Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass‐wasting events (>130 discrete ...events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (∼930 to ∼900 ka, ∼810 to ∼760 ka, and ∼190 to ∼120 ka) that coincide with periods of increased volcano instability and mass‐wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (∼130 ka) occurred toward the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea level rise (>5 m/ka). Available age data from other island arc volcanoes suggest a general correlation between the timing of large landslides and periods of rapid sea level rise, but this is not observed for volcanoes in intraplate ocean settings. We thus infer that rapid sea level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean‐island settings.
Key Points
Heightened volcanic activity on Montserrat at 120–190, 760–810, and 900–930 ka
Large landslides coincide with rapid sea level rise at island arc volcanoes
Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and ...central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimentation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to <0.07 W/m2 at distances >15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are similar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present‐day volcanism is confined to the region with the highest heat flow.
Key Points
Heat flow in the Lesser Antilles is similar to other volcanic arcs
No evidence for subsurface fluid flow
Volcanism is confined to the region with high heat flow
Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990–1995 eruption of Unzen volcano, Japan. Two different ...types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991–1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows.
Akita-Komagatake volcano, located at 30 km west of the volcanic front in the Northeast Japan arc, has been active in the recent 100,000 years with caldera-forming eruptions occurred around 13,000 ...years ago. The formation history of the pre-caldera stratocone has not been fully established, though it is inevitable to grasp the whole development scenario of the volcano, and also to mitigate potential volcanic hazards in the future. We reconstruct the stratigraphy of the lavas and pyroclastics that erupted during the stratocone building stage, by combining the new field and geomorphological observations with petrographic, lithologic and geochemical data. Geomorphology involves preservation degrees of original micro-geomorphic features on their surfaces, such as lava levees and lava wrinkles. We identify 38 eruptive units that made up the stratocone, including 31 units of low-K tholeiitic (TH) as the dominant magma series, with 4 units of calc-alkaline (CA) series, and additionally 3 units of MD (medium) series that show intermediate characteristics between TH and CA. The volcanic activity of the stratocone is divided into two stages on the basis of the distinctive eruption centers and their resultant contrastive edifices. The latter stage (stage 2) can be further divided into two substages, 2-1 and 2-2, respectively, because of contrastive preservations of micro-geomorphologic features on the lava surfaces. In stage 1, fluidal lava flows, mainly basalt to basaltic andesite in compositions, were effused from the southern crater to form the southern stratocone showing a shield-like gentle slope. There are several observations that suggest dormancy and/or erosion interval might be present between the stages 1 and 2; epiclastic deposits are characteristically recognized immediately below the lavas of the stage 2, and one of the deposits directly overlies a lava flow of the stage 1. The crater moved northward and commenced discharge magmas considerably silica-rich compositions compared with those erupted in the stage 1, and built up another steeper stratocone (northern edifices). Although, the northern edifices ware mainly developed in the stage 2-1, three lava flow units display distinctively better preservation of micro-geomorphic features on their surfaces. The freshness of these topography together with some tephrochronologic data suggest that the final stage (stage 2-2) must have lasted immediately before the caldera collapse occurred ca. 13,000 years ago.
Orbicular granite occurs at Minederayama in the Tsukuba Mountains, Ibaraki Prefecture, Japan. The orbs consist of a migmatitic core, originally pelitic hornfels, and a mantle that consists mainly of ...radial cordierite containing apatite inclusions and subordinate feldspar and micas. The orientation and size distribution of the orbs indicates that isolated orbs had floated within the host granitic magma. According to the geometrical selection theory of crystal growth, we propose that radial cordierite grew from the orb shells to the orb cores. Melting experiments demonstrate that the mineral assemblage of the orbicular granite reflects the incongruent melting of pelitic hornfels. Important factors in the formation of the orbicular granite are the unmixing between the incongruent melt of cordierite hornfels and host granite melt, and the infiltration of the incongruent melt into the host granite melt as the cordierite crystallized. The temperature of the host granite magma, as estimated from melting experiments and mineral assemblage, was above 730 °C at 0.3 GPa, which may represent superheated conditions for the host granitic magma. Cooling of the host granite magma placed the orbs under supercooled conditions as the radial cordierite grew from the shells of the orbs to the cores.
