Plinian-type Eruptions Research Articles

Radiocarbon dating, eruptive dynamics, and hazard implications of four Holocene Plinian-type eruptions recorded in Manizales city, Colombia

Urban centers settled under the influence of volcanic activity are common in the Colombian Andes, particularly those such as Manizales city (ca. 400,000 inhabitants) and other smaller towns on the western flank of the Central Cordillera. Several pumice-rich tephra layers widely dispersed beneath the city suggest multiple Plinian-type eruptions have occurred nearby. From this geological record, the four youngest, thickest, and most persistent layers in the region are the most representative of this activity. This work uses mapping, physical descriptions, radiocarbon dating, and calculation of physical parameters to: (1) define and characterize the tephra sequence, (2) determine the age of the eruptions, (3) define the source and eruptive dynamics associated with each of the events, and (4) give insights into the volcanic hazard for Manizales city. The deposits were named in this study as Lower Manizales (Lm), Middle-lower Manizales (Mlm), Middle-upper Manizales (Mum) and Upper Manizales (Um) tephra layers, together forming the “Manizales tephra sequence”. Mapping of the deposits revealed the lower layer originated from the little known Plazuela volcano (~ 15 km east of Manizales), while the others originated in the Cerro Bravo volcano (~ 25 km east of Manizales). The radiometric analysis indicates that the lowest tephra layer originated at 7879.5 ± 55.5 cal. years BP, while the three overlying layers were deposited at 7730 ± 55 cal. years BP, 5294 ± 19 cal. years BP, and 2759 ± 20 cal. years BP. All of these deposits were formed by Plinian-type eruptions, with column heights varying between 18 and 29 km. The minimum tephra volume emitted by each eruption was between 0.21 and 0.38 km3, and the emission rate 21.6 × 107 kg/s for the Lm layer, 3.38 × 107 kg/s for the Mlm layer, 6.27 × 107 kg/s for the Mum layer, and 5.16 × 107 kg/s for the Um layers. The results suggest that the region is strongly threatened by pyroclastic falls associated with several active volcanoes located in the Central Cordillera.

Pre-eruptive conditions and reheating of dacitic magma (Malinche Pumice II Plinian eruption) at La Malinche volcano, Central Mexico

The Malinche Pumice II is a fallout deposit produced by a Plinian-type eruption during the Pleistocene-Holocene at La Malinche volcano. La Malinche (4461 masl) is an active stratocone located in central Mexico, in the Trans-Mexican Volcanic Belt province. It is 26 km to the NE of the City of Puebla and 22 km to the SE of the City of Tlaxcala. The Malinche Pumice II was produced by one of the largest explosive eruptions of this volcano, involving a dacitic magma (63.6–65.3 wt% SiO2, anhydrous basis) with a mineral assemblage of plagioclase + amphibole + FeTi oxides and ± biotite phenocrysts. Larger plagioclase crystals are up to 1.2 mm in diameter, with resorbed margins and complex zoning (An26 to An43), whereas microcrysts are euhedral with homogeneous composition (An42 to An55). Amphibole is mainly magnesio-ferri-hornblende (Mg# = 0.65–0.82) and lesser amounts of pargasite. Geothermometry performed using FeTi oxides yields an average temperature of 873 ± 2 °C and oxygen fugacity of 1.8 units above the NNO buffer. Amphibole geothermobarometry yields a lower temperature (809 ± 19 °C) and a pressure of 230 ± 100 MPa, assuming 4.4 wt% of H2O. Considering an average density of 2650 kg/m3 for the upper crust in central Mexico, the storage region of the Malinche Pumice II dacitic magma was located at 8 ± 2 km depth. Biotite could be antecrystic or xenocrystic in origin, based on its reacted texture, large sizes (0.85–2 mm), and reaction rims. Similarly, pargasite that provides higher temperature estimates (900 ± 18 °C) and low anorthite plagioclase (An26) could also be antecrysts or xenocrysts. The Plinian eruption was likely triggered by an input of a hotter and deeper magma batch that overpressurized the dacitic magma system that eventually provoked vesiculation and fragmentation. The eruption shifted to a hydromagmatic style because of external water's entrainment that decreased the vesicularity of pumice. Our results are in good agreement with recent seismicity recorded at depths of ~7 km probably associated with activity of the Malinche's magmatic system.

