Pyroclastic Flow Deposits Research Articles

Sediment Yield from a Yarding Road Formed in Shirasu (Pyroclastic Flow Deposits) in an Abandoned Clear-Cut Plantation Forest in Kagoshima Prefecture

Sediment Yield from a Yarding Road Formed in Shirasu (Pyroclastic Flow Deposits) in an Abandoned Clear-Cut Plantation Forest in Kagoshima Prefecture

Discovery and its Significance of Pyroclastic Flow Deposits on the southern slope of Mt. Fuji at the Primary Stage of the Younger Fuji Volcano, Central Japan

Discovery and its Significance of Pyroclastic Flow Deposits on the southern slope of Mt. Fuji at the Primary Stage of the Younger Fuji Volcano, Central Japan

Glass Composition and Emplacement Mode of Koya Pyroclastic Flow Deposit and Its Proximal Equivalent

Glass Composition and Emplacement Mode of Koya Pyroclastic Flow Deposit and Its Proximal Equivalent

Juvenile-only clusters and behaviour of the Early Cretaceous dinosaur Psittacosaurus

It has hitherto been hard to prove that any association of juvenile dinosaurs represents original behaviour rather than sedimentary accumulation, and it has been hard also to determine the ages of such juveniles. A previously described specimen, which consists of an “adult” Psittacosaurus with 34 fully articulated juveniles, turns out to be a composite: the “adult” skull probably has been added, and in any case it is below breeding age. Other juvenile-only clusters have been reported, but the best examples that likely reflect behaviour rather than sedimentary accumulation are specimens from the Early Cretaceous Lujiatun beds in NE China, which were entombed beneath pyroclastic flow deposits. A remarkable juvenile-only cluster of Psittacosaurus shows clear evidence of different ages (five 2-year olds and one 3-year old) based on bone histological analysis. These juveniles may have associated together as a close-knit, mixed-age herd either for protection, to enhance their foraging, or as putative helpers at...

The ~ 31 ka rhyolitic Plinian to sub-Plinian eruption of Tlaloc Volcano, Sierra Nevada, central Mexico

Tlaloc is a late Pleistocene stratovolcano located NE of México City. It is the northernmost volcano of the N–S Sierra Nevada Volcanic Range, which consists from north to south of Tlaloc, Telapón, Iztaccíhuatl, and Popocatépetl volcanoes. Tlaloc has always been considered the oldest (and extinct) volcano of the Sierra Nevada Volcanic Range. Recent field data revealed that Tlaloc was very active during late Pleistocene through a series of explosive eruptions. One of these eruptions produced the Multilayered White Pumice (MWP) a rhyolitic pyroclastic sequence. The eruption began with a 24-km high Plinian column MWP-F1 that was dispersed to the NE by prevailing winds. It was interrupted by fountaining of the column with the generation of a pyroclastic density current that emplaced MWP-S1 layer. Then, followed five unstable sub-Plinian columns (MWP-F2 to F6) that reached altitudes between 16 and 19km. Fall deposits as a whole are 1m thick at 12km from the vent, cover a minimum area of 577km2 for a total volume of 4.68km3 (DRE 1.58km3). The eruption ejected a total mass of 3.45×1012kg at different mass discharges. The last sub-Plinian column (MWP-F6) collapsed and produced dense pyroclastic density currents that deposited pumiceous pyroclastic flows (MWP-PF) following main ravines to the north and east of the vent. These density currents filled gullies with 23m-thick deposits at a distance of 12km from the vent totaling a minimum DRE volume of 0.2km3. Pyroclastic flow deposits charred tree trunks that yielded an age of 31,490+1995/−1595yrB.P. that closely date the age of the eruption. Rain during this phase of the eruption generated syn-eruptive lahars (MWP-DF). Post-eruptive lahars (MWP-ED) finally swept the volcano flanks.The MWP deposits consist of abundant white pumice (up to 96vol.%), rare gray pumice, cognate lithics, accidental altered lithics, xenocrysts. White and gray pumice clasts contain phenocrysts of quartz, plagioclase, sanidine, biotite, rare Fe–Ti oxides, monazite, zircon and apatite. Xenocrysts are represented by plagioclase, microcline, orthoclase and quartz likely coming from a deeper plutonic body. Both pumices have a rhyolitic composition (74.98±1wt.% SiO2 in water free basis) representing one of the most acidic products of Tlaloc and the entire Sierra Nevada Volcanic Range. The rhyolitic liquid was likely extracted from a mush-like plutonic body reaching the minimum predicted vesicularity values for Plinian fragmentation for which the triggering mechanism of the MWP eruption is attributed to the high viscosity of the magma that provoked overpressure of the magmatic system. The parameters studied suggest that eruption behavior evolved from Plinian to sub-Plinian eruptive columns over time due to conduit dynamics rather than variations in magma properties of the MWP products.

