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1.
Formation of high-grade ignimbrites Part II. A pyroclastic suspension current model with implications also for low-grade ignimbrites 总被引:2,自引:2,他引:0
Armin Freundt 《Bulletin of Volcanology》1999,60(7):545-567
Analogue experiments in part I led to the conclusion that pyroclastic flows depositing very high-grade ignimbrite move as
dilute suspension currents. In the thermo–fluid–dynamical model developed, the degree of cooling of expanded turbulent pyroclastic
flows dynamically evolves in response to entrainment of air and mass loss to sedimentation. Initial conditions of the currents
are derived from column-collapse modeling for magmas with an initial H2O content of 1–3 wt.% erupting through circular vents and caldera ring-fissures. The flows spread either longitudinally or
radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content, temperature,
bottom slope and coarser initial grain size. Progressive dilution by entrainment and sedimentation causes pyroclastic currents
to transform into buoyant ash plumes at the runout distance. The ash plumes reach stratospheric heights and distribute 30–80%
of the erupted material as widespread co-ignimbrite ash. Pyroclastic suspension currents with initial mass fluxes of 107-1012 kg/s can spread for tens of kilometers with only limited cooling, although they move as supercritical, strongly entraining
currents for the eruption conditions considered here. With increasing eruption mass flux, cooling during passage through the
fountain diminishes while cooling during flow transport increases. The net effect is that eruption temperature exerts the
prime control on emplacement temperature. Pyroclastic suspension currents can form welded ignimbrite across their entire extent
if eruption temperature is To>1.3.Tmw, the minimum welding temperature. High eruption rates, a large fraction of fine ash, and a ring-fissure vent favor the formation
of extensive high-grade ignimbrite. For very hot eruptions producing sticky, partially molten pyroclasts, analysis of particle
aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation, which reduces runout.
As a result, very high-grade ignimbrites cannot spread more than a few tens of kilometers from their source. In cooler pyroclastic
currents, particles do not aggregate, and the sedimentation process may involve re-entrainment of particles, which potentially
leads to more extensive cooling and longer runout; such effects, however, are only significant when net erosion of substrate
occurs. Model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite
masses and aspect ratios. The model also provides an alternative explanation of the observed decrease in H/Lratios with ignimbrite
mass.
Received: 10 May 1998 / Accepted: 21 October 1998 相似文献
2.
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
3.
The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of
the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this
event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products
of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or
lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations
in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative
atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have
generated ∼250 Mt of H2SO4 aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements.
Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine
concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and
15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried
by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development
of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki
are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years.
The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved
in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over
Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements
of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of
aerosols estimated by other methods.
Received: 30 May 1995 / Accepted: 19 April 1996 相似文献
4.
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
5.
Thomas C. Pierson 《Bulletin of Volcanology》1998,60(2):98-109
Travel times for wet volcanic mass flows (debris avalanches and lahars) can be forecast as a function of distance from source
when the approximate flow rate (peak discharge near the source) can be estimated beforehand. The near-source flow rate is
primarily a function of initial flow volume, which should be possible to estimate to an order of magnitude on the basis of
geologic, geomorphic, and hydrologic factors at a particular volcano. Least-squares best fits to plots of flow-front travel
time as a function of distance from source provide predictive second-degree polynomial equations with high coefficients of
determination for four broad size classes of flow based on near-source flow rate: extremely large flows (>1 000 000 m3/s), very large flows (10 000–1 000 000 m3/s), large flows (1000–10 000 m3/s), and moderate flows (100–1000 m3/s). A strong nonlinear correlation that exists between initial total flow volume and flow rate for "instantaneously" generated
debris flows can be used to estimate near-source flow rates in advance. Differences in geomorphic controlling factors among
different flows in the data sets have relatively little effect on the strong nonlinear correlations between travel time and
distance from source. Differences in flow type may be important, especially for extremely large flows, but this could not
be evaluated here. At a given distance away from a volcano, travel times can vary by approximately an order of magnitude depending
on flow rate. The method can provide emergency-management officials a means for estimating time windows for evacuation of
communities located in hazard zones downstream from potentially hazardous volcanoes.