Eruption type probability and eruption source parameters at Cotopaxi and Guagua Pichincha volcanoes (Ecuador) with uncertainty quantification

Future occurrence of explosive eruptive activity at Cotopaxi and Guagua Pichincha volcanoes, Ecuador, is assessed probabilistically, utilizing expert elicitation. Eight eruption types were considered for each volcano. Type event probabilities were evaluated for the next eruption at each volcano and for at least one of each type within the next 100 years. For each type, we elicited relevant eruption source parameters (duration, average plume height, and total tephra mass). We investigated the robustness of these elicited evaluations by deriving probability uncertainties using three expert scoring methods. For Cotopaxi, we considered both rhyolitic and andesitic magmas. Elicitation findings indicate that the most probable next eruption type is an andesitic hydrovolcanic/ash-emission (~ 26–44% median probability), which has also the highest median probability of recurring over the next 100 years. However, for the next eruption at Cotopaxi, the average joint probabilities for sub-Plinian or Plinian type eruption is of order 30–40%—a significant chance of a violent explosive event. It is inferred that any Cotopaxi rhyolitic eruption could involve a longer duration and greater erupted mass than an andesitic event, likely producing a prolonged emergency. For Guagua Pichincha, future eruption types are expected to be andesitic/dacitic, and a vulcanian event is judged most probable for the next eruption (median probability ~40–55%); this type is expected to be most frequent over the next 100 years, too. However, there is a substantial probability (possibly >40% in average) that the next eruption could be sub-Plinian or Plinian, with all that implies for hazard levels.

Integrated hazards maps of the Tacaná Volcanic complex, Mexico-Guatemala: Ashfall, block-and-ash flows, and lahars

The Tacaná Volcanic Complex (TVC) is an active volcano, located on the border between Mexico and Guatemala, in the westernmost part of the Central American Volcanic Arc. The TVC has produced effusive and explosive activity during the past 300 ka through the emplacement of lava flows, pyroclastic deposits, debris avalanches and lahars. During the last 10 ka, at least 11 eruptions have occurred, with three small phreatic explosions documented in 1855–1856, 1949 and 1986. This work presents volcanic hazards maps of the TVC that integrate the results of analyzing two eruptive style scenarios: dome-collapses and Plinian-type eruptions and the lahars triggered by both these hazards. The extents and expected distributions for each of these phenomena were assessed using numerical models and the results were calibrated with recent field data. The simulation results were merged to prepare three new volcanic hazards maps, which depict on a color scale the hazard level forecasted for each phenomenon (high – red, moderate – orange, and low – yellow). The maps show that the total extension of block-and-ash flows will be restricted towards the volcano base, reaching a maximum distance towards Guatemala of ~5 km, and ~10 km in Mexico; while lahars could reach up to 50 km downstream along the main rivers that drains the complex (i.e. Coatán, Cahoacán and Suchiate rivers) that would flood important urban centers mostly over Mexican territory. Ashfall would cover an area of ca. 8000 km2, inhabited by ~2 million people in both countries. Hence, renewed eruptive activity of the TVC would be catastrophic not only because of the loss of human life but also for its adverse impact on economic activity based on corn, coffee, and banana plantations, together with the disruption of communication and transportation systems between Mexico and Central America.

The Use of a Numerical Weather Prediction Model to Simulate Near-Field Volcanic Plumes

In this paper, a state-of the art numerical weather prediction (NWP) model is used to simulate the near-field plume of a Plinian-type volcanic eruption. The NWP model is run at very high resolution (of the order of 100 m) and includes a representation of physical processes, including turbulence and buoyancy, that are essential components of eruption column dynamics. Results are shown that illustrate buoyant gas plume dynamics in an atmosphere at rest and in an atmosphere with background wind, and we show that these results agree well with those from theoretical models in the quiescent atmosphere. For wind-blown plumes, we show that features observed in experimental and natural settings are reproduced in our model. However, when comparing with predictions from an integral model using existing entrainment closures there are marked differences. We speculate that these are signatures of a difference in turbulent mixing for uniform and shear flow profiles in a stratified atmosphere. A more complex implementation is given to show that the model may also be used to examine the dispersion of heavy volcanic gases such as sulphur dioxide. Starting from the standard version of the weather research and forecasting (WRF) model, we show that minimal modifications are needed in order to model volcanic plumes. This suggests that the modified NWP model can be used in the forecasting of plume evolution during future volcanic events, in addition to providing a virtual laboratory for the testing of hypotheses regarding plume behaviour.

