The Sarikavak Tephra, Galatea, north central Turkey: a case study of a Miocene complex plinian eruption deposit

The Sarikavak Tephra from the central Galatean Volcanic Province (Turkey) represents the deposit of a complex multiple phase plinian eruption of Miocene age. The eruptive sequence is subdivided into the Lower-, Middle-, and Upper Sarikavak Tephra (LSKT, MSKT, USKT) which differ in type of deposits, lithology and eruptive mechanisms.The Lower Sarikavak Tephra is characterised by pumice fall deposits with minor interbedded fine-grained ash beds in the lower LSKT-A. Deposits are well stratified and enriched in lithic fragments up to >50 wt% in some layers. The upper LSKT-B is mainly reversely graded pumice fall with minor amounts of lithics. It represents the main plinian phase of the eruption. The LSKT-A and B units are separated from each other by a fine-grained ash fall deposit. The Middle Sarikavak Tephra is predominantly composed of cross-bedded ash-and-pumice surge deposits with minor pumice fall deposits in the lower MSKT-A and major pyroclastic flow deposits in the upper MSKT-B unit. The Upper Sarikavak Tephra shows subaerial laminated surge deposits in USKT-A and subaqueous tephra beds in USKT-B.Isopach maps of the LSKT pumice fall deposits as well as the fine ash at the LSKT-A/B boundary indicate NNE–SSW extending depositional fans with the source area in the western part of the Ovaçik caldera. The MSKT pyroclastic flow and surge deposits form a SW-extending main lobe related to paleotopography where the deposits are thickest.Internal bedding and lithic distribution of the LSKT-A result from intermittent activity due to significant vent wall instabilities. Reductions in eruption power from (partial) plugging of the vent produced fine ash deposits in near-vent locations and subsequent explosive expulsion of wall rock debris was responsible for the high lithic contents of the lapilli fall deposits. A period of vent closure promoted fine ash fall deposition at the end of LSKT-A. The subsequent main plinian phase of the LSKT-B evolved from stable vent conditions after some initial gravitational column collapses during the early ascent of the re-established eruption plume. The ash-and-pumice surges of the MSKT-A are interpreted as deposits from phreatomagmatic activity prior to the main pyroclastic flow formation of the MSKT-B.     

The Ottaviano eruption of Somma-Vesuvio (8000 y B.P.): a magmatic alternating fall and flow-forming eruption

The Ottaviano eruption occurred in the late neolithic (8000 y B.P.). 2.40 km³ of phonolitic pyroclastic material (0.61 km³ DRE) were emplaced as pyroclastic flow, surge and fall deposits. The eruption began with a fall phase, with a model column height of 14 km, producing a pumice fall deposit (LA). This phase ended with short-lived weak explosive activity, giving rise to a fine-grained deposit (L1), passing to pumice fall deposits as the result of an increasing column height and mass discharge rate. The subsequent two fall phases (producing LB and LC deposits), had model column heights of 20 and 22 km with eruption rates of 2.5 × 10⁷ and 2.81 × 10⁷ kg/s, respectively. These phases ended with the deposition of ash layers (L2 and L3), related to a decreasing, pulsing explosive activity. The values of dynamic parameters calculated for the eruption classify it as a sub-plinian event. Each fall phase was characterized by variations in the eruptive intensity, and several pyroclastic flows were emplaced (F1 to F3). Alternating pumice and ash fall beds record the waning of the eruption. Finally, owing to the collapse of a eruptive column of low gas content, the last pyroclastic flow (F4) was emplaced.     

Pyroclastic deposits of the November 13, 1985 eruption of Nevado del Ruiz volcano, Colombia

