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1.
We examine the basic characteristics of inflations at Semeru Volcano, Indonesia, to clarify the pressurization process prior to two different styles of explosive eruptions: Vulcanian eruptions and gas bursts. Analysis of data obtained from tilt meters installed close to the active crater allows clarification of the common features and the differences between the two styles of eruptions. To improve the signal-to-noise ratio and to determine the mean characteristics of the inflations, we stack tilt signals obtained from eruptions of different magnitudes and evaluate the maximum amplitude of the seismic signal associated with these eruptions. Vulcanian eruptions, which explosively release large amounts of ash, are preceded by accelerating inflation about 200–300 s before the eruption, which suggests volume expansion of the gas phase. In contrast, gas bursts, which rapidly effuse water steam accompanied by explosive sounds, follow non-accelerating changes of inflation starting 20 s before each emission. Tilt amplitudes increase with the magnitude of eruptions for both eruption styles. This suggests that the volume and/or pressure of magma or gas stored in the conduit before eruptions controls the magnitude of volcanic eruptions. These results further suggest that the magnitude of eruptions can be predicted from geodetic measurements of volcano inflation.  相似文献   

2.
El Chichón volcano is an andesite stratovolcano in southern México. It erupted in March 1982, after about 550 years of quiescence. The 1982 eruption of El Chichón has not been followed by the growth of a lava dome within the newly formed crater. This is rather anomalous since the construction of a new dome after the destruction of an old one is a common process during the eruptions at andesite and dacite volcanoes. To discuss this anomalous aspect of the El Chichón eruption, some regularity in the process of re-awakening of dormant (here defined as a period of quiescence of more than 100 years) andesite and dacite volcanoes are studied based on the seismic activity recorded at the volcanoes Bezymianny, Mount St. Helens, El Chichón, Unzen, Pinatubo and Soufrière Hills. Three stages were identified in the re-awakening activity of these volcanoes: (1) preliminary seismic activity, leading up to the first phreatic explosion; (2) activity between the first and the largest explosions; (3) post-explosion dome-building process. The eruptions were divided into two groups: low-VEI (Volcanic Explosivity Index) and the long duration stage-1 events (Unzen, 1991 and Soufrière Hills volcano, 1995) and high-VEI and the short duration stage-1 events (Bezymianny, 1956; Mount St. Helens, 1980; El Chichón, 1982 and Pinatubo, 1992). The comparative analysis of the seismo-eruptive activity of two eruptions of the second group, the 1980 of Mt. St. Helens and the 1982 of El Chichón, produced an explanation the absence of new dome building during the 1982 eruption of El Chichón volcano. It may be explained in terms of the unusually rapid emission of gas and water from the magmatic and hydrothermal system beneath the volcano during a relatively short sequence of large explosions that could have sharply increased the viscosity of the magma making impossible its exit to the surface.  相似文献   

3.
 Many basaltic and andesitic polygenetic volcanoes have cyclic eruptive activity that alternates between a phase dominated by flank eruptions and a phase dominated by eruptions from a central vent. This paper proposes the use of time-series diagrams of eruption sites on each polygenetic volcano and intrusion distances of dikes to evaluate volcano growth, to qualitatively reconstruct the stress history within the volcano, and to predict the next eruption site. In these diagrams the position of an eruption site is represented by the distance from the center of the volcano and the clockwise azimuth from north. Time-series diagrams of Mauna Loa, Kilauea, Kliuchevskoi, Etna, Sakurajima, Fuji, Izu-Oshima, and Hekla volcanoes indicate that fissure eruption sites of these volcanoes migrated toward the center of the volcano linearly, radially, or spirally with damped oscillation, occasionally forming a hierarchy in convergence-related features. At Krafla, terminations of dikes also migrated toward the center of the volcano with time. Eruption sites of Piton de la Fournaise did not converge but oscillated around the center. After the convergence of eruption sites with time, the central eruption phase is started. The intrusion sequence of dikes is modeled, applying crack interaction theory. Variation in convergence patterns is governed by the regional stress and the magma supply. Under the condition that a balance between regional extension and magma supply is maintained, the central vent convergence time during the flank eruption phase is 1–10 years, whereas the flank vent recurrence time during the central eruption phase is greater than 100 years owing to an inferred decrease in magma supply. Under the condition that magma supply prevails over regional extension, the central vent convergence time increases, whereas the flank vent recurrence time decreases owing to inferred stress relaxation. Earthquakes of M≥6 near a volcano during the flank eruption phase extend the central vent convergence time. Earthquakes during the central eruption phase promote recurrence of flank eruptions. Asymmetric distribution of eruption sites around the flanks of a volcano can be caused by local stress sources such as an adjacent volcano. Received: 18 March 1996 / Accepted: 14 January 1997  相似文献   