The unexpected explosive sub-Plinian eruption of Calbuco volcano (22–23 April 2015; southern Chile): Triggering mechanism implications

Plinian-type eruptions are extremely hazardous, producing pyroclastic fallout and flows extending many kilometres from the vent. The most commonly invoked eruption trigger for Plinian-type eruptions is the intrusion of fresh magma, generally associated with precursory ground deformation and seismicity days/weeks before eruption. Closed-system internal triggering has also been proposed, such as protracted crystallisation of magma, which can produce a build-up of exsolved volatiles and thus pressurise the system prior to eruption. On 22–23 April 2015 Calbuco volcano, Chile, produced a sub-Plinian eruption with <3 h seismic precursory activity and no clear deformation signals in the preceding months. Here, we show that petrological and geochemical evidence do not support a hypothesis of eruption triggering due to pre-eruptive intrusion of fresh magma, but instead are consistent with an internal trigger. We found that basaltic andesitic magma was stored at depths between 8 and 12 km (i.e. 230–320 MPa) beneath Calbuco volcano before the 2015 eruption. The stored magma had an initial temperature of 900–950 °C, was water-saturated (5.5–6.5 wt% H2O) and formed phenocrysts of titanomagnetite, orthopyroxene, clinopyroxene and plagioclase cores (An78–93). Gradual cooling of the magma chamber produced thermal gradients and magma convection, evidenced by plagioclase overgrowth rims (An58–77) and blocky microlites (25–250 μm). Our interpretation is that this continuing crystallisation induced second boiling and an over-pressurisation of the system, leading to the rapid onset of the 2015 eruption. Petrological and geochemical evidence therefore shows that a closed-system magma chamber can evolve into a highly explosive eruption with very little precursory warning, posing a challenge for current volcano monitoring paradigms. We propose that internal triggering should be carefully considered as a mechanism for unexpected sub-Plinian eruptions, prompting a potential revision of existing hazard management strategies.

Pómez Bosque de Tlalpan, producto de una erupción de gran magnitud en el margen suroeste de la cuenca de México

In this contribution we describe a white pumice fall deposit, informally named pómez Bosque de Tlalpan (PBT), found in several outcrops in the southern Mexico basin. The most representative sequence occurs at the Bosque de Tlalpan park, Delegación Tlalpan, in Mexico City. In this site the deposit is 3 m thick, massive, and contains 80–90 vol. % of pumice clasts, predominantly of coarse lapilli-sized and some block-sized fragments. The PBT has a dacitic composition (64.9–66 wt. % SiO2, on anhydrous basis) of calc-alkaline affinity, and a mineral assemblage represented by plagioclase &gt; amphibole &gt; orthopyroxene &gt; biotite, ±Fe-Ti oxides, with quartz in lesser proportions, and zircon as accesory mineral, in a vesicular and glassy matrix. According to data collected from seven outcrops, thickness and clast size of the deposit decrease towards the NE and therefore the vent source should be located to the SW of Bosque de Tlalpan. Possible sources are the volcanoes Ajusco and San Miguel, which are part of the Sierra de Las Cruces volcanic range, and are located in this direction at ~11 km from Bosque de Tlalpan. The age of the PBT deposit was determined by radiocarbon dating of underlying paleosoils that yielded 25,730 ± 130 to 37,450 ± 330 yrs B.P. (Late Pleistocene). Additionally, thermoluminscence dating of one sample yielded an age of 30,300 ± 5,000 years, similar to the radiocarbon ages. Hence, the PBT represents the youngest reported deposit from the Sierra de Las Cruces volcanic range. Although only seven outcrops were described in this work, we interpret the PBT deposit as produced by a plinian-type eruption, based on its massive structure, its ca. 3 m thickness, and the presence of pumice clasts in coarse lapilli to block sizes, as well as 3.5 cm sized lithics, at 11 km from the possible vent source, characteristic of this kind of deposits. We discard the monogenetic volcanoes from the Chichinautzin volcanic field as the vent source of the PBT deposit, because this kind of volcanoes is relatively mafic in composition and do not produce plinian-type deposits.

