Plinian Phase Research Articles

Young pumice deposits on Nisyros, Greece

The island of Nisyros (Aegean Sea) consists of a silicic volcanic sequence upon a base of mafic-andesitic hyaloclastites, lava flows, and breccias. We distinguish two young silicic eruptive cycles each consisting of an explosive phase followed by effusions, and an older silicic complex with major pyroclastic deposits. The caldera that formed after the last plinian eruption is partially filled with dacitic domes. Each of the two youngest plinian pumice falls has an approximate DRE volume of 2–3 km3 and calculated eruption column heights of about 15–20 km. The youngest pumice unit is a fall-surge-flow-surge sequence. Laterally transitional fall and surge facies, as well as distinct polymodal grainsize distributions in the basal fall layer, indicate coeval deposition from a maintained plume and surges. Planar-bedded pumice units on top of the fall layer were deposited from high-energy, dry-steam propelled surges and grade laterally into cross-bedded, finegrained surge deposits. The change from a fall-to a surge/flow-dominated depositional regime coincided with a trend from low-temperature argillitic lithics to high-temperature, epidote-and diopside-bearing lithic clasts, indicating the break-up of a high-temperature geothermal reservoir after the plinian phase. The transition from a maintained plume to a surge/ash flow depositional regime occurred most likely during break-up of the high-temperature geothermal reservoir during chaotic caldera collapse. The upper surge units were possibly erupted through the newly formed ringfracture.

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.

Nevado del Ruiz volcano (Colombia): pre-eruption observations and the November 13, 1985 catastrophic event

An Italian volcanological mission visited the Nevado del Ruiz volcano a few weeks before the catastrophic eruption of November 13, 1985. Fumarolic gases of Arenas crater were sampled and analysed and proved to be basically a mixture of CO 2 and SO 2, indicative of a direct magmatic input to the surface environment. The existence of high lahar risk was stressed in the mission's report delivered on October 22, 1985 and some simple preparedness measures were recommended to local authorities. Some observations of the November 13 pyroclastic sequence on the summit glacier suggest that the climax of the ice melting occurred before the beginning of the sustained column (Plinian phase) and was related to the early emplacement of pyroclastic flows.

Water contents, temperatures and diversity of the magmas of the catastrophic eruption of Nevado del Ruiz, Colombia, November 13, 1985

The petrology of the highly phyric two-pyroxene andesitic to dacitic pyroclastic rocks of the November 13, 1985 eruption of Nevado del Ruiz, Colombia, reveals evidence of: (1) increasingly fractionated bulk compositions with time; (2) tapping of a small magma chamber marginally zoned in regard to H 2O contents (1 to 4%), temperature (960–1090°C), and amount of residual melt (35 to 65%); (3) partial melting and assimilation of degassed zones in the hotter less dense interior of the magma chamber; (4) probable heating, thermal disruption and mineralogic and compositional contamination of the magma body by basaltic magma “underplating”; and (5) crustal contamination of the magmas during ascent and within the magma chamber. Near-crater fall-back or “spill-over” emitted in the middle of the eruptive sequence produced a small pyroclastic flow that became welded in its central and basal portions because of ponding and thus heat conservation on the flat glaciated summit near the Arenas crater. The heterogeneity of Ruiz magmas may be related to the comparatively small volume (0.03 km 3) of the eruption, nearly ten times less than the 0.2 km 3 of the Plinian phase of Mount St. Helens, and probable steep thermal and P H 2O gradients of a small source magma chamber, estimated at 300 m long and 100 m wide for an assumed ellipsoidal shape.

