Pyroclastic Surge Research Articles

Geological aspects of the 2003–2004 eruption of Anatahan Volcano, Northern Mariana Islands

Anatahan Volcano, Northern Mariana Islands, began erupting in May–June 2003. A series of subplinian explosive eruptions of andesite magma began at the Eastern Crater in the eastern part of the summit caldera on the evening of 10 May. Brown tephra was sent mainly westward by strong winds. Small-scale pyroclastic surges were discharged eastward outside the caldera in late May. An andesite lava dome that had once filled the inner crater was fragmented by phreatomagmatic explosions in the middle of June. The phreatomagmatic explosions probably occurred due to interaction of the magma head with groundwater around the crater, and abundant very fine ash (“gray tephra”) was discharged within the caldera and over most of the island. The volume of eruption products of the May–June eruption was estimated to be 1.4 × 10 7 m 3 dense-rock-equivalent. Erupted pumices and lava are aphyric andesite and are variously colored depending on their vesicularity. The SiO 2 contents of erupted materials decreased slightly with time. The fine gray ash is depleted in alkalies, probably due to leaching by acid hydrothermal fluids during explosions. Seismic activity resumed in late March 2004, and small strombolian-like explosions were repeated in May and June 2004. About half of the inner crater was filled with new scoria and lava.

Extrusive carbonatites: A brief review

49 known extrusive carbonatite occurrences are listed with brief details of their tectonic setting, structure, lithologies, associated silicate rocks, chemistry and presence or absence of included mantle materials. Half the occurrences appear to be related to tephra cones, tuff rings, diatremes and maars and the rest occur within strato-volcanoes. Pyroclastic carbonatitic rocks are present at all the localities, with carbonatite lava flows occurring at only 14 of them. The pyroclastic rocks, which include fallout tephra and deposits from pyroclastic surges and flows and products of phreatomagmatic eruptions, vary from rocks composed principally of carbonate to varieties with as little as 20% igneous carbonate. The most abundant silicate rocks associated with extrusive carbonatites are melilite-bearing rocks, nephelinite and/or ijolite, and phonolite and/or nepheline syenite; seven occurrences have no associated silicate rocks. 16 occurrences, most of them associated with small extrusive centres, contain mantle xenoliths or megacrysts, details of which are tabulated, with spinel lherzolite the most abundant rock type, but amphibole, phlogopite and garnet are also recorded. The lack of such materials in intrusive carbonatites may reflect their less energetic environment of emplacement. It is proposed that carbonatites are essentially of two types: (a) those rising energetically and rapidly from the mantle, which form small explosion craters, ash or tuff cones, or diatremes, have only low-volume associated silicate rocks, and entrain mantle debris, and (b) those which occur in strato-volcanoes, are associated with large volumes of silicate rocks and follow a more complex genesis, probably involving ponding and differentiation (separation from carbonate-bearing silicate magma) at higher levels in the mantle and/or crust. Most of the classic intrusive carbonatite complexes probably fall into the second category.

Chronology and products of the 2000 eruption of Miyakejima Volcano, Japan

Lateral migration of magma away from Miyakejima volcanic island, Japan, generated summit subsidence, associated with summit explosions in the summer of 2000. An earthquake swarm beneath Miyakejima began on the evening of 26 June 2000, followed by a submarine eruption the next morning. Strong seismic activity continued under the sea from beneath the coast of Miyakejima to a few tens of kilometers northwest of the island. Summit eruptive event began with subsidence of the summit on 8 July and both explosions and subsidence continued intermittently through July and August. The most intense eruptive event occurred on 18 August and was vulcanian to subplinian in type. Ash lofted into the stratosphere fell over the entire island, and abundant volcanic bombs were erupted at this time. Another large explosion took place on 29 August. This generated a low-temperature pyroclastic surge, which covered a residential area on the northern coast of the island. The total volume of tephra erupted was 9.3×106 m3 (DRE), much smaller than the volume of the resulting caldera (6×108 m3). Migration of magma away from Miyakejima was associated with crustal extension northwest of Miyakejima and coincident shrinkage of Miyakejima Island itself during July–August 2000. This magma migration probably caused stoping of roof rock into the magma reservoir, generating subsurface cavities filled with hydrothermal fluid and/or magmatic foam and formation of a caldera (Oyama Caldera) at the summit. Interaction of hydrothermal fluid with ascending magma drove a series of phreatic to phreatomagmatic eruptions. It is likely that new magma was supplied to the reservoir from the bottom during waning stage of magma’s migration, resulting in explosive discharge on 18 August. The 18 August event and phreatic explosions on 29 August produced a conduit system that allowed abundant SO2 emission (as high as 460 kg s−1) after the major eruptive events were over. At the time of writing, inhabitants of the island (about 3,000) have been evacuated from Miyakejima for more than 3 years.