The November 13, 1985 eruption of Nevado del Ruiz produced a series of pyroclastic flows and surges that eroded channels on the surface of the summit glacier and generated lahars which descended down most of the rivers that drain the volcano. The stratigraphy of the proximal pyroclastic deposits indicates that there were at least four episodes to the eruption. Episode I, deposited an unusual surge consisting of small pieces of ice mixed with ash and exhibiting planar stratification. Ballistically emplaced fragments are also intercalated with this unit. During Episode II, at least two pyroclastic flows were erupted. Their deposits contain the most evolved pumice of the entire eruption; SiO₂ content of matrix glass ranges between 74.5 and 74.9%. Episode III is marked by the emplacement of a welded tuff with an average SiO₂ content of about 66% in the matrix glass. The final Episode IV was characterized by the development of a high-altitude eruption column and the emplacement of several nonwelded pyroclastic flows. Banded pumice are common in the pyroclastic flow as well as in the pumice fall deposits. Co-existing dark and light pumice bands differ in SiO₂ content by 3.5% and in general are similar to the composition of the welded pumice from Episode III.The compositional zonation of the pyroclastic deposits from Episode I to IV suggests that a nearsurface compositionally-stratified portion of the magma body was tapped during Episode II. During Episodes III and IV the main body of magma was involved although the coexistence of the compositionally distinct pumice clasts at similar stratigraphic levels argues for mixing of magma from different levels in the chamber during the eruptive process.     

Plinian activity during the early eruptive history of the Sabatini Volcanic District, Central Italy

The early activity of the Sabatini Volcanic District (SVD; central Italy) was characterised by highly explosive eruptions that produced widespread subplinian and plinian fall deposits. In this study, four major eruptive units—informally named as units A, B, C and D—were recognised in the 514–449 ka age interval. In particular, unit D was emplaced during the early phase of the 449 ka Tufo Rosso a Scorie Nere pyroclastic flow-forming eruption, the most important event in the whole SVD activity history. Estimates of relevant eruptive parameters indicate tephra fall volumes up to 4 km³ for individual units, peak eruption column heights in the range of 14–29 km and corresponding mass eruption rates of 7.8×10⁶–1.3×10⁸ kg/s. Isopach and isopleth maps of fallout deposits—as well as the distribution of the proximal lag-breccia of the Tufo Rosso a Scorie Nere—consistently indicate a common vent area, which does not correspond to any volcanic centre identified up to now in the SVD. This was located along NE–SW-trending tectonic lineaments that also controlled the location of the other major volcanic centres of the SVD. The characterisation by means of field aspects, grain size, componentry and density and chemical composition of juvenile clasts, renders the studied fall deposits as valuable stratigraphic markers for the SVD and well beyond it. In fact, their wide areal dispersals toward the E and SE may allow correlations on a regional scale for the Quaternary successions of intermountain basins of the Central Apennine and the Adriatic Sea basin successions. Finally, the correct identification of distal tephra from plinian and co-ignimbrite plumes and their attribution to specific explosive eruptions of the SVD and the other volcanic districts of the Roman Province—rather than to local intra-Apennine centres—provides crucial implications for geodynamic reconstructions.     

The Avellino plinian eruption of Somma-Vesuvius (3760 y.B.P.): the progressive evolution from magmatic to hydromagmatic style

A detailed stratigraphic analysis of the Avellino plinian deposit of the Somma-Vesuvius volcano shows a complicated eruptive sequence controlled by a combination of magmatic and hydromagmatic processes. The role of external water on the eruptive dynamics was most relevant in the very early phase of the eruption when the groundwater explosively interacted with a rising, gas-exolving magma body creating the first conduit. This phase generated pyroclastic surge and phreatoplinian deposits followed by a rapidly increasing discharge of a gas-rich, pure magmatic phase which erupted as the most violent plinian episode. This continuing plinian phase tapped the magma chamber, generating about 2.9 km³ of reverse-graded fallout pumice, more differentiated at the base and more primitive at the top (white and gray pumice). A giant, plinian column, rapidly grew up reaching a maximum height of 36 km.The progressive magma evacuation at a maximum discharge rate of 10⁸ kg/s that accompanied a decrease of magmatic volatile content in the lower primitive magma allowed external water to enter the magma chamber, resulting in a drastic change in the eruptive style and deposit type. Early wet hydromagmatic events were followed by dry ones and only a few, subordinated magmatic phases. A thick, impressive sequence of pyroclastic surge bedsets of over 430 km² in area with a total volume of about 1 km³ is the visible result of this hydromagmatic phase.     