P53 岩石学的にみた立山火山第2期噴火の再区分とマグマ供給系(2) : 鉱物組成を用いた火砕流堆積物と広域テフラの対比(ポスターセッション)

P53 岩石学的にみた立山火山第2期噴火の再区分とマグマ供給系(2) : 鉱物組成を用いた火砕流堆積物と広域テフラの対比(ポスターセッション)

P37 宮城県鳴子火山起源の火砕流堆積物の岩石学的検討(ポスターセッション)

P37 宮城県鳴子火山起源の火砕流堆積物の岩石学的検討(ポスターセッション)

P81 新燃岳享保噴火に伴う火砕流の分布及び産状(ポスターセッション)

P81 新燃岳享保噴火に伴う火砕流の分布及び産状(ポスターセッション)

Variable eruptive styles in an ancient monogenetic volcanic field: examples from the Permian Levín Volcanic Field (Krkonoše Piedmont Basin, Bohemian Massif)

*Corresponding author The Permian pyroclastic deposits of the Levin Volcanic Field within the Krkonose Piedmont Basin were studied in terms of volcanology. Pyroclastic rocks are exposed in two quarries and the study was supported with a 30 m deep borehole K1 penetrating these rocks. The pyroclastic rocks are altered but preserved textures enable reconstruction of eruptive styles. The volcanic sequence exposed in the abandoned Hvězda quarry starts with a phreato-Strombolian pyroclastic rocks rich in basaltic scoria, cuspate glass shards, armoured- and accretionary lapilli overlain with mafic lava. Subsequent activity was phreatomagmatic in style and produced fall-out of accretionary lapilli and accumulation of base surge deposits. Overlying subhorizontally bedded matrix-supported pyroclastic deposits are rich in scoria and contain spindle-shaped bombs. These rocks are interpreted as mafic pyroclastic flow deposits related to Strombolian eruptions. A similar succession capped by scoriaceous fall-out deposit was documented in the K1 borehole. A coherent mafic volcanic rock (lava or sill) terminates the succession exposed in the Hvězda quarry. Agglutinates at the base of the Studenec quarry were produced during Hawaiian eruptions building up a spatter cone. The cone most probably dammed a stream and created an ephemeral lake. Increasing influence of water on eruptive styles is documented in overlying pyroclastic deposits of phreatomagmatic eruptions. Subsequent lava flowed into the lake. Quenching of the lava resulted in formation of pillows enclosed in hyaloclastite breccia. Further up in the exposure, transition of pillow to massive lava has been recognized. Overlaying pyroclastic deposits are matrix-supported and rich in scoria and spindle-shaped bombs. Similarly to the Hvězda quarry, these are interpreted as mafic pyro clastic flow deposits. Volcanic activity in the Levin Volcanic Field was characterized by Hawaiian, Strombolian, phreato-Strombolian and phreatomagmatic eruptions accompanying lava effusions and possibly sills emplacement. The lavas were emitted in both subaerial and subaquatic conditions. The character and distribution of volcaniclastic facies suggests existence of several monogenetic volcanoes.

Volcanic history of the Conchagua Peninsula (eastern El Salvador)

