Tephra event stratigraphy and emplacement of volcaniclastic sediments, Mogán and Fataga stratigraphic intervals, Part II: origin and emplacement of volcaniclastic layers
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We subdivided volcaniclastic layers drilled during Leg 157 around Gran Canaria at distances up to 70 km from the shore of the island at Hole 953C, 955A, and 956B deposited between 14 and ~11.5 Ma into >100 volcaniclastic units at each site.Most volcaniclastic layers are <20 cm thick, but complex turbidite units up to 1.5 m thick make up 10% to 20% of all volcaniclastic units in Holes 953C and 956B.We distinguish several types of clasts: felsic vitroclasts, (1) bubble-wall/junction shards, (2) brown nonvesicular felsic shards, (3) welded tuff clasts, (4) pumice shards, and (5) sideromelane shards.Mineral phases comprise anorthoclase and lesser amounts of plagioclase, calcic and sodic amphibole (kaersutite, richterite, and edenite), clinopyroxene (titanaugite to aegirine), hypersthene, minor enstatite, phlogopite, Fe/Ti oxides, sphene, chevkinite, apatite, and zircon.Xenocrysts are dominantly titanaugite derived from the subaerial and submarine shield basalts.Lithoclasts are mainly tachylitic and crystalline basalt, the latter most common in Hole 953C, and fragments of felsic lava and ignimbrite.Bioclasts consist of open planktonic foraminifers and nannofossil ooze in the highly vitric layers, while filled planktonic foraminifers, benthic foraminifers, and a variety of shallow water calcareous and siliceous fossils and littoral skeletal debris are common in the basal coarser grained parts of turbidites.Volcaniclastic sedimentation during the time interval 14-9 Ma was governed dominantly by direct and indirect volcanic processes rather than by climate and erosion.Most volcaniclastic units thought to represent ignimbrite eruptions consist of a coarse basal part in which pumice and large brown nonvesicular and welded tuff shards and crystals dominate, and an upper part that commonly consists of thin turbidites highly enriched in bubble-wall shards.The prominent coarser grained and vitroclast-rich volcaniclastic layers were probably emplaced dominantly by turbidity currents immediately following entry of ash flows into the sea.The brown, blocky and splintery, dense, completely welded, dominantly angular to subrounded, partially to completely welded tuff shards are thought to have formed by quench fragmentation (thermal shock) as the hot pyroclastic flows entered the sea, fragmentation of cooling ignimbrite sheets forming cliffs along the shore, and water vapor explosions in shallow water.Well-sorted beds dominated by bubble-wall/junction shards may have formed by significant sorting processes during turbidite transport into the deep (300-4000 m) marine basins north and south of Gran Canaria.Some may also have been generated largely by grinding of pumice rafts and fallout and/or by interface-shearing of coignimbrite ash clouds traveling over the water surface.Generally fresh sideromelane shards that occur dispersed in many felsic volcaniclastic layers and in one hyaloclastite layer are mostly nonvesicular and blocky.They indicate submarine basaltic eruptions at water depths of several hundred meters on the slope of Gran Canaria synchronously with felsic ash flow eruptions on land.Most sideromelane shards are slightly evolved (4-6 wt% MgO), but shards in some layers are mafic (6-8 wt% MgO).Most shards have alkali basaltic compositions.The dense, iron-rich, moderately evolved basaltic magmas are thought to be the direct parent magmas for the trachytic to rhyolitic magmas of the Mogán Group.They were probably unable to erupt beneath the thick, low-density lid of the felsic magma reservoir below the large caldera but were erupted through lateral dikes onto the flanks of the submarine cone.Tholeiitic shards occur low in the stratigraphic section where peraluminous K-poor magmas were erupted, a correlation that supports the parental