Home About us Contact
|
Home About us Contact | ||
|
|
||
Volcano
Kinds of Volcano Selected AbstractsON THE GEOCHRONOLOGICAL METHOD VERSUS FLOW SIMULATION SOFTWARE APPLICATION FOR LAHAR RISK MAPPING: A CASE STUDY OF POPOCATÉPETL VOLCANO, MEXICOGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 3 2010ESPERANZA MUÑOZ-SALINAS ABSTRACT. Lahars are hazardous events that can cause serious damage to people who live close to volcanic areas; several were registered at different times in the last century, such as at Mt St Helens (USA) in 1980, Nevado del Ruiz (Colombia) in 1985 and Mt Pinatubo (Philippines) in 1990. Risk maps are currently used by decision-makers to help them plan to mitigate the hazard-risk of lahars. Risk maps are acquired based on a series of tenets that take into account the distribution and chronology of past lahar deposits, and basically two approaches have been used: (1) The use of Flow Simulation Software (FSS), which simulates flows along channels in a Digital Elevation Model and (2) The Geochronological Method (GM), in which the mapping is based on the evaluation of lahar magnitude and frequency. This study addresses the production of a lahar risk map using the two approaches (FSS and GM) for a study area located at Popocatépetl volcano , Central Mexico. Santiago Xalitzintla, a town located on the northern flank of Popocatépetl volcano, where volcanic activity in recent centuries has triggered numerous lahars that have endangered local inhabitants, has been used for the case study. Results from FSS did not provide satisfactory findings because they were not consistent with lahar sediment observations made during fieldwork. By contrast, the GM produced results consistent with these observations, and therefore we use them to assess the hazard and produce the risk map for the study area. [source] LIVING RATIONALLY UNDER THE VOLCANO?INTERNATIONAL ECONOMIC REVIEW, Issue 1 2007AN EMPIRICAL ANALYSIS OF HEAVY DRINKING AND SMOKING This study investigates whether models of forward-looking behavior explain the observed patterns of heavy drinking and smoking of men in late middle age in the Health and Retirement Study better than myopic models. We develop and estimate a sequence of nested models that differ by their degree of forward-looking behavior. Our empirical findings suggest that forward looking models fit the data better than myopic models. These models also dominate other behavioral models based on out-of-sample predictions using data of men aged 70 and over. Myopic models predict rates of smoking for old individuals, which are significantly larger than those found in the data on elderly men. [source] Ritter Island Volcano,lateral collapse and the tsunami of 1888GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2003Steven N. Ward SUMMARY In the early morning of 1888 March 13, roughly 5 km3 of Ritter Island Volcano fell violently into the sea northeast of New Guinea. This event, the largest lateral collapse of an island volcano to be recorded in historical time, flung devastating tsunami tens of metres high on to adjacent shores. Several hundred kilometres away, observers on New Guinea chronicled 3 min period waves up to 8 m high, that lasted for as long as 3 h. These accounts represent the best available first-hand information on tsunami generated by a major volcano lateral collapse. In this article, we simulate the Ritter Island landslide as constrained by a 1985 sonar survey of its debris field and compare predicted tsunami with historical observations. The best agreement occurs for landslides travelling at 40 m s,1, but velocities up to 80 m s,1 cannot be excluded. The Ritter Island debris dropped little more than 800 m vertically and moved slowly compared with landslides that descend into deeper water. Basal friction block models predict that slides with shorter falls should attain lower peak velocities and that 40+ m s,1 is perfectly compatible with the geometry and runout extent of the Ritter Island landslide. The consensus between theory and observation for the Ritter Island waves increases our confidence in the existence of mega-tsunami produced by oceanic volcano collapses two to three orders of magnitude larger in scale. [source] Tyatya Volcano, southwestern Kuril arc: Recent eruptive activity inferred from widespread tephraISLAND ARC, Issue 4 2002MITSUHIRO NAKAGAWA Abstract Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low- and medium-K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and caad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short-lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub-Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km3), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions. [source] Mid,Cretaceous Episodic Magmatism and Tin Mineralization in Khingan-Okhotsk Volcano,Plutonic Belt, Far East RussiaRESOURCE GEOLOGY, Issue 1 2002Kohei SATO Abstract: Age of magmatism and tin mineralization in the Khingan-Okhotsk volcano,plutonic belt, including the Khingan, Badzhal and Komsomolsk tin fields, were reviewed in terms of tectonic history of the continental margin of East Asia. This belt consists mainly of felsic volcanic rocks and granitoids of the reduced type, being free of remarkable geomagnetic anomaly, in contrast with the northern Sikhote-Alin volcano,plutonic belt dominated by oxidized-type rocks and gold mineralization. The northern end of the Khingan-Okhotsk belt near the Sea of Okhotsk, accompanied by positive geomagnetic anomalies, may have been overprinted by magmatism of the Sikhote-Alin belt. Tin,associated magmatism in the Khingan-Okhotsk belt extending over 400 km occurred episodically in a short period (9510 Ma) in the middle Cretaceous time, which is coeval with the accretion of the Kiselevka-Manoma complex, the youngest accretionary wedge in the eastern margin of the Khingan-Okhotsk accretionary terranes. The