Les éruptions de Mars–Avril 1982 du Volcan Chichonal

El Chichonal Volcano, within the Sierra of Chiapas (Mexico), became active on March 28, 1982. The summit dome was blasted away, leaving a 1000 m wide and 290 m deep crater in its place. Plinian eruption of March 28–29 produced air fall deposits. More vigorous activity on April 3–4 produced air falls, surges and pyroclastic flows. Villages within 6 km radius were entirely destroyed. The explosive eruptions of Chichonal produced large amounts of K-rich andesitic ash and pumice. The juvenile magma shows little compositional variability. Much of eruptive products is highly porphyritic pumice containing andesine, hornblende, clinopyroxene, biotite and anhydrite phenocrysts.     

The Averno 2 fissure eruption: a recent small-size explosive event at the Campi Flegrei Caldera (Italy)

The Averno 2 eruption (3,700 ± 50 a B.P.) was an explosive low-magnitude event characterized by magmatic and phreatomagmatic explosions, generating mainly fall and surge beds, respectively. It occurred in the Western sector of the Campi Flegrei caldera (Campanian Region, South Italy) at the intersection of two active fault systems, oriented NE and NW. The morphologically complex crater area, largely filled by the Averno lake, resulted from vent activation and migration along the NE-trending fault system. The eruption generated a complex sequence of pyroclastic deposits, including pumice fall deposits in the lower portion, and prevailing surge beds in the intermediate-upper portion. The pyroclastic sequence has been studied through stratigraphical, morphostructural and petrological investigations, and subdivided into three members named A through C. Member A was emplaced during the first phase of the eruption mainly by magmatic explosions which generated columns reaching a maximum height of 10 km. During this phase the eruption reached its climax with a mass discharge rate of 3.2 10⁶ kg/s. Intense fracturing and fault activation favored entry of a significant amount of water into the system, which produced explosions driven by variably efficient water-magma interaction. These explosions generated wet to dry surge deposits that emplaced Member B and C, respectively. Isopachs and isopleths maps, as well as areal distribution of ballistic fragments and facies variation of surge deposits allow definition of four vents that opened along a NE oriented, 2 km long fissure. The total volume of magma extruded during the eruption has been estimated at about 0.07 km³ (DRE). The erupted products range in composition from initial, weakly peralkaline alkali-trachyte, to last-emplaced alkali-trachyte. Isotopic data and modeling suggest that mixing occurred during the Averno 2 eruption between a more evolved, less radiogenic stored magma, and a less evolved, more radiogenic magma that entered the shallow reservoir to trigger the eruption. The early phases of the eruption, during which the vent migrated from SW to the center of the present lake, were fed by the more evolved, uppermost magma, while the following phases extruded the less evolved, lowermost magma. Integration of the geological and petrological results suggests that the Averno 2 complex eruption was fed from a dyke-shaped shallow reservoir intruded into the NE-SW fault system bordering to the west the La Starza resurgent block, within the caldera floor.     

Stratigraphy, chemistry, and eruptive dynamics of the 12.4 ka plinian eruption of Apoyeque volcano, Managua, Nicaragua

Apoyeque volcano, located 9 km northwest of Managua city, erupted explosively at 12.4 ka. The Plinian eruption deposited a widespread pumice fall deposit known as the Upper Apoyeque Tephra (UAq). The UAq is massive, reversely graded, and consists of white juvenile pumice (~78 vol.%), a variety of cognate lithics and accidental altered lithics. The whole-rock pumice composition is rhyodacitic (SiO₂?=?66.9–68.5 wt.%) with a mineral paragenesis of plagioclase, orthopyroxene, clinopyroxene, amphibole, titanomagnetite, and ilmenite in a rhyolitic glass groundmass (SiO₂?=?74.4?±?0.6 wt.%). The deposit’s dispersal axis is to the south, with the deposit covering a minimum area of 877 km² within the 50 cm isopach and has a total volume of 3 km³ (dense rock equivalent, 1.15 km³). The eruption column reached a maximum height of ca.28 km. The eruption ejected a total mass of 3?×?10¹² kg at an average rate of 2?×?10⁸ kg/s, and based on available models, we infer duration of almost 4 h. Petrographic and geochemical characteristics suggest that the eruption was triggered by magma mixing.     

