Lahar Deposits Research Articles

Sistemas de información geográfica y cartografía geomorfológica aplicados al inventario de deslizamientos y cartografía de susceptibilidad en la cuenca del río El Estado, Pico de Orizaba, México

With the aim of raising awareness on the prevention of landslide disasters, this work develops a methodology that incorporates geomorphological mapping into the mapping of landslide susceptibility using Geographic Information Systems (gis) and Multiple Logistic Regression (mlr). In Mexico, some studies have evaluated the stability of hillsides using gis. However, these studies set a general framework and guidance (that includes basic concepts and explanations of landslide classification, triggering mechanisms, criteria, considerations, and analysis for landslide hazard reconnaissance, etc.) for preparing a landslide atlas at state and city levels. So far, these have not developed a practical and standardized approach incorporating geomorphological maps into the landslide inventory using gis. This paper describes the analysis conducted to develop an analytical technique and morphometric analysis for a multi-temporal landslide inventory. Three data management levels are used to create gis thematic layers. For the first level, analogue topographic, geological, land-use, and climate paper are converted to raster format, georeferenced, and incorporated as gis thematic layers. For the second level, five layers are derived from topographic elevation data: slope angles, slope curvature, contributing area, flow direction, and saturation. For the third level, thematic maps are derived from the previous two levels of data: a hypsometric map (heuristically classified to highlight altimetric levels), a reclassified slope map (allowing to highlight differences in relief), and a morphographic map (derived from a heuristic reclassification of the slope map to highlight volcanic landforms). The theoretical aspects of geomorphological mapping contribute to set the conceptual basis to support landslide mapping. The gis thematic layers provide context and establish an overall characterization of landslide processes within the watershed. Through the retrieval and on-off switching of layers in the gis system, a base map is created to assist in the digitizing of landslides and the modeling of susceptibility. A landslide inventory is created from aerial photographs, field investigations, and all the above gis thematic layers. El Estado river watershed on the southwestern flank of Pico de Orizaba volcano has been selected as study area. The watershed is located in the southwestern slope of Citlaltepetl or Pico de Orizaba volcano. Geological (the stream channel of El Estado river erodes Tertiary and Quaternary lavas, disjointed volcanoclastic materials such as pyroclastic flows, fall deposits, lahars deposits, and alluvium) and geomorphological factors (steep slopes, energy relief, and vertical erosion) in combination with high seasonal rainfall (annual rainfall averages 1000-1100 mm/yr at > 4000 m a.s.l. and 927 mm/yr at <1500 m a.s.l.), and the high degree of weathering, make the study area susceptible to landslides. To assess landslide susceptibility, a landslide inventory map and geomorphometric cartography (altimetry, slope and geomorphography) were reviewed, and field work was conducted. In the study area, more than one hundred landslides were mapped. Shallow landslides (including debris slides and debris flows) are the predominant type. Shallow landslides predominate on hills capped with ash and pyroclastic deposits. The second major landslide process includes rock falls (which occur where the stream erodes lava flows and lahars) and deep-seated landslides (which occur in ash and pyroclastic deposits where lava flows act as a slip plane). In parallel, the spatial geodatabase of landslides was constructed from standardized gis datasets. Pertinent attributes are recorded on a geo-dataset. These include 1) mass wasting process, 2) level of certainty of the observation, 3) photo identification date, 4) landslide size, 5) landslide activity, 6) landslide parts (head, evacuation zone, deposit), 7) slope shape, 8) field slope gradient, 9) map gradient measured from the 10 m digital elevation model (dem), 10) landslide delivery, 11) land use, 12) elevation at which the landslide started, 13) aerial photograph identification number, 14) landslide area, and 15) researcher comments. Each attribute is standardized by the geo-dataset domains in the gis system. With this information the landslide susceptibility is modeled using mlr within a gis platform. mlr is used to examine the relation between land sliding and several independent variables (elevation, slope, contributing area, land use, geology, and terrain curvature) to create the susceptibility map. With six independent variables, the multiple logistic model susceptibility map tends to overpredict landslides at a 10 m pixel resolution. However, the model is statistically valid and able to predict 79.81% of the existing landslides. The implementation of a landslide inventory and susceptibility mapping techniques demonstrate the feasibility of the method for use in other volcanic areas of Mexico.