The impacts of pyroclastic surges on buildings at the eruption of the Soufri�re Hills volcano, Montserrat

We investigated the impacts on buildings of three pyroclastic surges that struck three separate villages on 25 June, 21 September and 26 December, 1997, during the course of the andesitic dome building eruption of the Soufriere Hills Volcano, Montserrat, which began on 18 July, 1995. A detailed analysis of the building damage of the 26 December event was used to compare the findings on the flow and behaviour of dilute pyroclastic density currents (PDCs) with the classical reports of PDCs from historical eruptions of similar size. The main characteristics of the PDC, as inferred from the building damage, were the lateral loading and directionality of the current; the impacts corresponded to the dynamic pressure of the PDC, with a relatively slow rate of rise and without the peak overpressure or a shock front associated with explosive blast; and the entrainment of missiles and ground materials which greatly added to the destructiveness of the PDC. The high temperature of the ash, causing the rapid ignition of furniture and other combustibles, was a major cause of damage even where the dynamic pressure was low at the periphery of the current. The vulnerability of buildings lay in the openings, mainly windows, which allowed the current to enter the building envelope, and in the flammable contents, as well as the lack of resistance to the intense heat and dynamic pressure of some types of vernacular building construction, such as wooden chattel houses, rubble masonry walls and galvanised steel-sheet roofs. Marked variability in the level of damage due to dynamic pressure (in a range 1–5 kPa, or more) was evident throughout most of the impact area, except for the zone of total loss, and this was attributable to the effects of topography and sheltering, and projectiles, and probably localised variations in current velocity and density. A marked velocity gradient existed from the outer part to the central axis of the PDC, where buildings and vegetation were razed to the ground. The gradient correlated with the impacts due to lateral loading and heat transfer, as well as the size of the projectiles, whilst the temperature of the ash in the undiluted PDC was probably uniform across the impact area. The main hazard characteristics of the PDCs were very consistent with those described by other authors in the classic eruptions of Pelee (1902), Lamington (1951) and St Helens (1980), despite differences in the eruptive styles and scales. We devised for the first time a building damage scale for dynamic pressure which can be used in research and in future volcanic emergencies for modelling PDCs and making informed judgements on their potential impacts.

Long‐runout pyroclastic surge on a Cretaceous alluvial plain, Republic of Korea

AbstractPyroclastic surge is a dilute and turbulent flow of volcanic gas and tephra that is commonly generated during explosive volcanic eruptions and can threaten lives along its flow paths. Assessing its travel distance and delineating future volcanic hazards have therefore been major concerns of volcanologists. Historical eruptions show that most pyroclastic surges travel a few tens of kilometres or less from their sources. Aeolian or aquagene processes have therefore been evoked for the emplacement of supposed surge deposits much beyond this distance. Here we show that a Cretaceous tuff bed in Korea was emplaced by an exceptionally powerful pyroclastic surge that flowed as far as the most powerful pyroclastic flows that formed the low‐aspect‐ratio ignimbrites (LARI). This has significant implications for interpreting ancient volcanic eruptions and delineating volcanic hazards by pyroclastic surges, and casts intriguing questions on the eruption dynamics and physics of long‐runout pyroclastic surges and their distinction from LARI‐forming pyroclastic flows.

Inland-directed base surge generated by the explosive interaction of pyroclastic flows and seawater at Soufrière Hills volcano, Montserrat

The largest and most intense lava-dome collapse during the eruption of Soufriere Hills volcano, Montserrat, 1995–2004, occurred 12–13 July 2003. The dome collapse involved around 200 x 106 m3 of material and was associated with a phenomenon previously unknown at this volcano. Large pyroclastic flows at the peak of the dome collapse interacted explosively with seawater at the mouth of the Tar River Valley and generated a hot, dry base surge that flowed 4 km inland and 300 m uphill. The surge was destructive to at least 25 m above the ground and it carbonized vegetation. The resulting two-layer deposits were as much as 0.9 m thick. Although the entire collapse lasted 18 h, the base surge greatly increased the land area affected by the dome collapse in a few minutes at the peak of the event, illustrating the complex nature of the interaction between pyroclastic flows and seawater.

