Observations
A monitoring camera at ~ 3 km NW of Shindake operated by the Japan Meteorological Agency (Fig. 1) recorded the details of the PDC which flowed along the northwestern slope of Shindake. The image showed that the jet of pyroclastic materials appeared from the crater rim at 09:59′39″. The jet turned to an eruption column which rose from the summit crater of Shindake immediately after the onset of the eruption (Fig. 2A). A part of the eruption column also collapsed to generate the PDC which ran down along the slope (Fig. 2B). The front of the PDC appeared at the base of the eruption column from 09:59′57″, ~ 18 s after the onset of the eruption and migrated along the slope. The PDC spread on the western flank of Shindake formed several blanches (Fig. 2C). The PDC flowed ~ 2.4 km along the valley of the Mukaehama River and reached the coastal line at around 10:00′52″, 75 s after the onset of the eruption (Fig. 2D). These photographs show significant outflow to west and northwest; however, there are minor outflows in all direction. The average speed of the PDC is 42 m/s. The ash-rich plume rose from the area covered by the PDC and then drifted southwestward.
Distribution of PDC
Distribution of the PDC (Fig. 3) was reconstructed by comparing the video images of the PDC with several aerial images obtained within 1 week from the eruption. The area covered with the PDC is recognizable as the region showing various types and degrees of damages observed mainly in the forest vegetation. The outer margin of the discolored area is commonly sharp and can be clearly identified (Fig. 4A). Distribution of the discolored vegetation is consistent with the area covered with pyroclastic flow in the video image. In contrast, remarkable damage to the vegetation was not observed in the area covered only with the ash-fall deposit from the eruption column.
Although the PDC flowed toward all directions from the summit (Fig. 3), three major flows are distinct, i.e., in the northwest direction along the Mukaehama River, in the southwest direction along some minor valleys, and a smaller flow occurred in the eastern direction along the Nanakama River. Among them, the flow along the Mukaehama River is the longest and can be traced for ~ 2.4 km along the river to the coastal line.
The area suffered from the PDC is classified into five zones ranging from “a” to “e” in descending order (Fig. 3), corresponding to the thickness of the deposit and the degrees of the damage to the vegetation (Fig. 4A).
Zone a is the proximal area of the source crater, < 500 m from the rim of the source crater. The area is covered with thick and coarse pyroclastic deposit including large blocks (Fig. 4B). Thickness of the deposit in this zone was estimated > 1 m judging from the aerial photographs though it is poorly confirmed. The original vegetation less than 0.5–1 m height was completely destroyed and covered by the deposit in Zone a. Because Zone a distributes in the vicinity of the source crater, the pyroclastic deposit in Zone a consists of the mixture of ballistic blocks, fallout materials, and the deposit from PDC.
Zone b surrounds Zone a and distributes mainly in the steep slope in the eastern and western side of Shindake. The original vegetation was completely removed by the blast of PDC, and the basement rocks of lava are exposed in Zone b (Fig. 4B). Though Zone b may be also covered with thin layer of volcanic ash after the eruption, the rainfall washed out the deposit immediately after the eruption.
Zone c is the area covered with coarse-grained pyroclastic deposit (Fig. 4C). Maximum thickness of the deposit in Zone c exceeds 1 m. Zone c distributes mainly in the northwestern and southwestern flank of Shindake (Fig. 3). Deposit in Zone c is volcanic breccia which is characterized with the presence of large blocks of lava fragments more than 30 cm in diameter. Though most trees were broken and fell down in Zone c, many standing trees were buried in Zone c in the southwestern flank of Shindake (Fig. 4C).
Zone d is the area covered with thin pyroclastic deposit, which mainly consists of lapilli and volcanic ash with coarse-grained sand size. Zone d was elongated along the main valleys and went straight through over minor ridgelines and slopes. The thickness of deposit in Zone d is typically less than 10 cm. In Zone d, most trees were broken and fell by the PDC blast (Fig. 4D, E).
Zone e is the outermost part of the area covered with the PDC. The deposit in Zone e mainly consists of lapilli and volcanic ash with coarse-grained sand size. Very thin ash deposit (< 5 cm) is typically found in Zone e. Though most of the trees kept standing, their leaves and fine branches of trees were partially broken and removed (Fig. 4E). The leaves of trees have been completely browned in this zone.
Zone f is outside of the area covered with PDC and covered by the ash fall. The leaves of the trees in Zone f kept greenish color (Fig. 4E). No mechanical damage by PDC on the vegetation was found in this zone. Thin deposits of very fine ash are found in Zone f.
