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1.
Shiveluch Volcano, located in the Central Kamchatka Depression, has experienced multiple flank failures during its lifetime,
most recently in 1964. The overlapping deposits of at least 13 large Holocene debris avalanches cover an area of approximately
200 km2 of the southern sector of the volcano. Deposits of two debris avalanches associated with flank extrusive domes are, in addition,
located on its western slope. The maximum travel distance of individual Holocene avalanches exceeds 20 km, and their volumes
reach ∼3 km3. The deposits of most avalanches typically have a hummocky surface, are poorly sorted and graded, and contain angular heterogeneous
rock fragments of various sizes surrounded by coarse to fine matrix. The deposits differ in color, indicating different sources
on the edifice. Tephrochronological and radiocarbon dating of the avalanches shows that the first large Holocene avalanches
were emplaced approximately 4530–4350 BC. From ∼2490 BC at least 13 avalanches occurred after intervals of 30–900 years. Six
large avalanches were emplaced between 120 and 970 AD, with recurrence intervals of 30–340 years. All the debris avalanches
were followed by eruptions that produced various types of pyroclastic deposits. Features of some surge deposits suggest that
they might have originated as a result of directed blasts triggered by rockslides. Most avalanche deposits are composed of
fresh andesitic rocks of extrusive domes, so the avalanches might have resulted from the high magma supply rate and the repetitive
formation of the domes. No trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from
the crater; perhaps witnesses exaggerated or misinterpreted the events.
Received: 18 August 1997 / Accepted: 19 December 1997 相似文献
2.
Alexander Belousov 《Bulletin of Volcanology》1996,57(8):649-662
On 30 March 1956 a catastrophic directed blast took place at Bezymianny volcano. It was caused by the failure of 0.5 km3 portion of the volcanic edifice. The blast was generated by decompression of intra-crater dome and cryptodome that had formed
during the preclimactic stage of the eruption. A violent pyroclastic surge formed as a result of the blast and spread in an
easterly direction effecting an area of 500 km2 on the lower flank of the volcano. The thickness of the deposits, although variable, decreases with distance from the volcano
from 2.5 m to 4 cm. The volume of the deposit is calculated to be 0.2–0.4 km3. On average, the deposits are 84% juvenile material (andesite), of which 55% is dense andesite and 29% vesicular andesite.
On a plot of sorting vs median diameter (Inman coefficients) the deposits occupy the area between the fall and flow fields.
In the proximal zone (less than 19 km from the volcano) three layers can be distinguished in the deposits. The lower one (layer
A) is distributed all over the proximal area, is very poorly sorted, enriched in fragments of dense juvenile andesite and
contains an admixture of soil and uncharred plant remains. The middle layer (layer B) is distributed in patches tens to hundreds
of metres across on the surface of layer A. Layer B is relatively well sorted as a result of a very low content of fine fractions,
and it contains rare charred plant remains. The uppermost layer (layer C) forms still smaller patches on the surface of layer
B. Layer C is characterized by intermediate sorting, is enriched in vesicular juvenile andesitic fragments, and contains a
high percentage of the fine fraction and very rare plant remains which are thoroughly charred. Maximum clast size decreases
from layer A to layer C. The absence of internal cross bedding is a characteristic of all three layers. In the distal zone
(more than 19 km from the volcano) stratigraphy changes abruptly. Deposit here consists of one layer 26 to 4 cm in thickness,
is composed of wavy laminated sand with a touch of gravel, is well sorted and contains uncharred plant remains. The Bezymianny
blast deposits are not analogous with known types of pyroclastic surges, with the exception of the directed blast deposits
of the Mount St.Helens eruption of 18 May 1980. The peculiarities of deposits from these two eruptions allow them to be separated
into a special type: blast surge. This type of surge is formed when failure of volcanic edifice relieves the pressure from
an inter-crater dome and/or cryptodome. A model is proposed to explain the peculiarities of the formation, transportation
and emplacement of the Bezymianny blast surge deposits.
Received: 19 December 1994 / Accepted: 12 December 1995 相似文献
3.
Depositional features and transportation mechanism of valley-filling Iwasegawa and Kaida debris avalanches, Japan 总被引:1,自引:1,他引:0
The depositional features of two valley-filling debris avalanche deposits were studied to reveal their transportation and
depositional mechanisms. The valley-filling Iwasegawa debris avalanche deposit (ca. 0.1 km3) is distributed along the valleys at the southeastern foot of Tashirodake Volcano, northern Honshu, Japan. Debris-avalanche
blocks range in size from <35 m proximally to <10 m in the distal zone and consist dominantly of fragile materials. Debris-avalanche
matrix percentages increase from 35–60% in the proximal zone to 95% in the distal zone. The debris-avalanche matrix is greater
in volume (80–90%) at the bottom and margins of the deposit. Normal grading of large clasts and reverse grading of wood logs
and branches occur within the debris-avalanche matrix. Preferred orientation of 311 wood logs and branches within the deposit
coincide with the interpreted local flow direction. The basal part of the deposit is characterized by (1) erosional features
and incorporated clasts of underlying material; (2) a higher proportion (30–50%) of incorporated clasts than the upper part;
and (3) reverse grading of clasts.
