Spatter Deposits Research Articles

The Morphology of Martian Pyroclastic Ramparts and Their Use in Determining Vent‐Proximal Eruption Dynamics

AbstractHigh‐resolution investigations of Late Amazonian volcanic landforms provide previously unrevealed insights into the dynamics of Martian volcanic eruptions. On Earth, the formation of vent‐proximal accumulations of spatter deposits is attributed to low‐intensity lava fountaining episodes representing eruptions on the very edge of explosive activity. Martian spatter deposits form small‐scale volcanic landforms that are rarely reported, and thus the dynamics of Martian mafic explosive eruptions are still not fully constrained. We conducted high‐resolution Context Camera‐based mapping coupled with a stereo‐pair‐generated digital elevation model to reconstruct the eruptive history of a fissure system and its associated products south of Ascraeus Mons, Mars. The studied volcanic fissure clearly demonstrates both explosive and effusive deposits and, in addition, is spatially associated with a lava channel. For the first time, these observations allowed us to conduct a comparative analysis of vent‐proximal volcanic products and reconstruct the late‐stage eruption dynamics of a fissure system. We found that the spatial distribution of the pyroclastic (spatter) rampart along the fissure vent is heterogeneous and generated using dynamic eruption processes. Moreover, the lava channel fed from the fissure vent shows evidence of successive lava overspills whose emplacement was topographically controlled. These observations suggest that, in contrast to the general inference that Amazonian‐age volcanism mainly involves effusive eruptions, explosive‐origin landforms might have been overlooked. Therefore, we argue that high‐resolution mapping of pyroclastic deposits may provide critical insights into understanding the dynamic nature of Martian fissure eruptions and explosive‐associated volatile release during the last stages of eruptions.

Vector Magnetometry Using Remotely Piloted Aircraft Systems: An Example of Application for Planetary Exploration

Geomagnetic prospection is an efficient and environmentally friendly geophysical method for the analysis of the magnetic minerals’ distribution in the subsurface. High-resolution measurements require on-ground campaigns. However, these activities might imply high costs, risk and time consumption. Some more recent works have started to use magnetometers on-board remote piloted aircrafts. Normally, they fly at a constant altitude and use scalar probes. This configuration permits the determination of the magnitude of the magnetic field but not the direction, and requires advanced techniques for in-depth interpretation of the sources. In this manuscript, we describe the accommodation of a system for vector magnetometry in a drone whose flight altitude follows the elevation of the terrain. This singularity improves the capability of interpretation, including constraints in dating due to the record of the geomagnetic field. The work consists of the design, development and implementation of a solidary payload system anchored to the body of the platform in order to determine the vector magnetic field. It describes the details of the system and the performance characteristics obtained after the calibration, as well as its demonstration via a field campaign in the spatter deposits of Cerro Gordo volcano in Campos de Calatrava volcanic province in Spain.

Variability of Spatter Morphology in Pyroclastic Deposits in Southern Idaho, as Correlated to Thermal Conditions and Eruptive Environment

AbstractTo investigate the thermal conditions of spatter eruptions on Earth, other planets, and moons we measured and categorized the physical characteristics of clasts in spatter deposits in Southern Idaho, USA. Physical characteristics of clasts, such as aspect ratio and degree of welding, are influenced by the style of eruption, distance from the vent, and lava properties. The correlation between morphological characteristics and the degree of fusion constrains thermal conditions of unwitnessed spatter‐producing eruptions on Earth and extraterrestrial volcanoes. We categorize spatter deposits into three groups, as defined by their emplacement conditions: primary vent, distal (spatter exposed in noneruptive cracks on the flanks of primary vents), and hornito (secondary spatter from a nonprimary cone); and show the variability of clast morphology within the three categories. Primary vent deposits typically show a wide range of fused perimeters between clasts (18–82%), lower aspect ratios (0.20–0.49), and larger void space, both between clasts (4–21%) and within clasts (referred to as hollow cores, up to 93%). Hornitos typically have less fusion (14–22%) between clasts, higher aspect ratios (0.45–0.49), and lower proportions of void space (6–10%). Distal outcrops are easily distinguished by the lack of void space (<8%) and the very high aspect ratio of clasts (close to 1). When chemical composition and outcrop proximity are accounted for, the accumulation rate appears to be primarily responsible for spatter clast morphology. Our results are based on field measurements of clasts; however, digital imagery could be used to categorize and interpret pyroclastic deposits in planetary systems by robotic spacecraft.

