured. Various approaches to improve the model forecast were tested, including updating plume height and SO2 flux source terms based on measurements. These approaches did not unambiguously improve the model performance but suggest that improvements might be achieved in more-polluted conditions.

Emplacement of lava flows on eroded terrain, part I: The case of the Tiretaine valley (Chaîne des Puys, France)

Lava flows often erupt onto eroded landscapes, flowing across plateaus, into valleys and basins, and down escarpments. With the Tiretaine lava flow system in the Chaîne des Puys (Central France), lava flowed over the complex topography of an ancient rift margin. Erupted 41.3 ± 1.3 ka ago onto the granite plateau of the Massif Central, flow initially advanced over a surface that sloped 2°E. On reaching a fault scarp that delimits the plateau, flows descended the 6° slopes of a ravine that cuts the scarp E-W. In part 1 of this work we thus consider the case where lava is emplaced onto relatively flat terrain, then passes over a break-in-slope onto steeper slopes. To do this, we carried out mapping and facies analysis down this ∼6-km-long system, collecting 24 samples on which we completed textural and geochemical analyses. Results reveal three lava facies:(i)a 3 × 2 km proximal zone of inflated pāheohoe with a thickness of 25–50 m;(ii)a channelized medial zone with simple rubble levees nested in the ravine; and(iii)a valley-ponded distal zone with tubes and well-formed columnar jointing.Through simple mass balance calculations, and analogy with systems displaying similar facies, we find that the proximal inflated zone was fed at relatively low (1–5 m3/s) effusion rates. This built the proximal portion of the flow field over a period of 1–6 years, forming a volume of flow-field stored, degassed lava. Failure of the eastern edge of the proximal flow field at the head of the Tiretaine ravine fed high (215–315 m3/s) effusion rate flow down the ravine, to build a channelized flow system in just a few hours. However, the presence of a lava dam at the mouth of the ravine impeded further flow and resulted in ponding. This ponded volume then cooled in-place over a period of 10–11 years. Breaks-in-slope, and the resulting lava–topography interaction, thus created a system of reservoirs and transfers, with different flow rates, dynamics and emplacement time scales. For our case, where the lava flow system is mostly in a UNESCO World Heritage site, the study provides scientific support for Geoheritage efforts, outreach, and initiatives to support protection of geologically important sites.

Characterization of volcanic reservoirs; insights from the Badejo and Linguado oil field, Campos Basin, Brazil

The Badejo and Linguado oil fields are hosted within non-conventional volcanic reservoirs which produced commercial hydrocarbons from the Lower Cretaceous Cabiúnas Formation, Campos Basin, offshore Brazil. Despite over 30 years of production, limited characterization of the nature and reservoir properties of the volcanic reservoirs have been presented to date. A comprehensive reappraisal of the Cabiúnas Formation volcanic reservoirs is presented in this study incorporating extensive existing and new data including core descriptions, laboratory petrophysical analyses, petrography, QEMSCAN, SEM, wireline, and microtomography results from five cored intervals of wells spanning the main reservoirs of the Badejo and Linguado fields. Volcanic facies analyses of the sequences reveal a predominance of subaerial effusive basic volcanic rocks interbedded with sediment and in several cases comprising peperites revealing intricate lava-sediment interaction products. Four volcano-sedimentary units are identified, showing alternations between low (compound pahoehoe lava dominated Units 1 and 3) and high (tabular rubbly pahoehoe lava dominated Units 2 and 4) effusion rates. Paleoenvironmental conditions also varied between units with extensive oxidation present in Unit 1 inferred to relate to extended periods of subaerial exposure and weathering in an arid environment. Overlying Units 2–4 reveal an increase in humidity evidenced by an increase in the presence of well sorted fine-grained sediment interlayers, peperites, and non-marine ostracods. Both original facies and alteration reveal key controls on reservoir properties. Extensive weathering and alteration of Unit 1 caused pervasive filling of original porosity (vesicles and fractures) and resulted in reservoir degradation. Lesser weathering of overlying Units 2, 3 and 4, resulted in improved reservoir properties which can be clearly linked to volcanic intra-facies including vesiculated and autobrecciated lava flow tops which commonly reveal extensive oil staining. This study reveals the intricate interplay between primary volcanic facies and subsequent alteration history in dictating volcanic reservoir properties in a successful offshore oil field development.

