New paleomagnetic pole and magnetostratigraphy of Faroe Islands flood volcanics, North Atlantic igneous province

A paleomagnetic sampling was carried out along four sections (altogether 86 lava flows, 548 samples) in the North Atlantic Igneous Province outcropping in Faroe Islands, Denmark. The four polarity zones in the 700-m-thick exposed part of the Faroes lower formation can be correlated with the geomagnetic polarity time scale as C26n-C25r-C25n-C24r. The seven lava flows erupted during C25n indicate a very low eruption rate in the upper part of the Faroes lower formation of ∼1/70 kyr. The Faroes middle and upper formations (composite thickness ∼2300 m) are all reversely magnetized corresponding to C24r. The eruption rate at the onset of middle formation volcanism was very high as evidenced by several thick lava sequences recording essentially spot readings of the paleomagnetic field. The shift in eruption rate between the Faroes lower and middle formations and evidence that onset of the Faroes middle formation volcanism took place in C24r are of particular importance, placing onset of middle formation volcanism in close temporal relation to North Atlantic continental break-up and the late Paleocene thermal maximum. After grouping flows recording the same field directions, we obtained 43 independent readings of the paleomagnetic field, yielding a paleomagnetic pole with coordinates 71.4°N, 154.7°E (A₉₅=6.0°, K=14, N=43); age 55-58 Ma. The pole is supported by a positive reversal test. Paleosecular variation, estimated as the angular standard deviation of the virtual geomagnetic pole distribution 21.7°+3.9°/−2.8°, is close to expected for the given age and paleolatitude. Our new Faroes paleomagnetic pole is statistically different from the majority of previously published poles from the British and Faroes igneous provinces, and we suggest that these older data should be used with care.     

Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Komatiites of the 3.5-Ga Komati Formation are ultramafic lavas (>23% MgO) erupted in a submarine, lava plain environment. Newly discovered vesicular komatiites have vesicular upper crusts disrupted by synvolcanic structures that are similar to inflation-related structures of modern lava flows. Detailed outcrop maps reveal flows with upper vesicular zones, 2-15 m thick, which were (1) rotated by differential inflation, (2) intruded by dikes from the interior of the flow, (3) extended, forming a flooded graben, and/or (4) entirely engulfed. The largest inflated structure is a tumulus with 20 m of surface relief, which was covered by a compound flow unit of spinifex flow lobes. The lava that inflated and rotated the upper vesicular crust did not vesiculate, but crystallized as a thick spinifex zone with fist-size skeletal olivine. Instead of representing rapidly cooled lava, the spinifex zone cooled slowly beneath an insulating upper crust during inflation. Overpressure of the inflating lava may have inhibited vesiculation. This work describes the oldest vesicular komatiites known, illustrates the first field evidence for inflated structures in komatiite flows, proposes a new factor in the development of spinifex zones, and concludes that the inflation model is useful for understanding the evolution of komatiite submarine flow fields.     

Role of magma mixing during the recent activity of La Fossa di vulcano (Aeolian Islands, Italy)

Relatively homogeneous trachytes have been erupted for approximately 3800 years at la Fossa di Vulcano. From the Punte Nere eruptive cycle up to the Palizzi cycle the products varied little, while after the Palizzi cycle (1600 + 1000 a B.P.) to the latest eruption, 1888–1890 AD, a spectrum of compositions, with rhyolite dominating, characterized the erupted products.A stratigraphic sequence, starting with the Palizzi lava flow, has been studied, focussing the attention on lavas and volcanic bombs, to define the role that magma mixing processes have played in the recent history of La Fossa di Vulcano. Textural and chemical analyses of whole rocks, glass, groundmass, and mineral phases indicate that only the breadcrust bombs, erupted during the 1888–1890, show evidence of mixing between trachytic and rhyolitic end-members. Interestingly, in the deposits of the same eruption, trachytic bombs also occur.The lava flows erupted before 1888–1890 display general features suggesting that they entrained crystals and lava fragments during magma ascent. During the 1888–1890 eruption the trachytic bombs were erupted before the breadcrust bombs, which have a more evolved and hybrid composition. These characteristics, together with the change of the nature of the products after the Palizzi cycle, require a complex volcanological model for the recent history of la Fossa di Vulcano.     

