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1.
Michele Dragoni Maurizio Bonafede Enzo Boschi 《Journal of Volcanology and Geothermal Research》1986,30(3-4)
It is widely recognized that lavas behave as Bingham liquids, which are characterized by a yield stress σ and a plastic viscosity η. We consider two models describing downslope flows of a Bingham liquid with different aspect ratios A (= flow height/flow width): model 1 with A 1 and model 2 with A ≈ 1. Sufficiently uphill with respect to the front, such flows can be considered as laminar and locally isothermal. For both models, we obtain analytically the steady-state solution of the Navier-Stokes equations and the constitutive equation for a Bingham liquid. We study the flow height and velocity as functions of flow rate, rheological parameters and ground slope. It is found that such flows remain in the Newtonian regime at low yield stresses (σ 103dyne/cm2), but the transition to the Bingham regime also depends on flow rate and occurs at higher values of σ for higher flow rates: for instance, a high aspect ratio flow (model 2) is still very close to the Newtonian regime at σ = 104 dyne/cm2, if the flow rate is greater than 105 g/s. In the Bingham regime, flow heights are generally greater and flow velocities are smaller than in the Newtonian regime; moreover, flow heights are independent of flow rate, so that a change in flow rate results exclusively in a velocity change. After assuming a specific temperature dependence of σ and η between the solidus and the liquidus temperatures of an ideal Bingham liquid (1000°C and 1200 °C respectively), flow heights and velocities are examined as functions of temperature along the flow. Several effects observed in lava flows are predicted by these models and allow a more quantitative insight into the behaviour of lava flows. 相似文献
2.
L. Lodato L. Spampinato A. Harris S. Calvari J. Dehn M. Patrick 《Bulletin of Volcanology》2007,69(6):661-679
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
3.
The evolution of lava flows emplaced on Mount Etna (Italy) in September 2004 is examined in detail through the analysis of morphometric measurements of flow units. The growth of the main channelized flow is consistent with a layering of lava blankets, which maintains the initial geometry of the channel (although levees are widened and raised), and is here explicitly related to the repeated overflow of lava pulses. A simple analytical model is introduced describing the evolution of the flow level in a channelized flow unit fed by a fluctuating supply. The model, named FLOWPULSE, shows that a fluctuation in the velocity of lava extrusion at the vent triggers the formation of pulses, which become increasingly high the farther they are from the vent, and are invariably destined to overflow within a given distance. The FLOWPULSE simulations are in accordance with the observed morphology, characterized by a very flat initial profile followed by a massive increase in flow unit cross-section area between 600 and 700 m downflow. The modeled emplacement dynamics provides also an explanation for the observed substantial “loss” of the original flowing mass with increasing distance from the vent. 相似文献
4.
Factors which control lava flow length are still not fully understood. The assumption that flow length as mainly influenced by viscosity was contested by Walker (1973) who proposed that the length of a lava flow was dependent on the mean effusion rate, and by Malin (1980) who concluded that flow length was dependent on erupted volume. Our reanalysis of Malin's data shows that, if short duration and tube-fed flows are eliminated, Malin's Hawaiian flow data are consistent with Walker's assertion. However, the length of a flow can vary, for a given effusion rate, by a factor of 7, and by up to 10 for a given volume. Factors other than effusion rate and volume are therefore clearly important in controlling the lengths of lava flows. We establish the relative importance of the other factors by performing a multivariate analysis of data for recent Hawaiian lava flows. In addition to generating empirical equations relating flow length to other variables, we have developed a non-isothermal Bingham flow model. This computes the channel and levee width of a flow and hence permits the advance rates of flows and their maximum cooling-limited lengths for different gradients and effusion rates to be calculated. Changing rheological properties are taken into account using the ratio of yield strength to viscosity; available field measurements show that this varies systematically from the vent to the front of a lava flow. The model gives reasonable agreement with data from the 1983–1986 Pu'u Oo eruptions and the 1984 eruption of Mauna Loa. The method has also been applied to andesitic and rhyolitic lava flows. It predicts that, while the more silicic lava flows advance at generally slower rates than basaltic flows, their maximum flow lengths, for a given effusion rate, will be greater than for basaltic lava flows. 相似文献
5.