Besides the damage by PDC, impact craters of ballistic blocks are found within ~ 2 km from the source crater (area “ic” in Fig. 3). Particularly, many impact craters distribute in the northern side of Shindake (Fig. 4F).
Damages
The PDC caused mechanical damages due to the dynamic pressure of the blast and also thermal damages due to the hot materials contained in the PDC. The mechanical damages caused by the blast were mainly found on the forest. All trees in the inner portion of the Zone d were broken and falling toward the down-flow. In the peripheral part of the Zone d, most trees kept only main trunk and thick branches, whereas minor branches and leaves were completely lost (Fig. 5b). Several impact marks formed by the collision of pyroclastic fragments in the PDC were also found on the surface of trees and wood poles (Fig. 5c).
Thermal damage was also recognized in most areas covered with the PDC. Dieback of vegetation in the area covered by the PDC (Zones d and e) was characterized by the browned foliage. The uniform discoloration of the vegetation in the area covered with the PDC (Fig. 4) indicates that the dieback of the vegetation was caused chiefly by the thermal impact rather than the chemical damage (Efford et al. 2014). One inhabitant received a burn injury at the marginal portion of the PDC in Zone d in Mukaehama area (Fig. 3). Plastic plates attached on the electric wire pillar were deformed toward the downstream side of the PDC in the Zone d of the Mukaehama area (Fig. 5d). The plates were most deformed at the axial part of the PDC and the deformation degree decreased toward the marginal portion of the PDC. A nylon wire found in the deposit at Loc. 1 was also partially melted and deformed. These evidences indicate that the PDC had enough temperature to make thermal effect on these items. However, these thermal effects were relatively limited comparing to magmatic PDC. No carbonized trees, except for an incomplete carbonization on the skin of a pine trees buried in the PDC deposit of Zone c, were found in the area covered by PDC. This was consistent with the absence of forest fire.
Deposits
Deposits of the PDC were divided into two types based on the thickness and lithofacies of the deposit. The first type, which characterizes most of the PDC deposit, is a thin layer of coarse-grained volcanic ash which was formed by the pyroclastic surge. The second type is a thick layer of poorly sorted volcanic breccia, which deposited from block-and-ash flows.
Thin volcanic ash layer
The first type of deposit is a layer consisting of volcanic ash and fine lapilli. This distributes in Zones d and e, which occupy ~ 85% of the distribution area of the PDC. Figure 6a, b shows the occurrence of the first type in Zone d (Mukaehama area: Loc. 1 in Fig. 3) and Zone e (the summit of Furudake: Loc. 2). Maximum thicknesses of the deposit at Mukaehama and Furudake, which is the peripheral part of Zones d and e, are ~ 4 and ~ 2 cm, respectively. The thickness of the deposit is relatively constant at each locality, but locally thickened on the upwind side of the obstacles. No remarkable dune or ripple structure is recognized in the deposit. These deposits are also recognized in the underfloor space of the buildings and the insides of roofed bunkers, suggesting lateral transportation. Widely spread and thin distribution of the deposit indicates that this deposit was formed by a pyroclastic surge which is characterized with diluted flow.
The deposit exhibits upward-fining grading from fine lapilli at the base of the deposit to medium sand toward the top, with fine volcanic ash at the uppermost part of the deposit in both localities 1 and 2. In the median diameter (Mdφ) versus sorting coefficient (σφ) diagram (Walker 1983), the deposits were plotted within the area of “pyroclastic surge deposit” (Fig. 7). The deposits contained many fragments of woods and leaves, probably incorporated during the transportation of PDC.
A thin deposit of lapilli-free, consisting of volcanic sand, was also recognized in the outermost part of PDC (Zone e) where the mechanical damage on the trees was weak. Thickness of the deposit was less than 1 cm at Loc. 3 in the western flank of Shindake.
Thick breccia deposit
The thick layer of volcanic breccia mainly distributes in Zone c (Fig. 6c, d). The deposit consists of poorly sorted mixture of blocks of lavas, lapilli, and volcanic ash. The deposit exhibits matrix-supported structure without clear bedding structure (Fig. 6d). The maximum thickness of the deposit exceeds 2 meters. The thick breccia forms valley-fill deposit in some gullies on the slope of Shindake and also forms fans at the exit of these gullies in the western flanks of Shindake (Fig. 4C). The fan consists of a cluster of branching lobes. Individual lobe at Loc. 4 is 3–6 m in width, several 10 s meters in length and maximum 1 m in thickness (Fig. 6c). Large blocks were concentrated on the top and the tip of the lobe (Figs. 4C, 6c). The deposit consists of the fragments of lava with various degrees of hydrothermal alteration. Trees buried in the lobes remained upright, particularly in the marginal portion of the deposit (Fig. 6c). The valley-fill distribution, fan-formation and poorly sorted massive structure of the breccia support that they were produced from high-density block-and-ash flow. Though Zone a is also covered with coarse-grained thick deposit, their detailed lithofacies and structure are still unknown because of the lack of field observation in the proximal area of the active crater.
