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1.
Sakurajima volcano: a physico-chemical study of the health consequences of long-term exposure to volcanic ash 总被引:1,自引:1,他引:0
S. E. Hillman C. J. Horwell A. L. Densmore D. E. Damby B. Fubini Y. Ishimine M. Tomatis 《Bulletin of Volcanology》2012,74(4):913-930
Regular eruptions from Sakurajima volcano, Japan, repeatedly cover local urban areas with volcanic ash. The frequency of exposure
of local populations to the ash led to substantial concerns about possible respiratory health hazards, resulting in many epidemiological
and toxicological studies being carried out in the 1980s. However, very few mineralogical data were available for determination
of whether the ash was sufficiently fine to present a respiratory hazard. In this study, we review the existing studies and
carry out mineralogical, geochemical and toxicological analyses to address whether the ash from Sakurajima has the potential
to cause respiratory health problems. The results show that the amount of respirable (<4 μm) material produced by the volcano
is highly variable in different eruptions (1.1–18.8 vol.%). The finest samples derive from historical, plinian eruptions but
considerable amounts of respirable material were also produced from the most recent vulcanian eruptive phase (since 1955).
The amount of cristobalite, a crystalline silica polymorph which has the potential to cause chronic respiratory diseases,
is ~3–5 wt.% in the bulk ash. Scanning electron microscope and transmission electron microscope imaging showed no fibrous
particles similar to asbestos particles. Surface reactivity tests showed that the ash did not produce significant amounts
of highly reactive hydroxyl radicals (0.09–1.35 μmol m−2 at 30 min.) in comparison to other volcanic ash types. A basic toxicology assay to assess the ability of ash to rupture the
membrane of red blood cells showed low propensity for haemolysis. The findings suggest that the potential health hazard of
the ash is low, but exposure and respiratory conditions should still be monitored given the high frequency and durations of
exposure. 相似文献
2.
Cristobalite in a rhyolitic lava dome: evolution of ash hazard 总被引:1,自引:1,他引:0
Claire J. Horwell Jennifer S. Le Blond Sabina A. K. Michnowicz Gordon Cressey 《Bulletin of Volcanology》2010,72(2):249-253
Prolonged and heavy exposure to particles of respirable, crystalline silica-rich volcanic ash could potentially cause chronic,
fibrotic disease, such as silicosis, in individuals living in areas of frequent ash fall. Here, we show that the rhyolitic
ash erupted from Chaitén volcano, Chile, in its dome-forming phase, contains increased levels of the silica polymorph cristobalite,
compared to its initial plinian eruption. Ash erupted during the initial, explosive phase (2–5 May 2008) contained approximately
2 wt.% cristobalite, whereas ash generated after dome growth began (from 21 May 2008) contains 13–19 wt.%. The work suggests
that active obsidian domes crystallise substantial quantities of cristobalite on time-scales of days to months, probably through
vapour-phase crystallisation on the walls of degassing pathways, rather than through spherulitic growth in glassy obsidian.
The ash is fine-grained (9.7–17.7 vol.% <4 μm in diameter, the respirable range) and the particles are mostly angular. Sparse,
fibre-like particles were confirmed to be feldspar or glass. 相似文献
3.