episodic magmatism is in contrast with the Cretaceous-Paleogene long,lasted magmatism in Sikhote,Alin, indicating the two belts are essentially different arcs, rather than juxtaposed arcs derived from a single arc. The tin-associated magmatism may have been caused by the subduction of a young and hot back-arc basin, which is inferred from oceanic plate stratigraphy of the coeval accre-tionary complex and its heavy mineral assemblage of immature volcanic arc provenance. The subduction of the young basin may have resulted in dominance of the reduced-type felsic magmas due to incorporation of carbonaceous sediments within the accretionary complex near the trench. Subsequently, the back-arc basin may have been closed by the oblique collision of the accretionary terranes in Sikhote,Alin, which was subjected to the Late Cretaceous to Paleogene magmatism related to another younger subduction system. These processes could have proceeded under transpressional tectonic regime due to oblique subduction of the paleo-Pacific plates under Eurasian continent. [source] People of the Volcano: Andean Counterpoint in the Colca Valley of Peru , By Noble David Cook with Alexandra Parma CookTHE HISTORIAN, Issue 1 2009Cristina Moya No abstract is available for this article. [source] Volcanoes and earthquakes 18GEOLOGY TODAY, Issue 5 2000Article first published online: 7 JUL 200 First page of article [source] Censusing the mountain gorillas in the Virunga Volcanoes: complete sweep method versus monitoringAFRICAN JOURNAL OF ECOLOGY, Issue 3 2010Maryke Gray Abstract The mountain gorillas (Gorilla beringei beringei) of the Virunga Volcanoes Range of Rwanda, Uganda, and the Democratic Republic of Congo are one of the most endangered ape populations in the world. Following a dramatic decline during the 1960s, and relative stability in the 1970s, the population steadily increased during the 1980s. Due to political instability and war, a complete census had not been conducted since 1989. Here we compare the results of a complete census using the ,sweep method' conducted in 2003 with those from a monitoring program, to estimate the size and distribution of the gorilla population. A total of 360 gorillas were counted from census measurements and known habituated groups. Based on quantitative assessments of the census accuracy, we calculated that an additional 20 gorillas were not counted, leading to an estimated population of 380 individuals, and a 1.15% annual growth rate since 1989. The Ranger Based Monitoring programme yielded similar results. The encouraging results must be viewed with caution, however, because the growth was concentrated almost entirely in one section of the Virungas. Additionally, the distribution of gorilla groups was negatively correlated with the frequency of human disturbances, which highlights the need to continue strengthening conservation efforts. Résumé Les gorilles de montagne Gorilla gorilla beringei de l'aire de répartition des Volcans Virunga, au Rwanda, en Ouganda et en République Démocratique du Congo, sont une des populations de grands singes les plus menacées du monde. Après un déclin dramatique dans le courant des années 1960, et une stabilité relative dans les années 1970, la population s'est fermement reconstituée au cours des années 1980. Mais en raison de l'instabilité politique et de la guerre, il n'y avait plus eu de recensement total depuis 1989. Nous comparons ici les résultats d'un recensement complet effectué en 2003 par la méthode du balayage topologique (sweep method) avec ceux d'un programme de suivi, afin d'estimer la taille et la distribution de la population de gorilles. Nous avons dénombré un total de 360 gorilles en reprenant les chiffres du recensement et ceux des groupes habitués connus. En nous basant sur l'évaluation quantitative de la précision du recensement, nous avons calculé que 20 gorilles supplémentaires n'avaient pas été comptés, ce qui porte l'estimation à 380 individus et signifie un taux de croissance annuel de 1,15% depuis 1989. Le Programme de Suivi basé sur les gardes a obtenu des résultats comparables. Pourtant, ces résultats encourageants doivent être interprétés avec prudence étant donné que la croissance s'est presque entièrement concentrée sur une seule section des Virunga. De plus, la distribution des groupes de gorilles était négativement liée à la fréquence des perturbations humaines, ce qui souligne la nécessité de poursuivre le renforcement des mesures de conservation. [source] Satellite Remote Sensing as a Tool in Lahar Disaster ManagementDISASTERS, Issue 2 2002Norman Kerle At least 40,000 deaths have been attributed to historic lahars (volcanic mudflows). The most recent lahar disaster occurred in 1998 at Casita volcano, Nicaragua, claiming over 2,500 lives. Lahars can cover large areas and be highly destructive, and constitute a challenge for disaster management. With infrastructure affected and access frequently impeded, disaster management can benefit from the synoptic coverage provided by satellite imagery. This potential has been recognised for other types of natural disasters, but limitations are also known. Dedicated satellite constellations for disaster response and management have been proposed as one solution. Here we investigate the utility of currently available and forthcoming optical and radar sensors as tools in lahar disaster management. Applied to the Casita case, we find that imagery available at the time could not have significantly improved disaster response. However, forthcoming satellites, especially radar, will improve the situation, reducing the benefit of dedicated constellations. [source] Community Resilience and Volcano Hazard: The Eruption of Tungurahua and Evacuation of the Faldas in EcuadorDISASTERS, Issue 1 2002Graham A. Tobin Official response to explosive volcano hazards usually involves evacuation of local inhabitants to