Emplacement dynamics and hydrothermal alteration of the Atengo ignimbrite, southern Sierra Madre Occidental, northwestern Mexico

The Sierra Madre Occidental is a thick continental arc related to the subduction of the Farallon plate beneath North America resulting in a very intense and widespread Cretaceous to Cenozoic magmatic and tectonic activity. The 28 My old Atengo ignimbrite outcrops in the southern Sierra Madre Occidental, northwestern Mexico. From 12 sites that belong to various pyroclastic and lava flows emplaced during two pulses in the Oligocene (ca. 32–28 Ma) and Early Miocene (ca. 24–20 Ma), 97 rock specimens were drilled. The mineralogical and rock magnetic properties of the Atengo ignimbrite are compared with the surrounding volcanic rocks to identify the eruption mechanism, and with the El Castillo Ignimbrite, Veracruz, Mexico, to understand the depositional conditions. The comparisons reveal that the Atengo ignimbrite erupted from a single source, but less violently than the El Castillo ignimbrite, and cooled rapidly, inhibiting the formation of subhedral grains. The source of the Atengo Ignimbrite was a Plinian-type eruption, and the characteristic mineralogical and textural properties of each flow are related to different stages of the Plinian-type eruption. Further more, hydrothermal fluids were active during the last stages of volcanism, and caused moderate to intense alteration, especially in the ignimbrites, where high permeability aided the movement of hydrothermal fluids.

High-K Mafic Plinian Eruptions of Volcán de Colima, Mexico

The last Plinian-type eruption of Volcan de Colima, Mexico, occurred in 1913; this resulted in the removal of the top 100 m of the edifice and the deposition of a tephra layer that blanketed the slopes of the Colima Volcanic Complex (CVC). Road-cuts on the flanks of the nearby Nevado de Colima edifice expose pre-1913 air-fall tephra, pyroclastic flow and ash-rich surge deposits resulting from numerous highly explosive events throughout the Holocene. The majority of the pumice and scoria fallout deposits are medium-K subalkaline basaltic andesite and andesite in composition, defining a clear major element differentiation trend. In contrast, three newly discovered scoria fallout deposits are high-K subalkaline, transitional to alkaline, basaltic andesite in composition and are characterized by the presence of phlogopite; these deposits have high MgO (up to 7·9 wt %), K2O (up to 2·6 wt %) and P2O5 (up to 0·67 wt %) contents. They are also strongly enriched in fluid- and melt-mobile large ion lithophile elements (LILE; Rb, Ba, K, Sr and Th) and light REE (LREE; La, Ce, Pr and Nd) relative to the majority of the Colima tephra fallout deposits. Strontium and Nd isotope systematics reveal that the high-K mafic scoria have more radiogenic Sr (87Sr/86Sr = 0·70365–0·70408) and less radiogenic Nd (143Nd/144Nd = 0·51279–0·51294) compared with the majority of the subalkaline tephras (87Sr/86Sr = 0·70338–0·70371 and 143Nd/144Nd = 0·51290–0·51295). Two-component mixing models, using whole-rock geochemical data, indicate the importance of magma mixing in the petrogenesis of the Colima magmas, with addition of up to 50% by volume of an alkaline mafic magma component in the most potassic magmas. This is supported by mineral chemistry and textural data, which reveal multiple episodes of decompression and magma mingling within a shallow crustal magma storage region. The presence of these potassic tephra fall deposits among the otherwise prevailing medium-K subalkaline stratigraphy indicates that pulses of K-rich alkaline mafic magmas periodically enter the CVC plumbing system on timescales of a few thousand years and may trigger Plinian explosive eruptions.

A volcanogenic intensification factor of stratosphere-troposphere exchange

A mechanism of volcanic aerosol transport to the stratosphere after Plinian-type eruptions, when the maximal height of the emission does not exceed the tropopause altitude, is suggested. The NOAA HYSPLIT trajectory model and open global temperature data are used in order to show the role of the volcanic gas-ash clouds in the temperature change of the upper troposphere and lower stratosphere, tropopause destruction, and, as a consequence, intensification of the stratosphere-troposphere exchange. An abnormal increase in surface ozone concentrations is recorded during the passage of volcanic clouds.

