Mt. Fuji, located in central Japan near the triple junction of the Philippine Sea, Eurasia (or Amurian), and North American (or Okhotsk) plates, is one of the arc volcanoes associated with the ...subduction of the Pacific plate. Mt. Fuji has unique features including an average eruption rate in the last 100,000 years of 4–6 km3/ka, which is much higher than that of other volcanoes along the same arc (0.01 and 0.1 km3/ka). Basaltic rocks dominate in Mt. Fuji, while other arc volcanoes are dominated by intermediate and felsic compositions, and Mt. Fuji has erupted both explosively and effusively, with the two largest eruptions in the last 2000 years having different styles; the 864–866 CE Jogan eruption was effusive, while the 1707 Hoei eruption, the most recent eruption, was explosive. Mt. Fuji is not only interesting from a scientific point of view, but also important from a societal point of view because it is only 100 km from the Tokyo metropolitan region, one of the biggest cities in the world hosting more than 30 million people. The Hoei eruption resulted in more than a few tens of millimeters of ashfall in Tokyo by more than 150 mm. With this background, Mt. Fuji has been studied intensively using geological and geophysical methods to understand the evolution, magma plumbing system, and current activity of the volcano. This article synthesizes the current knowledge about Mt. Fuji. In particular, we provide background information required to understand the Jogan and Hoei eruptions, and discuss how to assess the size, timing, and style of the next eruption. Statistical insights suggest that the next eruption of Mt. Fuji is more likely to be voluminous with a long precursor, given a repose time of more than 300 years.
A field survey based on outcrop and hand-dug trench observations was conducted in the Maruyamasawa-Fumarolic-Geothermal-Area (MFG) of the Zao Volcano. Powder X-ray diffraction and microscopic ...analyses of samples collected from the MFG provide new possible links between underground magmatic–hydrothermal activity and fumaroles in the volcanic system. The MFG is a fumarolic area (solfatara) with evidence of intense geothermal activity, such as hot and cold springs, fumaroles precipitating native sulfur, and ongoing fumarolic alteration. The MFG occurs on a talus slope that mainly comprises detritus from Late Cretaceous granitoids mixed with blocks of the ca. 1 Ma Robanomimiiwa Pyrocalstics (Rmp). The Rmp is well exposed on a cliff behind the talus slope, and it unconformably overlies the granitoids with a NW-striking, SW-dipping contact. The southwestward extension of the Rmp-granitoids unconformity is projected laying approximately 500 m below the Okama crater, which is 800–1000 masl. The mineral assemblages of the MFG altered rocks are mainly 1) cristobalite + native sulfur ± pyrite, 2) cristobalite + quartz + alunite + kaolin-group minerals + smectite, and 3) quartz-only (intensely silicified) rocks. These mineral associations indicate a typical shallow acidic environment between steam-heated and fumarolic alteration zones. Three sandy layers (Layers 1 to 3 from bottom to top) are recognized in the hand-dug trench: Layer 1 is brown, poorly sorted and well-consolidated, Layer 2 is pale gray to gray and moderately sorted, and Layer 3 is pale gray and poorly sorted. The 14C dating of a paleosol underlying Layer 3 constrains the deposition event to around 437 ± 20 yrBP (2σ: 1429–1470). The presence of gray basaltic andesite (Rmp), granitic fragments (granitoids), and altered rocks (MFG constituent) in these layers suggests detrital remobilization from the MFG talus slope by rainfall (snowmelt) that emplaced thin lahar deposits. Furthermore, the detrital cristobalite and alunite in the L1-L3 matrix are minerals of fumarolic origin contained in the MFG altered rocks. These results indicate that the fumarolic activity and alteration of the MFG occurred before the deposition of these layers.
Based on this information, we propose the following model for the MFG activity: (1) The Rmp-granitoids unconformity behaves as an open pathway for ascending volcanic fluids and controls the MFG geothermal features. (2) The MFG was formed over 600 years ago, probably during early Okama crater activity at the 1200s CE. (3) Volcanic fluids have ascended through the unconformity over the last several hundred years. Therefore, careful monitoring of the MFG is crucial for detecting surface anomalies that could indicate imminent eruptions of the Zao Volcano.