The most recent major eruption event of the Zao volcano comprised a series of phreatic eruption episodes on 15 and 19 February, 22 August, and 27–28 September 1895, with several precursory vulcanian ...eruptions during February–July 1894. All were generated at the Okama crater lake located inside the Umanose caldera. The eruption products consist mainly of hydrothermally altered ash with altered blocks, except for ash from 1984. The eruption deposits of 1895 are divided lithologically into six layers (1–6). Comparison of the document with the lithofacies of deposits shows that layers 1, 2, 3–4, and 5–6 were correlated respectively with eruption episodes of 15 February (episode 1), 19 February (episode 2), 22 August (episode3), and 27–28 September (episode 4). During these four episodes, ca. 0.5%, 0.5%, 1.5%, and 98% of the total mass of the products had been discharged. Based on lithologic, stratigraphic, granulometric, and component analyses and on distributional features for these layers, the following depositional mechanisms were inferred. Layers 1, 3, and 4 were formed mainly from their related small pyroclastic density currents, whereas layer 2 resulted mainly from a small pyroclastic fall. In contrast, layers 5 and 6 are larger-scale near-vent pyroclastic fall deposits from ash clouds and eruption clouds, which might have included some juvenile fragments. The three early episodes in 1985 led to the climactic episode of 27–28 September. Furthermore, the andesitic magma chamber at <3kb depth, which caused the 1894 vulcanian eruptions, became a hydrothermal alteration source for the 1895 erupted materials. The chamber was re-activated before 1895 eruption by injection of basaltic magmas from greater depth. The injection reached maximum at the climactic event. The inferred course of that series of eruption episodes provides useful information to predict future volcanic phreatic-type eruptions at this volcano.
► We revealed the eruption sequence of 1895 eruption of Zao volcano in Tohoku Japan, as well as 1894 precursory eruptions. ► Mode of emplacement of each eruption was examined by geologic data and documents. ► Eruptions occurred at February, March, August and September in 1895, which became bigger toward September climactic eruption. ► Ascent of andesitic magmas would trigger the eruption of the sub-volcanic hydrothermal altered materials. ► Crater diameter and eruptive energy of the climatic eruption were also estimated.
Phreatic (non-juvenile) eruptions are the most common type of magmatic activity on Earth. Here we review the characteristics of phreatic eruptions, which occur when overheated water is rapidly ...vaporized. Tephra layers produced by phreatic eruptions are composed mainly of clay-rich volcanic ash with variably altered lapilli and volcanic blocks. A single phreatic eruption can last between one hour and one day; however, eruptions may occur successively over a period of years to decades. The total volume of tephra produced by a phreatic eruption is typically 104-6m3, maximum <108 m3. Phreatic eruptions may be accompanied by diverse phenomena, including: tephra fallout, ejected rock fragments, low-temperature pyroclastic flows, and syneruptive-spouted type lahars. There are few detailed descriptions of low-temperature (~100°C) pyroclastic flows and syneruptive-spouted type lahars associated with phreatic eruptions. Detailed studies of phreatic phenomena are required, as it remains difficult to identify and reconstruct these processes based on the characteristics of the deposits.
Phreatic (non-juvenile) eruptions are the most common type of magmatic activity on Earth. Here we review the characteristics of phreatic eruptions, which occur when overheated water is rapidly ...vaporized. Tephra layers produced by phreatic eruptions are composed mainly of clay-rich volcanic ash with variably altered lapilli and volcanic blocks. A single phreatic eruption can last between one hour and one day; however, eruptions may occur successively over a period of years to decades. The total volume of tephra produced by a phreatic eruption is typically 104-6m3, maximum <108 m3. Phreatic eruptions may be accompanied by diverse phenomena, including: tephra fallout, ejected rock fragments, low-temperature pyroclastic flows, and syneruptive-spouted type lahars. There are few detailed descriptions of low-temperature (~100°C) pyroclastic flows and syneruptive-spouted type lahars associated with phreatic eruptions. Detailed studies of phreatic phenomena are required, as it remains difficult to identify and reconstruct these processes based on the characteristics of the deposits.
We performed systematic sampling and description of the Adatara-Dake tephra outcrop on the summit of Adatara volcano, Fukushima Prefecture, Japan in order to reveal the detailed eruption sequence and ...temporal evolution of the magma system of this volcano that erupted 120ka BP. Even though there is no recognizable eruption hiatus represented by a paleosoil layer, pyroclastic fall characteristics at the outcrop permit to divide Adatara-Dake tephra into 19 layers: A to R; from bottom to top. The earlier layers (A to L) are characterized by successive pumice fall deposits, intercalated by thin volcanic sand layers. The later layers (M to R) are rich in scoria fall and are partly welded, with agglutinate found in layers O and R. Representative clasts from each layer were analyzed to determine the grain size distribution, componentry, modal composition and whole rock chemistry. Layer M can be correlated petrologically and petrographically with the lower part of Yugawa pyroclastic flow deposit on the eastern foot and the Motoyama pyroclastic flow deposit on the western flank of the volcano. Similarly, the upper part of Yugawa pyroclastic flow correlate with layer N, meanwhile the upper part of Motoyama pyroclastic flow correlate with layers O or R. There is an increase in the lithic fraction with decreasing sorting in layer M, which we interpret to indicate increasing enlargement of vent during the phase of this layer. The scoria/pumice volume ratio also increases remarkably in layer M, suggesting that vent enlargement and sudden increase of mafic magma during phase M caused the column collapse that generated the Yugawa and Motoyama pyroclastic flows. The mafic magma composition changed after layer O, with the FeO*/MgO ratios becoming less than 2.1. After this change in chemistry in phases O to R, pyroclastic flows were continuously generated and agglutinated deposits were formed at the summit. This suggests that the eruption style of the final phase abruptly changed to relatively low column height.
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