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41.
Submarine explosive eruptions are generally considered to become less likely with increasing depth due to the increasing hydrostatic
pressure of the overlying water column. Volcaniclastic deposits from the North Arch volcanic field, north of Oahu, have textural
characteristics of explosive fragmentation yet were erupted in water depths greater than 4,200 m.
The most abundant volcaniclastic samples from North Arch are clast-supported with highly vesicular, angular pyroclasts. They
are most likely near-vent pyroclastic fall deposits formed in eruption columns of limited height. Interbedded with highly
vesicular pillow lava, they form low (50 to 200 m), steep-sided cones around the vents. Less common are stratified samples
with graded bedding; one such sample includes a layer of roughly aligned, platy, bubble-wall glass fragments (resembling littoral
limu o Pele) that may have been deposited by density currents. In addition to bubble-wall glass shards, numerous glass fragments
with spherical, delicate spindle and ribbon shapes, and Pele's hair-like glass strands occur in the finer size fraction (<0.5 mm)
of some samples. They are probably more distal fallout. Another sample, consisting of glass fragments dispersed in a marine
clay matrix, was apparently reworked and deposited farther from the vents by bottom currents.
Glass compositions include low-(∼0.4-0.6 wt%) and medium-K2O (>0.6 wt%) alkalic basalt, basanite, and nephelinite. Sulfur and chlorine abundances are high, reaching a maximum of 1,800
and 1,300 ppm, respectively. The ubiquitous presence of limu o Pele fragments, regardless of glass composition, suggests that
bursts of Strombolian-like activity accompanied most eruptions. Coalescing vesicles observed in larger pyroclasts and some
pillow lava suggests accumulation of volatiles. Since the great hydrostatic pressure makes steam expansion impossible, a volatile-rich,
supercritical magmatic fluid probably drove the eruptions. If these volatile-rich magmas had erupted in shallow water or subaerially,
tall fountains would most likely have resulted. The great hydrostatic pressure (>40 MPa) limited fountain and eruption column
heights. 相似文献
42.
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
43.
The majority of tephra generated during the paroxysmal 1883 eruption of Krakatau volcano, Indonesia, was deposited in the sea within a 15-km radius of the caldera. Two syneruptive pyroclastic facies have been recovered in SCUBA cores which sampled the 1883 subaqueous pyroclastic deposit. The most commonly recovered facies is a massive textured, poorly sorted mixture of pumice and lithic lapilli-to-block-sized fragments set in a silty to sandy ash matrix. This facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the Krakatau islands on the basis of grain size and component abundances. A less common facies consists of well-sorted, planarlaminated to low-angle cross-bedded, vitric-enriched silty ash. Entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated, deflated components of pyroclastic flows as they traveled over water. The basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction, high temperature of emplacement, and lack of traction structures, and no significant hydraulic sorting of components. The laminated facies was most likely deposited from low-concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater. The Krakatau deposits are the first well-documented example of true submarine pyroclastic flow deposition from a modern eruption, and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences. 相似文献
44.
甘肃文县上震旦统重力流沉积及其构造古地理的意义 总被引:2,自引:0,他引:2
分布于甘肃文县一带的原“碧口群”据岩性、层位特征及微古植物化石厘定为震旦系,并可进一步划分出南沱组、陡山沱组、灯影组。在陡山沱组发育一套陆源浊积岩和深水相泥质岩、硅质岩组合,底部夹有多层火山碎屑岩及火山熔岩;灯影组发育碳酸盐岩浊积岩层和重力滑动层。这些浊积岩能够在地质记录中得以保存且厚度可达数百米,说明该区当时受到一个相对活动的从深水海沟演化到较浅水弧后盆地这一沉积构造环境的控制,从而说明扬子地台北缘应是具有拉张活动性质的被动大陆边缘。 相似文献
45.
Shallow landslides in pyroclastic soils: a distributed modelling approach for hazard assessment 总被引:3,自引:0,他引:3
Paolo Frattini Giovanni B. Crosta Nicoletta Fusi Paolo Dal Negro 《Engineering Geology》2004,73(3-4):277-295
An effective assessment of shallow landslide hazard requires spatially distributed modelling of triggering processes. This is possible by using physically based models that allow us to simulate the transient hydrological and geotechnical processes responsible for slope instability. Some simplifications are needed to address the lack of data and the difficulty of calibration over complex terrain at the catchment's scale. We applied two simple hydrological models, coupled with the infinite slope stability analysis, to the May 1998 landslide event in Sarno, Southern Italy. A quasi-dynamic model (Barling et al., 1994) was used to model the contribution to instability of lateral flow by simulating the time-dependent formation of a groundwater table in response to rainfall. A diffusion model [Water Resour. Res. 36 (2000) 1897] was used to model the role of vertical flux by simulating groundwater pressures that develop in response to heavy rainstorms. The quasi-dynamic model overestimated the slope instability over the whole area (more than 16%) but was able to predict correctly slope instability within zero order basins where landslides occurred and developed into large debris flows. The diffusion model simulated correctly the triggering time of more than 70% of landslides within an unstable area amounting to 7.3% of the study area. These results support the hypothesis that both vertical and lateral fluxes were responsible for landslide triggering during the Sarno event, and confirm the utility of such models as tools for hazard planning and land management. 相似文献
46.