Received: 5 June 1997 / Accepted: 2 February 1998 相似文献
6.
The Chillos Valley Lahar (CVL), the largest Holocene debris flow in area and volume as yet recognized in the northern Andes,
formed on Cotopaxi volcano's north and northeast slopes and descended river systems that took it 326 km north–northwest to
the Pacific Ocean and 130+ km east into the Amazon basin. In the Chillos Valley, 40 km downstream from the volcano, depths
of 80–160 m and valley cross sections up to 337 000 m2 are observed, implying peak flow discharges of 2.6–6.0 million m3/s. The overall volume of the CVL is estimated to be ≈3.8 km3. The CVL was generated approximately 4500 years BP by a rhyolitic ash flow that followed a small sector collapse on the north
and northeast sides of Cotopaxi, which melted part of the volcano's icecap and transformed rapidly into the debris flow. The
ash flow and resulting CVL have identical components, except for foreign fragments picked up along the flow path. Juvenile
materials, including vitric ash, crystals, and pumice, comprise 80–90% of the lahar's deposit, whereas rhyolitic, dacitic,
and andesitic lithics make up the remainder. The sand-size fraction and the 2- to 10-mm fraction together dominate the deposit,
constituting ≈63 and ≈15 wt.% of the matrix, respectively, whereas the silt-size fraction averages less than ≈10 wt.% and
the clay-size fraction less than 0.5 wt.%. Along the 326-km runout, these particle-size fractions vary little, as does the
sorting coefficient (average=2.6). There is no tendency toward grading or improved sorting. Limited bulking is recognized.
The CVL was an enormous non-cohesive debris flow, notable for its ash-flow origin and immense volume and peak discharge which
gave it characteristics and a behavior akin to large cohesive mudflows. Significantly, then, ash-flow-generated debris flows
can also achieve large volumes and cover great areas; thus, they can conceivably affect large populated regions far from their
source. Especially dangerous, therefore, are snow-clad volcanoes with recent silicic ash-flow histories such as those found
in the Andes and Alaska.
Received: 28 April 1997 / Accepted: 19 August 1997 相似文献
7.
Four Late Holocene pyroclastic units composed of block and ash flows, surges, ashfalls of silicic andesite and dacite composition,
and associated lahar deposits represent the recent products emitted by domes on the upper part of Nevado Cayambe, a large
ice-capped volcano 60 km northeast of Quito. These units are correlated stratigraphically with fallout deposits (ash and lapilli)
exposed in a peat bog. Based on 14C dating of the peat and charcoal, the following ages were obtained: ∼910 years BP for the oldest unit, 680–650 years BP for
the second, and 400–360 years BP for the two youngest units. Moreover, the detailed tephrochronology observed in the peat
bog and in other sections implies at least 21 volcanic events during the last 4000 years, comprising three principal eruptive
phases of activity that are ∼300, 800, and 900 years in duration and separated by repose intervals of 600–1000 years. The
last phase, to which the four pyroclastic units belong, has probably not ended, as suggested by an eruption in 1785–1786.
Thus, Cayambe, previously thought to have been dormant for a long time, should be considered active and potentially dangerous
to the nearby population of the Interandean Valley.
Received: 5 July 1997 / Accepted: 21 October 1997 相似文献
8.
Karen Fontijn Gerald G. J. Ernst Costanza Bonadonna Marlina A. Elburg Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2011,73(9):1353-1368
The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit
from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall
deposit with an inversely graded base. Empirical models suggest a maximum eruption column height H
T of 30.5–35 km with an associated peak mass discharge rate of 2.8–4.8 × 108 kg/s. Analytical calculations result in H
T values of 33 ± 4 km (inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2.3 to 6.0 × 108 kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of
thickness data yields volume estimates between 3.2 and 5.8 km3 (corresponding to an erupted mass of 1.1–2.0 × 1012 kg), whereas analytical derivation yields an erupted mass of 1.1 × 1012 kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption.
The plume corner is estimated to have been ca. 11–12 km from the vent. After an opening phase with gradually increasing intensity,
a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic
density current deposits. 相似文献
9.
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
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.