Pre-eruptive conditions of dacitic magma erupted during the 21.7 ka Plinian event at Nevado de Toluca volcano, Central Mexico

The Nevado de Toluca volcano in Central Mexico has been active over the last ca. 42ka, during which tens of km3 of pyroclastic material were erupted and two important Plinian-type eruptions occurred at ca. 21.7ka (Lower Toluca Pumice: LTP) and ca. 10.5ka (Upper Toluca Pumice: UTP). Samples from both the LTP and UTP contain plagioclase, amphibole, iron-titanium oxides, and minor anhedral biotite, set in a vesicular, rhyolitic, glassy matrix. In addition, UTP dacites contain orthopyroxene. Analysis of melt inclusions in plagioclase phenocrysts yields H2O contents of 2–3.5wt.% for LTP and 1.3–3.6wt.% for UTP samples. Ilmenite–ulvospinel geothermometry yields an average temperature of ~868°C for the LTP magma (hotter than the UTP magma, ~842°C; Arce et al., 2006), whereas amphibole–plagioclase geothermometry yields a temperature of 825–859°C for the LTP magma. Water-saturated experiments using LTP dacite suggest that: (i) amphibole is stable above 100MPa and below 900°C; (ii) plagioclase crystallizes below 250–100MPa at temperatures of 850–900°C; and (iii) pyroxene is stable only below pressures of 200–100MPa and temperatures of 825–900°C. Comparison of natural and experimental data suggests that the LTP dacitic magma was stored at 150–200MPa (5.8–7.7km below the volcano summit). No differences in pressure found between 21.7ka and 10.5ka suggest that these two magmas were stored at similar depths. Orthopyroxene produced in lower temperature LTP experiments is compositionally different to those found in UTP natural samples, suggesting that they originated in two different magma batches. Whole-rock chemistry, petrographic features, and mineral compositions suggest that magma mixing was responsible for the generation of the dacitic Plinian LTP eruption.

The White River Ash: largest Holocene Plinian tephra

The White River Ash is a bi-lobate tephra in eastern Alaska, Yukon Territory, and western Northwest Territories. Plinian-type eruptions produced the north lobe ∼1900 years BP and the larger east lobe ∼1250 years BP (14C years). Present evidence favors the vent for the east lobe to be beneath the Klutlan Glacier. East lobe pumice is not present atop Mt. Churchill, so the pumice there must belong to the north lobe and is also likely to have come from a vent beneath the Klutlan Glacier. Isopachs of the east lobe, now known to stretch as far east as Great Bear Lake, indicate an east lobe volume of ∼47 km3. Thickness and grain size of the east lobe decay in exponential fashion, producing straight line plots when the thickness half-distance and clast half-distance are plotted against the square root of the isopach area, the proximal slope being steeper than the distal. The east lobe eruption is indicated to have been into a wind of about 10 m/s and to have produced an eruptive cloud height of ∼45 km. The eruption rate was at least 2.8 × 108 kg/s.

