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
The size, shape and orientation of plagioclase crystals have been quantified in a tuff and series of andesite lavas from
the active Egmont volcano (Mt. Taranaki), New Zealand. Linear crystal size distributions (CSDs) show that if the magma had
several components, then only one provided the crystals. The slope of the CSD indicates that the earliest lavas measured had
a residence time of ∼50 years in the magma chamber for a growth rate of 10–11 cm/s. Subsequent lavas had slightly longer residence times (50–75 years), but the following series returned to 50-year residence
times. The youngest magmas, from both Egmont summit and the parasitic Fantham's Peak, have the shortest residence times of
∼30 years. Variations in residence time may reflect changes in the magma chamber shape or depth, or the temperature of the
surrounding rocks. Crystal shapes and zonation suggest that crystallization occurred in a bottle-shape magma chamber, and
not in a narrow conduit. If future eruptions use the same magma chamber as the most recent eruptions, then a delay of approximately
30 years can be expected between refilling and eruption.
Received: 25 October 1995 / Accepted: 19 April 1996 相似文献
The Levantine Basin in the South-eastern Mediterranean Sea is a world class site for studying the initial stages of salt tectonics driven by differential sediment load, because the Messinian evaporites are comparatively young, the sediment load varies along the basin margin, they are hardly tectonically overprinted, and the geometry of the basin and the overburden is well-defined. In this study we analyse depositional phases of the evaporites and their structural evolution by means of high-resolution multi-channel seismic data. The basinal evaporites have a maximum thickness of about 2 km, precipitated during the Messinian Salinity Crisis, 5.3–5.9 Ma ago. The evaporite body is characterized by 5 transparent layers sequenced by four internal reflections. We suggest that each of the internal reflection bands indicate a change of evaporite facies, possibly interbedded clastic sediments, which were deposited during temporal sea level rises. All of these internal reflections are differently folded and distorted, proving that the deformation was syn-depositional. Thrust angles up to 14° are observed. Backstripping of the Pliocene–Quaternary reveals that salt tectonic is mainly driven by the sediment load of the Nile Cone. The direction of lateral salt displacement is mainly SSW–NNE and parallel to the bathymetric trend. Apparent rollback anticlines off Israel result rather from differential subsidence than from lateral salt displacement. In the south-eastern basin margin the deposition of the Isreali Slump Complex (ISC) is coeval with the onset of salt tectonic faulting, suggesting a causal link between slumping processes and salt tectonics.
The superposition of ‘thin-skinned’ tectonics and ‘thick-skinned’ tectonics becomes apparent in several locations: The fold belt off the Israeli Mediterranean slope mainly results from active strike-slip tectonics, which becomes evident in faults which reach from the seafloor well below the base of the evaporites. Owing to the wrenching of the crustal segments which are bounded by deep-rooted fault lines like the Damietta–Latakia, Pelusium and Shelf Edge Hinge line the setting is transpressional south of 32°N, where the fault lines bend further towards the west. This adds a component of ‘thick-skinned’ transpression to the generally ‘thin-skinned’ compressional regime in the basin. Above 1.5 km of evaporites, a mud volcano is observed with the mud source seemingly within the evaporite layer. At the eastern Cyprus Arc, the convergence zone of the African and the Anatolian plates, deep-rooted compression heavily deformed the base of the evaporites, whereas at the Eratosthenes Seamount mainly superficial compression affecting the Post-Messinian sediments and the top of the evaporites is observed. 相似文献
Kick'em Jenny is a submarine volcano situated 9 kilometres north of Grenada in the Lesser Antilles. A preliminary study suggests that the volcano is a prime candidate for tsunamigenic eruptions on a potentially hazardous scale, possibly affecting the whole of the Eastern Caribbean region. The uniqueness of individual volcanic eruptions means that attempts to generalise tsunamigenic mechanisms are extremely tentative. However, the theory of underwater explosion generated water waves is applicable to submarine volcanoes to model explosive eruptions. Using this theory, initial maximum ocean surface displacements are calculated for Kick'em Jenny hydroeruptions, corresponding to various event magnitudes (up to a worst-case scenario eruption on the scale of Krakatau, 1883). Wave propagation theories are then applied to the resulting tsunami wave dispersion, before beach shoaling equations are used to estimate the maximum tsunami run-up at adjacent coastal areas. Maps of the region have been prepared showing the paths of the wave-fronts (ray-tracing), travel times and maximum wave run-up amplitudes along coastlines. Finally, an attempt is made to assess how great a hazard the volcano represents, by considering the probability of each magnitude event occurring. 相似文献
The existence of a lateral volcanic zoning in the Kuril island arc (KIA) was confirmed by the analysis of the 1: 50 000 scale maps of the abundances of natural radioactive elements (NRE). Trends of the NRE ratios were calculated for the magmatic complexes of the KIA, and their relation to fluidization processes was proved. Based on aerogeophysical data (maps of NRE ratios and the magnetic field), the main cyclic sequence of the formation of active volcanoes and magmatism was distinguished. Multicyclic volcanoes related to mantle sources usually show the predominance of calc-alkaline volcanic complexes, the occurrence of magnetite precipitation processes, and weak development of metasomatic aureoles. Other volcanoes connected to crustal sources are characterized by the development of calcic complexes and extensive hydrothermal-metasomatic systems. 相似文献
The use of a hand-held thermal camera during the 2002–2003 Stromboli effusive eruption proved essential in tracking the development
of flow field structures and in measuring related eruption parameters, such as the number of active vents and flow lengths.
The steep underlying slope on which the flow field was emplaced resulted in a characteristic flow field morphology. This comprised
a proximal shield, where flow stacking and inflation caused piling up of lava on the relatively flat ground of the vent zone,
that fed a medial–distal lava flow field. This zone was characterized by the formation of lava tubes and tumuli forming a
complex network of tumuli and flows linked by tubes. Most of the flow field was emplaced on extremely steep slopes and this
had two effects. It caused flows to slide, as well as flow, and flow fronts to fail frequently, persistent flow front crumbling
resulted in the production of an extensive debris field. Channel-fed flows were also characterized by development of excavated
debris levees in this zone (Calvari et al. 2005). Collapse of lava flow fronts and inflation of the upper proximal lava shield made volume calculation very difficult. Comparison
of the final field volume with that expecta by integrating the lava effusion rates through time suggests a loss of ~70% erupted
lava by flow front crumbling and accumulation as debris flows below sea level. Derived relationships between effusion rate,
flow length, and number of active vents showed systematic and correlated variations with time where spreading of volume between
numerous flows caused an otherwise good correlation between effusion rate, flow length to break down. Observations collected
during this eruption are useful in helping to understand lava flow processes on steep slopes, as well as in interpreting old
lava–debris sequences found in other steep-sided volcanoes subject to effusive activity. 相似文献