How do eruptions trigger lahars

Most of the ashfall deposits are distributed east of the vents because of the prevailing westerly winds Fig. Aerial photographs suggest a thin layer of ash mantles the southern slope Oikawa et al. Fumaroles have been commonly observed around vents in the Jigokudani valley before and after the eruption. On the syn-eruptive lahar, aerial photographs taken around the Jigokudani vents approximately 2—4 h and 1 day after the eruption Kaneko et al. Sasaki et al. They interpreted from the aerial photographs that the mudflow expelled from the vents directly and continuously travelled 5 km to the Akagawa River as a lahar.

However, uncertainty about these observations arises because the interpretation was not supported by any field evidence. Alternative possibility for the origin of the syn-eruptive lahar, such as transformation of pyroclastic density currents, has not been discussed by the previous studies.

The two rivers run parallel on the volcanic slope, although only the Akagawa River catchment holds active Jigokudani vents in the head of the river valley. Ontake Volcano is under a temperate climatic condition with seasonal snow cover from November to May around the top of the mountain.

The annual maximum snow depth, the annual mean air temperature, and the annual precipitation are 76 cm, 7. In order to understand the amount of water input to the Nigorigawa River catchment, the present authors carried out meteorological observations at Tanohara on the southeastern slope, m a.

Obtained rainfall data and rate of snowmelt are based on the measurements of energy balance components at the snow surface e. Snow started to accumulate at the end of November, with a maximum thickness of 2 m by the middle of March, which was completely melted on April 20, , at the Tanohara observation site Fig. At least three ROS events resulting in major stage changes during the snow-melting season were observed in records of a time-lapse camera at the Akagawa River set by the authors Fig.

Wire sensors, which were set up for a debris flow alert in the Nigorigawa River system Fig. This means that the rest of heavy rainfall and ROS events caused no or minimal mass transport to have resulted in any significant geomorphological changes to the river during the period. After April 20, , to the end of observation period in , rainstorms as well as ROS conditions were observed, but major lahars did not occur.

Wire sensors did not detect any lahars after the ROS-triggered lahar on April 20, Several analyses, in the field and in laboratories, were performed in this study. Analyses for this study consisted of 1 geological and sedimentological surveys to identify lahar deposits, 2 river image capturing by a time-lapse camera over a month period from November to November , and 3 grainsize of lahar deposits, and 4 XRF X-ray fluorescence and XRD X-ray diffraction analyses of lahar component particles.

Distribution, thickness, sedimentary facies, and lithological characteristics of lahar deposits in the Akagawa—Nigorigawa River valley were described in the field. Regular monthly to bimonthly field survey after the eruption enabled the identification of new lahar deposits with timing of their emplacement, and to document the drastic geomorphic changes of the Akagawa—Nigorigawa River over the period of the study.

Since November , the authors deployed an automatic camera time-lapse camera in the Akagawa River at 4. The camera captures one shot every 10 min during daytime. Samples for grainsize analysis were collected only for those materials that are finer than approximately 10 cm in size. Lahar deposits contain larger clasts tens of cm to 1 m in size ; therefore, the samples represent predominantly matrix fraction of the whole sediments. Sample quantities of about — g of dry weight for meter-thick deposits and less than g for decimeter- to centimeter-thick deposits were collected.

Five portions were randomly collected from each sample, and each portion was measured five times therefore a total 25 runs for one sample with duration of 15 s for each run. Averages of the total runs represent grainsize for each sample.

The accelerating voltage and tube current were 50 kV and 50 mA. Data for major element and total sulfur as SO 3 content are semiquantitative by the FP fundamental parameter method with a standardless technique Takase and Nagahashi Sulfur content in each sample is high enough to discuss comparison among individual samples.

Trace element contents were quantified by the calibration curve method Nagahashi and Nakazawa For muddy sediments, XRD analysis by Rigaku Ultima IV at Niigata University was performed to understand the mineral composition and to discuss the origin and relation of the phreatic eruption with lahars.

Samples were prepared for bulk analysis powdered and for fine fraction clay mineral analysis after separation by hydraulic settling and centrifuge. Some samples were treated with ethylene glycol and HCl to identify the minerals overlapping in certain cell parameters.

The first rainstorm event after the eruption occurred on October 2—3, , with total However, this rainstorm with maximum 6. The second rainstorm associated with a typhoon the 18th typhoon in year in Japan was 8 days after the eruption, on October 5, and did trigger a lahar.

