Dispersal Axis Research Articles

Holocene eruptions within the Katla volcanic system, south Iceland: Characteristics and environmental impact

Holocene volcanism within the Katla volcanic system is characterized by: 1) explosive (hydromagmatic) basaltic eruptions along volcanic fissures within the Mýrdalsjökull caldera; 2) explosive silicic eruptions from vents associated with the caldera and 3) predominantly effusive basaltic eruptions involving both the central volcano and the fissure swarm. Typical Katla eruptions are accompanied by basaltic tephra fall, lightning and glacial floods (jökulhlaups) of meltwater, ice and volcanic debris. Twenty eruptions have occurred in the last 11 centuries. The volume of airborne tephra varies by three orders of magnitude, with an estimated volume of 1.5 km³ of freshly fallen tephra from the largest historical Katla eruption. The length of documented eruptions varies from 2 weeks to over 5 months. The average repose period since 1500 AD is 47 years with maximum deviations of 33 and 34 years. All Katla eruptions during the last 400 years have begun in the springfall season. At least 12 silicic Katla eruptions are known from the period ca. 1700 BP and 6600 BP. The silicic magma was most likely erupted by hydromagmatic explosive eruptions. The tephra dispersal axes indicate vent locations within the caldera or along the caldera rim. The volume of airborne silicic tephra varies by orders of magnitude, the largest and most widespread is tephra layer UN with uncompacted tephra volume of 0.3 km³. Intervals between the silicic eruptions have varied from ca. 100 to ca. 1000 14C yrs. Two major “fires” and 5–10 relatively minor, partly effusive eruptions have occurred during the Holocene. The 10th century Eldgjá and the 6800 BP Hólmsá fires are the largest known Holocene eruptions within the Katla system. A ≥75 km long, discontinuous and partly subglacial eruptive fissure was active during the Eldgjá eruption. The opening phase on most fissure segments was explosive, followed by an effusive phase on the subaerial segments. The eruption produced a voluminous basaltic tephra layer with a minute silicic component, two major lava fields and possibly a hyaloclastic flow deposit. Large jökulhlaups accompanied the eruption. The combined volume of erupted material may exceed 19 km³ DRE. Eruptions on the Katla system have caused extensive environmental changes during the past 1100 years. The Eldgjá fires radically changed the landscape, hydrology and utilization potential of large areas in South Iceland. Since then, jökulhlaups accompanying eruptions within the caldera have escaped eastwards, raising the Mýrdalssandur plain and extending its coastline southward.

Characterization of a subpicosecond x-ray streak camera for ultrashort laser-produced plasmas experiments

We present results of the characterization of an ultrafast x-ray streak camera, based on Photonis (formerly Philips Photonics) P860 tube, developed for use in ultrashort laser-produced plasma research. The streak camera presented here (called PX1) has been extensively characterized with continuous and pulsed x-ray sources. Time resolution of 350 fs in the keV x-ray range has been achieved, while maintaining a high spatial resolution of 40 μm along a direction perpendicular to the time dispersion axis. It is shown that the streak camera response is lower when the photocathode is illuminated by a pulsed source than when used with a continuous one. This effect seems to be related to a change in the phosphor response. The camera has been used to achieve high-resolution subpicosecond time-resolved spectroscopy of ultrashort laser plasmas allowing the measurements of K-shell line emission durations of 700 fs.

Plinian pumice fall deposit of the Campanian Ignimbrite eruption (Phlegraean Fields, Italy)

A plinian pumice fall deposit associated with the Campanian Ignimbrite eruption (36 ka, Phlegraean Fields caldera, Italy) occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km 2 between Benevento and the Sorrentina peninsula. In the thickest stratigraphic section at Voscone (130 cm), 45 km east of the Phlegraean caldera centre (Pozzuoli), the deposit consists of two units: the lower fall unit (LFU) is well sorted, exhibits reverse size grading and is composed of equidimensional light-grey pumice clasts with very subordinate accidental lithics; the upper fall unit (UFU) is from well to poorly sorted, crudely stratified, richer in lithics and composed of both equidimensional and prolate pumice clasts. The two fall units show slightly different dispersal axis: N90° for the LFU and N95° for the UFU. Volumes calculated with the method of Pyle (1989) are about 8 km 3 for the LFU and 7 km 3 for the UFU. The maximum height of the eruptive columns are estimated, using the model of the maximum lithic clasts dispersal, at 44 km for the LFU and 40 km for the UFU, classifying both fall units as ultraplinian in character. Reverse size grading within the LFU suggests an increase of the height of the column and magma discharge rate with time. Moderate sorting and crude stratification of the UFU are consistent with short-period oscillation of the column, possibly associated with repeated partial column collapses. Sharp increases in lithic content at the transition to UFU and within the UFU suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit/vent erosion. The phase of column instability preceded the emplacement of widely dispersed pyroclastic flow. The ultraplinian nature of the fall fits well with the wide dispersal of the Campanian Ignimbrite with an estimated aspect ratio of 3–4×10 −4 (LARI).

