Repose Period Research Articles

Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano

Caldera-forming volcanic eruptions are low-frequency, high-impact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales. Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained. Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the 'Minoan' caldera-forming eruption of Santorini volcano, Greece, which occurred in the late 1600s BC. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption. Despite the large volume of erupted magma (40-60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicic magma batches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems.

Influência do índice glicêmico na glicemia em exercício físico aeróbico

OBJETIVO: Avaliar a influência do índice glicêmico (IG) na resposta glicêmica antes e durante o exercício físico decorrente a diferentes sessões experimentais pré-exercício. MÉTODOS: Doze homens adultos realizaram três sessões experimentais pré-exercício: de alto índice glicêmico (AIG); de baixo índice glicêmico (BIG); e em estado de jejum, sendo nesta última oferecida duas formas diferenciadas de hidratação durante o exercício: água e bebida carboidratada. RESULTADOS: Durante o período de repouso, o tipo de refeição, de AIG ou de BIG, interferiu no comportamento da glicemia, ambas tendendo ao "efeito rebote". Durante o exercício, não foi observada diferença na resposta glicêmica entre as quatro ações testadas; contudo, a intervenção com bebida carboidratada manteve constante a glicemia ao longo dos 60 min do exercício. CONCLUSÃO: O IG é determinante na resposta glicêmica ao longo de uma hora antes do exercício, porém não interfere na resposta glicêmica durante a atividade.

The Influence of Ecoclimatic Conditions on Wood Maturation and Bud's Viability, at Some Grape Varieties for Quality Wine, from Vineyard Târnave

During vine's repose period, temperature is the mai n stress factor, which influences plant growth and development, as well as the level and quality of next year's yield. In this paper, we re made observations regarding temperatures during the year 2009 and the level of critic temperature during the winter of 2009-2010, in order to establi sh the quantity of carbohydrates from vine shoots and bud viability for several grape varieties: Musc at Ottonel, Sauvignon blanc, Traminer roz and Neuburger. The critic period for vine was in Januar y, due to 7 consecutive days of frost. Carbohydrates content in vine shoots oscillated bet ween 12.09 g% (Neuburger) and 13.25 g% (Muscat Ottonel). Best viability for bearing buds was regis ter for Muscat Ottonel variety 59.56% and the lowest one for Neuburger variety 25.90%. At Neuburger variety, the highest percentage of viable buds is found in the upper third (37.29% main buds and 5 0% secondary buds).

Testing forecasts of a new Bayesian time-predictable model of eruption occurrence

In this paper we propose a model to forecast eruptions in a real forward perspective. Specifically, the model provides a forecast of the next eruption after the end of the last one, using only the data available up to that time. We focus our attention on volcanoes with open conduit regime and high eruption frequency. We assume a generalization of the classical time predictable model to describe the eruptive behavior of open conduit volcanoes and we use a Bayesian hierarchical model to make probabilistic forecasts. We apply the model to Kilauea volcano eruptive data and Mount Etna volcano flank eruption data. The aims of the proposed model are: (1) to test whether or not the Kilauea and Mount Etna volcanoes follow a time predictable behavior; (2) to discuss the volcanological implications of the time predictable model parameters inferred; (3) to compare the forecast capabilities of this model with other models present in literature. The results obtained using the MCMC sampling algorithm show that both volcanoes follow a time predictable behavior. The numerical values inferred for the parameters of the time predictable model suggest that the amount of the erupted volume could change the dynamics of the magma chamber refilling process during the repose period. The probability gain of this model compared with other models already present in literature is appreciably greater than zero. This means that our model provides better forecast than previous models and it could be used in a probabilistic volcanic hazard assessment scheme.

