Magma Budget From Lava and Tephra Volumes Erupted During the 25-26 October 2013 Lava Fountain at Mt Etna
Daniele AndronicoBoris BehnckeEmanuela De BeniA. CristaldiSimona ScolloManuela LópezMaria Deborah Lo Castro
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Determining the volume of the various types of products of a highly frequent active volcano can be very difficult, especially if most of them are deposited on a growing volcanic cone. The New South-East Crater (NSEC) of Mt Etna, Italy, may be considered one of the best case studies because of tens of paroxysmal episodes which it produced in the last few years. On 25-26 October 2013, a lava fountain at the NSEC produced magma jets up to 500 m high, a maximum ~8 km high column, a multilobate lava flow field 1.3 to 1.5 km long, and almost 30 m of growth in height of the NSEC cone. Mapping of explosive and effusive deposits allowed us to calculate the total volume of erupted products, including lava flows, proximal and distal tephra fallout, and the amount of coarse pyroclastics on the cone. The estimation of the latter products was also confirmed subtracting digital elevation models (DEMs) obtained at different stages of the NSEC growth. Results show that the volume of tephra fallout away from the cone was only <5 % of the total erupted magma, while the total volume of pyroclasts (distal plus proximal fallout) is about a third of the lava volume. Our analysis suggests that, at least for the studied event, three fourth of the involved magma was already partially degassed and thus emitted as lava flows. Hence, the main distinctive character of lava fountains at Etna, i.e. formation of eruption column and propagation of tephra-laden volcanic plumes to tens of km away from the volcano, would not contribute significantly to the final budget of erupted magma of the 25-26 October 2013 eruption. We finally propose that the same magma dynamics probably occur also during most of the common lava fountain episodes.Keywords:
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Chaitén Volcano erupted unexpectedly in May 2008 in one of the largest eruptions globally since the 1990s. It was the largest rhyolite eruption since the great eruption of Katmai Volcano in 1912, and the first rhyolite eruption to have at least some of its aspects monitored. The eruption consisted of an approximately 2-week-long explosive phase that generated as much as 1 km3 bulk volume tephra (~0.3 km3 dense rock equivalent) followed by an approximately 20-month-long effusive phase that erupted about 0.8 km3 of high-silica rhyolite lava that formed a new dome within the volcano’s caldera. Prior to its eruption, little was known about the eruptive history of the volcano or the hazards it posed to society. This edition of Andean Geology contains a selection of papers that discuss new insights on the eruptive history of Chaitén Volcano, and the broad impacts of and new insights obtained from analyses of the 2008-2009 eruption. Here, we summarize the geographic, tectonic, and climatic setting of Chaitén Volcano and the pre-2008 state of knowledge of its eruptive history to provide context for the papers in this edition, and we provide a revised chronology of the 2008-2009 eruption.
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Explosive volcanic eruption is one of the most hazardous natural phenomena. During explosive eruptions, a mixture of volcanic ash and gases is ejected from a volcanic vent into the atmosphere. For hazard risk assessment, it is important to comprehensively explain various observed data during eruptions and to understand the dynamics of explosive eruptions and the mechanism of volcanic ash dispersal. We have developed a pseudo-gas model of eruption cloud dynamics and ash dispersal. Our model has successfully reproduced the heights of eruption cloud and the distribution of fall deposits during large eruptions such as the Pinatubo 1991 eruption and those during small eruptions such as the Shinmoe-dake 2011 eruption. For more accurate estimates of volcanic hazard risks, two-way coupled models of multiphase flow are required.
