Volcanic Eruption and Nature of the Pyroclastic Deposits
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This article deals with the mechanism of eruption and transportation of the pyroclastic material and the nature of the resultant deposits from the geological standpoint.In Japan, the method of tephrochronology is best applied to pyroclastic deposits of the Quaternary central volcanoes and those related to the Krakatoan calderas. Most of the rocks are andesitic in composition with subordinate amount of basalt and dacite.Three modes of volcanic eruption may be distinguished: 1) projection of pyroclastic materials which form pyroclastic fall deposits, 2) eruption of pyroclastic flows, and 3) outflow of lava flows or extrusion of dome and spine. Table 1 shows characteristic features of the deposits formed by the three modes of volcanic eruption.Tephra, as originally defined by Thorarinsson, signifies only the air-fall pyroclastic materials and its relation to pyroclastic flow is not clear. In this article, all the pyroclastic materials directly connected with volcanic eruptions, irrespective of their origin (i. e. essential, accessory, or accidental) and of their mode of emplacement, are included in the term tephra. The chronology using the deposits of pyroclastic flows are included in the tephrochronology.The small-scale vesiculation occurring at or close to the top of the magma column results in the so-called Strombolian and Vulcanian eruptions. Larger scale vesiculation with longer time duration leads to the Plinian eruption. The greatest vesiculation takes place within the magma reservoir resulting in the formation of a depression caldera. The larger the size of eruption column, the more effective the sorting of the erupted pyroclastic fragments. The larger and denser particles fall first and closer to the vent while the smaller and more vesicular fragments fall farther away. Consequently the deposits of pyroclastic falls are well sorted and exhibit pronounced lateral regular grading in texture and composition. This is in strong contrast with the poorly sorted character of pyroclastic flow deposits, in which all particles travel en masse in a state of turbulent flow.Welding of the deposit is not uncommon in the pyroclastic flow deposits while it is rare in pyroclastic fall deposits except those deposited near the vents of basaltic eruptions.To reconstruct past eruptions from volcanic deposits, it may be necessary to establish definite correlation between stratigraphic units by which volcanic deposits are grouped and time duration by which specific eruptive activity is grouped. A single eruptive cycle, the deposits of which represent such a time unit, is defined as a series of eruptive events limited by fairly long intervals of quiescence. Historic examples indicate that the duration of a single eruptive cycle ranges from a day to several years in most cases. The intervening periods are generally far longer than the duration of single eruptive cycle.From many examples of single eruptive cycles, a rule has been established: the degree of vesiculation of magma gradually decreases toward the end of the cycle. This is expressed in successive eruption of pyroclastic fall, pyroclastic flow, and lava flow from the same vent in case of felsic magma, and of pyroclastic fall and lava flow in case of mafic magma, which fact may indicate that the original magma column responsible for the eruptive cycle was more enriched in volatiles in the upper part than the lower.The close correlation between the recorded sequence of single eruptive cycles and the reultant beds of volcanic materials is described for a few examples. The beds produced by a single cycle of witnessed eruption conformably superpose each other and do not include a layer representing weathering break. It is stressed that such a group of beds of volcanic ejecta, volcanic deposits of a single eruptive cycle, should be taken into account as a stratigraphic unit when precise tephrochronology is undertaken.Keywords:
Peléan eruption
Pyroclastic fall
Strombolian eruption
Caldera
Lava dome
Dense-rock equivalent
Tephrochronology
Volcanic plateau
Phreatomagmatic eruption
Effusive eruption
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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Bandai volcano is located in the southern part of Tohoku-Honshu arc, Japan, and has been active from about 300 ka. Most recently, the volcano erupted in 1888 and the phreatic eruption caused volcanic body collapse and produced huge debris avalanche deposit. Here we present new data of the tephrochronology and volcanic geology of Bandai volcano and discuss its growth history. The tephra-loam association in this area consists of the Hayama and the Mineyama Loam Formations. Sixty-three layers of tephra are recognized in the Mineyama Loam Formation, and seventy-seven layers of tephra in the Hayama Loam Formation. The volcanic activity is classified into seven stages based on tephrochronology: Stage 1: 300 ka≤(presumed age), Stage 2: 300-280 ka, Stage 3: 250-230 ka, Stage 4: 170-85 ka, Stage 5: 75-57 ka, Stage 6: 36-28 ka, Stage 7: 24-0 ka. Pyroxene andesite lavas and tephras are eruptions of Bandai volcano throughout its activity, and more than 13 large avalanche deposits are found in Stages 2, 5, 6, and 7 including 1888 debris avalanche. Modes of eruptions were almost sub-plinian with lava effusions from Stages 2 to 3, whereas sub-plinian was subsequently followed by vulcanian with lava effusions from Stages 5 to 7. Sub-plinian eruptions occurred in the earliest phase of Stages 5, 6, and pumice falls with occasional pumice flows were associated. Stage 4 consists of two eruption types. Large debris avalanches were commonly produced related with the sub-plinian eruption, except for 1888 eruption. Bandai volcano is a complex of at least five stratocones, and resurge of volcanic activity caused collapse of pre-existed volcanic body. This cyclic feature is considered to be the behavior of the volcano.