噴出物から推定した有珠山１６６３年噴火の推移

The AD1663 ejecta of Usu volcano, western Hokkaido, northern Japan, have a thickness of approximately 200, cm and are divisible into 7 units on the eastern foot of Usu volcano. The basal deposit, Unit-A, is composed of fine-grained, poorly sorted pumice and ash with well-developed stratification. It contains base surge beds near the sources, and is interpreted as phreatomagmatic in origin. Unit-B is typical in many respects of the product of plinian activity. It is characterized by coarse grain size and wide dispersal, combined with low lithic contents and poor development of internal stratification. This fallout deposit (Usu-b tephra) can be seen in the eastern regions of Usu volcano, and has a thickness of about 4 cm at 200 km from the vent. Unit-C, Unit-E, Unit-F, and Unit-G have characteristics that are satisfactorily interpreted as a consequence of phreatomagmatic activity : fine grain size near source, poor sorting, well-developed stratification and base surge bedforms. Unit-D is typical of phreatic deposits, being composed largely of non-juvenile lithic fragments.According to historical records, the eruption was preceeded by earthquake swarms for 3 days from August 13, 1663. The first eruption (Unit-A) occurred on August 16, followed by an eruption associated with a large amount of pumice and ash (Unit-B) on August 17. The explosive eruptions lasted approximately 20 days.The petrographic properties of the plinian product differ considerably from those of the phreatomagmatic products. The pumice clasts of Unit-B contain abundant glass and little phenocryst (plagioclase, orthopyroxene, and hornblende). The glass has a relatively high refractive index (n = 1.490-1.492) and a low SiO2 content. Orthopyroxene in Unit-B has a relatively high refractive index (γ = 1.735-1.740) and low Mg#. The phreatomagmatic products are rich in massive glass. This glass has a relatively low refractive index (n = 1.475-1.490) and a high SiO2 content, with values differing among horizons. The compositional diversity of the 1663 products indicates the existence of some magma. Presumably, their mixing triggered the 1663 eruption of Usu volcano.

Determination of burial age of the "Augustus' villa" (Italy)

Based on the combination of petrologic characteristics of the deposits and carbon-14 dating of charcoal pieces, we determined the burial age of the ancient Roman villa, believed to belong to the first Roman Emperor Augustus, at the northern foot of Mt. Vesuvius. Volcanic deposits covering the site consist of three geological formations related to the eruptive activity of Mt. Vesuvius. Juvenile material of the lowest formation show the same whole-rock chemical composition as that of the AD472 eruption, and carbon-14 ages of charcoal in and below the deposits coincide with AD472, indicating that the villa was first buried by the AD472 eruption. The villa itself is thought to have been already abandoned, judging from the depositional relation of the Plinian-fall and pyroclastic surge deposits to the damaged building frames.

Addressing complexity in laboratory experiments: the scaling of dilute multiphase flows in magmatic systems

The kinematic and dynamic scaling of dilute multiphase mixtures in magmatic systems is the only guarantee for the geological verisimilitude of laboratory experiments. We present scaling relations that can provide a more complete framework to scale dilute magmatic systems because they explicitly take into account the complexity caused by the feedback between particles (crystal, bubble, or pyroclast) and the continuous phase (liquid or gas). We consider three canonical igneous systems: magma chambers, volcanic plumes, and pyroclastic surges, and we provide estimates of the proposed scaling relations for published experiments on those systems. Dilute magmatic mixtures can display a range of distinct dynamical regimes that we characterize with a combination of average (Eulerian) properties and instantaneous (Lagrangian) variables. The Eulerian properties of the mixtures yield the Reynolds number ( Re), which indicates the level of unsteadiness in the continuous phase. The Lagrangian acceleration of particles is a function of the viscous drag and gravity forces, and from these two forces are derived the Stokes number ( S T ) and the stability number ( Σ T ), two dimensionless numbers that describe the dynamic behavior of the particles within the mixture. The compilation of 17 experimental studies relevant for pyroclastic surges and volcanic plumes indicates that there is a need for experiments above the mixing transition ( Re>10 4) and for scaling S T and Σ T . Among the particle dynamic regimes present in surges and plumes, some deserve special attention, such as the role of mesoscale structures on transport and sedimentary processes or the consequences of the transition to turbulence on particle gathering and dispersal. The compilation of seven experimental studies relevant to magma bodies indicates that, in the laminar regime, crystals mostly follow the motion of the melt, and thus the physical state of the system can be approximated as single phase. In the transition to turbulence, magmas can feature spatially heterogeneous distributions of laminar regions and important velocity gradients. This heterogeneity has a strong potential for crystals sorting. In conclusion, the Re–S T –Σ T framework demonstrates that, despite numerous experimental studies on processes relevant to magmatic systems, some and perhaps most geologically important parameter ranges still need to be addressed at the laboratory scale.