Mayotte 2018 eruption likely sourced from a magmatic mush

Despite significant advancements in understanding crustal melt transport, determining the shallow magmatic architecture at any given volcanic system remains a significant challenge with geophysical methods alone. In this study, we present a new conceptual model combining previously studied models for geomechanical magma reservoirs and dyke-shaped conduits. Supplementary to previous work, we include a model for porous mush adjacent to a visco-elastic magma reservoir during the eruption. This addition enables physically consistent magma recharge into the elastic magma reservoir by porous flow instead of employing an arbitrary mantle magma recharge parameterization. We compare our conceptual model with an illustrative test case - the submarine 2018 Mayotte eruption in the Comoros Islands (∼6.55 km3 Bulk Rock Volume, ∼3.5 km water depth, ∼35 km estimated deep magma reservoir). We estimate a magma effusion rate history using a new continuous-time inversion of surface deformation data. A single magma reservoir cannot match the observed eruptive history for any reservoir geometry or crustal and magmatic material properties. However, the presence of a porous mush adjacent to the magma reservoir helps to reconcile the model with the observations. The additional magma flux from the mush region sustains the long eruption at a low effusion rate, as is observed off Mayotte, with a decreasing effusion rate after few months of eruptions and a low persistent effusion rate since then. The mush flow dynamics are a direct consequence of reduced pressure in the magma reservoir during the eruption. Thus, our model provides a parsimonious and physically motivated explanation of the Mayotte eruptive history. Since many historical basaltic eruptions worldwide, such as sometime observed at Piton de la Fournaise, Volcan Arenal, and Laki, have a similar effusion rate history shape as Mayotte, magmatic mushes may be a key component for explaining long-lived basaltic eruptions.

Unsupervised Machine Learning on Domes in the Lunar Gardner Region: Implications for Dome Classification and Local Magmatic Activities on the Moon

Lunar volcanic domes are essential windows into the local magmatic activities on the Moon. Classification of domes is a useful way to figure out the relationship between dome appearances and formation processes. Previous studies of dome classification were manually or semi-automatically carried out either qualitatively or quantitively. We applied an unsupervised machine-learning method to domes that are annularly or radially distributed around Gardner, a unique central-vent volcano located in the northern part of the Mare Tranquillitatis. High-resolution lunar imaging and spectral data were used to extract morphometric and spectral properties of domes in both the Gardner volcano and its surrounding region in the Mare Tranquillitatis. An integrated robust Fuzzy C-Means clustering algorithm was performed on 120 combinations of five morphometric (diameter, area, height, surface volume, and slope) and two elemental features (FeO and TiO2 contents) to find the optimum combination. Rheological features of domes and their dike formation parameters were calculated for dome-forming lava explanations. Results show that diameter, area, surface volume, and slope are the selected optimum features for dome clustering. 54 studied domes can be grouped into four dome clusters (DC1 to DC4). DC1 domes are relatively small, steep, and close to the Gardner volcano, with forming lavas of high viscosities and low effusion rates, representing the latest Eratosthenian dome formation stage of the Gardner volcano. Domes of DC2 to DC4 are relatively large, smooth, and widely distributed, with forming lavas of low viscosities and high effusion rates, representing magmatic activities varying from Imbrian to Eratosthenian in the northern Mare Tranquillitatis. The integrated algorithm provides a new and independent way to figure out the representative properties of lunar domes and helps us further clarify the relationship between dome clusters and local magma activities of the Moon.

The 1974 West Flank Eruption of Mount Etna: A Data-Driven Model for a Low Elevation Effusive Event