The lahar was detected in the Nigorigawa River at p. The wire sensor was placed at 1. Cumulative rainfall from early morning to 5 p. A third rainstorm on October 13—14 was again associated with a typhoon. Although heavier rainfall of total mm during the day and intensive rain over 6 h exceeding 10 mm every hour were recorded, the rain event did not trigger a lahar. Aerial photo-images captured by the Geospatial Information Authority, Japan, and Google Earth images provide evidence that the first and second rainstorms and the lahar caused high degradation and deposition with major geomorphic changes on the southern flank of the edifice and in the river courses Fig.

On October 1, before the rainstorms, gray areas near the vents and farther downslope indicate mantling by ash. Images of the river valley, where a pyroclastic density current descended, also show mantling by ash Fig. Images taken 2 days after the lahar on October 7 show extensive erosion of the ash on the southern flank, with underlying brown to reddish brown altered rock and deposits then exposed by the rain and lahar Fig.

These changes correspond with the aerial observation by Oikawa et al. The river channel in downstream areas also shows changes. Comparison of aerial photographs of the confluence area of the Akagawa and Shirakawa Rivers Fig.

Aerial photographs and field investigation revealed that overbank flows reached the confluence with the Shirakawa River Fig. The lahar deposits with boulders were distributed down to the confluence of Nigorigawa and Denjogawa Rivers, and fine-grained flows travelled further downstream Fig. The total volume of the October lahar deposits is roughly estimated as 2.

Aerial photographs of the catchment of Akagawa and Shirakawa Rivers. Much of the slope-covered ashfall materials were swept away by the rainstorm. Arrows indicate areas where major geomorphological changes erosion and overbank deposition by the lahar were identified.

The October lahar left deposits not only in the main river channel, but also spilled sediments over banks onto side terraces Figs. The deposits were recognized between 3 km from the vents in Jigokudani Loc. S in Fig. The gray-colored fine ash deposit possibly derived from ash cloud of the pyroclastic density current overlain by the lahar deposits is recognized at Loc.

The ash deposit mantles underlying pre-eruptive deposits. Field photographs showing the occurrence of the October lahar muddy, cohesive debris flow: MF deposits. The photograph was taken on November 12, , when the river was turbid. Photograph May 30, showing turbidity of the river. Schematic sections showing the stratigraphy of the October rain-triggered and April ROS-triggered lahar deposits. Thickness of the lahar deposits in the Akagawa and Nigorigawa Rivers varies between 1 and 3 m for in-channel facies Fig.

In-channel facies is defined as deposits that rest directly on fluvial gravel, whereas overbank deposits overlie soil and vegetation developed on banks and terraces Figs. Occasionally, run-up deposits on sidewall of the valley up to 1 to 1.

Mud lines were left on trees and leaves. On the surface of deposits wrinkles and ridges formed by compression are present Fig. Overbank deposits are identified on a sabo-dam check dam at Loc. Facies, texture, and composition of in-channel and overbank deposits components less than 8 mm are usually similar in an outcrop scale vertical and lateral , and from the proximal to distal locations, however, thickness and content of clasts larger than 8 mm decreased toward downstream Figs.

The presence of boulders in the deposits diminishes beyond the confluence with the Denjogawa River where the valley widens. Key outcrop sections and grainsize characteristics of the October rain-triggered lahar and April ROS-triggered lahar deposits a in the Akagawa River, b in the Nigorigawa River, c in the Shirakawa River, and d at summit of the volcano.

Sample IDs beside the sections correspond to those in tables and appendices. The lahar deposits are muddy matrix supported, massive, and unstratified Fig. Approximately 40 days after the lahar event November 12, , the deposits were still fluidal, i. Locally 2-cm-thick sole layers matrix material without clasts occupy the base of sequence.

The poor sorting nature does not change with distance Figs. Pyrite grains in matrix are observable in hand specimens. The deposits contain granules to cobbles; however, cm-sized to 1-m-sized boulders are also included. Clasts are mainly composed of unaltered andesite with subordinate amounts of white-colored hydrothermally altered rocks of angular shape up to 3 cm in diameter. Juvenile materials such as pumice, scoria, and fresh glass shards are not recognized.

Standing trees, wood logs, and shrubs that had been part of the riparian forests at the time of eruption were incorporated with the lahar deposits. Grainsize characteristics of pre-eruptive, eruptive and post-eruptive lahar deposits in the Akagawa and Nigorigawa Rivers and around Ontake Volcano. Note that the samples from debris flow deposits represent grainsize less than cobble because of limitation of sampling of larger clasts at the field. The clay content slightly decreases toward downstream Fig.

No major facies changes in vertical sections and proximal to distal sections Fig. The absence of flow transformation is more common in cohesive debris flows than cohesionless debris flows Scott et al. Brown-colored lithic fragments, quartz, and pyrite are present.

Trace amounts of orthopyroxene and clinopyroxene are recognized. Some of these are typical of eruption material derived from hydrothermally altered areas in Japanese volcanoes Ohba and Kitade The composition of the lahar samples is similar to that of primary ashfall deposits of the eruption at Ontake Table 2 ; Minami et al.