The interaction of ash flows with ridges 总被引:1,自引:0,他引:1
Using both laboratory experiments and theoretical models, we examine the different flow regimes that may develop when an
ash flow encounters a ridge. For very small ridges, all the flow may pass over the ridge. For intermediate-size ridges, the
flow may be partially blocked, with a fraction of the flow reflected upstream as a travelling bore. In this case, the remainder
of the flow, which does pass over the ridge, is hydraulically controlled at the ridge crest. Finally, if the ridge is sufficiently
high, then the flow will be totally blocked. New laboratory experiments show that the sedimentation patterns associated with
these flow regimes may be very different. Most importantly, flows that involve partial blocking and the formation of upstream
propagating bores display enhanced sedimentation upstream of the ridge, analogous to valley-ponded and caldera-fill deposits.
In contrast, under some circumstances, if the flow is able to scale a ridge, the deposit may be relatively unaffected by the
presence of the ridge. The minimum ridge height that leads to total blocking of the flow increases with mass eruption rate
and has a complex variation with distance from the source. In a one-dimensional channel, the minimum ridge height that causes
blocking increases with distance downstream. This is because the flow becomes less dense through sedimentation of particles
and entrainment of air and so requires less energy to scale a ridge of a particular height. In axisymmetric flow, the minimum
ridge height initially decreases with distance downstream as the flow spreads radially, but subsequently increases as the
flow becomes less dense through sedimentation and entrainment. A new quantitative model of dilute ash flows propagating over
ridges indicates that flows with mass fluxes in excess of 108–109 kg/s can partially scale barriers as high as 1000 m at distances of tens of kilometres from the source, whereas smaller flows
are likely to be totally blocked by such an obstacle. Our results shed new insight on the possible long-range transport mechanism
of several large flows including the Ata, Fisher and Aniakchak pyroclastic flows.
Received: 20 December 1996 / Accepted: 15 March 1998 相似文献
12.
During the 1944 eruption of Vesuvius a sudden change occurred in the dynamics of the eruptive events, linked to variations
in magma composition. K-phonotephritic magmas were erupted during the effusive phase and the first lava fountain, whereas
the emission of strongly porphyritic K-tephrites took place during the more intense fountain. Melt inclusion compositions
(major and volatile elements) highlight that the magmas feeding the eruption underwent differentiation at different pressures.
The K-tephritic volatile-rich melts (up to 3 wt.% H2O, 3000 ppm CO2, and 0.55 wt.% Cl) evolved to reach K-phonotephritic compositions by crystallization of diopside and forsteritic olivine
at total fluid pressure higher than 300 MPa. These magmas fed a very shallow reservoir. The low-pressure differentiation of
the volatile-poor K-phonotephritic magmas (H2O<1 wt.%) involved mixing, open-system degassing, and crystallization of leucite, salite, and plagioclase. The eruption was
triggered by intrusion of a volatile-rich magma batch that rose from a depth of 11–22 km into the shallow magma chamber. The
first phase of the eruption represents the partial emptying of the shallow reservoir, the top of which is within the volcanic
edifice. The newly arrived magma mixed with that resident in the shallow reservoir and forced the transition from the effusive
to the lava fountain phase of the eruption.
Received: 14 September 1998 / Accepted: 10 January 1999 相似文献
13.
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
14.
Jean-Claude Thouret Franck Lavigne Hiroshi Suwa Bambang Sukatja Surono 《Bulletin of Volcanology》2007,70(2):221-244
Mt. Semeru, the highest mountain in Java (3,676 m), is one of the few persistently active composite volcanoes on Earth, with
a plain supporting about 1 million people. We present the geology of the edifice, review its historical eruptive activity,
and assess hazards posed by the current activity, highlighting the lahar threat. The composite andesite cone of Semeru results
from the growth of two edifices: the Mahameru ‘old’ Semeru and the Seloko ‘young’ Semeru. On the SE flank of the summit cone,
a N130-trending scar, branched on the active Jonggring-Seloko vent, is the current pathway for rockslides and pyroclastic
flows produced by dome growth. The eruptive activity, recorded since 1818, shows three styles: (1) The persistent vulcanian
and phreatomagmatic regime consists of short-lived eruption columns several times a day; (2) increase in activity every 5
to 7 years produces several kilometer-high eruption columns, ballistic bombs and thick tephra fall around the vent, and ash
fall 40 km downwind. Dome extrusion in the vent and subsequent collapses produce block-and-ash flows that travel toward the
SE as far as 11 km from the summit; and (3) flank lava flows erupted on the lower SE and E flanks in 1895 and in 1941–1942.