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N: Implications for ridge crest plumbing and decadal changes in magma chamber compositions

Detailed mapping, sampling, and geochemical analyses of lava flows erupted from an ∼18 km long section of the northern East Pacific Rise (EPR) from 9°46′N to 9°56′N during 2005–2006 provide unique data pertaining to the short‐term thermochemical changes in a mid‐ocean ridge magmatic system. The 2005–2006 lavas are typical normal mid‐oceanic ridge basalt with strongly depleted incompatible trace element patterns with marked negative Sr and Eu/Eu* anomalies and are slightly more evolved than lavas erupted in 1991–1992 at the same location on the EPR. Spatial geochemical differences show that lavas from the northern and southern limits of the 2005–2006 eruption are more evolved than those erupted in the central portion of the fissure system. Similar spatial patterns observed in 1991–1992 lavas suggest geochemical gradients are preserved over decadal time scales. Products of northern axial and off‐axis fissure eruptions are consistent with the eruption of cooler, more fractionated lavas that also record a parental melt component not observed in the main suite of 2005–2006 lavas. Radiogenic isotopic ratios for 2005–2006 lavas fall within larger isotopic fields defined for young axial lavas from 9°N to 10°N EPR, including those from the 1991–1992 eruption. Geochemical data from the 2005–2006 eruption are consistent with an invariable mantle source over the spatial extent of the eruption and petrogenetic processes (e.g., fractional crystallization and magma mixing) operating within the crystal mush zone and axial magma chamber (AMC) before and during the 13 year repose period. Geochemical modeling suggests that the 2005–2006 lavas represent differentiated residual liquids from the 1991–1992 eruption that were modified by melts added from deeper within the crust and that the eruption was not initiated by the injection of hotter, more primitive basalt directly into the AMC. Rather, the eruption was driven by AMC pressurization from persistent or episodic addition of more evolved magma from the crystal mush zone into the overlying subridge AMC during the period between the two eruptions. Heat balance calculations of a hydrothermally cooled AMC support this model and show that continual addition of melt from the mush zone was required to maintain a sizable AMC over this time interval.

Avaliação do Desempenho e Eficiência Energética de Um Secador para Café de Fluxos Concorrentes e Contracorrentes

This study was done to evaluate the performance and energy efficiency of a coffee grain dryer using concurrent and counter-current air flow. The first concurrent flow drying stage was separated from the second counter-current flow stage by a repose chamber. The first drying treatment was characterized by intermittent drying with mixing with the air at 45°C, and the second by intermittent drying with continuous reflux at the air at 70°C. In the first drying system initial and final moisture contents were 33.9 ± 5.0 and 11.8 ± 0.7% (w.b.), respectively, compared to 29.0 ± 3.6 and 11.6 ± 1.0% in the second treatment. The grain temperature remained within the recommended values for coffee. Intermittent drying, with continuous mixing, at 70°C consumed less energy compared to the intermittent drying with intermittent mixing at 45°C, and there was greater specific energy consumption in tests where the drying was concluded in few hours after the repose period.

MAIN MORPHO PHYSIOLOGIC INDEX STUDY AND INCIDENCE OF SOME DIGESTIVE ENDOPARASYTOSIS IN CATTLE

Investigation done regarding the main morpho-productive traits and incidence of some digestive endoparasytosis on a 70 cattle livestock, of R.S breed and crossbred in an exploitation family association type, reveals aspects as: milk production, on total lactation, has an average value of 4583,97±185,23 kg, with maximal value in the 2nd lactation, of 5254,4±468,6 kg and minimal value of 3668,0±428,7 kg. Fat percentage with an average of 3,68±0,07, is superior to STAS value (3,5%). Mammary repose period of time is framed in normal limits (X 39,9 days), with maximal values in the 5th lactation (48,5 days) and minimal in the 4th one (34,7 days). Incidence of eimeria and trichostrongylidosis presents different values, function of age category. So, in eimeriosis the extent is maximal in dairy cows (30,0%) and minimal in young stock (16,6%), in trichostrongylidosis we have a maximal incidence in pregnant heifer (50,0%) and minimal in young stock (30,0%). Coprology elimination quantum of eimeria ovocyists presents maximal values in dairy cows (600 OPG) and minimal in young stock (350 OPG), and in trichostrongylidosis the maximal level of coprology eggs eliminations is in pregnant heifers (900 OPG) and minimal in young stock (450 OPG). In hepatic trematodosis the parasite extent is in fascioliasis with maximal values in pregnant heifers (75%) and minimal values in dairy cows (30%), and in dicrocoeliosis maximal values obtained are in young stock and dairy cows (50%) and minimal, in pregnant heifers (25%).