Transport and deposition of pyroclastic material from the ∼1000 A.D. caldera-forming eruption of Volcán Ceboruco, Nayarit, Mexico

The complex eruption sequence from the ∼1000 A.D. caldera-forming eruption of Volcan Ceboruco, known as the Jala Pumice, offers an exceptional opportunity to examine how pyroclastic material is transported and deposited from pyroclastic density currents over variable topography. Three main pyroclastic surge deposits (S1, S2, and S3) and two pyroclastic flow deposits (Marquesado and North-Flank PFDs) were emplaced during this eruption. Pyroclastic surge deposits are massive, planar, or cross-bedded, poor-to-well sorted, and display fluctuations in thickness, median diameter, sorting, and lithology as a function of distance, topography, and flow dynamics. Marquesado pyroclastic flow deposits reveal lateral variations from massive, poorly sorted deposits located within 5 km of Ceboruco to planar bedded, moderately well sorted deposits located >15 km away over the nearly horizontal topography to the south of Ceboruco. North-Flank pyroclastic flow deposits also reveal lateral variations from massive, poorly sorted deposits located within 4 km of Ceboruco to planar bedded, moderately well sorted deposits located 8 km away atop an escarpment that steeply rises 230 m from the northern valley floor. Field observations, granulometric analyses, component analyses, and crystal sedimentation calculations along flow-parallel sampling transects all suggest that both surges and flows were density stratified currents, where deposition occurred from a basal region of higher particle concentration that was supplied from an overlying dilute layer that transports particles in suspension. This supports the idea of a transition between “flow” and “surge” end members with variations in particle concentration. Topography greatly affects the transport and depositional capacity of the pyroclastic density currents as a result of “blocking”, either by topographic obstacles or by abrupt breaks at the base of volcano slopes, whereas the origin of Jala Pumice surge deposits (phreatomagmatic versus magmatic) appears to have little impact on their flow dynamics.

Eruptive dynamics and caldera collapse during the Onano eruption, Vulsini, Italy

The Onano explosive eruption of the Latera Volcanic Complex (Vulsini Volcanoes, Quaternary potassic Roman Comagmatic Region, Italy) provides an interesting example of multiple changes of eruptive style that were concomitant with a late phase of collapse of the polygenetic Latera Caldera. This paper reports a reconstruction of the event based on field analysis, laboratory studies of grain size and density of juvenile clasts, and re-interpretation of available subsurface geology data. The Onano eruption took place in a structurally weak area, corresponding to a carbonate substrate high bordered by the pre-existing Latera caldera and Bolsena volcano-tectonic depression, which controlled the ascent and eruption of a shoshonitic-phonotephritic magma through intersecting rim fault systems. Temporal changes of magma vesiculation, fragmentation and discharge rate, and consequent eruptive dynamics, were strongly controlled by pressure evolution in the magma chamber and changing vent geometry. Initially, pumice-rich pyroclastic flows were emplaced, followed by spatter- and lithic-rich flows and fallout from energetic fire-fountaining. The decline of magma pressure due to the partial evacuation of the magma chamber induced trapdoor collapse of the chamber roof, which involved part of the pre-existing caldera and external volcano slopes and eventually led to the present-day caldera. The widening of the vent system and the emplacement of the main pyroclastic flow and associated co-ignimbrite lag breccia marked the eruption climax. A sudden drop of the confining pressure, which is attributed to a pseudo-rigid behaviour of the magma chamber wall rocks during a phase of rapid magma drainage, led to extensive magma vesiculation and fragmentation. The disruption of the magma chamber roof and waning magma pressure in the late eruption stage favoured the explosive interaction of residual magma with groundwater from the confined carbonate aquifer. Pulsating hydrostatic and magma pressures produced alternating hydromagmatic pyroclastic surges, strombolian fallout and spatter flows.