Procesos y riesgos volcánicos

Los magmas se generan en cuatro ambientes geodinamicos diferentes: a) en los margenes de placa constructivos, en los que se incluyen las dorsales centro-oceanicas y las cuencas tras-arco; b) en los margenes de placa destructivos, como los arcos-isla y los margenes continentales activos; c) en zonas de intraplaca oceanica, y d) en zonas de intraplaca continental. En las dorsales, en los arcos-isla intraoceanicos y en las islas oceanicas los magmas solo se pueden generar en el manto superior, ya que en estas areas no existe corteza continental, pero en los margenes continentales activos y en las areas de intraplaca continental la corteza puede jugar un papel mas o menos importante. A su vez, las dos zonas del manto superior en las que potencialmente se pueden originar magmas son la litosfera y la astenosfera. El mecanismo por el que se originan los magmas en cada uno de estos ambientes geodinamicos es diferente. En los margenes de placa constructivos la astenosfera asciende adiabaticamente y funde. En los margenes destructivos la generacion de magmas esta relacionada con la subduccion. Finalmente, en areas de intraplaca continental el magmatismo esta asociado a fenomenos de extension producidos por fenomenos tales como la indentacion y la delaminacion o bien esta relacionado con plumas mantelicas, como ocurre en general en areas de intraplaca oceanica. El tipo de erupcion depende de la composicion del magma, y sobre todo de su temperatura, viscosidad y contenido en gases. En general, los magmas basalticos (que tienen contenidos en SiO2 comprendidos entre 45 y 52%, temperaturas de 1.000-1.200 °C, viscosidades del orden de 10-102 Pa·s y baja abundancia de gases) extruyen de forma tranquila o moderadamente explosiva, dando lugar a erupciones de tipo hawaiano o estromboliano. Por el contrario, los magmas acidos (que tienen contenidos en SiO2 superiores al 63%, temperaturas de 700 a 900 °C, viscosidades entre 106 y 108 Pa·s y elevado contenido en gases) y los traquitico-fonoliticos lo hacen de forma explosiva, generando erupciones de tipo vulcaniano y pliniano. A escala global las erupciones volcanicas y los fenomenos asociados son mas infrecuentes y generan menos victimas y danos que otros riesgos naturales. Segun la informacion recopilada en la mas reciente y completa base de datos (Witham, 2005), de los 176 volcanes/areas volcanicas que se incluyen en la misma, mas de la mitad han sido responsables de mas de un incidente en el siglo XX, y de las 491 erupciones que han tenido lugar en este periodo de tiempo del orden del 50% han producido muertes. De los diferentes materiales emitidos, las coladas y oleadas piroclasticas fueron las principales causas de muerte, seguidas de los lahares, que a su vez fueron la principal causa de heridos. Por el contrario, las lavas y los piroclastos de caida generaron un numero relativamente pequeno de muertes y heridos, si bien los piroclastos de caida fueron responsables del mayor numero de personas que perdieron su casa y que tuvieron que ser evacuadas. Por otra parte, como las erupciones de los volcanes asociados a zonas de subduccion son, en general, mas explosivas y por ende mas peligrosas que las de los volcanes de areas de intraplaca, las regiones en las que mas muertes se produjeron en el siglo XX fueron el Caribe, Sudamerica, America Central y el Sudeste asiatico.

Transient dynamics of vulcanian explosions and column collapse.

Several analytical and numerical eruption models have provided insight into volcanic eruption behaviour, but most address plinian-type eruptions where vent conditions are quasi-steady. Only a few studies have explored the physics of short-duration vulcanian explosions with unsteady vent conditions and blast events. Here we present a technique that links unsteady vent flux of vulcanian explosions to the resulting dispersal of volcanic ejecta, using a numerical, axisymmetric model with multiple particle sizes. We use observational data from well documented explosions in 1997 at the Soufrière Hills volcano in Montserrat, West Indies, to constrain pre-eruptive subsurface initial conditions and to compare with our simulation results. The resulting simulations duplicate many features of the observed explosions, showing transitional behaviour where mass is divided between a buoyant plume and hazardous radial pyroclastic currents fed by a collapsing fountain. We find that leakage of volcanic gas from the conduit through surrounding rocks over a short period (of the order of 10 hours) or retarded exsolution can dictate the style of explosion. Our simulations also reveal the internal plume dynamics and particle-size segregation mechanisms that may occur in such eruptions.

Conduit flow and fragmentation

Abstract This chapter provides a critical overview of syn-eruptive processes between magma chamber and vent in magmatic, sustained, Plinian-type eruptions. Phreatomagmatic effects are largely neglected. The main sources of information derive from textural studies of ejecta, theoretical models and physical experiments of eruption dynamics. Textural studies commonly find bimodal vesicle populations, indicative of several discrete nucleation events. The degree of disorder increases with explosivity. Dynamical parameters, such as nucleation density and rate and bubble growth rate, can be inferred from studies of the moments of bubble size distributions. Vesicularity in pumices is observed to vary significantly both within and between deposits, which suggests that the vesicularity at fragmentation is affected by the flow dynamics. Vesicularity variations correlate most closely with changes in magmatic composition and viscosity, but not with discharge rate. Conduit flow models can be broadly grouped into first- and second-generation models; the former generally impose a lithostatic pressure gradient and a constant Newtonian viscosity, whereas the latter include equations for the rheological changes that take place during vesiculation and solve for the pressure. Second-generation models derive highly non-lithostatic pressure gradients with the result that most of the vesiculation occurs at a high rate over a short distance just prior to fragmentation. The mechanisms of brittle and ductile fragmentation have been investigated in separate studies in non-vesiculating magmas, but which mechanism operates in explosive eruptions is not known. Dynamical laboratory experiments provide observations of the physical processes operating in conduit flows. Gas-expansion experiments have shown that it is possible to generate violent explosions by unloading in cool magmatic materials. Expanding dusty flows are found to be stable only if the bulk density increases with height. Exsolution experiments have demonstrated that acceleration precedes fragmentation and that gas evolution is enhanced by advection and bubble deformation. Deformed vesicles similar to those found in ‘woody’ pumice have been generated in an analogue system that has similar rheology to that found in vesiculating magmas. Large-scale exsolution experiments suggest that explosive volcanic eruptions are inherently heterogeneous: the fluctuations in discharge rate and discrete pulses and shocks commonly observed are a consequence of the large physical scale of volcanic systems. The effect of the magma chamber and conduit geometry has also been investigated. Eruption of material from a spherical flask up a narrow cylindrical tube generates quasi-steady flow conditions after an initial transient during which the discharge rate grows, as frequently observed in volcanic eruptions. The fragmentation surface does not propagate down into the magma chamber.