The 322 ka Tiribí Tuff: stratigraphy, geochronology and mechanisms of deposition of the largest and most recent ignimbrite in the Valle Central, Costa Rica

The Tiribí Tuff covered much of the Valle Central of Costa Rica, currently the most densely populated area in the country (∼2.4 million inhabitants). Underlying the tuff, there is a related well-sorted pumice deposit, the Tibás Pumice Layer. Based on macroscopic characteristics of the rocks, we distinguish two main facies in the Tiribí Tuff in correlation to the differences in welding, devitrification, grain size, and abundance of pumice and lithic fragments. The Valle Central facies consists of an ignimbritic plateau of non-welded to welded deposits within the Valle Central basin and the Orotina facies is a gray to light-bluish gray, densely to partially welded rock, with yellowish and black pumice fragments cropping out mainly at the Grande de Tárcoles River Gorge and Orotina plain. This high-aspect ratio ignimbrite (1:920 or 1.1×10⁻³) covered an area of at least 820 km² with a long runout of 80 km and a minimum volume outflow of 25 km³ (15 km³ DRE). Geochemically, the tuff shows a wide range of compositions from basaltic-andesites to rhyolites, but trachyandesites are predominant. Replicate new ⁴⁰Ar/³⁹Ar age determinations indicate that widespread exposures of this tuff represent a single ignimbrite that was erupted 322±2 ka. The inferred source is the Barva Caldera, as interpreted from isopach and isopleth maps, contours of the ignimbrite top and geochemical correlation (∼10 km in diameter). The Tiribí Tuff caldera-forming eruption is interpreted as having evolved from a plinian eruption, during which the widespread basal pumice fall was deposited, followed by fountaining pyroclastic flows. In the SW part of the Valle Central, the ignimbrite flowed into a narrow canyon, which might have acted as a pseudo-barrier, reflecting the flow back towards the source and thus thickening the deposits that were filling the Valle Central depression. The variable welding patterns are interpreted to be a result of the lithostatic load and the influence of the content and size of lithic fragments.     

Complex changes in eruption dynamics during the 79 AD eruption of Vesuvius

The 79 AD eruption of Vesuvius included 8 eruption units (EU1–8) and several complex transitions in eruptive style. This study focuses on two important transitions: (1) the abrupt change from white to gray pumice during the Plinian phase of the eruption (EU2 to EU3) and (2) the shift from sustained Plinian activity to the onset of caldera collapse (EU3 to EU4). Quantification of the textural features within individual pumice clasts reveals important changes in both the vesicles and groundmass crystals across each transition boundary. Clasts from the white Plinian fall deposit (EU2) present a simple story of decompression-driven crystallization followed by continuous bubble nucleation, growth and coalescence in the eruptive conduit. In contrast, pumices from the overlying gray Plinian fall deposit (EU3) are heterogeneous and show a wide range in both bubble and crystal textures. Extensive bubble growth, coalescence, and the onset of bubble collapse in pumices at the base of EU3 suggest that the early EU3 magma experienced protracted vesiculation that began during eruption of the EU2 phase and was modified by the physical effects of syn-eruptive mingling-mixing. Pumice clasts from higher in EU3 show higher bubble and crystal number densities and less evidence of bubble collapse, textural features that are interpreted to reflect more thorough mixing of two magmas by this stage of the eruption, with consequent increases in both vesiculation and crystallization. Pumice clasts from a short-lived, high column at the onset of caldera collapse (EU4) continue the trend of increasing crystallization (enhanced by mixing) but, unexpectedly, the melt in these clasts is more vesicular than in EU3 and, in the extreme, can be classified as reticulite. We suggest that the high melt vesicularity of EU4 reflects strong decompression following the partial collapse of the magma chamber.Editorial responsibility: D.B. Dingwell     

长白山天池火山千年大喷发不同颜色浮岩的岩石化学特征

长白山天池火山大约 1 000年前的大喷发,形成了巨厚的火山碎屑流堆积层,其主要组成是浮岩与火山灰。以往的研究普遍认为其中的浮岩为灰白色,属流纹质。笔者在考察中发现了不少黑色及少量其它颜色的浮岩,系统地采集了各色样品作浮岩化学成分分析,结果表明,灰白色浮岩与黑色浮岩分别为流纹质和粗面质,灰色浮岩属于粗面质但靠近流纹质端元。它们都来源于地壳岩浆房,是岩浆房内不同分异演化阶段的产物,它们同时喷出说明岩浆房内具有分带性及不同性质岩浆的混合     