•New interpretation of the Maruyamasawa-Fumarolic-Geothermal-Area (MFG) of the Zao Volcano.•A field survey and sample analyses revealed origin of the fumarolic activity.•The MFG is located above the unconformity contact between the old volcanic rocks.•The contact acts as an active pathway for the upwelling vapor-rich volcanic fluids.•The fluids have used this path during the last several hundred years.
Two-thirds of the 111 active volcanoes in Japan are covered with snow for several months during winter and demonstrate high hazard and risk potentials associated with snow-related lahars during and ...after eruptions. On 23 January 2018, a sudden phreatic eruption occurred at the ski field on Kusatsu-Shirane (Mt. Motoshirane) volcano, Japan. This new vent eruption from the snow-clad pyroclastic cone required forecasting of future snow-related lahars and crisis hazards zonation of downslope areas including Kusatsu town, a popular tourist site for skiing and hot springs. In order to achieve a prompt hazard assessment for snow-related lahars, a multidisciplinary approach was carried out involving characterization of proximal tephra deposits, snow surveys, and numerical lahar flow simulations using the Titan2D model. To determine the input parameters for the flow model, the consideration of snow water equivalent (SWE) immediately after the eruption (on 29 January) and in the post-eruptive period (on 12 March), was significant. In the case of Kusatsu-Shirane volcano during the winter of 2018, linear relationships between altitude and SWE, obtained at different elevations, were used to estimate the snow volume around the new vents. Several scenarios incorporating snow and snowmelt (water), with or without the occurrence of a new eruption, were simulated for the prediction of future lahars. Three lahar scenarios were simulated, including A) rain-on-snow triggered, B) ice/snow slurry, and C) full snowmelt triggered by a new eruption, and indicated the flow paths (inundation areas) and travel distances. These were useful for lahar hazard zonation and identification of potential high-risk areas. Since the input parameters required for the Titan2D flow model can be relatively easily determined, the model was suitable for the 2018 eruption at Motoshirane where historical and geological lahar records are not available for calibration. The procedure used in the study will enable rapid lahar prediction and hazard zonation at snow-clad volcanoes. Further consideration for simulating a cohesive-type flow, which was predicted by the primary deposits containing large amounts of clay minerals and could not be expressed in the Titan2D flow model, is necessary.
Lahar phenomena and the accompanying eruptions at Zao Volcano, NE Japan, have been recorded historically from the 13th to 20th centuries. However, no studies have been conducted on lahar deposits. ...This study focused on the lahar deposits of the last ca. 8000 years from fluvial terraces (terraces I, II, and III in descending order of elevation) distributed along the Nigorikawa River at the eastern foot of the volcano. The lithologic, granulometric, and component features of the lahar deposits revealed gravelly non-cohesive debris flow, sandy hyperconcentrated flow, and muddy cohesive debris flow lahar deposits. The sandy and muddy matrix of the lahar deposits mostly originated in the scoriaceous magmatic and phreatic eruption deposits and/or phreatic-eruption-related products in the Zao youngest stage. The clasts of older lavas and basement rocks of all the lahar deposits were entrained during transportation through the river. Radiocarbon (14C) dating indicates the depositional ages of the lahar deposits in terraces I, II, and III to be ca. 8–6 ka, ca. 3.5–2.5 ka ca. and 1700–1900 CE, respectively. The two gravelly and three muddy lahar units of terrace I, two muddy units of terrace II, and at least two gravelly and three muddy units of terrace III correlate with the reported magmatic and phreatic eruptions. Based on chronostratigraphic positions and lithology, the 1895 CE phreatic eruptions may have triggered the two uppermost muddy units in terrace III. However, the gravelly and sandy units of terrace II revealed the thickest deposits, and are widely distributed among the sections of the study area, with no corresponding magmatic eruption deposits in the proximal area. Unknown large-scale magmatic eruptions or crater lake outbursts could have triggered the lahars that formed the sequence. This study revealed that lahar events occurred repeatedly during the past 8000 years and flowed down to distal area. These results indicate the likely occurrence of lahars, especially during and after the eruption at Zao Volcano.