Spatter matters \u2013 distinguishing primary (eruptive) and secondary (non-eruptive) spatter deposits

Spatter is a common pyroclastic product of hawaiian fountaining, which typically forms vent-proximal ramparts or cones. Based on textural characteristics and field relations of spatter from the 1969 Mauna Ulu eruption of Kīlauea, Hawai’i, three spatter types were identified: (1) Primary spatter deposited as spatter ramparts and isolated cones during the peak of episode 1; (2) Late-stage spatter comprising dense, small volume, vent proximal deposits, formed at the end of episode 1; (3) Secondary spatter preserved in isolated mounds around tectonic ground cracks that we interpret to have formed by the disruption of overlying lava. We propose that not all spatter deposits are evidence of primary magmatic fountaining. Rather, deposits can be “secondary” in nature and associated with lava drain-back, disruption, and subsequent ejection from tectonic cracks. Importantly, these secondary pyroclastic deposits are difficult to distinguish from primary eruptive features based on field relations and bulk clast vesicularity alone, allowing for the potential misinterpretation of eruption vents, on Earth and in remotely sensed planetary data, thereby misinforming hazard maps and probabilistic assessments. Here, we show that vesicle number density provides a statistically-robust metric by which to discriminate primary and secondary spatter, supporting accurate identification of eruptive vents.

Boninite and boninite-series volcanics in northern Zambales ophiolite: doubly vergent subduction initiation along Philippine Sea plate margins

Abstract. A key component of subduction initiation rock suites is boninite, a high-magnesium andesite that is uniquely predominant in western Pacific forearc terranes and in select Tethyan ophiolites such as Oman and Troodos. We report, for the first time, the discovery of low-calcium, high-silica boninite in the middle Eocene Zambales ophiolite (Luzon Island, Philippines). Olivine–orthopyroxene microphyric high-silica boninite, olivine–clinopyroxene–phyric low-silica boninite and boninitic basalt occur as lapilli fall deposits and pillow lava flows in the upper volcanic unit of the juvenile arc section (Barlo locality, Acoje Block) of the Zambales ophiolite. This upper volcanic unit overlies a lower volcanic unit consisting of basaltic andesite, andesite to dacitic lavas and explosive eruptive material (subaqueous pahoehoe and lobate sheet flows, agglutinate and spatter deposits) forming a low-silica boninite series. The overall volcanic stratigraphy of the extrusive sequence at Barlo resembles holes U1439 and U1442 drilled by IODP Expedition 352 in the Izu–Ogasawara (Bonin) trench slope. The presence of depleted proto-arc basalts in the Coto Block (45 Ma) (Geary et al., 1989), boninite and boninite series volcanics in Barlo (Acoje Block (44 Ma)) and simultaneous and post-boninite moderate-Fe arc tholeiites in Sual and Subic areas of the Acoje Block (44–43 Ma) indicate that the observed subduction initiation stratigraphy in the Izu–Ogasawara–Mariana forearc is also present in the Zambales ophiolite. Paleolatitudes derived from tilt-corrected sites in the Acoje Block place the juvenile arc of northern Zambales ophiolite in the western margin of the Philippine Sea plate. In this scenario, the origin of Philippine Sea plate boninites (IBM and Zambales) would be in a doubly vergent subduction initiation setting.