Morphology and propagation styles of Miocene submarine basanite lavas at Stanley, northwestern Tasmania, Australia

Miocene submarine basanite pillows, lava lobes, megapillows and sheet lavas in the Stanley Peninsula, northwestern Tasmania, Australia, are well-preserved in three dimensions. The pillows have ropy wrinkles, transverse wrinkles, symmetrical wrinkles, contraction cracks and three types of spreading cracks on their surfaces, and concentric and radial joints in the interior. The lava lobes have ropy wrinkles and contraction cracks on their surfaces. The megapillows are cylindrical with a smoothly curved upper surface and steep sides, and are characterized by distinct radial columnar joints in the interior. They are connected to pillows that propagate radially from its basal margin. The sheet lavas are tabular and have vertical columnar joints in the interior. The largest sheet lava shows a remarkable gradation from a lower 5-m-thick pillow facies to an upper massive facies. The pillows, lava lobes, megapillows and sheet lavas are inferred to have been emplaced completely below sea level but in a shallow marine environment. Their morphological features suggest that the pillows grew by episodic rupture of a near-solid crust and emergence of hot lava, whereas the lava lobes propagated by continuous stretching of the outer skin at the flow front. The megapillows and sheet lavas were master feeder channels by which molten lava was conveyed to the advancing pillows. The sheet lavas propagated by repeated processes of pillow formation and overriding by an upper massive part. Alternating pillow and massive facies commonly found in ocean-floor drill cores and exposed in cross-section in many subaqueous volcanic successions may have formed by propagation of pillows from the basal margins of advancing sheet lavas.     

Deformation structures associated with the Tazo landslide (La Gomera, Canary Islands)

Deformation structures below the basal plane of gravitational slides can provide useful information about the state of stress undergone by rocks prior to the sliding process and about the triggering forces acting at each particular sliding event. In the present work we conducted a structural analysis of the rocks below the surface of the gravitational slide of Tazo (La Gomera, Canary Islands) and determined the epigenetic processes involved in the filling of the amphitheatre. We also inferred the possible triggering phenomena related to the Tazo landslide. The rocks located below the surface of the gravitational slide of Tazo -i.e., the basaltic lava flows, sills and dikes of the Lower Old Edifice and the submarine volcanic rocks, gabbros, pyroxenites and dikes of the Basal Complex of La Gomera- are strongly deformed close to this sliding surface. The lava flows and dikes of the Lower Old Edifice are folded, with fault breccias and gouges, and locally foliated, defining the sliding surface. The dikes of the Basal Complex are also folded, and the gabbros and pyroxenites are affected by a large number of small faults. In the Basal Complex, the sliding surface is defined by a foliated granular gouge. In the damage zone, the Basal Complex rocks show an incipient fracture cleavage. The sliding amphitheatre has been filled by the debris avalanche or cohesive debris flow generated within the slide, as well as by later debris flows, hyperconcentrated flows, sheet flows, and by interspersed lava flows from the Upper Old Edifice. We suggest here that the collapse of the north-western flank of the Lower Old Edifice at Tazo could in part have been triggered by continuous magma injection, associated with the emplacement of dikes in a rift zone with an ENE-WSW direction, enhanced by the mechanical weakness of the Basal Complex unit, which was affected by hydrothermal metamorphism under greenschist facies conditions and by the displacement along the Montaña de Alcalá and Guillama normal faults, which are deeply entrenched in the altered rocks of the Basal Complex.     

Seismic attenuation in Faroe Islands basalts

We analysed vertical seismic profiling (VSP) data from two boreholes at Glyvursnes and Vestmanna on the island of Streymoy, Faroe Islands, to determine the magnitude and causes of seismic attenuation in sequences of basalt flows. The work is part of SeiFaBa, a major project integrating data from vertical and offset VSP, surface seismic surveys, core samples and wireline log data from the two boreholes. Values of effective seismic quality factor (Q) obtained at Glyvursnes and Vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image. This observation is consistent with results from other VSP experiments in the North Atlantic region. We demonstrate that the most likely cause of the low values of effective Q at Glyvursnes and Vestmanna is a combination of 1D scattering and intrinsic attenuation due to seismic wave‐induced fluid flow within pores and micro‐cracks. Tests involving 3D elastic wave numerical modelling with a hypothetical basalt model based on field observations, indicate that little scattering attenuation is caused by lateral variations in basalt structure.     