relationship.Heterogeneity in glass and crystal populations in the absence of other evidence for an epiclastic origin, probably largely reflect systematic primary compositional heterogeneity of most of the ignimbrites, which become more mafic toward the top.This gross compositional zonation is destroyed at the land/sea interface, where the ignimbrites are likely to have resulted in a chaotic buildup of large, quickly cooled, and fragmented mounds of hot ignimbrite.Post-emplacement, erosional mixing is probably reflected in volcaniclastic layers that are well bedded, contain a large amount of shallow water skeletal debris and rounded basaltic lithoclast, and show a wide spectrum of glass and mineral compositions.Basaltic lithoclasts are much more common in volcaniclastic layers at Hole 953C, probably because the northeastern shield basalts were highly dissected in this older part of the composite shield volcano prior to the beginning of ignimbrite volcanism at 14 Ma.As a result, many ignimbrites may have been channeled into the sea via deep canyons.In contrast, erosion was minimal during Mogán time in the southern half of the island, which was gently sloping and practically undissected, leading to concentric sedimentation on the volcanic apron.In general, the submarine, syn-ignimbrite turbidites have preserved a number of characteristics from the pristine stage of ash flow emplacement-especially shape and vesicularity of primary particles and the transient glassy state-that are lacking in the subaerial ignimbrites that cooled and devitrified at high temperatures.,The early Pleistocene Tugm Tephra Bed in the Niigata region and the Kd18 Tephra Bed in the Boso Peninsula were correlated to the Ashino Pyroclastic Flow Deposit in the Aizu region. They contain bubble-junction type glass shards and high quartz in common. Chemical composition of glass shard and orthopyroxene (Mg#=63.6-66.0) of these tephra is also coincident. These tephras are expected to be found as marker beds in Japan.
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We subdivided volcaniclastic layers drilled during Leg 157 around Gran Canaria at distances up to 70 km from the shore of the island at Hole 953C, 955A, and 956B deposited between 14 and ~11.5 Ma into >100 volcaniclastic units at each site.Most volcaniclastic layers are <20 cm thick, but complex turbidite units up to 1.5 m thick make up 10% to 20% of all volcaniclastic units in Holes 953C and 956B.We distinguish several types of clasts: felsic vitroclasts, (1) bubble-wall/junction shards, (2) brown nonvesicular felsic shards, (3) welded tuff clasts, (4) pumice shards, and (5) sideromelane shards.Mineral phases comprise anorthoclase and lesser amounts of plagioclase, calcic and sodic amphibole (kaersutite, richterite, and edenite), clinopyroxene (titanaugite to aegirine), hypersthene, minor enstatite, phlogopite, Fe/Ti oxides, sphene, chevkinite, apatite, and zircon.Xenocrysts are dominantly titanaugite derived from the subaerial and submarine shield basalts.Lithoclasts are mainly tachylitic and crystalline basalt, the latter most common in Hole 953C, and fragments of felsic lava and ignimbrite.Bioclasts consist of open planktonic foraminifers and nannofossil ooze in the highly vitric layers, while filled planktonic foraminifers, benthic foraminifers, and a variety of shallow water calcareous and siliceous fossils and littoral skeletal debris are common in the basal coarser grained parts of turbidites.Volcaniclastic sedimentation during the time interval 14-9 Ma was governed dominantly by direct and indirect volcanic processes rather than by climate and erosion.Most volcaniclastic units thought to represent ignimbrite eruptions consist of a coarse basal part in which pumice and large brown nonvesicular and welded tuff shards and crystals dominate, and an upper part that commonly consists of thin turbidites highly enriched in bubble-wall shards.The prominent coarser grained and vitroclast-rich volcaniclastic layers were