Tephrochronology
Dense-rock equivalent
Peléan eruption
Pumice
Effusive eruption
Phreatomagmatic eruption
Volcanic hazards
Phreatic eruption
Vulcanian eruption
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Many timed observations make it possible to subdivide the 9‐hour Plinian eruption of Mount St. Helens on May 18, 1980, into six phases, defined by eruption style. The phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits. The suite of pyroclastic deposits indicates that the eruption became more pumice‐rich and compositionally diverse with time, perhaps owing to concurrent eruption of less evolved, gas‐poor parts of the magma body with the more evolved, gas‐rich parts. The paroxysmal phase I (0832–0900) consisted of landslides, lithic pyroclastic flows of a lateral blast and other explosions, and a weak pre‐Plinian column. Phase I pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer. The early Plinian phase II (0900–1215) consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks (1010–1035), a brief period of lithic ash ejection (1035–1100), and a pumice‐rich pulse that accompanied growth in height of the eruption column (1100–1215). Deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer, the lower lithic ash layer, and the middle pumice lapilli layer, all of which consist of evolved white dacitic pumice (63–64% SiO 2 ). During the early ash flow phase III (1215–1500) the height of the eruption column decreased, vertical ejection of tephra ceased, and pyroclastic flows were fed from intermittent fountains. Phase III deposits consist of a poorly exposed sequence (.≤12 m) of ash flow tuff that consist of many thin flow units (≤2 m each) containing pumiceous white dacite (63–64% SiO 2 ) and denser, gray silicic andésite (61–62% SiO 2 ), and fine‐grained ash cloud deposits interbedded with a nongraded middle pumice ash layer. The climactic phase IV (1500–1715) developed in two stages: fountain‐fed pyroclastic flows, followed by a short pulse (1625–1715) of vigorous vertical ejection of tephra. These stages were accompanied by the peak seismic energy release and peak eruption column height, respectively. Climactic deposits consist of a thick (≤35 m) sequence of thick, lapilli‐rich ash flow sheets (4–12 m each) with white and gray pumice, and streaky scoria bands (60% SiO 2 ) in pumice breccia clasts, and the reversely‐graded, upper pumice lapilli layer that is interbedded with fine‐grained ash cloud deposits. During the late ash flow phase V (1715–1815) eruption intensity waned but included a brief episode of small pyroclastic flows (1745–1815). Phase V deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice, and minor fine‐ash deposits. Phase VI activity (1815 to May 19, 1980) consisted of a low‐energy ash plume, with transient increases in intensity, while seismicity continued at depth. Sustained vertical discharge of phase II prodeced evolved dacitewith high S/Cl ratios. Ash flow activity of phase III is attributed to decreases in gas content, indicated by reduced S/Cl ratios and increased clast density of the less evolved, gray pumice. Climactic events are attributed to vent clearing and exhaustion of the evolved dacite.