4.
We report a compilation of data recorded at a distant tiltmeter station (RER) during recent episodes of dyke emplacement and eruption (2003–2007) at Piton de La Fournaise volcano (La Réunion Island). This sensitive station provides useful information for evaluating the extent of deformation. Distinct responses of this station were recorded based on the eruption type. Dykes feeding summit eruptions did not significantly influence the RER tiltmeter signals, whereas dykes feeding large distal eruptions (with vents located more than 4 km from the summit) generated up to 1.4 μrad of tilt, an amplitude 2 to 4 times greater than for proximal eruptions (0.3–0.7 μrad) on the flanks of the summit cone. The distinct tilt amplitude is directly linked to the location, depth, and volume of the dyke. Comparison with summit tiltmeters reveals that up to one-third to half of the RER tilt signal associated to dyke propagation is recorded when the dyke is still below the summit crater. Thus, before large distal eruptions, more than 0.5 μrad of tilt is recorded in less than 20 min when the dyke is below the summit crater (i.e. a few minutes/hours before the beginning of the eruption). We can thus propose for the RER station a threshold value of 0.5 μrad which, when reached as a dyke rises beneath the summit crater, suggests a high likelihood of a large distal eruption. The distant RER tiltmeter station thus appears to be a powerful tool for forecasting the type of eruption that is likely to occur, and can contribute to the early detection of large distal eruptions at Piton de La Fournaise, which are the most dangerous to inhabitants. For volcano monitoring, installation of high precision distant tiltmeters along the lower slopes of a volcano may provide warnings of large eruptions with enough lead time to allow for short-term hazards mitigation efforts.  相似文献   

5.
After 33 years of repose, one of the most active volcanoes of the Kurile island arc—Sarychev Peak on Matua Island in the Central Kuriles—erupted violently on June 11, 2009. The eruption lasted 9 days and stands among the largest of recent historical eruptions in the Kurile Island chain. Satellite monitoring of the eruption, using Moderate Resolution Imaging Spectroradiometer, Meteorological Agency Multifunctional Transport Satellite, and Advanced Very High Resolution Radiometer data, indicated at least 23 separate explosions between 11 and 16 June 2009. Eruptive clouds reached altitudes of generally 8–16 km above sea level (ASL) and in some cases up to 21 km asl. Clouds of volcanic ash and gas stretched to the north and northwest up to 1,500 km and to the southeast for more than 3,000 km. For the first time in recorded history, ash fall occurred on Sakhalin Island and in the northeast sector of the Khabarovsky Region, Russia. Based on satellite image analysis and reconnaissance field studies in the summer of 2009, the eruption produced explosive tephra deposits with an estimated bulk volume of 0.4 km3. The eruption is considered to have a Volcanic Explosivity Index of 4. Because the volcano is remote, there was minimal risk to people or infrastructure on the ground. Aviation transport, however, was significantly disrupted because of the proximity of air routes to the volcano.  相似文献   