The architecture, eruptive history, and evolution of the Table Rock Complex, Oregon: From a Surtseyan to an energetic maar eruption

The Table Rock Complex (TRC; Pliocene–Pleistocene), first documented and described by Heiken [Heiken, G.H., 1971. Tuff rings; examples from the Fort Rock-Christmas Lake valley basin, south-central Oregon. J. Geophy. Res. 76, 5615-5626.], is a large and well-exposed mafic phreatomagmatic complex in the Fort Rock–Christmas Lake Valley Basin, south-central Oregon. It spans an area of approximately 40 km 2, and consists of a large tuff cone in the south (TRC1), and a large tuff ring in the northeast (TRC2). At least seven additional, smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions. The first period of activity, TRC1, initiated with a Surtseyan-style eruption through a 60–70 m deep lake. The TRC1 deposits are dominated by multiple, 1-2 m thick, fining upward sequences of massive to diffusely-stratified lapilli tuff with intermittent zones of reverse grading, followed by a finely-laminated cap of fine-grained sediment. The massive deposits are interpreted as the result of eruption-fed, subaqueous turbidity current deposits; whereas, the finely laminated cap likely resulted from fallout of suspended fine-grained material through a water column. Other common features are erosive channel scour-and-fill deposits, massive tuff breccias, and abundant soft sediment deformation due to rapid sediment loading. Subaerial TRC1 deposits are exposed only proximal to the edifice, and consist of cross-stratified base-surge deposits. The eruption built a large tuff cone above the lake surface ending with an effusive stage, which produced a lava lake in the crater (365 m above the lake floor). A significant repose period occurred between the TRC1 and TRC2 eruptions, evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences. The TRC2 eruption was the last and most energetic in the complex. General edifice morphology and a high percentage of accidental material suggest eruption through saturated TRC1 deposits and/or playa lake sediments. TRC2 deposits are dominated by three-dimensional dune features with wavelengths 200–500 m perpendicular to the flow, and 20–200 m parallel to the direction of flow depending on distance from source. Large U-shaped channels (10–32 m deep), run-up features over obstacles tens of meters high, and a large (13 m) chute-and-pool feature are also identified. The TRC2 deposits are interpreted as the products of multiple, erosive, highly-inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions.

The Pro-collusive Effect of Increasing the Repose Period for Price Fixing Agreements

Intuitively, extending the period of repose for price fixing agreements enhances the effectiveness of competition policy enforcement. This paper proofs this intuition wrong. As extending the repose period reduces cartel members' defection payoff while it leaves unaltered expected compliance profits, it induces cartels to be more stable internally.

The Pro-collusive Effect of Increasing the Repose Period for Price Fixing Agreements

Abstract Intuitively, extending the period of repose for price fixing agreements enhances the effectiveness of competition policy enforcement. This paper proves this intuition wrong. As extending the repose period reduces cartel members' defection payoff while it leaves unaltered expected compliance profits, it induces cartels to be more stable internally.