Nature and significance of small volume fall deposits at composite volcanoes: Insights from the October 14, 1974 Fuego eruption,Guatemala 总被引:3,自引:2,他引:1
W. I. Rose S. Self P. J. Murrow C. Bonadonna A. J. Durant G. G. J. Ernst 《Bulletin of Volcanology》2008,70(9):1043-1067
The first of four successive pulses of the 1974 explosive eruption of Fuego volcano, Guatemala, produced a small volume (∼0.02 km3 DRE) basaltic sub-plinian tephra fall and flow deposit. Samples collected within 48 h after deposition over much of the dispersal
area (7–80 km from the volcano) have been size analyzed down to 8 φ (4 μm). Tephra along the dispersal axis were all well-sorted
(σ
φ = 0.25–1.00), and sorting increased whereas thickness and median grain size decreased systematically downwind. Skewness varied
from slightly positive near the vent to slightly negative in distal regions and is consistent with decoupling between coarse
ejecta falling off the rising eruption column and fine ash falling off the windblown volcanic cloud advecting at the final
level of rise. Less dense, vesicular coarse particles form a log normal sub-population when separated from the smaller (Mdφ < 3φ or < 0.125 mm), denser shard and crystal sub-population. A unimodal, relatively coarse (Mdφ = 0.58φ or 0.7 mm σ
φ = 1.2) initial grain size population is estimated for the whole (fall and flow) deposit. Only a small part of the fine-grained,
thin 1974 Fuego tephra deposit has survived erosion to the present day. The initial October 14 pulse, with an estimated column
height of 15 km above sea level, was a primary cause of a detectable perturbation in the northern hemisphere stratospheric
aerosol layer in late 1974 to early 1975. Such small, sulfur-rich, explosive eruptions may substantially contribute to the
overall stratospheric sulfur budget, yet leave only transient deposits, which have little chance of survival even in the recent
geologic record. The fraction of finest particles (Mdφ = 4–8φ or 4–63 μm) in the Fuego tephra makes up a separate but minor size mode in the size distribution of samples around
the margin of the deposit. A previously undocumented bimodal–unimodal–bimodal change in grain size distribution across the
dispersal axis at 20 km downwind from the vent is best accounted for as the result of fallout dispersal of ash from a higher
subplinian column and a lower “co-pf” cloud resulting from pyroclastic flows. In addition, there is a degree of asymmetry
in the documented grain-size fallout pattern which is attributed to vertically veering wind direction and changing windspeeds,
especially across the tropopause. The distribution of fine particles (<8 μm diameter) in the tephra deposit is asymmetrical,
mainly along the N edge, with a small enrichment along the S edge. This pattern has hazard significance. 相似文献
4.
WANG Xiaorui GAO Shan LIU Xiaoming YUAN Honglin HU Zhaochu ZHANG Hong WANG Xuance 《中国科学D辑(英文版)》2006,49(9):904-914
The Yixian Formation at Sihetun in western Liao- ning Province has attracted considerable attention over the last two decades due to discovery of a wide range of well-preserved ‘feathered’ dinosaurs and primitive bird fossils[1―4]. This formation is dominated by vol- canic rocks, with fossil-bearing lacustrine sedimentary rocks at the upper part of the section[4]. The sedimen- tary rocks contain thin layers of tuff. According to previous studies[4], the total thickness of the Yixian Form… 相似文献
5.
Surface area, porosity and water adsorption properties of fine volcanic ash particles 总被引:1,自引:1,他引:0
Our understanding on how ash particles in volcanic plumes react with coexisting gases and aerosols is still rudimentary, despite the importance of these reactions in influencing the chemistry and dynamics of a plume. In this study, six samples of fine ash (<100 m) from different volcanoes were measured for their specific surface area, as, porosity and water adsorption properties with the aim to provide insights into the capacity of silicate ash particles to react with gases, including water vapour. To do so, we performed high-resolution nitrogen and water vapour adsorption/desorption experiments at 77 K and 303 K, respectively. The nitrogen data indicated as values in the range 1.1–2.1 m2/g, except in one case where a as of 10 m2/g was measured. This high value is attributed to incorporation of hydrothermal phases, such as clay minerals, in the ash surface composition. The data also revealed that the ash samples are essentially non-porous, or have a porosity dominated by macropores with widths >500 Å. All the specimens had similar pore size distributions, with a small peak centered around 50 Å. These findings suggest that fine ash particles have relatively undifferentiated surface textures, irrespective of the chemical composition and eruption type. Adsorption isotherms for water vapour revealed that the capacity of the ash samples for water adsorption is systematically larger than predicted from the nitrogen adsorption as values. Enhanced reactivity of the ash surface towards water may result from (i) hydration of bulk ash constituents; (ii) hydration of surface compounds; and/or (iii) hydroxylation of the surface of the ash. The later mechanism may lead to irreversible retention of water. Based on these experiments, we predict that volcanic ash is covered by a complete monolayer of water under ambient atmospheric conditions. In addition, capillary condensation within ash pores should allow for deposition of condensed water on to ash particles before water reaches saturation in the plume. The total mass of water vapour retained by 1 g of fine ash at 0.95 relative water vapour pressure is calculated to be ~10–2 g. Some volcanic implications of this study are discussed.Editorial responsibility: J. Gilbert 相似文献
6.
