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1.
La Soufrière of Guadeloupe is a dangerous volcano characterized over the last decade by moderate seismic and fumarolic unrest. In the last 15,000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank collapse events sometimes associated with a magmatic eruption. We propose a new age of 1530 A.D. and a new eruptive scenario for the last magmatic eruption on the basis of a novel statistical analysis of radiocarbon age dates, and new field and geochemical data. This eruption is the only magmatic eruption likely to have occurred in Guadeloupe during the last 1400 years. The eruption mainly involved an andesitic magma which, in the first phase of the eruption, partially mixed with a slightly more differentiated magma stored in a small and shallow magma chamber. Ascent of magma to the surface generated a partial collapse of the hydrothermally altered edifice that increased the magma discharge and led to a sub-plinian phase with scoria fallout and column-collapse pyroclastic flows followed by near-vent pyroclastic scoria fountains. The eruption ended with growth of a lava dome. Our revised interpretation of the last magmatic eruption of La Soufrière constitutes the most likely key to a future magmatic eruption scenario for this volcano which displays strong evidence of unrest since 1992.  相似文献   

2.
Since the end of the last magmatic eruption (1890), activity of La Fossa (southern Tyrrhenian Sea, Italy) has consisted of fumarolic emissions of fluctuating intensity. Fluids are discharged principally at two fumarolic fields located in the northern rim of the active crater and at the beach sited at its northern foot. Increased thermal, seismic and geochemical activity has been recorded since 1978, when an earthquake of M=5.5 occurred in the region. This paper combines available geophysical and geochemical information in order to develop a tentative interpretation of two episodes of apparent unrest which occurred in 1985 and 1987–1988, enhancing the risk of renewal of the eruptive activity. The 1985 unrest consisted essentially of a sharp build up of the internal pressure in the shallow hydrothermal system, which was induced by the injection of hot gases of magmatic origin. The crater fumaroles displayed significant increases in CO2 and other acid species, but their outlet temperature did not change. Conversely, the 1987–1988 episode was characterized by appreciable modifications at the crater fumaroles, with only secondary effects at the fumarole system of the beach. The sliding of part of the eastern flank of the La Fossa cone into the sea occurred on 20 April 1988, when the region was affected by crustal dilatation producing a seismic sequence of relatively high intensity. Both episodes of unrest were accompanied by increases of local microseismic activity, which affected the nothern sector of the island in 1985, and the southern one in 1988. Finally, a phase of appreciable areal contraction was detected in 1990, probably due to the effect of the cooling and crystallization of magma at relatively shallow depths, accompanying the increased thermal activity at the crater fumaroles. Regional tectonic stress seems to play an important role in the transition of the volcanic system from a phase of relative stability to a phase of apparent unrest, inducing the heating and the expansion of shallow hydrothermal fluids. Available information is insufficient to indicate whether or not the volcano is building towards the renewal of a magmatic eruption, and there is no evidence to hypothesize episodes of significant magma migration. The frequency of measurements of many parameters needs to be increased in order to learn more about the temporal relationships between geochemical and geophysical variations preceding and accompanying periods of increased thermal activity. This will probably be a valid tool for recognizing short-term precursors of a future eruption, reducing the risk of false alarms.  相似文献   

3.
The 2002–03 Mt Etna flank eruption began on 26 October 2002 and finished on 28 January 2003, after three months of continuous explosive activity and discontinuous lava flow output. The eruption involved the opening of eruptive fissures on the NE and S flanks of the volcano, with lava flow output and fire fountaining until 5 November. After this date, the eruption continued exclusively on the S flank, with continuous explosive activity and lava flows active between 13 November and 28 January 2003. Multi-disciplinary data collected during the eruption (petrology, analyses of ash components, gas geochemistry, field surveys, thermal mapping and structural surveys) allowed us to analyse the dynamics of the eruption. The eruption was triggered either by (i) accumulation and eventual ascent of magma from depth or (ii) depressurisation of the edifice due to spreading of the eastern flank of the volcano. The extraordinary explosivity makes the 2002–03 eruption a unique event in the last 300 years, comparable only with La Montagnola 1763 and the 2001 Lower Vents eruptions. A notable feature of the eruption was also the simultaneous effusion of lavas with different composition and emplacement features. Magma erupted from the NE fissure represented the partially degassed magma fraction normally residing within the central conduits and the shallow plumbing system. The magma that erupted from the S fissure was the relatively undegassed, volatile-rich, buoyant fraction which drained the deep feeding system, bypassing the central conduits. This is typical of most Etnean eccentric eruptions. We believe that there is a high probability that Mount Etna has entered a new eruptive phase, with magma being supplied to a deep reservoir independent from the central conduit, that could periodically produce sufficient overpressure to propagate a dyke to the surface and generate further flank eruptions.Editorial responsibility: J. Donnelly-Nolan  相似文献   