The 26.5 ka Oruanui eruption, New Zealand: an introduction and overview

The 26.5 ka Oruanui eruption, from Taupo volcano in the central North Island of New Zealand, is the largest known ‘wet’ eruption, generating 430 km³ of fall deposits, 320 km³ of pyroclastic density–current (PDC) deposits (mostly ignimbrite) and 420 km³ of primary intracaldera material, equivalent to 530 km³ of magma. Erupted magma is >99% rhyolite and <1% relatively mafic compositions (52.3–63.3% SiO₂). The latter vary in abundance at different stratigraphic levels from 0.1 to 4 wt%, defining three ‘spikes’ that are used to correlate fall and coeval PDC activity. The eruption is divided into 10 phases on the basis of nine mappable fall units and a tenth, poorly preserved but volumetrically dominant fall unit. Fall units 1–9 individually range from 0.8 to 85 km³ and unit 10, by subtraction, is 265 km³; all fall deposits are of wide (plinian) to extremely wide dispersal. Fall deposits show a wide range of depositional states, from dry to water saturated, reflecting varied pyroclast:water ratios. Multiple bedding and normal grading in the fall deposits show the first third of the eruption was very spasmodic; short-lived but intense bursts of activity were separated by time breaks from zero up to several weeks to months. PDC activity occurred throughout the eruption. Both dilute and concentrated currents are inferred to have been present from deposit characteristics, with the latter being volumetrically dominant (>90%). PDC deposits range from mm- to cm-thick ultra-thin veneers enclosed within fall material to >200 m-thick ignimbrite in proximal areas. The farthest travelled (90 km), most energetic PDCs (velocities >100 m s⁻¹) occurred during phase 8, but the most voluminous PDC deposits were emplaced during phase 10. Grain size variations in the PDC deposits are complex, with changes seen vertically in thick, proximal accumulations being greater than those seen laterally from near-source to most-distal deposits. Modern Lake Taupo partly infills the caldera generated during this eruption; a 140 km² structural collapse area is concealed beneath the lake, while the lake outline reflects coeval peripheral and volcano–tectonic collapse. Early eruption phases saw shifting vent positions; development of the caldera to its maximum extent (indicated by lithic lag breccias) occurred during phase 10. The Oruanui eruption shows many unusual features; its episodic nature, wide range of depositional conditions in fall deposits of very wide dispersal, and complex interplay of fall and PDC activity.     

Variability in eruptive dynamics associated with caldera collapse: an example from two successive eruptions at Batur volcanic field,Bali, Indonesia

Batur volcanic field (BVF) in Bali, Indonesia, underwent two successive caldera-forming eruptions, CI and CII (29,300 and 20,150 years b.p., respectively) that resulted in the deposition of dacitic ignimbrites. The respective ignimbrites show contrasted stratigraphies, exemplify the variability of dynamics associated with caldera-forming eruptions and provide insights into the possible controls exerted by caldera collapse mechanisms. The Ubud Ignimbrite is widespread and covers most of southern Bali. The deposits consist dominantly of pyroclastic flow with minor pumice fall deposits. The intra-caldera succession comprises three distinct, partially to densely welded cooling units separated by non-welded pyroclastic flow and fall deposits. The three cooling units consist of pyroclastic flow deposits only and together represent up to 16 distinct flow units, each including a thin, basal, lithic-rich breccia. This eruption was related to a 13.5×10 km caldera (CI) with a minimum collapsed volume of 62 km³. The floor of caldera CI is inferred to have a piecemeal geometry. The Ubud Ignimbrite is interpreted as the product of a relatively long-lasting, pulsating, collapsing fountain that underwent at least two time breaks. A stable column developed during the second time break. Discharge rate was high overall, but oscillatory, and increased toward the end of the eruption. These dynamics are thought to reflect sequential collapse of the CI structure. The Gunungkawi Ignimbrite is of more limited extent outside the source caldera and occurs only in central southern Bali. The Gunungkawi Ignimbrite proximal deposits consist of interbedded accretionary lapilli-bearing ash surge, ash fall, pumice lapilli fall and thin pyroclastic flow deposits, overlain by a thick and massive pyroclastic flow deposit with a thick basal lag breccia. The caldera (CII) is 7.5×6 km in size, with a minimum collapsed volume of 9 km³. The CII eruption included two distinct phases. During the first, eruption intensity was low to moderate and an unstable, essentially phreatomagmatic column developed. During the second phase, the onset of caldera collapse drastically increased the eruption intensity, resulting in column collapse. The caldera floor is believed to have subsided rapidly, producing a single, short-lived burst of high eruption intensity that resulted in the deposition of the uppermost massive pyroclastic flow.Editorial responsibility: T. Druitt     