The valley-filling Kaida debris avalanche deposit (50 000 y B.P., >0.3 km3) is distributed along the valleys at the eastern-southeastern foot of Ontake Volcano, central Japan. Debris-avalanche blocks
range in size from <25 m proximally to <7 m in the medial zone. Debris-avalanche matrix percentages increase from 50–70% in
the proximal zone to 80% in the distal zone. The debris-avalanche matrix is more abundant (80–90%) at the bottom part of the
deposit. Deformation structures observed in the debris-avalanche blocks include elongation, folding, conjugate reverse faults,
and numerous minor faults in unconsolidated materials. Lithic components within the debris-avalanche matrix tend to have a
higher percentage of plucked clasts from the adjacent underlying formations.
A Bingham "plug flow" model is consistent with the transportation and depositional mechanisms of the valley-filling debris
avalanches. In the plug of the debris avalanche, fragile blocks were transported without major rupturing due to relatively
small shear stresses in regions of small strain rate. The debris-avalanche matrix was mainly produced by shearing at the bottom
and margins of the avalanche. Valley-filling debris avalanches tend to have smaller debris-avalanche blocks and larger amounts
of debris-avalanche matrix than do unconfined debris avalanches. These differences may be due to disaggregation of debris-avalanche
blocks by shearing against valley walls and interaction between debris-avalanche blocks and valley walls. Oriented wood logs
and branches, reverse grading of clasts at the base, and a higher proportion of incorporated clasts at the base are interpreted
to result from shearing along the bottom and valley walls.
Received: 25 March 1998 / Accepted: 10 October 1998 相似文献
4.
Hideo Hoshizumi Kozo Uto Kazunori Watanabe 《Journal of Volcanology and Geothermal Research》1999,89(1-4)
During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km3, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano. 相似文献
5.
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
6.
Usu volcano has erupted nine times since 1663. Most eruptive events started with an explosive eruption, which was followed by the formation of lava domes. However, the ages of several summit lava domes and craters remain uncertain. The petrological features of tephra deposits erupted from 1663 to 1853 are known to change systematically. In this study, we correlated lavas with tephras under the assumption that lava and tephra samples from the same event would have similar petrological features. Although the initial explosive eruption in 1663 was not accompanied by lava effusion, lava dome or cryptodome formation was associated with subsequent explosive eruptions. We inferred the location of the vent associated with each event from the location of the associated lava dome and the pyroclastic flow deposit distribution and found that the position of the active vent within the summit caldera differed for each eruption from the late 17th through the 19th century. Moreover, we identified a previously unrecognized lava dome produced by a late 17th century eruption; this dome was largely destroyed by an explosive eruption in 1822 and was replaced by a new lava dome during a later stage of the 1822 event at nearly the same place as the destroyed dome. This new interpretation of the sequence of events is consistent with historical sketches and documents. Our results show that petrological correlation, together with geological evidence, is useful not only for reconstructing volcanic eruption sequences but also for gaining insight into future potential disasters. 相似文献
7.
M. Nakagawa K. Wada T. Thordarson C. P. Wood J. A. Gamble 《Bulletin of Volcanology》1999,61(1-2):15-31
Ruapehu volcano erupted intermittently between September and November 1995, and June and July 1996, producing juvenile andesitic
scoria and bombs. The volcanic activity was characterized by small, sequential phreatomagmatic and strombolian eruptions.
The petrography and geochemistry of dated samples from 1995 (initial magmatic eruption of 18 September 1995, and two larger
events on 23 September and 11 October), and from 1996 (initial and larger eruptions on 17–18 June) suggest that episodes of
magma mixing occurred in separate magma pockets within the upper part of the magma plumbing system, producing juvenile andesitic
magma by mixing between relatively high (1000–1200 °C)- and low (∼1000 °C)- temperature (T) end members. Oscillatory zoning
in pyroxene phenocrysts suggests that repeated mixing events occurred prior to and during the 1995 and 1996 eruptions. Although
the 1995 and 1996 andesitic magmas are products of similar mixing processes, they display chronological variations in phenocryst
clinopyroxene, matrix glass, and whole-rock compositions. A comparison of the chemistry of magnesian clinopyroxene in the
four tephras indicates that, from 18 September through June 1996, the tephras were derived from at least two discrete high-temperature
(high-T) batches of magma. Crystals of magnesian clinopyroxene in the 23 September and 11 October tephras appear to be derived
from different high-T magma batches. Whole-rock and matrix-glass compositions of all tephras are consistent with their derivation
from distinct mixed melts. We propose that, prior to 1995 there was a shallow low-temperature (low-T) magma storage system
comprising crystal-rich mush and remnant magma from preceding eruptive episodes. Crystal clots and gabbroic inclusions in
the tephras attest to the existence of relict crystal mush. At least two discrete high-T magmas were then repeatedly injected
into the mush zone, forming discrete and mixed magma pockets within the shallow system. The intermittent 1995 and 1996 eruptions
sequentially tapped these magma pockets.