New results of detailed geological mapping, K–Ar dating and geochemical study of the Conchagua Peninsula in eastern El Salvador are presented. Volcanism in the area was controlled by intersection of three tectonic structures, the trenchparallel Central Graben, perpendicular Comayagua Graben, and the Guayape Fault Zone. The age of the volcanic activity spans from Miocene to Quaternary, however, the volcano itself is extinct. The basement is built of the welded rhyolitic Playitas ignimbrite, which extends as far as to the Island of Zacatillo. The pyroclastic rocks of La Union unit (mean K–Ar age: 13.3 ± 3.7 Ma) display signs of mingling between basaltic and dacitic magmas (banded pumice, deposits containing both mafic scoria and felsic pumice fragments), and this is interpreted as a result of eruptions triggered by injection of a basaltic magma into a dacitic magma chamber. Lavas and pyroclastic flow deposits of the subsequent Pozo unit are poorly exposed and strongly altered. Following effusive activity hereby defined as Pilon Lavas was dominated by andesite and basaltic andesite lavas. Pleistocene volcanic activity is represented by the Pre-Conchagua edifice (1.6 ± 0.6 Ma), Cerro Montoso, El Bable and Juana-Pancha. Regarding the trace-element composition, some lavas of the PreConchagua – Juana-Pancha are distinct from common volcanic front products (lower Zr/Nb, Th/Nb, Ba/Nb), resembling the lavas of Tegucigalpa volcanic field to the north, which is located behind the volcanic arc in the Comayagua Graben. Behind-arc extensional tectonics could have facilitated the magma genesis via decompression melting of the mantle wedge. The current Conchagua Volcano consists of two cones, Ocotal and Banderas, built by repeated Strombolian eruptions associated with effusions of basaltic lavas. The uppermost unit consists of a white tuff preserved in the sedimentary fill of several tectonic valleys west of Conchagua. The white tuff was interpreted as distal fall-out of the Tierra Blanca Joven eruption of the Ilopango Caldera.

The volcano-tectonic evolution of the Miocene Santa Lucía Volcano, Boaco district, Nicaragua

*Corresponding author The present-day Santa Lucia caldera is an erosional relic of a Late Oligocene to Early Miocene stratoshield volcano located in the south-western part of the Chortis Block in Central Nicaragua. Six main lithological units were recognized: (Unit I) dacitic ignimbrite of Boaco type, which represents the basement of the Santa Lucia caldera; (Unit II) dacitic ignimbrite of Fonseca type, locally intercalated with epiclastic and dacitic lavas; (Unit III) “lower” andesite lavas; (Unit IV) blocky, lithic-rich pyroclastic flow deposits, (Unit V) “upper” andesite and basalt lavas, and (Unit VI) epic lastic rocks (lahar deposits). On the basis of field mapping, petrological and geochemical data, a new model for the evolution of the Santa Lucia Volcano is presented. The first stage consisted of a series of strong Sub-Plinian eruptions, which produced thick ignimbri te units. These events destroyed the pre-existing volcanic edifice. The second stage was dominated by large explosive eruptions producing mainly non-welded dacitic–andesitic ignimbrites. The next resulted in the formation of andesitic lava flows and minor tephra fall-out deposits, covered by voluminous basaltic lavas. Lahars probably triggered by volcanic and/or seismic events represent the final stage of volcanic activity. From the Miocene onwards, volcanic rocks became deeply weathered and locally eroded. Strongly weathered rocks are susceptible to landslides, and surficial modification by post-volcanic slope movements is observed on the slopes of the Santa Lucia Volcano. Identification of the lithological units and their comparison with wider Tertiary volcanic areas in Central Nicaragua will contribute to the knowledge of the regional volcanic stratigraphy and evolution.

LiDAR derived morphology of the 1993 Lascar pyroclastic flow deposits, and implication for flow dynamics and rheology

Abstract Pumice flows are potentially destructive volcanic events that derive from eruption column collapse and whose dynamics are poorly understood. The challenges in studying these flows include the lack of constraints on the dynamics, kinematics and initial conditions that control their emplacement. We present a morphological study of the distal deposits (lobes) of the pumice flows resulting from the 1993 eruption at Lascar, Chile. The surface geometry of the lobes was measured in detail using a LiDAR device, which allowed for detailed characterisation of their morphology, consisting of central channel and lateral levees, and terminal frontal snout. In particular we find that the ratio of channel/levee height as a function of the ratio of the distance between levees/total width of the lobe has a characteristic curve for these pumice flow lobes. Our analysis of several of the Lascar pumice lobe deposits (south east sector) identified several dimensionless groups of the available parameters which, when compared against published results from both experimental and numerical investigations, allowed us to constrain crucial kinematic and dynamic information on the terminal phase of the pumice flows. Notably, we estimate the velocity of the terminal phase of pumice flows to be 5–10 m/s. Froude numbers of 1.5–2 are comparable with values found for experimental granular flows. Height–width aspect ratios for the levee–channel section of the pumice lobes are similar to those for experimental flows although these same aspect ratios for the snout are much larger for the natural deposits than their small-scale analogues. Finally, we discuss the possible emplacement dynamics of the terminal Lascar 1993 pumice flows. A pseudo-Reynolds number based on the velocity estimation is found to be up to 100 times larger for the pumice flows than experimental-scale flows. This suggests that the flow-retarding frictional forces for large-scale flows are relatively unimportant compared to flows at smaller scales. Mechanical effects such as fluidisation, mobilisation of material lying on the slopes over which they propagate and lubrication due to polydispersivity could provide an explanation for their ability to propagate on shallow slopes (6–11°).