The recent pumice eruptions of Mt. Pelée volcano, Martinique. Part I: Depositional sequences, description of pumiceous deposits

Abstract Mount Pelee is one of the most active volcanoes of the Lesser Antilles arc, with more than twenty eruptions over the last 5000 years. Both nuee ardente-type eruptions, which are well known, and pumice eruptions, although little known, are very common in the stratigraphic record. The four younger pumice eruptions, P4 (2440 y.B.P.), P3 (2010 y.B.P.), P2 (1670 y.B.P.) and P1 (650 y.B.P.) can be used to reconstruct the eruption sequences. The various pumiceous deposits can be described as fine lithic ash layer, Plinian fall deposits, pumice and ash flow deposits with associated ash cloud fall deposits, and pumice surge deposits. Three kinds of depositional sequences have been defined. The distinctions between them are based on the occurrence of an initial Plinian phase and the generation of intraflow pyroclastic surges. The pumice eruptions of Mt. Pelee are small in intensity and magnitude, as expressed by the dispersal of their products and by the total mass of erupted material which is estimated to be less than 1 km3 in each case. The pumice fall deposits have dispersal characteristics of small Plinian eruptions, close to the sub-Plinian type. Nevertheless, the probability of an occurrence of a new pumice eruption at Mt. Pelee is high, and the widespread distribution of pumice deposits around the volcano suggests that such an eruption is a major volcanic risk during the present stage of activity.

One-dimensional adiabatic flow of equilibrium gas–particle mixtures in long vertical ducts with friction

The equations of the steady, adiabatic, one-dimensional flow of an equilibrium mixture of a perfect gas and incompressible particles, in constant-area ducts with friction, are derived taking into account the effects of gravity and of the finite volume of the particles. As is the case for a pure gas, the mixture is shown to be subject to the phenomenon of choking, and the possibility of an adiabatic heating of the mixture in a subsonic expansion is also theoretically predicted for certain flow inlet conditions. The model may be used to approximately describe the conditions existing in portions of volcanic conduits during the Plinian phases of explosive eruptions. Some results of the numerical integration of the equations for a typical application of this type are briefly discussed, thus showing the potential of the model for carrying out rapid analyses of the influence of the main geometrical and flow parameters describing the problem. A non-volcanological application is also analysed to illustrate the possibility of the adiabatic heating of the mixture.

A numerical simulation of the Plinian Fall Phase of 79 A.D. eruption of Vesuvius

The tephra transport and fallout during the Plinian phase of the 79 A.D. eruption of Vesuvius has been numerically simulated by using an advection‐diffusion model based on a continuity equation for the mass concentration in which wind field, atmospheric diffusion, and gravity settling are considered. The solution of the equation has been obtained on a nonuniform three‐dimensional grid to optimize the computation time and to reduce the errors. The input data are (1) the total erupted mass, (2) the variation with time of the column height, (3) the particle population in terms of the settling velocity, and (4) the wind field. The vertical mass distribution of the particles which leave the column is given by an empirical formula (Suzuki, 1983), whose parameters are obtained from data of Carey and Sigurdsson (1987) and best fitting with field data. Two distinct phases have been distinguished in the eruption: (1) related to white phonolitic pumice fallout and (2) dominated by the emission of gray tephritic‐phonolite pumice. According to the literature the total duration of the pumice fallout was about 19 hours with column height ranging between about 14 and 32 km. The “white phase” has been estimated to have had 9 hours duration, while the “gray phase” lasted 10 hours. Magma discharge rate has been calculated assuming a fourth power relationship with column height. Total erupted volumes of 1.0 and 2.6 km3 of dense rock equivalent products have been assumed for the white and the gray phases, respectively (Sigurdsson et al., 1985). The particle population of the cloud has been assumed to correspond to that of a pyroclastic flow deposit related to a nearly total column collapse. The wind field results from a 60° clockwise rotation of the presently most frequent wind distribution field during the summer reduced in speed module of about 40%. The simulated ground thicknesses generally agree with the actual ones. The computed granulometric spectra at specific localities are in excellent agreement with the measured ones. As a whole, the simulation was successful. This fact emphasizes the significant consequences that the used model with a suitable choice of the input parameters can have for the establishment of a truly probabilistic air fall hazard map of Vesuvius.