Low elevation flank eruptions represent highly hazardous events due to their location near, or in, communities. Their potentially high effusion rates can feed fast moving lava flows that enter populated areas with little time for warning or evacuation, as was the case at Nyiragongo in 1977. The January–March 1974 eruption on the western flank of Mount Etna, Italy, was a low elevation effusive event, but with low effusion rates. It consisted of two eruptive phases, separated by 23 days of quiescence, and produced two lava flow fields. We describe the different properties of the two lava flow fields through structural and morphological analyses using UAV-based photogrammetry, plus textural and rheological analyses of samples. Phase I produced lower density (∼2,210 kg m−3) and crystallinity (∼37%) lavas at higher eruption temperatures (∼1,080°C), forming thinner (2–3 m) flow units with less-well-developed channels than Phase II. Although Phase II involved an identical source magma, it had higher densities (∼2,425 kg m−3) and crystallinities (∼40%), and lower eruption temperatures (∼1,030°C), forming thicker (5 m) flow units with well-formed channels. These contrasting properties were associated with distinct rheologies, Phase I lavas having lower viscosities (∼103 Pa s) than Phase II (∼105 Pa s). Effusion rates were higher during Phase I (≥5 m3/s), but the episodic, short-lived nature of each lava flow emplacement event meant that flows were volume-limited and short (≤1.5 km). Phase II effusion rates were lower (≤4 m3/s), but sustained effusion led to flow units that could still extend 1.3 km, although volume limits resulted from levee failure and flow avulsion to form new channels high in the lava flow system. We present a petrologically-based model whereby a similar magma fed both phases, but slower ascent during Phase II may have led to greater degrees of degassing resulting in higher cooling-induced densities and crystallinities, as well as lower temperatures. We thus define a low effusion rate end-member scenario for low elevation effusive events, revealing that such events are not necessarily of high effusion rate and velocity, as in the catastrophic event scenarios of Etna 1669 or Kilauea 2018.

Generation, ascent and eruption of magma on the Moon: New insights into source depths, magma supply, intrusions and effusive/explosive eruptions (Part 2: Predicted emplacement processes and observations)

Generation, ascent and eruption of magma on the Moon: New insights into source depths, magma supply, intrusions and effusive/explosive eruptions (Part 2: Predicted emplacement processes and observations)

Complex explosive volcanic activity on the Moon within Oppenheimer crater

Oppenheimer crater is a floor-fractured crater located within the South Pole–Aitken basin on the Moon, and exhibits more than a dozen localized pyroclastic deposits associated with the fractures. Localized pyroclastic volcanism on the Moon is thought to form as a result of intermittently explosive Vulcanian eruptions under low effusion rates, in contrast to the higher-effusion rate, Hawaiian-style fire fountaining inferred to form larger regional deposits. We use Lunar Reconnaissance Orbiter Camera images and Diviner Radiometer mid-infrared data, Chandrayaan-1 orbiter Moon Mineralogy Mapper near-infrared spectra, and Clementine orbiter Ultraviolet/visible camera images to test the hypothesis that the pyroclastic deposits in Oppenheimer crater were emplaced via Vulcanian activity by constraining their composition and mineralogy. Mineralogically, we find that the deposits are variable mixtures of orthopyroxene and minor clinopyroxene sourced from the crater floor, juvenile clinopyroxene, and juvenile iron-rich glass, and that the mineralogy of the pyroclastics varies both across the Oppenheimer deposits as a whole and within individual deposits. We observe similar variability in the inferred iron content of pyroclastic glasses, and note in particular that the northwest deposit, associated with Oppenheimer U crater, contains the most iron-rich volcanic glass thus far identified on the Moon, which could be a useful future resource. We propose that this variability in mineralogy indicates variability in eruption style, and that it cannot be explained by a simple Vulcanian eruption. A Vulcanian eruption should cause significant country rock to be incorporated into the pyroclastic deposit; however, large areas within many of the deposits exhibit spectra consistent with high abundances of juvenile phases and very little floor material. Thus, we propose that at least the most recent portion of these deposits must have erupted via a Strombolian or more continuous fire fountaining eruption, and in some cases may have included an effusive component. These results suggest that localized lunar pyroclastic deposits may have a more complex origin and mode of emplacement than previously thought.

Emplacement of submarine lava flow fields: A geomorphological model from the Niños eruption at the Galápagos Spreading Center

In the absence of any direct observations of an active submarine eruption at a mid-ocean ridge (MOR), our understanding of volcanic processes there is based on the interpretation of eruptive products. Submarine lava flow morphology serves as a primary indicator of eruption and emplacement processes; however, there is typically a lack of visual observations and bathymetric data at a scale and extent relevant to submarine lava flows, which display meter to submeter-scale morphological variability. In this paper, we merge submersible-based visual observations with high-resolution multibeam bathymetry collected by an autonomous underwater vehicle (AUV) and examine the fine-scale geomorphology of Ninos, a submarine lava flow field at the Galapagos Spreading Center (GSC).We identify separate morphological facies (i.e., morphofacies) within the lava flow field, each having distinct patterns of lava flow morphology and volcanic structures. The spatial and stratigraphic arrangement of morphofacies suggests that they were emplaced sequentially as the eruption progressed, implying that the Ninos eruption consisted of at least three eruptive phases. We estimate eruption parameters and develop a chronological model that describes the construction of the Ninos lava flow field. An initial phase with high effusion rates emplaced sheet flows, then an intermediate phase emplaced a platform of lobate lavas, and then an extended final phase with low effusion rates emplaced a discontinuous row of pillow lava domes. We then compare this model to mapped lava flow fields at other MORs. Despite disparities in scale, the morphological similarities of volcanic features at MORs with different spreading rates suggest common emplacement processes that are primarily controlled by local magma supply.