Samer K. Abi Kaed Bey Douglas P. Connelly François-Eric Legiret Andy J. K. Harris Matthew C. Mowlem 《Ocean Dynamics》2011,61(10):1555-1565
In this work, we describe a high-resolution fluorometric shipboard analyser and an improved method to determine NH4+ in oligotrophic seawater. The limit of detection is <5 nM, calculated with 95% confidence level using the weighted regression
line applied to the standard addition method using real samples prepared with low nutrient seawater from the Atlantic. The
results are summarised and cross-compared with spiked artificial seawater (ASW) and spiked Milli-Q water samples. The analyser
has a precision of ±1–4% with a high performance over a wide range from 5 nM to 25 μM. The methodology of NH4+ analysis is based on the fluorescent product formed between o-pthaldialdehyde and NH4+ in the presence of sulfite. Due to the high resolution of the developed system, we were able to study in depth the sensitivity
of the method to salinity, amines, amino acids and potential interferences from particles/algae. The method was found to be
sensitive to salinity variations, reducing the signal by up to 85% at 5 nM; this effect decreased at higher concentrations
of ammonium. It was noted that the interference from amines at low concentrations was negligible; however, at either high
amino acid or high amine concentrations, the signal was depressed. To test for the effect of particles on the system, the
system was tested with samples containing phytoplankton (Dunaliella primolecta) cells at different concentrations prepared with ASW to simulate the effect of a phytoplankton bloom. This experiment assessed
the potential impact of both particles and other potential fluorescence interferences from cells and/or ammonium leaching
from cells. This experiment showed that a phytoplankton bloom could potentially have an impact of up to 12% on the signal
of interest. Thus, we propose that this method is suitable for oligotrophic environments rather than coastal and eutrophic
environments. The reagent was found to be stable for 17 days and standards of 1 μM were stable for 6 days under laboratory
conditions. The developed analyser was successfully demonstrated in the North Atlantic Ocean, in an area of oligotrophic,
low NH4+ oceanic waters. 相似文献
7.
ZHANG HongFei XIAO Long ZHANG Li YUAN HongLin & JIN LanLan State Key Laboratory of Geological Processes Mineral Resources China University of Geosciences Wuhan China Faculty of Earth Sciences China University of Geosciences Wuhan China State Key Laboratory of Continental Dynamics Northwest University Xi’an China 《中国科学D辑(英文版)》2007,50(7):972-983
This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma),Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These granitoids are enriched in Al (Al2O3:14.56%―16.48%) and Sr (352 μg/g―1047 μg/g),and depleted in Y (<16 μg/g) and HREE (e.g. Yb<1.61 μg/g),resulting in high Sr/Y (36.3―150) and (La/Yb)N (7.8―36.3) ratios and strongly fractionationed REE patterns. The Indosinian granotoids show initial Sr isotopic ratios (ISr) from 0.70419 to 70752,εNd(t) values from-3.1 to -8.5,and initial Pb isotopic ratios 206Pb/204Pb=17.891-18.250,207Pb/204Pb=15.494-15.575,and 208Pb/204Pb=37.788-38.335. Their geochemi-cal signatures indicate that the granitoids are adakitic. However,they are distinct from some adakites,generated by partial melting of subducted oceanic slab and/or underplated basaltic lower crust,be-cause they have high K (K2O: 1.49%―3.84%) and evolved Nd isotopic compositions,with older Nd iso-topic model ages (TDM=1.06―1.83 Ga). Geochemical and Sr-Nd isotopic compositions suggest that the magmas of the Insoninian adakitic rocks in the Bikou block were derived from partial melting of thick-ened basaltic lower crust. Combined with regional analyses,a lithospheric delamination model after collision between the North China and South China plates can account for the Indosinian adakitic magma generation. On the other hand,based on the Pb-Sr-Nd isotopic probing to the magma sources of the adakitic rocks,it is suggested that there is an unexposed continent-type basement under the exposed Bikou Group volcanic rocks. This can constrain on the Bikou Group volcanic rocks not to be MORB-or OIB-type. 相似文献
8.