safe shelters. Enforcement is often difficult and problems can be exacerbated when major eruptions do not ensue. Families are deprived of livelihoods and pressure to return to hazardous areas builds. Concomitantly, prevailing socioeconomic and political conditions limit activities and can influence vulnerability. This paper addresses these issues, examining an ongoing volcano hazard (Tungurahua) in Ecuador where contextual realities significantly constrain responses. Fieldwork involved interviewing government officials, selecting focus groups and conducting surveys of evacuees in four locations: a temporary shelter, a permanent resettlement, with returnees and with a control group. Differences in perceptions of risk and health conditions, and in the potential for economic recovery were found among groups with different evacuation experiences. The long-term goal is to develop a model of community resilience in long-term stress environments. [source] Prokaryotic diversity and metabolically active microbial populations in sediments from an active mud volcano in the Gulf of MexicoENVIRONMENTAL MICROBIOLOGY, Issue 10 2006Robert J. Martinez Summary In this study, ribosomes and genomic DNA were extracted from three sediment depths (0,2, 6,8 and 10,12 cm) to determine the vertical changes in the microbial community composition and identify metabolically active microbial populations in sediments obtained from an active seafloor mud volcano site in the northern Gulf of Mexico. Domain-specific Bacteria and Archaea 16S polymerase chain reaction primers were used to amplify 16S rDNA gene sequences from extracted DNA. Complementary 16S ribosomal DNA (crDNA) was obtained from rRNA extracted from each sediment depth that had been subjected to reverse transcription polymerase chain reaction amplification. Twelve different 16S clone libraries, representing the three sediment depths, were constructed and a total of 154 rDNA (DNA-derived) and 142 crDNA (RNA-derived) Bacteria clones and 134 rDNA and 146 crDNA Archaea clones obtained. Analyses of the 576 clones revealed distinct differences in the composition and patterns of metabolically active microbial phylotypes relative to sediment depth. For example, ,- Proteobacteria rDNA clones dominated the 0,2 cm clone library whereas ,-Proteobacteria dominated the 0,2 cm crDNA library suggesting , to be among the most active in situ populations detected at 0,2 cm. Some microbial lineages, although detected at a frequency as high as 9% or greater in the total DNA library (i.e. Actinobacteria, ,- Proteobacteria), were markedly absent from the RNA-derived libraries suggesting a lack of in situ activity at any depth in the mud volcano sediments. This study is one of the first to report the composition of the microbial assemblages and physiologically active members of archaeal and bacterial populations extant in a Gulf of Mexico submarine mud volcano. [source] Inorganic sulphate extraction from SO2 -impacted AndosolsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2005T. Delfosse Summary Sulphate sorption on to the surface of short-range ordered minerals and precipitation of Al-hydroxy sulphate contribute to the acid neutralizing capacity of soils. The correct measurement of total inorganic sulphate is thus essential in soils that are accumulating SO42, anions. We extracted SO42, by various solutions, namely 0.005 m Ca(NO3)2, 0.016 m KH2PO4, 0.5 m NH4F and 0.2 m acidic NH4 -oxalate (pH 3), from Vitric and Eutric Andosols exposed to prolonged deposition of acid and SO2 from an active volcano (Masaya, Nicaragua). We attributed sulphate extractable by KH2PO4 (20,3030 mg kg,1) to anion-exchangeable SO42,, which was much smaller than NH4F- and oxalate-extractable SO42, (400,9680 and 410,10 480 mg kg,1, respectively). Our results suggest the occurrence of a sparingly soluble Al-hydroxy-mineral phase extractable by both NH4F and oxalate. The formation of Al-hydroxy minerals would result from the combination of enhanced weathering caused by strong acid loading and simultaneous occurrence of large SO42, concentrations in soil solution. Oxalate extracted slightly more inorganic SO42, than did NH4F, this additional amount of SO42, correlating strongly with oxalate-extractable Si and Fe contents. Preferential occlusion of SO42, by short-range ordered minerals, especially ferrihydrite, explains this behaviour. If we exclude the contribution of occluded sulphate then oxalate and NH4F mobilize similar amounts of SO42, and are believed to mobilize all of the inorganic SO42, pool. [source] ON THE GEOCHRONOLOGICAL METHOD VERSUS FLOW SIMULATION SOFTWARE APPLICATION FOR LAHAR RISK MAPPING: A CASE STUDY OF POPOCATÉPETL VOLCANO, MEXICOGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 3 2010ESPERANZA MUÑOZ-SALINAS ABSTRACT. Lahars are hazardous events that can cause serious damage to people who live close to volcanic areas; several were registered at different times in the last century, such as at Mt St Helens (USA) in 1980, Nevado del Ruiz (Colombia) in 1985 and Mt Pinatubo (Philippines) in 1990. Risk maps are currently used by decision-makers to help them plan to mitigate the hazard-risk of lahars. Risk maps are acquired based on a series of tenets that take into account the distribution and chronology of past lahar deposits, and basically two approaches have been used: (1) The use of Flow Simulation Software (FSS), which simulates flows along channels in a Digital Elevation Model and (2) The Geochronological Method (GM), in which the mapping is based on the evaluation of lahar magnitude and frequency. This study addresses the production of a lahar risk map using the two approaches (FSS and GM) for a study area located at Popocatépetl volcano , Central Mexico. Santiago Xalitzintla, a town located on the northern flank of Popocatépetl volcano, where volcanic activity in recent centuries has triggered