Lapilli
Pumice
Peléan eruption
Pyroclastic fall
Dense-rock equivalent
Volcanic ash
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Citations (110)
Peléan eruption
Pyroclastic fall
Dense-rock equivalent
Pumice
Effusive eruption
Volcanic ash
Phreatic eruption
Lava dome
Volcanic hazards
Phreatomagmatic eruption
Lateral eruption
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Citations (10)
Pumice
Peléan eruption
Pyroclastic fall
Dense-rock equivalent
Silicic
Magma chamber
Caldera
Phreatic eruption
Vulcanian eruption
Phreatomagmatic eruption
Lapilli
Effusive eruption
Lateral eruption
Cite
Citations (63)
On 13 to 14 February 2014 a ~4 h long, VEI 4 eruption occurred at Kelut volcano (Java, Indonesia). Pyroclastic density currents (PDCs) and extensive ash fall led to 7 fatalities and disruption to flights across the Asia-Pacific region. New sedimentological descriptions of the pyroclastic deposits from the 2014 eruption were compared with eyewitness and satellite reports to elucidate temporal variations in eruptive dynamics. The stratigraphy of the deposits is presented in 3 stages, associated with two eruptions that occurred approximately ~15–30 min apart. Stage 1 PDC deposits originate from the smaller onset eruption. The PDC deposits from Stage 2, and tephra fall deposits from Stage 3 originate from the second, plinian eruption. During the onset eruption (Stage 1), low energy PDCs were produced that ran out to <2.6 km. Basal layers show characteristics of deposits similar to ash-cloud surges that carried dominantly fine ash and crystal fragments. These are capped by deposits typical of high-particle concentrated pyroclastic flows. All Stage 1 deposits have high contents of dense lithic fragments (up to 44% by vol.), sourced from the 2007–2008 lava dome and conduit walls, indicating that the eruption onset was driven by an explosive release of gas-overpressure below the vent-capping dome. Increases in the magma flux and transition to a more constant eruption led to a growing eruption column during the ~2-hour long Stage 2 plinian eruption. Pumice rich (>70% by vol.) PDC deposits ran out to 4.7 km from the vent. The deposits reflect an increased output of fresh fragmented magma, and some conduit widening evidenced by dense lithic fragments. Vent instabilities and incorporation of dense material into basal margins of the plume led to the marginal collapse and formation of these PDCs. Stage 3 occurred in the final hour at the peak of the plinian eruption, around 01:00 to 02:00, and produced reversely graded lapilli fall deposits with ≤90 vol% pumice from a 26 km-high plume. This indicates that there was a sustained flux of juvenile magma to the now open vent system, and expansion and fragmentation of the gas-rich magma was at its most efficient. Our study of the eruptive sequence of Kelut provides some constraints on predicted patterns for future explosive activity, critical for further hazard assessment of the volcano. Since 1901 Kelut has erupted on intervals of 1 to 23 years, and the 2014 event characterises a typical “explosive” style of eruption that alternates regularly with effusive dome-formation and collapse events. This pattern depends on the dynamics of magma renewal to the system, and degassing conditions in the shallow magma reservoir and upper conduit. If this pattern holds, and the next eruption occurs within the next two decades, a return of prolonged dome growth could be anticipated.
Peléan eruption
Dense-rock equivalent
Pyroclastic fall
Lava dome
Phreatic eruption
Effusive eruption
Dome (geology)
Volcanic hazards
Phreatomagmatic eruption
Lateral eruption
Pumice
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Citations (26)
This article deals with the mechanism of eruption and transportation of the pyroclastic material and the nature of the resultant deposits from the geological standpoint.In Japan, the method of tephrochronology is best applied to pyroclastic deposits of the Quaternary central volcanoes and those related to the Krakatoan calderas. Most of the rocks are andesitic in composition with subordinate amount of basalt and dacite.Three modes of volcanic eruption may be distinguished: 1) projection of pyroclastic materials which form pyroclastic fall deposits, 2) eruption of pyroclastic flows, and 3) outflow of lava flows or extrusion of dome and spine. Table 1 shows characteristic features of the deposits formed by the three modes of volcanic eruption.Tephra, as originally defined by Thorarinsson, signifies only the air-fall pyroclastic materials and its relation to pyroclastic flow is not clear. In this article, all the pyroclastic materials directly connected with volcanic eruptions, irrespective of their origin (i. e. essential, accessory, or accidental) and of their mode of emplacement, are included in the term tephra. The chronology using the deposits of pyroclastic flows are included in the tephrochronology.The small-scale vesiculation occurring at or close to the top of the magma column results in the so-called Strombolian and Vulcanian eruptions. Larger scale vesiculation with longer time duration leads to the Plinian eruption. The greatest vesiculation takes