The Agnano–Monte Spina eruption (4100 years BP) in the restless Campi Flegrei caldera (Italy)

The Agnano–Monte Spina tephra (AMST), dated at 4100 years BP by 40 Ar/ 39 Ar and 14 C AMS techniques, is the product of the highest-magnitude eruption in the Campi Flegrei caldera (CFc) during its last epoch of activity (4800–3800 years BP). The sequence alternates magmatic and phreatomagmatic pyroclastic-fallout, -flow and -surge beds and bedsets. Two main pumice-fallout deposits with variable easterly-to-northeasterly dispersal axes are about 10 cm thick at 42 km from the vent area. High particle concentration pyroclastic currents were confined to the caldera depression; lower concentration flows overtopped the morphological boundary of the caldera and traveled at least 15 km over the surrounding plain. The unit is subdivided into six members, named A through F in stratigraphic sequence, based upon their sedimentological characteristics. Isopachs and isopleths maps suggest a vent location in the Agnano plain. A volcano-tectonic collapse begun during the course of the eruption, took place along the faults of the northeastern sector of the resurgent block within the CFc, and generated the Agnano plain. The early erupted trachytic magma had a homogeneous alkali–trachytic composition, whereas later-erupted magma shows small-scale hetereogeneities. Trace elements and Sr-isotope compositions, indicate that two isotopically distinct magmas, one alkali–trachytic and the other trachytic, were tapped and partially mixed during the eruption. The small volume (1.2 km 3 DRE) of erupted magma and the structural position of the vent suggest that the eruption was fed by a dyke intruded along a normal fault in the sector of the resurgent block under a tensional stress regime.

Hemispherical Analyser with 2-D PSD for Zero-degree Auger Projectile Spectroscopy

Details of a new high gain zero-degree Auger projectile electron spectrograph using a hemispherical analyser and a 2-dimensional position sensitive detector (PSD) with multichannel plates and a resistive anode encoder are presented. A four-element lens mounted at the entrance of the analyser, provides a virtual slit for the incoming electrons by focusing them while at the same time decelerating them to improve their energy resolution. Electrons enter through an aperture at a position R0 which is displaced (along the energy dispersion axis) with respect to the commonly used central entrance position at ½(R1 + R2). The analyser has an acceptance energy range of 20% and an energy resolution of 0.9%. An ion-optics trajectory simulation indicates improved focusing properties for this off-center position thus avoiding the need for cumbersome fringing field correction schemes. Test measurements of high resolution projectile Auger spectra produced in 21.7 MeV collisions of F8+ and F7+ projectiles with H2 and He are presented.

Holocene plinian eruption of La Virgen volcano, Baja California, Mexico

A plinian eruption occurred approximately 6500 yr ago at La Virgen Volcano, the youngest volcano of the Tres Virgenes Volcanic Complex (TVVC), located in Baja California, Mexico. Deposits of the eruption suggest a sequence of events that started with the opening of the volcanic conduit, and development of a plinian eruption column up to 18 km in height. This eruption column produced a fallout deposit with a dispersal axis toward the southwest, an areal extent of about 500 km 2, and a minimum volume of 1.14 km 3. Vulcanian activity (hydromagmatic) followed the plinian phase, producing pyroclastic surge and fallout deposits. The eruptive activity ceased after a basaltic-andesite lava flow was emplaced closing the eruptive activity. Petrological and geochemical evidence indicates that the eruption was triggered by magma mixing processes. Our studies confirm that La Virgen is a dormant volcano with the potential for future violent eruptions. The present study provides important information for the construction of a volcanic hazards map. Significant hazards are presented to the population living within a distance of 30 km from the volcano, together with the interstate road connecting the entire peninsula of Baja California, which runs at a distance of only 3 km from the volcano.