numerous lahars that have endangered local inhabitants, has been used for the case study. Results from FSS did not provide satisfactory findings because they were not consistent with lahar sediment observations made during fieldwork. By contrast, the GM produced results consistent with these observations, and therefore we use them to assess the hazard and produce the risk map for the study area. [source] Eyjafjallajökull volcano causes widespread disruption to European air trafficGEOLOGY TODAY, Issue 3 2010Ralf Gertisser It started with the usual fire fountains and lava flows that are typical of Iceland's basaltic fissure eruptions, quickly turned into one of the country's major tourist attractions, and when they thought it was all over, the first eruption of Eyjafjallajökull volcano in 200 years suddenly brought Europe's air traffic to a halt. [source] New procedures to decompose geomagnetic field variations and application to volcanic activitiyGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2008Ikuko Fujii SUMMARY We report the development of numerical procedures for extracting long-term geomagnetic field variations caused by volcanic activity from an observed geomagnetic field by using statistical methods. The newly developed procedures are to estimate the trend from the observed data, as well as variations of non-volcanic origin such as periodic components, components related to external geomagnetic variations and observational noise. We also aim at referring to data obtained at a remote standard geomagnetic observatory rather than using a temporarily installed reference site for reasons of data quality. Two different approaches,a Bayesian statistical method and a Kalman filter method,are applied to decompose the geomagnetic field data into four components for comparison. The number of filter coefficients and the degree of condition realizations are optimized on the basis of minimization of the information criteria. The two procedures were evaluated by using a synthetic data set. Generally, the results of both methods are equally sufficient. Subtle differences are seen at the first, several data points due to arbitrarily selected initial values in the case of the Kalman filter method and at the smaller residual for the Bayesian statistical method. The largest differences are in computation time and memory size. The Kalman filter method runs a thousand times faster on a testing workstation and requires less memory than the Bayesian method. The Kalman filter method was applied to the total intensity data at Kuchi-erabu-jima volcano. The result suggests that the procedure works reasonably well. [source] Analytical and 3-D numerical modelling of Mt. Etna (Italy) volcano inflationGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2005A. Bonaccorso SUMMARY Since 1993, geodetic data obtained by different techniques (GPS, EDM, SAR, levelling) have detected a consistent inflation of the Mt. Etna volcano. The inflation, culminating with the 1998,2001 strong explosive activity from summit craters and recent 2001 and 2002 flank eruptions, is interpreted in terms of magma ascent and refilling of the volcanic plumbing system and reservoirs. We have modelled the 1993,1997 EDM and GPS data by 3-D pressurized sources to infer the position and dimension of the magma reservoir. We have performed analytical inversions of the observed deformation using both spheroidal and ellipsoidal sources embedded in a homogeneous elastic half-space and by applying different inversion methods. Solutions for these types of sources show evidence of a vertically elongated magma reservoir located 6 km beneath the summit craters. The maximum elevation of topography is comparable to such depth and strong heterogeneities are inferred from seismic tomography; in order to assess their importance, further 3-D numerical models, employing source parameters extracted from analytical models, have been developed using the finite-element technique. The deformation predicted by all the models considered shows a general agreement with the 1993,1997 data, suggesting the primary role of a pressure source, while the complexities of the medium play a minor role under elastic conditions. However, major discrepancies between data and models are located in the SE sector, suggesting that sliding along potential detachment surfaces may contribute to amplify deformation during the inflation. For the first time realistic features of Mt. Etna are studied by a 3-D numerical model characterized by the topography and lateral variations of elastic structure, providing a framework for a deeper insight into the relationships between internal sources and tectonic structures. [source] The hydroelectric problem of porous rocks: inversion of the position of the water table from self-potential dataGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2004A. Revil SUMMARY The self-potential (SP) method is a fast and cheap reconnaissance tool sensitive to ground water flow in unconfined aquifers. A model based on the use of Green's functions for the coupled hydroelectric problem yields an integral equation relating the SP field to the distribution of the piezometric head describing the phreatic surface and to the electrical resistivity contrast through this phreatic surface. We apply this model to SP data measured on the south flank of the Piton de la Fournaise volcano, a large shield volcano located on Réunion island, Indian ocean. The phreatic surface, inverted with the help of the Simplex algorithm from the SP data, agrees well with the available information in this area [one borehole and electromagnetic (EM) data]. This interpretation scheme, which we call electrography, has many applications to the crucial problem of water supply in volcanic areas where drilling is expensive. [source] Dynamical similarity of explosions at Stromboli volcanoGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004S. De