Chronology and pyroclastic stratigraphy of the May 18, 1980, Eruption of Mount St. Helens, Washington

Many timed observations make it possible to subdivide the 9‐hour Plinian eruption of Mount St. Helens on May 18, 1980, into six phases, defined by eruption style. The phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits. The suite of pyroclastic deposits indicates that the eruption became more pumice‐rich and compositionally diverse with time, perhaps owing to concurrent eruption of less evolved, gas‐poor parts of the magma body with the more evolved, gas‐rich parts. The paroxysmal phase I (0832–0900) consisted of landslides, lithic pyroclastic flows of a lateral blast and other explosions, and a weak pre‐Plinian column. Phase I pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer. The early Plinian phase II (0900–1215) consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks (1010–1035), a brief period of lithic ash ejection (1035–1100), and a pumice‐rich pulse that accompanied growth in height of the eruption column (1100–1215). Deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer, the lower lithic ash layer, and the middle pumice lapilli layer, all of which consist of evolved white dacitic pumice (63–64% SiO2). During the early ash flow phase III (1215–1500) the height of the eruption column decreased, vertical ejection of tephra ceased, and pyroclastic flows were fed from intermittent fountains. Phase III deposits consist of a poorly exposed sequence (.≤12 m) of ash flow tuff that consist of many thin flow units (≤2 m each) containing pumiceous white dacite (63–64% SiO2) and denser, gray silicic andésite (61–62% SiO2), and fine‐grained ash cloud deposits interbedded with a nongraded middle pumice ash layer. The climactic phase IV (1500–1715) developed in two stages: fountain‐fed pyroclastic flows, followed by a short pulse (1625–1715) of vigorous vertical ejection of tephra. These stages were accompanied by the peak seismic energy release and peak eruption column height, respectively. Climactic deposits consist of a thick (≤35 m) sequence of thick, lapilli‐rich ash flow sheets (4–12 m each) with white and gray pumice, and streaky scoria bands (60% SiO2) in pumice breccia clasts, and the reversely‐graded, upper pumice lapilli layer that is interbedded with fine‐grained ash cloud deposits. During the late ash flow phase V (1715–1815) eruption intensity waned but included a brief episode of small pyroclastic flows (1745–1815). Phase V deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice, and minor fine‐ash deposits. Phase VI activity (1815 to May 19, 1980) consisted of a low‐energy ash plume, with transient increases in intensity, while seismicity continued at depth. Sustained vertical discharge of phase II prodeced evolved dacitewith high S/Cl ratios. Ash flow activity of phase III is attributed to decreases in gas content, indicated by reduced S/Cl ratios and increased clast density of the less evolved, gray pumice. Climactic events are attributed to vent clearing and exhaustion of the evolved dacite.

Emplacement of small-volume pyroclastic flows at Laacher See (East-Eifel, Germany)

We distinguish three eruptive units of pyroclastic flows (T1, T2, and T3; T for trass) within the late Quaternary Laacher See tephra sequence. These units differ in the chemical/mineralogical composition of the essential pyroclasts ranging from highly differentiated phonolite in T1 to mafic phonolite in T3. T1 and T2 flows were generated during Plinian phases, and T3 flows during a late Vulcanian phase. The volume of the pyroclastic flow deposits is about 0.6 km3. The lateral extent of the flows from the source vent decreases from > 10 km (T1) to 1000 N/m2 is consistent with the divergence of lithic size/distance curves from purely Newtonian models; the transport of lithics must be treated as in a Bingham fluid. The flow temperature probably decreased from T1 (300°–500°C) to T3 (<200°C). A large-scale longitudinal variation in the flow units from proximal through medial to distal facies dominantly reflects temporal changes during the progressive collapse of an eruption column. Only a small amount of fallout tephra was generated in the T1 phase of eruption. The pyroclastic flows probably formed from relatively low ash fountains rather than from high Plinian eruption columns.

