Multiple origins of obsidian pyroclasts and implications for changes in the dynamics of the 1300 B.P. eruption of Newberry Volcano,USA

The pyroclastic deposits of the 1300 B.P. eruption of Newberry Volcano, OR, USA, contain minor amounts of obsidian (1–6 wt.%). The volatile (H₂O and CO₂) contents and textures of these clasts vary considerably. FTIR measurements of H₂O in obsidian pyroclasts range from 0.1 to 1.5 wt.% indicating equilibration pressures ≤20 MPa. CO₂ contents are low (<10 ppm) except in clasts that also contain xenolith powder that provided a local CO₂ source. Obsidian clasts exhibit a range of color and textural types that changed in relative proportion as the eruption progressed. Together these data indicate that there were multiple origins of obsidian and that the dominant source changed during the eruption. Early in the eruption, obsidian was almost entirely black or grey (microlite-bearing) and probably derived from dikes or wall rock fractures filled with vanguard magma or tuffisite that, together with wall rocks, were eroded and incorporated into the eruption column as the vent widened. Later in the eruption, following a brief cessation of activity, the proportion of obsidian to wallrock lithic clasts increased and new types of obsidian dominated, types that represent remnants of a shallow conduit plug, welded fallback material from within the conduit, and sheared and degassed magma from near the conduit walls. Analysis of bubble shapes preserved within obsidian indicates that shear stresses and shear rates varied by over two orders of magnitude, with maxima of 88 kPa and 10^−2.3 s⁻¹, respectively, based on an assumed magma temperature of 850°C. Furthermore, the highest shear rates and stresses, and the shortest flow times (10–20 min), are preserved in clasts that also contain wall rock. The longest deformation times (5 and 8 h) correspond to two microlite-rich clasts, suggesting that the higher microlite content results from slower ascent rates and/or longer magma residence times at shallow levels. Differences between obsidian pyroclasts from the Newberry eruption and those of the Mono Craters may relate to the nature of the conduit feeding the two events. From this comparison, we conclude that obsidian can provide information on time scales and mechanisms of pre-fragmentation magma ascent.     

The impacts of pyroclastic surges on buildings at the eruption of the Soufrière 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 Soufrière 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 Pelée (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. Editorial responsibility: T. Druitt     

NUMERICAL STUDY OF THE INFLUENCE OF WAVES AND TIDE-SURGE INTERACTION ON TIDE-SURGES IN THE BOHAI SEA

Abstract The author‘s combined numerical model consisting of a third generation shallow water wave model and a 3-D tide-surge model with wave-dependent surface wind stress were used to study the influence of waves on fide-surge motion. For the typical weather case, in this study, the magnitude and mechanism of the influence of waves on tide-surges in the Bohai Sea were revealed for the first time. The results showed that although consideration of the wave-dependent surface wind stresses raise slightly the traditional surface wind stress, due to the accumulated effects, the computed results are improved on the whole. Storm level maximum modulation can reach 0.4 m. The results computed by the combined model agreed well with the measured data.     

Numerical Modelling of Storm Surge in the Head Bay of Bengal Using Location Specific Model

The head Bay of Bengal region, which covers part of Orissa and west Bengal in India as well as Bangladesh, is one of the most vulnerable regions of extreme sea levels associated with severe tropical cyclones which cause extensive damage. There has been extensive loss of life and property due to extreme events in this region. Shallow nature of the Bay, presence of Ganga-Brahmaputra-Meghna deltaic system and high tidal range are responsible for storm surges in this region. In view of this a location specific fine resolution numerical modelis developed for the simulation of storm surges. To represent mostof the islands and rivers in this region a 3km grid resolution is adopted. Several numerical experiments are carried out to compute the storm surges using the wind stress forcings representative of 1974, 1985, 1988, 1989, 1991, 1994 and 1999 cyclones, which crossed this region. The model computed surges are in good agreement with the available observations/estimates.     

On the use of NOAA's storm surge model,SLOSH, in managing coastal hazards — the experience in Puerto Rico

A numerical-dynamic, tropical storm surge model, SLOSH (Sea, Land, and Overland Surges from Hurricanes), was originally developed for real-time forecasting of hurricane storm surges on continental shelves, across inland water bodies and along coastlines and for inland routing of water -either from the sea or from inland water bodies. The model is two-dimensional, covering water bodies and inundated terrain. In the present version available at the University of Puerto Rico a curvilinear, polar coordinate grid scheme is used. The grid cells are approximately 3.2 × 3.2 km in size.The model has been used in a revision of all coastal Flood Insurance Rate Maps (FIRM) for Puerto Rico and the U.S. Virgin Islands, and in hurricane evacuation studies. The FIRM's, since they are based on the 100 year stillwater elevation, are also used by the state Planning Board for regulatory purposes. The hurricane evacuation studies are used by emergency planners and personnel to assign shelters, escape routes, and delimit coastal zones that need to be evacuated during a hurricane threat.Recently, the acquisition of data from hurricane Hugo has allowed the first comparison of model results and observations for Puerto Rico and the other islands.     