Complexities of plinian fall deposition at vent: an example from the 1912 Novarupta eruption (Alaska)

An extremely proximal ejecta ring, with exposures to within 100 m of vent, was deposited during later-stage plinian fall activity during the 1912 Novarupta eruption in Alaska. One bed in the ejecta ring (bed S) contains predominantly andesitic clasts which serve to delineate the striking contrast in thinning rates along dispersal axis of the ejecta ring [Pyle b t values of 70 m (bed S alone) or 190 m (whole ejecta ring)] and the coeval dacitic plinian fall deposits [Pyle b t values of 4 km (proximal) and 37 km (medial-distal)]. The locally deposited andesitic and dacitic clasts of the ejecta ring are interpreted as products of an irregular ‘collar’ of low-fountaining ejecta partially sheathing the core of higher-velocity dacitic ejecta that fed the stable, converting 23-km-high column. The presence of such an extremely proximal accumulation of ejecta appears to be a feature common to several other historic eruptions that generated widespread fall deposits. This feature in part accounts for conflicts between measured and calculated values for thickness maxima in plinian fall deposits and suggests that modifications may be required of existing models for plinian eruption columns.

Plinian fall deposits from Vulsini Volcanic District (Central Italy)

Stratigraphic investigations of the Vulsini Volcanic District indicate the existence of five prominent plinian fall deposits, deposited over a period of about 0.34 Ma. The oldest deposit (‘Basal pumices’) crops out mainly in the peripheral areas and is one of the largest plinian events in the Vulsini District, with a volume of about 9 km3 and a calculated column height exceeding 30 km. Subcircular patterns of isopachs and isopleths around the Bolsena lake indicate emplacement in wind-free conditions and suggest a source vent location in the northernmost sector of the present Bolsena lake, where it is possible that an older central volcanic structure existed. The four younger plinian fall deposits are related to Bolsena activity. The oldest of these, named the ‘Ponticello’ and ‘Orvieto-Bagnoregio’ pumices, are mainly distributed on the north-eastern sector of the Vulsini District. Their volumes are an order of magnitude lower than that of the basal unit, and the estimated column heights do not exceed 20 km. The younger ‘Ospedaletto’ pumice deposit has a NE-SW dispersal axis, whereas the youngest ‘Casetta’ pumice deposit is found only in the north-eastern sector of the district. Their estimated volumes are respectively 1.2 and 0.1 km3, whereas the inferred vent areas appear to be located slightly to the east of the Ponticello and Orvieto-Bagnoregio pumice deposit source areas. The chronology of the Ospedaletto and Casetta pumices indicates that the final plinian activity from the Bolsena complex is contemporaneous with the Latera activity. The decreasing volumes from the oldest to the youngest units, together with the progressive shifting northeastwards of the source vents, may be related to the volcano-tectonic subsidence of the Bolsena area. The source vents for the post-Basal pumices events are mainly clustered just north of Bolsena village where the greatest displacement during subsidence occurred. Accordingly, source vent shifting appears to be related to the progressive opening of normal faults from the inner to the outer margin of the Bolsena depression.

Tephra from the Minoan eruption of Santorini in sediments of the Black Sea

THE explosive eruption of Santorini volcano in the Aegean Sea about 3,300 years ago is of considerable archaeological and volcanological significance1–5. Here we report the discovery of tephra from this Minoan event in laminated sediments of the Black Sea. This finding provides constraints on the distribution of debris from the eruption. We estimate a minimum fallout area of 2×l06 km2 extending from the Black Sea in the north to the southeastern Mediterranean Sea. The main dispersal axis trends through southern Turkey, in agreement with other studies of Minoan tephra6,7. The tephra deposits should provide a useful reference horizon for assessing the chronology of Black Sea sediments, which has been much debated8–15.

Ashfall dispersal for the 16 September 1986, eruption of Lascar, Chile, calculated by a turbulent diffusion model

A two dimensional model for the dispersal of volcanic ash, modified from that of Suzuki [1983], is applied to the 16 September 1986, eruption of Lascar, in northern Chile. The result is an isopach map predicting ground concentrations of ash. The map shows two local maxima along distinct dispersal axes and predicts that the deposit was detached from the source. Predicted concentrations are in agreement with a single reported thickness measurement.