Sediment transport in headwaters of a volcanic catchment—Kamchatka Peninsula case study

Due to specific environmental conditions, headwater catchments located on volcanic slopes and valleys are characterized by distinctive hydrology and sediment transport patterns. However, lack of sufficient monitoring causes that the governing processes and patterns in these areas are rarely well understood. In this study, spatiotemporal water discharge and sediment transport from upstream sources was investigated in one of the numerous headwater catchments located in the lahar valleys of the Kamchatka Peninsula Sukhaya Elizovskaya River near Avachinskii and Koryakskii volcanoes. Three different subcatchments and corresponding channel types (wandering rivers within lahar valleys, mountain rivers within volcanic slopes and rivers within submountain terrains) were identified in the studied area. Our measurements from different periods of observations between years 2012–2014 showed that the studied catchment was characterized by extreme diurnal fluctuation of water discharges and sediment loads that were influenced by snowmelt patterns and high infiltration rates of the easily erodible lahar deposits. The highest recorded sediment loads were up to 9∙104 mg/L which was related to an increase of two orders of magnitude within a one day of observations. Additionally, to get a quantitative estimate of the spatial distribution of the eroded material in the volcanic substrates we applied an empirical soil erosion and sediment yield model–modified universal soil loss equation (MUSLE). The modeling results showed that even if the applications of the universal erosion model to different non-agricultural areas (e.g., volcanic catchments) can lead to irrelevant results, the MUSLE model delivered might be acceptable for non-lahar areas of the studied volcanic catchment. Overall the results of our study increase our understanding of the hydrology and associated sediment transport for prediction of risk management within headwater volcanic catchments.

米代川流域で発見された十和田火山AD915噴火後のラハール堆積物と埋没建物

米代川流域で発見された十和田火山AD915噴火後のラハール堆積物と埋没建物

Economic risk assessment of Cotopaxi volcano, Ecuador, in case of a future lahar emplacement

The Cotopaxi volcano in north-central Ecuador is one of the most dangerous volcanoes of the world due to its ability to generate far-reaching lahars, which are able to destroy infrastructure and endanger a high amount of population living close to the main fluvial drainages of this volcano. An economic evaluation of the infrastructure, which is above or close to the historic lahar deposits, has been undertaken in order to determine the values of direct and indirect damages of a potential future lahar. Such strategic infrastructure includes habituated and constructed areas, highways and hydroelectric plants besides many other structures or properties. Indirect damages include losses in revenues in water, food, energy and gasoline supply among others. The total losses of such damages are approximately 17 billon USD, representing 16.8 % of the Ecuadorian GDP. Simultaneously, the construction of mitigation structures able to retain lahars is calculated to be of about 150 million USD. The calculated benefit–cost ratio (1:114) is significant, entailing a cost reduction in 9912 %, and leads to the conclusion that the construction of mitigation structures is the most optional and justified choice for the authorities in order to protect life and prevent a high loss of strategic infrastructure.

Volcanological study of the middle Miocene Okiduse Volcanic Group, Woodlark Island (Muyuw), eastern Papua

ABSTRACTWoodlark Island (Muyuw) is located in a tectonically complex region, one of the few places on Earth where continental breakup is occurring ahead of seafloor spreading. Rifting commenced in the late Miocene (8.8–6 Ma) and is associated with the westward-propagating Woodlark Basin Spreading Centre. The island comprises approximately 850 km2 of raised Pleistocene coral reef and associated sediments with a central, moderately elevated range underlain by the middle Miocene calc-alkaline to shoshonitic Okiduse Volcanic Group (new name). It provides an exposure of upper Cenozoic geology in close proximity to the spreading centre. The Okiduse Volcanic Group is host to most of the island's historical gold and silver production and recently defined mineral resources totalling 1.75 Moz gold. This study uses facies analysis of pyroclastic deposits to develop a detailed geological map of the Okiduse Volcanic Group, with a revision and reinterpretation of the unit. Facies associations suggest that two major volcanic centres erupted synchronously during the middle Miocene (14–12 Ma), referred to as the Watou Mountain Eruptive Centre (new name) and the Uvarakoi Caldera (new name). The mafic–intermediate Watou Mountain Eruptive Centre formed during frequent small eruptions of widely varying style. Strombolian, subplinian, vulcanian and dome-related explosive eruptions occurred, alternating with extrusion of block and ash flow deposits and lava domes. Pyroclastic deposits were rapidly reworked from the steep cone, and were redeposited in a series of coalescing aprons surrounding the volcano. The felsic Uvarakoi Caldera formed during a series of violent explosive eruptions by rapid removal of magma from the underlying chamber, followed by collapse. Plinian and possibly phreatoplinian eruptions, as a result of magma–water mixing in the surface environment, resulted in widely dispersed, highly fragmented tuff deposits. The caldera was modified by widespread erosion following eruptions, resulting in fluvial, laharic and slope-wash deposits. This study highlights lithological controls (porosity and permeability) by various units within the Okiduse Volcanic Group on ore deposition.