Andrea Borgia Scott Linneman Daniel Spencer Luis Diego Morales Jose Brenes Andre 《Journal of Volcanology and Geothermal Research》1983,19(3-4)
Arenal Volcano has effused basaltic andesite lava flows nearly continuously since September, 1968. The two different kinds of material in flows, lava and lava debris, have different rheologic properties and dynamic behavior. Flow morphology depends on the relationship between the amount and distribution of the lava and the debris, and to a lesser extent the ground morphology.Two main units characterize the flows: the channel zone and the frontal zone. The channel zone consists of two different units, the levées and the channel proper. A velocity profile in the channel shows a maximum value at the plug where the rate of shear is zero, and a velocity gradient increasing outward until, at the levées, the velocity becomes zero. Cooling produces a marked temperature gradient in the flow, leading to the formation of debris by brittle fracture when a critical value of shear rate to viscosity is reached. When the lava supply ceases, much of this debris and part of the lava is left behind after the flow nucleus drains out, forming a collapsed channel.Processes at the frontal zone include levée formation, debris formation, the change in shape of the front, and the choice of the flow path. These processes are controlled primarily by the rheological properties of the lava.Frontal zone dynamics can be understood by fixing the flow front as the point of reference. The lava flows through the channel into the front where it flows out into the levées, thereby increasing the length of the channel and permitting the front to advance. The front shows a relationship of critical height to the yield strength (τ0) surface tension, and slope; its continued movement is activated by the pressure of the advancing lava in the channel behind. For an ideal flow (isothermal, homogeneous, and isotropic) the ratio of the section of channel proper to the section of levées is calculated and the distance the front will have moved at any time tx can be determined once the amount of lava available to the front is known. Assuming that the velocity function of the front {G(t)} during the collapsing stage is proportional to the entrance pressure of the lava at the channel-front boundary, an exponential decrease of velocity through time is predicted, which shows good agreement with actual frontal velocity measurements taken on two flows. Local variations in slope have a secondary effect on frontal velocities.Under conditions of constant volume the frontal zone can be considered as a machine that consumes energy brought in by the lava to perform work (front advancement). While the front will use its potential energy to run the process, the velocity at which it occurs is controlled by the activation energy that enters the system as the kinetic energy of the lava flowing into the front. A relation for the energy contribution due to frontal acceleration is also derived. Finally the entrance pressure, that permits the front to deform, is calculated. Its small value confirms that the lava behaves very much like a Bingham plastic. 相似文献
6.
M. Carmen Solana 《Bulletin of Volcanology》2012,74(10):2397-2413
Historic and recent (last 2,000?years) eruptions on the active volcanic island of Tenerife have been predominantly effusive, indicating that this is the most probable type of activity to be expected in the near future. In the past, lava flow invasion caused major damage on the island, and as the population and infrastructure have increased dramatically since the last eruption, lava flows are the most important short-term volcanic risk on Tenerife. Hence, an understanding of lava flow behaviour is vital to manage risks from lava flows and minimise future losses on the island. This paper focuses on the lava flows from the historic eruptions in Tenerife, providing new data on the volumes emitted, advance rates and the timing of the emplacement of flows. The studies show three main stages in the development of unconfined flow fields: the first stage, corresponding to the fast advance of the initial fronts during the first 24?C36?h of eruption (reaching calculated velocities of up to 1.1?m/s); the second stage, in which fronts stagnate; and a third stage, in which secondary lava flows develop from breakouts 4?C7?days after the initial eruption and farther extend the flow field (velocities of up to 0.02?m/s have been calculated for this stage). The breakouts identified originated at sites both proximal and distal to the vent and, in both cases, caused damage through lengthening and widening the original flow field. Hence, the probability of damage from lavas to land and property is highest during stages 1 and 3, and this should be accounted for when planning the response to a future effusive eruption. Tenerife??s lava flows display a similar behaviour to that of lava flows on volcanoes characterised by basaltic effusive activity (such as Etna or Kilauea), indicating the possibility of applying forecasting models developed at those frequently active volcanoes to Tenerife. 相似文献
7.