The Christmas Mountains caldera complex, Trans-Pecos Texas

The Christmas Mountains caldera complex developed approximately 42 Ma ago over an elliptical (8×5 km) laccolithic dome that formed during emplacement of the caldera magma body. Rocks of the caldera complex consist of tuffs, lavas, and volcaniclastic deposits, divided into five sequences. Three of the sequences contain major ash-flow tuffs whose eruption led to collapse of four calderas, all 1–1.5 km in diameter, over the dome. The oldest caldera-related rocks are sparsely porphyritic, rhyolitic, air-fall and ash-flow tuffs that record formation and collapse of a Plinian-type eruption column. Eruption of these tuffs induced collapse of a wedge along the western margin of the dome. A second, more abundantly porphyritic tuff led to collapse of a second caldera that partly overlapped the first. The last major eruptions were abundantly porphyritic, peralkaline quartz-trachyte ash-flow tuffs that ponded within two calderas over the crest of the dome. The tuffs are interbedded with coarse breccias that resulted from failure of the caldera walls. The Christmas Mountains caldera complex and two similar structures in Trans-Pecos Texas constitute a newly recognized caldera type, here termed a laccocaldera. They differ from more conventional calderas by having developed over thin laccolithic magma chambers rather than more deep-seated bodies, by their extreme precaldera doming and by their small size. However, they are similar to other calderas in having initial Plinian-type air-fall eruption followed by column collapse and ash-flow generation, multiple cycles of eruption, contemporaneous eruption and collapse, apparent pistonlike subsidence of the calderas, and compositional zoning within the magma chamber. Laccocalderas could occur else-where, particularly in alkalic magma belts in areas of undeformed sedimentary rocks.

Plio-Pleistocene volcanism and sedimentary facies changes at Gadeb prehistoric site, Ethiopia

Volcanic ashes, resulting from Plinian type eruptions, are regularly interbedded with fluvio-lacustrine successions of the Plio-Pleistocene Upper Webi Shebele Basin in the southeastern uplands of Ethiopia. Pumiceous and vitric ashes of rhyolitic composition dominate. Interbedded with the lacustrine succession are also scoriaceous ashes of alkali olivine basalt composition and minor amounts of mixed scoria. The majority of ashes interbedded with the lake sequence is primary air-fall material that underwent only minor degrees of reworking. Within the overlying fluviatile sequence primary ashes decrease in abundance, and crystalrich volcanoclastic deposits (»crystal tuffs«) take their place.

