Hydrological and geochemical change related to volcanic activity of Usu volcano, Japan
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Stratovolcano
Caldera
Fumarole
Phreatic eruption
Abstract The Himeji–Yamasaki region in the Inner Zone of southwest Japan is underlain mainly by Late Cretaceous volcanic rocks called the Ikuno Group or the Hiromine and Aioi Groups. A new stratigraphic and geochronological study shows that the volcanic rocks in this area consist of 15 eroded caldera volcanoes between 82 and 65 Ma; they are, in order of decreasing age, the Hiromine, Hoden, Ibo, Okawachi, Seppikosan, Hayashida, Shinokubi, Fukusaki, Kurooyama, Ise, Fukadanigawa, Nagusayama, Matobayama, Yumesaki and Mineyama Formations. These calderas vary in diameter from 1 to 20 km and are bounded by steep unconformities; they coalesce and overlap each other. The individual caldera fills are composed mainly of single voluminous pyroclastic flow deposits, which are often interleaved with debris avalanche deposits and occasionally underlie lacustrine deposits. The intracaldera pyroclastic flow deposits are made up of massive, welded or non‐welded tuff breccia to lapilli tuff, and are characterized by their great thickness. The debris avalanche deposits are ill‐sorted breccia, generated by the collapse of the caldera wall toward the caldera floor during the pyroclastic‐flow eruption. The large calderas that are more than 10 km in diameter contain original values of approximately 100 km 3 of intracaldera pyroclastic flow deposits. These large calderas are similar to the well‐known Valles‐type calderas in their dimensions, although it is uncertain whether their caldera floors are coherent plates or incoherent pieces. Conversely, the small calderas have diatreme‐like subsurface structures. The variety of the caldera volcanoes in this area is caused by the difference in the volume of caldera‐forming pyroclastic eruptions, as the large and small calderas coexisted. The caldera‐forming eruption rates in Late Cretaceous southwest Japan, including the studied area, were similar to those in late Cenozoic central Andes and northeast Honshu arc, Japan, but obviously smaller than those of late Cenozoic intracratonic caldera clusters in western North America and the Quaternary extensional volcanic arcs in Taupo, New Zealand. The widespread Late Cretaceous felsic igneous rocks in southwest Japan were generated by a long‐term accumulation of low‐rate granitic magmatism at the eastern margin of the Eurasian Plate.
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Lapilli
Breccia
Diatreme
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Each of the three phases of the 2006 eruption at Augustine Volcano had a distinctive eruptive style and flowage deposits. From January 11 to 28, the explosive phase comprised short vulcanian eruptions that punctuated dome growth and produced volcanowide pyroclastic flows and more energetic hot currents whose mobility was influenced by efficient mixing with and vaporization of snow. Initially, hot flows moved across winter snowpack, eroding it to generate snow, water, and pyroclastic slurries that formed mixed avalanches and lahars, first eastward, then northward, and finally southward, but subsequent flows produced no lahars or mixed avalanches. During a large explosive event on January 27, disruption of a lava dome terminated the explosive phase and emplaced the largest pyroclastic flow of the 2006 eruption northward toward Rocky Point. From January 28 to February 10, activity during the continuous phase comprised rapid dome growth and frequent dome-collapse pyroclastic flows and a lava flow restricted to the north sector of the volcano. Then, after three weeks of inactivity, during the effusive phase of March 3 to 16, the volcano continued to extrude the lava flow, whose steep sides collapsed infrequently to produce block-and-ash flows. The three eruptive phases were each unique not only in terms of eruptive style, but also in terms of the types and morphologies of deposits that were produced, and, in particular, of their lithologic components. Thus, during the explosive phase, low-silica andesite scoria predominated, and intermediate- and high-silica andesite were subordinate. During the continuous phase, the eruption shifted predominantly to high-silica andesite and, during the effusive phase, shifted again to dense low-silica andesite. Each rock type is present in the deposits of each eruptive phase and each flow