Ciro Del Negro Luigi Fortuna Alexis Herault Annamaria Vicari 《Bulletin of Volcanology》2008,70(7):805-812
Since the mechanical properties of lava change over time, lava flows represent a challenge for physically based modeling.
This change is ruled by a temperature field which needs to be modeled. MAGFLOW Cellular Automata (CA) model was developed
for physically based simulations of lava flows in near real-time. We introduced an algorithm based on the Monte Carlo approach
to solve the anisotropic problem. As transition rule of CA, a steady-state solution of Navier-Stokes equations was adopted
in the case of isothermal laminar pressure-driven Bingham fluid. For the cooling mechanism, we consider only the radiative
heat loss from the surface of the flow and the change of the temperature due to mixture of lavas between cells with different
temperatures. The model was applied to reproduce a real lava flow that occurred during the 2004–2005 Etna eruption. The simulations
were computed using three different empirical relationships between viscosity and temperature. 相似文献
8.
We introduce a 3D model for near-vent channelized lava flows. We assume the lava to be an isothermal Newtonian liquid flowing
in a rectangular channel down a constant slope. The flow velocity is calculated with an analytical steady-state solution of
the Navier-Stokes equation. The surface velocity and the flow rate are calculated as functions of the flow thickness for different
flow widths, and the results are compared with those of a 2D model. For typical Etna lava flow parameters, the influence of
levees on the flow dynamics is significant when the flow width is less than 25 m. The model predicts the volume flow rate
corresponding to the surface velocity, taking into account that both depend on flow thickness. The effusion rate is a critical
parameter to evaluate lava flow hazard. We propose a model to calculate the effusion rate given the lava flow width, the topograhic
slope, the lava density, the surface flow velocity, and either the lava viscosity or the flow thickness.
Received: 20 January 1998 / Accepted: 8 January 1999 相似文献
9.
Massimiliano Favalli Giuseppe D. Chirico Paolo Papale Maria Teresa Pareschi Enzo Boschi 《Bulletin of Volcanology》2009,71(4):363-374
The 2002 eruption of Nyiragongo volcano constitutes the most outstanding case ever of lava flow in a big town. It also represents
one of the very rare cases of direct casualties from lava flows, which had high velocities of up to tens of kilometer per
hour. As in the 1977 eruption, which is the only other eccentric eruption of the volcano in more than 100 years, lava flows
were emitted from several vents along a N–S system of fractures extending for more than 10 km, from which they propagated
mostly towards Lake Kivu and Goma, a town of about 500,000 inhabitants. We assessed the lava flow hazard on the entire volcano
and in the towns of Goma (D.R.C.) and Gisenyi (Rwanda) through numerical simulations of probable lava flow paths. Lava flow
paths are computed based on the steepest descent principle, modified by stochastically perturbing the topography to take into
account the capability of lava flows to override topographic obstacles, fill topographic depressions, and spread over the
topography. Code calibration and the definition of the expected lava flow length and vent opening probability distributions
were done based on the 1977 and 2002 eruptions. The final lava flow hazard map shows that the eastern sector of Goma devastated
in 2002 represents the area of highest hazard on the flanks of the volcano. The second highest hazard sector in Goma is the
area of propagation of the western lava flow in 2002. The town of Gisenyi is subject to moderate to high hazard due to its
proximity to the alignment of fractures active in 1977 and 2002. In a companion paper (Chirico et al., Bull Volcanol, in this issue, 2008) we use numerical simulations to investigate the possibility of reducing lava flow hazard through the construction of protective
barriers, and formulate a proposal for the future development of the town of Goma. 相似文献
10.