Intra-cone plumbing system and eruptive dynamics of small-volume basaltic volcanoes: A case study in the Calatrava Volcanic Field

The Manoteras volcano (Tortonian to Pleistocene, Calatrava Volcanic Field, Spain) is composed of a scoria and spatter cone surrounded by a field of pahoehoe lava. The volcanic cone is made essentially of vitreous lapilli-tuffs with intercalations of vitreous tuffs and spatter deposits, without any intercalations of lava flows. Erosion has uncovered an intra-cone plumbing system formed by coherent dykes and pyroclastic dykes (mixed-type dykes). This dyke swarm reflects processes of intrusion at the end of the eruption or even post-eruption. All the volcanic products are nephelinitic in composition. The main dyke is up to 3.4m thick and has an exposed length of ~1000m. It is composed mostly of coherent nephelinite with some pyroclastic sections at its northern extremity. This dyke is regarded as a feeder dyke of the volcano, although the upper parts of the dike have been eroded, which prevents the observation of the characteristics and nature of the possible overlying vent(s). Mixed-type dykes could also have acted as small linear vents and indicate that the magma fragmentation level during final waning stages of the eruption was located inside the volcanic cone. The pyroclastic deposits that make up the volcanic cone at the current exposure level were probably developed during a major phase of violent Strombolian style that formed the scoria cone, followed by a Hawaiian phase that formed the summital intracrater spatter deposit. Three central-type vents have been identified: one at the highest point of the remnant volcanic cone (summital vent), from where the earlier explosive eruptions took place, and the other two at the fringe of the cone base, from where emissions were only effusive. The lava flows were emitted from these boccas through the scoria cone feeding the lava field. The results obtained, based on careful field observations, add substantial complexity to the proposed eruptive models for small-volume basaltic volcanoes as it appears evident that there may exist and evolution through time from central conduit settings to fissure eruptions. Moreover, it is shown that intracone plumbing systems can integrate coherent and clastic dykes of variable thicknesses, which, in some cases could represent feeder dykes.

Extrusive carbonatite outcrops – A source of chemical elements imbalance in topsoils of oceanic volcanic islands

Extrusive carbonatite ash tuff (Tantum) and spatter (Monte Miranda) deposits, and topsoils (incipient and shallow) from both sites of Brava Island (Cape Verde archipelago) were studied by neutron activation analysis, X-ray diffraction and Mössbauer spectroscopy. The ash tuff deposit near Tantum, here reported for the first time, corresponds to an extrusive calciocarbonatite with high W, Th, U and REE contents and a positive Eu anomaly. Similar REE patterns and W concentrations were also found in the spatter deposit of Monte Miranda. Magnetite is the major iron containing phase in both extrusive carbonatite deposits. High W and REE concentrations associated with the presence of calcite prevail in topsoils with extrusive carbonatite influence. In the topsoil developed on extrusive carbonatite ashes, magnetite is well preserved particularly in the coarser fractions probably due to the recent age of the parent material associated with the semi-arid climate of the island; a significant chemical heterogeneity of the whole samples as well as of the different size fractions of these topsoils was found, with an enrichment of Co, Zn, Ga, As, Br, Rb, Ce, and particularly Sb and W in the clay-size fraction, which may be relevant for the study of environmental health issues in Brava Island.

Pulsed Nd: YAG laser drilling of aerospace materials (Ti-6Al-4V)