Submarine evidence of a debris avalanche deposit on the eastern slope of Santorini volcano, Greece

Hummocky seafloor features were discovered on the eastern flank of Santorini volcano, Greece. Multibeam bathymetric mapping, airgun seismic profiling, side scan sonar survey, and remotely operated vehicle (ROV) dives have been carried out to characterize the nature of the hummocks. These hummocks appear to be composed of several tens of blocks that are up to several hundred meters in diameter, and are the surface expression of a much larger deposit than is observed in the bathymetry. The sidescan and airgun data show that the deposit covers an area of approximately 6km wide by 20km long, and is up to 75m thick. We estimate the total volume of the deposit to be approximately 4.4×109m3. Sampling of these blocks show they are composed of pyroclastic flow deposits produced during the Minoan eruption of Santorini (ca. 3600BP). We propose that the deposit is the result of a multi-stage landslide event that was caused by one of the several large earthquakes or volcanic eruptions that have occurred in the vicinity of Santorini since the Minoan eruption. One or more of these events likely triggered the destabilization of a part of the eastern flank of Santorini, which led to a debris avalanche, depositing blocks and forming a hummocky terrain at the base of the island's slope. The mass movement later evolved into a turbulent suspension flow that traveled 20km or more from the presumed initial failure. Given the size of the landslide deposit, it might have a tsunami potentially affecting the islands across the southern Aegean Sea. The understanding of earthquake–landslide dynamics has important implications for hazard assessment in this seismically active, historical, and highly populated region of the world.

The 1963–1964 eruption of Agung volcano (Bali, Indonesia)

The February 1963 to January 1964 eruption of Gunung Agung, Indonesia’s largest and most devastating eruption of the twentieth century, was a multi-phase explosive and effusive event that produced both basaltic andesite tephra and andesite lava. A rather unusual eruption sequence with an early lava flow followed by two explosive phases, and the presence of two related but distinctly different magma types, is best explained by successive magma injections and mixing in the conduit or high level magma chamber. The 7.5-km-long blocky-surfaced andesite lava flow of ∼0.1 km3 volume was emplaced in the first 26 days of activity beginning on 19 February. On 17 March 1963, a major moderate intensity (∼4 × 107 kg s−1) explosive phase occurred with an ∼3.5-h-long climax. This phase produced an eruption column estimated to have reached heights of 19 to 26 km above sea level and deposited a scoria lapilli to fine ash fall unit up to ∼0.2 km3 (dense rock equivalent—DRE) in volume, with Plinian dispersal characteristics, and small but devastating scoria-and-ash flow deposits. On 16 May, a second intense 4-h-long explosive phase (2.3 × 107 kg s−1) occurred that produced an ∼20-km-high eruption column and deposited up to ∼0.1 km3 (DRE) volume of similar ash fall and pyroclastic flow deposits, the latter of which were more widespread than in the March phase. The two magma types, porphyritic basaltic andesite and andesite, are found as distinct juvenile scoria populations. This indicates magma mixing prior to the onset of the 1963 eruption, and successive injections of the more mafic magma may have modulated the pulsatory style of the eruption sequence. Even though a total of only ∼0.4 km3 (DRE volume) of lava, scoria and ash fall, and scoria-and-ash pyroclastic flow deposits were produced by the 1963 eruption, there was considerable local damage caused mainly by a combination of pyroclastic flows and lahars that formed from the flow deposits in the saturated drainages around Agung. Minor explosive activity and lahar generation by rainfall persisted into early 1964. The climactic events of 17 March and 16 May 1963 managed to inject ash and sulfur-rich gases into the tropical stratosphere.