Storage and interaction of compositionally heterogeneous magmas from the 1986 eruption of Augustine Volcano, Alaska

Compositional heterogeneity (56–64 wt% SiO2 whole-rock) in samples of tephra and lava from the 1986 eruption of Augustine Volcano, Alaska, raises questions about the physical nature of magma storage and interaction beneath this young and frequently active volcano. To determine conditions of magma storage and evolutionary histories of compositionally distinct magmas, we investigate physical and chemical characteristics of andesitic and dacitic magmas feeding the 1986 eruption. We calculate equilibrium temperatures and oxygen fugacities from Fe-Ti oxide compositions and find a continuous range in temperature from 877 to 947°C and high oxygen fugacities (ΔNNO=1–2) for all magmas. Melt inclusions in pyroxene phenocrysts analyzed by Fourier-transform infrared spectroscopy and electron probe microanalysis are dacitic to rhyolitic and have water contents ranging from <1 to ∼7 wt%. Matrix glass compositions are rhyolitic and remarkably similar (∼75.9–76.6 wt% SiO2) in all samples. All samples have ∼25% phenocrysts, but lower-silica samples have much higher microlite contents than higher-silica samples. Continuous ranges in temperature and whole-rock composition, as well as linear trends in Harker diagrams and disequilibrium mineral textures, indicate that the 1986 magmas are the product of mixing between dacitic magma and a hotter, more mafic magma. The dacitic endmember is probably residual magma from the previous (1976) eruption of Augustine, and we interpret the mafic endmember to have been intruded from depth. Mixing appears to have continued as magmas ascended towards the vent. We suggest that the physical structure of the magma storage system beneath Augustine contributed to the sustained compositional heterogeneity of this eruption, which is best explained by magma storage and interaction in a vertically extensive system of interconnected dikes rather than a single coherent magma chamber and/or conduit. The typically short repose period (∼10 years) between Augustine's recent eruptive pulses may also inhibit homogenization, as short repose periods and chemically heterogeneous magmas are observed at several volcanoes in the Cook Inlet region of Alaska.

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

The emplacement of the Manaslu leucogranite body (Nepal, Himalaya) has been modelled as the accretion of successive sills. The leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection, and by a thin (<100 m) upper thermal aureole. These characteristics were used to constrain the maximum magma emplacement rate. Models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period. Additionally, the hypothesis of a tectonic top contact, with unroofing limiting heat transfer during magma emplacement, was evaluated. In this latter case, the upper limit for the emplacement rate was estimated at 3·4 mm/year (or 1·5 Myr for 5 km of granite). Geological and thermobarometric data, however, argue against a major role of fault activity in magma cooling during the leucogranite emplacement. The best model in agreement with available geochronological data suggests an emplacement rate of 1 mm/year for a relatively shallow level of emplacement (granite top at 10 km), uninterrupted by a long repose period. The thermal aureole temperature and thickness, and the isotopic heterogeneities within the leucogranite, can be explained by the accretion of 20–60 m thick sills intruded every 20 000–60 000 years over a period of 5 Myr. Under such conditions, the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites.

Chronology of Vesuvius? activity from A.D. 79 to 1631 based on archeomagnetism of lavas and historical sources

The activity of Vesuvius between A.D. 79 and 1631 has been investigated by means of precise archaeomagnetic dating of primary volcanic deposits and taking into account the stratigraphy of lavas and tephra, historical written accounts, archaeological evidence related to the developing urbanisation, and radiocarbon ages. We found that the historical records are highly useful in constraining the timing of the main events, even if the data are often too scarce and imprecise for ascertaining the details of all phases of activity, especially their magnitude and emplacement of all the deposit types. In addition, some eruptions that took place in the 9th and 10th centuries appear to be unnoticed by historians. The archaeomagnetic study involved 26 sites of different lavas and 2 pyroclastic deposits. It shows that within the 15 centuries which elapsed between A.D. 79 and 1631, the effusive activity of Vesuvius clustered in the relatively short period of time between A.D. 787 and 1139 and was followed by a 5-century-long repose period. During this time Vesuvius prepared itself for the violent explosive eruption of 1631. The huge lavas shaping the morphology of the coast occurred largely through parasitic vents located outside the Mount Somma caldera. One of these parasitic vents is located at low elevation, very close to the densely inhabited town of Torre Annunziata. Among the various investigated lavas, a number of which were previously attributed to the 1631 eruption, none is actually younger than the 12th century. Therefore it is definitively concluded that the destructive 1631 event was exclusively explosive.