type, and lithologic proportions are unique and consistent within the deposits that correspond to each eruptive phase. The chief factors that influenced pyroclastic currents and the characteristics of their deposits were genesis, grain size, and flow surface. Column collapse from short-lived vulcanian blasts, dome collapses, and collapses of viscous lavas on steep slopes caused the pyroclastic currents documented in this study. Column-collapse flows during the explosive phase spread widely and probably were affected by vaporization of ingested snow where they overran snowpack. Such pyroclastic currents can erode substrates formed of snow or ice through a combination of mechanical and thermal processes at the bed, thus enhancing the spread of these flows across snowpack and generating mixed avalanches and lahars. Grain-size characteristics of these initial pyroclastic currents and overburden pressures at their bases favored thermal scour of snow and coeval fluidization. These flows scoured substrate snow and generated secondary slurry flows, whereas subsequent flows did not. Some secondary flows were wetter and more laharic than others. Where secondary flows were quite watery, recognizable mixed-avalanche deposits were small or insignificant, and lahars were predominant. Where such flows contained substantial amounts of snow, mixed-avalanche deposits blanketed medial reaches of valleys and formed extensive marginal terraces and axial islands in distal reaches. Flows that contained significant amounts of snow formed cogenetic mixed avalanches that slid across surfaces protected by snowpack, whereas water-rich axial lahars scoured channels. Correlations of planimetric area (A) versus volume (V) for pyroclastic deposits with similar origins and characteristics exhibit linear trends, such that A=cV2/3, where c is a constant for similar groups of flows. This relationship was tested and calibrated for dome-collapse, column-collapse, and surgelike flows using area-volume data from this study and examples from Montserrat, Merapi, and Mount St. Helens. The ratio A/V2/3=c gives a dimensionless measure of mobility calibrated for each of these three types of flow. Surgelike flows are highly mobile, with c≈520; column-collapse flows have c≈150; and dome-collapse flows have c≈35, about that of simple rock avalanches. Such calibrated mobility factors have a potential use in volcano-hazard assessments.
Lava dome
Stratovolcano
Peléan eruption
Lahar
Effusive eruption
Pyroclastic fall
Phreatic eruption
Phreatomagmatic eruption
Scoria
Dome (geology)
Strombolian eruption
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Fumarole
Stratovolcano
Caldera
Phreatic eruption
Lava dome
Breccia
Dacite
Silicic
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This study shows the stratigraphy, petrography, whole-rock chemistry and paleomagnetic polarity of pyroclastic flow deposits during the latest Pliocene to Middle Pleistocene in the southeastern foot area of the Hakkoda Caldera, Northeast Japan. Six pyroclastic flow deposits are identified in this area : Kumanosawa Pyroclastic Flow Deposits (Ks), Takatoge Pyroclastic Flow Deposits (Tk), Osegawa Pyroclastic Flow Deposits (Os), Hakkoda-Ose Pyroclastic Flow Deposits (Hto), Hakkoda 1st-stage Pyroclastic Flow Deposits (Ht1) and Hakkoda 2nd-stage Pyroclastic Flow Deposits (Ht2), in order of decreasing age. These pyroclastic flow deposits have dacitic to rhyolitic compositions, and show distinct modal compositions and whole-rock major element chemistry in each. Based on stratigraphy, topography and paleomagnetic polarity, eruptive ages of the pyroclastic flow deposits are estimated to be as follows : Ks, 1.95-1.77 Ma; Tk, 1.77-1.07 Ma ; Os and Hto, 0.99-0.78 Ma ; Ht1, 0.76 Ma ; Ht2, 0.40 Ma. The source calderas of the Ks, Tk, Os, and Hto can be estimated from petrological features. The source of the Ks is neither the Hakkoda Caldera nor Okiura Caldera, and is probably an unidentified caldera. The source of the Tk may be a low gravity anomaly area "Nenokuchi Caldera" located northeast of Towada Caldera. The source of the Os is not the Okiura Caldera. The source of the Hto is the Hakkoda Caldera. This study suggests that the two large-scale pyroclastic flow deposits during 1-2 Ma erupted from previously unidentified calderas (one is possibly Nenokuchi Caldera). This proposes a revised volcanic history in which the activities of caldera volcanoes overlap each other in the Hakkoda-Towada Volcanic Region.