Pyroclastic flows recur every 5 years on average while large-scale lahars exceeding 5 million m3 each have occurred at least five times since 1884. Lumajang, a city home to 85,000 people located 35 km E of the summit,
was devastated by lahars in 1909. In 2000, the catchment of the Curah Lengkong River on the ESE flank shows an annual sediment
yield of 2.7 × 105 m3 km−2 and a denudation rate of 4 105 t km−2 yr−1, comparable with values reported at other active composite cones in wet environment. Unlike catchments affected by high magnitude
eruptions, sediment yield at Mt. Semeru, however, does not decline drastically within the first post-eruption years. This
is due to the daily supply of pyroclastic debris shed over the summit cone, which is remobilised by runoff during the rainy
season. Three hazard-prone areas are delineated at Mt. Semeru: (1) a triangle-shaped area open toward the SE has been frequently
swept by dome-collapse avalanches and pyroclastic flows; (2) the S and SE valleys convey tens of rain-triggered lahars each
year within a distance of 20 km toward the ring plain; (3) valleys 25 km S, SE, and the ring plain 35 km E toward Lumajang
can be affected by debris avalanches and debris flows if the steep-sided summit cone fails. 相似文献
15.
J.-C. Thouret K. E. Abdurachman J.-L. Bourdier S. Bronto 《Bulletin of Volcanology》1998,59(7):460-480
In contrast to most twentieth-century eruptions of Kelud volcano (eastern Java), the 10 February 1990 plinian eruption was
not accompanied by lake-outburst lahars. However, at least 33 post-eruption lahars occurred between 15 February and 28 March
1990. They swept down 11 drainage systems and travelled as far as 24 km at an estimated mean peak velocity in the range of
4–11 m s–1. The deposits (volume ≥30 000 000 m3) were approximately 7 m thick 2 km from vent, and 3 m thick 10 km from vent, on the volcaniclastic apron surrounding the
volcano. Subtle but significant sedimentological differences in the deposits relate to four flow types: (a) Early, massive
deposits are coarse, poorly sorted, slightly cohesive, and commonly inversely graded. They are inferred to record hot lahars
that incorporated pumice and scoria from pyroclastic-flow deposits, probably by rapid remobilization of hot proximal pyroclastic
flow deposits by rainfall runoff. Sedimentary features, such as clasts subparallel to bedding and thick, reversely to ungraded
beds, suggest that these flows were laminar. (b) Abundant, very poorly sorted deposits include non-cohesive, clast-supported,
inversely graded beds and ungraded, finer-grained, and cohesive matrix-supported beds. These beds display layering and vertical
segregation/density stratification, suggesting unsteady properties of pulsing debris flows. They are interpreted as deposited
from segments of flow waves at a middle distance downstream that incorporated pre-eruption sediments. Sedimentological evidence
suggests unsteady flow properties during progressive aggradation. (c) Fine-grained, poorly sorted and ungraded deposits are
interpreted as recording late hyperconcentrated streamflows that formed in the waning stage of an overflow and transformed
downcurrent into streamflows. (d) Ungraded, crudely stratified deposits were emplaced by flows transitional between hyperconcentrated
flows and streamflows that traveled farther downvalley (as far as 27 km from the vent). At Kelud, the transformation of flow
and behavior occurs within only 10 km of the source, at the apex of the alluvial fans. The rapid change of flow behavior is
attributed to the low fines content and to the unsteady flow regime, which may be due to: (a) the rapid deposition of bedload,
owing to the break in channel gradient close to the vent and to changes in channel cross-section and roughness; and (b) the
very low silt+clay content in the non-cohesive deposits. These deposits mix with water to produce streamflows.
Received: 27 June 1997 / Accepted: 5 January 1998 相似文献
16.
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
17.
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
18.