•The lahar deposit sequence in Zao Volcano during the past ca. 8000 years was revealed.•More than seven gravelly, one sandy, and eight muddy debris flow units were distinguished.•Some of the units were deduced to originate in the eruption induced lahar.•Especially several cohesive debris flow deposits were caused by 1895 CE phreatic eruption.•Lahar hazard risks are remarkably high during and after the eruption at Zao Volcano.
The lahars are one of the most hazardous volcanic phenomena causing the third greatest causalities among the volcanic hazards since the 16th century worldwide. Lahars can flow down a long distance ...and cause tremendous disaster at the foot of volcanoes often beyond the areas of primary volcanic impacts of pyroclastic fall and pyroclastic density currents. Therefore, the research on lahar history of active volcanoes approaching from an analysis of a geological record in distal volcanic regions is significant for lahar hazard risk evaluation. Zao volcano has high risks of future eruptions, because volcanic tremors have been detected since 2013. The presence of a crater lake at the summit area, and steep slopes at the high altitude of Zao indicates high potential energy for future lahars, if triggered by an eruption starting underneath the crater lake. This study firstly reports the existence of lahar deposits at the western foot of Zao and discusses the depositional features and the origin of these as well as the lahar hazard risk at this volcano. The lahar deposits were exposed during the archaeological excavation of the Fujiki ruin, western foot of the Zao volcano. Two major lahar units, L1 and L2, were recognized. Based on the
14
C dating and stratigraphic relationships, the ages of units L1 and L2 were estimated to be <ca. 4.0 and ca. 4.6 cal ka, respectively. The lithology, granulometry, and componentry features of the lahar deposits revealed the depositional features and the source materials. The upper part of L1 (L1-1) unit and lower part of L2 (L2-2) unit were deposited from a hyperconcentrated flow, whereas, the lower part of L1 (L1-2) unit and upper part of L2 (L2-1) unit were formed by a debris flow. The sources of both units were phreatomagmatic eruption products that may have erupted shortly before the lahar events. This implies that these eruptions were the most plausible trigger for the lahars. These results suggest that lahar risk will increase especially after the phreatomagmatic eruptions as well as phreatic eruptions, even in the western foot of this volcano.
Vulcanian activity is one of the most common eruption styles of arc andesitic volcanism on Earth. It ejects and deposits volcanic bombs around the source crater. Although paleomagnetic studies of ...volcanic bombs are limited, such studies can potentially provide more opportunities for high-resolution paleomagnetic dating of volcanic activity. In this study, paleomagnetic dating was applied to large (> 1 m) volcanic bombs around active craters in the Azuma volcano group, NE Japan. Oriented samples were collected from the interior parts of five large volcanic bombs situated on gentle slopes, a few hundred meters from the source crater. More than six core samples were collected from each bomb and all samples were subjected to a range of rock magnetic experiments, including anisotropy of magnetic susceptibility (AMS) and thermal/alternating field demagnetization (THD/AFD) analyses. The Characteristic Remanent Magnetization (ChRM) directions for specimens from all bombs were well-defined, have small α
95
(< 2.5º), and are in close agreement with each other. Comparing our measured overall mean direction (D
m
= 355.5º, I
m
= 49.8º, α
95
= 1.6º) with modeled geomagnetic field estimates and a reference secular variation curve for this area (using MATLAB-based archaeomagnetic dating tool), we suggest that the volcanic bombs were produced in the historical Meiji period (1893–1895 CE) eruption. In addition, a combination of the data of ChRM, AMS, thermomagnetic analyses, hysteresis measurement, and XRF analysis indicates that the volcanic bombs were derived from a plug of lava in the conduit under the solidification point (ca. 800 °C), but above the Curie point of the titanomagnetite remanence carrier (around 300 °C). We show that volcanic bombs can be powerful for paleomagnetic dating if certain sampling conditions, such as quantity, situation, size and portion are satisfied.