Characteristics of the syneruptive-spouted type lahar generated by the September 2014 eruption of Mount Ontake, Japan

Mount Ontake erupted at 11:52 am on September 27, 2014, which generated pyroclastic density currents, ballistic projectiles, ash falls, and a small-scale lahar that spouted directly from craters formed by the eruption. Because this lahar may have been generated by water released from within these craters, we refer to this lahar as a “syneruptive-spouted type lahar” in this study. The lahar of the 2014 eruption was small relative to the other syneruptive type lahars reported in the past that were snowmelt type or crater lake breakout type lahars. Nevertheless, in the 2014 event, the syneruptive-spouted type lahar extended approximately 5 km downstream from the Jigokudani crater via the Akagawa River, with an estimated total volume of ~1.2 × 105 m3. We have reviewed other representative syneruptive-spouted type lahars that have been reported in Japan. The syneruptive-spouted type lahar attributed to the September 2014 eruption had the longest runout distance and largest volume of all cases studied. The mineral assemblage identified from samples of the lahar deposits is similar to that of ash-fall deposits from the same eruption. Previous workers deduced that the ash was derived mainly from shallow depths (within 2 km of the surface). The syneruptive-spouted type lahar deposits are therefore also considered to have originated from shallow depths. A syneruptive-spouted type lahar is a small-scale phenomenon that causes little direct damage to infrastructure, but has long-term influence on water quality. Increases in turbidity and decreases in pH are expected to occur in the Mount Ontake area downstream of Nigorisawa after heavy rainfall events in the future. Therefore, the potential indirect (but long term) damage of syneruptive-spouted type lahars should be considered for hazard mapping and planning volcanic disaster prevention measures.

Volcaniclastic fan facies and reservoir characteristics: a case study of Guantao Formation in the No. 1 and No. 2 structures in the Nanpu Sag, Bohai Bay Basin, East China

This paper investigates volcaniclastic fan facies and reservoir characteristics of Guantao Formation in the No. 1 and No. 2 structures in the Nanpu Sag, Bohai Bay basin, Eastern China. Core and log data from the Guantao 3 and 4 Formations were analyzed in order to characterize volcaniclastic fan facies. Comprehensive analyses show that volcaniclastic fan sediments have been characterized as rapid proximal deposition of well-rounded and poorly sorted large-grain sediments. During the developmental processes of the volcanic fans, shallow to moderately deep lacustrine mudstones are deposited on the volcanoes. They vertically form superimposed sedimentary bodies that were characterized by laterally alternating fingers of volcaniclastic fan sediments and interchannel mudstone. Differences in pyroclastic supply and the volcanic terrain during the deposition account for the variations in volcanic rocks distribution. Distribution of volcaniclastic fans also exhibited characteristic features. Volcaniclastic fans around the volcanic highlands show variable distribution of volcanic rocks. The deep valley sizes of the debris flows and streams indicate variable sediment supply. The extent and thickness of the lahar deposits from the volcaniclastic debris flows indicate variable size of sedimentation processes. Most volcaniclastic fans are composed of debris-transported deposits having small clasts. The bed thickness increases with increasing distance from the volcanic source. The volcaniclastic rocks show high content of intermediate–basic volcanic rocks debris. The original and dissolved intergranular and dissolved intragranular porosities in the volcaniclastic rocks provide the best reservoir properties. Identification of lithofacies and their assemblages, and volcaniclastic sedimentary facies buried within volcanos and adjacent regions provide a better understanding that can be applied to similar rift basins. A method to identify depositional geometry and accumulation patterns of volcaniclastic fans was employed. This method assisted in constructing depositional models, reconstructing the depositional history of volcaniclastic fans and the related history of volcanic activity based on geological and geophysical data within volcanic areas in rift basins.