Martino SUMMARY We analyse Strombolian explosions recorded with broadband seismometers to quantify the degree of similarity among explosions. First, we construct the trajectory space. This reconstruction is analogous to that of phase space but, unlike the phase space which relies on the analysis of a single trajectory of a dynamic system, the trajectory space takes into account many trajectories of the dynamic process to study the overlapping properties. Based on the scaling of distances between each pair of histories, it is possible to evaluate the dimension of the trajectory space. We consider the different explosions as different trajectories and, after normalizing all the records, we find that the scaling region is spread over less than one order of magnitude. This absence of scaling implies that all the trajectories cover the same attractor in the trajectory space, and that the trajectories are generated by the same dynamic system. Accordingly, we conclude that all the events are very similar to each other. This result is confirmed by the energy distribution of these events. Standard techniques of energy estimation are inadequate in this case, and we propose an evaluation method based on a quantity that is proportional to energy. The distribution function of this quantity, calculated on 580 events, displays a log-normal behaviour with very low variability (less than two orders of magnitude in energy). These results provide quantitative support for the observation that there is a great degree of similarity among Strombolian explosions and support the idea that a dynamic model underlying these events can be elaborated through the study of just one event. [source] Ritter Island Volcano,lateral collapse and the tsunami of 1888GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2003Steven N. Ward SUMMARY In the early morning of 1888 March 13, roughly 5 km3 of Ritter Island Volcano fell violently into the sea northeast of New Guinea. This event, the largest lateral collapse of an island volcano to be recorded in historical time, flung devastating tsunami tens of metres high on to adjacent shores. Several hundred kilometres away, observers on New Guinea chronicled 3 min period waves up to 8 m high, that lasted for as long as 3 h. These accounts represent the best available first-hand information on tsunami generated by a major volcano lateral collapse. In this article, we simulate the Ritter Island landslide as constrained by a 1985 sonar survey of its debris field and compare predicted tsunami with historical observations. The best agreement occurs for landslides travelling at 40 m s,1, but velocities up to 80 m s,1 cannot be excluded. The Ritter Island debris dropped little more than 800 m vertically and moved slowly compared with landslides that descend into deeper water. Basal friction block models predict that slides with shorter falls should attain lower peak velocities and that 40+ m s,1 is perfectly compatible with the geometry and runout extent of the Ritter Island landslide. The consensus between theory and observation for the Ritter Island waves increases our confidence in the existence of mega-tsunami produced by oceanic volcano collapses two to three orders of magnitude larger in scale. [source] Retrieving geomagnetic secular variations from lava flows: evidence from Mounts Arso, Etna and Vesuvius (southern Italy)GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002Alberto Incoronato Summary Mean directions of magnetization from Mounts Arso (Ischia Island, Gulf of Naples), Etna and Vesuvius lava flows have been determined based on very stringent linearity criteria. These indicate that, regardless of the source volcano, the lava flow mean directions of magnetization form a common path, the SISVC (Southern Italy Secular Variation Curve). This curve enables a reassessment of the age of eruption of several lavas. A date of AD 1169 is demonstrated to be the only possible time of emplacement for one Etna lava flow previously assigned an age of AD 812/1169. It is also demonstrated that two Etna lava flows, which, according to the literature, were emplaced in AD 1536 and 1595 respectively, were actually both emplaced around AD 1037. Three other Etna lava flows, one ascribed to AD 1566 and two to AD 1595, were actually emplaced between AD 1169 and 1284/85. The same time window also holds for a Vesuvius lava flow for which only an upper time threshold was previously available. Only one of the studied flows needs further sampling and analysis to verify whether this flow has been affected by a complete remagnetization or has an erroneous historical dating. The applied procedure seems to be the most appropriate one in carrying out palaeomagnetic surveys of lava flows, as also suggested by the broad agreement with some 17th and 19th century measurements of the geomagnetic field in Rome, relocated to Etna, and is likely to improve knowledge of past history of a volcano significantly. [source] Scattering behaviour at Merapi volcano (Java) revealed from an active seismic experimentGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2001Ulrich Wegler SUMMARY The seismic structure of the stratovolcano Merapi (Java, Indonesia) was studied using an active seismic experiment. Three 3 km long seismic profiles each consisting of up to 30 three-component seismometers with an interstation distance of 100 m were built up in an altitude range between 1000 and 2000 m above sea level. The detailed study of the seismic properties of the propagation media in active volcanic regions is important to understand the natural seismic signals used for eruption forecasting. The seismic experiment at Merapi therefore concentrates on the heterogeneous structure within a radius of 5 km from the active dome, where the sources of most of the natural volcanic seismic events are located. The cone of