The role of viscosity in the emplacement of high-temperature acidic flows of Serra Geral Formation in Torres Syncline (Rio Grande do Sul State, Brazil)

The acidic flows from Serra Geral Formation in Torres Syncline, Rio Grande do Sul, Brazil, are on the top of a volcanic sequence composed by a complex facies association of compound, simple and rubbly pahoehoebasic flows, acidic lava domes, and tabular acidic lava flows. The origin and emplacement conditions of the acidic volcanic rocks are discussed in this paper based on petrology, on calculated apatite saturation thermometry temperatures, and on estimated viscosity data. The liquidus temperatures for metaluminous rhyodacite to rhyolite samples are about 1,067.5 ± 25oC in average. The viscosity (η) values vary from 105 to 106 Pas for anhydrous conditions, suggesting the emplacement of high-temperature - low-viscosity lava flows and domes. The occurrence of acidic lava domes above simple pahoehoe flows as flow-banded vitrophyres was under low effusion rates, in spite of their high temperature and low viscosities, which are reflected in their small height. The emplacement of lava domes has continued until the eruption of rubbly pahoehoe flows and the geometry of these deposits rugged the relief. Presence of tabular acidic lava flows covering the landscape indicates that it was under high effusion rates conditions and such flows had well-insulated cooled surface crusts. The capacity to attain greater distances and overpass relief obstacles is explained not only by high effusion rates, but also by very low viscosities at the time of emplacement.

The mode of emplacement of Neogene flood basalts in eastern Iceland: Facies architecture and structure of simple aphyric basalt groups

Simple flows (tabular) in the Neogene flood basalt sections of Iceland are described and their mode of emplacement assessed. The flows belong to three aphyric basalt groups: the Kumlafell group, the Hólmatindur group and the Hjálmadalur group. The groups can be traced over 50km and originate in the Breiðdalur–Thingmuli volcanic zone. The groups have flow fields that display mixed volcanic facies architecture and can be classified after dominating type morphology. The Kumlafell and the Hólmatindur groups have predominantly simple flows of pāhoehoe and rubbly pāhoehoe morphologies with minor compound or lobate pāhoehoe flows. The Hjálmadalur group has simple flows of rubbly pāhoehoe, but also includes minor compound or lobate flows of rubble and 'a'ā. Simple flows are most common in the distal and medial areas from the vents, while more lobate flows in proximal areas. The simple flows are formed by extensive sheet lobes that are several kilometers long with plane-parallel contacts, some reaching thicknesses of ~40m (aspect ratios <0.01). They have overlapping contacts and are free of tubes and inflation structures. Their internal structure consists generally of a simple upper vesicular crust, a dense core and a thin basal vesicular zone. The brecciated flow-top is formed by clinker and crustal rubble, the clinker often welded or agglutinated. The simple flows erupted from seemingly short-lived fissures and have the characteristics of cooling-limited flows. We estimate the effusion rates to be ~105m3/s for the simple flows of the Kumlafell and Hólmatindur groups and ~104m3/s for the Hjálmadalur group. The longest flows advanced 15–20km from the fissures, with lava streams of fast propagating flows inducing tearing and brecciation of the chilled crust. Compound or lobate areas appear to reflect areas of low effusion rates or the interaction of the lava with topographic barriers or wetlands, resulting in chaotic flowage. Slowing lobes with brecciated flow-tops developed into 'a'ā flows. The groups interdigitated with lava groups from the Reyðarfjörður volcanic zone to the east, and the exhumed Breiðdalur–Thingmuli volcanic zone which appear to have formed as a flank lineament parallel the main rift zone. Flood basalt volcanism in flank areas may support a mantle anomaly more pronounced and/or perhaps more widespread in the Neogene of Iceland than today. Eruptions of simple flows have not been observed in modern times and are significant for models of crustal accretion in Iceland and other Large Igneous Provinces.