Volcanic Deposits and Volcanic Hazard in Santo Domingo de Heredia, Costa Rica

The present research aims to investigate more precisely about the geology of the Eastern region of the Santo Domingo County. Santo Domingo is part of the structural plateau in the center of Costa Rica, which is located at the foot of the Cordillera Volcánica Central (CVF) [Central Volcanic Front] and is covered by volcanic deposits. On this plateau, called Central Valley, is the highest percentage of the population of the country and therefore, a large sector of the Costa Rican population is exposed to volcanic eruptions of the volcanoes in the CVF. For existing the national system for risk management and a law that demands actions to local authorities to prevent and mitigate disaster, it is necessary to identify the threats that exist in the cantons (counties) of Costa Rica. This will serve to take the prevention and mitigation actions necessary to reduce the impact of volcanic eruptions in the area of Santo Domingo.The research method consisted of review and analysis of previous works through literature research, data collection and analysis of boreholes from records of water-supply wells and open pits, and field work to better know the geology of the area. The results indicate that there are deposits of powerful volcanic eruptions of pyroclastic fall deposits (volcanic ash and lapilli) that mostly form clayey soils and lahars deposits that practically covers the entire territory. Underlying these deposits there is a pyroclastic flow deposit (ignimbrite), followed by lapilli tephra (a layer of pumice of at least 2 meters thick) that mark a change in the volcanic activity. Such pyroclastic flow is overlaid by an igneous presumably sub-volcanic activity of andesites interlayered with ancient tuffs, with a considerable thickness of over 350 meters according with borehole data and the exposure recognition on Pará river study sites. According to site locations (P1 to P23) of volcaniclastic deposits, there is evidence of an important environmental impact caused by the last eruptions of the CVF volcanoes. The real and current volcanic threat to the population of the County is the fall-out of ash emitted from the Turrialba and Irazú volcanoes. From local observations along the Virilla and Pará rivers sections, there is no evidence of younger pyroclastic flows overlying the volcanic sequence.

Biotic interactions in an exceptionally well preserved osmundaceous fern rhizome from the Early Jurassic of Sweden

A remarkably well permineralized osmundaceous rhizome from the Early Jurassic of southern Sweden yields evidence of an array of interactions with other organisms in its immediate environment. These include epiphytism by a herbaceous heterosporous lycopsid; putative oribatid mite herbivory and detritivory (petiole and detritus borings and coprolites); potential pathogenic, saprotrophic or mycorrhizal interactions between fungi and the host plant and its epiphytes; parasitism or saprotrophy by putative peronosporomycetes; and opportunistic or passive mycophagy by oribatid mites evidenced by fungal spores in coprolites. A combination of abrupt burial by lahar deposits and exceedingly rapid permineralization by precipitation of calcite from hydrothermal brines facilitated the exquisite preservation of the rhizome and its component community of epiphytes, herbivores, saprotrophs and parasites. Ancient ferns with a rhizome cloaked by a thick mantle of persistent leaf bases and adventitious roots have a high potential for preserving macro-epiphytes and associated micro-organisms, and are especially promising targets for understanding the evolution of biotic interactions in forest understorey ecosystems.