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A DISCUSSION ON THE TYPE OF THE QIXIANGZHAN ERUPTION OF CHANGBAISHAN TIANCHI VOLCANO,NORTHEAST CHINA
Changbaishan Tianchi volcano is one of the most dangerous active volcanoes in Northeast Asia.It experienced three periods of large-scale eruptions since the Holocene,i.e.the Tianwenfeng period at about 5,000 years ago,the Qixiangzhan period at about 4,000 years ago and the Millennium eruption at about 1,100 years ago,respectively.The type of Tianwenfeng and Millennium eruptions is commonly accepted to be a typical Plinian eruption.However,there arises a considerable debate about the type of Qixiangzhan eruption as whether it is effusive or explosive.In high-resolution remote-sensing images,the morphology of the products of Qixiangzhan eruption looks like a lava flow,which erupts from Qixiangzhan parasitic crater,and flows along the northern slope of the volcanic cone about 5.4km in length and 400~800m in width.However,the recent researches by the author have revealed that the Qixiangzhan eruption should be a small-scale pulsed explosive eruption.The main evidence is as follows:1) The bulk-rock composition of Qixiangzhan eruption products is characterized by high SiO2(≥71%) and Na2O + K2O(≥10%) contents representative of alkaline magma,which has high viscosity,low flowing ability and extremely high potential of explosive eruption; 2) Field observations show that the Qixinagzhan eruption products appear as thin layers about 2~5cm in thickness,the central part of which is welded stronger than the edge,significantly different from the massive or slaggy structures of lava flow.In addition,flame structures indicative of explosive eruption are well developed in the volcanic deposits around parasitic crater; 3) Microscopic observation reveals that most of the phenocrysts in the Qixiangzhan eruption products were severely broken by explosion to form angular grains with well developed micro-cracks.The vesicles in the Qixiangzhan eruption products are irregular in shape and have rough margin,different significantly from the elliptical and smooth margin vesicles commonly observed in lava flow; 4) Stereomicroscopic observation shows that the Qixiangzhan eruption products are composed of clastic particles and exhibit grain-supported texture with well developed irregular vesicles.Based on the above analyses,we may conclude that the Qixiangzhan eruption can be assigned to a small-scale pulsed explosive eruption.During the explosive eruption,a large number of fine pyroclastic particles flowed down the mountain slope as a high speed pyroclasstic flow to form thin layer of ignimbrite under the action of high temperature and strong shear forces.Over many times of explosive eruptions,layer upon layer of ignimbrite were accumulated,resulting in a shape just like lava flow.Therefore,all the three large eruptions of Changbaishan Tianchi volcano in Holocene can be assigned to explosive eruption,rather than the previously proposed model of explosive-effusive-explosive explosions.
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Holocene volcanism within the Katla volcanic system is characterized by: 1) explosive (hydromagmatic) basaltic eruptions along volcanic fissures within the Mýrdalsjökull caldera; 2) explosive silicic eruptions from vents associated with the caldera and 3) predominantly effusive basaltic eruptions involving both the central volcano and the fissure swarm. Typical Katla eruptions are accompanied by basaltic tephra fall, lightning and glacial floods (jökulhlaups) of meltwater, ice and volcanic debris. Twenty eruptions have occurred in the last 11 centuries. The volume of airborne tephra varies by three orders of magnitude, with an estimated volume of 1.5 km³ of freshly fallen tephra from the largest historical Katla eruption. The length of documented eruptions varies from 2 weeks to over 5 months. The average repose period since 1500 AD is 47 years with maximum deviations of 33 and 34 years. All Katla eruptions during the last 400 years have begun in the springfall season. At least 12 silicic Katla eruptions are known from the period ca. 1700 BP and 6600 BP. The silicic magma was most likely erupted by hydromagmatic explosive eruptions. The tephra dispersal axes indicate vent locations within the caldera or along the caldera rim. The volume of airborne silicic tephra varies by orders of magnitude, the largest and most widespread is tephra layer UN with uncompacted tephra volume of 0.3 km³. Intervals between the silicic eruptions have varied from ca. 100 to ca. 1000 14C yrs. Two major “fires” and 5–10 relatively minor, partly effusive eruptions have occurred during the Holocene. The 10th century Eldgjá and the 6800 BP Hólmsá fires are the largest known Holocene eruptions within the Katla system. A ≥75 km long, discontinuous and partly subglacial eruptive fissure was active during the Eldgjá eruption. The opening phase on most fissure segments was explosive, followed by an effusive phase on the subaerial segments. The eruption produced a voluminous basaltic tephra layer with a minute silicic component, two major lava fields and possibly a hyaloclastic flow deposit. Large jökulhlaups accompanied the eruption. The combined volume of erupted material may exceed 19 km³ DRE. Eruptions on the Katla system have caused extensive environmental changes during the past 1100 years. The Eldgjá fires radically changed the landscape, hydrology and utilization potential of large areas in South Iceland. Since then, jökulhlaups accompanying eruptions within the caldera have escaped eastwards, raising the Mýrdalssandur plain and extending its coastline southward.
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