Lithofacies analysis of basic lava flows of the Paraná igneous province in the south hinge of Torres Syncline, Southern Brazil

The Paraná igneous province records the volcanism of the earlier Cretaceous that preceded the fragmentation of the Gondwana supercontinent. Historically, investigations of these rocks prioritized the acquisition of geochemical and isotopic data, considering the volcanic pile as a monotonous succession of tabular flows. This work provides a detailed analysis of the emplacement conditions of these basic volcanic rocks, applying the facies analysis method integrated to petrographic and geochemical data. The Torres Syncline is a NW–SE tectonic structure, located in southern Brazil, where a thick sequence of the Paraná–Etendeka volcanic rocks is well preserved. This study was performed in the south hinge of the syncline, where the basaltic lava flows are divided into three lithofacies associations: early compound pahoehoe, early simple pahoehoe and late simple rubbly. The first lavas that erupted were more primitive compound pahoehoe flow fields composed of olivine basalts with higher MgO contents and covered the sandstones of the Botucatu Formation. The emplacement of compound pahoehoe flow fields is possibly related to intermittent low effusion rates, whereas the emplacement of simple pahoehoe is related to sustained low effusion rates with continuous supply. The thick simple rubbly lavas are associated with high effusion rates and were formed during the main phase of volcanism in the area. The absence of paleosoils between the lavas and lithofacies associations suggests that the successive emplacement of the lava flows occurred in a relatively short time gap. Geochemically, the lithofacies associations are low-TiO2 and belong to Gramado magma type. The lavas of the south hinge of the Torres Syncline have a similar evolution when compared to other Continental Basaltic Provinces with earlier compound flows at the base and thicker simple flows in the upper portions.

Preeruptive flow focussing in dikes feeding historical pillow ridges on the Juan de Fuca and Gorda Ridges

Linear, hummocky pillow mound volcanism dominates at slow and intermediate spreading rate mid‐ocean ridges. Volcanic hummocks are thought to be formed by low effusion rates or as a result of flow focussing during effusive fissure style eruptions in which the initial dike intercepts the seafloor and erupts along its entire length. In this study, high‐resolution autonomous underwater vehicle (AUV) bathymetry is used to accurately map the extents of four historical fissure eruptions of the Juan de Fuca and Gorda ridges: on the North Gorda, North Cleft, and CoAxial ridge segments. The four mapped eruptions take the form of pillow mounds, which are similar in both lithology and dimension to hummocks on the Mid‐Atlantic Ridge. Pillow mounds may be isolated, or coalesce to form composite mounds, aligned as ridges or as clustered groups. In three of the four mapped sites, the eruptions were discontinuous along their lengths, with pillow mounds and composite mounds commonly separated by areas of older seafloor. This style of discontinuous eruption is inconsistent with typical en echelon fissure eruptions and is probably due to a mildly overpressured, fingering dike intersecting the seafloor along parts of its length.

Emplacement of pillow lavas from the ~ 2.8 Ga Chitradurga Greenstone Belt, South India: A physical volcanological, morphometric and geochemical perspective

The physical volcanology and morphometric analyses of pillowed lava flows from the Chitradurga basin of Chitradurga Greenstone Belt, South India have been undertaken. In the Chitradurga hills individual pillowed flows alternate with massive submarine sheet flows. The pillows from such flows are separated by chert and occur as spheroidal, elongated or reniform units that are devoid of vesicles, vesicle bands or pipe vesicles. The Mardihalli flow is exposed as a small elongated mound in the basin and consists of a massive core that is draped by pillows along the flow crest and flanks. The pillows from Mardihalli occur as spheroidal to elongate units with smooth, spalled or wrinkled surfaces with vesicular interiors. Repeated budding of larger pillows have produced a series of interconnected pillow units indicating fluid lava that was emplaced on steeply dipping flanks. Based on the morphological features the pillowed flows from the Chitradurga basin were emplaced at low effusion rates (≤5m3/s). Pillows in these flows formed from low viscosity lavas that underwent negligible to moderate inflation due to rapid chilling. Sporadic occurrences of pillow breccias, hyaloclastite and chert breccias in the pillowed flow fields indicate disruption of pillows due to lava surges and slumping. It is envisaged that the Chitradurga basin witnessed distinct episodes of submarine tholeiite eruptions that produced pillowed lavas that variably interacted with sea water to produce geochemistries. The field and stratigraphic relationships of the volcanics and associated clastic sediments suggest that the pillow lavas were emplaced in a shallow marine marginal inter/back arc basin.