Pyroclastic Flow Hazard at Volcán Citlaltépetl

Volcan Citlaltepetl (Pico de Orizaba) with an elevation of 5,675 m is the highest volcano in North America. Its most recent catastrophic events involved the production of pyroclastic flows that erupted approximately 4,000, 8,500, and 13,000 years ago. The distribution of mapped deposits from these eruptions gives an approximate guide to the extent of products from potential future eruptions. Because the topography of this volcano is constantly changing computer simulations were made on the present topography using three computer algorithms: energy cone, FLOW2D, and FLOW3D. The Heim Coefficient (μ), used as a code parameter for frictional sliding in all our algorithms, is the ratio of the assumed drop in elevation (H) divided by the lateral extent of the mapped deposits (L). The viscosity parameter for the FLOW2D and FLOW3D codes was adjusted so that the paths of the flows mimicked those inferred from the mapped deposits. We modeled two categories of pyroclastic flows modeled for the level I and level II events. Level I pyroclastic flows correspond to small but more frequent block-and-ash flows that remain on the main cone. Level II flows correspond to more widespread flows from catastrophic eruptions with an approximate 4,000-year repose period. We developed hazard maps from simulations based on a National Imagery and Mapping Agency (NIMA) DTED-1 DEM with a 90 m grid and a vertical accuracy of ±30 m. Because realistic visualization is an important aid to understanding the risks related to volcanic hazards we present the DEM as modeled by FLOW3D. The model shows that the pyroclastic flows extend for much greater distances to the east of the volcano summit where the topographic relief is nearly 4,300 m. This study was used to plot hazard zones for pyroclastic flows in the official hazard map that was published recently.

Holocene eruptions within the Katla volcanic system, south Iceland: Characteristics and environmental impact

Holocene volcanism within the Katla volcanic system is characterized by: 1) explosive (hydromagmatic) basaltic eruptions along volcanic fissures within the Mýrdalsjökull caldera; 2) explosive silicic eruptions from vents associated with the caldera and 3) predominantly effusive basaltic eruptions involving both the central volcano and the fissure swarm. Typical Katla eruptions are accompanied by basaltic tephra fall, lightning and glacial floods (jökulhlaups) of meltwater, ice and volcanic debris. Twenty eruptions have occurred in the last 11 centuries. The volume of airborne tephra varies by three orders of magnitude, with an estimated volume of 1.5 km³ of freshly fallen tephra from the largest historical Katla eruption. The length of documented eruptions varies from 2 weeks to over 5 months. The average repose period since 1500 AD is 47 years with maximum deviations of 33 and 34 years. All Katla eruptions during the last 400 years have begun in the springfall season. At least 12 silicic Katla eruptions are known from the period ca. 1700 BP and 6600 BP. The silicic magma was most likely erupted by hydromagmatic explosive eruptions. The tephra dispersal axes indicate vent locations within the caldera or along the caldera rim. The volume of airborne silicic tephra varies by orders of magnitude, the largest and most widespread is tephra layer UN with uncompacted tephra volume of 0.3 km³. Intervals between the silicic eruptions have varied from ca. 100 to ca. 1000 14C yrs. Two major “fires” and 5–10 relatively minor, partly effusive eruptions have occurred during the Holocene. The 10th century Eldgjá and the 6800 BP Hólmsá fires are the largest known Holocene eruptions within the Katla system. A ≥75 km long, discontinuous and partly subglacial eruptive fissure was active during the Eldgjá eruption. The opening phase on most fissure segments was explosive, followed by an effusive phase on the subaerial segments. The eruption produced a voluminous basaltic tephra layer with a minute silicic component, two major lava fields and possibly a hyaloclastic flow deposit. Large jökulhlaups accompanied the eruption. The combined volume of erupted material may exceed 19 km³ DRE. Eruptions on the Katla system have caused extensive environmental changes during the past 1100 years. The Eldgjá fires radically changed the landscape, hydrology and utilization potential of large areas in South Iceland. Since then, jökulhlaups accompanying eruptions within the caldera have escaped eastwards, raising the Mýrdalssandur plain and extending its coastline southward.