4.
Data collected by a pressure sensor provide new insights into the 1999 eruption of Shishaldin volcano, Unimak Island, Alaska. On 19 April 1999, after 3 months of unrest and an extended period of low-level Strombolian activity, Shishaldin experienced a Subplinian eruption (ash plume to >16 km), followed by several episodes of strong Strombolian explosions. Acoustic data from the pressure sensor allow us to investigate the details of an eruption which was instrumentally well recorded, but with few visual observations. In the 12 h prior to the Subplinian phase, the pressure sensor detected a series of small, repeated pulses with a constant spectral peak at 2–3 Hz. The amplitude and occurrence rate of the pulses both grew such that the signal became a nearly continuous hum just before the Subplinian eruption. This humming signal may represent gas release from rising magma. The main Subplinian phase was heralded by (1) the abrupt end of the humming signal, (2) several pulses of low-frequency sound interpreted as ash bursts, and (3) a dramatic increase in seismic tremor amplitude. The change in acoustic signature at this time allows us to precisely time the start of the Subplinian eruption, previously approximated as the time of strongest tremor increase. The 50-min Subplinian phase actually contained several bursts of signal, each of which may represent a discrete volume of magma passing through the system. Following the Subplinian event, the pressure sensor recorded four discrete episodes of Strombolian gas explosions on 19–20 April and another on 22–23 April. Four of the five episodes were accompanied by strong seismic tremor; the fifth has not been previously recognized and was not associated with anomalous tremor amplitudes. In time series these events are similar to explosions recorded at other volcanoes but in general they are much larger, with maximum amplitudes of >65 Pa at 6.5 km from the vent, and they have low (0.7–1.5 Hz) peak frequencies. These large explosions occurred at rates of 3–20 per minute for 1–5 h in each episode. The explosions were accompanied by a small (<5 km above sea level) ash plume and only minor amounts of ejecta were produced. Thus, the explosion activity was dominated by gas release.  相似文献   

5.
Following the 2001 and 2002–2003 flank eruptions, activity resumed at Mt. Etna on 7 September 2004 and lasted for about 6 months. This paper presents new petrographic, major and trace element, and Sr–Nd isotope data from sequential samples collected during the entire 2004–2005 eruption. The progressive change of lava composition allowed defining three phases that correspond to different processes controlling magma dynamics inside the central volcano conduits. The compositional variability of products erupted up to 24 September is well reproduced by a fractional crystallization model that involves magma already stored at shallow depth since the 2002–2003 eruption. The progressive mixing of this magma with a distinct new one rising within the central conduits is clearly revealed by the composition of the products erupted from 24 September to 15 October. After 15 October, the contribution from the new magma gradually becomes predominant, and the efficiency of the mixing process ensures the emission of homogeneous products up to the end of the eruption. Our results give insights into the complex conditions of magma storage and evolution in the shallow plumbing system of Mt. Etna during a flank eruption. Furthermore, they confirm that the 2004–2005 activity at Etna was triggered by regional movements of the eastern flank of the volcano. They caused the opening of a complex fracture zone extending ESE which drained a magma stored at shallow depth since the 2002–2003 eruption. This process favored the ascent of a different magma in the central conduits, which began to be erupted on 24 September without any significant change in eruptive style, deformation, and seismicity until the end of eruption.  相似文献   

6.
The November 13, 1985 eruption of Nevado del Ruiz produced a series of pyroclastic flows and surges that eroded channels on the surface of the summit glacier and generated lahars which descended down most of the rivers that drain the volcano. The stratigraphy of the proximal pyroclastic deposits indicates that there were at least four episodes to the eruption. Episode I, deposited an unusual surge consisting of small pieces of ice mixed with ash and exhibiting planar stratification. Ballistically emplaced fragments are also intercalated with this unit. During Episode II, at least two pyroclastic flows were erupted. Their deposits contain the most evolved pumice of the entire eruption; SiO2 content of matrix glass ranges between 74.5 and 74.9%. Episode III is marked by the emplacement of a welded tuff with an average SiO2 content of about 66% in the matrix glass. The final Episode IV was characterized by the development of a high-altitude eruption column and the emplacement of several nonwelded pyroclastic flows. Banded pumice are common in the pyroclastic flow as well as in the pumice fall deposits. Co-existing dark and light pumice bands differ in SiO2 content by 3.5% and in general are similar to the composition of the welded pumice from Episode III.The compositional zonation of the pyroclastic deposits from Episode I to IV suggests that a nearsurface compositionally-stratified portion of the magma body was tapped during Episode II. During Episodes III and IV the main body of magma was involved although the coexistence of the compositionally distinct pumice clasts at similar stratigraphic levels argues for mixing of magma from different levels in the chamber during the eruptive process.  相似文献   