Merapi volcano consists of different materials changing on a small scale due to the layering of eruptive material. Additionally, the topography of the erosion valleys leads to an irregular deposition, which cannot be described by a simple 1-D layering. These inhomogeneities have a strong influence on seismic signals. The direct P and S waves are attenuated quickly and show only small amplitudes on seismograms. The energy lost from the direct waves, however, is not changed into heat but scattered and can be observed as seismic coda following the direct waves. The observed seismograms show a spindle-like amplitude increase after the direct P phase. This shape of the envelope can be explained by the diffusion model. According to this model there are so many strong inhomogeneities that the direct wave can be neglected and all energy is concentrated in multiple scattered waves. Besides the envelope, the coherence and polarization properties of the wavefield also indicate strong scattering. Only the first onset shows coherence over a station spacing of 100 m, whereas the late phases carrying the major part of the energy are mainly incoherent. The horizontal components of the seismograms have larger amplitudes than the vertical component, but within the horizontal plane the polarization is almost arbitrary, corresponding to waves arriving from scatterers located arbitrarily in space. As a result of the inversion using the diffusion model we obtain values of the S -wave scattering attenuation coefficient, ,s, and the S -wave intrinsic absorption coefficient, ,i. In the frequency range of 4,20 Hz used in this study the scattering attenuation is at least one order of magnitude larger than the intrinsic absorption (,s,,i). The mean free path of S waves is as low as 100 m (,s,1,100 m). The scattering coefficient is independent of frequency (,s,f0.0), whereas the coefficient of intrinsic attenuation increases with increasing frequency (,i,f1.6). The natural seismic signals at Merapi volcano show similar characteristics to the artificial shots. The first onsets have only small amplitudes and the energy maximum arrives delayed compared to the direct waves. Therefore, these signals appear to be strongly affected by multiple scattering also. [source] Crustal structure of central and northern Iceland from analysis of teleseismic receiver functionsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Fiona A. Darbyshire We present results from a teleseismic receiver function study of central and northern Iceland, carried out during the period 1995,1998. Data from eight broad-band seismometers installed in the SIL network operated by the Icelandic Meteorological Office were used for analysis. Receiver functions for each station were generated from events for a wide range of backazimuths and a combination of inversion and forward modelling was used to infer the crustal structure below each station. The models generated show a considerable variation in the nature and thickness of the crust across Iceland. The thinnest crust (20,21 km) is found in the northern half of the Northern Volcanic Zone approximately 120 km north of the centre of the Iceland mantle plume. Thicker crust (24,30 km) is found elsewhere in northern and central Iceland and the thickest crust (37 km) is found close to the plume centre. Velocity,depth profiles show a distinct division of the crust into two main sections, an upper high-velocity-gradient section of thickness 2,8 km and a lower crustal section with small or zero overall velocity gradient. The thickness of the upper crust correlates with the tectonic structure of Iceland; the upper crust is thickest on the flanks of the northern and central volcanic rift zones and thinnest close to active or extinct central volcanoes. Below the Krafla central volcano in northeastern Iceland the receiver function models show a prominent low-velocity zone at 10,15 km depth with minimum shear wave velocities of 2.0,2.5 km s,1. We suggest that this feature results from the presence of partially molten sills in the lower crust. Less prominent low-velocity zones found in other regions of Iceland may arise from locally high temperatures in the crust or from acidic intrusive bodies at depth. A combination of the receiver function results and seismic refraction results constrains the crustal thickness across a large part of Iceland. Melting by passive decompression of the hot mantle below the rift zone in northern Iceland forms a crust of thickness ,20 km. In contrast, the larger crustal thickness below central Iceland probably arises from enhanced melt production due to active upwelling in the plume core. [source] Searching for a significant correlation between volcanic tremor amplitude and SO2 emissions at Mount Etna volcano, SicilyGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2000Sabrina Leonardi Summary A strong correlation between the amplitude of volcanic tremor and the flux of SO2 has been found at Mount Etna volcano, Sicily, corresponding to enhanced volcanic activity in the period 1987,1995. We therefore suggest that tremors and SO2 emissions have a common physical origin linked to the magma dynamics of the volcano. [source] Hydrodynamics and geomorphic work of jökulhlaups (glacial outburst floods) from Kverkfjöll volcano, IcelandHYDROLOGICAL PROCESSES, Issue 6 2007Jonathan L. Carrivick Abstract Jökulhlaups (glacial outburst floods) occur frequently within most glaciated regions of the world and cause rapid landscape change, infrastructure damage, and human disturbance. The largest jökulhlaups known to have occurred during the Holocene within Iceland drained from the northern margin of Vatnajökull and along the Jökulsá á Fjöllum. Some of these jökulhlaups originated from Kverkfjöll volcano and were routed through anastomosing, high gradient and hydraulically rough channels. Landforms and sediments preserved within