probably emplaced dominantly by turbidity currents immediately following entry of ash flows into the sea.The brown, blocky and splintery, dense, completely welded, dominantly angular to subrounded, partially to completely welded tuff shards are thought to have formed by quench fragmentation (thermal shock) as the hot pyroclastic flows entered the sea, fragmentation of cooling ignimbrite sheets forming cliffs along the shore, and water vapor explosions in shallow water.Well-sorted beds dominated by bubble-wall/junction shards may have formed by significant sorting processes during turbidite transport into the deep (300-4000 m) marine basins north and south of Gran Canaria.Some may also have been generated largely by grinding of pumice rafts and fallout and/or by interface-shearing of coignimbrite ash clouds traveling over the water surface.Generally fresh sideromelane shards that occur dispersed in many felsic volcaniclastic layers and in one hyaloclastite layer are mostly nonvesicular and blocky.They indicate submarine basaltic eruptions at water depths of several hundred meters on the slope of Gran Canaria synchronously with felsic ash flow eruptions on land.Most sideromelane shards are slightly evolved (4-6 wt% MgO), but shards in some layers are mafic (6-8 wt% MgO).Most shards have alkali basaltic compositions.The dense, iron-rich, moderately evolved basaltic magmas are thought to be the direct parent magmas for the trachytic to rhyolitic magmas of the Mogán Group.They were probably unable to erupt beneath the thick, low-density lid of the felsic magma reservoir below the large caldera but were erupted through lateral dikes onto the flanks of the submarine cone.Tholeiitic shards occur low in the stratigraphic section where peraluminous K-poor magmas were erupted, a correlation that supports the parental relationship.Heterogeneity in glass and crystal populations in the absence of other evidence for an epiclastic origin, probably largely reflect systematic primary compositional heterogeneity of most of the ignimbrites, which become more mafic toward the top.This gross compositional zonation is destroyed at the land/sea interface, where the ignimbrites are likely to have resulted in a chaotic buildup of large, quickly cooled, and fragmented mounds of hot ignimbrite.Post-emplacement, erosional mixing is probably reflected in volcaniclastic layers that are well bedded, contain a large amount of shallow water skeletal debris and rounded basaltic lithoclast, and show a wide spectrum of glass and mineral compositions.Basaltic lithoclasts are much more common in volcaniclastic layers at Hole 953C, probably because the northeastern shield basalts were highly dissected in this older part of the composite shield volcano prior to the beginning of ignimbrite volcanism at 14 Ma.As a result, many ignimbrites may have been channeled into the sea via deep canyons.In contrast, erosion was minimal during Mogán time in the southern half of the island, which was gently sloping and practically undissected, leading to concentric sedimentation on the volcanic apron.In general, the submarine, syn-ignimbrite turbidites have preserved a number of characteristics from the pristine stage of ash flow emplacement-especially shape and vesicularity of primary particles and the transient glassy state-that are lacking in the subaerial ignimbrites that cooled and devitrified at high temperatures.,
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In the historic period, several large eruptions were recorded from Kirishima, Sakurajima and Kaimondake volcanoes in southern Kyushu, Japan. Estimated dates of volcanic activity were established on these volcanoes through historical documentation of major eruption events. This study presents the correspondence between these documents and the records of AMS 14 C dating of soils underlying tephra layers. We conclude that AMS 14 C dates of soil materials can be useful in correlating tephra layers with documentary records of eruption.