Volatiles contents, degassing and crystallisation of intermediate magmas at Volcan de Colima, Mexico, inferred from melt inclusions

In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical infor- mation on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly H2O) and evolution of intermediate arc magmas at Volcan de Coli- ma, Mexico. The melt inclusions have maximal H2O contents (B4 wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxene- hosted melt inclusions can preserve H2O contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6 m 3 s -1 ). The absence of noticeable H2O loss in some of the inclusions requires post-entrapment diffusion coefficients (B1 9 10 -13 m 2 s -1 ) at least several order of magnitude smaller than experimentally determined H ? diffusion coefficient in pyroxenes. The H2O content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive H2O reequil- ibration, are observed around 1 and 0.2 wt%. H2O diffusive loss is also consistent with the manifest lack of correlations between H2O and CO2 or S contents. The absence of tex- tural evidence supporting post-entrapment H2O loss sug- gests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion popu- lation accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that H2O diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapour- saturated magmas.

Evolution of crust- and core-dominated lava flows using scaling analysis

We present a simple tool to evaluate the dominant dynamical regime of a lava flow and to estimate the order of magnitude of the main rheological parameter (viscosity or yield strength) controlling the length of the lava flow with time. We consider three dynamical regimes: a Newtonian viscous regime, a yield strength-dominated regime and a crust-dominated regime. For each of these regimes, we present a scaling analysis to derive relationships between front position and time, emitted volume, slope, width of the flow and rheological properties. We apply the resulting equations to published data from eruptions of 10 lava flows with a range of compositions and conditions. Comparisons of the fits of the models to the data reveal that short-lived, high effusion rate eruptions are dominated by the internal viscosity of the lava, whereas low effusion rate or long-lived eruptions are dominated by the yield strength in the growing crust. Finally, blocky lavas with very high initial crystal contents are dominated by the internal yield strength. The evolution of some flows can be approximated with only two viscosity values: an early low lava viscosity stage and a later higher viscosity stage. The increase in viscosity is attributed to the initial disequilibrium conditions of the magma at the vent with further degassing and cooling triggering crystallisation of the lava flow. For yield strength-dominated flows yield strength is always within an order of magnitude of 105 Pa. This study provides a practical framework for predicting the evolution of the length of lava flows from estimates of the crystal content of the erupting lava and its effusion rate.

Effect of slope on development of pahoehoe flow fields: Evidence from Mount Etna

We describe and compare two types of pahoehoe on Mount Etna: one similar to an Hawaiian inflated flow field consisting of tumuli, lava rises and lava rise pits; and the other a coalescing complex of rootless centres of ephemeral boccas fed from lava tubes and superimposed on an initial 'a'a flow field. The former we define as a primary pahoehoe field that results from relatively slow advance rate of lava over an almost horizontal surface. The latter is a result of an eruption of long enough duration to allow tube formation over a substantial portion of the flow field over steeper slopes; at breaks in slope in the underlying topography centres of ephemeral boccas develop giving rise to low effusion rate pahoehoe lavas covering the original channelled 'a'a surface. These we refer to as secondary pahoehoe flow fields.

´A´ā lava flows in the Deccan Volcanic Province, India, and their significance for the nature of continental flood basalt eruptions

Newly identified ´a´ā lava flows outcrop intermittently over an area of ~110 km2 in the western Deccan Volcanic Province (DVP), India. They occur in the upper Thakurvadi Formation in the region south of Sangamner. The flows, one of which is compound, are 15–25 m thick, and exhibit well-developed basal and flow-top breccias. The lavas have microcrystalline groundmasses and are porphyritic or glomerocrystic and contain phenocrysts of olivine, clinopyroxene or plagioclase feldspar. They are chemically similar to compound pāhoehoe flows at a similar stratigraphic horizon along the Western Ghats. Petrographic and geochemical differences between ´a´ā flows at widely spaced outcrops at the same stratigraphic horizon suggest that they are the product of several eruptions, potentially from different sources. Their presence in the DVP could suggest relative proximity to vents. This discovery is significant because ´a´ā lavas are generally scarce in large continental flood basalt provinces, which typically consist of numerous inflated compound pāhoehoe lobes and sheet lobes. Their scarcity is intriguing, and may relate to either their occurrence only in poorly preserved or exposed proximal areas or to the flat plateau-like topography of flood basalt provinces that may inhibit channelization and ´a´ā formation, or both. In this context, the ´a´ā flow fields described here are inferred to be the products of eruptions that produced unusually high-effusion-rate lavas compared to typical flood basalt eruptions. Whether these phases were transitional to lower intensity, sustained eruptions that fed extensive low effusion rate pāhoehoe flow fields remains unclear.