The May 18, 1980 eruption of Mount St. Helens: 2. Modeling of dynamics of the Plinian Phase

The Plinian phase of the May 18, 1980, Mount St. Helens eruption is modeled as a steady state discharge of dacitic magma from a reservoir at 7–10 km depth at a rate of 1.94×107 kg/s. Properties of the magma, including preeruption volatile content (4.6% in the melt), temperature (920°–940° C), and confining pressure (190–250 MPa) are constrained by petrologic studies. Mass eruption rate, magma viscosity, and independent estimates of magma ascent velocity suggest a 95‐m‐diameter conduit at a depth below vapor saturation. Dispersal of pyroclasts indicate a minimum exit velocity of roughly 200 m/s during the Plinian phase. An upper limit of 330 m/s is obtained from the total amount of exsolved volatiles. Model‐derived vent diameters based on 0.1‐MPa exit pressure, petrologically inferred magma properties, and known mass eruption rate range from 105 to 135 m with a flared configuration. The calculated vent diameter, mass eruption rate, and exit velocity define conditions close to the transition between convective column rise and column collapse. These results are consistent with the sporadic generation of pyroclastic flows during a period characterized mainly by a sustained eruption column.

Vent Evolution and Lag Breccia Formation during the Cape Riva Eruption of Santorini, Greece

The 18,500 yr. b.p. Cape Riva (CR) eruption of Santorini vented several km3 or more of magma, generating four eruption units: a basal Plinian fall deposit (CR-A) and three pyroclastic flow deposits (CR-B to CR-D upwards). CR-B and CR-D are welded ignimbrites; CR-C consists predominantly of up to 25 m thick coarse, lithic-rich co-ignimbrite lag breccias resulting from a climactic phase of the eruption. The initial Plinian phase occurred from a localized vent in N Santorini, and subsequent column collapse resulted in emplacement of CR-B. Towards the end of CR-B, new conduits were activated and pyroclastic flows discharged from multiple vents to generate the lag breccias (CR-C). CR-D probably records a return to a localized vent as the eruption waned. The eruption sampled a zoned magma chamber containing rhyodacite overlying andesite, and leaks of these magmas were manifested as the Skaros-Therasia lavas preceding the CR eruption. Plinian and initial ignimbrite stages occurred while the magma chamber was ove...

Caldera development during the Minoan eruption, Thira, Cyclades, Greece

The well‐known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption of 1400 B.C., Thira consisted of three Java shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half. Andesitic lavas formed the overlapping shields of the north and were contemporaneous with and, in many places, interbedded with the southern tuff deposits. Although there appears to be little difference between the composition of magmas erupted, differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial, producing lava flows, whereas in the south, eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs. Many of these tuffs are plastered onto the walls of what appears to have been an older caldera, most probably associated with an eruption of rhyodacitic tephra 100,000 years ago. The Minoan eruption of about 1400 B.C. had four distinct phases, each reflecting a different vent geometry and eruption mechanism. The Minoan activity was preceded by minor eruptions of fine ash. (1) The eruption began with a Plinian phase, from subaerial vent(s) located on the easternmost of the lava shields. (2) Vent(s) grew toward the SW into the flooded depression. Subsequent activity deposited large‐scale base surge deposits during vent widening by phreatomagmatic activity. (3) The third eruptive phase was also phreatomagmatic and produced 60% of the volume of the Minoan Tuff. This activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding. This deposit contains 5–40% lithic fragments that are typical of the westernmost lava shield and appears to have been erupted when caldera collapse began. (4) The last phase consisted of eruption of ignimbrites from vent(s) on the eastern shield, not yet involved in collapse. Collapse continued after eruption of the ignimbrites with foundering of the eastern half of the caldera. Total volume of the collapse was about 19 km3, overlapping the older caldera to form the caldera complex visible today. Intracaldera eruptions have formed the Kameni Islands along linear vents concomitant with vents that may have been sources for the Minoan Tuff.