Volcanic geomorphologists have investigated various relationships between eruption rate and morphologic parameters of lava flows, particularly with regard to preferred statistical correlations and the conditions under which they are valid. Here we employ two simple models for lava flow heat loss by Stefan-Boltzmann radiation to derive eruption rate versus planimetric area relationships. Both of these models predict a linear relationship between eruption rate and planimetric area, modulated by distinct prefactors potentially sensitive to compositional and temperature differences among different flows. Regardless of any theoretical considerations, we show that eruption rate is highly correlated with planimetric area for the Hawaiian basaltic flows analyzed in this work. Moreover, this observed correlation is superior to those from other obvious combinations of eruption rate and flow dimensions. On the basis of the theoretical models for lava flow heat loss, the correlations obtained here suggest that the surfaces of Hawaiian flows radiate at an effective temperature much less than the inner parts of the flowing lava in agreement with numerous field observations. This work also indicates that eruption rate versus planimetric area correlations can be markedly degraded when data from different vents, volcanoes and epochs are combined. These previously unrecognized sensitivities identified by the thermal loss modeling may have contributed to past unresolved debates on relationships between eruption rates and morphologic dimensions. 相似文献
11.
We sampled basaltic lava flows and underlying dacitic tuff deposits in or near lava tubes of the Cave Basalt, Mount St. Helens, Washington to determine whether the Cave Basalt lavas contain geochemical evidence of substrate contamination by lava erosion. The samples were analyzed using a combination of wavelength-dispersive X-ray fluorescence spectrometry and inductively-coupled plasma mass spectrometry. The results indicate that the oldest, outer lava tube linings in direct contact with the dacitic substrate are contaminated, whereas the younger, inner lava tube linings are uncontaminated and apparently either more evolved or enriched in residual liquid. The most heavily contaminated lavas occur closer to the vent and in steeper parts of the tube system, and the amount of contamination decreases with increasing distance downstream. These results suggest that erosion by lava and contamination were limited to only the initially emplaced flows and that erosion was localized and enhanced by vigorous laminar flow over steeper slopes. After cooling, the initial Cave Basalt lava flows formed an insulating lining within the tubes that prevented further erosion by later flows. This interpretation is consistent with models of lava erosion that predict higher erosion rates closer to sources and over steeper slopes. A greater abundance of xenoliths and xenocrysts relative to xenomelts in hand samples indicates that mechanical erosion rather than thermal erosion was the dominant erosional process in the Cave Basalt, but further sampling and petrographic analyses must be performed to verify this hypothesis.Editorial responsibility: J. Donnelly-Nolan 相似文献
12.
In an attempt to model the effect of slope on the dynamics of lava flow emplacement, four distinct morphologies were repeatedly produced in a series of laboratory simulations where polyethylene glycol (PEG) was extruded at a constant rate beneath cold sucrose solution onto a uniform slope which could be varied from 1° through 60°. The lowest extrusion rates and slopes, and highest cooling rates, produced flows that rapidly crusted over and advanced through bulbous toes, or pillows (similar to subaerial “toey” pahoehoe flows and to submarine pillowed flows). As extrusion rate and slope increased, and cooling rate decreased, pillowed flows gave way to rifted flows (linear zones of liquid wax separated by plates of solid crust, similar to what is observed on the surface of convecting lava lakes), then to folded flows with surface crusts buckled transversely to the flow direction, and, at the highest extrusion rates and slopes, and lowest cooling rates, to leveed flows, which solidified only at their margins. A dimensionless parameter, Ψ, primarily controlled by effusion rate, cooling rate and flow viscosity, quantifies these flow types. Increasing the underlying slope up to 30° allows the liquid wax to advance further before solidifying, with an effect similar to that of increasing the effusion rate. For example, conditions that produce rifted flows on a 10° slope result in folded flows on a 30° slope. For underlying slopes of 40°, however, this trend reverses, slightly owing to increased gravitational forces relative to the strength of the solid wax. Because of its significant influence on heat advection and the disruption of a solid crust, slope must be incorporated into any quantitative attempt to correlate eruption parameters and lava flow morphologies. These experiments and subsequent scaling incorporate key physical parameters of both an extrusion and its environment, allowing their results to be used to interpret lava flow morphologies on land, on the sea floor, and on other planets. 相似文献
13.
More than 40 late Cenozoic monogenetic volcanoes formed a volcanic belt striking NNW from Keluo, through Wudalianchi to Erkeshan in NE China. These volcanoes belong to a unified volcano system, namely Wudalianchi volcanic belt(WVB for short). Based on the volcanic evolution history and the nature of monogenetic volcanic system, we estimate that the volcanic system of WVB is still active and has the potential to erupt again. Hence, this paper studied the temporal-spatial distribution and volcanic eruption types to evaluate the possible eruption hazard types and areas of influence in the future.