Integrated field, satellite and petrological observations of the November 2010 eruption of Erta Ale

Erta Ale volcano, Ethiopia, erupted in November 2010, emplacing new lava flows on the main crater floor, the first such eruption from the southern pit into the main crater since 1973, and the first eruption at this remote volcano in the modern satellite age. For many decades, Erta Ale has contained a persistently active lava lake which is ordinarily confined, several tens of metres below the level of the main crater, within the southern pit. We combine on-the-ground field observations with multispectral imaging from the SEVIRI satellite to reconstruct the entire eruptive episode beginning on 11 November and ending prior to 14 December 2010. A period of quiescence occurred between 14 and 19 November. The main eruptive activity developed between 19 and 22 November, finally subsiding to pre-eruptive levels between 8 and 15 December. The estimated total volume of lava erupted is ??0.006?km³. The mineralogy of the 2010 lava is plagioclase?+?clinopyroxene?+?olivine. Geochemically, the lava is slightly more mafic than previously erupted lava lining the caldera floor, but lies within the range of historical lavas from Erta Ale. SIMS analysis of olivine-hosted melt inclusions shows the Erta Ale lavas to be relatively volatile-poor, with H₂O contents ??1,300?ppm and CO₂ contents of ??200?ppm. Incompatible trace and volatile element systematics of melt inclusions show, however, that the November 2010 lavas were volatile-saturated, and that degassing and crystallisation occurred concomitantly. Volatile saturation pressures are in the range 7?C42?MPa, indicating shallow crystallisation. Calculated pre-eruption and melt inclusion entrapment temperatures from mineral/liquid thermometers are ??1,150?°C, consistent with previously published field measurements.     

The influence of palaeoenvironment and lava flux on the emplacement of submarine, near-shore Late Permian basalt lavas, Sydney Basin (Australia)

Lava flux and a low palaeoslope were the critical factors in determining the development of different facies in the Late Permian Blow Hole flow, which comprises a series of shoshonitic basalt lavas and associated volcaniclastic detritus in the southern Sydney Basin of eastern Australia. The unit consists of a lower lobe and sheet facies, a middle tube and breccia facies, and an upper columnar-jointed facies. Close similarities in petrography and geochemistry between the basalt lavas from the three facies suggest similar viscosities at similar temperatures. Sedimentological and palaeontological evidence from the sedimentary units immediately below the Blow Hole flow suggests that the lower part of the volcanic unit was emplaced in a cold water, shallow submarine environment, but at least the top of the uppermost lava was subaerial with some palaeosol development. The lower lobe and sheet facies was emplaced on a low slope (<2°) in a lower to middle shoreface environment with water depths of 20–25 m. Lava may have transgressed from subaerial to subaqueous and was emplaced relatively passively with lava flux sufficiently high and uniform to form lobes and sheets rather than pillows. The middle unit probably originated from a subaerial vent and flowed into a shallow (10–15 m) submarine environment, and wave action probably interacted with the advancing lava front to form a lava delta. Lava flux was sufficiently high to produce well-developed, subcircular lava tubes, which lack evidence for thermal erosion. In some areas, lava ‘burrowed’ into the unconsolidated, water-saturated lava delta and sand pile to produce intrusive contacts. The upper columnar-jointed unit represents a ponded facies probably emplaced initially in water depths <5 m but whose top was subaerial.     

Faroe Shelf Water

Faroe Shelf Water (FSW) is the water mass that occupies the shallow parts of the Faroe Shelf, surrounding the Faroe Islands (62°N, 7°W). Intensive tidal mixing induces a high degree of homogeneity and the circulation system allows a partial isolation from surrounding waters. This water mass, therefore, supports a unique ecosystem of great importance for commercial fish stocks and studies have shown a clear dependence of the ecosystem on the physical processes that maintain this system and control the exchange between the FSW and the off-shelf waters. In order to identify and quantify these processes, a large observational dataset has been analysed and related to alternative theories. From this analysis, the extent and properties of the FSW have been quantified and the degree of stratification explained in terms of the Simpson–Hunter theory. The residual clockwise circulation system, which is responsible for the partial isolation from off-shelf waters, is shown to be mainly generated by tidal rectification. The typical exchange rate of water between the FSW and the off-shelf regimes has been determined by the use of simple models based on the heat and the salt budgets but the actual exchange rate is found to vary considerably in time and space. These results support earlier suggestions that this exchange is the main limiting factor for the phytoplankton spring bloom on the Faroe Shelf and that variations in exchange rate are responsible for the large inter-annual variation in spring bloom timing and intensity. The observations indicate that the on-shelf/off-shelf exchange intensity is not symmetrically distributed around the shelf, but rather concentrated around the narrow southern tip of the Faroe Shelf, where off-shelf waters during intensive exchange events may be imported all the way to the shore.     