Volcanological study of the great Tambora eruption of 1815

The 1815 eruption of Tambora is one of the largest explosive volcanic events of the past 10 000 yr. By a conservative estimate, 175 km3 of nepheline-normative trachyandesitic pyroclastic material (equivalent to about 50 km3 of dense rock) was erupted in 24 h. Major activity began with a brief plinian phase followed by pyroclastic flows; much of the ignimbrite that was produced shows evidence of deposition by turbulent flows. A 6×7-km caldera, about 1 km deep, formed at the end of the pyroclastic flow phase; caldera volume closely matches that of magma ejected. Plinian and co-ignimbrite ash fall >1 cm thick covered >500 000 km2 of the Java Sea and surrounding islands, accounting for more than two-thirds of the magma volume erupted. This distal tephra is known only from 1815 reports, indicating that some great eruptions, especially those from island volcanoes, may be virtually undetectable in the geologic record without supporting information, such as deep-sea core data. Land-based volume estimates may be insufficient to characterize eruption magnitude in many cases.

Sedimentology of the Minoan deep-sea tephra layer in the Aegean and Eastern Mediterranean

The Minoan eruption of Santorini resulted in deposition of pyroclastic material over a large area of the Aegean Sea and Eastern Mediterranean. The eruptive activity commenced with a plinian phase, which was followed by a phreatomagmatic phase, and ended with numerous pyroclastic flows. We present a sedimentological study of the Minoan ash deposits found in cores taken throughout the region. The cores are divided into three groups: (1) those from the sub-marine slopes immediately surrounding Santorini; (2) those from the Aegean Sea where the topography consists of many steep-sided basins; and (3) those from the Eastern Mediterranean where the topography is relatively flat and smooth. This last group is designated the Distal Ash Zone. The deposits from the Aegean Sea have been remobilised as turbidity currents. Grain-size distribution analysis reveals that these deposits are bimodal, the Md ø of the coarse population increasing away from source, whilst that of the fine population remains uniform irrespective of distance from source. The bimodality is caused by a combination of aggregation of fine particles in ash clouds produced during the last two phases of the eruptive activity and mixing of the coarse plinian and fine phreatomagmatic and co-ignimbrite deposits within the turbidity current. An unusual feature found in two of the cores is a coarse, density sorted layer beneath the Bouma “a” division of several turbidite units. This is interpreted as an example of a flow head deposit as described by Sparks and Wilson (1983). The deposits from the Distal Ash Fall Zone are fine grained and unimodal, containing phreatomagmatic and co-ignimbrite material. They show only very minor remobilisation of the original air-fall ash, due to the flat topography of the area. The Md ø of the deposit remains uniform irrespective of distance from source and is very similar to that of the fine population found in the bimodal Aegean deposits. This is interpreted as the result of aggregation of fines in the phreatomagmatic and co-ignimbrite ash clouds. The deposits in both areas have been extensively bioturbated. A study of dispersed ash in the cores to reconstruct the original thickness of the ash layers suggests that ash layers less than 0.62 cm thick will be completely dispersed in the surrounding sediment, whilst those thicker than 2.5 cm will be preserved as discrete layers, the base being unaffected by bioturbation. The deposits on the submarine slopes of Santorini were not originally deposited as air-fall ash, but are derived from the erosion of coastal exposures of non-welded ignimbrite. They form storm-sand layers and are typical of the type of deposit that may be formed by rapid erosion of fresh pyroclastic material after the eruption of an island-arc volcano.

The Hekla eruption 1980–1981

The sixteenth eruption of Hekla since 1104 began on August 17th, 1980, after the shortest repose period on record, only ten years. The eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the Hekla volcanic ridge. The production rate declined rapidly after the first day and the eruption stopped on August 20th. A total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity. In the following seven months steam emissions were observed on the volcano. Activity was renewed on April 9th 1981, and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope.