7.
Jun-Ichi  Kimura  Mamiko  Tateno  Isaku  Osaka 《Island Arc》2005,14(2):115-136
Abstract   The geology and geochemistry of pyroclastic flows and fallout tephras formed during the Karasugasen dome eruption in the Daisen–Hiruzen Volcano Group in southwest Japan have been examined in detail. The Karasugasen lava dome erupted at about 26 ka. The eruption began with a vulcanian ash fall, and this was followed by at least eight block and ash flows and a pumice flow. The block and ash flows were produced by the successive collapses of a growing lava dome. This main eruption phase was followed by an eruption of vulcanian ash falls, and finally ended with a sub-Plinian pumice fall. This eruption sequence is typical of the Daisen Volcano during the last three eruption events, which occurred at 58, 26 and 17 ka. The magma produced during the Karasugasen eruption was a typical adakite, with extremely high Sr/Y ratios and low HREE/LREE ratios compared to normal arc lavas. The chemistry of the Karasugasen lavas is almost identical to other Daisen–Hiruzen lavas that were produced from eruptions over an interval of a million years. The continuous supply of a huge amount of adakitic magma (>100 km3) for such a long period suggests a massive homogeneous source material, such as molten Philippine Sea Plate slab. Slab melting is a plausible mechanism for the production of the adakitic lavas at Karasugasen, and hence the Daisen–Hiruzen Volcano Group.  相似文献   

8.
The 2013 eruption of Pavlof Volcano, Alaska began on 13 May and ended 49 days later on 1 July. The eruption was characterized by persistent lava fountaining from a vent just north of the summit, intermittent strombolian explosions, and ash, gas, and aerosol plumes that reached as high as 8 km above sea level and on several occasions extended as much as 500 km downwind of the volcano. During the first several days of the eruption, accumulations of spatter near the vent periodically collapsed to form small pyroclastic avalanches that eroded and melted snow and ice to form lahars on the lower north flank of the volcano. Continued lava fountaining led to the production of clastogenic lava flows that extended to the base of the volcano, about 3–4 km beyond the vent. The generation of fountain-fed lava flows was a dominant process during the 2013 eruption; however, episodic collapse of spatter accumulations and formation of hot spatter-rich granular avalanches was a more efficient process for melting snow and ice and initiating lahars. The lahars and ash plumes generated during the eruption did not pose any serious hazards for the area. However, numerous local airline flights were cancelled or rerouted, and trace amounts of ash fall occurred at all of the local communities surrounding the volcano, including Cold Bay, Nelson Lagoon, Sand Point, and King Cove.  相似文献   

9.
地震、形变、火山气体地球化学等观测结果表明2002~2005年长白山天池火山经历了1次扰动事件。长白山站地震台(CBS台)记录到了扰动事件前后连续稳定的宽频带地震观测资料。前人的观测研究结果认为长白山天池火山扰动期间的火山地震类型主要为构造型火山地震,伴随少量的谐频型地震。本文通过匹配滤波技术,对1999~2007年扰动事件前后CBS台单台三分量地震观测数据进行模板扫描,获得3763个清晰的火山地震事件,其中谐频(HS)事件125个,构造(VT)事件3618个,并发现长周期(LP)事件20个。进而将火山扰动期间火山地震事件分为3种类型:构造型事件、长周期事件和谐频型事件,并提出2002~2005年长白山天池火山扰动机制模型:深源地震-火山能量传递模型,即汪清深源地震能量释放和传递,引发长白山火山区岩石圈应力状态波动。地幔岩浆房受应力干扰后,岩浆通道打开,少量岩浆侵入地壳岩浆房。岩浆混合脱气导致地壳岩浆房升压,引起顶部岩石微破裂,产生构造型火山地震,气体和流体填充这些裂隙,从而产生LP和HS型火山地震事件。  相似文献   

10.
Systematic analyses of the major-element chemistry of products of several eruptions during syn-and post-caldera stages of Izu-Oshima volcano were compiled. Comparisons of the products of large-scale eruptions in 1338?, 1421? and 1777–1778, of intermediate-scale eruptions in 1950–1951 and 1986, and of small-scale eruptions in 1954, 1964 and 1974 clearly show the existence of two types of magmas. One is “plagioclase-controlled” and the other is “differentiated” magma (multimineral-controlled); i.e. the bulk chemistry of the first magma type is controlled by plagioclase addition or removal, while that of the second type is controlled by fractionation of plagioclase, orthopyroxene, clinopyroxene, and titanomagnetite. Eruptions of Izu-Oshima volcano have occurred at the summit and along the flanks. Summit eruptions tap only plagioclase-controlled magmas, while flank eruptions supply both magma types. It is considered unlikely that both magma types would coexist in the same magma chamber based on the petrology. In the case of the 1986 eruption, the flank magma was isolated sometime in the past from the summit magma chamber or central conduit, and formed small magma pockets, where further differentiation occurred due to relatively rapid cooling. In a period of quiescence prior to the 1986 eruption, new magma was supplied to the summit magma chamber, and the summit eruption began. The dike intrusion or fracturing around the small magma pockets triggered the flank eruption of the differentiated magma. This model can be applied to the large-scale flank eruption in 1338(?) which erupted differentiated magmas. In 1421(?), the flank eruption tapped plagioclase-controlled magma. In this case, the isolated magmas from the summit magma chamber directly penetrated the flank without differentiation.  相似文献   

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