6.
We report the stratigraphic sequence of the 2005 eruption of Ilamatepec volcano together with sedimentological and chemical analyses of its products.Structural and textural characteristics of the deposits indicate that the eruption was driven by a small-volume rhyolitic intrusion at shallow levels, which resulted first in the collapse of the existing hydrothermally altered fan of previous deposits inside the crater lake, driving phreatic explosions with launching of blocks on ballistic trajectories; later the magma interacted with lake waters producing several hydromagmatic pyroclastic density currents (PDCs). These flows were energetic enough to knock down pine trees up to distances of 1.8 km from the crater in the E-NE sector of the volcano. Finally, ejection of ballistic blocks that landed on previously emplaced, wet pyroclastic density current deposits, caused the generation of a lahar that flowed down the steep eastern flank toward the El Jabillal gully. Subsequent lahars occurred as a result of intense rain caused by hurricane Stan.Radiocarbon ages on paleosols and charcoal fragments, separating previous volcanogenic sequences, indicate that similar eruptions have occurred more frequently in the past centuries, than previously thought.The new data confirms that Ilamatepec volcano is one of the most active volcanoes in El Salvador. Nevertheless, more detailed studies of the eruptive sequence of Ilamatepec volcano are mandatory to establish future eruptive patterns.  相似文献   

7.
Microgravity measurements and levelling surveys on volcanoes are not always easy to make, but are useful for studying volcanic processes quantitatively. Gravity changes associated with volcanic activity are not always significant. Precision of microgravity measurements depend critically on the procedures adopted, and those applied in the present paper are described. Levelling technique is now orthodox, and some empirical laws relating ground deformation to volcanic activity are deduced from the accumulated data. Gravity changes occur at the same time and places as ground deformations. The relationship between microgravity and height changes are discussed from the standpoint of analyzing the data obtained on volcanoes. The observational results obtained on four volcanoes in Japan are separately analyzed because each volcano exhibits different patterns of gravity changes and deformations. During the 1977–1982 activity of Usu volcano, deformation was accompanied by microgravity changes frequently observed at a particular benchmark at the base of the volcano for about five years. The gravity changes prove to be not a direct effect of magma movements but to be caused by the deformations of ground strata and aquifers around the benchmark. The 1983 eruption of Miyakejima volcano was associated with local gravity changes around the eruptive fissures due to magma intrusion which was approximately modelled. Similarly the 1986 eruption of Ooshima volcano caused gravity changes on the volcano, but these were poorly correlated with elevation changes and their origins were not uniquely interpreted. To detect gravity changes associated with the activity of Sakurajima volcano, an equigravity point was selected at the north of the volcano besides the gravity points on and around the volcano itself. The probable gradual accumulation of magmas beneath the volcano for eight years is substantiated by observed microgravity and elevation changes.  相似文献   

