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1.
A. Costa F. Dell’Erba M. A. Di Vito R. Isaia G. Macedonio G. Orsi T. Pfeiffer 《Bulletin of Volcanology》2009,71(3):259-273
Tephra fallout associated with renewal of volcanism at the Campi Flegrei caldera is a serious threat to the Neapolitan area.
In order to assess the hazards related with tephra loading, we have considered three different eruption scenarios representative
of past activity: a high-magnitude event similar to the 4.1 ka Agnano-Monte Spina eruption, a medium-magnitude event, similar
to the ∼3.8 ka Astroni 6 eruption, and a low-magnitude event similar to the Averno 2 eruption. The fallout deposits were reconstructed
using the HAZMAP computational model, which is based on a semi-analytical solution of the two-dimensional advection–diffusion–sedimentation
equation for volcanic tephra. The input parameters into the model, such as total erupted mass, eruption column height, and
bulk grain-size and components distribution, were obtained by best-fitting field data. We carried out tens of thousands simulations
using a statistical set of wind profiles, obtained from NOAA re-analysis. Probability maps, relative to the considered scenarios,
were constructed for several tephra loads, such as 200, 300 and 400 kg/m2. These provide a hazard assessment for roof collapses due to tephra loading that can be used for risk mitigation plans in
the area. 相似文献
2.
Phil Shane 《Bulletin of Volcanology》1998,60(3):224-238
Grain-specific analyses of Fe–Ti oxides and estimates of eruption temperature (T) and oxygen fugacity (fO2) have been used to fingerprint rhyolitic fall and flow deposits that are important for tephrostratigraphic studies in and
around the Taupo volcanic zone of North Island, New Zealand. The analysed Fe–Ti oxides commonly occur in the rims of orthopyroxene
crystals and appear to reflect equilibrium immediately prior to eruption because of geochemical correlation with the co-existing
glass phase. The composition of the spinel phase is particularly diagnostic of eruptive centre for post-65 ka events and can
be used to distinguish many tephra beds from the same volcano. The 29 different units examined were erupted over a wide range
in T (690–990°C) and Δ log fO2 (–0.1 to 2.0). These parameters are closely related to the mafic mineral assemblage, with hydrous mineral-bearing units displaying
higher fO2. Such trends are superimposed on larger differences in fO2 that are related to eruptive centre. At any given temperature, all post-65 ka Okataina centre tephra have higher fO2 values than post-65 ka Taupo centre tephra. This provides a useful criterion for identifying the volcanic source. There are
no temporal T and fO2 trends in the tephra record; over intervals >20 ka, however, tephra sequences from Taupo centre form characteristic T-fO2 buffer trends mirroring the glass chemistry. Individual eruptive events display uniform spinel and rhombohedral phase compositions
and thus narrow ranges in T (± <20°C) and log fO2 (± <0.5), allowing these features to identify individual magma batches. These criteria can help distinguish tephra deposits
of similar bulk or glass composition that originated from the same volcano. Distal fall deposits record the same T-fO2 conditions as the proximal ignimbrite and enable distal–proximal correlation. Lateral and vertical compositional and T-fO2 variability displayed in large volume (>100 km3) ignimbrites, such as the Oruanui, Rotoiti and Ongatiti, is similar to that found in a single pumice clast and thus mainly
reflects analytical error; however, thermal gradients of ca. 50°C may occur in some units.
Received: 6 April 1998 / Accepted: 16 June 1998 相似文献
3.
Judy Fierstein 《Bulletin of Volcanology》2007,69(5):469-509
At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula
are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National
Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin
and from Shelikof Strait to Bristol Bay (∼8,500 km2). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes
area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka;
the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and
tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers
with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions
to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event
are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta
and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy
Mountain summit dome (<250 14C years B.P.). East Mageik has erupted most frequently during Holocene time with seven explosive events (9,400 to 2,400 14C years B.P.) preserved as tephra layers. Mount Martin erupted entirely during the Holocene, with lava coulees (>6 ka), two
tephras (∼3,700 and ∼2,700 14C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large
explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive
activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time
(mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash
has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 14C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite
ash from Kaguyak’s caldera-forming event (5,800 14C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 14C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500
14C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 14C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations
supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing
substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available. 相似文献
4.