Disrupted vegetation as a response to Jurassic volcanism in southern Sweden

Abstract Central Skåne (Scania) in southern Sweden hosts evidence of extensive Jurassic volcanism in the form of mafic volcanic plugs and associated volcaniclastic deposits that entomb well-preserved macro-plant and spore–pollen assemblages. Palynological assemblages recovered from the Höör Sandstone are of Hettangian–Pliensbachian age and those from the overlying lahar deposits are dated as Pliensbachian–early Toarcian (?). Palynomorph assemblages from these units reveal significantly different ecosystems, particularly with respect to the gymnospermous components that represented the main canopy plants. Both palynofloras are dominated by osmundacean, marattiacean and cyatheacean fern spore taxa but, whereas the Höör Sandstone hosts abundant Chasmatosporites spp. pollen produced by plants related to cycadophytes, the volcanogenic deposits are dominated by cypress family pollen ( Perinopollenites ) with an understorey component rich in putative Erdtmanithecales (or possibly Gnetales), and collectively representing vegetation of disturbed habitats. Permineralized conifer wood attributed to Protophyllocladoxylon sp., belonging to plants that probably produced the abundant Perinopollenites grains, is abundant in the volcanigenic strata, and shows sporadic intraseasonal and multi-year episodes of growth disruption. Together with the relatively narrow but marked annual growth rings, and the annual and mean sensitivity values that span the complacent–sensitive domains, these features suggest growth within Mediterranean-type biomes subject to episodic disturbance.

Tracing the evolution of 2010 Merapi volcanic deposits (Indonesia) based on object-oriented classification and analysis of multi-temporal, very high resolution images

ABSTRACT We compare identification, delineation and recording of freshly erupted deposits around active volcanoes from very high resolution optical images with that done by traditional geologic mapping. Object-oriented classification (OOC) and normalized difference spectral indices of vegetation, moisture and soil redness (NDVI, NDWI and NDRSI) have been applied to sub-metric GeoEye-1 and Pleiades images to identify and map pyroclastic and lahar deposits and trace their spatio-temporal evolution over two years, following the 2010 eruption of Merapi Volcano, Indonesia. We could identify several categories of pyroclastic depositional areas, and also map the damaged forest and destroyed cultivated terraces and settlements in the Gendol and Opak River basins on the south flank of the volcano. More than 75% of erupted deposits were delineated semi-automatically unlike the ground geological map based on photo-interpretation of the 2010 GeoEye image and field observations. A temporal image analysis, using bivariate scatter diagrams of the three spectral indices between 2010 and 2012 and a combination of NDWI and NDVI, separated areas affected by surges from unscathed vegetation. Use of NDRSI and NDWI allowed us to differentiate overbank Pyroclastic Density Current deposits from wet lahar deposits. NDRSI values close to 0 refer to scoria-rich pyroclastic deposits. About 40% of the devastated upper catchment was recolonized by vegetation between 2010 and 2012. The recovery also took place in the forested valley margins affected by ash-cloud surges. The morphometric analysis of the initial drainage network, digitized from the 2011–2012 images, demonstrated (1) the resurfacing of pristine 2010 PDC deposits by runoff and (2) incision or remobilization by lahars. It took two years following the eruption in the rugged upper catchment devastated by high-energy surges to fully develop the hydrographic network. It is, however, still rudimentary on gently sloping fans created by overbank PDC deposits in the middle valley, thus suggesting the importance of slope gradient, grain size, permeability and thickness of deposits. As much as 35% of the 2010 PDC deposits, emplaced in the vicinity of the river channels, were remobilized by lahars over the two post-eruption rainy seasons and also by constant mining activities. Studies on the erosion of the pyroclastic deposits after 2012 need to concentrate on the upper reach of the catchment on the south flank.

Paleoflow directions of a subaqueous lahar deposit around the Miocene Keserűs Hill lava dome complex (North Hungary) as constrained by photo-statistics and anisotropy of magnetic susceptibility (AMS)

A twofold fabric analysis by using photo-statistics on rock surfaces and low-field anisotropy of magnetic susceptibility (AMS) is presented, making it possible to infer paleoflow directions, which in turn helps to constrain the primary volcanic geomorphology of a deeply eroded mid-Miocene field (Keserűs Hill lava dome group, Visegrád Mountains, North Hungary). The analyses were carried out on the Rám Hill Pumiceous Sandstone (RHPS) that, based on its petrographic features, is considered as a subaqueously emplaced laharic deposit.The observed anisotropic frequencies of clast a-axis azimuths, the good agreement of fabric directions obtained for all and most elongated samples (elongation>2.5), the good clustering of the declinations of the K1 susceptibilities largely corresponding with the result of image analysis, allow to infer reliable flow directions. The obtained large-scale paleoflow paths show a quasi-radial pattern around the central part of the Visegrád Mountains, which quantitatively confirms the previous hypothesis on the volcanic structure, namely a central edifice-topped syn-eruptive topography.The RHPS is characterized by a clast fabric (revealed by photostatistics on cutted rock surfaces considering the lapilli-sized fraction) as strong as in subaerial PDC deposits, but by a significantly weaker magnetic fabric. The weak magnetic fabric is supposed to be the result of rock heterogeneity, thus the weakening effect of abundant lithic clasts with strong internal magnetic susceptibility anisotropy, different from the shape anisotropy of the clasts and the flow-related magnetic fabric of the fine-grained matrix.