The Soufrière Hills Volcano, Montserrat, erupting since 18 July 1995, intensified its degassing in early 1996 with the continuing
growth of the lava dome inside the summit crater. During this period of increased activity, between 11 and 18 March 1996,
we measured gases and particles within the visible plume to determine whether at that time it posed a health risk to the population
of Plymouth, the capital town, which is 5 km southwest (downwind) and was then still occupied. Gravimetric measurements were
made of total suspended particles (TSP) and particles having an aerodynamic diameter of less than 10 μm (PM10). Measurements were made of sulphur dioxide (SO2), hydrochloric acid (HCl), hydrofluoric acid (HF), nitric acid (HNO3), acetic acid (CH3COOH), formic acid (HCOOH), and particulate sulphate (SO4
2–), chloride (Cl–), nitrate (NO3
–), fluoride (F–), methanesulphonate (CH3SO3
–), acetate (CH3COO–), formate (HCOO–), ammonium (NH4
+), sodium (Na+) and acidity (H+). Trace metals having human health implications [chromium (Cr), nickel (Ni), cobalt (Co), copper (Cu), zinc (Zn), arsenic
(As), selenium (Se), cadmium (Cd), tin (Sn), mercury (Hg) and lead (Pb)] were also determined. Mean concentrations of HCl,
SO2 and HF obtained in the town of Plymouth were 14.0, 5.9 and 0.8 ppbv, respectively. Corresponding concentrations in the mixed
plume on the crater edge were 533, 168 and 22 ppbv. There were no direct emissions of HNO3, although nitrate was detected in coarse particles at the source. Higher concentrations of CH3COOH and HCOOH were measured close to the crater. Mean TSP and PM10 were 64 and 15 μg m–3 in Plymouth, and 455 and 47 μg m–3 on the upper volcano slope. Aerosols were highly acidic at the source but rapidly neutralised during transport. Trace metals
were enriched in the aerosol relative to crater surface material. The concentrations of the acid gases, sulphur dioxide in
particular, and particles were found to be too small to pose a health hazard at the time of these measurements, when relatively
modest volcanic activity was occurring.
Received: 9 September 1998 / Accepted: 29 August 1999 相似文献
9.
Controls on magma permeability in the volcanic conduit during the climactic phase of the Kos Plateau Tuff eruption (Aegean Arc) 总被引:1,自引:1,他引:0
X-ray computed microtomography (μCT) was applied to pumices from the largest Quaternary explosive eruption of the active South
Aegean Arc (the Kos Plateau Tuff; KPT) in order to better understand magma permeability within volcanic conduits. Two different
types of pumices (one with highly elongated bubbles, tube pumice; and the other with near spherical bubbles, frothy pumice)
produced synchronously and with identical chemical composition were selected for μCT imaging to obtain porosity, tortuosity,
bubble size and throat size distributions. Tortuosity drops on average from 2.2 in frothy pumice to 1.5 in tube pumice. Bubble
size and throat size distributions provide estimates for mean bubble size (~93–98 μm) and mean throat size (~23–29 μm). Using
a modified Kozeny-Carman equation, variations in porosity, tortuosity, and throat size observed in KPT pumices explain the
spread found in laboratory measurements of the Darcian permeability. Measured difference in inertial permeability between tube and frothy pumices can also be partly explained by the same variables but require an additional
parameter related to the internal roughness of the porous medium (friction factor f
0
). Constitutive equations for both types of permeability allow the quantification of laminar and turbulent gas escape during
ascent of rhyolitic magma in volcanic conduits. 相似文献
10.