Transitions between fall phases and pyroclastic density currents during the AD 79 eruption at Vesuvius: building a transient conduit model from the textural and volatile record

The magmatic phase of the AD 79 eruption of Vesuvius produced alternations of fall and pyroclastic density current (PDC) deposits. A previous investigation demonstrated that the formation of several PDCs was linked with abrupt increases in the proportion of denser juvenile clasts within the eruptive column. Under the premise that juvenile clast density is controlled by vesiculation processes within the conduit, we investigate the processes responsible for these variations at or close to fragmentation levels. Pumice textures (vesicle sizes, numbers, and connectivity combined with crystal textures) from the AD 79 PDC deposits are compared to those from interbedded fall samples. Both PDC and fall deposits preserve textures that represent a full spectrum of degassing and outgassing processes, from bubble nucleation to collapse. Combining the textural and volatile (groundmass H₂O) data, we derive a conduit model that satisfies all the textural and physical observations made for this phase of the eruption: lateral vesicularity/density stratifications are produced by maturing of bubble textures with superimposed localized shearing of bubble-rich magmas, which enhance outgassing of H₂O. The incorporation of denser slower-moving magma from the conduit margins (??lateral magma density gradient??) is likely to be responsible for the higher abundances of dense juvenile pumice that triggered partial column collapses. We also illustrate how variations in the fragmentation depth (tapping a ??vertical magma density gradient??) can be responsible for variations in erupted clast density distributions, and potentially in the extent of degassing/outgassing.     

The 1631 eruption of Vesuvius

Contemporary accounts of the violent eruption of Vesuvius in 1631 are reviewed, and recorded events are correlated with resulting volcanic deposits. Field study of the deposits in the proximal area revealed the presence of tephra falls, pyroclastic flows and lava, with subordinate surge deposits. A total volume of 1.1 km³ (0.55 km³ DRE) of phono-tephritic to phonolitic magma was ejected during 24 hours.The different magma compositions correspond with a transition from a lower, white, aphyric, highly vesiculated pumice (layer 1) to an upper, gray, crystal-rich, poorly vesiculated pumice (layer 3), showing reverse grading. Isopach and isopleth maps of the tephra-falls have been constructed to determine changes in the eruptive style and temporal evolution of the eruption column which reached a maximum height of 16 to 28 km.The recorded column height variations show a change in the mass discharge rate (8.9 × 10⁶ kg/s to 8.2 × 10⁷ kg/s) and the occurrence of pyroclastic flows during the deposition of the weakly vesiculated, dense pumice of the upper part of layer 3. Pyroclastic flows are crystal-rich and show St. Vincent-type features. The explosive phase demolished the upper part of the pre-existing cone, and debris flows invaded the southern side of the volcano. In the afternoon of December 17, 1631 an outbreak of lava flow from a southern lateral fracture system occurred, and effusion of lava continued up to midnight of December 18. Intermittent steam blasts continued to the end of December, when the eruption ended and Mount Vesuvius entered a solfataric phase. The earthquakes that had marked both the pre-eruptive and eruptive phases, continued, however, well into March 1632.     

Correlation of rhyolitic pyroclastic eruptive units from the Taupo volcanic zone by Fe–Ti oxide compositional data