Mary E. MacKay Scott K. Rowland Peter J. Mouginis-Mark Harold Garbeil 《Bulletin of Volcanology》1998,60(4):239-251
We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness
of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average
40–60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of
flow thickness, which we used to calculate the volume (up to 1 km3 for an individual flow, and 1.8 km3 for all the identified flows) and yield strength of several flows (23–124 kPa). Using the DEM we estimated apparent viscosity
based on the spacing of large folds (1.2×1012 to 5.5×1012 Pa s for surface viscosity, and 7.5×1010 to 5.2×1011 Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures
such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable
in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images.
Received: 6 September 1997 / Accepted: 15 May 1998 相似文献
19.
Eruptive style of the young high-Mg basaltic-andesite Pelagatos scoria cone,southeast of México City 总被引:1,自引:1,他引:0
The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico
City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long
lava flow, and continued with a phase of alternating and/or simultaneous explosive and effusive activity that built a 50-m-high
scoria cone on the western end of the fissure and formed a compound lava flow-field near the vent. The eruption ended with
the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater. Products
consist of a microlite-rich high-Mg basaltic andesite. Samples were analyzed to determine the magma’s initial properties as
well as the effects of degassing-induced crystallization on eruptive style. Although distal ash fallout deposits from this
eruption are not preserved, a recent quarry exposes a large section of the scoria cone. Detailed study of exposed layers allows
us to elucidate the mode of cone-building activity. Petrological and textural data, combined with models calibrated by experimental
work and melt-inclusion analyses of similar magmas elsewhere, indicate that the magma was initially hot (>1,200°C), gas-rich
(up to 5 wt.% H2O), crystal-poor (~10 vol.% Fo90 olivine phenocrysts) and thus poorly viscous (40–80 Pa s). During the early phase, low magma ascent velocity at the fissure
vent allowed low-viscosity magma to degas and crystallize during ascent, producing lava flows with elevated crystal contents
at T < 1,100°C, and blocky surfaces. Later, the closure of the fissure by cooling dikes focused the magma flow at a narrow section
of the fissure. This led to an increased magma ascent velocity. Rapid and shallow degassing (<3 km deep) triggered ~40 vol.%
microlite crystallization. Limited times for gas-escape and higher magma viscosity (6 × 105–4 × 106 Pa s) drove strong explosions of highly (60–80 vol.%) and finely vesicular magma. Coarse clasts broke on landing, which implies
brittle behavior due to complete solidification. This requires sufficient time to cool and in turn implies ejection heights
of over 1 km, which is much higher than “normal” Strombolian activity. Hence, magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics
of shallow degassing. The long-lasting eruptions of Jorullo and Paricutin, which produced similar magmas in western México,
were more explosive. This can be related to higher magma fluxes and total erupted volumes. Implications of this study are
important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the Trans-Mexican Volcanic Belt. 相似文献
20.
Variations in column height and magma discharge during the May 18, 1980 eruption of Mount St. Helens
S. Carey H. Sigurdsson J.E. Gardner W. Criswell 《Journal of Volcanology and Geothermal Research》1990,43(1-4)
Peak eruption column heights for the B1, B2, B3 and B4 units of the May 18, 1980 fall deposit from Mount St. Helens have been determined from pumice and lithic clast sizes and models of tephra dispersal. Column heights determined from the fall deposit agree well with those determined by radar measurements. B1 and B2 units were derived from plinian activity between 0900 and about 1215 hrs. B3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs. A brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height (19 km) during deposition of B4.Variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass-balance calculations based on the volume of pyroclastic flows and coignimbrite ash. Peak magma discharge occurred during the period 1215–1630 hrs, when pyroclastic flows were generated by collapse of low fountains through the crater breach. Pyroclastic flow deposits and the widely dispersed co-ignimbrite ash account for 77% of the total erupted mass, with only 23% derived from plinian discharge.A shift in eruptive style at noon on May 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite. Increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw-up and tapping of deeper levels in the magma reservoir, as predicted by theoretical models of magma withdrawal. Return to plinian activity late in the afternoon, when magma discharge decreased, is consistent with theoretical predictions of eruption column behavior. The dominant generation of pyroclastic flows during the May 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable, convective eruption column to a collapsing one. 相似文献