Total mass of the PDC deposit
Total volume of the PDC deposits was evaluated on the basis of their distribution. In Fig. 3, the area covered by the PDC was estimated to be 5.2 km2. The PDC deposits were distributed in the Zones c, d, and e (Fig. 3).
The average thicknesses in the Zones e, d, and c are assumed from field observation as 0.01, 0.05, and 1.0 m, respectively, and the volumes of the PDC deposit were also calculated as 1.1 × 105, 3.8 × 104, and 3.6 × 104 m3, respectively. The total mass of the PDC deposit was calculated by employing the average density of the deposit in dry condition to be 1500 kg m−3 for Zones a and c, and 1000 kg m−3 for Zones d and e, based on the measurement of the block-and-ash deposit at Loc. 4 in Fig. 3 (1520 kg/m3) and the thin-ash deposit at Loc. 1 (995–1045 kg/m3). The total mass of the PDC deposit was evaluated to be 2.4 × 108 kg, including 1.7 × 108 kg for the block-and-ash flow deposit in Zone c and 7 × 107 kg for pyroclastic surge deposit in Zones d and e.
The proximal deposit distributed in the Zone a was evaluated as 5.0 × 108 kg, assuming that the deposit with 1 m and 1500 kg/m3 covers an area 0.33 km2. The combined analysis of the numerical simulation of volcanic ash plume and the ground survey of the ash-fall deposit suggests that the ash cloud contained ~ 6 × 108 kg of ash particles (Tanaka and Iguchi 2016). The total mass of the erupted materials from the May 29 eruption was thus estimated as ~ 1.3 × 109 kg.
Components
We investigated particle components of the thin volcanic ash layer formed by the pyroclastic surge, the thick breccia deposits, and the ash-fall deposit. Samples were collected from the Mukaehama area (Loc. 1 in Fig. 3), the western flank of Shindake (Loc. 4 in Fig. 3), and in the Zone f in the western flank of Shindake (Loc. 3 in Fig. 3). The coarser grains (> 2 mm) were investigated under the optical microscope.
Hydrothermally altered whitish rock fragments occupied more than half of the coarser grains of all deposits. They were subdivided into silicified dense rock fragments and porous fragments consisting of sulfide, clay, and silicate minerals. The fragments of crystalline lavas derived likely from the vent wall were also recognized. Some fragments exhibited a pink coloring due to oxidization.
Small amounts of least-altered fragments of lava were also recognized in these deposits. The grains had an angular outline surrounded by brittle-fractured surfaces. These grains exhibit grayish color and had semi-glossy surfaces when observed under the optical microscope. Some grains also had microcracks on the surface. The interior of these grains was highly crystallized. These grains contained plagioclase, orthopyroxene, magnetite, and tridymite as the groundmass minerals. The groundmass glass remained in tiny interstices between the crystals. Some groundmass glasses also contained microbubbles.
The fine components (< 63 μm) of the PDC deposits were analyzed with an X-ray diffractometer (XRD) installed at the Geological Survey of Japan. The XRD analysis identified that the major components of the PDC deposits are natroalunite, jarosite, crystallite, and tridymite, as well as the dominant plagioclase, which is a representative mineral of fresh volcanic rock. Under the optical microscope, pyrite crystals were also found.
These observations indicate that the PDC deposits contain abundant blocky rock fragments with various degrees of hydrothermal alterations. The presence of hydrothermally altered minerals and the absence of vesiculated juvenile materials in these PDC deposits indicate that the 2015 eruption occurred from the hydrothermal system sealed inside the edifice of Shindake. The mineral assemblage of the hydrothermally altered minerals indicates low pH environment of the hydrothermal system.
The PDC deposit exposed on the ground surface was weathered rapidly after the eruption. The sulfide minerals in the hydrothermally altered materials in the deposit can be broken down to generate ferrihydrite minerals and gypsum under such ground surface condition. In fact, the PDC deposit, originally grayish color just after the eruption, turned brownish in color owing to the precipitation of the secondary ferrihydrite ~ 1.5 years after the eruption.