The geology of Mount Hasan stratovolcano, central Anatolia, Turkey

Mount Hasan is a double-peaked stratovolcano, located in Central Anatolia, Turkey. The magmas erupted from this multi-caldera complex range from basalt to rhyolite, but are dominated by andesite and dacite. Two terminal cones (Big Mt. Hasan and Small Mt. Hasan) culminate at 3253 m and 3069 m respectively. There are four evolutionary stages in the history of the volcanic complex (stage 1: Kecikalesi volcano, 13 Ma, stage 2: Palaeovolcano, 7 Ma, stage 3: Mesovolcano and stage 4: Neovolcano). The eruptive products consist of lava flows, lava domes, and pyroclastic rocks. The later include ignimbrites, phreatomagmatic intrusive breccias and nuées ardentes, sometimes reworked as lahars. The total volume is estimated to be 354 km³, the area extent 760 km². Textural and mineralogical data suggest that both magma mixing and fractional crystallization were involved in the generation of the andesites and dacites. The magmas erupted from the central volcanoes show a transition with time from tholeite to calc-alkaline. Three generations of basaltic strombolian cones and lava flows were emplaced contemporaneously with the central volcanoes. The corresponding lavas are alkaline with a sodic tendency.     

Video and seismic observations of Strombolian eruptions at Erebus volcano,Antarctica

Between 1986 and 1990 the eruptive activity of Erebus volcano was monitored by a video camera with on-screen time code and recorded on video tape. Corresponding seismic and acoustic signals were recorded from a network of 6 geophones and 2 infrasonic microphones. Two hundred Strombolian explosions and three lava flows which were erupted from 7 vents were captured on video. In December 1986 the Strombolian eruptions ejected bombs and ash. In November 1987 large bubble-bursting Strombolian eruptions were observed. The bubbles burst when the bubble walls thinned to ∼ 20 cm. Explosions with bomb flight-times up to 14.5 s were accompanied by seismic signals with our local size estimate, “unified magnitudes” (m_u), up to 2.3. Explosions in pools of lava formed by flows in the Inner Crater were comparatively weak.     

Phreatomagmatic to Strombolian eruptive activity of basaltic cinder cones: Montaña Los Erales, Tenerife, Canary Islands

Phreatomagmatic activity results from the interaction of magma and external water during a volcanic eruption and is a frequent eruptive phenomenon worldwide. Such ‘fuel-coolant’ reactions change the eruptive dynamics, thus generating particles that reflect the degree of explosivity. Different eruptive phases may thus be identified from deposits, allowing us to reconstruct conditions that prevailed in the past and use these to predict the level of explosivity in a given geological setting in the future.A detailed study of the deposits from Montaña Los Erales, a 70 m-high Quaternary cinder cone belonging to a rift-related chain of vents in the Bandas del Sur region, in Southeast Tenerife, was undertaken. Field observations on excavated sections and SEM analysis of tephra suggest that the eruption style changed progressively from an initial phreatomagmatic phase, through a transitional stage, to one that was entirely Strombolian. To investigate the causes and the nature of these changes in eruptive style, products from each major unit were analysed for their morphology using hand specimen observations, secondary electron microscopy, backscatter electron microscopy, and reflected light microscopy to examine fragment size variation, fragment morphologies, vesicularity, and the level of secondary hydrous alteration (e.g. palagonitisation and zeolitisation). Study results demonstrate that the initial phase of activity was largely driven by magma–water interaction, where magma may have interacted with a lens of fresh ground- or surface water. With proceeding eruptive activity the water became exhausted, giving rise to an entirely Strombolian eruptive style.Examples of phreatomagmatic activity that occur on typical rift-related basaltic vent alignments are not infrequent in the Canary Islands. These vent systems usually erupt in Strombolian fashion, producing scoria and lava flows that do not generally extend far beyond the vent area. However, aligned feeders may intersect different strata, structural features, and different hydrological situations, thus giving rise to activity that is less predictable in setting, intensity, and duration. The occurrence of phreatomagmatism in an otherwise low-explosivity basaltic eruptive environment increases the need for awareness of the geo-hydrological situation in volcanically active areas. Studying the past eruptive history is therefore essential to derive realistic scenarios for future vulnerability evaluation and risk assessment, especially in densely populated areas like the Canary Islands.     