Tephrochronology
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The surrounding area of the Shikotsu and the Kuttara volcanoes, and Ishikari-Lowland, in the southwestern Hokkaido, are covered with thick pyroclastic deposits of the late Pleistocene. These pyroclastic deposits have been subdivided into units according to facies changes. consequently each unit does not necessarily represented a single eruption. In the present paper, the products of one eruption (tephra formations) are separated from one another by recognition of intervening "volcanic ash soils" which are considered to be the most suitable for indicating quiet period, owing to its low depositional rate and generality.In the Ishikari-Lowland, the previously defined two groups of tephra formations (En-c and Spfl · Spfa 1; Spfa 7, 8, 9 and 10) are reinterpreted to represent the products of two eruptions, because the volcanic ash soils are absent among them. And three tephra formations are newly recognized. Twenty two tephra formations constitute the late Pleistocene pyroclastic deposits in the Ishikari-Lowland.The pyroclastic deposits distributed around the Kuttara volcano are subdivided into twelve tephra formations and one scoria fall group. The tephra formations derived from the Kuttara volcano are as follows in ascending order : Kt-8, 7, 6, 5, 4, Kt-Hy, Kt-3, Kt-Tk scoria fall group, Kt-2, Kt-1. The Kt-8 …-1 are mainly composed of plinian pumice fall deposits and pumice flow deposits generated by large and moderate scale explosive eruptions.The correlation of the tephra formations in the surrounding area of the Kuttara volcano and the Ishikari-Lowland is attempted with several petrographic parameters and stratigraphic position. The following tephra formations are correlated each other : Kt-8 and Aafa 1, Kt-5 and Mpfa 2b, Kt-4 and Mpfa 2a, Kt-3 and Spfa 4, Kt-Tk and Spfa 3, Kt-1 and Spfa 2. In addition, it is proved that the following tephra formations spread to the Ishikari-Lowland : Kt-7, Kt-Hy, Nj-Os.On the Shikotsu volcano, three explosive eruptions preceded a large scale caldera forming eruption (Spfl · Spfa 1 : Ca. 32 ka), and after a short repose time, present post caldera activity started. The Kuttara Volcano was the site of seven explosive eruptions during the period from 70-40 ka ; then the stratovolcano was formed by erupting Kt-Tk scoria fall group, and the summit caldera was formed by the last two explosive eruption (Kt-1 and 2).
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Pyroclastic fall
Peléan eruption
Lapilli
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Abstract Measurements of volcanic tephra fallout deposits provide useful information about the magnitude and intensity of explosive volcanic eruptions and potential for remobilization of deposits as dangerous volcanic flows. However, gathering information in the vicinity of erupting craters is extremely dangerous, and moreover, it is often quite difficult to determine deposit thickness proximal to volcanic craters because the thickness of the deposit is too great to easily measure; thus, airborne remote sensing technologies have generally been utilized during the intermission between eruptions. As an alternative tool, a muographic tephra deposit monitoring system was developed in this work. Here we report the performance of this system by applying the muographic data acquired at Sakurajima volcano, Japan as an example. By assuming the average density of the deposit was 2.0 g cm −3 , the deposit thicknesses measured with muography were in agreement with the airborne results, indicating that volcanic fallout built up within the upper river basin, showed its potential for monitoring the episodic tephra fallouts even during eruptions.
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Pyroclastic fall
Peléan eruption
Dense-rock equivalent
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Pyroclastic fall
Peléan eruption
Fragmentation
Vulcanian eruption
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Tephrochronology
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The Indonesian Young Toba Tuff (YTT), classically dated around 74 ka BP, is considered as a short-lived explosive cataclysmic super-eruption. The huge amounts of ash and SO2 emitted are likely to have triggered a volcanic winter which accelerated the transition to the last glaciation, and may have induced a human genetic bottleneck. However, the global climatic impact of the YTT or its duration are hotly debated. The present work offers a new interpretation of the Toba volcanic complex eruptive history. Analysing the BAR94-25 marine core proximal to the Toba volcanic center and combining it with high-resolution tephrostratigraphy and δ18O stratigraphy, we show that the Toba complex produced a volcanic succession that consists of at least 17 distinct layers of tephra and cryptotephra. Textural and geochemical analyses show that the tephra layers can be divided in 3 main successive volcanic activity phases (VAP1 to VAP3) over a period of ~ 50 kyr. The main volcanic activity phase, VAP2, including the YTT, is likely composed of 6 eruptive events in an interval whose total duration is ~ 10 ka. Thus, we suggest that the eruptive model of the Toba volcano must be revised as the duration of the Toba volcanic activity was much longer than suggested by previous studies. The implications of re-estimating the emission rate and the dispersion of ashes and SO2 include global environmental reconstitutions, climate change modelling and possibly human migration and evolution.
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