Post-eruptive lahars at Kali Putih following the 2010 eruption of Merapi volcano,Indonesia: occurrences and impacts

Following the 2010 VEI 4 eruption of Merapi volcano, more than 250 lahars were triggered during two rainy seasons from October 2010 to March 2012. This high number of post-eruption lahars mainly occurred in the Kali (valley) Putih watershed and was mostly associated with high-magnitude rainstorms. A lahar occurring on January 8, 2011, caused significant damage to homes in several communities, bridges, sabo dams, and agricultural crops. The aims of this contribution are to document the impacts of lahars on the Kali Putih watershed and specifically (1) to analyze the lahar frequency during the period of 1969–2012 on an inter-annual and intra-annual basis and to determine the link between the volume of tephra and the frequency of lahars; (2) to detail the lahar trajectory and channel evolution following the January 8th lahar; (3) to map the spatial distribution of the thickness and geomorphic effects of the lahar deposit; and (4) to determine the impacts of the lahar on the infrastructure (sabo dams and roads) and settlements in the distal area of the volcano. The Kali Putih watershed has experienced 62 lahars, which represent 22% of all lahars triggered on 17 rivers at Merapi between 2010 and 2012. The main geomorphic impacts are: (1) excessive sedimentation in valleys, settlements and agricultural areas; (2) undercutting of the river banks by as much as 50 m, accompanied by channel widening; and (3) abrupt changes in the river channel direction in the distal area (15–20 km downstream of the volcano). About 19 sabo dams were damaged, and 3 were totally destroyed. Over 307 houses were damaged, and the National Road Yogyakarta–Semarang was regularly cut (18 times during approximately 25 days). Although the sabo dams on Kali Putih were originally constructed to protect distal areas from lahar damage, they had little effect on the 2010–2012 rain-triggered lahars. The underlying design of those dams along this river is one of the main reasons for the major destruction in this sector of the volcano’s lower slope. The catch basin capacity of the sabo dam was only 1.75?×?10⁶ m³, whereas the total volume of the 2010–2011 lahars exceeded 5?×?10⁶ m³. In order to prepare for future lahars, the government has invested in significant mitigation measures, ranging from structural approaches (e.g., building new sabo dams and developing an early warning system) to non-structural approaches (e.g., contingency and preparedness planning and hazard education).

     

长白山火山1000年前火山泥流堆积及其灾害

长白山火山１０００年前大喷发,火山泥流堆积物沿松花江中上游分布。距火山较近的是火山泥流的岩屑流堆积,远离火山变为火山泥流的超高密度流堆积。探讨了火山泥流的成因,并指出长白山火山一旦再次爆发,火山泥流将是主要的火山灾害,沿松花江中上游可能造成巨大破坏,并可能危及鸭绿江、图们江中、上游。     

长白山天池火山地质学研究的若干进展与灾害分析

通过以减轻火山灾害为目的的天池火山锥体顶部地区地质填图工作，发现了天池火山锥体附近不同期次火山泥石流，部分火山泥石流显示的高温定位特征指示了其与千年大喷发的成因联系。这些火山泥石流构成了严重的火山泥石流灾害，天池火山锥体近顶部大型滑坡体的发现则指示了天池火山另一种重要的灾害类型。滑坡体堆积物结构上可分为3种类型。天池火山千年大喷发时不同成分与物性的岩浆混合作用十分发育，指示了天池火山喷发前不同岩浆批的混合与共喷发机理。本文还论述了天池火山近代历史记录喷发物的分布与鉴别特征。     

Predicting Lahar-Inundation Zones: Case Study in West Mount Pinatubo, Philippines

This paper demonstrates techniques for pre-eruption prediction of lahar-inundation zones in areas where a volcano has not erupted within living memory and/or where baseline geological information about past lahars could be scarce or investigations to delimit past lahars might be incomplete. A lahar source (or proximal lahar-inundation) zone is predicted based on ratio of vertical descent to horizontal run-out of eruptive deposits that spawn lahars. Immediate post-eruption distal lahar-inundation zones are predicted based on “pre-eruption” distal lahar-inundation zones and on spatial factors derived from a digital elevation model. Susceptibility to distal lahar-inundation is estimated by weights-of-evidence, by logistic regression and by evidential belief functions. Predictive techniques are applied using a geographic information system and are tested in western part of Pinatubo volcano (Philippines). Predictive maps are compared with a forecast volcanic-hazard map through validation against a field-based volcanic-hazard map. The predictive model of proximal lahar-inundation zone has “true positive” prediction accuracy, “true negative” prediction accuracy, “false positive” prediction error and “false negative” prediction error that are similar to those of the forecast volcanic-hazard map. The predictive models of distal lahar inundation zones have higher “true positive” prediction accuracy and lower “false negative” prediction error than the forecast volcanic-hazard map, although the latter has higher “true negative” prediction accuracy and lower “false positive” prediction error than the former. The results illustrate utility of proposed predictive techniques in providing geo-information could be used, howbeit with caution, for planning to mitigate potential lahar hazards well ahead of an eruption that could generate substantial source materials for lahar formation.     