Modeling of pyroclastic flows of Colima Volcano, Mexico: implications for hazard assessment

The 18–24 January 1913 eruption of Colima Volcano consisted of three eruptive phases that produced a complex sequence of tephra fall, pyroclastic surges and pyroclastic flows, with a total volume of 1.1 km 3 (0.31 km 3 DRE). Among these events, the pyroclastic flows are most interesting because their generation mechanisms changed with time. They started with gravitanional dome collapse (block-and-ash flow deposits, Merapi-type), changed to dome collapse triggered by a Vulcanian explosion (block-and-ash flow deposits, Soufrière-type), then ended with the partial collapse of a Plinian column (ash-flow deposits rich in pumice or scoria,). The best exposures of these deposits occur in the southern gullies of the volcano where Heim Coefficients (H/L) were obtained for the various types of flows. Average H/L values of these deposits varied from 0.40 for the Merapi-type (similar to the block-and-ash flow deposits produced during the 1991 and 1994 eruptions), 0.26 for the Soufrière-type events, and 0.17–0.26 for the column collapse ash flows. Additionally, the information of 1991, 1994 and 1998–1999 pyroclastic flow events was used to delimit hazard zones. In order to reconstruct the paths, velocities, and extents of the 20th Century pyroclastic flows, a series of computer simulations were conducted using the program FLOW3D with appropriate Heim coefficients and apparent viscosities. The model results provide a basis for estimating the areas and levels of hazard that could be associated with the next probable worst-case scenario eruption of the volcano. Three areas were traced according to the degree of hazard and pyroclastic flow type recurrence through time. Zone 1 has the largest probability to be reached by short runout (<5 km) Merapi and Soufrière pyroclastic flows, that have occurred every 3 years during the last decade. Zone 2 might be affected by Soufriere-type pyroclastic flows (∼9 km long) similar to those produced during phase II of the 1913 eruption. Zone 3 will only be affected by pyroclastic flows (∼15 km long) formed by the collapse of a Plinian eruptive column, like that of the 1913 climactic eruption. Today, an eruption of the same magnitude as that of 1913 would affect about 15,000 inhabitants of small villages, ranches and towns located within 15 km south of the volcano. Such towns include Yerbabuena, and Becerrera in the State of Colima, and Tonila, San Marcos, Cofradia, and Juan Barragán in the State of Jalisco.

Numerical study of pyroclastic surges

The dynamics of pyroclastic surges accompanied by co-ignimbrite plumes is investigated numerically. The numerical simulations are performed with a newly developed numerical model, which is based on the Navier–Stokes equations for time-dependent flows of a compressible fluid in two-dimensional Cartesian coordinates. We regard pyroclastic surges as dilute turbulent suspensions in which hot gases and fine solid particles are homogeneously mixed owing to vigorous turbulence. In other words, the gas–particle mixture is treated as a single-phase fluid whose bulk density is represented by averaging the density of each component in the numerical model. We focus on the effect of buoyancy forces generated by the thermal expansion of the air mixed into pyroclastic surges from the calm surroundings. For our purpose, the numerical model is designed to simulate relatively simple flows spreading over a horizontal flat surface. Topographic irregularity and the sedimentation process of solid particles are neglected in the present simulations. The motion of pyroclastic surges is generated by the instantaneous release of a gas–particle mixture whose density is initially larger than the ambient air density and changes nonlinearly with the temperature and concentration of suspended solid particles. Turbulent mixing is evaluated by adopting the Smagorinsky model. By employing cubic interpolated pseudo-particle (CIP) method and C-CUP method, we obtain the fine structure of flows. The behavior of calculated flows agrees fairly well with observed pyroclastic surges in nature. The current head, which remains hot and dense, keeps spreading over a horizontal surface at a speed of about 20 m s −1. The spreading speed is of the order of the speed of a gravity current that excludes the influence of thermal expansion. Besides, turbulent mixing between the basal dense layer and the ambient air is enhanced by the successive development of an interfacial less-dense layer. This results in the formation of a number of buoyant plumes rising above a horizontally spreading current. Consequently, the tails of the current thickens as time progresses. A parametric study shows that the initial temperature of a gas–particle mixture should be higher than about 600 K when buoyant plumes occur owing to the thermal expansion of mixed air. The result is quantitatively interpreted by introducing a diagram that describes the relationship among the bulk density, temperature and concentration of solid particles suspended in pyroclastic surges.