Forecasting sizes and repose times of future extreme volcanic events

Research Article| April 01, 1998 Forecasting sizes and repose times of future extreme volcanic events David M. Pyle David M. Pyle 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (4): 367–370. https://doi.org/10.1130/0091-7613(1998)026<0367:FSARTO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation David M. Pyle; Forecasting sizes and repose times of future extreme volcanic events. Geology 1998;; 26 (4): 367–370. doi: https://doi.org/10.1130/0091-7613(1998)026<0367:FSARTO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract By using rank-order statistics, it is possible to predict the likely scale of future “extreme” volcanic events (eruptions larger than a given threshold size, or repose periods longer than a given time) on both a local and global scale. When ranked by diminishing size, the largest and most devastating volcanic eruptions in terms of volume, or consequent fatalities, can be described by power-law functions. In turn, this approach permits projection of, for example, (1) the likely size of the next volcanic eruption larger than the A.D. 1815 eruption of Tambora (∼90 km3 of magma); (2) the size of the next eruption of the Taupo volcano, New Zealand, that is larger than the eruption of A.D. 186 (∼82 km3 of tephra); and (3) the likely duration of the current repose period of the Taupo volcano (4000 ± 1200 years). This approach can also be employed on shorter time scales, for example to provide quantitative criteria to assess when a period of activity has ceased. With the establishment of procedures for making projections such as these, volcano hazard assessment may be placed onto a more rigorous and quantitative basis. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Global volcanic earthquake swarm database and preliminary analysis of volcanic earthquake swarm duration

Global data from 1979 to 1989 pertaining to volcanic earthquake swarms have been compiled into a custom-designed relational database. The database is composed of three sections: 1) a section containing general information on volcanoes, 2) a section containing earthquake swarm data (such as dates of swarm occurrence and durations), and 3) a section containing eruption information. The most abundant and reliable parameter, duration of volcanic earthquake swarms, was chosen for preliminary analysis. The distribution of all swarm durations was found to have a geometric mean of 5.5 days. Precursory swarms were then separated from those not associated with eruptions. The geometric mean precursory swarm duration was 8 days whereas the geometric mean duration of swarms not associated with eruptive activity was 3.5 days. Two groups of precursory swarms are apparent when duration is compared with the eruption repose time. Swarms with durations shorter than 4 months showed no clear relationship with the eruption repose time. However, the second group, lasting longer than 4 months, showed a significant positive correlation with the log10 of the eruption repose period. The two groups suggest that different suites of physical processes are involved in the generation of volcanic earthquake swarms.

The intensity and magnitude of Holocene plinian eruptions from Mount St. Helens volcano

Dispersal characteristics of the T, We, Wn, Pu, Ps, Ye, Yn and Yb plinian fall deposits of Mount St. Helens have been measured at 80 sites downwind of the volcano in order to model eruption dynamics and atmospheric transport. Isopleth contours for the sizes of maximum pumice and lithic clasts are used to calculate peak eruption column heights and intensities (magma discharge) based on a theoretical model of tephra dispersal. New proximal thickness measurements are combined with an empirical distal extrapolation, based on studies of 53 plinian deposits, to calculate the magnitude (erupted mass) of each eruption. Layer Yn (3510 y r B.P.) represents the highest intensity and largest magnitude eruption at Mount St. Helens in post-glacial times. Modeling suggests column height grew to about 31 km before gradually declining at the end of the plinian phase (~ 26 hours). Several intraplinian surge deposits are present in the upper part of the fall layer close to the volcano and up to 15 km to the northeast of Mount St. Helens. Peak intensity of the plinian phase was 10 8 kg/s and the total erupted volume was 4 km 3 (DRE of magma). Small plinian-style eruptions are represented by layers such as Ps and Pu of the Pine Creek eruptive period (3000-2500 yr B.P.) and have intensities of only ~ 10 6 kg/s. When compared with plinian eruptions from other volcanoes, the Holocene eruptions of Mount St. Helens span from the lower to the middle part of the known range in intensity and magnitude and are typical of events derived from intermediate-sized stratovolcanoes. There is also a general correlation between the intensity of plinian eruptions within eruptive cycles and the repose period prior to each cycle. This relationship may be related to a time-dependent process for the accumulation of differentiated and volatile-rich magma within the chamber beneath Mount St. Helens.