Volcanic hazards in the surroundings of Pico de Orizaba,eastern Mexico

Volcanic hazards from Pico de Orizaba volcano are presented here tor the first time. Some 1.3 million people live within the hazard zone, which in the most severe case would encompass the Mexican Gulf coast, east of the volcano. Three major cities located in the eastern part of the hazard zone account for 800 000 of this population and about 200 000 people live within a 20 km radius of the volcano. Probability calculations are presented as an attempt to quantify the hazards in the surroundings of the volcano. Such quantification can be of use in planning for future land use within the hazard zones.A zone of about 10 km radius centred on the top crater is a high hazard zone for gravity-driven flows and fallout ejecta. For large volume eruptions, the radius could be extended to 120 km to the east and 60 km to the west. The asymmetrical distribution is related to the topography of the volcano. Hazards from Pyroclastic-fall deposits are principally to the west of the volcano, since easterly winds are dominant in the area lava-flow hazards are greatest within a 10 km radius from the summit crater. Pyroclastic flow hazards are high up to 20 km from the volcano summit.In the case of reactivation of the volcano, melting of a glacier covering the summit of Pico de Orizaba having a volume equivalent to some 45 × 10⁹ litres of water, would produce lahars which would descend the flanks of the volcano.     

The first ascent to the volcano Cotopaxi in Ecuador by Wilhelm Reiss (1838–1908)

The volcano Cotopaxi in South America is 5,897 m high and is one of the highest active volcanoes of the world and the second highest volcano in the Andes after the Chimborazo (6,310 m). In Ecuador, there are more than 20 volcanoes contributing to the spectacular mountain range diving this country between the western and eastern lowlands. There have been more than 50 reports of volcanic activity at Cotopaxi since 1738, among which those from the years 1744, 1768, and 1877 are the largest. During the 1877 eruption, the whole summit glacier collapsed and a huge mudflow spread out for more than 100 km and flooded the city of Latacunga. Five years prior to this catastrophic event, the German geologist Wilhelm Reiss from the University of Heidelberg ascended Cotopaxi for the first time together with his supporter Angel M. Escobar from Columbia.     

Glacier Peak,Washington: A potentially hazardous cascade volcano

Recent field studies of postglacial volcanic deposits at Glacier Peak indicate the volcano has erupted more often, more voluminously, and more recently than previously thought. These past eruptions produced pyroclastic flows, extensive lahars, and widely distributed tephra falls. Analysis of the magnitude of past eruptions and the distribution of volcanic sediments indicates that future eruptions at Glacier Peak as large as those of the last several thousand years would dramatically affect people and property downstream and downwind from the volcano. Pyroclastic flows and lateral blasts would primarily affect uninhabited valleys within a few tens of kilometers of the volcano. Lahars and floods constitute the major hazard to populated areas from future eruptions, and could affect areas at low elevation along valley floors and in the Puget lowland as far as 100 km downvalley west of the volcano. Air-fall tephra from future eruptions will probably be deposited primarily east of Glacier Peak because of prevailing westerly winds.     

长白山火山灾害及其对大型工程建设的影响

长白山火山是世界著名的活火山,历史时期有过多次喷发,有再次爆发的危险.长白山火山最大的一次爆发发生在公元1199-1200年,这次大爆发的火山灰最远到达距其1 000km远的日本北部.依据这次大爆发由火山喷发空中降落堆积物、火山碎屑流和火山泥流造成的巨大火山灾害,预测了长白山火山未来爆发火山灾害的类型、强度和范围,并编制了长白山火山未来爆发火山喷发空中降落堆积物灾害预测图、火山碎屑流灾害预测图和火山泥流灾害预测图.该研究可预防和减轻火山灾害,指导核电站等大型工程选址.     