New seismo-stratigraphic and marine magnetic data of the Gulf of Pozzuoli (Naples Bay, Tyrrhenian Sea, Italy): inferences for the tectonic and magmatic events of the Phlegrean Fields volcanic complex (Campania)

A detailed reconstruction of the stratigraphic and tectonic setting of the Gulf of Pozzuoli (Naples Bay) is provided on the basis of newly acquired single channel seismic profiles coupled with already recorded marine magnetics gathering the volcanic nature of some seismic units. Inferences for the tectonic and magmatic setting of the Phlegrean Fields volcanic complex, a volcanic district surrounding the western part of the Gulf of Naples, where volcanism has been active since at least 50 ka, are also discussed. The Gulf of Pozzuoli represents the submerged border of the Phlegrean caldera, resulting from the volcano-tectonic collapse induced from the pyroclastic flow deposits of the Campanian Ignimbrite (35 ka). Several morpho-depositional units have been identified, i.e., the inner continental shelf, the central basin, the submerged volcanic banks and the outer continental shelf. The stratigraphic relationships between the Quaternary volcanic units related to the offshore caldera border and the overlying deposits of the Late Quaternary depositional sequence in the Gulf of Pozzuoli have been highlighted. Fourteen main seismic units, both volcanic and sedimentary, tectonically controlled due to contemporaneous folding and normal faulting have been revealed by geological interpretation. Volcanic dykes, characterized by acoustically transparent sub-vertical bodies, locally bounded by normal faults, testify to the magma uprising in correspondence with extensional structures. A large field of tuff cones interlayered with marine deposits off the island of Nisida, on the western rim of the gulf, is related to the emplacement of the Neapolitan Yellow Tuff deposits. A thick volcanic unit, exposed over a large area off the Capo Miseno volcanic edifice is connected with the Bacoli-Isola Pennata-Capo Miseno yellow tuffs, cropping out in the northern Phlegrean Fields.

Hornblende, cummingtonite, and biotite hydrogen isotopes: Direct evidence of slab-derived fluid flux in silicic magmas of the Taupo Volcanic Zone, New Zealand

Hornblende, cummingtonite, and biotite δD values from rhyolitic and mingled/mixed mafic rocks from the Central Taupo Volcanic Zone were determined in an effort to characterize the isotopic composition of the dissolved fluid of magmas in this predominantly rhyolitic volcanic zone. Hand samples were obtained from pyroclastic flow, airfall, and lava flow deposits as well as co-eruptive cognate plutonic blocks. Mineral δD values range from −88 to −35‰ SMOW (Standard Mean Ocean Water). Cummingtonite phenocryst δD overlaps with hornblende from the same rocks, while biotite values are consistently ~7‰ more negative than hornblende phenocrysts from the same hand sample. Calculated volcanic steam δD values (ca. −46 to −25‰) are consistent with an island arc crustal source (ca. −40 to −15‰), with a negligible to absent mantle (ca. −100 to −60‰) component.Earlier researchers suggested that the Taupo Volcanic Zone fluid flux included dilute concentrations of slab-derived phases (i.e. CO2, Cl) similar to back-arc volcanic systems at other convergent margins. However, the island arc type fluid composition estimated from the minerals we studied is similar to hornblende and biotite compositions from pyroclastic deposits and volcanic fumaroles from the Kamchatka and Japanese arcs. The silicic rocks from the 2.0–1.0Ma Mangakino caldera, the most inland caldera system we studied, have the lowest δD hornblende and biotite, suggesting that silicic volcanism, distal to the subduction trench, was derived from a back-arc like (i.e. mixture of mantle and slab-fluid). However, we found that the currently active more proximal locus of silicic volcanism reflects typical arc-like fluid compositions. Our results further suggest that the composition of volcanic steam migrating through the upper crust was similar in composition to regional meteoric waters (ca. −45 to −23‰).

Marine-continental tephra correlations: Volcanic glass geochemistry from the Marsili Basin and the Aeolian Islands, Southern Tyrrhenian Sea, Italy