Hetu C. Sheth Jyotiranjan S. Ray Rajneesh Bhutani Alok Kumar R. S. Smitha 《Bulletin of Volcanology》2009,71(9):1021-1039
Barren Island (India) is a relatively little studied, little known active volcano in the Andaman Sea, and the northernmost
active volcano of the great Indonesian arc. The volcano is built of prehistoric (possibly late Pleistocene) lava flows (dominantly
basalt and basaltic andesite, with minor andesite) intercalated with volcaniclastic deposits (tuff breccias, and ash beds
deposited by pyroclastic falls and surges), which are exposed along a roughly circular caldera wall. There are indications
of a complete phreatomagmatic tephra ring around the exposed base of the volcano. A polygenetic cinder cone has existed at
the centre of the caldera and produced basalt-basaltic andesite aa and blocky aa lava flows, as well as tephra, during historic
eruptions (1787–1832) and three recent eruptions (1991, 1994–95, 2005–06). The recent aa flows include a toothpaste aa flow,
with tilted and overturned crustal slabs carried atop an aa core, as well as locally developed tumuli-like elliptical uplifts
having corrugated crusts. Based on various evidence we infer that it belongs to either the 1991 or the 1994–95 eruptions.
The volcano has recently (2008) begun yet another eruption, so far only of tephra. We make significantly different interpretations
of several features of the volcano than previous workers. This study of the volcanology and eruptive styles of the Barren
Island volcano lays the ground for detailed geochemical-isotopic and petrogenetic work, and provides clues to what the volcano
can be expected to do in the future. 相似文献
5.
Records of Toba eruptions in the South China Sea 总被引:1,自引:0,他引:1
Three layers of volcanic tephra, sampled from ODP 1143 Site in the South China Sea, were observed at the mcd depth of 5.55
m, 42.66 m, and 48.25 m, and named, in this paper, layers of A, B, and C, respectively. All of these tephra layers have an
average thickness of ca. 2 cm. They were constrained in age of ca. 0.070 Ma, ca. 0.80 Ma, and ca. 1.00 Ma, respectively, by
the microbiostratigraphy data. These tephra layers were predominated by volcanic glass shards with a median grain size of
70–75 μm in diameter. Major chemical compositions analyzed by EMPA and comparison with the previous data from other scatter
areas suggest that these three layers of tephra can correspond to the three layers of Toba tephra, YTT, OTT, and HDT, respectively,
erupting during the Quaternary. The occurrence of these tephra layers in the South China Sea implies that the Toba eruptions
often occurred in the summer monsoon seasons of the South China Sea during the Quaternary, and that the strength of eruptions
was probably stronger than that previously estimated. 相似文献
6.
The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main
deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced
varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these
units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and
scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial
age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller
Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together
cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as
well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated
with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial
lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding
lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y.
Received: 24 June 1996 / Accepted: 6 December 1996 相似文献
7.
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
8.
Richard S. Fiske Katharine V. Cashman Atsushi Shibata Kazuki Watanabe 《Bulletin of Volcanology》1998,59(4):262-275
A new and detailed bathymetric map of the Myojinsho shallow submarine volcano provides a framework to interpret the physical
volcanology of its 1952–1953 eruption, especially how the silicic pyroclasts, both primary and reworked, enlarged the volcano
and were dispersed into the surrounding marine environment. Myojinsho, 420 km south of Tokyo along the Izu–Ogasawara arc,
was the site of approximately 1000 phreatomagmatic explosions during the 12.5-month eruption. These explosions shattered growing
dacite domes, producing dense clasts that immediately sank into the sea; minor amounts of pumice floated on the sea surface
after some of these events. The Myojinsho cone has slopes of almost precisely 21° in the depth range 300–700 m.We interpret
this to be the result of angle-of-repose deposition of submarine pyroclastic gravity flows that traveled downslope in all
directions. Many of these gravity flows resulted from explosions and associated dome collapse, but others were likely triggered
by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone. Tephra was repeatedly
carried into air in subaerial eruption columns and fell into the sea within 1–2 km of the volcano's summit, entering water
as deep as 400 m. Because the fall velocity of single particles decreased by a factor of ∼30 in passing from air into the
sea, we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra. Gravitational
instabilities within these tephra-choked regions could have formed vertical density currents that descended at velocities
greater than those of the individual particles they contained. Upon reaching the sea floor, many of these currents probably
continued to move downslope along Myojinsho's submarine slopes. Fine tephra was elutriated from the rubbly summit of the volcano
by upwelling plumes of heated seawater that persisted for the entire duration of the eruption. Ocean currents carried this
tephra to distal areas, where it presumably forms a pyroclastic component of deep-sea sediment.