Causes and consequences of bimodal grain-size distribution of tephra fall deposited during the August 2006 Tungurahua eruption (Ecuador) 总被引:2,自引:2,他引:0
Julia Eychenne Jean-Luc Le Pennec Liliana Troncoso Mathieu Gouhier Jean-Marie Nedelec 《Bulletin of Volcanology》2012,74(1):187-205
The violent August 16–17, 2006 Tungurahua eruption in Ecuador witnessed the emplacement of numerous scoria flows and the deposition
of a widespread tephra layer west of the volcano. We assess the size of the eruption by determining a bulk tephra volume in
the range 42–57 × 106 m3, which supports a Volcanic Explosivity Index 3 event, consistent with calculated column height of 16–18 km above the vent
and making it the strongest eruptive phase since the volcano’s magmatic reactivation in 1999. Isopachs west of the volcano
are sub-bilobate in shape, while sieve and laser diffraction grain-size analyses of tephra samples reveal strongly bimodal
distributions. Based on a new grain-size deconvolution algorithm and extended sampling area, we propose here a mechanism to
account for the bimodal grain-size distribution. The deconvolution procedure allows us to identify two particle subpopulations
in the deposit with distinct characteristics that indicate dissimilar transport-depositional processes. The log-normal coarse-grained
subpopulation is typical of particles transported downwind by the main volcanic plume. The positively skewed, fine-grained
subpopulation in the tephra fall layer shares close similarities with the elutriated co-pyroclastic flow ash cloud layers
preserved on top of the scoria flow deposits. The area with the higher fine particle content in the tephra layer coincides
with the downwind prolongation of the pyroclastic flow deposits. These results indicate that the bimodal distribution of grain
size in the Tungurahua fall deposit results from synchronous deposition of lapilli from the main plume and fine ash elutriated
from scoria flows emplaced on the western flank of the volcano. Our study also reveals that inappropriate grain-size data
processing may produce misleading determination of eruptive type. 相似文献
11.
In April 2010, volcanic ash from the Eyjafjalla volcano in Iceland strongly impacted aviation in Europe. In order to prevent a similar scenario in the future, a threshold value for safe aviation based on actual mass concentrations was introduced (2 mg m−3 in Germany). This study contrasts microphysical and optical properties of volcanic ash and mineral dust and assesses the detectability of potentially dangerous ash layers (mass concentration larger than 2 mg m−3) from a pilot’s perspective during a flight. Also the possibility to distinguish between volcanic ash and other aerosols is investigated. The visual detectability of airborne volcanic ash is addressed based on idealized radiative transfer simulations and on airborne observations with the DLR Falcon gathered during the Eyjafjalla volcanic ash research flights in 2010 and during the Saharan Mineral Dust Experiments in 2006 and 2008. Mineral dust and volcanic ash aerosol both show an enhanced coarse mode (>1 μm) aerosol concentration, but volcanic ash aerosol additionally contains a significant number of Aitken mode particles (<150 nm) not present in mineral dust. Under daylight clear-sky conditions and depending on the viewing geometry, volcanic ash is visible already at mass concentrations far below what is currently considered dangerous for aircraft engines. However, it is not possible to visually distinguish volcanic ash from other aerosol layers or to determine whether a volcanic ash layer is potentially dangerous (mass concentration larger or smaller than 2 mg m−3). Different appearances due to microphysical differences of both aerosol types are not detectable by the human eye. Nonetheless, as ash concentrations can vary significantly over distances travelled by an airplane within seconds, this visual threat evaluation may contribute greatly to the short-term response of pilots in ash-contaminated air space. 相似文献
12.