Detecting the development of active lava flow fields with a very‐long‐range terrestrial laser scanner and thermal imagery

Regular topographic surveys of active lava flows could provide significant insight into the development of flow fields, but data of sufficient accuracy, spatial extent and repeat frequency to quantify the processes involved have yet to be acquired. Here, we report results from the use of a new very‐long‐range terrestrial laser scanner (TLS) on active lavas at Mount Etna, Sicily. The scanner proved capable of providing useful topographic data from volcanic terrain at ranges up to ∼3500 m, with laser returns from ash‐covered slopes as well as from lava. Despite very low effusion rates (&lt;1 m3s−1), topographic changes associated with the emplacement and inflation of new flows and the inflation of a tumulus were detected. Irregular data spacing resulting from oblique views makes the interpretation of laser‐derived digital elevation models alone difficult, but fusing topographic data with thermal images allows active flow features to be clearly visualized.

Construction dynamics of a lava channel

We use a kinematic GPS and laser range finder survey of a 200 m-long section of the Muliwai a Pele lava channel (Mauna Ulu, Kilauea) to examine the construction processes and flow dynamics responsible for the channel–levee structure. The levees comprise three packages. The basal package comprises an 80–150 m wide ′a′a flow in which a ∼2 m deep and ∼11 m wide channel became centred. This is capped by a second package of thin (<45 cm thick) sheets of pahoehoe extending no more than 50 m from the channel. The upper-most package comprises localised ′a′a overflows. The channel itself contains two blockages located 130 m apart and composed of levee chunks veneered with overflow lava. The channel was emplaced over 50 h, spanning 30 May–2 June, 1974, with the flow front arriving at our section (4.4 km from the vent) 8 h after the eruption began. The basal ′a′a flow thickness yields effusion rates of 35 m3 s−1 for the opening phase, with the initial flow advancing across the mapped section at ∼10 m/min. Short-lived overflows of fluid pahoehoe then built the levee cap, increasing the apparent channel depth to 4.8 m. There were at least six pulses at 90–420 m3 s−1, causing overflow of limited extent lasting no more than 5 min. Brim-full flow conditions were thus extremely short-lived. During a dominant period of below-bank flow, flow depth was ∼2 m with an effusion rate of ∼35 m3 s−1, consistent with the mean output rate (obtained from the total flow bulk volume) of 23–54 m3 s−1. During pulses, levee chunks were plucked and floated down channel to form blockages. In a final low effusion rate phase, lava ponded behind the lower blockage to form a syn-channel pond that fed ′a′a overflow. After the end of the eruption the roofed-over pond continued to drain through the lower blockage, causing the roof to founder. Drainage emplaced inflated flows on the channel floor below the lower blockage for a further ∼10 h. The complex processes involved in levee–channel construction of this short-lived case show that care must be taken when using channel dimensions to infer flow dynamics. In our case, the full channel depth is not exposed. Instead the channel floor morphology reflects late stage pond filling and drainage rather than true channel-contained flow. Components of the compound levee relate to different flow regimes operating at different times during the eruption and associated with different effusion rates, flow dynamics and time scales. For example, although high effusion rate, brim-full flow was maintained for a small fraction of the channel lifetime, it emplaced a pile of pahoehoe overflow units that account for 60% of the total levee height. We show how time-varying volume flux is an important parameter in controlling channel construction dynamics. Because the complex history of lava delivery to a channel system is recorded by the final channel morphology, time-varying flow dynamics can be determined from the channel morphology. Developing methods for quantifying detailed flux histories for effusive events from the evidence in outcrop is therefore highly valuable. We here achieve this by using high-resolution spatial data for a channel system at Kilauea. This study not only indicates those physical and dynamic characteristics that are typical for basaltic lava flows on Hawaiian volcanoes, but also a methodology that can be widely applied to effusive basaltic eruptions.