Received: 1 April 1998 / Accepted: 22 December 1998 相似文献
8.
Michael O. Garcia Ken H. Rubin Marc D. Norman J. Michael Rhodes David W. Graham David W. Muenow Khalil Spencer 《Bulletin of Volcanology》1998,59(8):577-592
Samples of basalt were collected during the Rapid Response cruise to Loihi seamount from a breccia that was probably created
by the July to August 1996 Loihi earthquake swarm, the largest swarm ever recorded from a Hawaiian volcano. 210Po–210Pb dating of two fresh lava blocks from this breccia indicates that they were erupted during the first half of 1996, making
this the first documented historical eruption of Loihi. Sonobuoys deployed during the August 1996 cruise recorded popping
noises north of the breccia site, indicating that the eruption may have been continuing during the swarm. All of the breccia
lava fragments are tholeiitic, like the vast majority of Loihi's most recent lavas. Reverse zoning at the rim of clinopyroxene
phenocrysts, and the presence of two chemically distinct olivine phenocryst populations, indicate that the magma for the lavas
was mixed just prior to eruption. The trace element geochemistry of these lavas indicates there has been a reversal in Loihi's
temporal geochemical trend. Although the new Loihi lavas are similar isotopically and geochemically to recent Kilauea lavas
and the mantle conduits for these two volcanoes appear to converge at depth, distinct trace element ratios for their recent
lavas preclude common parental magmas for these two active volcanoes. The mineralogy of Loihi's recent tholeiitic lavas signify
that they crystallized at moderate depths (∼8–9 km) within the volcano, which is approximately 1 km below the hypocenters
for earthquakes from the 1996 swarm. Taken together, the petrological and seismic evidence indicates that Loihi's current
magma chamber is considerably deeper than the shallow magma chamber (∼3–4 km) in the adjoining active shield volcanoes.
Received: 21 August 1997 / Accepted: 15 February 1998 相似文献
9.
Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced
by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle
Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation
permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest
that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine,
unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational
with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580 °C and are interpreted as aa flow
breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche
bedding beneath flows, were emplaced at varying temperatures between 200 and 550 °C and are interpreted as forming part of
block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor,
well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises
constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic
properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after
acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550 °C.
These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia
formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor
and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting
of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature,
although some deposits contain clasts possibly emplaced at temperatures as high as 525 °C. These breccias are interpreted
as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic
and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite
volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather
than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent
lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely
many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described
as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic
beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences.
Received: 1 October 1998 / Accepted: 28 December 1998 相似文献
10.
Pliocene–Recent volcanic outcrops at Seal Nunataks and Beethoven Peninsula (Antarctic Peninsula) are remnants of several
monogenetic volcanoes formed by eruption of vesiculating basaltic magma into shallow water, in an englacial environment. The
diversity of sedimentary and volcanic lithofacies present in the Antarctic Peninsula outcrops provides a clear illustration
of the wide range of eruptive, transportational and depositional processes which are associated with englacial Surtseyan volcanism.
Early-formed pillow lava and glassy breccia, representing a pillow volcano stage of construction, are draped by tephra erupted
explosively during a tuff cone stage. The tephra was resedimented around the volcano flanks, mainly by coarse-grained sediment
gravity flows. Fine-grained lithofacies are rare, and fine material probably bypassed the main volcanic edifice, accumulating
in the surrounding englacial basin. The pattern of sedimentation records variations in eruption dynamics. Products of continuous-uprush
eruptions are thought to be represented by stacks of poorly bedded gravelly sandstone, whereas better bedded, lithologically
more diverse sequences accumulated during periods of quiescence or effusive activity. Evidence for volcano flank failure is
common. In Seal Nunataks, subaerial lithofacies (mainly lavas and cinder cone deposits) are volumetrically minor and occur
at a similar stratigraphical position to pillow lava, suggesting that glacial lake drainage may have occurred prior to or
during deposition of the subaerial lithofacies. By contrast, voluminous subaerial effusion in Beethoven Peninsula led to the
development of laterally extensive stratified glassy breccias representing progradation of hyaloclastite deltas.
Received: 5 February 1996 / Accepted: 17 January 1997 相似文献