This paper studies the influence of Nd:YAG (neodymium-doped yttrium aluminium garnet) laser process parameters on laser drilled hole quality. Ti-6Al-4V of 1 mm and 3 mm thickness were used as the workpiece substrate. The principal findings are mainly based on minimising the taper angle in laser drilled holes, reducing the heat affected zone and reducing the production of spatter. Identification of key process variables associated with laser drilling process is accomplished by trial experimentation. Using the identified key process variables, further experiments were then performed with the assistance of statistical design of experiment (DOE) to find the interaction and individual effects of various laser process parameters on laser drilled hole quality. The lowest taper angle of 1.8 degrees was achieved with use of nitrogen as the assist gas. Furthermore, from the laser process observations, it was found that laser power significantly affects the quality of the laser drilled hole. Increase in laser power would increase the hole size and result in more spatter on the entry hole surfaces. The nozzle focus position substantially influenced the laser drilled hole size. The amount of spatter deposits increased with decrease in the nozzle offset. Increase in laser frequency significantly increased the exit diameter, which resulted in smaller taper angle. Number of pulse required to drill through a workpiece depends on the material properties and physical properties of the material. For 1mm Ti-6Al-4V, a minimum of two pulses was required to successfully removed the material during drilling and a minimum of 4 pulses was required to drill through the same material with 3mm thickness.

Arrested diatreme development: Standing Rocks East, Hopi Buttes, Navajo Nation, USA

Maar–diatreme volcanoes, defined by their relatively large pyroclastic debris-filled subsurface structures and craters that cut into the pre-eruptive land surface, are typically found in small-volume mafic to ultramafic monogenetic volcanic fields. Diatremes are associated with strong explosions throughout most of their development, focused along feeder dikes and generally attributed to magma–water interaction, or high magmatic volatiles. Detailed mapping of the magnificently exposed Standing Rocks East (SRE) diatreme shows evidence of additional eruptive complexity, and offers new insights into how the plumbing and vent structures of small-volume volcanoes evolve during an eruption.SRE is part of a larger, basanitic volcanic complex that includes several diatremes formed along a series of irregular, offset NW–SE trending dikes exposed 300m below the pre-eruptive land surface. Its similarly oriented elliptical-shaped diatreme structure comprises predominantly country rock lithic-rich breccia of coarse inhomogeneously mixed wall-rock blocks sourced from above and below the current surface, plus sparse juvenile material. Domains of pyroclastic deposits crosscut the country rock breccia deposits, and the best exposed is the NW massif rising 35m above the current erosional surface. It represents a cross-section of an evolving crater floor, and comprises matrix-rich lapilli tuff and spatter deposits cut by irregularly distributed dikes, some with very complex textures. The most significant deposit, in terms of volume, is an unbedded lapilli tuff that is poorly sorted and has a well-mixed population of wall-rock and juvenile clast varieties, thus resembling deposits typical of diatremes. It is overlain by and locally intercalated with spatter deposits, and this irregular contact demarcates the base of what was during eruption an uneven, evolving crater floor. The generally massive, variably welded spatter deposits constitute mostly lapilli-sized juvenile clasts with fluidal, folded-over shapes and ropy surfaces, subordinate thermally altered wall-rock and variegated domains of lapilli tuff.SRE shows a progressive transition from fissure to diatreme, and overall evolution from more explosive to weakly explosive eruption styles recorded at the conduit-crater transition. Diatreme development was initiated by deep-quarrying explosive eruptions along a fissure to form the country rock-rich breccia. Only parts of the fissure remained active as magma feeding the highly explosive eruptions along the fissure localized into discrete point sources forming the matrix-rich lapilli tuff deposits. These superimposed deposits record the passage of multiple debris-jets and subvertical fallback from shallow cratering arising from explosions triggered by magma–water interaction at numerous, discrete sites. However, instead of continuing to build a well-formed diatreme, the system switched to weak spattering with intermittent explosive activity and near-surface dike emplacement into the unconsolidated anisotropic, pyroclastic debris of the crater floor. Dominant spatter from strombolian-style bursts accumulated on the topographically varied, evolving unstable syn-eruptive crater floor, and led to local failure and remobilization. This study demonstrates how the combination of fissure behavior and sensitivity of the shallow plumbing system to local conditions during an eruption can lead to a decrease in eruptive footprint within the diatreme structure, and an overall decrease in explosivity resulting in the arrested development of an immature diatreme.