Cliff notch formation along the sub-tropical La Paz Peninsula, Southern Baja California, Mexico

Abstract Most work on cliff notches in the Tropics and Subtropics has been on calcareous substrates that experience chemical or biochemical corrosion and the erosional and protectional effects of boring and encrusting marine organisms. There are deep notches in coarse andesitic lahar deposits along the subtropical western coast of the La Paz Peninsula in Baja California, Mexico. Many notch-forming mechanisms cannot operate here. There are no visible marine organisms at the cliff foot, the waves are generally too weak to dislodge rock fragments, there is little loose material for abrasion, and the substratum is resistant to corrosion. To determine the processes of notch formation, and their rates of erosion due to weathering and debris removal, rock samples were alternatively immersed in water for 90 min and then dried in air for one or two weeks to simulate conditions in the upper high tidal zone, corresponding to the deepest part of the notches in the field. These experiments ran for 17 months and used either fresh water to determine the efficacy of wetting and drying, or synthetic sea water to include the additional effect of salt weathering. The results suggested that salt weathering is more effective in this environment than wetting and drying and, because one week of drying was more effective than two weeks, that optimum notch development is probably closer to the neap than to the spring high tidal levels. As the experimental rock breakdown could also result from chemical weathering, rock samples were analyzed using inductively coupled plasma optical emission spectroscopy and mass spectrometry. The results suggested that, despite some Mg and Ca leaching, chemical weathering is not important in this area. Although the dominant notch-forming mechanism is salt weathering, field evidence implied that its efficacy rapidly increases with exposure to wave action, probably because of more frequent and stronger spray and splash and possibly because of the premature removal of loosened, weathered grains.

Shore platform and cliff notch transitions along the La Paz Peninsula, southern Baja, Mexico

Increasing exposure to wave action produces a northerly transition from high tidal notches to shore platforms inthe andesitic lahar deposits of the La Paz Peninsula, southern Baja, Mexico. Twenty-four notches were surveyedand wear pins were cemented into the apex of each notch. Thirty-six transverse micro-erosion meter (TMEM)stations were installed on three surveyed platforms. Field measurements were made over a 2.5 year period.The wear pins suggested notch backwearing (horizontal erosion) is <2mm yr -1 . The shore platforms were fairlynarrow (a few tens of metres) and steeper (1o) than most platforms in similar microtidal environments, reflectinga weak wave environment and resistant rocks. Mean TMEM downwearing (vertical erosion) rates for each of thethree platforms ranged from 0.14mm yr -1 to 0.42mm yr -1 . There was a good relationship between notch height(difference in elevation between the floor and the roof at the front of the notch) and exposure to wave action, butnotch depth is time-dependent and the relationship with exposure was not statistically significant. Notch heightwas also related to the orientation and wave fetch of the site. Field evidence suggested that the notches were notproduced by bioerosion or chemical weathering but by alternate wetting and drying or salt weathering from hightidal immersion and wave-generated splash and spray. Coastal morphology is fairly well adjusted to present sealevel although notch occurrence in the upper portion of the high tidal zone suggests that there is slow tectonic uplift in this region.