these channels permit palaeoflow reconstructions. Kverkfjöll jökulhlaups were reconstructed using palaeocompetence (point measurements), slope,area (one-dimensional), and depth-averaged two-dimensional (2D) hydrodynamic modelling techniques. The increasing complexity of 2D modelling required a range of assumptions, but produced information on both spatial and temporal variations in jökulhlaup characteristics. The jökulhlaups were volcanically triggered, had a linear-rise hydrograph and a peak discharge of 50 000,100 000 m3 s,1, which attenuated by 50,75% within 25 km. Frontal flow velocities were ,2 m s,1; but, as stage increased, velocities reached 5,15m s,1. Peak instantaneous shear stress and stream power reached 1 × 104 N m,2 and 1 × 105 W m,2 respectively. Hydraulic parameters can be related to landform groups. A hierarchy of landforms is proposed, ranging from the highest energy zones (erosional gorges, scoured bedrock, cataracts, and spillways) to the lowest energy zones (of valley fills, bars, and slackwater deposits). Fluvial erosion of bedrock occurred in Kverkfjallarani above ,3 m flow depth, ,7m s,1 flow velocity, ,1 × 102 N m,2 shear stress, and 3 × 102 W m,2 stream power. Fluvial deposition occurred in Kverkfjallarani below ,8 m flow depth, 11 m s,1 flow velocity, 5 × 102 N m,2 shear stress, and 3 × 103 W m,2 stream power. Hence, erosional and depositional ,envelopes' have considerable overlap, probably due to transitional flow phenomena and the influence of upstream effects, such as hydraulic ponding and topographic constrictions, for example. Holocene Kverkfjöll jökulhlaups achieved geomorphic work comparable to that of other late Pleistocene ,megafloods'. This work was a result of steep channel gradients, topographic channel constrictions, and high hydraulic roughness, rather than to extreme peak discharges. The Kverkfjöll jökulhlaups have implications for landscape evolution in north-central Iceland, for water-sediment inputs into the North Atlantic, and for recognizing jökulhlaups in the rock record. 2D hydrodynamic modelling is likely to be important for hazard mitigation in similar landscapes and upon other glaciated volcanoes, because it only requires an input hydrograph and a digital elevation model to run a model, rather than suites of geomorphological evidence and field-surveyed valley cross-sections, for example. Copyright © 2006 John Wiley & Sons, Ltd. [source] Impacts of a volcanic eruption on the forest bird community of Montserrat, Lesser AntillesIBIS, Issue 2 2007B. DALSGAARD Volcanic eruptions are an important and natural source of catastrophic disturbance to ecological communities. However, opportunities to study them are relatively rare. Here we report on the effects of the eruption of the Soufrière Hills volcano on the forest bird community of the Lesser Antillean island of Montserrat. The island's species-poor avifauna includes 11 restricted-range species, including the Critically Endangered endemic Montserrat Oriole Icterus oberi. Analysis of monitoring data from 1997 to 2005 indicates that counts of most species were substantially lower following major ashfalls. However, this effect was short-lived, with rapid population recovery in subsequent years. Furthermore, levels of seasonal rainfall appear to have been at least as important in determining population trends as ashfall. Overall, most species were at least as abundant at the end of the study as at the start, and no forest bird species have been extirpated from Montserrat. We discuss potential ecological drivers of ashfall impacts on populations: there is some evidence that terrestrial foragers were most severely affected. [source] Tyatya Volcano, southwestern Kuril arc: Recent eruptive activity inferred from widespread tephraISLAND ARC, Issue 4 2002MITSUHIRO NAKAGAWA Abstract Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low- and medium-K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and caad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short-lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub-Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km3), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions. [source] Variations of magnetic susceptibility and fine quartz accumulation rate in Daisen loam over the past 200 000 years: Interaction between winter and summer monsoons in south-west JapanISLAND ARC, Issue 2 2001Jun-Ichi Kimura Abstract A loam section near Daisen volcano, South-west Japan, has been examined for low-field magnetic susceptibility (MS) and fine quartz accumulation rate. Fission track dating of tephra layers interbedded in the deposit shows that the loam age ranges from about 200 ka to the Present. The MS was measured for both bulk sample and the < 63 ,m fine fraction. Fine quartz contents in the < 63 ,m fraction were also determined using acid-alkali digestions and recalculated to derive fine quartz accumulation rate (Rqz). Grain size analysis was then carried out on the separated fine quartz. Low-field MS varies from low frequency magnetic suspectibility (,(LF)) 5 to 100 (× 10,6 m3/kg) for bulk samples and from 1 to 30 for fine fractions. The fine fraction ,(LF) variation correlated with Chinese loess MS stratigraphy, which indicated changes in pedogenic enhancement of the MS and is reflected by summer monsoon intensity. The Rqz are high in cool climate stages, with volumes between 0.2 and 0.4 (× 10,2 kg/m2 per yr), whereas in warm stages the rate falls to about 0.1. These values compare well with those reported from the Hokkaido and Kanto areas, suggesting the fine quartz originates from tropospheric dust. The strong winter monsoons during glacial stages alternated with weak summer monsoons as a result of a southward shift of the jet stream. In interglacials, summer monsoons were stronger. Seasonal alternating monsoons appear to have operated in South-west Japan through