Stratigraphy, age and correlation of middle Pleistocene silicic tephras in the Auckland region, New Zealand: A prolific distal record of Taupo Volcanic Zone volcanism

Coastal sections in the Auckland region reveal highly carbonaceous and/or highly weathered clay‐dominated cover‐bed successions with numerous discrete distal volcanic ash (tephra) layers, fluvially reworked siliciclastic (tephric) deposits, and two widely distributed pyroclastic density current (PDC) deposits generated from explosive silicic volcanism within the Taupo Volcanic Zone (TVZ). The younger of the two PDC deposits (informally named Waiuku tephra) is glass‐isothermal plateau fission‐track (ITPFT) dated at 1.00 ± 0.03 Ma and occurs in a normal polarity interval interpreted as the Jaramillo Subchron. Waiuku tephra is correlated with Unit E sourced from the Mangakino Volcanic Centre of the TVZ. Waiuku tephra can be subdivided into two distinctive units enabling unequivocal field correlation: a lower stratified unit (dominantly pyroclastic surge with fall component) and an upper massive to weakly stratified unit (pyroclastic flow). At many sites in south Auckland, Waiuku tephra retains basal “surge‐like” beds (<1.4 m thickness). This provides clear evidence for primary emplacement and is an exceptional feature considering the c. 200 km this PDC has travelled from its TVZ source area. However, at many other Auckland sites, Waiuku tephra displays transitional sedimentary characteristics indicating lateral transformation from hot, gas‐supported flow/surge into water‐supported mass flow and hyperconcentrated flow (HCF) deposits. The older PDC deposit is dated at 1.21 ± 0.09 Ma, is enveloped by tephras that are ITPFT‐dated at 1.14 ± 0.06 Ma (above) and 1.21 ± 0.06 Ma (below), respectively, and occurs below a short normal polarity interval (Cobb Mountain Subchron) at c. 1.19 Ma. This PDC deposit, correlated with Ongatiti Ignimbrite sourced from the Mangakino Volcanic Centre of TVZ, has laterally transformed from a gas‐supported, fine‐grained pyroclastic flow deposit at Oruarangi, Port Waikato, into a water‐supported volcaniclastic mass flow deposit farther north at Glenbrook Beach. The occurrence of Ongatiti Ignimbrite in Auckland significantly extends its northward distribution. Large numbers of post‐ and pre‐Ongatiti rhyolitic tephra layers, ranging in age from c. 1.31 to 0.53 Ma, are also recognised in the region, with some up to 0.5 m in compacted fallout thickness. Although some tephras can be attributed to known TVZ eruptions (e.g., Ahuroa/Unit D), many have yet to be identified in proximal source areas and remain uncorrelated. However, some can be reliably correlated to tephra layers occurring in marine to nearshore sequences of Wanganui Basin and deep‐sea cores retrieved east of North Island. The identification of previously unrecognised mid‐Pleistocene TVZ‐sourced tephra deposits in the Auckland region, and their correlation to the offshore marine record, represent an advance in the construction of a higher resolution history for the TVZ where, close to eruptive source, the record is fragmentary and obscured by deep burial, or erosion, or both.