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Southren Kyushu has been the region of intense volcanism at least since Pliocene time. One of the most characteristic features is the prevalence of the large-scale pyroclastic flow eruptions which originated from such gigantic calderas as Aira, Ata, Kikai and Kakuto.There exist a considerable number of literature on the stratigraphic sequence and distributions of the pyroclastic flow deposits in South Kyushu. However, relatively small number of reports are available on air-fall tephra deposits, which are useful for establishing Quaternary chronology both of source volcanoes and of marine or fluvial sediments in the coastal regions such as the Miyazaki Plain. In this study, each bed of maker-tephras which erupted during the time from ca. 100, 000 to 25, 000y.B.P., is precisely discriminated and described in the northern part of the Osumi Peninsula, Kagoshima Prefecture first. And then each tephra is traced northeastward along the main axis of distributions to the Miyazaki Plain.Of many tephras, the following four well-dated tephras are used as fundamental timemakers because of their widespread occurence; Ata pyroclastic flows, originated from Ata caldera in 95, 000-90, 000y.B.P. ; Kikai-Tozurahara ash falls, originated from Kikai caldera in 75, 000y.B.P. ; Aso-4 pyroclastic flows, originated from Aso caldera in 70, 000y.B.P.; Ito pyroclastic flows and AT ash, originated from Aira caldera in 22, 000-21, 000y.B.P. Several air-fall tephras from the Aira and Kirishima volcanic centers are identified in detail and roughly dated from their stratigraphic positions between these fundamental maker-beds.About 75, 000-70, 000y.B.P., explosive activity of Aira caldera occurred resulting in the formation of plinian pumice fall deposit, Fukuyama pumice falls, which is found from the Osumi Peninsula to the Miyazaki Plain. During ca. 60, 000-25, 000y.B.P., intermittent eruptions occurred forming five sheets of tephras, of which the Iwato eruption was greatest in producing pumice falls, pyroclastic surges and pyroclastic flows. Iwato pumice falls mantle extensive area from the Osumi Peninsula to the Miyazaki Plain. Cataclysmic eruption occurred from Aira caldera, producing Osumi pumice falls, Tsurnaya and Ito pyroclastic flows and AT ash 22, 000-21, 000y.B.P. Most of these eruptions were accompanied with phreatomagmatic ones.Eruptive history of Kirishima volcano is divided into two stages deduced from the tephra sequence. At ca. 40, 000 y.B.P., older stage of activity started with ejection of relatively felsic pumice falls, Iwaokoshi pumice fall, and graded to more mafic and frequent eruptions, Awaokoshi scoria fall. Younger stage began with the plinian eruption of Kobayashi pumice fall at ca. 15, 000y.B.P.Of many terraces in Miyazaki Plain, Sanzaibaru terrace is the most extensive one and is accompanied with transgressive marine deposits. Stratigraphic relation with tephra sequence shows that Sanzaibaru terrace was emerged before the Ata pyroclastic flow eruption, ca. 95, 000y.B.P., probably indicating the Last Interglacial Stage. Most of terraces younger than Sanzaibaru are of fluvial origin, except for Nyutabaru II and probably III terraces which are partly of marine origin, and are largely devided into two groups, older and younger. Older terraces, Nyutabaru terrace group, formed during the time from the Ata eruption to the Aso-4 eruption, were chracterized by the profiles with more gentle gradient. Younger ones which were chracterized by the profiles with steeper gradient, were formed after the Aso-4 eruption and before the Kobayashi pumice fall. The difference of their profiles reflects the sea level after the maximum stage in the Last Interglacial Age.
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This chapter contains sections titled: Introduction Precaldera Events The Osumi Pumice Fall Tsumaya Pyroclastic Flow Kamewarizaka Breccia Ito Pyroclastic Flow Nature of the Magma Formation of the Aira Caldera Post-Aira Caldera Activity Funnel-Shaped Underground Structure of the Aira Caldera and Other Japanese Calderas Conclusion
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