Major, minor and trace element analysis of volcanic glass in proximal and distal (<2mm) tephra deposits underpins tephrochronology. This approach has been tested in the Aeolian Islands and the Tyrrhenian Sea using juvenile clasts in pyroclastic fall and flow deposits. Geochemical data are used to link marine tephras in the Marsili Basin (core TIR2000-C01) to explosive eruptions of (1) Lipari (Monte Pilato; 776calAD); (2) Vulcano; and (3) Campi Flegrei (Soccavo 1; 11,915–12,721calyears BP). Whether a polymictic coarse grained volcaniclastic turbidite in the Marsili Basin originated from collapse on Salina remains unresolved because multi-elemental analysis raises doubt about the published correlation to the Pollara region.It is evident that correlation of proximal continental and distal marine tephras, at a high level of confidence, requires a full complement of major, minor and trace element data. In conjunction with considerations of the mineralogy and morphology of juvenile deposits these data help define petrological lineages such that precise provenance can be established. Whilst a precise proximal–distal match must be based on identical major, minor and trace element concentrations it is clear that resurgent activity from a single volcano can produce magmas with identical compositions. In such cases stratigraphic relationships must complement any geochemical study. Occasionally proximal stratigraphies may be unrepresentative of the complete eruptive history because of a lack of exposure due to burial by more recent effusive and explosive activity, or sector collapse which can remove vital stratigraphy particularly on volcanic islands.

The pyroclastic dyke and welded crystal tuff of the Morro dos Gatos alkaline intrusive complex, State of Rio de Janeiro, Brazil

The present article reports the geologic, lithologic, and petrographic characteristics of the pyroclastic dyke and phenocryst-rich welded crystal tuff of the Morro dos Gatos alkaline intrusive body, State of Rio de Janeiro, Brazil. The intrusive complex has an extension of 1.2 x 0.8 km occupying an area of 0.5 km². It is constituted mainly by monzonite and trachyte. At the south flank of the massif, there exits a pyroclastic dyke and at the northwest flank, an intrusive welded crystal tuff. The pyroclastic dyke is 40 to 80 cm wide and intrudes into the basement porphyritic granite. This rock is constituted by welded tuff breccia with large amount of lapilli-size angular lithic fragments of trachyte. Along the contact plane of this dyke, there is an intercalation of the trachyte. The crystal tuff is characterised by abundant alkaline feldspar phenocrysts and mineral fragments of alkaline feldspar, quartz, biotite, and amphibole, showing a crystal matrix-supported texture. Large phenocrysts are aggregations of idiomorphic alkaline feldspar crystals with a general size of 2 mm and a maximum one of 6 mm. The matrix is composed mainly of mineral fragments of less than 1 mm in general size. The size-frequency diagram for the mineral fragments suggests that they have been originated from an explosive eruption. The matrix is strongly welded and completely devitrified showing a cryptocrystalline texture. The crystals were concentrated by sorting with the help of the volcanic gas ascending in the volcanic conduit during a continuous and effusive phase of the eruption. The limited special distribution of the pyroclastic rocks and small outcrop size indicate that they are not constituent of pyroclastic flow deposits, but of subvolcanic conduit and fissure.

応力依存剛性変化モデルを用いたしらす地山における超大断面トンネルの挙動分析

Shirasu, which is defined as the non-welded part of pyroclastic flow deposits, is widely distributed in the southern part of Kyushu Island. Shirasu is regarded as soft ground when the mountain tunneling method is applied because its unconfined compression strength is about 50 kN/m2. Nevertheless, it is well known that Shirasu tunnels are comparatively stable. In order to make this mechanism clean, the authors have already proposed a numerical analysis method considering geotechnical nonlinear properties of Shirasu ground. In this paper, they adopted this method to a tunnel with an extremely large section in Shirasu ground and confirmed the validity of this method by comparing the numerical results with those obtained from the field measurement. Further, they analyzed the peculiar behavior of the tunnel with the extremely large section.

秋田県谷地地すべり地域の地形発達過程

谷地地すべり地域において，林道切土法面を観察し，火砕流堆積物の分布範囲，火砕流堆積物と地すべり移動地塊の内部構造の確認を行うとともに，テフラ分析を行った。その結果，火砕流堆積物は二種類あり，それらは前期更新世に噴出した湯浜火砕流堆積物もしくは約20万年前以前に噴出した不動山火砕流堆積物と同定される二つのユニットであると判断された。火砕流堆積物については，泥岩礫の中に混入している，破砕泥岩あるいは別の火砕流堆積物と急角度で接している，破砕泥岩の上を覆っているなどの3つの状態が観察された。このような内部構造の観察結果と空中写真による微地形判読結果を総合すると，当該斜面では約20万年前以前から地すべりが発生しており，さらに火砕流堆積物の堆積後にそれらを巻き込んだ地すべりと現在の微地形を形成した地すべりが存在することが明らかとなった。