8.
Quantifying the potential ash fall hazards from re-awakening volcanoes is a topic of great interest. While methods for calculating the probability of eruptions, and for numerical simulation of tephra dispersal and fallout exist, event records at most volcanoes that re-awaken sporadically on decadal to millennial cycles are inadequate to develop rigorous forecasts of occurrence, much less eruptive volume. Here we demonstrate a method by which eruption records from radiocarbon-dated sediment cores can be used to derive forecasting models for ash fall impacts on electrical infrastructure. Our method is illustrated by an example from the Taranaki region of New Zealand. Radiocarbon dates, expressed as years before present (B.P.), are used to define an age-depth model, classifying eruption ages (with associated errors) for a circa 1500–10 500 year B.P. record at Mt. Taranaki (New Zealand). In addition, data describing the youngest 1500 years of eruption activity is obtained from directly dated proximal deposits. Absence of trend and apparent independence in eruption intervals is consistent with a renewal model using a mix of Weibulls distributions, which was used to generate probabilistic forecasts of eruption recurrence. After establishing that interval length and tephra thickness were independent in the record, a thickness–volume relationship (from [Rhoades, D.A., Dowrick, D.J., Wilson, C.J.N., 2002. Volcanic hazard in New Zealand: Scaling and attenuation relations for tephra fall deposits from Taupo volcano. Nat. Hazards, 26:147–174]) was inverted to provide a frequency–volume relationship for eruptions. Monte Carlo simulation of the thickness–volume relationship was then used to produce probable ash fall thicknesses at any chosen site. Several critical electrical infrastructure sites in the Taranaki Region were analysed. This region, being the only gas and condensate-producing area in New Zealand, is of national economic importance, with activities in and around the area depending on uninterrupted power supplies. Forecasts of critical ash thicknesses (1 mm wet and 2 mm dry) that may cause short-circuiting, surges or power shutdowns in substations show that the annual probabilities of serious impact are between ~ 0.5% and 27% over a 50 year period. It was also found that while large eruptions with high ash plumes tend to affect “expected” areas in relation to prevailing winds, the direction impacts of small ash falls are far less predictable. In the Taranaki case study, areas out of normal downwind directions, but close to the volcano, have probabilities of impact for critical thicknesses of 1–2 mm of around half to 60% of those in downwind directions and therefore should not be overlooked in hazard analysis. Through this method we are able to definitively show that the potential ash fall hazard to electrical infrastructure in this area is low in comparison to other natural threats, and provide a quantitative measure for use in risk analysis and budget prioritisation for hazard mitigation measures.  相似文献   

9.
Two explosive eruptions occurred on 2 January 1996 at Karymsky Volcanic Center (KVC) in Kamchatka, Russia: the first, dacitic, from the central vent of Karymsky volcano, and the second, several hours later, from Karymskoye lake in the caldera of Akademia Nauk volcano. The main significance of the 1996 volcanic events in KVC was the phreatomagmatic eruption in Karymskoye lake, which was the first eruption in this lake in historical time, and was a basaltic eruption at the acidic volcanic center. The volcanic events were associated with the 1 January Ms 6.7 (Mw 7.1) earthquake that occurred at a distance of about 9–17 km southeast from the volcanoes just before the eruptions. We study the long-term (1972–1995) and short-term (1–2 January 1996) characteristics of crustal deformations and seismicity before the double eruptive event in KVC. The 1972–1995 crustal deformation was homogeneous and characterized by a gradual extension with a steady velocity. The seismic activity in 1972–1995 developed at the depth interval from 0 to 20 km below the Akademia Nauk volcano and spread to the southeast along a regional fault. The seismic activity in January 1996 began with a short sequence of very shallow microearthquakes (M ~0) beneath Karymsky volcano. Then seismic events sharply increased in magnitude (up to mb 4.9) and moved along the regional fault to the southeast, culminating in the Ms 6.7 earthquake. Its aftershocks were located to the southeast and northwest from the main shock, filling the space between the two active volcanoes and the ancient basaltic volcano of Zhupanovsky Vostryaki. The eruption in Karymskoye lake began during the aftershock sequence. We consider that the Ms 6.7 earthquake opened the passageway for basic magma located below Zhupanovsky Vostryaki volcano that fed the eruption in Karymskoye lake.  相似文献   

10.
We report electric potential gradient measurements carried out at Sakurajima volcano in Japan during: (1) explosions which generated ash plumes, (2) steam explosions which produced plumes of condensing gases, and (3) periods of ashfall and plume-induced acid rainfall. Sequential positive and negative deviations occurred during explosions which generated ash plumes. However, no deflections from background were found during steam explosions. During periods of ashfall negative electric potential gradients were observed, while positive potential gradients occurred during fallout of plume-induced acid rain from the same eruption. These results suggest that a dipole arrangement of charge develops within plumes such that positive charges dominate in the volcanic gas-rich top and negative charges in the following ash-rich part of the plume. The charge polarity may be reversed for other volcanoes (Hatakeyama and Uchikawa 1952). We suggest that charge is generated by fracto-emission (Donaldson et al. 1988) processes probably during magma fragmentation within the vent, rather than by frictional effects within the plume.  相似文献   