かん国うつ陵島のテフラ

A trachytic tephra fall deposit, the Ulreung-Oki ash, was first discovered at the Torihama archaeological site, in the northern Kinki district, where it forms whitish ash layer in strata bearing artifacts of the earliest Jomon ceramic culture. Occurrence of alkali feldspar and specific amphibole (probably kaersutite) clearly shows that this tephra is alkalic in composition. Correlatives of this tephra have been recognized at many localities in central Honshu and in piston cores from adjacent to Ulreung island to Oki bank. The grain-size distribution clearly suggests the source to be Ulreung volcano.This paper mainly deals with late Quaternary eruptive history of Ulreung volcano based on tephra studies in Ulreung island.Ulreung or Dagelet island is a large Quaternary stratovolcano situated in the west Japan Sea, 130km off the eastern coast of the Korean peninsula. Though the volcano has been considerably dissected, it has a remarkable caldera with a longer diameter of 3.5km and a central cone. The caldera has had a series of silicic tephra eruptions, forming a sequence of airfall and airflow deposits in late Quaternary. At least seven coarse-grained airfall units are discriminated and named from upper to lower as U-1, U-2, …and U-7.Approximate dating of these tephras is possible because the two time-marker tephras of Kyushu origin are found within the soils between the pumice falls: the Aira-Tn ash (AT, 21, 000-22, 000 YBP) between U-4 and U-5, and the Kikai-Akahoya ash (K-Ah, 6, 300 YBP) between U-1 and U-2.These Ulreung tephras are trachytic in composition, containing considerable amounts of alkali feldspar, kaersutite, biotite and clinopyroxene as phenocrysts. Chemical composition of pumiceous glass for these tephras is completely different from that of such subalkalic tephras as AT and K-Ah, etc. However, compositional variation from layer to layer is too small to be distinguished each other.In the late Quaternary age before ca. 22, 000 YBP, many plinian type eruptions took place producing such pumice falls as U-5, 6 and 7, which are not well preserved in the island due to erosion but in abyssal sediments in the Japan Sea. The three pumice falls, U-2, -3 and -4, recognized between the AT and K-Ah ashes were respectively formed by plinian, phreato-plinian and plinian eruptions. The last two are associated with pyroclastic flow deposits. This multicyclic explosive volcanism should result in the formation of caldera.Of the younger three tephras, U-2 seems to represent the largest recent eruption of the caldera. Distribution and stratigraphic position of this tephra indicates that U-2 can be correlated with the U-Oki ash. Three radiocarbon dates of ca. 9, 300 YBP were obtained at the Torihama site and south port of Osaka for this ash. U-Oki or U-2 is used as an excellent time-marker in analyzing paleo-sea level and paleo -oceanographic environments in the Japan Sea.The youngest activity of Ulreung volcano took place after 6, 300 YBP, forming the U-1 scoria falls, Al-bong cinder cone and lava flows.

Formation of the Green Tuff, Pantelleria

The strongly peralkaline Green Tuff, Pantelleria, is an example of a thin, densely welded air-fall tuff which mantles an area of at least 85 km2. Offshore the tuff is correlated with the Y-6 ash layer in the central Mediterranean Sea, and the total volume of the eruption is estimated at 7 km3 D.R.E. New petrological data suggests that the tuff was erupted from a zoned magma chamber containing a cooler, more fractionated upper zone relative to be bulk of the magma. Analysis of the distribution of accessory lithic fragments in terms of existing models of eruption dynamics indicates emplacement by a plinian-type eruption. It is shown that, due to the low viscosity of pantelleritic ejecta, dense welding can occur at moderate tephra accumulation rates and a rate of the order of 1 cm/minute is suggested for the Green Tuff; this yields an estimate for the eruption duration of rather less than one day. It is predicted that welded tuff should be formed during large plinian eruptions of pantelleritic magma, and therefore that welded airfall tuffs should be common in areas of peralkaline volcanism.

Motutere Tephra Formation and redefinition of Hinemaiaia Tephra formation, Taupo Volcanic Centre, New Zealand

Abstract Motutere and Hinemaiaia Tephra Formations are moderately widespread rhyolitic tephras erupted from near the southeast margin of Lake Taupo. The 2 multiple-bedded formations constitute a prolonged eruptive sequence interrupted by a short hiatus. The eruptive sequence comprises a plinian-type eruption of coarse pumice lapilli (Motutere Tephra (new name), volume c. 1 km3), and further eruptions of a more widespread vitric ash (Hinemaiaia Tephra, volume c. 3 km3) apparently related to the growth of a rhyolite dome. Isopach data for both tephras place the eruptive vent near the southeast margin of Lake Taupo in the vicinity of Te Rangiita. Bathymetry and geophysical data suggest the presence of a rhyolite dome 2 km offshore and within the vent area.Two unpublished radiocarbon dates of 5370 ± 90 years B.P. (NZ4846A) for Motutere Tephra and 4650 ± 80 years B.P. (NZ4574A) (old T½) for Hinemaiaia Tephra establish the age of these tephras.