Eruption conditions of spatter deposits

Spatter is an eruptive product that forms within a narrow range of thermal conditions: it must be hot enough to deform and agglutinate, but not so hot that clasts completely re-fuse and remobilize as clastogenic lava. This narrow thermal window of spatter-forming conditions allows for quantitative prediction of cooling rates and accumulation rates. Cooling and accumulation rates then provide information that enables estimates of eruption parameters for inaccessible and prehistoric deposits. High-temperature experiments conducted on basaltic scoria from Devil’s Garden, Oregon have revealed the eruption temperature was ~1130°C. The strength welds formed between experimental clasts is shown to depend on cooling rate. Natural samples are compared to the experimental samples by measuring tensile strength and welded area between clasts. The weld strength in natural deposits yields estimates of cooling rates that range between 2.5°C and 48°C/min, with the majority of the samples grouping between 7°C and 14°C/min. Thermal models based on these cooling rates yield spatter accumulation rates of 0.5–1.8m/h in the Devil’s Garden spatter deposits. We provide a general model for cooling and accumulation rates for spatter cones, ramparts, and hornitos, which allow estimation of the factors that control basaltic eruptive products.

Post-depositional intrusion and extrusion through a scoria and spatter cone of fountain-fed nephelinite lavas (Las Herrerías volcano, Calatrava, Spain)

The effusion of lava flows from the base of volcanic cones is a common process in continental monogenetic basaltic fields. However, there are no descriptions in the literature of the structures that channel the lava output through the cone. The Pliocene-Quaternary Las Herrerias volcano (Calatrava, Spain) was constructed from the superposition of a nephelinite spatter and scoria cone and related lavas over a maar. Different eruptive styles contributed to the construction of the current volcanic cone: phreatomagmatic, Hawaiian, Strombolian and violent Strombolian. Quarrying has exposed a lava pond inside the volcanic cone, the intrusion of lava through cone-forming pyroclastic deposits and the ponding of associated lava flows within the maar crater. Low fire-fountaining activity formed a lava pond that stagnated within the crater protected by the cone’s highly welded spatter deposits at its base and overlying scoria deposits. Once the pressure exerted by ponding lava and overlying pyroclastic deposits exceeded the yield strength of the damming rock walls, the melt oozed out through fractures to form an intricate network of dikes and sills and extruded through the lower part of the volcanic cone, forming lava flows. We propose that lava intrusions similar to those here described may represent the final stage of feeding systems for lava flows associated with scoria and spatter cones.

Reconnaissance Volcanic Geology of the Batamote Mountains, Ajo, Arizona

The Batamote Mountains represent a 14-16-million-year-old eroded shield volcano complex in the Basin and Range province near Ajo, Arizona. Analysis of Landsat 7 Enhanced Thematic Mapper “Plus” data and fieldwork enabled mapping the geology at 1:24,000 scale. Volcanic deposits and field relations indicate that mid-Tertiary eruptions initially involved mild explosions from at least six vents, evidenced by near-surface intrusive units surrounded by ash, oxidized cinders, and spatter deposits. The associated lava flows partly cover the pre-Batamote basement silicic volcanic rocks. At least three subsequent effusive phases emplaced multiple flows from a minimum of six vents on the cinder cone deposits and earlier lava flows. The last volcanic event involved fire-fountains that produced several short, thin, agglutinated flows, forming the steeper summit of the volcano. Extension and normal faulting caused rocks in the central portion of the Batamote Mountains to collapse; subsequent erosion of summit material left the amphitheater morphology seen today.