Volcanic risk ranking for Auckland, New Zealand. II: Hazard consequences and risk calculation

In a companion paper, a methodology for ranking volcanic hazards and events in terms of risk was presented, and the likelihood and extent of potential hazards in the Auckland Region, New Zealand investigated. In this paper, the effects of each hazard are considered and the risk ranking completed. Values for effect are proportions of total loss and, as with likelihood and extent, are based on order of magnitude. Two outcomes were considered – building damage and loss of human life. In terms of building damage, tephra produces the highest risk by an order of magnitude, followed by lava flows and base surge. For loss of human life, risk from base surge is highest. The risks from pyroclastic flows and tsunami are an order of magnitude smaller. When combined, tephra fall followed by base surge produces the highest risk. The risks from lava flows and pyroclastic flows are an order of magnitude smaller. For building damage, the risk from Mt. Taranaki volcano, 280 km from the Auckland CBD, is largest, followed by Okataina volcanic centre, an Auckland volcanic field eruption centred on land, then Tongariro volcanic centre. In terms of human loss, the greatest risk is from an Auckland eruption centred on land. The risks from an Auckland eruption centred in the ocean, Okataina volcanic centre, and Taupo volcano are more than an order of magnitude smaller. When combined, the risk from Mt. Taranaki remains highest, followed by an Auckland eruption centred on land. The next largest risks are from the Okataina and Tongariro volcanic centres, followed by Taupo volcano. Three alternative situations were investigated. As multiple eruptions may occur from the Auckland volcanic field, it was assumed that a local event would involve two eruptions. This increased risk of a local eruption occurring on land so that it was equal to that of an eruption from Mt. Taranaki. It is possible that a future eruption may be of a similar, or larger size, to the previous Rangitoto eruption. Risk was re-calculated for local eruptions based on the extent of hazards from Rangitoto. This increased the risk of lava flow to greater than that of base surge, and the risk from an Auckland land eruption became greatest. The relative probabilities used for Mt. Taranaki volcano and the Auckland volcanic field may only be minimum values. When the probability of these occurring was increased by 50%, there was no change in either ranking.

Temporal variation of eruption rate during the 1978 activity of Usu Volcano, northern Japan, revealed by the eruptive deposits and volcanic tremor

We estimate the temporal variation of mass eruption rates during a single eruptive event occurring during the 1978 activity of Usu Volcano, northern Japan. The 1978 Usu eruption had several hundred phreatic to phreatomagmatic events. Eruptive activity started without explosion-type earthquakes and gradually increased in intensity. Events were several minutes to hours in duration. Major phreatomagmatic events can be divided into waxing and waning phases. Tephra beds corresponding to each event are characterized by alternating layers of gravelly diamicts and small, proximal pyroclastic surge deposits. We determined the mass of beds of each event in the field and compared bed mass with the temporal variation of tremor amplitude for each event as determined using a single three-component short period seismometer located 2.4 km from the vent. The number of diamict layers is consistent with the number of waxing-waning phases of tremor amplitude. Large tremor amplitudes correlate with the most voluminous diamict layers. Mass-eruption rates are calculated from the mass of each layer and the identified duration estimated from analysis of eruption tremor. We find that the eruption rate within a phase varies from one to three orders of magnitude during a phreatomagmatic event.

Interaction between particles transported by fallout and surge in the deposits of the Agnano–Monte Spina eruption (Campi Flegrei, Southern Italy)

The Agnano–Monte Spina Tephra (AMST) is a complex sequence of beds generated by contrasting fragmentation and transportation dynamics. The 4.1 ka eruption was accompanied by a volcano-tectonic collapse, part of which is the present Agnano plain. The pyroclastic sequence is subdivided into members and submembers, each characterized by different lithological and sedimentological features. Plinian/subplinian fallout deposits frequently alternate with base-surge beds of phreatomagmatic origin. Analysis of lateral facies variations and vertical facies associations of correlated layers of submembers B2, D2 and E2 reveals that during some eruption phases the contrasting eruptive dynamics were almost contemporaneous. Base-surge deposits of submember B2 formed during the declining stage of a plinian column. They resulted from highly energetic and steady pyroclastic density currents that traveled long distances from the vent area and surmounted topographic obstacles such as the Posillipo hill (∼150 m) and the northern sector of the Camaldoli hill (∼250 m). Submember D2 pyroclastic density currents formed when contemporaneous fallout from an eruptive column was present. Coarse particles from the column settled throughout the pyroclastic density current, determining a significant increase of the solid load of the base surge. The consequent increase of supply rate from the transportation to the depositional zone of the base surge led to the formation of unsteady flows that could not efficiently transport the solid load and did not have the ability to travel long distances. Base-surge deposits of submember E2 were fed by a pulsating phreatomagmatic activity, which was punctuated by a short-lived fallout phase. Fallout material, which was incorporated as an additional load to the base surges, was partially transported by low-energy, steady pyroclastic density currents that traveled over the Agnano plain but did not surmount either the Camaldoli or the Posillipo hills. Only the very fine material, in continuous suspension in the upper, no-shearing part of the base-surge cloud, was dispersed higher in the atmosphere and quietly settled over a large area outside the caldera rim. The phreatomagmatic origin of base surges, which contrasts with the magmatic origin of fallout activity, suggests that the pyroclastic density currents of the Agnano–Monte Spina eruption did not result from eruption column collapses. They were most likely related to radially spreading clouds which were contemporaneous with fallout activity but issued from distinct zones in the vent area. The turbulent nature and the high expansion of base surges made them capable, under certain conditions, of passing over high topographic obstacles, with hazardous effects in distal areas.