the past 200 000 years. [source] Colonization of an island volcano, Long Island, Papua New Guinea, and an emergent island, Motmot, in its caldera lake.JOURNAL OF BIOGEOGRAPHY, Issue 11-12 2001Abstract Biogeographical context Long Island, in Vitiaz Strait, is 55 km north of New Guinea, 60 km from Umboi Island and 125 km from New Britain. After its explosive caldera-forming eruption in about 1645, Long is being recolonized by animals and plants. Through renewed volcanic activity in the 1950s a new island emerged from Long's caldera lake, 4 km from the nearest lake shore and was recreated by eruptions in 1968. Long Island thus provides the opportunity to study a nested pair of natural colonization sequences. The geological background, eruptive history, course and results of the seventeenth century eruption, and the geographical features and climate of Long Island are summarized. Existing knowledge of Long's recolonization, confined almost entirely to surveys of its avifauna in 1933 and 1972, is reviewed. The geological history of Motmot is outlined, and published knowledge of its colonization by animals and plants from 1968 to 1988 is summarized. The 1999 expedition and aims An expedition to Long Island and Motmot in 1999 set out to investigate the hitherto little-known flora and present vertebrate fauna of Long Island and to survey the entire flora and fauna of Motmot for comparison with the results of previous surveys. The methods used in the 1999 survey are described, and the papers setting out the results briefly introduced. [source] Colonization of an island volcano, Long Island, Papua New Guinea, and an emergent island, Motmot, in its caldera lake.JOURNAL OF BIOGEOGRAPHY, Issue 11-12 2001Abstract Aim Long Island erupted catastrophically in c. 1645 with the probable destruction of its entire biota. While several expeditions have visited the island since, no survey of its flora has been published. In 1968 a small island, Motmot, emerged from its caldera lake. Motmot has been surveyed several times but not since 1988. The aim of this study was to investigate the colonization by vascular plants of this interesting nested pair of islands. Location Long Island is 55 km off the northeast coast of Papua New Guinea (PNG) and Motmot lies in its fresh water caldera lake, Lake Wisdom, c. 4 km from the rim of the surrounding volcano. Methods We conducted a complete survey of the vascular plants on Motmot and made ad hoc collections on Long Island, including surveying three small plots, over 15 days in 1999. In addition, we incorporated data from several collections lodged at the regional herbarium in Lae. Data on seed dispersal syndromes and plant habit were obtained from the literature. Results We recorded 305 species of vascular plants from Long Island, but most trees were small and the species diversity was low. Motmot was still very sparsely vegetated and only forty-five species of vascular plants were found. Communities on Motmot were unspecialized and common species widespread. The flora of Motmot was not significantly different from that on Long Island in terms of the number of species amongst higher taxa, seed dispersal syndromes or plant habits. Main conclusions Low rainfall combined with very porous soils may be responsible for the small stature and low diversity of the forest on Long Island's caldera rim. The absence of specialist littoral species on Motmot, as a result of its landlocked situation, is probably responsible for the very slow species recruitment on the island. This community appears to be functionally important for colonization by comparison with other small volcanic islands in a marine situation. [source] Colonization of an island volcano, Long Island, Papua New Guinea, and an emergent island, Motmot, in its caldera lake.JOURNAL OF BIOGEOGRAPHY, Issue 11-12 2001Abstract Location, Aim Long Island, 55 km north of New Guinea, erupted explosively in the seventeenth century and has been recolonized by animals and plants. Effectively in 1968, an island, Motmot, emerged from Long's 13 km-diameter fresh-water caldera lake, about 4 km from the nearest shore. A nested pair of colonization sequences is thus available for study. Long Island has been recolonized by birds, and Motmot has been colonized, presumably from the surrounding ring of Long Island. In order to monitor further colonization of Long Island following bird surveys in 1933 and 1972, and to survey the birds of Motmot, Motmot and the western parts of Long were visited for 15 days in 1999. Results In the last 66 years one species probably has become extinct; in the past 27 years three species have colonized and three, present in 1972, were not recorded in 1999. We assess the present number of nonmigrant land bird species as fifty. On Motmot, we recorded eight species, three breeding, and found two more only as prey remains (three raptor species were seen on the island). Main conclusions Long Island's avifauna still has youthful features. Species with good colonizing ability predominate, and there are no island-endemic subspecies. There is a persistently high proportion of supertramps; none of the ten supertramps has been replaced by later-colonizing ,high-S' species. A quasi-equilibrium of about fifty species, fourteen fewer than the predicted equilibrium number for the island, has persisted for at least seven decades and may be seen as that appropriate to the present prolonged stage of Long's floral and vegetational development. Compared with the younger avifauna of Krakatau, Long's avifauna has a higher proportion of nectarivores, predators of large invertebrates and small vertebrates, and ground-foraging insectivore-herbivores, and a lower proportion of the insectivore-frugivore guild. Motmot's small avifauna consists predominantly of species obtaining subsistence from outside the island. [source] | |||||||||||||||||||||||||||||||||||||