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We compare eruptive dynamics, effects and deposits of the Bezymianny 1956 (BZ), Mount St Helens 1980 (MSH), and Soufrière
Hills volcano, Montserrat 1997 (SHV) eruptions, the key events of which included powerful directed blasts. Each blast subsequently
generated a high-energy stratified pyroclastic density current (PDC) with a high speed at onset. The blasts were triggered
by rapid unloading of an extruding or intruding shallow magma body (lava dome and/or cryptodome) of andesitic or dacitic composition.
The unloading was caused by sector failures of the volcanic edifices, with respective volumes for BZ, MSH, and SHV c. 0.5,
2.5, and 0.05 km3. The blasts devastated approximately elliptical areas, axial directions of which coincided with the directions of sector
failures. We separate the transient directed blast phenomenon into three main parts, the burst phase, the collapse phase,
and the PDC phase. In the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into
the atmosphere, with much of the expansion having an initially lateral component. The erupted material fails to mix with sufficient
air to form a buoyant column, but in the collapse phase, falls beyond the source as an inclined fountain, and thereafter generates
a PDC moving parallel to the ground surface. It is possible for the burst phase to comprise an overpressured jet, which requires
injection of momentum from an orifice; however some exploding sources may have different geometry and a jet is not necessarily
formed. A major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be
attributed to shock waves within an overpressured jet, or alternatively to dynamic pressures and shocks within the energetic
collapse and PDC phases. Internal shock structures related to unsteady flow and compressibility effects can occur in each
phase. We withhold judgment about published shock models as a primary explanation for the damage sustained at MSH until modern
3D numerical modeling is accomplished, but argue that much of the damage observed in directed blasts can be reasonably interpreted
to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified PDC.
This view is reinforced by recent modeling cited for SHV. In distal and peripheral regions, solids concentration, maximum
particle size, current speed, and dynamic pressure are diminished, resulting in lesser damage and enhanced influence by local
topography on the PDC. Despite the different scales of the blasts (devastated areas were respectively 500, 600, and >10 km2 for BZ, MSH, and SHV), and some complexity involving retrogressive slide blocks and clusters of explosions, their pyroclastic
deposits demonstrate strong similarity. Juvenile material composes >50% of the deposits, implying for the blasts a dominantly
magmatic mechanism although hydrothermal explosions also occurred. The character of the magma fragmented by explosions (highly
viscous, phenocryst-rich, variable microlite content) determined the bimodal distributions of juvenile clast density and vesicularity.
Thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater, and laterally
from the axial region. The proximal stratigraphy of the blast deposits comprises four layers named A, B, C, D from bottom
to top. Layer A is represented by very poorly sorted debris with admixtures of vegetation and soil, with a strongly erosive
ground contact; its appearance varies at different sites due to different ground conditions at the time of the blasts. The
layer reflects intense turbulent boundary shear between the basal part of the energetic head of the PDC and the substrate.
Layer B exhibits relatively well-sorted fines-depleted debris with some charred plant fragments; its deposition occurred by
rapid suspension sedimentation in rapidly waning, high-concentration conditions. Layer C is mainly a poorly sorted massive
layer enriched by fines with its uppermost part laminated, created by rapid sedimentation under moderate-concentration, weakly
tractive conditions, with the uppermost laminated part reflecting a dilute depositional regime with grain-by-grain traction
deposition. By analogy to laboratory experiments, mixing at the flow head of the PDC created a turbulent dilute wake above
the body of a gravity current, with layer B deposited by the flow body and layer C by the wake. The uppermost layer D of fines
and accretionary lapilli is an ash fallout deposit of the finest particles from the high-rising buoyant thermal plume derived
from the sediment-depleted pyroclastic density current. The strong similarity among these eruptions and their deposits suggests
that these cases represent similar source, transport and depositional phenomena. 相似文献
2.
We examine an eruptive sequence in late 2007 at Bezymianny Volcano to characterize the magmatic plumbing system and eruption-related seismicity. Earthquake locations reveal seismicity below and offset to the north of the volcano along a tectonic fault. Based on historical seismicity, the magma chamber is postulated to have a top at about 6 km depth. Minor dome explosions, large sub-plinian eruptions and dome collapses are analyzed using an automated event classification scheme. Low-frequency tremor, interpreted as gas escape, and low-frequency earthquakes are a dominant proportion of the energy released. We also examine multiplet earthquakes whose behavior during the study period changed significantly and systematically before the largest eruption, demonstrating the potential of tracking multiplets to assess changing conditions with the conduit. 相似文献
3.
V.O. Davydova P.Yu. Plechov V.D. Shcherbakov A.B. Perepelov 《Russian Geology and Geophysics》2018,59(9):1087-1099
Bezymianny is an active andesitic volcano of the Klyuchevskaya group, and its eruptive products are xenolith- and enclave-bearing basaltic andesites and dacites. Here we report the first occurrence of clinopyroxene-plagioclase high-potassium basaltic trachyandesite xenoliths (51.84-53.00 wt.% SiO2, 0.45-1.95 wt.% K2O) crystallized in the temperature range 1120-840 °C in products of modern eruptions (2007, 2011, 2012). Basaltic trachyandesite differ systematically in petrologic and geochemical characteristics from all previously studied rocks from the Bezymianny volcano. They correspond to the clinopyroxene-plagioclase porphyry rocks from the foot of the Tolbachik volcanoes. 相似文献
4.
An explosive eruption occurred at the summit of Bezymianny volcano (Kamchatka Peninsula, Russia) on 11 January 2005 which
was initially detected from seismic observations by the Kamchatka Volcanic Eruption Response Team (KVERT). This prompted the
acquisition of 17 Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite images of the volcano over
the following 10 months. Visible and infrared data from ASTER revealed significant changes to the morphology of the summit
lava dome, later seen with field based thermal infrared (TIR) camera surveys in August 2005. The morphology of the summit
lava dome was observed to have changed from previous year’s observations and historical accounts. In August 2005 the dome
contained a new crater and two small lava lobes. Stepped scarps within the new summit crater suggest a partial collapse mechanism
of formation, rather than a purely explosive origin. Hot pyroclastic deposits were also observed to have pooled in the moat
between the current lava dome and the 1956 crater wall. The visual and thermal data revealed a complex eruption sequence of
explosion(s), viscous lava extrusion, and finally the formation of the collapse crater. Based on this sequence, the conduit
could have become blocked/pressurized, which could signify the start of a new behavioural phase for the volcano and lead to
the potential of larger eruptions in the future. 相似文献
5.
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
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