The Eruption of Vesuvius in A.D. 79: Reconstruction from Historical and Volcanological Evidence

Reinterpretation of the volcanological and historical evidence shows that the eruption of Vesuvius in A.D. 79 consisted of two main phases. initial 18 to 20hour Plinian phase caused extensive pumice-fall south of the volcano, resulting in the slow accumulation of a pumice layer up to 2.8 m. thick over Pompeii and other regions to the south. Much of the population fled the area during this non-lethal phase. On the second day of activity the Pelkan phase occurred, when a series of nudes ardentes or hot ash-avalanches swept down the south and west flanks of the volcano, affecting the region as far as Misenum, 30 km. to the west. first of two nudes ardentes which inundated Pompeii overwhelmed and asphyxiated those who remained above ground in the city and their bodies were immediately interred in the fine-grained deposit. effects of the Pelkan activity were even more severe west and northwest of Pompeii, resulting in burial of the cities of Oplontis and Herculaneum by a series of nue A. Maiuri, Herculaneum (Rome 1977) 13. 2 S.R.J. Sparks and G.P.L. Walker, The Ground Surge Deposit: a Third Type of Pyroclastic Rock, Nature 241 (1973) 62-64; S.R.J. Sparks, S. Self and G.P.L. Walker, Geology 1 (1973) 115-18; S.R.J. Sparks, Dusts of Destruction, New Scientist (1973) 134-36. 3A. Rittmann, Vulkane und ihre Tiitigkeit (Stuttgart 1960) 48. 4 Maiuri 1958 (supra n. 1) 69. 5G.A. Macdonald, Volcanoes (Englewood Cliffs 1972) 234; F.M. Bullard, Volcanoes of the Earth (Austin 1976) 200; P. Francis, Volcanoes (New York 1976) 63. 6 A. Helprin, Mont Pelke and the Tragedy of Martinique (New York 1903) 120; E.T. Merrill, Notes on the Eruption of Vesuvius in 79 A.D., AJA 22 (1918) 304-309; E.T. Merrill, Further Note on the Eruption of Vesuvius in 79 A.D., AJA 24 (1920) 262-68. This content downloaded from 157.55.39.35 on Mon, 29 Aug 2016 05:31:16 UTC All use subject to http://about.jstor.org/terms 40 H. SIGURDSSON, S. CASHDOLLAR AND S.R.J. SPARKS [AJA 86 after the disastrous nu&e ardente eruption of Mt. Pel&e in 1902, and later revived by Sparks and Walker,7 and Lirer et al.8 VOLCANOLOGICAL EVIDENCE Products of the eruption of Vesuvius in A.D. 79 form distinct deposits on the southwestern and southern slopes of the volcano and on the Sarno plain as far south as Castellammare (ill. 1). We r. Air-fall deposits accumulate gradually during the explosive phase of a volcanic eruption and characteristically mantle the topography with a wellbedded, uniform pumice layer which shows an exponential decrease in thickness with increasing distance from the volcano. Their distribution is entirely controlled by the effect of the prevailing wind direction on the vertical eruption column, which may reach a height of tens of kilometers above the vol-

Plinian eruptions and their products

Plinian eruptions are amongst the most powerful of explosive volcanic events, and the extensive pumice deposits which they produce have an exceptionally wide dispersal because of the great eruptive plume height. Historical data on 12 plinian eruptions, and available quantitative data on the deposits of these and 37 other plinian eruptions are collated in this review to characterise further the plinian eruptive style and its products and to establish the known limits of their variation. The deposit volumes have been recomputed according to a standard procedure to provide a better basis for comparison, and they vary over 4 orders of magnitude to reach a maximum of about 100 km3. Almost all volcanic magma compositions apart from the most mafic are represented among the juvenile products; rhyolitic and dacitic deposits account for 80% of the total volume and basaltic ones less than 1%. Compositional zoning is very common. Plinian eruptions are of open vent type and produce deposits which tend to be homogeneous in grain size and constitution through their thickness. Considerable departures from homogeneity often however exist. Finer grained beds which often interrupt the continuity can be produced by a number of different mechanisms, the features of which are summarised. In a significant proportion of plinian deposits the finer beds are the deposits of intraplinian pyroclastic flows, or are related to such flows; pyroclastic flows such as may be attributable to column collapse thus do not form exclusively at the end of the plinian phase. The most recent work indicates that major phreatoplinian eruptions dominated by the copious inflow of water into the vent can produce deposits quite as widely dispersed and as voluminous as the biggest plinian eruptions, and it appears that the characteristics of the grain size populations of the two types tend to converge in the most powerful eruptions.