Grain-specific analyses of Fe–Ti oxides and estimates of eruption temperature (T) and oxygen fugacity (fO₂) have been used to fingerprint rhyolitic fall and flow deposits that are important for tephrostratigraphic studies in and around the Taupo volcanic zone of North Island, New Zealand. The analysed Fe–Ti oxides commonly occur in the rims of orthopyroxene crystals and appear to reflect equilibrium immediately prior to eruption because of geochemical correlation with the co-existing glass phase. The composition of the spinel phase is particularly diagnostic of eruptive centre for post-65 ka events and can be used to distinguish many tephra beds from the same volcano. The 29 different units examined were erupted over a wide range in T (690–990°C) and Δ log fO₂ (–0.1 to 2.0). These parameters are closely related to the mafic mineral assemblage, with hydrous mineral-bearing units displaying higher fO₂. Such trends are superimposed on larger differences in fO₂ that are related to eruptive centre. At any given temperature, all post-65 ka Okataina centre tephra have higher fO₂ values than post-65 ka Taupo centre tephra. This provides a useful criterion for identifying the volcanic source. There are no temporal T and fO₂ trends in the tephra record; over intervals >20 ka, however, tephra sequences from Taupo centre form characteristic T-fO₂ buffer trends mirroring the glass chemistry. Individual eruptive events display uniform spinel and rhombohedral phase compositions and thus narrow ranges in T (± <20°C) and log fO₂ (± <0.5), allowing these features to identify individual magma batches. These criteria can help distinguish tephra deposits of similar bulk or glass composition that originated from the same volcano. Distal fall deposits record the same T-fO₂ conditions as the proximal ignimbrite and enable distal–proximal correlation. Lateral and vertical compositional and T-fO₂ variability displayed in large volume (>100 km³) ignimbrites, such as the Oruanui, Rotoiti and Ongatiti, is similar to that found in a single pumice clast and thus mainly reflects analytical error; however, thermal gradients of ca. 50°C may occur in some units. Received: 6 April 1998 / Accepted: 16 June 1998     

The physical volcanology of the 1600 eruption of Huaynaputina, southern Peru

Volcán Huaynaputina is a group of four vents located at 16°36'S, 70°51'W in southern Peru that produced one of the largest eruptions of historical times when ~11 km³ of magma was erupted during the period 19 February to 6 March 1600. The main eruptive vents are located at 4200 m within an erosion-modified amphitheater of a significantly older stratovolcano. The eruption proceeded in three stages. Stage I was an ~20-h sustained plinian eruption on 19-20 February that produced an extensive dacite pumice fall deposit (magma volume ~2.6 km³). Throughout medial-distal and distal parts of the dispersal area, a fine-grained plinian ashfall unit overlies the pumice fall deposit. This very widespread ash (magma volume ~6.2 km³) has been recognized in Antarctic ice cores. A short period of quiescence allowed local erosion of the uppermost stage-I deposits and was followed by renewed but intermittent explosive activity between 22 and 26 February (stage II). This activity resulted in intercalated pyroclastic flow and pumice fall deposits (~1 km³). The flow deposits are valley confined, whereas associated co-ignimbrite ash fall is found overlying the plinian ash deposit. Following another period of quiescence, vulcanian-type explosions of stage III commenced on 28 February and produced crudely bedded ash, lapilli, and bombs of dense dacite (~1 km³). Activity ceased on 6 March. Compositions erupted are predominantly high-K dacites with a phenocryst assemblage of plagioclase>hornblende>biotite>Fe-Ti oxides-apatite. Major elements are broadly similar in all three stages, but there are a few important differences. Stage-I pumice has less evolved glass compositions (~73% SiO₂), lower crystal contents (17-20%), lower density (1.0-1.3 g/cm³), and phase equilibria suggest higher temperature and volatile contents. Stage-II and stage-III juvenile clasts have more evolved glass (~76% SiO₂) compositions, higher crystal contents (25-35%), higher densities (up to 2.2 g/cm³), and lower temperature and volatile contents. All juvenile clasts show mineralogical evidence for thermal disequilibrium. Inflections on a plot of log thickness vs area^1/2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system. The ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper-level winds, whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower-altitude wind field. Thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24-32 km and a total column height of 34-46 km. The estimated mass discharge rate for the ~20-h-long stage-I eruption is 2.4᎒⁸ kg/s and the volumetric discharge rate is ~3.6᎒⁵ m³/s. The pumice fall deposit has a dispersal index (Hildreth and Drake 1992) of 4.4, and its index of fragmentation is at least 89%, reflecting the dominant volume of fines produced. Of the 11 km³ total volume of dacite magma erupted in 1600, approximately 85% was evacuated during stage 1. The three main vents range in size from ~70 to ~400 m. Alignment of these vents and a late-stage dyke parallel to the NNW-SSE trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre-existing weaknesses. During stage I this fissure evolved into a large flared vent, vent 2, with a diameter of approximately 400 m. This vent was active throughout stage II, at the end of which a dome was emplaced within it. During stage III this dome was eviscerated forming the youngest vent in the group, vent 3. A minor extra-amphitheater vent was produced during the final event of the eruptive sequence. Recharge may have induced magma to rise away from a deep zone of magma generation and storage. Subsequently, vesiculation in the rising magma batch, possibly enhanced by interaction with an ancient hydrothermal system, triggered and fueled the sustained Plinian eruption of stage I. A lower volatile content in the stage-II and stage-III magma led to transitional column behavior and pyroclastic flow generation in stage II. Continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage III. No caldera collapse occurred because no shallow magma chamber developed beneath this volcano.     