Volcanic field characteristics and K-Ar radiometric data suggest two episodes of volcanism in the WVB, the Pliocene to early Pleistocene volcanism(4.59~1.00MaBP)and the middle Pleistocene to Holocene volcanism(0.79Ma to now). The early episode volcanoes are distributed only in the north of WVB(mainly in Keluo volcanic field), featured by effusive eruption, and mainly formed monogenetic shield, whose base diameter is large and slope is gentle. However, the late episode eruptions occurred over the entire WVB. The explosive eruption in this stage formed numerous relatively intact scoria cones of explosive origin. Meanwhile the effusive eruption formed widely distributed lava flows.
Both effusive eruption and explosive eruption are common in WVB. The effusive eruption formed monogenetic shields and lava flows. The resulting pahoehoe lava, aa lava and block lava appeared in WVB. There are three end-member types of explosive eruption driven by magmatic volatile. Violent Strombolian eruption has the highest degree of fragmentation and mass flux, characterized by eruption column. Strombolian eruption has the high degree of fragmentation, but low mass flux, featured by pulse eruption. Hawaiian eruption has low degree of fragmentation, but high in mass flux, generating large scoria cones. In addition, this paper for the first time found phreatomagmatic eruption in WVB, which formed tuff cone. Transitional eruptions are also common in WVB, which have certain characteristics among the end-member eruption types. Besides, certain volcanoes displayed multiple explosive eruption types during the whole eruption span.
According to the volcanic temporal-spatial distribution and eruption characteristics in WVB, the potential volcanic hazards in future are constrained. It appears that the violent Strombolian and Strombolian eruption will not have significant impact on aviation safety in the vertical direction. In the radial direction, the ejected volcanic bomb can reach as far as 1km from the vents and the fallout tephra may disperse downwind over a distance ranging from 1~10km. The major hazard of Hawaiian eruption and effusive eruption comes from lava flow, and its migration distance may reach 3.0~13.5km for pahoehoe lava and 2.9~14.9km for aa lava. The base surge in phreatomagmatic eruption can reach a velocity of 200~400m/s, and the migration distance is around 10km. This is a big threat that people should pay more attention to and take precautions in advance. Besides, it is necessary to strengthen the real-time observation of the volcanoes in the WVB, especially those formed in the late episode as well as near the active fault. 相似文献
14.
The 3-month long eruption of Asama volcano in 1783 produced andesitic pumice falls, pyroclastic flows, lava flows, and constructed a cone. It is divided into six episodes on the basis of waxing and waning inferred from records made during the eruption. Episodes 1 to 4 were intermittent Vulcanian or Plinian eruptions, which generated several pumice fall deposits. The frequency and intensity of the eruption increased dramatically in episode 5, which started on 2 August, and culminated in a final phase that began on the night of 4 August, lasting for 15 h. This climactic phase is further divided into two subphases. The first subphase is characterized by generation of a pumice fall, whereas the second one is characterized by abundant pyroclastic flows. Stratigraphic relationships suggest that rapid growth of a cone and the generation of lava flows occurred simultaneously with the generation of both pumice falls and pyroclastic flows. The volumes of the ejecta during the first and second subphases are 0.21 km3 (DRE) and 0.27 km3 (DRE), respectively. The proportions of the different eruptive products are lava: cone: pumice fall=84:11:5 in the first subphase and lava: cone: pyroclastic flow=42:2:56 in the second subphase. The lava flows in this eruption consist of three flow units (L1, L2, and L3) and they characteristically possess abundant broken phenocrysts, and show extensive "welding" texture. These features, as well as ghost pyroclastic textures on the surface, indicate that the lava was a fountain-fed clastogenic lava. A high discharge rate for the lava flow (up to 106 kg/s) may also suggest that the lava was initially explosively ejected from the conduit. The petrology of the juvenile materials indicates binary mixing of an andesitic magma and a crystal-rich dacitic magma. The mixing ratio changed with time; the dacitic component is dominant in the pyroclasts of the first subphase of the climactic phase, while the proportion of the andesitic component increases in the pyroclasts of the second subphase. The compositions of the lava flows vary from one flow unit to another; L1 and L3 have almost identical compositions to those of pyroclasts of the first and second subphases, respectively, while L2 has an intermediate composition, suggesting that the pyroclasts of the first and second subphases were the source of the lava flows, and were partly homogenized during flow. The complex features of this eruption can be explained by rapid deposition of coarse pyroclasts near the vent and the subsequent flowage of clastogenic lavas which were accompanied by a high eruption plume generating pumice falls and/or pyroclastic flows.Editorial responsibility: T. Druitt 相似文献
15.