13.
G. A. Pospelova L. V. Golovanova V. B. Dronochev V. A. Tselmovich 《Izvestiya Physics of the Solid Earth》2011,47(7):641-651
The results of magnetic and microprobe studies of the rock sequences in the Mezmaiskaya cave at the unique multilayer Paleolithic
site are discussed. The magnetic properties of rocks are analyzed for 17 layers (upwards from layer 4 to layer 1\1) dated
from over 73 ka ago to recent times. The rocks of layer 1C (Early Upper Paleolithic, ∼38 ka) are found to have the highest
magnetic susceptibility (K) (up to 2500 × 10−6 SI), which is related to the intensive activity of [Homo sapiens]. The minimum K corresponds to the rocks of layer 2, which is overlain by layer 1D. The sizes of magnetic grains vary throughout the rock
section. The largest grains are found in the middle part of the section in the Middle Paleolithic layers 2B3, 2B2, 2B1, 2A,
and 2. The superparamagnetic fraction is identified in all layers. This fact supports the view that the cave was open as early
as the formation of layer 4. According to the thermomagnetic data on the saturation magnetization and the temperature curves
of magnetic susceptibility, magnetite is the main carrier of the rock magnetization; some samples contain iron hydroxides.
Samples with iron sulfides (pyrite) are abundant. The study of the hysteresis parameters of rocks showed that the question
on whether sulfide-bearing rocks are suitable for reliable paleomagnetic determinations requires further laboratory research
into the origin of magnetite in the rocks. The chemical composition of rocks composing layer 2B3 and layers 1D (∼39 ka) and
2B1 (∼45 ka), in which the presence of volcanic ash has been previously established according to the presence of volcanic
glass, was determined by detailed microprobe analysis. A wide variety of chemical elements (up to 18 items) was recognized
in layers 1D and 2B1. The iron, titan, chrome, manganese content, and concentrations of other components vary from grain to
grain. The microprobe analysis of samples from layers 1D and 2B1 revealed a set of magnetic particles with compositions characteristic
of volcanic rocks, which supports the ash origin of these layers. Layer 2B3 is established not to be volcanic ash. The results
on the volcanic glass in the rocks of layers 1D and 2B1 were published by Golovanova and her colleagues in Current Anthropology in October 2010 [Golovanova et al., 2010]. 相似文献
14.
Abiotic effects of UV on planktonic P kinetics 总被引:1,自引:0,他引:1
Jeff M. Sereda Jeff J. Hudson William D. Taylor 《Aquatic Sciences - Research Across Boundaries》2009,71(2):127-134
15.
Gianfranco Di Vincenzo Laura Bracciali Paola Del Carlo Kurt Panter Sergio Rocchi 《Bulletin of Volcanology》2010,72(4):487-505
The AND-2A drillcore (Antarctic Drilling Program—ANDRILL) was successfully completed in late 2007 on the Antarctic continental
margin (Southern McMurdo Sound, Ross Sea) with the aim of tracking ice proximal to shallow marine environmental fluctuations
and to document the 20-Ma evolution of the Erebus Volcanic Province. Lava clasts and tephra layers from the AND-2A drillcore
were investigated from a petrographic and stratigraphic point of view and analyzed by the 40Ar–39Ar laser technique in order to constrain the age model of the core and to gain information on the style and nature of sediment
deposition in the Victoria Land Basin since Early Miocene. Ten out of 17 samples yielded statistically robust 40Ar–39Ar ages, indicating that the AND-2A drillcore recovered ≤230 m of Middle Miocene (∼128–358 m below sea floor, ∼11.5–16.0 Ma)
and >780 m of Early Miocene (∼358–1093 m below sea floor, ∼16.0–20.1 Ma). Results also highlight a nearly continuous stratigraphic
record from at least 358 m below sea floor down hole, characterized by a mean sedimentation rate of ∼19 cm/ka, possible oscillations
of no more than a few hundreds of ka and a break within ∼17.5–18.1 Ma. Comparison with available data from volcanic deposits
on land, suggests that volcanic rocks within the AND-2A core were supplied from the south, possibly with source areas closer
to the drill site for the upper core levels, and from 358 m below sea floor down hole, with the “proto-Mount Morning” as the
main source. 相似文献
16.
