Petrologic Study of Explosive Pyroclastic Eruption Stage in Shirataka Volcano, NE Japan: Synchronized Eruption of Multiple Magma Chambers
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Some of detailed petrologic studies on rock samples of middle to large sized explosive pyroclastic eruptions recently revealed that the eruptions were caused by simultaneous eruption of multiple distinct magma chambers beneath the volcanoes (e.g., Nakagawa et al. 2003: Shane et al. 2007). It is very important to examine the genetic relationships among the magmas to understand the magma feeding system which caused such explosive eruptions. The explosive pyroclastic eruption stage in Shirataka volcano, NE Japan (Fig. 1) is one of potential candidates for such kind of researches. The aim of this study is to reveal the magma feeding system beneath Shirataka volcano in the explosive pyroclastic eruption stage and examine the genetic relationships among magmas involved in the explosive eruption.Keywords:
Peléan eruption
Vulcanian eruption
Pyroclastic fall
Phreatic eruption
Effusive eruption
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Dense-rock equivalent
Vulcanian eruption
Effusive eruption
Peléan eruption
Phreatic eruption
Lateral eruption
Phreatomagmatic eruption
Volcanology
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The pyroclastic fall deposits of the Asama-Maekake volcano, such as A (1783AD), B ′ and B (12 century), and C (4 century), are mainly composed of pumice layers. On the other hand, ash fall derived from the recent vulcanian eruptions (e.g., 2004 and 2009 eruptions) is too small in scale to be preserved as a geologic unit. Ash particles from such small-scale eruptions are mainly lithic fragments originating from solidified lava in a shallow level of the conduit. After the 1783 eruption, repeated vulcanian eruptions have formed ash and soil mixtures on the flank of the volcano. Similar ash and soil mixtures are also recognized beneath A, B, C, and D pyroclastic fall deposits, respectively. These ash and soil mixtures contain lithic fragments as the ash component, indicating that vulcanian eruptions occurred repeatedly in the period between large-scale eruptions, similarly to the period after the 1783 eruption. Lithic ash layers are also interbedded with pumice fall layers of B′, B, and E pyroclasic fall deposits. There seem to be some cases of intermittent vulcanian and sub-plinian eruptions in the course of the large-scale eruption. In the case of the 1783 eruption, detailed reconstruction of the eruptive sequence is possible on the basis of correlation between the stratigraphy of the eruptive products and old documents. The large-scale sub-plinian eruption is considered to be associated with the formation of a pyroclastic cone in a proximal area owing to vigorous fountaining. Subsequently, large-scale clastogenic lava flows are generated throughout its climactic eruption. On the other hand, little information is available for eruptions before 1783 because of limited exposure and few old documents. Although the reconstruction accuracy for the eruptions in the 12th century is not as good as that for the 1783 eruption, these eruptions might have occurred with a different sequence from the 1783 eruption. Intermittent events of ash and pumice fall occurred in the initial stage of these eruptions. Phreatomagmatic eruption also occurred in the early stage of the 1128 eruption, resulting in a B ′ pyroclastic fall deposit. The existence of many units of pyroclastic flows in the 1108 eruption indicates that pyroclastic flow occurred on multiple occasions. Since the stratigraphic relationship between the B pyroclastic fall and these pyroclastic flows is unclear, the sequence of the eruption is still in question. Furthermore, little information is available for eruptions predating the 12 century. Comparative study of the distributions of pyroclastic fall deposits using isopach maps reveals that some fall units from B′ and B are larger in scale than that of the climactic pyroclastic fall deposit of the 1783 eruption. In addition, the A′ pyroclastic fall deposit is estimated to be comparable to or smaller than the preclimactic fall unit of the 1783 eruption. Although the isopach maps of A, B ′ , B, C, and E could be prepared, the accuracy of the isopach maps for the C and E pyroclastic fall deposits is insufficient. The preparation of an accurate map is difficult for deposits of older age. Consequently, at this point, the 1783 eruption is the only example in which the temporal variation in eruptive style and in eruptive volume can be discussed with high accuracy in the history of the Asama-Maekake volcano.
Peléan eruption
Pumice
Pyroclastic fall
Volcanic ash
Dense-rock equivalent
Effusive eruption
Vulcanian eruption
Phreatic eruption
Strombolian eruption
Lateral eruption
Phreatomagmatic eruption
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Some of detailed petrologic studies on rock samples of middle to large sized explosive pyroclastic eruptions recently revealed that the eruptions were caused by simultaneous eruption of multiple distinct magma chambers beneath the volcanoes (e.g., Nakagawa et al. 2003: Shane et al. 2007). It is very important to examine the genetic relationships among the magmas to understand the magma feeding system which caused such explosive eruptions. The explosive pyroclastic eruption stage in Shirataka volcano, NE Japan (Fig. 1) is one of potential candidates for such kind of researches. The aim of this study is to reveal the magma feeding system beneath Shirataka volcano in the explosive pyroclastic eruption stage and examine the genetic relationships among magmas involved in the explosive eruption.
Peléan eruption
Vulcanian eruption
Pyroclastic fall
Phreatic eruption
Effusive 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.
Phreatic eruption
Dense-rock equivalent
Volcanic hazards
Peléan eruption
Volcanic ash
Vulcanian eruption
Effusive eruption
Volcanology
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By research with field investigation and observation records of volcanic eruption,combining with the achievement in volcanic research home and abroad,the authors discuss the volcanic eruption features of Laoheishan and Huoshaoshan in Wudalianchi of Heilongjiang,China in volcanic genetic type,eruption mode,eruption symptom and other aspects,and point out that the Laoheishan and Huoshaoshan volcanoes are monogenetic in genetic type,the eruption mode is not a simple way of central eruption,but experienced fission eruption turn to central eruption.Through the analysis of volcanic eruption observational record and the comparison with foreign volcanoes,it is revealed that this volcanic eruption possesses precursor which features benefit the monitor and forecast future volcanic eruption.
Peléan eruption
Dense-rock equivalent
Vulcanian eruption
Effusive eruption
Lateral eruption
Phreatic eruption
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Phreatic eruption
Peléan eruption
Vulcanian eruption
Effusive eruption
Dense-rock equivalent
Phreatomagmatic eruption
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