The transition of spatter to lava-like body in lava fountain deposits: features and examples from the Cabezo Segura volcano (Calatrava, Spain)

The Cabezo Segura II volcanic cone (Calatrava volcanic province, Iberian microplate) comprises proximal wall deposits with a well defined crater wall unconformity and crater-fill deposits. The complex volcanic succession, that shows evidence of several eruptive episodes, was built by magmatic and hydrovolcanic explosions of different styles (Strombolian, Hawaiian, sub-Plinian and phreato-Strombolian) generated from a multiple feeder ultrabasic dyke. Intra-crater rock units at the volcano summit include spatter deposits together with up to 10m thick and more than 200m long lava-like bodies. Geological logs for the main lava-like bodies define a characteristic facies model that involves a central lava-like mass which grades vertically into a transition zone of apparently coherent spatter, then dense spatter and, finally, into vuggy spatter deposits. These units are inferred to have formed during pulsating lava fountain-type explosive eruptions; the depicted facies distribution being the result of progressive increase in welding grade and densification of the spatter in response to variations in the accumulation rate. Their field features may be used as a guide for the precise identification of vent sites in deposits of Hawaiian eruptions. Also, structures like those here recognised, that might have survived in lava-like flows, could be of help to identify when lava-producing eruptions represented an explosive Hawaiian event (lava fountains) and not a purely effusive event.

Paroxysmal activity at Stromboli: lessons from the past

The persistent normal activity of Stromboli is occasionally interrupted by sudden and highly energetic explosive events called Strombolian paroxysms. These phenomena together with landslide-generated tsunamis represent the most hazardous manifestations of present-day volcanic activity at Stromboli. The most recent paroxysms, on 5 April 2003 and 15 March 2007, have drawn attention to these energetic events because they significantly threatened inhabitants and tourists. Historical accounts and field evidence indicate, however, that even larger paroxysms, in terms of volume, dispersal of products and intensity of explosive phenomena, occurred in the recent past. During these paroxysms incipiently welded spatter deposits mantled the north and south rims of the Sciara del Fuoco down to low elevations, extending much farther than the similar deposits from recent observed events (5 April 2003 and 15 March 2007). In order to identify, characterize and discriminate among products of these outstanding spatter-forming eruptions, more than 50 stratigraphic sections were measured and sampled. Stratigraphic, sedimentological and radiometric (14C) data indicate that only two paroxysms produced spatter that reached very low elevations and inhabited areas: the first occurred in the 16th century and the last in AD 1930. Analysis of texture and deposit components reveals that the early phases of the two eruptions were driven by distinctly different eruptive dynamics. Both identified paroxysms are at least one order of magnitude greater than any similar event observed by monitoring systems at Stromboli. These two large paroxysms were the most powerful volcanic events at Stromboli in the last eighteen centuries.

Hawaiian and Strombolian style monogenetic volcanism in the extra-Andean domain of central-west Argentina

In the provinces of Mendoza and La Pampa (central-western Argentina), retroarc volcanism was active from the Miocene through historic times. There are approximately 40 basaltic eruptive centers between 36°S and 37°30′S to the East of 68°40′W. These centers were studied in order to define their volcanological evolution. The vents are constituted of seven lithofacies: i.e., (A-1) weakly welded lapilli and bomb beds; (A-2) agglutinated spatter and bomb beds; (A-3) agglutinated spatter beds; (B) ash beds; (C) lava flows; (D) conduit fill; and (E) dikes. Three eruptive episodes (phases) were identified. The first one generated agglutinated spatter and bomb beds (lithofacies A-2), which constitute the internal part of the edifices. These are followed by weakly welded lapilli and bomb beds (lithofacies A-1) and agglutinated spatter beds (lithofacies A-3). The final episode includes lava flows (lithofacies C), minor agglutinated spatter deposits (lithofacies A-3) and isolated lapilli and bomb clasts. We postulate Hawaiian style fountaining activity as responsible for the generation of agglutinated spatter and bombs beds, and variations between Strombolian and Hawaiian activity for the formation of successions of weakly welded lapilli and bomb beds and agglutinated spatter beds. The vent features and the homogeneity of the erupted magma suggest that the studied volcanoes were monogenetic.