Models for the origin of accretionary lapilli 总被引:1,自引:0,他引:1
Binding between initially cohesionless ash particles to form concentric accretionary lapilli is provided primarily by the capillary forces of liquid bridges from condensed moisture and by electrostatic attraction. Capillary forces are strong bonds if the particles are in close contact, but they decrease rapidly with increasing particle spacing. Electrostatic attraction between charged ash particles is much weaker but effective over larger distances, increasing the frequency of collision between them.Experimental results of liquid film binding of volcanic ash showed that agglomeration was most successful between 15 and 25 wt.%, defining the agglomeration window for the formation of accretionary lapilli. Below 5–10 wt.% and above about 25–30 wt.% of water, concentric agglomeration was inhibited. Particles <350 m could be selected from a wider particle population in the experiments using only small amounts of water, which can explain the growth of accretionary lapilli in pyroclastic surges around agglomeration nuclei. Experiments testing the behavior of volcanic ash in electric fields showed that ash clusters formed instantaneously when the ash entered the field between a corona discharge gun and a grounded metal plate. The maximum grain size incorporated into the artificial clusters was about 180 m but >90 wt.% of ash was <45 m.Accretionary lapilli form in turbulent ash clouds when particles carrying liquid films of condensed moisture collide with each other and when the binding forces exceed the grain dispersive forces. Larger particles >500 m act as agglomeration nuclei in surges, accreting ash <350 m around them. In pyroclastic flows the aggregates are thought to originate from already size-sorted ash at the interface between the lower avalanche part of the flow and its overriding elutriation cloud. The fine-grained rims around accretionary lapilli found close to source are interpreted to be accreted dominantly by electrostatic attraction of very fine ash similar to clustering in elutriation clouds. 相似文献
17.
In this paper, we present a complete morphological characterization of the ash particles erupted on 18 December 2002 from
Etna volcano, Italy. The work is based on the acquisition and processing of bidimensional digital images carried out by scanning
electron microscopy (SEM) to obtain shape parameters by image analysis. We measure aspect ratio (AR), form factor (FF), compactness
(CC), and rectangularity (RT) of 2065 ash particles with size between 0.026 and 1.122 mm. We evaluate the variation of these
parameters as a function of the grain-size. Ash particles with a diameter of <0.125 mm vary from mostly equant to very equant,
ash particles between 0.125 and 0.250 mm have an intermediate shape, and particles with diameters >0.250 mm are subelongate.
We find that, on average, particles with a diameter of <0.250 mm are subrounded, particles between 0.250 and 0.50 mm are subangular,
and particles >0.50 mm are angular. Using this morphological analysis and an empirical relation between the drag coefficient
(C
D) and Reynolds number (R
e) of Wilson and Huang (Earth Planet Sci Lett 44:311–324, 1979), we calculate the terminal settling velocities (V
WH). The comparisons between these velocities and those calculated with the formula of Kunii and Levenspiel (Fluidization engineering. Wiley, New York, pp 97, 1969) (V
KL), which considers ash particles as spheres, show that V
KL are in average 1.28 greater than V
WH. Hence, we quantify the systematic error on the spatial distribution of the mass computed around the volcano carried out
by tephra dispersal models when the particles are assumed to be spherical. 相似文献
18.