place within the magma reservoir resulting in the formation of a depression caldera. The larger the size of eruption column, the more effective the sorting of the erupted pyroclastic fragments. The larger and denser particles fall first and closer to the vent while the smaller and more vesicular fragments fall farther away. Consequently the deposits of pyroclastic falls are well sorted and exhibit pronounced lateral regular grading in texture and composition. This is in strong contrast with the poorly sorted character of pyroclastic flow deposits, in which all particles travel en masse in a state of turbulent flow.Welding of the deposit is not uncommon in the pyroclastic flow deposits while it is rare in pyroclastic fall deposits except those deposited near the vents of basaltic eruptions.To reconstruct past eruptions from volcanic deposits, it may be necessary to establish definite correlation between stratigraphic units by which volcanic deposits are grouped and time duration by which specific eruptive activity is grouped. A single eruptive cycle, the deposits of which represent such a time unit, is defined as a series of eruptive events limited by fairly long intervals of quiescence. Historic examples indicate that the duration of a single eruptive cycle ranges from a day to several years in most cases. The intervening periods are generally far longer than the duration of single eruptive cycle.From many examples of single eruptive cycles, a rule has been established: the degree of vesiculation of magma gradually decreases toward the end of the cycle. This is expressed in successive eruption of pyroclastic fall, pyroclastic flow, and lava flow from the same vent in case of felsic magma, and of pyroclastic fall and lava flow in case of mafic magma, which fact may indicate that the original magma column responsible for the eruptive cycle was more enriched in volatiles in the upper part than the lower.The close correlation between the recorded sequence of single eruptive cycles and the reultant beds of volcanic materials is described for a few examples. The beds produced by a single cycle of witnessed eruption conformably superpose each other and do not include a layer representing weathering break. It is stressed that such a group of beds of volcanic ejecta, volcanic deposits of a single eruptive cycle, should be taken into account as a stratigraphic unit when precise tephrochronology is undertaken.
Peléan eruption
Pyroclastic fall
Strombolian eruption
Caldera
Lava dome
Dense-rock equivalent
Tephrochronology
Volcanic plateau
Phreatomagmatic eruption
Effusive eruption
Cite
Citations (9)
Pyroclastic fall
Peléan eruption
Dense-rock equivalent
Cite
Citations (16)
Pumice
Peléan eruption
Pyroclastic fall
Phreatic eruption
Dense-rock equivalent
Volcanic ash
Cite
Citations (132)
Shikotsu volcano is a caldera volcano in southwestern Hokkaido. It is considered that a large-scale eruption started at ca. 60 ka (named as 60 ka Shadai eruption), mainly ejecting scoria fall and scoria flow deposits. The volcano repeated explosive eruptions every several thousand years, and after 10 ky of dormancy, a caldera-forming eruption took place at ca. 46 ka. Trench and boring surveys were carried out in the eastern part of Shikotsu caldera, and stratigraphy and changes in the components of the 60 ka Shadai eruption were reexamined. There are three types of juvenile materials in 60 ka eruption deposits: dacitic pumice, olivine-bearing andesitic scoria, and intermediate banded/gray pumice. As a result, tephra layers of the eruption are mainly classified into three units: Units A to C in ascending order. Soils and volcanic ash soils were not discovered among these layers, so these must derive from a single eruption sequence. Unit A consists of pumice fall deposits and ash fall deposits; Unit B of scoria fall deposits; and Unit C comprises a pyroclastic flow deposit and a following pyroclastic fall deposit. Each eruptive unit is subdivided into A1-A3, B1-B5 and C1-C2. Based on these eruptive units, the sequence of the 60 ka Shadai eruption is constructed as follows: Phase 1 was a pumiceous plinian eruption (A1, A2), and eruption rate abruptly decreased in A3. Phase 2 was a scoriaceous plinian eruption (B1-B5). Eruption rate was unstable in early Phase 2 (B1-B4); however, it gradually increased in late Phase 2 (intermittent eruption had repeated in B5). Consequently, a pyroclastic flow eruption occurred in Phase 3 (C1), followed by a plinian eruption (C2). Andesitic and mixed magmas were found to begin to erupt from Phase 2, while dacitic magma survived through the 60 ka Shadai eruption. These characteristics are difficult to explain based on the eruption from a zoned magma chamber or from a common eruptive vent. Dacitic and andesitic magmas are suggested to have existed in separated magma chambers, and dacitic magma at least was supplied through an independent vent system.
Peléan eruption
Pumice
Dense-rock equivalent
Scoria
Phreatic eruption
Pyroclastic fall
Effusive eruption
Lateral eruption
Caldera
Volcanic ash
Phreatomagmatic eruption
Lapilli
Vulcanian eruption
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Citations (5)