Eruption dynamics and magma withdrawal during the Plinian Phase of the Bishop Tuff Eruption, Long Valley Caldera

Variations in eruption column height, magma discharge (intensity), and geochemistry have been documented for the plinian phase of the Bishop Tuff eruption from the Long Valley caldera, California. Although exposures of the complete Bishop fall deposit are sparse, their widespread relationships, knowledge of the plinian vent location, and a reasonable estimate of the main dispersal axis allow for the construction of minimum clast isopleths. Column height and intensity are estimated to have increased from 23 km and 7×107 kg s−1 to a peak of 45 km and 7.5×108 kg s−1 at the end of the plinian phase. Variations in geochemistry of the plinian deposit reveal complex magma withdrawal processes during the Bishop plinian phase. Concentrations of many incompatible trace elements increase from the base to midlevel and then decrease to the top of the plinian deposit. This trend is mirrored by the variation in pumice density, which in turn reflects vesicularity. Based on the physical volcanology and compositional variations in the plinian deposit, a model is proposed for the temporal progression of magma withdrawal during the plinian phase. Relatively less‐evolved magma was first tapped by the plinian phase from the side of the magma chamber, with the conduit location being controlled by the Hilton Creek fault system. As eruption intensity increased, more‐evolved magma from the chamber roof was drawn down and tapped. Less‐evolved magma was again tapped for the duration of the plinian phase, possibly as withdrawal from the roof zone became less efficient. Renewed eruption of slightly more evolved magma by the first post‐plinian ignimbrites suggests that the vent migrated along the ring fault system to an area that had not experienced previous magma withdrawal.

Calibrated soft x-ray spectroscopy using transmission grating, pinhole and film

The combination of transmission diffraction grating (free-standing gold bars), pinhole and film can be used for quantitative measurements of the x-ray spectra in the 0.1–1.5-keV region by adopting the procedures described. The steps consist of film calibration and deconvolution of overlapping contributions from different spectral orders. In addition, spatial resolution is available in the direction perpendicular to the dispersion axis for each x-ray energy. The working example presented is the characterization of an x-ray source created by a laser-target interaction, for possible use in x-ray microscopy or x-ray lithography.

Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns

A theoretical model of clast fallout from convective eruption columns has been developed which quantifies how the maximum clast size dispersal is determined by column height and wind strength. An eruption column consists of a buoyant convecting region which rises to a heightH B where the column density equals that of the atmosphere. AboveH B the column rises further to a heightH T due to excess momentum. BetweenH T andH B the column is forced laterally into the atmosphere to form an upper umbrella region. Within the eruption column, the vertical and horizontal velocity fields can be calculated from exprimental and theoretical studies and consideration of mass continuity. The centreline vertical velocity falls as a nearly linear function over most of the column's height and the velocity decreases as a gaussian function radially away from the centreline. Both column height and vertical velocity are strong functions of magma discharge rate. From calculations of the velocity field and the terminal fall velocity of clasts, a series of particle support envelopes has been constructed which represents positions where the column vertical velocity and terminal velocity are equal for a clast of specific size and density. The maximum range of a clast is determined in the absence of wind by the maximum width of the clast support envelope. The trajectories of clasts leaving their relevant support envelope at its maximum width have been modelled in columns from 6 to 43 km high with no wind and in a wind field. From these calculations the shapes and areas of maximum grain size contours of the air-fall deposit have been predicted. For the no wind case the theoretical isopleths show good agreement with the Fogo A plinian deposit in the Azores. A diagram has been constructed which plots, for a particular clast size, the maximum range normal to the dispersal axis against the downward range. From the diagram the column height (and hence magma discharge rate) and wind velocity can be determined. Historic plinian eruptions of Santa Maria (1902) and Mount St. Helens (1980) give maximum heights of 34 and 19 km respectively and maximum wind speeds at the tropopause of m/s and 30 m/s respectively. Both estimates are in good agreement with observations. The model has been applied to a number of other plinian deposits, including the ultraplinian phase of theA.D. 180 Taupo eruption in New Zealand which had an estimated column height of 51 km and wind velocity of 27 m/s.

Influence of particle aggregation on deposition of distal tephra from the MAy 18, 1980, eruption of Mount St. Helens volcano

The May 18, 1980, eruption of Mount St. Helens (MSH) produced an extensive ashfall deposit in Washington, Idaho, and Montana with a minumum volume of 0.55 km3 (tephra). An unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ENE of MSH near Ritzville, Washington. Grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained (mean size, 22 μm), poorly sorted, polymodal, and rich in glass shards and pumice fragments. A computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics, particularly the thickness versus distance relationship. The model was constrained by observations of the eruption column height, elevation of major ash transport, lateral spreading of the eruption plume, and atmospheric wind structure in the vicinity of MSH. Results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central Washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause. For the model to fit the observed characteristics of the deposit, significant particle aggregation of ash finer than 63 μm must be invoked. The best fit occurs when ash less than 63 μm is aggregated into particles several hundred microns in diameter with a settling velocity of 0.35 m/s. Support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at Pullman, Washington, during the May 18 eruption (Sorem, 1982). The premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics, variations in component abundances, and thickness versus distance relationship of the May 18 MSH ash fall deposit.

