Eruption records in the terrestrial stratigraphy are often incomplete due to erosion after tephra deposition, limited exposure and lack of precise dating owing to discontinuity of strata. A lake ...system and sequence adjacent to active volcanoes can record various volcanic events such as explosive eruptions and subaqueous density flows being extensions of eruption triggered and secondary triggered lahars. A lacustrine environment can constrain precise ages of such events because of constant and continuous background sedimentation. A total of 71 subaqueous density flow deposits in a 28 m long core from Lake Inawashiro‐ko reveals missing terrestrial volcanic activity at Adatara and Bandai volcanoes during the past 50 kyr. Sedimentary facies, colour, grain size, petrography, clay mineralogy, micro X‐ray fluorescence analysis and chemistry of included glass shards characterize the flow event deposits and clarify their origin: (i) clay‐rich grey hyperpycnites, extended from subaerial cohesive lahars at Adatara volcano, with sulphide/sulphate minerals and high sulphur content which point to a source from hydrothermally altered material ejected by phreatic eruptions; and (ii) clay‐rich brown density flow deposits, induced by magmatic hydrothermal eruptions and associated edifice collapse at Bandai volcano, with the common presence of fresh juvenile glass shards and low‐grade hydrothermally altered minerals; whereas (iii) non‐volcanic turbidites are limited to the oldest large slope failure and the 2011 Tohoku‐oki earthquake events. The high‐resolution chronology of volcanic activity during the last 50 kyr expressed by lacustrine event deposits shows that phreatic eruption frequency at Adatara has roughly tripled and explosive eruptions at Bandai have increased by ca 50%. These results challenge hikers, ski‐fields and downstream communities to re‐evaluate the increased volcanic risks from more frequent eruptions and far‐reaching lahars, and demonstrate the utility of lahar and lacustrine volcanic density flow deposits to unravel missing terrestrial eruption records, otherwise the recurrence rate may be underestimated at many volcanoes.
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.
In association with the September 2014 phreatic eruption (VEI 1–2) at Ontake Volcano, a syn-eruptive and two post-eruptive lahars occurred in the Akagawa–Nigorigawa River, southern flank of the ...volcano. The present contribution describes and discusses the contrasting features of the two post-eruptive lahars, which caused a major impact on downstream river morphology, and re-examines the description of the syn-eruptive lahar in the previous study. The first post-eruptive lahar occurred 8 days after the eruption by the rainstorm (October 5, 2014, before the snowy season), and the second lahar was associated with the rain-on-snow (ROS) event on April 20, 2015, in the early spring of the snowmelt season. The October rain-triggered lahar, which can be interpreted as a cohesive debris flow, reached at least ~ 11 km downstream and left muddy matrix-rich sediments with high clay content (10–20 wt% of clay in matrix). The lahar deposits contain hydrothermally altered rock fragments, sulfide/sulfate minerals, and clay minerals and show extremely high total sulfur content (10–14 wt%) in matrix part, indicating source material from the September phreatic eruption deposits. The presence of “rain-triggered” clay-rich lahar and deposits originating from a single small phreatic eruption is important because usually such clay-rich lahars are known to occur in association with large-scale sector collapse and debris avalanches. The April ROS-triggered lahar was caused by the heavy rain and accompanying snow melting. The lahar was dilute and partly erosional and evolved into hyperconcentrated flow, which left fines-depleted sandy and gravelly deposits. Despite these lahars that originated from the same volcanic source and occurring within a 7-month period, the flow and resulting depositional characteristics are totally different. These different types of lahars after a single eruptive event need different simulations and mitigation of lahar hazards with timing (season) of the lahar onset. In comparison with rainfall intensity, snow-melting rate, and the contrasting lahars occurred in 2014/2015, it is postulated that the generation, size, and types of lahars can vary with the timing of eruption, whether it happens during the pre-snow season, snow season, or rainy season.