11.
We present results from a detailed analysis of seismic and infrasonic data recorded over a four day period prior to the Vulcanian eruptive event at Sakurajima volcano on May 19, 1998. Nearly one hundred seismic and infrasonic events were recorded on at least one of the nine seismic–infrasonic stations located within 3 km of the crater. Four unique seismic event types are recognized based on the spectral features of seismograms, including weak seismic tremor characterized by a 5–6 Hz peak mode that later shifted to 4–5 Hz. Long-period events are characterized by a short-duration, wide spectral band signal with an emergent, high-frequency onset followed by a wave coda lasting 15–20 s and a fundamental mode of 4.2–4.4 Hz. Values of Q for long-period events range between 10 and 22 suggesting that a gas-rich fluid was involved. Explosive events are the third seismic type, characterized by a narrow spectral band signal with an impulsive high-frequency onset followed by a 20–30 second wave coda and a peak mode of 4.0–4.4 Hz. Volcano-tectonic earthquakes are the fourth seismic type. Prior to May 19, 1998, only the tremor and explosion seismic events are found to have an infrasonic component. Like seismic tremor, infrasonic tremor is typically observed as a weak background signal. Explosive infrasonic events were recorded 10–15 s after the explosive seismic events and with audible explosions prior to May 19. On May 19, high-frequency impulsive infrasonic events occurred sporadically and as swarms within hours of the eruption. These infrasonic events are observed to be coincident with swarms of long-period seismic events. Video coverage during the seismic–infrasonic experiment recorded intermittent releases of gases and ash during times when seismic and acoustic events were recorded. The sequence of seismic and infrasonic events is interpreted as representing a gas-rich fluid moving through a series of cracks and conduits beneath the active summit crater.  相似文献   

12.
Many volcanic eruptions are shortly preceded by injection of new magma into a pre-existing, shallow (<10 km) magma chamber, causing convection and mixing between the incoming and resident magmas. These processes may trigger dyke propagation and further magma rise, inducing long-term (days to months) volcano deformation, seismic swarms, gravity anomalies, and changes in the composition of volcanic plumes and fumaroles, eventually culminating in an eruption. Although new magma injection into shallow magma chambers can lead to hazardous event, such injection is still not systematically detected and recognized. Here, we present the results of numerical simulations of magma convection and mixing in geometrically complex magmatic systems, and describe the multiparametric dynamics associated with buoyant magma injection. Our results reveal unexpected pressure trends and pressure oscillations in the Ultra-Long-Period (ULP) range of minutes, related to the generation of discrete plumes of rising magma. Very long pressure oscillation wavelengths translate into comparably ULP ground displacements with amplitudes of order 10−4–10−2 m. Thus, new magma injection into magma chambers beneath volcanoes can be revealed by ULP ground displacement measured at the surface.  相似文献   

13.
Fuego volcano, Guatemala is a high (3,800 m) composite volcano that erupts gas-rich, high-Al basalt, often explosively. It spends many years in an essentially open vent condition, but this activity has not been extensively observed or recorded until now. The volcano towers above a region with several tens of thousands of people, so that patterns in its activity might have hazard mitigation applications. We conducted 2 years of continuous observations at Fuego (2005–2007) during which time the activity consisted of minor explosions, persistent degassing, paroxysmal eruptions, and lava flows. Radiant heat output from MODIS correlates well with observed changes in eruptive behavior, particularly during abrupt changes from passive lava effusion to paroxysmal eruptions. A short-period seismometer and two low-frequency microphones installed during the final 6 months of the study period recorded persistent volcanic tremor (1–3 Hz) and a variety of explosive eruptions. The remarkable correlation between seismic tremor, thermal output, and daily observational data defines a pattern of repeating eruptive behavior: 1) passive lava effusion and subordinate strombolian explosions, followed by 2) paroxysmal eruptions that produced sustained eruptive columns, long, rapidly emplaced lava flows, and block and ash flows, and finally 3) periods of discrete degassing explosions with no lava effusion. This study demonstrates the utility of low-cost observations and ground-based and satellite-based remote sensing for identifying changes in volcanic activity in remote regions of underdeveloped countries.  相似文献   