Received: 5 December 1996 / Accepted: 17 September 1997 相似文献
9.
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
10.
Katla volcano,Iceland: magma composition,dynamics and eruption frequency as recorded by Holocene tephra layers 总被引:1,自引:0,他引:1
Bergrún Arna Óladóttir Olgeir Sigmarsson Gudrun Larsen Thor Thordarson 《Bulletin of Volcanology》2008,70(4):475-493
The Katla volcano in Iceland is characterized by subglacial explosive eruptions of Fe–Ti basalt composition. Although the
nature and products of historical Katla eruptions (i.e. over the last 1,100 years) at the volcano is well-documented, the
long term evolution of Katla’s volcanic activity and magma production is less well known. A study of the tephra stratigraphy
from a composite soil section to the east of the volcano has been undertaken with emphasis on the prehistoric deposits. The
section records ∼8,400 years of explosive activity at Katla volcano and includes 208 tephra layers of which 126 samples were
analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR)
derived from soil thicknesses between 14C-dated marker tephra layers. Temporal variations in major-element compositions of the basaltic tephra divide the ∼8,400-year
record into eight intervals with durations of 510–1,750 years. Concentrations of incompatible elements (e.g. K2O) in individual intervals reveal changes that are characterized as constant, irregular, and increasing. These variations
in incompatible elements correlate with changes in other major-element concentrations and suggest that the magmatic evolution
of the basalts beneath Katla is primarily controlled by fractional crystallisation. In addition, binary mixing between a basaltic
component and a silicic melt is inferred for several tephra layers of intermediate composition. Small to moderate eruptions
of silicic tephra (SILK) occur throughout the Holocene. However, these events do not appear to exhibit strong influence on
the magmatic evolution of the basalts. Nevertheless, peaks in the frequency of basaltic and silicic eruptions are contemporaneous.
The observed pattern of change in tephra composition within individual time intervals suggests different conditions in the
plumbing system beneath Katla volcano. At present, the cause of change of the magma plumbing system is not clear, but might
be related to eruptions of eight known Holocene lavas around the volcano. Two cycles are observed throughout the Holocene,
each involving three stages of plumbing system evolution. A cycle begins with an interval characterized by simple plumbing
system, as indicated by uniform major element compositions. This is followed by an interval of sill and dyke system, as depicted
by irregular temporal variations in major element compositions. This stage eventually leads to a formation of a magma chamber,
represented by an interval with increasing concentrations of incompatible elements with time. The eruption frequency within
the cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during
magma chamber stage. In accordance with this model, Katla volcano is at present in the first interval (i.e. simple plumbing
system) of the third cycle because the activity in historical time has been characterized by uniform magma composition and
relatively low eruption frequency. 相似文献
11.
Field, geochronologic, and geochemical evidence from proximal fine-grained tephras, and from limited exposures of Holocene
lava flows and a small pyroclastic flow document ten–12 eruptions of Mount Rainier over the last 2,600 years, contrasting
with previously published evidence for only 11–12 eruptions of the volcano for all of the Holocene. Except for the pumiceous
subplinian C event of 2,200 cal year BP, the late-Holocene eruptions were weakly explosive, involving lava effusions and at
least two block-and-ash pyroclastic flows. Eruptions were clustered from ∼2,600 to ∼2,200 cal year BP, an interval referred
to as the Summerland eruptive period that includes the youngest lava effusion from the volcano. Thin, fine-grained tephras
are the only known primary volcanic products from eruptions near 1,500 and 1,000 cal year BP, but these and earlier eruptions
were penecontemporaneous with far-traveled lahars, probably created from newly erupted materials melting snow and glacial
ice. The most recent magmatic eruption of Mount Rainier, documented geochemically, was the 1,000 cal year BP event. Products
from a proposed eruption of Mount Rainier between AD 1820 and 1854 (X tephra of Mullineaux (US Geol Surv Bull 1326:1–83, 1974))
are redeposited C tephra, probably transported onto young moraines by snow avalanches, and do not record a nineteenth century
eruption. We found no conclusive evidence for an eruption associated with the clay-rich Electron Mudflow of ∼500 cal year
BP, and though rare, non-eruptive collapse of unstable edifice flanks remains as a potential hazard from Mount Rainier.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
T. W. Sisson and J. W. Vallance contributed equally to this study. 相似文献
12.