In this study, the decadal evolution of a washover fan on the west coast of Denmark is examined from its initial generation in 1990 until 2015. Since its inception, the bare and flat washover fan surface has recovered and accreted slowly due to re-activation by overwash during surges and due to aeolian activity and dune formation, stimulated by vegetation growth. The volume of sand on the washover has increased steadily at an average rate of about 23 m3/yr per unit length of shoreline, and a total of 175,000 m3 of sand is now deposited on the fan, while at the same time the shoreline has receded by some 250 m. The evolution can be divided into three stages: 1) An initiation phase when storm surge levels and energetic wave conditions caused a breach in the foredunes and overwash processes formed a washover fan with a relatively low elevation above mean sea level; 2) An initial recovery phase during which waves supplied sand to the fan during frequent overwash activity and winds transported this sand into marginal dunes surrounding the fan; and 3) A later recovery phase when the surface of the fan had accreted to a level where vegetation could survive and trap sediment into new foredune growth across the fan. The rate of accretion has been overall linear but scales with neither annual overwash frequency, nor with aeolian transport potential. Instead, the linear accretion is more closely related to the steady onshore migration of nearshore bars that weld to the beach and provide a sand supply for transfer to the fan. The fan evolution demonstrates the importance of washover fans in preserving barrier resilience during transgressional phases caused by increasing mean sea level. © 2019 John Wiley & Sons, Ltd. 相似文献
47.
We present here a methodology implemented within a geographical information system (GIS) for hazard mapping of small volume
pyroclastic density currents (PDCs). This technique is implemented as a set of macros written in Visual Basic for Applications
(VBA) that run within GIS-software (i.e. ArcGIS). Based on the energy line concept, we calibrated an equation that relates
the volume (V) and the mobility (ΔH/L) of single PDCs using data from Soufrière Hills volcano (Montserrat) and Arenal volcano (Costa Rica). Maximum potential run-outs
can be predicted with an associated uncertainty of about 30%. Also based on the energy line concept and with data from Soufrière
Hills volcano and Mt. St. Helens (USA), we were able to calibrate an equation that predicts the flow velocity as a function
of the vertical distance between the energy line and the ground surface (Δh). Velocities derived in this way have an associated uncertainty of 3 m s−1. We wrote code to implement these equations and allow the automatic mapping of run-out and velocity with the inputs being
(i) the height and location of the vent (ii) the flow volume and (iii) a digital elevation model (DEM) of the volcano. Dynamic
pressure can also be estimated and mapped by incorporating the density of the pyroclastic density current (PDC). This computer
application allows the incorporation of uncertainties in the location of the vent and of statistical uncertainties expressed
by the 95% confidence limits of the regression model. We were able to verify predictions by the proposed methodology with
data from Unzen volcano (Japan) and Mayon volcano (The Philippines). The consistencies observed highlight the applicability
of this approach for hazard mitigation and real-time emergency management. 相似文献
48.
A coupled coastal-bay estuarine numerical model is described and applied to investigate the combination of wind-estuarine
driven circulation off the Orissa coast. The model is based on coupling of a 2-dimensional estuarine model with a 3-dimensional
coastal-bay model. The models are linked through the elevation at the interface. Using the coupled model, the numerical experiments
are carried out to elicit the dynamical linking between the estuarine outflow and the coastal ocean to simulate the ensuing
adjoining coastal circulation. During the southwest monsoon, it is noticed that the estuarine discharge from the northern
head-bay river system and the river systems that join the Bay of Bengal along the Orissa coast would sufficiently modify the
coastal circulation along the coast. Numerical experiments are also carried for the model simulation of surges generated by
the 1999 Orissa cyclone. It is shown that the estuarine system would influence significantly on surge development and associated
inundation through the rivers. 相似文献
49.
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
50.
Water storage mapping of pyroclastic covers through electrical resistivity measurements 总被引:1,自引:0,他引:1
The knowledge of the geological setting of pyroclastic covers and their water content distribution represents crucial information for stability analyses of slopes potentially subject to debris-flow phenomena. The study we here present would provide a contribution to this issue by means of an approach based on electrical resistivity measurements. Specifically, we describe the results of high-resolution 2D resistivity surveys carried out in a test area on Sarno Mountains (Campania Region – Southern Italy), where shallow landslides involving pyroclastic soils periodically occur triggered by critical rainfall events. We discuss the results in relation to the geology of the area in order to locate characteristic horizons of pyroclastic soils below the ground surface. Then, on the basis of a laboratory characterization of pyroclastic samples collected from the same test area at representative depths, we provide an estimation of the soil water content distribution in the field. Finally, we analyze temporal variations of the soil water content distribution by comparing the data of two surveys carried out in the autumnal and spring seasons, respectively. 相似文献