火山灰降下火砕物に挟まれている土壌について 浅間山天明テフラ下の土壌

Major activities of andesitic/dacitic volcanoes are investigated by the thickness and areal dispersal of airfall tephra. In the field work, we often observe blackish soil sandwiched between two fall units of major eruptions. It seems that such soil represents environment including volcanogeneous materials and florae during the repose period of major activities. At the flank of Asama Volcano, we find a soil layer sandwiched with the 1783 and the 1108 tephra deposits. The historical record of eruptive activities of Asama Volcano prior to the 20 th Century documented major eruptions in 1108, 1532, 1596, and 1783. However, we find only the thick tephra depoists of 1108 and 1783. The purpose of this study is to find any trace of eruptive ejecta in the sandwiched soil and to presume the scale of eruptive activities between the two major eruptive episodes.The sandwiched soils were sampled at 19 localities ranging from 3 to 35 km distance from the summit crater along the major axis of the 1783 tephra deposit. The main results are ; 1) the thickness of the soil is correlated with the thickness of the overlying 1783 tephra. This fact indicates that the main parent material was supplied from the summit crater of Asama Volcano by frequent eruptions during 675 year period. 2) At some soil profiles, four layers are distinguished. Upper two layers are black and blackish brown in color contrasted with the lower two layers of dark greyish yellow and dark olive brown. 3) Bulk density of the uppermost layer becomes small with increasing distance from the summit crater, which indicates the abundance of pumice increases with distance from the summit crater. 4) The volume ratio of pumice increases exponentially with the distance from the summit. 5) The 3 rd and the 4 th layers are much abundant in pumice compared with the 1 st and the 2 nd layers, which implies the difference of eruptive type between them. 6) The eruptive acitivities during 675 years after the 1108 eruption is suspected to be much violent than the activities after the 1783 eruption.Considering the historic records, it is suspected that the soil of the 3 rd and the 4 th layers could have been formed by the eruptive products during 1108-1532, mostly in 1532. After 64 year quiescence, awakening of violent eruptive activities started with more explosice one. It may be the reason why the 1 st and the 2 nd layers are abundant in lapilli than the lower layers.

Influence of magma composition on the eruptive activity of Mount St. Helens, Washington

The relation of eruptive intensity with magma composition and viscosity has been investigated for Mount St. Helens, Washington, where eruptive activity has ranged from basaltic lava flows to dacitic Plinian eruptions. The Plinian eruptions have varied in eruptive intensity from 10 6 to 10 8 kg/s, yet all erupted dacitic magma. These dacites, however, differ greatly in temperature, water content, and crystallinity, and thus magma viscosity varies by two orders of magnitude. The variation in viscosity is correlated inversely with intensity, demonstrating the control of composition on intensity. In addition, more mafic magmas erupted at lower intensities, showing that the wide range in eruptive behavior is linked to magma composition. Changes in composition result mainly from mixing of basaltic and dacitic magmas and occur in cycles. The rate of change during a cycle depends upon the length of the preceding repose period and the flux of basalt to the reservoir, because the supply of dacite has been relatively constant. When the flux of basalt is low, cycles progress from dacite to andesite over extended periods, whereas a higher flux leads to more rapid changes. Because composition controls eruptive intensity, the eruptive behavior of the volcano also varies through a cycle.