长白山天池地区全新世以来火山活动及其特征

长白山火山全新世规模最大的喷发活动发生在公元1199-1200年,即800年前的大爆发,被确定为普林尼或布里尼(Plinian)式喷发。这次大爆发形成体积巨大的、分布广泛的以空中降落堆积物为主的火山喷发碎屑堆积物,在长白山火山周围,远至日本都留下了地质记录。文章辨认并划分了这次大爆发火山碎屑物的成因类型:火山喷发空中降落堆积物(airfalltephra)、火山碎屑流(pyroclasticflow)状堆积物和火山泥流(lahar)堆积物,并且点、面结合,近、远和国内、国外兼顾,分析了这些火山碎屑物的主要特征、分布和相互关系,进而确定这些火山碎屑物分别属于两次普林尼式爆发。第1次(早期)普林尼式爆发称赤峰期,火山喷发模式为:普林尼式喷发柱(赤峰空落浮岩层)-火山碎屑流(长白山火山碎屑流层),随即主要由火山碎屑流诱发火山泥流(二道白河火山泥流层);第2次(晚期)普林尼式爆发称园池期,喷发模式为:普林尼式喷发柱(园池空落浮岩火山灰层)-火山碎屑流(冰场火山碎屑流层)。在层序上将气象站期碱流岩置于800年前大爆发火山碎屑物之下是正确的,其时代为晚更新世-全新世早期。     

TITAN2D simulations of pyroclastic flows at Cerro Machín Volcano,Colombia: Hazard implications

Cerro Machín is a dacitic tuff ring located in the central part of the Colombian Andes. It lies at the southern end of the Cerro Bravo–Cerro Machín volcanic belt. This volcano has experienced at least six major explosive eruptions during the last 5000 years. These eruptions have generated pyroclastic flows associated with Plinian activity that have traveled up to 8 km from the crater, and pyroclastic flows associated with Vulcanian activity with shorter runouts of 5 km from the source. Today, some 21,000 people live within a 8 km radius of Cerro Machín. The volcano is active with fumaroles and has shown increasing seismic activity since 2004, and therefore represents a potentially increasing threat to the local population. To evaluate the possible effects of future eruptions that may generate pyroclastic density currents controlled by granular flow dynamics we performed flow simulations with the TITAN2D code. These simulations were run in all directions around the volcano, using the input parameters of the largest eruption reported. The results show that an eruption of 0.3 km³ of pyroclastic flows from a collapsing Plinian column would travel up to 9 km from the vent, emplacing a deposit thicker than 60 m within the Toche River valley. Deposits >45 m thick can be expected in the valleys of San Juan, Santa Marta, and Azufral creeks, while 30 m thick deposits could accumulate within the drainages of the Tochecito, Bermellón, and Coello Rivers. A minimum area of 56 km² could be affected directly by this kind of eruption. In comparison, Vulcanian column-collapse pyroclastic flows of 0.1 km³ would travel up to 6 km from the vent depositing >45 m thick debris inside the Toche River valley and more than 30 m inside the valleys of San Juan, Santa Marta, and Azufral creeks. The minimum area that could be affected directly by this kind of eruption is 33 km². The distribution and thickness of the deposits obtained by these simulations are consistent with the hazard map presented by INGEOMINAS (Geological Survey of Colombia) in 2002. The composite map of the simulated flow deposits suggests that after major explosive events such as these, the generation of lahars is probable.     

Recent glacier variations on active ice capped volcanoes in the Southern Volcanic Zone (37°–46°S), Chilean Andes

Glaciers in the southern province of the Southern Volcanic Zone (SVZ) of Chile (37–46°S) have experienced significant frontal retreats and area losses in recent decades which have been primarily triggered by tropospheric warming and precipitation decrease. The resulting altitudinal increase of the Equilibrium Line Altitude or ELA of glaciers has lead to varied responses to climate, although the predominant volcanic stratocone morphologies prevent drastic changes in their Accumulation Area Ratios or AAR. Superimposed on climate changes however, glacier variations have been influenced by frequent eruptive activity. Explosive eruptions of ice capped volcanoes have the strongest potential to destroy glaciers, with the most intense activity in historical times being recorded at Nevados de Chillán, Villarrica and Hudson. The total glacier area located on top of the 26 active volcanoes in the study area is ca. 500 km². Glacier areal reductions ranged from a minimum of −0.07 km² a ⁻¹ at Mentolat, a volcano with one of the smallest ice caps, up to a maximum of −1.16 km² a ⁻¹ at Volcán Hudson. Extreme and contrasting glacier–volcano interactions are summarised with the cases ranging from the abnormal ice frontal advances at Michinmahuida, following the Chaitén eruption in 2008, to the rapid melting of the Hudson intracaldera ice following its plinian eruption of 1991. The net effect of climate changes and volcanic activity are negative mass balances, ice thinning and glacier area shrinkage. This paper summarizes the glacier changes on selected volcanoes within the region, and discusses climatic versus volcanic induced changes. This is crucial in a volcanic country like Chile due to the hazards imposed by lahars and other volcanic processes.     