Turbulent boundary layer shear flows as an approximation of base surges at Campi Flegrei (Southern Italy)

Small-volume base-surge deposits of Astroni and Agnano–Monte Spina eruptions at Campi Flegrei consist of sets of beds formed by the passage of multiple pyroclastic density currents. Each set is made by a fining-upward sequence consisting, from base to top, of: (1) an inversely graded layer made of bombs and lapilli; (2) a finely laminated layer with wavy bedforms made of coarse ash and fine lapilli; (3) a structureless fine-ash bed. Lack of discontinuities at layer contacts allows us to hypothesize that each set of beds represents the passage of one pyroclastic density current. The sequence formed as a function of the decreasing particle transportation capability of the base surge during the waxing–sustained–waning phases of flowage. We make the simplifying assumptions that the flow, on passing from the deposition of the inversely graded layer to the deposition of the finely laminated layer, had low particle concentration, was quasi-steady and incompressible. By such assumptions, some of the equations that regulate sediment mechanics are applicable to the base surges of Campi Flegrei to reconstruct flow density and shear velocity in between the deposition of the two layers. Structural and textural features of deposits stress the importance of shear and turbulence during transportation and suggest that the turbulent boundary layer shear flow is a good approximation to such pyroclastic density currents. We used the Coles formula to reconstruct velocity as a function of flow height during the formation of the finely laminated layer of Astroni and Agnano–Monte Spina base surges. The methodology we used allows the reconstruction of flow parameters just at a particular point in time and space of base-surge evolution, and does not provide the complete picture of the flow history of pyroclastic density currents. Nevertheless, flow velocity and density data, as obtained with the proposed approach, point to dynamic pressure values that are compatible with those reported for similar currents on other volcanoes. We conclude therefore that the results obtained give approximate but helpful information on the nature of such hazardous events at Campi Flegrei.

3D Simulations of a Pyroclastic Surge as an Example of a Compressible Suspension Flow

This paper demonstrates the usefulness of the CIP-combined unified procedure (C-CUP) to perform numerical simulations of a compressible flow of a suspension. As a conspicuous example of such a flow, we present three-dimensional numerical simulations of a pyroclastic surge, which spreads laterally over the ground surface during some volcanic eruptions as occurred at Unzen Volcano in 1991-1995. Since a pyroclastic surge suspends numerous solid particles, the density of a pyroclastic surge cannot be approximated by the equation of state for an ideal gas. On the other hand, a pyroclastic surge greatly changes its density as it heats and expands the air mixed from the calm surroundings. The above characteristics bring unique features into the motion of a pyroclastic surge. The features are successfully simulated by C-CUP with a slight modification owing to the fact that C-CUP can incorporate the equation of state of an arbitrary form.

Flow Transformations in Particulate Gravity Currents

The nature, thickness, and location of deposits from a particulate gravity current is strongly influenced by whether the flow is concentrated or dilute, whether it is laminar or turbulent, and whether it is supercritical or subcritical. These transitions are causally linked, because there is a clear contrast between a concentrated, laminar flow-type (e.g., pyroclastic flows and debris flows) and a dilute, turbulent flow-type (e.g., pyroclastic surges and turbidity currents). In this paper it is shown that the primary transition is from a dense to a less dense current, leading, in turn, to a transition from laminar to turbulent flow. This density transition can be caused by interface instability or by vigorous entrainment of ambient fluid at a hydraulic jump. It is also shown in this paper that hydraulic jumps occur at Froude numbers significantly different from unity. These concepts are confirmed by previously published data from a gravity current in the San Dimas Reservoir.