Graphical Abstract
The 2014 Mount Ontake eruption started just before noon on September 27, 2014. It killed 58 people, and five are still missing (as of January 1, 2016). The casualties were mainly caused by the impact ...of ballistic blocks around the summit area. It is necessary to know the magnitude of the block velocity and energy to construct a hazard map of ballistic projectiles and design effective shelters and mountain huts. The ejection velocities of the ballistic projectiles were estimated by comparing the observed distribution of the ballistic impact craters on the ground with simulated distributions of landing positions under various sets of conditions. A three-dimensional numerical multiparticle ballistic model adapted to account for topographic effect was used to estimate the ejection angles. From these simulations, we have obtained an ejection angle of
γ
= 20° from vertical to horizontal and
α
= 20° from north to east. With these ejection angle conditions, the ejection speed was estimated to be between 145 and 185 m/s for a previously obtained range of drag coefficients of 0.62–1.01. The order of magnitude of the mean landing energy obtained using our numerical simulation was 10
4
J.
Snow avalanches are catastrophic phenomena because of their destructive power. Therefore, it is very important to forecast the affected area of snow avalanches using numerical simulations. In our ...study, we focus on applying a numerical model to snow avalanches. The inertia-dependent flow friction model, which we call the “I-dependent” model, is a promising numerical model based on granular flow experiments and includes the local inertial effect. This model was introduced in previous studies as it predicts the shape and velocity of the granular flow accurately. We numerically investigated the particle diameter effect of the I-dependent model, and found that the smaller the particle diameter is, the faster the flow front velocity becomes. The final flow shape is similar to a crescent shape when the particle diameter is small. We applied this model to the ping-pong ball flow experiment, which imitated a snow avalanche on a ski jump slope. Comparing between the experimental and simulated results, the flow shape is better reproduced when the particle diameter is small, while the numerical simulation using a real ping-pong ball diameter did not show the clear crescent shape. Moreover, the relative error analysis shows that the best fit between experimental and simulated flow front velocity occurs when the particle diameter is larger than the actual size of a ping-pong ball. We conjecture that this discrepancy is mainly caused by aerodynamic effects, which, in this case, are large due to the low density of ping-pong balls. Therefore, it is necessary to explore the granular features of ping-pong balls or snow avalanches by conducting experiments, as done in previous experimental studies. Through such efforts, it may be possible to apply this I-dependent model to snow avalanches in the future.
Large pyroclasts—often called ballistic projectiles—cause many casualties and serious damage on people and infrastructures. One useful measure of avoiding such disasters is to numerically simulate ...the ballistic trajectories and forecast where large pyroclasts deposit. Numerical models are based on the transport dynamics of these particles. Therefore, in order to accurately forecast the spatial distribution of these particles, large pyroclasts from the 2015 Aso Strombolian eruptions were observed with a video camera. In order to extrapolate the mechanism of particle transport, we have analyzed the frame-by-frame images and obtained particle trajectories. Using the trajectory data, we investigated the features of Strombolian activity such as ejection velocity, explosion energy, and particle release depth. As gas flow around airborne particles can be one of the strongest controlling factors of particle transport, the gas flow velocities were estimated by comparing the simulated and observed trajectories. The range of the ejection velocity of the observed eruptions was 5.1–35.5 m/s, while the gas flow velocity, which is larger than the ejection velocity, reached a maximum of 90 m/s, with mean values of 25–52 m/s for each bursting event. The particle release depth, where pyroclasts start to move separately from the chunk of magmatic fragments, was estimated to be 11–13 m using linear extrapolation of the trajectories. Although these parabolic trajectories provide us with an illusion of particles unaffected by the gas flow, the parameter values show that the particles are transported by the gas flow, which is possibly released from inside the conduit.
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