长白山火山次生泥石流灾害危险范围预测

长白山火山次生泥石流是由长白山火山喷发引起的火口湖中的水沿长白山北坡缺口,以类似水库溃坝的形式突然溢出而形成的短时间、大体积的水流,是携带着地表的松散堆积物,沿着沟谷和山坡向下快速流动的一种类似洪流的特殊泥石流.笔者在野外地质调查和室内模拟试验的基础之上,采用FLOW-3D数值模拟软件,对长白山火山喷发引起的次生泥石流灾害进行大范围的数值模拟,并着重研究其对二道白河镇地区的影响程度,旨在为政府决策和防灾提供依据.结果表明:泥石流总体积为30.27亿m³时,二道白河镇将完全被泥石流淹没;不论哪种泥石流体积假设情况,泥石流都将到达二道白河镇,并对其造成危害;一旦火山爆发,二道白河镇居民可逃生时间只有30~42 min.     

从神话、传说与文字记录中探索中国活火山喷发的信息

火山喷发文字记录资料的考证可以给出历史上火山喷发的最为准确的时间限定，对喷发过程、灾害效应也都可给出极为详细、准确的措施。长白山天池火山喷发的满语资料是除了汉语、朝语之处另一种极为重要的资料来源，目前已从有关满语神话传说中得到了若干有重要意义的火山与火山学信息。本文列出了3类、14条天池火山喷发的神话与传说，从中可以得知数千年以来天池火山发生过多次猛烈的与温和的周期性的喷发。天池火山喷发与火山泥石流、喷发后洪水泛滥有密切关系，泛滥物波及到距离天池火山460km以过的嫩江流域及珲春一带。在中国近代史上还有若干次火山喷发记录，经考察较为确定的1951年昆仑山西部于田县卡尔达西火山喷发是一次地下岩浆上侵、地表汽爆炸与喷发的火山喷发事件。     

From sink to volcanic source: Unravelling missing terrestrial eruption records by characterization and high‐resolution chronology of lacustrine volcanic density flow deposits,Lake Inawashiro‐ko,Fukushima, Japan

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.     

Effusive Activity at Mount Etna Volcano (Italy) During the 20th Century: A Contribution to Volcanic Hazard Assessment

Mount Etna is an open conduit volcano, characterised by persistent activity, consisting of degassing and explosive phenomena at summit craters, frequent flank eruptions, and more rarely, eccentric eruptions. All eruption typologies can give rise to lava flows, which represent the greatest hazard by the volcano to the inhabited areas. Historical documents and scientific papers related to the 20th century effusive activity have been examined in detail, and volcanological parameters have been compiled in a database. The cumulative curve of emitted lava volume highlights the presence of two main eruptive periods: (a) the 1900–1971 interval, characterised by a moderate slope of the curve, amounting to 436 × 10⁶ m³ of lava with average effusion rate of 0.2 m³/s and (b) the 1971–1999 period, in which a significant increase in eruption frequency is associated with a large issued lava volume (767 × 10⁶ m³) and a higher effusion rate (0.8 m³/s). The collected data have been plotted to highlight different eruptive behaviour as a function of eruptive periods and summit vs. flank eruptions. The latter have been further subdivided into two categories: eruptions characterised by high effusion rates and short duration, and eruptions dominated by low effusion rate, long duration and larger volume of erupted lava. Circular zones around the summit area have been drawn for summit eruptions based on the maximum lava flow length; flank eruptions have been considered by taking into account the eruptive fracture elevation and combining them with lava flow lengths of 4 and 6 km. This work highlights that the greatest lava flow hazard at Etna is on the south and east sectors of the volcano. This should be properly considered in future land-use planning by local authorities.     