16.
The Llangorse volcanic field is located in northwest British Columbia, Canada, and comprises erosional remnants of Miocene to Holocene volcanic edifices, lava flows or dykes. The focus of this study is a single overthickened, 100-m-thick-valley-filling lava flow that is Middle-Pleistocene in age and located immediately south of Llangorse Mountain. The lava flow is basanitic in composition and contains mantle-derived peridotite xenoliths. The lava directly overlies a sequence of poorly sorted, crudely bedded volcaniclastic debris-flow sediments. The debris flow deposits contain a diverse suite of clast types, including angular clasts of basanite lava, blocks of peridotite coated by basanite, and rounded boulders of granodiorite. Many of the basanite clasts have been palagonitized. The presence and abundance of clasts of vesicular to scoriaceous, palagonitized basanite and peridotite suggest that the debris flows are syngenetic to the overlying lava flow and sampled the same volcanic vent during the early stages of eruption. They may represent lahars or outburst floods related to melting of a snow pack or ice cap during the eruption. The debris flows were water-saturated when deposited. The rapid subsequent emplacement of a thick basanite flow over the sediments heated pore fluids to at least 80–100°C causing in-situ palagonitization of glassy basanite clasts within the sediments. The over-thickened nature of the Llangorse Mountain lavas suggests ponding of the lava against a down-stream barrier. The distribution of similar-aged glaciovolcanic features in the cordillera suggests the possibility that the barrier was a lower-elevation, valley-wide ice-sheet. 相似文献
17.
18.
天池火山东北侧造盾玄武岩可划分出8个流动单元,熔岩流的流动距离主要集中在30~50km,熔岩流宽度以5km左右为主。通过由野外调查获得的天池火山东北侧不同熔岩流单元的地表坡度、熔岩流厚度等,结合温度、密度与黏度等物理参数,按照熔岩流速度公式恢复的头道组和早白山组0.5m厚晶体含量5%的玄武岩熔岩流流速集中在0~1m/s之间。晶体含量为30%、厚度为0.5m的晚白山组和老房子小山组玄武岩熔岩流的流动速度集中在0~0.12m/s之间。厚度增大至2m左右,晶体含量不变的头道组和早白山组的玄武岩熔岩流流动速度可加快至11m/s。天池火山2m厚的碱性熔岩流在12h内达到或接近了它的最远距离,而各组内2m厚拉斑玄武岩熔岩流在20h内接近了最远距离。0.5m厚的熔岩流在10d内接近最大距离。50km是预计的熔岩流长度,在未来制定减灾措施时,可将此长度作为重要依据之一。天池火山熔岩流灾害主要表现为熔岩流动时对房屋建筑、农田、道路、林地、电站的毁坏,火灾及大量的人口伤亡 相似文献
19.