Textural and geochemical constraints on eruptive style of the 79 <Emphasis Type="SmallCaps">ad</Emphasis> eruption at Vesuvius

The 79 ad Plinian eruption of Vesuvius produced first a white pumice fallout from a high steady eruptive column, and then a grey pumice fallout originating from an oscillatory eruptive column with several partial column collapse events after which there was a total column collapse. This first total collapse was followed by renewed Plinian activity and produced the last grey pumice (GP) fallout deposit of the eruption. Textural characteristics (vesicularity and microcrystallinity) of a complete sequence of the pumice fallout deposits are presented along with the major element compositions and residual volatile contents (H₂O, Cl) to constrain the degassing processes and the eruptive dynamics. Large variations in residual volatile contents exist between the different eruptive units. Textural features also strongly differ between white and grey pumices, but also within the grey pumices. The degassing processes were thus highly heterogeneous. We propose a new model of the 79 ad eruption in which pre-eruptive conditions (H₂O saturation, magma temperature and viscosity) are the critical controls on the diversity of the syn-eruptive degassing processes and hence the eruptive dynamics. Cl contents measured in melt inclusions show that only the white pumice and the upper part of the grey pumice magma were H₂O saturated prior to eruption. The white pumice eruptive units represent a typical closed-system degassing evolution, whereas the first grey pumice one, stored under similar pre-eruptive saturation conditions, follows a particular open-system degassing evolution. We suggest that the oscillatory regime that dominated the grey pumice eruptive phase is linked to pre-eruptive water undersaturation of most of the grey magma, and the associated time delays necessary for H₂O exsolution. We also suggest that the high residual H₂O content of the last grey pumice, deposited after the renewal of Plinian activity following the first total column collapse event, is due to syn-eruptive saturation of GP magma and reduced H₂O exsolution efficiency resulting from speciation of dissolved H₂O in the melt.     

Environmental hazards of fluoride in volcanic ash: a case study from Ruapehu volcano, New Zealand

The vent-hosted hydrothermal system of Ruapehu volcano is normally covered by a c. 10 million m³ acidic crater lake where volcanic gases accumulate. Through analysis of eruption observations, granulometry, mineralogy and chemistry of volcanic ash from the 1995–1996 Ruapehu eruptions we report on the varying influences on environmental hazards associated with the deposits. All measured parameters are more dependent on the eruptive style than on distance from the vent. Early phreatic and phreatomagmatic eruption phases from crater lakes similar to that on Ruapehu are likely to contain the greatest concentrations of environmentally significant elements, especially sulphur and fluoride. These elements are contained within altered xenolithic material extracted from the hydrothermal system by steam explosions, as well as in residue hydrothermal fluids adsorbed on to particle surfaces. In particular, total F in the ash may be enriched by a factor of 6 relative to original magmatic contents, although immediately soluble F does not show such dramatic increases. Highly soluble NaF and CaSiF₆ phases, demonstrated to be the carriers of ‘available’ F in purely magmatic eruptive systems, are probably not dominant in the products of phreatomagmatic eruptions through hydrothermal systems. Instead, slowly soluble compounds such as CaF₂, AlF₃ and Ca₅(PO₄)₃F dominate. Fluoride in these phases is released over longer periods, where only one third is leached in a single 24-h water extraction. This implies that estimation of soluble F in such ashes based on a single leach leads to underestimation of the F impact, especially of a potential longer-term environmental hazard. In addition, a large proportion of the total F in the ash is apparently soluble in the digestive system of grazing animals. In the Ruapehu case this led to several thousand sheep deaths from fluorosis.     

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.