I. V. Melekestsev E. V. Kartasheva T. P. Kirsanova A. A. Kuz’mina 《Journal of Volcanology and Seismology》2011,5(1):17-30
This study is the first to show, using data from the eruption of Koryakskii Volcano, Kamchatka that began in December 2008
and continued through 2009 that the water in permanent and temporary streams that start on the slopes of the volcanic cone
and in temporary lakes when contaminated with fresh tephra is a specific hazard factor related to long-continued hydrothermal-phreatic
eruptions on that volcano. This water is characterized by increased acidity (pH 4.1–4.35) and large amounts (up to 50–100
cm3/liter) of solid suspension and is unfit for drinking and irrigation. When combined with tephra, it probably produced mass
destruction of a number of animals who lived on the slopes and at the base of the volcano. The water contaminated with tephra
is an important component of the atmospheric mud flows occurring on Koryakskii Volcano; for several future years it will be
a potential source for enhancing the acidity of ground water in the volcanic edifice. 相似文献
13.
James D. L. White 《Bulletin of Volcanology》1996,58(4):249-262
The subaqueous phases of an eruption initiated approximately 85 m beneath the surface of Pleistocene Lake Bonneville produced
a broad mound of tephra. A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes
active prior to emergence of the volcano above lake level. At the base of the volcano and very near inferred vent sites are
fines-poor, well-bedded, broadly scoured beds of sideromelane tephra having local very low-angle cross-stratification (M1
lithofacies). These beds grade upward into lithofacies M3, which shows progressively better developed dunes and cross-stratification
upsection to its uppermost exposure approximately 10 m below syneruptive lake level. Both lithofacies were emplaced largely
by traction from relatively dilute sediment gravity flows generated during eruption. Intercalated lithofacies are weakly bedded
tuff and breccia (M2), and nearly structureless units with coarse basal layers above strongly erosional contacts (M4). The
former combines products of deposition from direct fall and moderate concentration sediment gravity flows, and the latter
from progressively aggrading high-concentration sediment gravity flows. Early in the eruption subaqueous tephra jetting from
phreatomagmatic explosions discontinuously fed inhomogeneous, unsteady, dilute density currents which produced the M1 lithofacies
near the vent. Dunes and crossbeds which are better developed upward in M3 resulted from interaction between sediment gravity
flows and surface waves triggered as the explosion-generated pressure waves and eruption jets impinged upon and occasionally
breached the surface. Intermingling of (a) tephra emplaced after brief transport by tephra jets within a gaseous milieu and
(b) laterally flowing tephra formed lithofacies M2 along vent margins during parts of the eruption in which episodes of continuous
uprush produced localized water-exclusion zones above a vent. M4 comprises mass flow deposits formed by disruption and remobilization
of mound tephra. Intermittent, explosive magma–water interactions occurred from the outset of the Pahvant eruption, with condensation,
entrainment of water and lateral flow marking the transformation from eruptive to "sedimentary" processes leading to deposition
of the mound lithofacies.
Received: 10 October 1995 / Accepted: 18 April 1996 相似文献
14.
Kenichiro Tani Richard S. Fiske Yoshihiko Tamura Yukari Kido Jiro Naka Hiroshi Shukuno Rika Takeuchi 《Bulletin of Volcanology》2008,70(5):547-562
Sumisu volcano was the site of an eruption during 30–60 ka that introduced ∼48–50 km3 of rhyolite tephra into the open-ocean environment at the front of the Izu-Bonin arc. The resulting caldera is 8 × 10 km
in diameter, has steep inner walls 550–780 m high, and a floor averaging 900 m below sea level. In the course of five research
cruises to the Sumisu area, a manned submersible, two ROVs, a Deep-Tow camera sled, and dredge samples were used to study
the caldera and surrounding areas. These studies were augmented by newly acquired single-channel seismic profiles and multi-beam
seafloor swath-mapping. Caldera-wall traverses show that pre-caldera eruptions built a complex of overlapping dacitic and
basaltic edifices, that eventually grew above sea level to form an island about 200 m high. The caldera-forming eruption began
on the island and probably produced a large eruption column. We interpret that prodigious rates of tephra fallback overwhelmed
the Sumisu area, forming huge rafts of floating pumice, choking the nearby water column with hyperconcentrations of slowly
settling tephra, and generating pyroclastic gravity currents of water-saturated pumice that traveled downslope along the sea
floor. Thick, compositionally similar pumice deposits encountered in ODP Leg 126 cores 70 km to the south could have been
deposited by these gravity currents. The caldera-rim, presently at ocean depths of 100–400 m, is mantled by an extensive layer
of coarse dense lithic clasts, but syn-caldera pumice deposits are only thin and locally preserved. The paucity of syn-caldera
pumice could be due to the combined effects of proximal non-deposition and later erosion by strong ocean currents. Post-caldera
edifice instability resulted in the collapse of a 15° sector of the eastern caldera rim and the formation of bathymetrically
conspicuous wavy slump structures that disturb much of the volcano’s surface. 相似文献
15.