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

Newly identified ??a?? 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?C25?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 p??hoehoe flows at a similar stratigraphic horizon along the Western Ghats. Petrographic and geochemical differences between ??a?? 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?? lavas are generally scarce in large continental flood basalt provinces, which typically consist of numerous inflated compound p??hoehoe 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?? formation, or both. In this context, the ??a?? 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 p??hoehoe flow fields remains unclear.     

The rio caliente ignimbrite: Analysis of a compound intraplinian ignimbrite from a major late quaternary Mexican eruption

The Rio Caliente ignimbrite is a multi-flow unit orcompound ignimbrite formed during a major late Quaternary explosive rhyolitic eruption of La Primavera volcano, Mexico. The eruption sequence of the ignimbrite is complex and it occurs between lower and upper plinian air-fall deposits. It is, therefore, anintraplinian ignimbrite. Air-fall layers, pyroclastic surge, mudflow and fluviatile reworked pumice deposits also occur interbedded between ignimbrite flow units. A chaotic near-vent facies of the ignimbrite includes co-ignimbrite lag breccias segregated from proximal pumice flows. The facies locates a central vent but one which could not have been associated with a well defined edifice. Many of the lithics in the exposed lag breccias and near-vent facies of the ignimbrite appear to be fragments of welded Rio Caliente ignimbrite, and indicate considerable vent widening, or migration, during the eruption. Nearer vent the ignimbrite is thickest and composed of the largest number of flow units. Here it is welded and is a simple cooling unit. Evidence suggests that it was only the larger thicker pumice flows that escaped to the outer parts of the sheet. Detailed analysis of four flow units indicates that the pumice flows were generally poorly expanded, less mobile flows which would be produced by collapse of low eruption columns. The analogy of a compound ignimbrite with a compound lava flow is, therefore, good — a compound lava flow forms instead of a simple one when the volumetric discharge rate (or intensity) is low, and in explosive eruptions this predicts lower eruption column heights. A corollary is that the ignimbrite has a high aspect ratio. The complex eruption sequence shows the reinstatement of plinian activity several times during the eruption after column collapse occurred. This, together with erosional breaks and evidence that solidified fragments of already welded ignimbrite were re-ejected, all suggest the eruption lasted a relatively significant time period. Nearly 90 km³ of tephra were erupted. The associated plinian pumice fall is one of the largest known having a volume of 50 km³ and the ignimbrite, plus a co-ignimbrite ash-fall, have a volume of nearly 40 km³. Published welding models applied to the reejected welded blocks indicate an eruption duration of 15-20d, and a maximum average magma-discharge rate of 1.4 × 10⁴ m³/s for the ignimbrite. This is low intensity when compared with available data from other ignimbrite-forming eruptions, and concurs with all the geological evidence presented. The total eruption duration was perhaps 15-31d, which is consistent with other estimates of the duration of large magnitude explosive silicic eruptions.     

Morphological and textural diversity of the Steens Basalt lava flows,Southeastern Oregon,USA: implications for emplacement style and nature of eruptive episodes

This study focuses on Middle Miocene tholeiitic flood basalt lava flows from the Oregon Plateau, northwestern USA (Steens Basalt), and is the first to comprehensively document and evaluate their morphology. Field observations of flows from several sections within and proximal to the main exposures at Steens Mountain have been supplemented with textural and geochemical data, and are used to offer preliminary insights into their emplacement. Compound pahoehoe flows of variable thickness appear to be common throughout the study area, laterally and vertically. These tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as Hawaii and the Snake River Plain. Classic a’a flows with brecciated upper and basal crusts are not abundant, but by no means uncommon. Flows with characters different from typical pahoehoe and a’a are also common. Such flows display a range in morphology; flows with preserved upper crusts but brecciated basal crusts, as well as those displaying well-developed flow-top breccias and preserved basal crusts (rubbly pahoehoe) are observed. The Steens Basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces. The abundant compound pahoehoe flows (often rich in plagioclase phenocrysts) were likely emplaced during slow but sustained eruptive episodes; their constituent lobes show clear evidence for endogenous growth. The relatively aphyric flows with brecciated surfaces (including a’a) hint at higher strain rates and/or higher viscosity, probably caused by higher effusion rates. A couple of sections are characterized by compositionally similar, but morphologically different flows that were possibly part of the same eruption. While differences in pre-eruptive topography could explain this, it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system. It is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems, and deserve closer scrutiny.     