Field-based studies of surficial volcanic deposits are commonly complicated by a combination of poor exposure and rapid lateral
variations controlled by unknown paleotopography. The potential of ground-penetrating radar (GPR) as an aid to volcanological
studies is shown using data collected from traverses over four well-exposed, Recent volcanic deposits in western Canada. The
deposits comprise a pumice airfall deposit (3–4 m thick), a basalt lava flow (3–6 m thick), a pyroclastic flow deposit (15 m
thick), and an internally stratified pumice talus deposit (60 m thick). Results show that GPR is effective in delineating
major stratigraphic contacts and hence can be used to map unexposed deposits. Different volcanic deposits also exhibit different
radar stratigraphic character, suggesting that deposit type may be determined from radar images. In addition, large blocks
within the pyroclastic deposits are detected as distinctive point diffractor patterns in the profiles, showing that the technique
has potential for providing important grain-size information in coarse poorly sorted deposits. Laboratory measurements of
dielectric constant (K') are reported for samples of the main rock types and are compared with values of K' for the bulk deposit as inferred from the field data. The laboratory values differ significantly from the "field" values
of K'; these results suggest that the effectiveness of GPR at any site can be substantially improved by initial calibration of
well-exposed locations.
Received: 10 May 1996 / Accepted: 27 December 1996 相似文献
19.
Strombolian explosive styles and source conditions: insights from thermal (FLIR) video 总被引:1,自引:3,他引:1
Matthew R. Patrick Andrew J. L. Harris Maurizio Ripepe Jonathan Dehn David A. Rothery Sonia Calvari 《Bulletin of Volcanology》2007,69(7):769-784
Forward Looking Infrared Radiometer (FLIR) cameras offer a unique view of explosive volcanism by providing an image of calibrated
temperatures. In this study, 344 eruptive events at Stromboli volcano, Italy, were imaged in 2001–2004 with a FLIR camera
operating at up to 30 Hz. The FLIR was effective at revealing both ash plumes and coarse ballistic scoria, and a wide range
of eruption styles was recorded. Eruptions at Stromboli can generally be classified into two groups: Type 1 eruptions, which
are dominated by coarse ballistic particles, and Type 2 eruptions, which consist of an optically-thick, ash-rich plume, with
(Type 2a) or without (Type 2b) large numbers of ballistic particles. Furthermore, Type 2a plumes exhibited gas thrust velocities
(>15 m s−1) while Type 2b plumes were limited to buoyant velocities (<15 m s−1) above the crater rim. A given vent would normally maintain a particular gross eruption style (Type 1 vs. 2) for days to
weeks, indicating stability of the uppermost conduit on these timescales. Velocities at the crater rim had a range of 3–101 m
s−1, with an overall mean value of 24 m s−1. Mean crater rim velocities by eruption style were: Type 1 = 34 m s−1, Type 2a = 31 m s−1, Type 2b = 7 m s−1. Eruption durations had a range of 6–41 s, with a mean of 15 s, similar among eruption styles. The ash in Type 2 eruptions
originates from either backfilled material (crater wall slumping or ejecta rollback) or rheological changes in the uppermost
magma column. Type 2a and 2b behaviors are shown to be a function of the overpressure of the bursting slug. In general, our
imaging data support a broadening of the current paradigm for strombolian behavior, incorporating an uppermost conduit that
can be more variable than is commonly considered. 相似文献
20.
Chris B. Folkes Shanaka L. de Silva Heather M. Wright Raymond A. F. Cas 《Bulletin of Volcanology》2011,73(10):1455-1486
By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components,
we show that the Cerro Galán volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr.
The Cerro Galán system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km3 (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO2). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides,
apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro
Galán ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite
contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide
and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially
hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice
is interpreted to represent the parental magmas to the Galán system emplaced into the upper crust from a deeper storage zone.
Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from
40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and
Yb between the Galán ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent
magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km3) and Real Grande Ignimbrite (∼390 km3) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km3), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute
this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like
magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume
ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galán
system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts.
A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance
of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly
identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma
source in a subduction zone volcanic setting. At Cerro Galán, we propose that there is a zone between mantle magma input and
upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures.
This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation,
storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve
only to slightly modify the geochemistry of the magmas. 相似文献