14.
A two-channel or split-window algorithm designed to correct for atmospheric conditions was applied to thermal images taken by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) of Lake Yugama on Kusatsu–Shirane volcano in Japan in order to measure the temperature of its crater lake. These temperature calculations were validated using lake water temperatures that were collected on the ground. Overall, the agreement between the temperatures calculated using the split-window method and ground truth is quite good, typically ± 1.5 °C for cloud-free images. Data from fieldwork undertaken in the summer of 2004 at Kusatsu–Shirane allow a comparison of ground-truth data with the radiant temperatures measured using ASTER imagery. Further images were analyzed of Ruapehu, Poás, Kawah Ijen, and Copahué volcanoes to acquire time-series of lake temperatures. A total of 64 images of these 4 volcanoes covering a wide range of geographical locations and climates were analyzed. Results of the split-window algorithm applied to ASTER images are reliable for monitoring thermal changes in active volcanic lakes. These temperature data, when considered in conjunction with traditional volcano monitoring techniques, lead to a better understanding of whether and how thermal changes in crater lakes aid in eruption forecasting.  相似文献   

15.
In order to clarify the time relation of the expansion of a gas pocket and failure of its overlying plug of lava during Vulcanian eruptions, infrasound records and video images of the Vulcanian eruption that occurred at Sakurajima volcano on January 2, 2007 were analyzed with respect to their origin times. Weak (≤3 Pa) and slowly increasing air pressure preceded the impulsive compression phase by 0.25–0.32 s, and a longer-period rarefaction phase of infrasound waves was recognized at all microphone stations. The velocity of the compression phase was assumed to be supersonic (ca. 400 m/s) up to 850 m above the crater bottom from other recent explosions. On the other hand, the propagation velocity of the preceding weak signal was regarded to be similar to the air sound velocity because the lack of impulsiveness is unlikely to be related to the main compression phase. Therefore, the estimated origin time of the main compression phase was delayed by 0.5–0.7 s from the preceding phase. The origin time of the preceding phase coincided with the onset of the isotropic expansion process of the pressurized gas pocket, which was obtained by the waveform inversion of the explosion earthquake. In contrast, the origin time of the main impulsive phase coincided with the time when the expansion rate reached its peak. This observation suggests that the volumetric increase of the gas pocket caused swelling of the surface of the crater bottom and its subsequent failure. When the expansion velocity exceeded a threshold level, the main impulsive compression phase radiated with a high velocity by the sudden releases of the pressurized gases. The volumetric change at the source was estimated to be 280–560 m3 from the preceding phase of the infrasound. This volume change indicates that the vertical displacement of the swelling ground was on the order of 1.0 m, assuming the radius of the lava plug was ca. 10 m.  相似文献   