This study reports on geomorphic and sedimentary evidence for a gigantic breakout flood from Towada caldera, Honshu Island, northeast Japan, as observed along the Oirase River downstream from the ...outlet of the caldera lake. A number of features of the Oirase River are inconsistent with its present discharge, including the occurrence of 1) hanging valleys and a horseshoe-shaped waterfall in the upstream gorge area, 2) boulder bars and scattered flood boulders, 3) a dry valley at a high elevation in the gorge, and 4) a large fan (the Sanbongi fan) in the downstream area of the river. The Sanbongi fan is composed of thick, lithic-rich hyperconcentrated flow deposits that include pumice clasts derived from the 15ka Towada–Hachinohe ignimbrite and well-rounded meter-sized (and outsized) boulders derived from bedrock of welded ignimbrite. The deposits are entirely aggradational, with no major channels, indicating the absence of a major hiatus during sedimentation. The depositional facies also indicate that a single sheet-like flood event deposited the sediment within the Sanbongi fan area. Based on the age of the Sanbongi fan, the flood occurred between 15 and 12ka, after eruption of the Towada–Hachinohe ignimbrite. The most probable water source for the flood is Towada caldera lake, as suggested by landforms and sediments along the lake outlet. A paleohydrological analysis indicates that at least 6km³ of water was released from the caldera at a peak discharge of >2×10⁴ to 3×10⁵m³s⁻¹ during the breakout flood. Although the Sanbongi fan was previously considered to be a “normal” alluvial fan that formed during a stage of low-stand sea level, the present results show that the formation of the fan was closely related to a catastrophic flood from Towada caldera rather than perennial fluvial activity, climate change, or a change in relative sea level.
The present contribution investigates the temporal changes in volcaniclastic sediment transport over the 2-year period after the 2014 eruption of Ontake Volcano in two small drainage basins where ...increased turbidity was observed immediately after the eruption. Two similar-sized catchments on the southern flank of the volcano, the Akagawa River (~ 4.4 km
2
) and the Shirakawa River (~ 2.9 km
2
) catchments, exhibited contrasting sediment delivery patterns and river water characteristics such as acidity and electric conductivity (EC). Increased turbidity, a high rate of suspended sediment supply, and elevated EC values were observed only in the Akagawa River, which hosts volcanic vents in its proximal part. The mineral assemblages and chemical characteristics of suspended sediment from the Akagawa River clearly indicate that the turbidity was derived from the erosion and reworking of primary eruptive material and lahar deposits. Previous airborne and remote surveys suggested the presence of primary ashfall and pyroclastic density currents in the upslope areas and valley heads of both the Akagawa and Shirakawa rivers. However, the river water characteristics and sediment transportation data of the present study clarify that the initial volcanic disturbance of the Shirakawa catchment was minor and limited. The influence of volcanic disturbance on the Akagawa River catchment continued for at least 10 months after the eruption and was also observed for an additional 9 months until the end of the snowmelt season in 2016. In the Akagawa River valley, two post-eruptive lahars that occurred during a 7-month period may have enhanced the removal of volcaniclastic deposits, and this remobilization may have resulted in diminished sediment delivery in the river after the lahar events. The results of this study provide information about the timing of the decline of suspended sediment delivery associated with small-scale eruptive activity, and such information may prove useful for evaluating the effects of other eruptions similar in size and character to the 2014 Ontake eruption. In addition, the approach adopted for monitoring rivers at downstream sites is clearly of utility for evaluating primary pyroclastic deposition and volcanic disturbance near inaccessible vent areas.
This study reports on geomorphic and sedimentary evidence for a gigantic breakout flood from Towada caldera, Honshu Island, northeast Japan, as observed along the Oirase River downstream from the ...outlet of the caldera lake. A number of features of the Oirase River are inconsistent with its present discharge, including the occurrence of 1) hanging valleys and a horseshoe-shaped waterfall in the upstream gorge area, 2) boulder bars and scattered flood boulders, 3) a dry valley at a high elevation in the gorge, and 4) a large fan (the Sanbongi fan) in the downstream area of the river. The Sanbongi fan is composed of thick, lithic-rich hyperconcentrated flow deposits that include pumice clasts derived from the 15
ka Towada–Hachinohe ignimbrite and well-rounded meter-sized (and outsized) boulders derived from bedrock of welded ignimbrite. The deposits are entirely aggradational, with no major channels, indicating the absence of a major hiatus during sedimentation. The depositional facies also indicate that a single sheet-like flood event deposited the sediment within the Sanbongi fan area. Based on the age of the Sanbongi fan, the flood occurred between 15 and 12
ka, after eruption of the Towada–Hachinohe ignimbrite. The most probable water source for the flood is Towada caldera lake, as suggested by landforms and sediments along the lake outlet. A paleohydrological analysis indicates that at least 6
km
3 of water was released from the caldera at a peak discharge of >
2
×
10
4 to 3
×
10
5
m
3
s
−
1
during the breakout flood. Although the Sanbongi fan was previously considered to be a “normal” alluvial fan that formed during a stage of low-stand sea level, the present results show that the formation of the fan was closely related to a catastrophic flood from Towada caldera rather than perennial fluvial activity, climate change, or a change in relative sea level.
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.
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.
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