On the significance of CO2 inclusions in plagioclase microphenocrysts in tholeiite from Moeraki,New Zealand 总被引:1,自引:0,他引:1
Liquid plus vapour inclusions of CO2 are widespread in plagioclase microphenocrysts in small tholeiitic intrusions and tephra of the Moeraki and adjacent areas
of northeast Otago, New Zealand. They imply the presence of immiscible CO2 droplets in the magma at depths of about 7–14 km. Their presence within 5 μm of the edges of microphenocrysts as little as
35 μm thick and 118 μm long indicates minimal feldspar crystal growth during the final ascent and quenching of the magma.
Delicate branching clusters of lath-like microphenocrysts escaped disruption during this ascent. Such CO2 inclusions are a potential source of ‘excess argon’ perturbing K-Ar age determinations.
Received March 24, 1993/Accepted September 10, 1993 相似文献
16.
Charles R. Stern 《Bulletin of Volcanology》2008,70(4):435-454
For regionally widespread Holocene tephra layers in southernmost Patagonia, correlations based on both chemical and chronological
data indicate their derivation from five large-volume (>1 km3) explosive eruptions of four different volcanoes in the southernmost Andes. Bulk-tephra and tephra-glass major and trace-element
chemistry and Sr isotopic ratios unambiguously distinguish different source volcanoes, and imply that two of the regionally
widespread tephra (MB1 and MB2) were derived from Mt. Burney (52°S), one (R1) from Reclus (51°S), one (A1) from Aguilera (50°S) and one (H1) from Hudson volcano (46°S). The H1 tephra derived from the Hudson volcano, which is located at the southern end of the Andean Southern Volcanic Zone (SVZ; 33–46°S),
contains distinctive greenish andesitic glass with FeO > 4.5 wt.% and TiO2 > 1.2 wt.%. In contrast, rhyolitic glass in tephra derived from the eruptions of Mt. Burney, Reclus and Aguilera volcanoes,
which are located in the Andean Austral Volcanic Zone (AVZ; 49–55°S), is clear and transparent and has significantly lower
FeO and TiO2. Tephra derived from these three AVZ volcanoes all contain plagioclase, orthopyroxene, minor clinopyroxene and amphibole.
Biotite occurs only in the Aguilera A1 tephra, which also has the highest bulk-tephra and tephra-glass K2O and Rb contents. Averages of new and published 14C ages determined on organic material in soil and sediment samples above and below these tephra constrain the uncalibrated
14C age of the R1 eruption of Reclus volcano to 12,685 ± 260 years BP, the MB1 and MB2 eruptions of Mt. Burney to 8,425 ± 500 and 3,830 ± 390 years BP, the Hudson H1 eruption to 6,850 ± 160 years BP, and the A1 eruption of Aguilera volcano to 3,000 ± 100 years BP. The volume of the largest of these eruptions, H1 of the Hudson volcano, is estimated as >18 km3. The volume of the Reclus R1 eruption is estimated at >10 km3, the Aguilera A1 eruption at between 4 and 9 km3, and the younger Mt. Burney MB2 eruption at ≥2.8 km3. The volume of the older MB1 Mt. Burney eruption is the least well constrained, but must have been larger than the younger MB2 eruption. The data indicate that the frequency of explosive activity of volcanic centers in the AVZ is lower than in the
southern SVZ. 相似文献
17.
Tanna island is part of a large volcanic complex mainly subsided below sea-level. On-land, two series of hydroclastic deposits
and ignimbrites overlie the subaerial remains of a basal, mainly effusive volcano. The ‘Older’ Tanna Ignimbrite series (OTI),
Late Pliocene or Pleistocene in age, consists of ash flows and ash- and scoria-flow deposits associated with fallout tephra
layers, overlain by indurated pumice-flow deposits. Phreatomagmatic features are a constant characteristic of these tuffs.
The ‘younger’ Late Pleistocene pyroclastics, the Siwi sequence, show basal phreatomagmatic deposits overlain by two successive
flow units, each comprising a densely welded layer and a nonwelded ash-flow deposit. Whole-rock analyses of 17 juvenile clasts
from the two sequences (vitric blocks from the phreatomagmatic deposits, welded blocks, scoriaceous bombs and pumices from
the ignimbrites) show basaltic andesite and andesite compositions (SiO2=53–60%). In addition, 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69% SiO2. Dominant compositions at ∼54, 56, 58.5 and 61–62% SiO2 characterize the glass from the OTI. Glass compositions in the lower – phreatomagmatic – deposits from the Siwi sequence
also show multimodal distribution, with peaks at SiO2=55, 57.5, 61–62 and 64% whereas the upper ignimbrite has a predominant composition at 61–62% SiO2. In both cases, mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic
signature of a voluminous layered chamber, the compositional gradient of which is the result of fractional crystallization.
During two major eruptive stages, probably related to two caldera collapses, the OTI and Siwi ignimbrites represent large
outpourings from these magmatic reservoirs. The successive eruptive dynamics, from phreatomagmatic to Plinian, emphasize the
role of water in initiating the eruptions, without which the mafic and intermediate magmas probably would not have erupted.
Received: February 19, 1993/Accepted October 10, 1993 相似文献
18.
Fabrizio Alfano Costanza Bonadonna Alain C. M. Volentik Charles B. Connor Sebastian F. L. Watt David M. Pyle Laura J. Connor 《Bulletin of Volcanology》2011,73(5):613-630
On May 1st 2008 Mount Chaitén (southern Chile) interrupted a long period of quiescence, generating a sequence of explosive
eruptions and causing the evacuation of Chaitén town located a few kilometers south of the volcano. The activity was characterized
by several explosive events each associated with plumes which reached up to about 19 km above sea level. The products were
dispersed across a wide area, with the finest ash reaching the Atlantic coast of Argentina. Our field observations in the
proximal-medial area (3–25 km from the vent) indicate that the May 2008 tephra deposit consists of numerous layers, most of
which can be correlated with individual eruptive events. These layers vary from extremely fine-grained ash to layers of lapilli
and blocks, composed of both juvenile and lithic material. Here we describe the stratigraphy and physical characteristics
of the May 2008 deposits, and propose a reconstruction of the timing of the May 2008 events. The deposits are mainly associated
with the three main explosive phases which occurred on 1st–2nd May, 3rd–5th May and 6th May, with an estimated bulk tephra
volume of 0.5–1.0 km3 (integration of both exponential and power-law fitting). For the 6th May event, represented by a layer composed mainly of
lithic lapilli and blocks (>2 mm), an isopleth map was compiled from which a 19 km plume height was determined, which is in
good agreement with satellite observations. 相似文献
19.
Non-welded, lithic-rich ignimbrites, hereintermed the Roque Nublo ignimbrites, are the most distinctive deposits of the Pliocene
Roque Nublo group, which forms the products of second magmatic cycle on Gran Canaria. They are very heterogeneous, with 35–55%
volume lithic fragments, 15-30% mildly vesiculated pumice, 5–7% crystals and 20–30% ash matrix. The vitric components (pumice
fragments and ash matrix) are largely altered and transformed into zeolites and subordinate smectites. The Roque Nublo ignimbrites
originated from hydrovolcanic eruptions that caused rapid and significant erosion of vents thus incorporating a high proportion
of lithic clasts into the eruption columns. These columns rapidly became too dense to be sustained as vertical eruption columns
and were transformed into tephra fountains which fed high-density pyroclastic flows. The deposits from these flows were mainly
confined to palaeovalleys and topographic depressions. In distal areas close to the coast line, where these palaeovalleys
widened, most of the pyroclastic flows expanded laterally and formed numerous thin flow units. The combined effect of the
magma–water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement
temperature of the Roque Nublo ignimbrites. This fact also explains the transition from pyroclastic flows into lahar deposits
observed in distal facies of the Roque Nublo ignimbrites. The existence of hydrovolcanic eruptions generating high-density
pyroclastic flows, unable to efficiently separate the water vapour from the vitric components during transport, also accounts
for the intense zeolitic alteration in these deposits.
Received: 5 November 1996 / Accepted: 3 March 1997 相似文献
20.
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