Snow-contact volcanic facies and their use in determining past eruptive environments at Nevados de Chillán volcano, Chile

Studies of the eruptive products from volcanoes with variable ice and snow cover and a long history of activity enable reconstruction of erupted palaeoenvironments, as well as highlighting the hazards associated with meltwater production, such as jökulhlaups and magma-water interaction. Existing difficulties include estimation of ice/snow thicknesses and discrimination between ice- and snow-contact lithofacies. We present field evidence from the Cerro Blanco subcomplex of Nevados de Chillán stratovolcano, central Chile, which has erupted numerous times in glacial and non-glacial periods and most recently produced andesitic lava flows in the 1861–1865 eruption from the Santa Gertrudis cone on the northwest flank of the volcano. The main period of lava effusion occurred during the winter of 1861 when the upper flanks of the volcano were reportedly covered in snow and ice. The bases and margins of the first lava flows produced are cut by arcuate fractures, which are interpreted as snow-contact features formed when steam generated from the melting of snow entered tensional fractures at the flow base. In contrast, the interiors and upper parts of these flows, as well as the overlying flow units, have autobrecciated and blocky textures typical of subaerial conditions, due to insulation by the underlying lava. Similar textures found in a lava flow dated at 90.0±0.6 ka that was emplaced on the northwest flank of Cerro Blanco, are also inferred to be ice and snow-contact features. These textures have been used to infer that a small valley glacier, overlain by snow, existed in the Santa Gertrudis Valley at the time of the eruption. Such reconstructions are important for determining the long-term evolution of the volcano as well as assessing future hazards at seasonally snow-covered volcanoes.     

A quantitative look at the demise of a basaltic vent: the death of Kupaianaha,Kilauea Volcano,Hawai'i

 The Kupaianaha vent, the source of the 48th episode of the 1983-to-present Pu'u 'O'o–Kupaianaha eruption, erupted nearly continuously from July 1986 until February 1992. This investigation documents the geophysical and geologic monitoring of the final 10 months of activity at the Kupaianaha vent. Detailed very low frequency (VLF) electromagnetic profiles across the single lava tube transporting lava from the vent were used to determine the cross-sectional area of the molten lava within the tube. Combined with measurements of lava velocity, these data provide an estimate of the lava output of Kupaianaha. In addition, lava temperatures (calculated from analysis of quenched glass) and bulk-rock chemistry were obtained for samples taken from the tube at the same site. The combined data set shows the lava flux from Kupaianaha vent declining linearly from 250 000 m³/day in April 1991 to 54 000 m³/day by November 1991. During that time surface breakouts of lava from weak points along the tube occurred progressively closer to the vent, consistent with declining efficiency in lava transport. There were no significant changes in lava temperature or in bulk MgO content during this period. Another eruptive episode (the 49th) began uprift of Kupaianaha on 8 November 1991 and erupted lava concurrently with Kupaianaha for 18 days. Lava flux from Kupaianaha decreased in response to this new episode, but the response was delayed by approximately 1 day. After 14 November 1991, lava velocities were no longer measurable in the tube because the lava stream beneath the skylight had crusted over; however, the VLF-derived electrical conductances documented the decreasing flux of molten lava through the tube. Kupaianaha remained active, but output continued to decrease until early February 1992 when the last active surface flows were seen. In November 1991 we used the linearly decreasing effusion rate to accurately predict the date for the death of the Kupaianaha vent. The linear nature of the decline in lava tube conductance and the delayed and slow response of the Waha'ula tube conductances to the 49th eruptive episode led us to speculate that (a) the Kupaianaha vent shut down because of a decrease in driving pressure and not because of a freeze-up of the vent, and (b) that Pu'u 'O'o, episode 49, and Kupaianaha were fed nearly vertically from a source deep within the rift zone. Received: 29 September 1995 / Accepted: 21 November 1995     

Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.     

Study of an uncommon lava sheet in the bolsena district (central Italy)

In the Bolsena volcanic district (Vulcani Vulsini) near the little town of Pitigliano, at the border between Latium and Tuscany, a lava sheet showing quite peculiar geological features has been examined in all its macro and microscopic aspects. Such volcanic deposit is an interesting example of the uncommon products originated by the Quaternary alkaline mediterranean volcanism. The Pitigliano sheet was erupted from the NW section of the Latera caldera, and covered an area of about 6×12 km mantaining an extremely small thickness (from a feet to few yards). This sheet is normally made of a leucitic lava entirely so densely microvesiculated as to be better called pumice. At the flanks, and expecially at the front of the sheet, the rock gets a piperno-like or welded tuff aspect. The different facies and the transitions between them have been carefully studied and interpreted.