High-spatial-resolution imagery helps map deposits of the large (VEI 4) 2010 Merapi Volcano eruption and their impact

The 26 October–23 November 2010 eruption is Merapi’s largest event (VEI 4) over the past 140 years. We used high-spatial-resolution (HSR) imagery from GeoEye, Pleiades, IKONOS, and SPOT5 satellites to assess the extent and effects of the pyroclastic density currents (PDCs) and subsequent lahars. We have tracked the geomorphic and structural (fracturing) changes of Merapi’s summit crater and dome between 2008 and 2012. The 4 September 2011 GeoEye image shows that due to the explosive eruption, the summit area lost about 10 × 106 m3. The eruption enlarged the SSE-trending Gendol breach to be 1.3 × 0.3 × 0.2 km. The 2010 tephra and PDC deposits covered about 26 km2 in the two catchments of Gendol and Opak Rivers on Merapi’s south flank, i.e., 60–75 % of the total PDC deposit area, with a total bulk volume of 45 × 106 m3. The tephra-fall deposit covered an area of about 1300 km2 with a range in volume of 18–21 × 106 m3. Supervised and object-oriented classification on HSR imagery enables us to map in detail the PDC deposits across the Gendol-Opak catchment. We delineated 16 spectrally and/or texturally distinct units of PDC deposits and compared them with previously published results. They encompass high-energy surge deposits within ca. 8 km of the summit, valley-confined PDC deposits channeled as far as 16.5 km in the Gendol River, and widespread overbank PDC with ash-cloud surge deposits on valley margins. Additional high-resolution data are provided to map and analyze flooded areas due to lahar activity in 2011–2012 on the south and west flanks. Subsequent overbank lahars impacted selective small areas in the populated ring plain, devastating villages along the Putih River on the southeast flank and the Gendol River. We have analyzed the morphometric parameters (channel cross-sectional capacity, longitudinal rate of channel confinement, and channel sinuosity) of the Gendol-Opak River channels that govern overbank and avulsion of lahars in the ring plain. The paper demonstrates the potential of HSR satellite imagery to outline tephra, PDC, and lahar deposits; map the geomorphic and structural evolution of the summit area of persistently active composite cones; and thus improve hazard assessment for Merapi, a volcano whose summit, slopes, and drainages have changed more since October 2010 than at any other time since 1931.

Geological and geotechnical characteristics of recent lahar deposits from El Misti volcano in the city area of Arequipa, South Peru

This study provides geotechnical characteristics of recent lahar deposits on which the city of Arequipa (South Peru) is built. Geological and sedimentological observations point out the existence of three types of lahar deposits in the Arequipa region: fine hyperconcentrated-flow deposits, coarse hyperconcentrated-flow deposits, and debris-flow deposits. The mineral components identified in the three types of lahars show that they are linked to the outcropping volcanic rocks around Arequipa city. Physical measurements (dry density, grain-size distribution, specific surface of the grains based on methylene blue tests) and mechanical tests (in situ dynamic cone penetration soundings, oedometric and Casagrande shear-box tests) were performed on the three main categories of soils. Our results highlight that hyperconcentrated-flow deposits are fine sand- and silt-rich deposits that lack clay particles. Their dry density is low (ρd = 1.25 g/cm3) and their friction angle is high (ϕ = 38°) which contribute to the peculiar dynamics of lahar flows and to their high erosive power. The low apparent density provides a better capacity for the debulking process, whereas the high friction angle takes part in the erosion process. Finally, the geotechnical properties observed here suggest that the high contents in silica pyroclastic particles and the lack of clay or fine particles control the rheological behavior of lahar deposits. We can also consider that the rheological behavior of lahars through time is complex and that existing older lahars can be remobilized by heavy rain or future stream flows and lahars.

Effects and Behavior of Pyroclastic and Lahar Deposits of the 2010 Merapi Eruption Based on High-resolution Optical Imagery

The 26 October-23 November 2010, eruption is Merapi's largest event (VEI 4) over the past 140 years. We tracked and identified the 2010 Merapi's PDC deposits in the most impacted catchment (South) using high-resolution optical (from GeoEye and SPOT-5 satellites as well as low altitude photograph) imagery. We show that high-resolution imagery enables mapping with unprecedented detail the effects of the 2010 eruption in the summit area and across the most devastated catchment on Merapi. We investigated the relationships between the morphology of the river channel, and the apparent behavior of the PDCs and lahars, as deduced from over-banking processes. The 2010 pyroclastic deposits cover an area of ∼27 km2 in the Gendol-Opak catchment, i.e. 35% of the total deposit area. We analyze how unconfined PDCs with over-bank and veneer facies, as well as two types of surges have mantled widespread areas on both sides of the Gendol valley which contain the confined PDC deposits. Geometric and geomorphic characteristics that allow over bank and veneer deposits beyond the main valley are: limited cross-sectional areas under 1500 m2 and the decreasing longitudinal rate of channel confinement. Subsequent lahars six months after the eruption have devastated several villages along the Gendol River 20km from the summit on the ring plain. Small areas down-valley was affected by over-bank lahars once pyroclastic deposits were remobilized 3.8km farther than the PDC front. The over-bank and avulsed lahars can be attributed to the limited capacity (200-250 m2) of river channels and meandering river (sinuosity index of 1.25) across the lowest-angle (<2°) ring plain. Lahars now threaten the area of the iconic Prambanan temples and towards the Yogyakarta airport farther down the Opak River.