The wire sensor at the time was set at almost the same place to the previous site Fig. At the Tanohara rain gauge at m. The warm, humid, and windy weather condition was due to an extratropical cyclone. It caused rainstorm and snow melting, and hence, the snowpack around the weather station completely disappeared by the end of the rainstorm Fig.

Snow depth at Tanohara became less than 10 cm after 6 p. The rate of water input was sufficient to trigger a lahar. The increase in flow discharge in the Akagawa River was observed by the time-lapse camera records Fig.

The preexisting depositional sequences emplaced by the October muddy lahar deposits and pre-eruptive terrace deposits were highly eroded and exhumed by the ROS lahar flow Figs. At Loc. The camera records and meteorological observations at Tanohara provide evidences that there were no ROS major flood events after April 20 to May Therefore, the sedimentary sequence observed on May 19 displays the ROS-triggered lahar deposits. Field photographs of the April ROS-triggered lahar deposits.

MF: October lahar deposits, p. The deposits are well sorted and parallel laminated Loc. The sedimentary facies is indicative of hyperconcentrated flow deposits Smith ; Pierson The deposits mainly consist of very coarse sand to granules with medium sand and fine pebbles. Clasts in lahar deposits are mainly of andesite. Angular fine pebbles to granules, in white color, of hydrothermally altered rock origin, are also present.

At the Loc. Gray mud drapes, less than 1 cm thick, are locally observed on the top of the sandy ROS lahar deposits Fig. The distal lahar deposits The deposits directly overlie the October lahar and pre-eruptive fluvial bar deposits Figs. The thickness of the ROS lahar deposits is clearly less than that of the October lahar deposits. Therefore, the total volume for the deposits is smaller than that of the October lahar.

Pyrite grains as well as white hydrothermally altered lithic fragments attached to smaller pyrite aggregates are found, but are less prominent than those in the October lahar deposits. Locally well-rounded semitransparent mineral grains weathered feldspars are present. Trace amounts of orthopyroxene and clinopyroxene grains are recognized. The total sulfur content in the ROS deposits varies from 0. The only exception is found from the mud drape part, which contains No typical cohesive debris flow deposits by the October lahar could be observed during the ground survey in the upper reach of the Shirakawa River.

There was a post-eruptive deposit consisting of three units in ascending order Fig. Pre-eruptive fluvial some terraced deposits in the Akagawa, Shirakawa, and Nigorigawa Rivers are described and analyzed similarly in order to compare with lahar event deposits Fig. They basically comprise well-rounded to sub-rounded andesite clasts pebble to boulder with sand matrix as parts of gravel and sand bars within channels.

Bulk chemistry shows low sulfur content 0. The air-borne survey Sasaki et al. A warning system triggered by sensors on the mountain near the Carbon and Puyallup River channels will activate sirens to warn residents downstream.

You can see a map of the evacuation routes at the Pierce County Emergency Management website. There has also been a hazard map created from past lahar activity for the region possibly impacted by a lahar from Mount Rainier. For even more information on lahars, visit the Cascade Volcano Observatory page on lahars and debris flows. Quick Links. Contact Us Email: pnsn uw.

Recent Earthquake Map. Recent Earthquake List. Seismic Stations Map. Volcanoes are a perfect setting for these events because of an abundance of steep, rocky rubble and a ready source of water in the form of rain, snow or ice. Lahars can flow many miles downstream from the volcano, making this the most threatening hazard in the Cascades. Lahar is an Indonesian word describing a mudflow or debris flow that originates on the slopes of a volcano.

Small debris flows are common in the Cascades, where they form during periods of heavy rainfall, rapid snow melt, and by shallow landsliding. These relatively small debris flows seldom move more than a few miles down valleys. In the Cascades, the word lahar is typically reserved for larger events that occur in conjunction with volcanic eruptions, and travel many miles down valleys and affect local communities.

Lahars can occur by rapid melting of snow and ice during eruptions, by liquefaction of large landslides also known as debris avalanches , by breakout floods from crater lakes, and by erosion of fresh volcanic ash deposits during heavy rains.

During and immediately following volcanic eruptions, lahars can pose the most severe hazard to populated valleys downstream from Cascades volcanoes. Visit individual volcano websites to learn more about specific Cascade lahar histories and hazards volcano drop down from CVO home page. The lahar flowed from the crater into the North Fork Toutle River valley and eventually reached the Cowlitz River 80 km 50 mi downstream.

The lahar also entered Spirit Lake, which can be seen in the lower left corner. About years ago, the collapse of weakened rocks caused a large lahar at Mount Rainier.