Climatological study of the main meteorological and marine disasters in Shanghai

In this paper, the main meteorological and meteorologically induced marine disasters in Shanghai are climatologically investigated on the basis of historical qualitative documentary records and quantitative data. Statistics show that disasters may be classified into six different kinds, of which three are fundamental, i.e. floods, winds, and storm surges. Emphasis is placed on the detailed analysis and discussion of floods, tropical cyclones, and Mei-yu which affect Shanghai and cause destructive calamities.Firstly, the classification of floods, their frequencies, and geographical distribution characteristics are described and the regulation of the climatic variation of floods is revealed. Secondly, the frequency, source regions, seasonal variations, tracks, and translatory speeds of tropical cyclones affecting Shanghai, and their induced strong winds, storm rainfalls, and storm surges are systematically studied. Many meaningful conclusions have been reached for providing the climatological background of Shanghai.The climatic change and its regulation of these main disasters in Shanghai have been basically clarified throughout this study.     

Stratigraphy of the Kos Plateau Tuff: product of a major Quaternary explosive rhyolitic eruption in the eastern Aegean, Greece

 The Kos Plateau Tuff (KPT) erupted during a moderate-volume explosive rhyolitic event approximately 161 ka from a source south of Kos in the eastern Aegean sea. Six major stratigraphic units have been identified, from A at the base, to F, uppermost. Unit A is a widespread vitric ash fall layer that is thickest (1.5 m), and most extensive, southeast of the source. Unit B is a 1- to 2-m-thick, low-angle cross-stratified armoured pumice lapilli and ash layer found on Kos. Unit C resembles unit B but includes a greater abundance of lithic lapilli, less fine ash, is only diffusely stratified and is on Kos and west of the source. Unit D includes a sequence of three non-welded, 1- to 20-m-thick ignimbrites that extend radially >38 km from the source in areas of low topography. Unit E is a sequence of two non-welded, 3- to 8-m-thick ignimbrites which occur radially from the vent regardless of topography, >64 km from source. Unit F has a 6-m-thick, basal, low-angle cross-stratified armoured pumice lapilli and ash part probably deposited radially from source. The upper part of unit F is a widespread >1-m-thick vitric ash fall layer, found to at least 50 km from the source. These six units represent a change in eruptive conditions from initial and final phreatomagmatic activity depositing fallout and internally stratified pyroclastic density current deposits to "dry" explosive during the more intense phases of the eruption which generated ignimbrites. Received: 8 June 1998 / Accepted: 14 January 1999     

Erosion by pyroclastic flows on Lascar Volcano, Chile

 Pyroclastic flows generated in the 19–20 April 1993 eruption of Lascar Volcano, Chile, produced spectacular erosion features. Scree and talus were stripped from the channels and steep slopes on the flanks of the volcano. Exposed bedrock and boulders suffered severe abrasion, producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders. Erosional furrows developed with wavelengths of 0.5–2 m and depths of 0.1–0.3 m. Furrows commonly nucleated downstream of large boulders or blocks, which are striated on the upstream side, and thereby produced crag-and-tail structures. Erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes. The pyroclastic flows are inferred to have segregated during movement into lithic-rich and pumice-rich parts. Lithic-rich deposits occur on slopes up to 14°, whereas pumice-rich deposits occur only on slopes less than 4°, and mainly at the margins and distal parts of the 1993 fan. The lithic-rich deposits contain large (up to 1 m) lithic clasts eroded from the substrate and transported from the vent, whereas pumice-rich deposits contain only small (typically <2 cm) lithic clasts. These observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena. The erosive features, distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions. In some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s. However, shorter striae at different angles, impact marks, segregation of the deposits into pumice- and lithic-rich parts, and mixing of bedrock-derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows. Received: 15 July 1996 / Accepted: 15 January 1997     

Source of Ash Zone 1 in the North Atlantic

Geochemical evidence shows that the silicic component of the widespread Ash Zone 1 in the North Atlantic is derived from a major ignimbrite-forming eruption which occurred at the Katla caldera in southern Iceland during the transition from glacial to interglacial conditions in Younger Dryas time. Both trace and major element evidence of the rhyolitic products excludes the Öræfajökull volcano as a source. The high-Ti basaltic component in the marine ash zone can also be attributed to contemporaneous eruption in the Katla volcanic complex. Dispersal of tephra from this event is primarily attributed to the generation of co-ignimbrite ash columns in the atmosphere, with ash fallout on both sea ice and on the ocean floor north and east of Iceland. Owing to the changing ocean circulation characteristics of the glacial regime, including suppression of the Irminger Current and a stronger North Atlantic Current, tephra was rafted on sea ice south into the central North Atlantic and deposited as dispersed Ash Zone 1. Sediments south of Iceland also show evidence of the formation of ash turbidites, generated either by the entrance of pyroclastic flows into the sea, or during discharge of jökulhlaups or glacier bursts from this subglacial eruption.     

Zoning for flowslide and debris flow in pyroclastic soils of Campania Region based on “infinite slope” analysis

Campania Region (Italy), one of the most densely populated areas in Europe, is probably the one with the highest risk of landslide. A large part of the region is covered by unsaturated cohesionless pyroclastic deposits subjected to rainfall-induced landslides. According to experience, these can display different features and magnitude. The most catastrophic landslides are liquefied debris flows which periodically occur on steep slopes, causing death and destruction in areas located downslope. Therefore, zoning of those areas which can be the source of liquefied debris flow is necessary. The paper reports some useful elements for zoning based on infinite slope analysis, accounting for the results of recent research on the mechanics of rainfall-induced landslides in pyroclastic soils.