Spatter

We have used a combination of field studies, laboratory analog experiments and computational fluid dynamics to determine the factors controlling the deformation and agglutination of spatter. Our field studies identify a spectrum of clast types produced by fire-fountains, ranging from brittle scoria through to fluidal droplets that coalesce. The clast types are related to conditions in the parent fire-fountain. There are a number of different outcomes for a clast impacting the ground, ranging from brittle fracture, through folding to fluidal splashing. Laboratory analog experiments and computational fluid dynamics were used to quantify both the outcome and the type of deformation experienced by viscous clasts impacting the ground. We find that a clast will splash on hitting the ground, forming daughter droplets, if its impact Reynolds number is greater than ca. 80. This condition is readily met for clasts of at least 10 cm diameter, falling at terminal speed, if the magma viscosity is up to 10 5 Pa s. Splashing is predicted to be a common process during deposition of inner-fountain clasts and therefore to play an important role in agglutination of near-vent spatter deposits.

Paleomagnetism of spatter lavas from Stromboli volcano (Aeolian Islands, Italy): Implications for the age of paroxysmal eruptions

We report on 270 paleomagnetic directions retrieved from 17 different spatter deposits spread over the northern and western flanks of the Stromboli volcano (Aeolian Islands, Italy). The spatter were emplaced during previously undated high‐energy Strombolian eruptions occurring after the third‐seventh century AD. Our paleomagnetic directions were superimposed over the paleosecular variation curve of the geomagnetic field for the last two millennia, obtained by reducing to the coordinates of Stromboli the archeomagnetic and magnetic observatory data from France and Italy. This comparison suggests that most of the spatter exposed in the northern flank of the volcano were emplaced during several eruptions occurring between ca. 1400 and 1600 AD, while the western flank of Stromboli was diffusely spread by spatter during the powerful XX century eruptions. Conversely, there is no evidence for paroxysmal fire fountain activity occurring before 1400 AD, and between 1600 and 1907 AD.

The thermal history of a spatter-fed lava flow: the 8-ka pantellerite flow of Mayor Island, New Zealand

The thermal history of the 8-ka spatter-fed lava flow of Mayor Island, New Zealand, has been investigated via relaxation geospeedometry. Cooling rates extant during the vitrification of the clastogenic pantellerite flow have been quantified. Cooling rates modelled for two obsidian layers at the top and the base of the flow, vary over four orders of magnitude. The flow base defined by a lower obsidian layer shows a gradual increase in cooling rates from 0.00072 K/min adjacent to the crystalline core of the flow to 0.017 K/min at the contact to a basal crumble breccia. Cooling rates of flow ridges formed by an upper obsidian layer vary between 0.001 and 6 K/min. Based on field observation and thermal modelling, cooling rates of Kelvins to tens of Kelvins per day for both upper and lower obsidians are difficult to interpret as simple cooling processes. Instead, we propose a thermal history involving thermal annealing as an alternative explanation for these slow effective cooling rates. Such thermal annealing appears to have facilitated welding and rheomorphism of the spatter deposit leading to the development of a coherent lava sheet. Based on the results obtained here, we present a quantitative scenario of the thermal history during the final stages of flow emplacement as a contribution to understanding the rheology of rheomorphic spatter deposits associated with explosive peralkaline rhyolitic volcanism and briefly address the hazard potential of such deposits.

Radiocarbon Dates for Lava Flows and Pyroclastic Deposits on São Miguel, Azores

We report 63 new radiocarbon analyses of samples from São Miguel, the largest (ca 62 × 13km) and most populous (ca 150,000 inhabitants) island in the Azores archipelago (Fig 1). The samples are mainly carbonized tree roots and other plant material collected from beneath 20 mafic lava flows and spatter deposits and from within and beneath 42 trachytic pyroclastic flow, pyroclastic surge, mudflow, pumice-fall, and lacustrine deposits and lava flows. One calcite date is reported. The samples were collected during geologic mapping of the entire island (Moore, in press A; sample locations are shown on this map). Nine 1:25,000-scale topographic maps, published in 1983 by the Portuguese Army Cartographic Service, cover the island; samples and locations described below refer to these named sheets.