16.
Despite the recent recognition of Mount Etna as a periodically violently explosive volcano, the hazards from various types of pyroclastic density currents (PDCs) have until now received virtually no attention at this volcano. Large-scale pyroclastic flows last occurred during the caldera-forming Ellittico eruptions, 15–16 ka ago, and the risk of them occurring in the near future is negligible. However, minor PDCs can affect much of the summit area and portions of the upper flanks of the volcano. During the past ~ 20 years, small pyroclastic flows or base-surge-like vapor and ash clouds have occurred in at least 8 cases during summit eruptions of Etna. Four different mechanisms of PDC generation have been identified during these events: (1) collapse of pyroclastic fountains (as in 2000 and possibly in 1986); (2) phreatomagmatic explosions resulting from mixing of lava with wet rock (2006); (3) phreatomagmatic explosions resulting from mixing of lava with thick snow (2007); (4) disintegration of the unstable flanks of a lava dome-like structure growing over the rim of one of the summit craters (1999). All of these recent PDCs were of a rather minor extent (maximum runout lengths were about 1.5 km in November 2006 and March 2007) and thus they represented no threat for populated areas and human property around the volcano. Yet, events of this type pose a significant threat to the lives of people visiting the summit area of Etna, and areas in a radius of 2 km from the summit craters should be off-limits anytime an event capable of producing similar PDCs occurs. The most likely source of further PDCs in the near future is the Southeast Crater, the youngest, most active and most unstable of the four summit craters of Etna, where 6 of the 8 documented recent PDCs originated. It is likely that similar hazards exist in a number of volcanic settings elsewhere, especially at snow- or glacier-covered volcanoes and on volcano slopes strongly affected by hydrothermal alteration.  相似文献   

17.
Large volcanic eruptions at dacitic or rhyolitic volcanoes often generate exceptional volumes of fine ash that mantles an area up to a million km2. These eruptions are characterized by extreme fragmentation of the magma and hence extraordinary dispersal of ash and are categorized as plinian, ultraplinian, or phreatoplinian events. Large-volume co-ignimbrites or co-plinian ashes are often produced by such eruptions. High fragmentation indices of > 90% are attributed to the violent eruption of silicic magma, especially if augmented by fuel-coolant reactions produced when abundant external water interacts with the magma. The present study documents a case where the fine ash (≤ 1 mm diameter) fall deposit related to the plinian phase of the eruption comprises the overwhelming bulk – about 87 wt.% of the eruptive products. This is another example demonstrating the predominance of a widespread, fine-grained, co-plinian ash which follows the initial coarser lapilli fall. Historical eruptions at two other Andean volcanoes Quizapu, (Chile) and Huaynaputina, (Peru), and at Santa Maria, (Guatemala) and Novarupta, (Alaska) produced similar ash fall sequences.  相似文献   

18.
Continuous tilt changes during the 2004–2005 effusive-explosive episodes at Volcán de Colima (México) were recorded simultaneously by two tiltmeters installed on opposite sides of the volcano at elevations of 2200 m and 3060 m above sea level. Data indicate that the 2004 lava extrusion was preceded by an inflation accompanied by a deflation. The 2005 explosion sequences were associated with a deflationary–inflationary tilt. The period between the 2004 extrusion and the 2005 explosions was characterized by an inflationary tilt during a 3 month period. Two deformation sources were located. The first was situated at a depth between 300 m and 1800 m beneath the crater at the northern flank of the volcano and was responsible for volcano deformation during the preliminary September 2004 stage, the October 2004 extrusion, and the initial stage of the transition period and the March 2005 explosion sequence. The second source was located at a depth between 1800 m and 2800 m beneath the crater at the southern flank of the volcano and was responsible for volcano deformation during the final stage of the transition period and the May–June 2005 explosion sequence.  相似文献   

19.
Analysis of ground-deformation data obtained at and around Sakurajima volcano during the 1914 eruption indicates that the deformation may be interpreted by assuming a model with two pressure sources, one shallow (about 2 km deep) and vertically directive and the other deep (about 8 km deep) and obliquely, directly beneath the volcano. This model is reasonable from the viewpoint of the volcanic processes.The local upheaval near the centers of eruption has scarcely recovered because it surpassed the elastic limit. The recovery of the regional depression after the eruption can be interpreted as pressure accumulation beneath the volcano. It may be concluded that the center of pressure would remain at the deeper source beneath the volcano, but that the pressure would change, resulting in surface deformation. The depression and its recovery suggest the presence of a pressure focus or a ‘magma reservoir’ beneath the volcano.  相似文献   

20.
The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic 3He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30?Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40?m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >102 to 106?Pa?s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma).  相似文献   

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