Depositional Processes and Temporal Component-change of Lahar Deposits at the Northern Foot of Chokai Volcano, NE Japan

Depositional Processes and Temporal Component-change of Lahar Deposits at the Northern Foot of Chokai Volcano, NE Japan

Hot clasts and cold blasts: thermal heterogeneity in boiling-over pyroclastic density currents

Abstract Partial thermal remanent magnetization data from clasts in pyroclastic density current (PDC) deposits provide information on the emplacement temperatures of both lithic and juvenile magmatic clasts contained in the deposits. We collected palaeomagnetic data from clasts in PDC deposits emplaced during historical eruptions of two volcanoes in Ecuador, the 2006 eruption at Tungurahua and the 1877 eruption at Cotopaxi. These eruptions were characterized by emplacement of PDCs mainly related to boiling-over activity. The deposits of these eruptions are similar and are characterized by cauliflower-textured juvenile scoria clasts up to 1 m in diameter and a diverse assemblage of lithic clasts surrounded by an unwelded ashy matrix. On the basis of progressive thermal demagnetization experiments, we infer that emplacement temperatures for most of the lithic clasts in PDC deposits are below 90 °C. In contrast, palaeomagnetic data from juvenile clasts from the same deposits provide emplacement temperatures higher than 540 °C. These data indicate the PDC were thermally heterogeneous over short length scales (decimetres) also after deposition. We hypothesize that PDCs emplaced by the boiling-over mechanism cool quickly owing to atmosphere entrainment, causing the juvenile clasts to form a rind that retains heat and that also prevents lithic clasts from appreciable heating. Several deposits on Cotopaxi, despite being morphologically similar to the PDC deposits, contain both cold lithic and juvenile clasts, which we interpret to be lahar deposits formed by PDCs travelling across glacial ice and snow. Rare deposits containing both hot lithic and hot juvenile clasts are classified as well-mixed, hot PDCs, and were erupted during a more energetic phase at Tungurahua.

U–Th dating of broken speleothems from Cacahuamilpa cave, Mexico: Are they recording past seismic events?

Cacahuamilpa cave is one of the largest karst systems in Central Mexico. The cave contains numerous massive speleothems broken and fallen following oriented directions, damaged during cataclysmic geological events. One fallen and two broken speleothems were sampled in the Cacahuamilpa cave for dating the rupture event using measured U–Th disequilibrium ages. A total of eight small carbonate cores were drilled perpendicular and longitudinal to the rupture surface. Results showed three groups of ages (weighted average): 0.95 ± 0.02 ka, 28.8 ± 0.2 ka and 88.0 ± 0.7 ka. This indicates that the construction of the Cacahuamilpa karst system, for which no absolute ages existed before this study, initiated at least since Late Pleistocene. The first two groups of ages might be related to two distinct episodes of intense seismic activity. Calculated minimum horizontal ground acceleration and frequency values of the seismic events needed to create the rupture of the stalagmites dated at 0.95 ± 0.02 ka and 28.8 ± 0.2 ka range between 1.3 and 2.0 m s−2 and between 13.4 and 20.8 Hz, respectively. These parameters are compatible with earthquakes of magnitude equal or higher than 7 M, with an epicentral distance between 50 and 100 km from the Cacahuamilpa cave. The stalagmite rupture dated at 88.0 ± 0.7 ka might result from the invasion of the cave by one of the older lahars deposits of the nearby volcano Nevado del Toluca, and successively fell by gravity instability.