Deep-sea evidence for distribution of tephra from the mixed magma eruption of the Soufrière on St. Vincent, 1902: Ash turbidites and air fall

The 7 May 1902 eruption of the Soufriere on St. Vincent in the Lesser Antilles island arc was a mixed magma eruption that produced both andesitic (65%) and dacitic magma (35%) of narrow compositional range. Evidence from land and deep-sea coring indicates that the eruption produced three types of mixed deposits that have been correlated by means of electron microprobe analyses of glass shards: air-fall ash in the Tobago Trough east of St. Vincent, pyroclastic flow deposits on flanks of the volcano, and an ash turbidite in the Grenada Trough west of St. Vincent, which formed during pyroclastic flow discharge into the sea off the west coast of the volcano and subsequent slumping of the coastline. The main dispersal axis and fallout pattern of ash as deduced from contemporary ships' logs and coring data, indicate that air-fall distribution was controlled by antitrade winds, with transport velocities of approximately 60 km/hr.

Explosive volcanic eruptions — a new classification scheme

A classification scheme is proposed based on measurements made on the resulting pyroclastic fall deposits, the significant parameters being the area of dispersal and degree of fragmentation of the material. An empirical measure of the first is the area enclosed by the 0.01 Tmax isopach (where Tmax is the maximum thickness of the deposit), called D, which ranges from less than 10 km2 for deposits of strongly cone-building type to more than 1000 km2 for deposits of strongly sheet-forming type. An empirical measure of the second is the percentage of material finer than 1 mm in the deposit, or more simply at the point where the 0.1 Tmax isopach crosses the dispersal axis. The latter value, called F, varies from less than 20 for deposits in which fragmentation was mainly achieved by the tearing apart of magma, to more than 80 where it was largely due to thermal shock resulting from the quenching of lava by water.

Remarks on Mr. Espy's theory of centripetal storms, including a refutation of his positions relative to the storm of September 3rd, 1821: with some notice of the fallacies which appear in his examinations of other storms

Somma-Vesuvius is a composite volcano on the southern margin of the Campanian Plain which has been active during the last 39 ka BP and which poses a hazard and risk for the main population center situated around its base. The fieldwork and data analysis on which this report is based are related to the eight Plinian eruptions that have occurred in the last 25 ka. For six of these eruptions, the fallout products were dispersed to the east–northeast, whereas deposits from the 25 ka Codola and AD 79 eruptions were dispersed in a south-easterly direction. During the AD 79 eruption, in particular, the dispersal axis migrated from the east–southeast to south–southeast. New high level wind data collected at the weather stations of the Aereonautica Militare data centres at Pratica di Mare (Rome) and Brindisi have been compiled to characterize the prevailing wind condition in the Somma-Vesuvius region. The common north-easterly dispersal directions of the Plinian eruptions are consistent with the distribution of ash by high-altitude winds from October to June. In contrast, the south-easterly trend of the AD 79 products appears to be anomalous, because the eruption is conventionally believed to have occurred on the 24th of August, when its southeast dispersive trend falls in a transitional period from the Summer to Autumnal wind regimes. In fact, the AD 79 tephra dispersive direction towards the southeast is not in agreement with the June–August high-altitude wind directions that are toward the west. This poses serious doubt about the date of the eruption and the mismatch raises the hypothesis that the eruption occurred in the Autumnal climatic period, when high-altitude winds were also blowing towards the southeast. New archaeological findings presented in this study definitively place the date of eruption in the Autumn, in good agreement with the prevailing high-altitude wind directions above Somma-Vesuvius.Moreover, wind data and past eruptive behaviour indicate that a future subplinian–Plinian eruption at Somma-Vesuvius has a good chance to occur when winds are blowing toward the eastern sectors (northeast–southeast), in the Autumnal–Winter period, and only a slightest chance in Summer, when winds are blowing toward the west, depositing ash fallout on the Neapolitan community.