The pyroclastic deposits of the 1875 eruption of Askja, Iceland

The 1875 explosive eruption of Askja, Iceland was part of a series of regional volcanic and tectonic events which took place in the northern rift zone in 1874 and 1875. These events were marked by regional seismicity, graben formation and a basaltic fissure eruption at Sveinagja, and the plinian eruption of Askja on 28-29 March. Crustal rifting caused basaltic magma to be mixed with rhyolitic magma, triggering the plinian eruption. A caldera, Oskjuvatn, was formed in Askja measuring 3 x 4 km and 267 m deep. Six distinguishable pyroclastic layers can be recognized. The main eruption began with a small sub-plinian pumice eruption forming layer B. The next phase produced a fine-grained, poorly sorted pumice and ash deposit with well developed stratification (layer C), which contains base surge beds near source and is interpreted as phreatomagmatic in origin. The main plinian phase of the eruption lasted 6 h and formed a coarse-grained, poorly bedded pumice-fall deposit (layer D) which contains 75% of the total ejecta. Late-stage explosions formed a layer of lithic clasts (layer E). Isopach and grain-size isopleth maps show that the vents migrated from south to north along a line 1.5 km long in the area now occupied by Oskjuvatn. The intensity and column height of the eruption increased with time as shown by reverse grading and an increasing dispersal index in successive layers. Most of the ejecta is composed of white rhyolitic pumice and ash. Lithics consist of rhyolitic obsidian, partially fused trondhjeimite, and basalt fragments: layer D contains 2.1 mass % lithics. All layers contain abundant grey pumice clasts consisting of intimate mixtures of dark brown basaltic and brown rhyolitic glasses. The mass percentage of mixed pumice in layer D is 4.7, of which 40 % is basaltic glass. These mixed pumice clasts are concentrated at distances of 30-80 km in layer D by aeolian sorting. A grey, crystal-rich, andesitic pumice occurs as inclusions in the white pumice. Layer D shows a systematic decrease in median grain diameter, but no change in cr^ with distance from source. Layer C shows no change in median grain diameter, but a decrease in with distance from source. Phreatomagmatic deposits such as layer C can be readily distinguished from plinian deposits on a Md$ against cr^ diagram, on a against a* (skewness) diagram and on the F against D plot of Walker (1973). The downwind, coarse-tail grading in layer C is attributed to fall-out of fine ash as clumps and aggregates. The total grain-size distributions of both layers D and C show bimodality. In layer D a minor mode in the ash size classes reflects secondary processes of fragmentation by collisions in the vent and column, whereas the major mode is due to disruption of magma by expanding gases. In layer C the fine mode is dominant and represents extensive fragmentation by explosive interaction with water. Field and grain-size studies of layer D show that impact breakage is of major importance near source.

The interaction between volcanic gases and tephra: Fluorine adhering to tephra of the 1970 hekla eruption

The mass distribution and sorting of tephra produced in the plinian phase of the 1970 Hekla eruption was controlled by the particle size distribution, the height of the eruption column, and velocity of transport. Near the volcano the mass distribution of soluble fluorine was controlled by particle size of the deposits, but approaches the mass distribution of the tephra at longer distances. Adsorbed soluble fluorine reaches a maximum at a distance from the volcano determined by the velocity of the transporting medium. SEM studies show the soluble fluorine to be chemically adsorbed on the surface of tephra particles. The adsorption is shown by experiment to occur at temperatures below 600°C in the cooling eruption column. Evaluation of reactions in the eruption column leads to the conclusion that formation of water soluble compounds adhering to tephra is principally controlled by environmental factors and to a lesser degree by the composition of the volcanic gas phase.

The roseau ash: Deep-sea tephra deposits from a major eruption on Dominica, lesser antilles arc

Abstract Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 105 km2 with a volume of 13 km3 (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km3 of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash. Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km3 of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment.