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1.
Daniela Mele Roberto Sulpizio Pierfrancesco Dellino Luigi La Volpe 《Bulletin of Volcanology》2011,73(3):257-278
New volcanological studies allow reconstruction of the eruption dynamics of the Pomici di Mercato eruption (ca 8,900 cal.
yr B.P.) of Somma-Vesuvius. Three main Eruptive Phases are distinguished based on two distinct erosion surfaces that interrupt
stratigraphic continuity of the deposits, indicating that time breaks occurred during the eruption. Absence of reworked volcaniclastic
deposits on top of the erosion surfaces suggests that quiescent periods between eruptive phases were short perhaps lasting
only days to weeks. Each of the Eruptive Phases was characterised by deposition of alternating fall and pyroclastic density
current (PDC) deposits. The fallout deposits blanketed a wide area toward the east, while the more restricted PDC deposits
inundated the volcano slopes. Eruptive dynamics were driven by brittle magmatic fragmentation of a phonolitic magma, which,
because of its mechanical fragility, produced a significant amount of fine ash. External water did not significantly contribute
either to fragmentation dynamics or to mechanical energy release during the eruption. Column heights were between 18 and 22 km,
corresponding to mass discharge rates between 1.4 and 6 × 107 kg s−1. The estimated on land volume of fall deposits ranges from a minimum of 2.3 km3 to a maximum of 7.4 km3. Calculation of physical parameters of the dilute pyroclastic density currents indicates speeds of a few tens of m s−1 and densities of a few kg m−3 (average of the lowermost 10 m of the currents), resulting in dynamic pressures lower than 3 kPa. These data suggest that
the potential impact of pyroclastic density currents of the Pomici di Mercato eruption was smaller than those of other Plinian
and sub-Plinian eruptions of Somma-Vesuvius, especially those of 1631 AD and 472 AD (4–14 kPa), which represent reference
values for the Vesuvian emergency plan. The pulsating and long-lasting behaviour of the Pomici di Mercato eruption is unique
in the history of large explosive eruptions of Somma-Vesuvius. We suggest an eruptive scheme in which discrete magma batches
rose from the magma chamber through a network of fractures. The injection and rise of the different magma batches was controlled
by the interplay between magma chamber overpressure and local stress. The intermittent discharge of magma during a large explosive
eruption is unusual for Somma-Vesuvius, as well as for other volcanoes worldwide, and yields new insights for improving our
knowledge of the dynamics of explosive eruptions. 相似文献
2.
The Middle Scoria deposit represents an explosive eruption of basaltic andesite magma (54 wt. % SiO2) from Okmok volcano during mid-Holocene time. The pattern of dispersal and characteristics of the ejecta indicate that the
eruption opened explosively, with ash textural evidence for a limited degree of phreatomagmatism. The second phase of the
eruption produced thick vesicular scoria deposits with grain texture, size and dispersal characteristics that indicate it
was violent strombolian to subplinian in style. The third eruptive phase produced deposits with a shift towards grain shapes
that are dense, blocky, and poorly vesicular, and intermittent surge layers, indicating later transitions between magmatic
(violent strombolian) to phreatomagmatic (vulcanian) eruptive styles. Isopach maps yield bulk volume estimates that range
from 0.06 to 0.43 km3, with ~ 0.04 to 0.25 km3 total DRE. The associated column heights and mass discharge values calculated from isopleth maps of individual Middle Scoria
layers are 8.5 – 14 km and 0.4 to 45 × 106 kg/s. The Middle Scoria tephras are enriched in plagioclase microlites that have the textural characteristics of rapid magma
ascent and relatively high degrees of effective undercooling. Those textures probably reflect the rapid magma ascent accompanying
the violent strombolian and subplinian phases of the eruption. In the later stages of the eruption, the plagioclase microlite
number densities decrease and textures include more tabular plagioclase, indicating a slowing of the ascent rate. The findings
on the Middle Scoria are consistent with other explosive mafic eruptions, and show that outside of the two large caldera-forming
eruptions, Okmok is also capable of producing violent mafic eruptions, marked by varying degrees of phreatomagmatism. 相似文献
3.
R. Cioni A. Bertagnini D. Andronico P. D. Cole F. Mundula 《Bulletin of Volcanology》2011,73(7):789-810
We describe the products of the hitherto poorly known 512 AD eruption at Vesuvius, Italy. The deposit records a complex sequence
of eruptive events, and it has been subdivided into eight main units, composed of stratified scoria lapilli or thin subordinate
ash-rich layers. All the units formed by deposition from tephra fallout, pyroclastic density currents of limited extent being
restricted to the initial stages of the eruption (U2). The main part of the deposit (U3 and U5) is characterized by a striking
grain size alternation of fine to coarse lapilli, similar to that often described for mid-intensity, explosive eruptions.
The erupted products have a phonotephritic composition, with progressively less evolved composition from the base to the top
of the stratigraphic sequence. Based on different dispersal, sedimentological and textural features of the products, we identify
five phases related to different eruptive styles: opening phase (U1, U2), subplinian phase (U3 to U5), pulsatory phreatomagmatic
phase (U6), violent strombolian phase (U7) and final ash-dominated phase (U8). A DRE volume of 0.025 km3 has been calculated for the total fallout deposit. Most of the magma was erupted during the subplinian phase; lithic dispersal
data indicate peak column heights of between 10 and 15 km, which correspond to a mass discharge rate (MDR) of 5 × 106 kg s−1. The lower intensity, violent strombolian phase coincided with the eruption of the least evolved magma; a peak column height
of 6–9 km, corresponding to an MDR of 1 ×10 6 kg s −1, is estimated from field data. Phreatomagmatic activity played a minor role in the eruption, only contributing to the ash-rich
deposits of U1, U4, U6 and U8. 相似文献
4.
G. Rolandi S. Maraffi P. Petrosino L. Lirer 《Journal of Volcanology and Geothermal Research》1993,58(1-4)
The Ottaviano eruption occurred in the late neolithic (8000 y B.P.). 2.40 km3 of phonolitic pyroclastic material (0.61 km3 DRE) were emplaced as pyroclastic flow, surge and fall deposits. The eruption began with a fall phase, with a model column height of 14 km, producing a pumice fall deposit (LA). This phase ended with short-lived weak explosive activity, giving rise to a fine-grained deposit (L1), passing to pumice fall deposits as the result of an increasing column height and mass discharge rate. The subsequent two fall phases (producing LB and LC deposits), had model column heights of 20 and 22 km with eruption rates of 2.5 × 107 and 2.81 × 107 kg/s, respectively. These phases ended with the deposition of ash layers (L2 and L3), related to a decreasing, pulsing explosive activity. The values of dynamic parameters calculated for the eruption classify it as a sub-plinian event. Each fall phase was characterized by variations in the eruptive intensity, and several pyroclastic flows were emplaced (F1 to F3). Alternating pumice and ash fall beds record the waning of the eruption. Finally, owing to the collapse of a eruptive column of low gas content, the last pyroclastic flow (F4) was emplaced. 相似文献
5.
Lucio Lirer Paola Petrosino Ines Alberico 《Journal of Volcanology and Geothermal Research》2001,112(1-4)
Volcanological analysis of the 10 000 yr
–1538
explosive activity at Campi Flegrei shows that the most common explosive eruptions are characterized by the emplacement of flow or surge deposits, originating from the interaction between magma and shallow and/or sea water. The minimum volumes of pyroclastic products range between 0.04 and 0.7 km3; the proximal areas covered by these products range from 3–4 to 40–50 km2. The pyroclastic flow and surge deposits occurring inside the caldera have been strongly controlled by pre-existent morphology; because of this, the area of present Napoli city was blanketed by approximately 5 m of pyroclastic deposits, during the last 5000 yr.Previous analysis suggests that the presence of even very low topographic obstacles may influence pyroclastic density current run out such that future eruptive deposits would mainly be confined inside the caldera rim. We suggest that a future eruption at Campi Flegrei would not seriously involve the urbanized area of Napoli city located on the hills. On the contrary, the plains located on the eastern side of the caldera (Fuorigrotta, Bagnoli) would be the most damaged area. 相似文献
6.
Gianluca Sottili Danilo M. Palladino Mario Gaeta Matteo Masotta 《Bulletin of Volcanology》2012,74(1):163-186
Maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water.
Here, we provide information on a number of maars in the ultrapotassic Sabatini Volcanic District (SVD, Roman Province) as
young as ∼90 ka. The SVD maars are characterised in terms of crater and ejecta ring morphologies, eruptive successions and
magma compositions, in light of the local substrate settings, with the aim of assessing magma–water interaction conditions,
eruption energetics and genetic mechanisms. Feeder magmas spanned the whole SVD differentiation trend from trachybasalts–shoshonites
to phonolites. From the ejected lithic fragments from aquifer rocks, the range of depth of magma–water explosive interaction
is estimated to have been mostly at ∼400–600 m below ground level, with a single occurrence of surficial interaction in palustrine–lacustrine
environment. In particular, the interaction with external water may have triggered the explosive behaviour of poorly differentiated
magmas, whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for
the most differentiated magma batches during low-flux ascent in an incipiently fragmented state. Crater sizes, ejecta volumes
and ballistic data allow a reconstruction of the energy budget of SVD maar-forming eruptions. Erupted tephra volumes from
either monogenetic or polygenetic maars ranged 0.004–0.07 km3 during individual maar-forming eruptions, with corresponding total magma thermal energies of 8 × 1015–4 × 1017 J. Based on energy partitioning and volume balance of erupted magmas and lithic fractions vs. crater holes, we consider the
different contributions of explosive excavation of the substrate vs. subsidence in forming the SVD maar craters. Following
available models based on crater sizes, highly variable fractions (5–50%) of the magma thermal energies would have been required
for crater excavation. It appears that subsidence may have played a major role in some SVD maars characterised by low lithic
contents, whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation. 相似文献
7.
Magma supply, magma discharge and readjustment of the feeding system of mount St. Helens during 1980
The landslide and cataclysmic eruption of Mount St. Helens on May 18, 1980 triggered a sequence of explosive eruptions over the following five months. The volume of explosive products from each of these eruptions decreased uniformly over this period, and the character for each eruption progressed from a fairly continuous eruption lasting more than eight hours on May 18 to a series of short bursts, some of which were spaced 12 hours apart, on October 16–18. The transition in the character of these eruption sequences can be explained by a difference between the magma supply rate and the magma discharge rate from a shallow reservoir.The magma supply rate (MSR) is the rate with which magma is supplied to the level where disruption due to vesiculation occurs. It is determined by dividing the dense-rock-equivalent volume of eruptive products by the total duration of each eruption sequence. The magma discharge rate (MDR) is the rate with which the disrupted magma is discharged through the vent. It is determined by dividing the volume of erupted products by the duration of each explosive burst. The relative magnitude of these two quantities controls the temporal evolution of an explosive event. When MDRMSR the explosive phase of the eruption lasts for several hours as a single continuous event. When MDR>MSR, an eruption is characterized by a series of short explosive bursts at intervals of several minutes to several days. The MSR of the eruptions of 1980 decreased with time from 5500 m' s−1 on May 18 to 7 m3 s−2 on October 16–18 and approximately fits an exponential decay. The MDR for the same events remained approximately constant at 2000 m3 s−1. Each explosive event has been followed by an aftershock-like series of earthquakes located beneath the volcano at depth mostly between 7 and 14 km. The seismic energy released during each of these series is proportional to the corresponding volume of erupted magma. Deformation data between June and November, 1980 indicate a subsidence of the volcanic structure which can be modeled by a volume collapse of 0.25 km3 located at 9 km depth.We propose a model in which magma is supplied from depths of 7–14 km through a narrow conduit during each eruption. It erupts to the surface at a uniform rate during each eruption. The deep seismic activity following each eruption is related to a readjustment and volume decrease in the deep feeding system. The decrease of the MSR over time is explained by an increase in the viscosity of a progressively water-depleted magma. The amount of water necessary to explain the observed decrease of the MSR is of the order of 4.6%. 相似文献
8.
O.A. Braitseva I.V. Melekestsev V.V. Ponomareva V.Yu. Kirianov 《Journal of Volcanology and Geothermal Research》1996,70(1-2)
The largest Plinian eruption of our era and the latest caldera-forming eruption in the Kuril-Kamchatka region occurred about cal. A.D. 240 from the Ksudach volcano. This catastrophic explosive eruption was similar in type and characteristics to the 1883 Krakatau event. The volume of material ejected was 18–19 km3 (8 km3 DRE), including 15 km3 of tephra fall and 3–4 km3 of pyroclastic flows. The estimated height of eruptive column is 22–30 km. A collapse caldera resulting from this eruption was 4 × 6.5 km in size with a cavity volume of 6.5–7 km3. Tephra fall was deposited to the north of the volcano and reached more than 1000 km. Pyroclastic flows accompanied by ash-cloud pyroclastic surges extended out to 20 km. The eruption was initially phreatomagmatic and then became rhythmic, with each pulse evolving from pumice falls to pyroclastic flows. Erupted products were dominantly rhyodacite throughout the eruption. During the post-caldera stage, when the Shtyubel cone started to form within the caldera, basaltic-andesite and andesite magma began to effuse. The trigger for the eruption may have been an intrusion of mafic magma into the rhyodacite reservoir. The eruption had substantial environmental impact and may have produced a large acidity peak in the Greenland ice sheet. 相似文献
9.
Holocene explosive activity of Hudson Volcano, southern Andes 总被引:3,自引:1,他引:2
Fallout deposits in the vicinity of the southern Andean Hudson Volcano record at least 12 explosive Holocene eruptions, including
that of August 1991 which produced ≥4 km3 of pyroclastic material. Medial isopachs of compacted fallout deposits for two of the prehistoric Hudson eruptions, dated
at approximately 3600 and 6700 BP, enclose areas at least twice that of equivalent isopachs for both the 1991 Hudson and the
1932 Quizapu eruptions, the two largest in the Andes this century. However, lack of information for either the proximal or
distal tephra deposits from these two prehistoric eruptions of Hudson precludes accurate volume estimates. Andesitic pyroclastic
material produced by the 6700-BP event, including a 1 10-cm-thick layer of compacted tephra that constitutes a secondary
thickness maximum over 900 km to the south in Tierra del Fuego, was dispersed in a more southerly direction than that of the
1991 Hudson eruption. The products of the 6700-BP event consist of a large proportion of fine pumiceous ash and accretionary
lapilli, indicating a violent phreatomagmatic eruption. This eruption, which is considered to be the largest for Hudson and
possibly for any volcano in the southern Andes during the Holocene, may have created Hudson's 10-km-diameter summit caldera,
but the age of the caldera has not been dated independently.
Received: 31 January 1997 / Accepted: 29 October 1997 相似文献
10.
Susan L. Donoghue Alan S. Palmer Elizabeth McClelland Kate Hobson Robert B. Stewart Vincent E. Neall Jèrôme Lecointre Richard Price 《Bulletin of Volcanology》1999,61(4):223-240
The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km3 of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this
volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone,
where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped
into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic
flow deposits). These eruptions identify a brief (<ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define
as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history
and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year
period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered
by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring
plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation
of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal
welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously
considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important
new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre.
Received: 5 July 1998 / Accepted: 12 March 1999 相似文献
11.
Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 105 km2 with a volume of 13 km3 (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km3 of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash.Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km3 of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment. 相似文献
12.
Claude Robin Pablo Samaniego Jean-Luc Le Pennec Michel Fornari Patricia Mothes Johannes van der Plicht 《Bulletin of Volcanology》2010,72(9):1109-1129
Fieldwork, radiometric (40Ar/39Ar and 14C) ages and whole-rock geochemistry allow a reconstruction of eruptive stages at the active, mainly dacitic, Pichincha Volcanic
Complex (PVC), whose eruptions have repeatedly threatened Quito, most recently from 1999 to 2001. After the emplacement of
basal lavas dated at ∼1100 to 900 ka, the eruptive activity of the old Rucu Pichincha volcano lasted from ∼850 ka to ∼150 ka
before present (BP) and resulted in a 15 × 20 km-wide edifice, which comprises three main building stages: (1) A lower stratocone
(Lower Rucu, ∼160 km3 in volume) developed from ∼850 to 600 ka; (2) This edifice was capped by a steeper-sided and less voluminous cone (the Upper
Rucu, 40–50 km3), the history of which started 450–430 ka ago and ended around 250 ka with a sector collapse; (3) A smaller (8–10 km3) but more explosive edifice grew in the avalanche amphitheatre and ended Rucu Pichincha's history about 150 ka ago. The Guagua
Pichincha volcano (GGP) was developed from 60 ka on the western flank of Rucu with four growth stages separated by major catastrophic
events. (1) From ∼60 to 47 ka, a basal effusive stratocone developed, terminating with a large ash-and-pumice flow event.
(2) This basal volcano was followed by a long-lasting dome building stage and related explosive episodes, the latter occurring
between 28–30 and 22–23 ka. These first two stages formed the main GGP (∼30 km3), a large part of which was removed by a major collapse 11 ka BP. (3) Sustained explosive activity and viscous lava extrusions
gave rise to a new edifice, Toaza (4–5 km3 in volume), which in turn collapsed around 4 ka BP. (4) The ensuing amphitheatre was partly filled by the ∼1-km3 Cristal dome, which is the historically active centre of the Pichincha complex. The average output rate for the whole PVC
is 0.29 km3/ka. Nevertheless, the chronostratigraphic resolution we obtained for Lower Rucu Pichincha and for the two main edifices of
Guagua Pichincha (main GGP and Toaza), leads to eruptive rates of 0.60–0.65 km3/ka during these construction stages. These output rates are compared to those of other mainly dacitic volcanoes from continental
arcs. Our study also supports an overall SiO2 and large-ion lithophile elements enrichment as the PVC develops. In particular, distinctive geochemical signatures indicate
the involvement of a new magma batch at the transition between Rucu and Guagua. At the GGP, the same phenomenon occurs at
each major collapse event marking the onset of the ensuing magmatic stage. Since the 11-ka-BP collapse event, this magmatic
behaviour has led to increasingly explosive activity. Four explosive cycles of between 100 and 200 years long have taken place
at the Cristal dome in the past 3.7 ka, and repose intervals between these cycles have tended to decrease with time. As a
consequence, we suggest that the 1999–2001 eruptive period may have initiated a new eruptive cycle that might pose a future
hazard to Quito (∼2 million inhabitants). 相似文献
13.
The Pebble Creek Formation (previously known as the Bridge River Assemblage) comprises the eruptive products of a 2350 calendar
year B.P. eruption of the Mount Meager volcanic complex and two rock avalanche deposits. Volcanic rocks of the Pebble Creek
Formation are the youngest known volcanic rocks of this complex. They are dacitic in composition and contain phenocrysts of
plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. The eruption was episodic, and the
formation comprises fallout pumice (Bridge River tephra), pyroclastic flows, lahars and a lava flow. It also includes a unique
form of welded block and ash breccia derived from collapsing fronts of the lava flow. This Merapi-type breccia dammed the
Lillooet River. Collapse of the dam triggered a flood that flowed down the Lillooet Valley. The flood had an estimated total
volume of 109 m3 and inundated the Lillooet Valley to a depth of at least 30 m above the paleo-valley floor 5.5 km downstream of the blockage.
Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks (an older formation making up part of the Mount
Meager volcanic complex) underlie and overlie the primary volcanic units of the Formation. Both rock avalanches are unrelated
to the 2350 B.P. eruption, although the post-eruption avalanche may have its origins in the over-steepened slopes created
by the explosive phase of the eruption. Much of the stratigraphic complexity evident in the Pebble Creek Formation results
from deposition in a narrow, steep-sided mountain valley containing a major river.
Received: 20 January 1998 / Accepted: 29 September 1998 相似文献
14.
N. A. Malik 《Journal of Volcanology and Seismology》2011,5(4):268-277
An explosive eruption occurred on Bezymyannyi Volcano December 24, 2006. The distribution of ashfall deposits over the peninsula
was studied. The tephra was investigated for its chemical, mineral, and grain-size composition, its water-soluble complex
was studied. About 30 000 tons of water-soluble substances along with 7 million tons of ash entered the environment over an
area of over 8000 km2. Information is supplied for this eruption from various sources and its geological impact has been estimated. The total volume
of erupted pyroclastics was 0.01–0.014 km3, of which 0.004 km3 consisted of ash and 0.006–0.01 km3 were pyroclastic flow deposits. 相似文献
15.
Ármann Höskuldsson Níels Óskarsson Rikke Pedersen Karl Grönvold Kristín Vogfjörð Rósa Ólafsdóttir 《Bulletin of Volcanology》2007,70(2):169-182
The 18th historic eruption of Hekla started on 26 February, 2000. It was a short-lived but intense event, emitting basaltic
andesitic (55.5 wt% SiO2) pyroclastic fragments and lava. During the course of the eruption, monitoring was done by both instruments and direct observations,
together providing unique insight into the current activity of Hekla. During the 12-day eruption, a total of 0.189 km3 DRE of magma was emitted. The eruptive fissure split into five segments. The segments at the highest altitude were active
during the first hours, while the segments at lower altitude continued throughout the eruption. The eruption started in a
highly explosive manner giving rise to a Subplinian eruptive column and consequent basaltic pyroclastic flows fed by column
collapses. After the explosive phase reached its maximum, the eruption went through three more phases, namely fire-fountaining,
Strombolian bursts and lava effusion. In this paper, we describe the course of events of the eruption of Hekla and the origin
of its magma, and then show that the discharge rate can be linked to different style of eruptive activity, which are controlled
by fissure geometry. We also show that the eruption phases observed at Hekla can be linked with inferred magma chamber overpressure
prior to the eruption. 相似文献
16.
C. Silva Parejas T. H. Druitt C. Robin H. Moreno J.-A. Naranjo 《Bulletin of Volcanology》2010,72(6):677-692
The Pucón eruption was the largest Holocene explosive outburst of Volcán Villarrica, Chile. It discharged >1.0 km3 of basaltic-andesite magma and >0.8 km3 of pre-existing rock, forming a thin scoria-fall deposit overlain by voluminous ignimbrite intercalated with pyroclastic
surge beds. The deposits are up to 70 m thick and are preserved up to 21 km from the present-day summit, post-eruptive lahar
deposits extending farther. Two ignimbrite units are distinguished: a lower one (P1) in which all accidental lithic clasts
are of volcanic origin and an upper unit (P2) in which basement granitoids also occur, both as free clasts and as xenoliths
in scoria. P2 accounts for ∼80% of the erupted products. Following the initial scoria fallout phase, P1 pyroclastic flows
swept down the northern and western flanks of the volcano, magma fragmentation during this phase being confined to within
the volcanic edifice. Following a pause of at least a couple of days sufficient for wood devolatilization, eruption recommenced,
the fragmentation level dropped to within the granitoid basement, and the pyroclastic flows of P2 were erupted. The first
P2 flow had a highly turbulent front, laid down ignimbrite with large-scale cross-stratification and regressive bedforms,
and sheared the ground; flow then waned and became confined to the southeastern flank. Following emplacement of pyroclastic
surge deposits all across the volcano, the eruption terminated with pyroclastic flows down the northern flank. Multiple lahars
were generated prior to the onset of a new eruptive cycle. Charcoal samples yield a probable eruption age of 3,510 ± 60 14C years BP. 相似文献
17.
David A. Clague Jonathan T. Hagstrum Duane E. Champion Melvin H. Beeson 《Bulletin of Volcanology》1999,61(6):363-381
The tube-fed pāhoehoe lava flows covering much of the northeast flank of Kīlauea Volcano are named the 'Ailā'au flows. Their
eruption age, based on published and six new radiocarbon dates, is approximately AD 1445. The flows have distinctive paleomagnetic
directions with steep inclinations (40°–50°) and easterly declinations (0°–10°E). The lava was transported ∼40 km from the
vent to the coast in long, large-diameter lava tubes; the longest tube (Kazumura Cave) reaches from near the summit to within
several kilometers of the coast near Kaloli Point. The estimated volume of the 'Ailā'au flow field is 5.2±0.8 km3, and the eruption that formed it probably lasted for approximately 50 years. Summit overflows from Kīlauea may have been
nearly continuous between approximately AD 1290 and 1470, during which time a series of shields formed at and around the summit.
The 'Ailā'au shield was either the youngest or the next to youngest in this series of shields. Site-mean paleomagnetic directions
for lava flows underlying the 'Ailā'au flows form only six groups. These older pāhoehoe flows range in age from 2750 to <18,000
BP, and the region was inundated by lava flows only three times in the past 5000 years. The known intervals between eruptive
events average ∼1600 years and range from ∼1250 years to >2200 years. Lava flows from most of these summit eruptions also
reached the coast, but none appears as extensive as the 'Ailā'au flow field. The chemistry of the melts erupted during each
of these summit overflow events is remarkably similar, averaging approximately 6.3 wt.% MgO near the coast and 6.8 wt.% MgO
near the summit. The present-day caldera probably formed more recently than the eruption that formed the 'Ailā'au flows (estimated
termination ca. AD 1470). The earliest explosive eruptions that formed the Keanakāko'i Ash, which is stratigraphically above
the 'Ailā'au flows, cannot be older than this age.
Received: 10 October 1998 / Accepted: 12 May 1999 相似文献
18.
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
19.
Degassing and microlite crystallization during pre-climactic events of the 1991 eruption of Mt. Pinatubo, Philippines 总被引:2,自引:2,他引:0
Dacite tephras produced by the 1991 pre-climactic eruptive sequence at Mt. Pinatubo display extreme heterogeneity in vesicularity,
ranging in clast density from 700 to 2580 kg m–3. Observations of the 13 surge-producing blasts that preceded the climactic plinian event include radar-defined estimates
of column heights and seismically defined eruptive and intra-eruptive durations. A comparison of the characteristics of erupted
material, including microlite textures, chemical compositions, and H2O contents, with eruptive parameters suggests that devolatilization-induced crystallization of the magma occurred to a varying
extent prior to at least nine of the explosive events. Although volatile loss progressed to the same approximate level in
all of the clasts analyzed (weight percent H2O=1.26-1.73), microlite crystallization was extremely variable (0–22%). We infer that syn-eruptive volatile exsolution from
magma in the conduit and intra-eruptive separation of the gas phase was facilitated by the development of permeability within
magma residing in the conduit. Correlation of maximum microlite crystallinity with repose interval duration (28–262 min) suggests
that crystallization occurred primarily intra-eruptively, in response to the reduction in dissolved H2O content that occurred during the preceding event. Detailed textural characterization, including determination of three-dimensional
shapes and crystal size distributions (CSD), was conducted on a subset of clasts in order to determine rates of crystal nucleation
and growth using repose interval as the time available for crystallization. Shape and size analysis suggests that crystallization
proceeded in response to lessening degrees of feldspar supersaturation as repose interval durations increased. We thus propose
that during repose intervals, a plug of highly viscous magma formed due to the collapse of vesicular magma that had exsolved
volatiles during the previous explosive event. If plug thickness grew proportionally to the square root of time, and if magma
pressurization increased during the eruptive sequence, the frequency of eruptive pulses may have been modulated by degassing
of magma within the conduit. Dense clasts in surge deposits probably represent plug material entrained by each subsequent
explosive event.
Received: 4 December 1997 / Accepted: 13 September 1998 相似文献
20.
R. Sulpizio R. Cioni M. A. Di Vito D. Mele R. Bonasia P. Dellino 《Bulletin of Volcanology》2010,72(5):539-558
The stratigraphic succession of the Pomici di Avellino Plinian eruption from Somma-Vesuvius has been studied through field
and laboratory data in order to reconstruct the eruption dynamics. This eruption is particularly important in the Somma-Vesuvius
eruptive history because (1) its vent was offset with respect to the present day Vesuvius cone; (2) it was characterised by
a distinct opening phase; (3) breccia-like very proximal fall deposits are preserved close to the vent and (4) the pyroclastic
density currents generated during the final phreatomagmatic phase are among the most widespread and voluminous in the entire
history of the volcano. The stratigraphic succession is, here, divided into deposits of three main eruptive phases (opening,
magmatic Plinian and phreatomagmatic), which contain five eruption units. Short-lived sustained columns occurred twice during
the opening phase (Ht of 13 and 21.5 km, respectively) and dispersed thin fall deposits and small pyroclastic density currents onto the volcano
slopes. The magmatic Plinian phase produced the main volume of erupted deposits, emplacing white and grey fall deposits which
were dispersed to the northeast. Peak column heights reached 23 and 31 km during the withdrawal of the white and the grey
magmas, respectively. Only one small pyroclastic density current was emplaced during the main Plinian phase. In contrast,
the final phreatomagmatic phase was characterised by extensive generation of pyroclastic density currents, with fallout deposits
very subordinate and limited to the volcano slopes. Assessed bulk erupted volumes are 21 × 106 m3 for the opening phase, 1.3–1.5 km3 for the main Plinian phase and about 1 km3 for the final phreatomagmatic phase, yielding a total volume of about 2.5 km3. Pumice fragments are porphyritic with sanidine and clinopyroxene as the main mineral phases but also contain peculiar mineral
phases like scapolite, nepheline and garnet. Bulk composition varies from phonolite (white magma) to tephri-phonolite (grey
magma). 相似文献