Pyroclastic flows from the 1991 eruption of Unzen volcano,Japan 总被引：1，自引：0，他引：1
Pyroclastic flows from Unzen were generated by gravitational collapse of the growing lava dome. As soon as the parental lobe failed at the edge of the dome, spontaneous shattering of lava occurred and induced a gravity flow of blocks and finer debris. The flows had a overhanging, tongue-like head and cone- or rollershaped vortices expanding outward and upward. Most of the flows traveled from 1 to 3 km, but some flows reached more than 4 km, burning houses and killing people in the evacuated zone of Kita-kamikoba on the eastern foot of the volcano. The velocities of the flows ranged from 15 to 25 m/s on the gentle middle flank. Observations of the flows and their deposits suggest that they consisted of a dense basal avalanche and an overlying turbulent ash cloud. The basal avalanche swept down a topographic low and formed to tongue-like lobe having well-defined levees; it is presumed to have moved as a non-Newtonian fluid. The measured velocities and runout distances of the flows can be matched to a Bingham model for the basal avalanche by the addition of turbulent resistance. The rheologic model parameters for the 29 May flow are as follows: the density is 1300 kg/m3, the yield strength is 850 Pa, the viscosity is 90 Pa s, and the thickness of the avalanche is 2 m. The ash cloud is interpreted as a turbulent mixing layer above the basal avalanche. The buoyant portions of the cloud produced ash-fall deposits, whereas the dense portions moved as a surge separated from the parental avalanche. The ash-cloud surges formed a wide devastated zone covered by very thin debris. The initial velocities of the 3 June surges, when they detached from avalanches, are determined by the runout distance and the angle of the energy-line slope. A comparison between the estimated velocities of the 3 June avalanches and the surges indicates that the surges that extended steep slopes along the avalanche path, detached directly from the turbulent heads of the avalanches. The over-running surge that reached Kita-Kamikoba had an estimated velocity higher than that of the avalanche; this farther-travelled surge is presumed to have been generated by collapse of a rising ash-cloud plume. 相似文献
20.
A. J. Warden 《Bulletin of Volcanology》1967,30(1):277-318
The eruption commenced on July 7th 1963 with activity at the summit crater which had been dormant for at least 50 years. Production of lava spatte r characterised the opening stages of the eruption, and although hot lava blocks avalanched down the north-eastern slope no flows were produced. In August a crater opened at a height of approximately 1,000 metres at the head of a north-west trending fissure, the site of the 1960 eruption. Intermittent lava fountaining up to a height of 600 feet took place at the crater which was active throughout the remainder of the eruption, and viscous steep-sided tongues of «aa» lava flowed from it. A new east-west trending fissure 200 feet deep and 400 feet wide opened in September at a height of approximately 240 metres and extended up the slope to a point approximately 660 metres above sea level. From this fissure lavas of more fluid character though identical in mineral composition to tongues issuing from the flank crater flowed into the sea until mid November when activity at the fissure ceased. Whilst the fissure was active gas issued from a vent located immediately beyond it’s uper end. The slopes above the anchorage at Tematu were the site of subsidiary activity. Four small fissures opened at heights of up to 180 metres above sea level from mid-October to February 1964 producing short tongues of «aa» lava which flowed into the water. Emission of small ash clouds at sporadic intervals was noted at a crater situated in the highest fissure during a visit in December, 1963. There was a change from activity of «Strombolian type» with associated production of lava flows at the flank crater from November 1963 when the proportion of ash emitted increased. Ash emission became the predominant type of activity throughout the remainder of the eruption. Although the interval between successive outbursts lengthened progressively during 1964 the activity reached a climax on April 8th when the ash column attained a height of 30,000 feet, the maximum recorded during the course of the eruption. There was also an increase in July culminating in the production of a dense ash cloud 15 miles in diameter on the 26th. The activity entered a new phase in July 1964 when fissures producing lava tongues opened not only on the northern slopes but on the east side of the volcano as well. Activity continued on the opposite side to the north-west quadrant in which it had previously been localised when a fissure with a small crater at it’s head appeared in September on the south-east slopes a few hundred metres above sea level. The infrequency of outbursts during 1965 suggests that the present cycle of activity is waning, and that the volcano will soon become quiescent once more. Structures of interest in the lava flows include channels and tunnels. Hypersthene andesite was produced simultaneously with tholeiitic olivine bearing basalt during the opening stages of the eruption although the lavas produced later were all of the latter type. It is suggested that the hypersthene andesite was formed by magmatic differentiation of an olivine-bearing basalt parent magma, the lighter more acid fraction being tapped first at the beginning of the eruption. Such differentiation could account for similar basalt-andesite associations in older volcanic sequences within the central area. 相似文献