Multi-scale topographic analysis of Merbabu and Merapi volcanoes using wavelet decomposition

With the increasing availability of data, geo-sciences have experienced deep changes in handling and processing it. One of the presently explored research directions concerns the systematic decomposition and understanding of topographical features, without the subjective interactions of humans. This can eventually lead to fully automated algorithms for topographic analysis and understanding. This paper aims at being a contribution to this broad research area for the specific cases of stratovolcanoes, whose general geometry are very similar to a perfect cone. More specifically, this paper addresses two issues: (1) is it possible to separate erosion features (local variations) from structural features (large variations) on stratovolcanoes, through mathematical expression; (2) can information on volcanic activity??intensity, age, etc.??be retrieved from a topographic analysis? The study has been conducted from two volcanoes in Central Java (Indonesia): the Merapi and the Merbabu. The DEM of these volcanoes has been sampled using concentric circles with a radius ranging from 500 to 5,000?m (horizontal distance) to the summit. The data conversion and sampling was performed in ArcMap^?, while the data analysis was carried out with Matlab^?, using Discrete Meyer wavelet decomposition. Results provide an insight on large-scale topographic variations (long-wave wavelet) that have been separated from rapidly varying topographic features such as lahar channels (short-wave wavelets). Observations proved that flanks where the most recent volcanic activity occur??like at Merapi Volcano on the S-SE flank??present a very low variability of long-wave variations, whereas short-wave variations are important. The author argues that this feature is due to highly erosive lahars that dig the valleys combined with a recent production of material and volcanic growth keeping the overall structure regular. Flanks with lesser activity are characterized at the two volcanoes by important long-wave variations??most certainly due to long-term differential erosion??and different level of short-waves variations. Comparing the two volcanoes, results show that the valleys of Merapi and Merbabu volcanoes are deeply incised, indicating recent periods of high activity, with reworkable material eroded by lahars and other channels deepening processes. The topography of the summit area of Merapi Volcano is smoother than at Merbabu Volcano, where deep erosion features extend up to the summit area. This difference is most certainly due by the material production at the summit of Merapi Volcano. Developing such classification is important for automated mapping and computer recognition of volcanic past activities and their impacts on landscapes. It is the based for the development of decision trees that assist computer assisted and automated computer vision.     

A risk analysis for floods and lahars: case study in the Cordillera Central of Colombia

The glacier-covered Nevado del Tolima in the Colombian Cordillera Central is an active volcano with potential lahars that might be more hazardous than those on Nevado del Ruiz. Furthermore, rainfall-triggered floods and landslides notoriously and severely affect the region. For effective disaster prevention, a risk analysis is of primary importance. We present here a risk analysis methodology that is based on the assessment of lahar and rainfall-related flood hazard scenarios and different aspects of vulnerability. The methodology is applied for populated centres in the Combeima valley and the regional capital Ibagué (~500,000 inhabitants). Lahar scenarios of 0.5, 1, 5, and 15?million m³ volume are based on melting of 1, 2, 10, and 25?% of ice, firn and snow, respectively, due to volcanic activity and subsequent lahar formation. For flood modelling, design floods with a return period of 10 and 100?years were calculated. Vulnerability is assessed considering physical vulnerability, operationalized by market values of dwelling parcels and population density, whereas social vulnerability is expressed by the age structure of the population and poverty. Standardization of hazard and vulnerability allows for the integration into a risk equation, resulting in five-level risk maps, with additional quantitative estimate of damage. The probability of occurrence of lahars is low, but impacts would be disastrous, with about 20,000 people and more directly exposed to it. Floods are much more recurrent, but affected areas are generally smaller. High-risk zones in Ibagué are found in urban areas close to the main river with high social vulnerability. The methodology has proven to be a suitable tool to provide a first overview of spatial distribution of risk which is considered by local and regional authorities for disaster risk reduction. The harmonization of technical-engineering risk analysis and approaches from social sciences into common reference concepts should be further developed.     

New evidence concerning the Plio-Pleistocene landscape evolution of southern Santa Cruz region

Remnants of an old aggradational landscape, Cerro Cuadrado Proglacial, are preserved on top of the high mesetas Pampa Alta and La Meseta on both sides of the upper Santa Cruz river valley, South Patagonia.A first dissection of the mesetas, attributable to extended river erosion, predates the expansion of glacier lobes down the piedmontane area. The glacial advance is represented by the moraines of Pampa Alta Glaciation displayed on the top of Meseta Pampa Alta. Glacifluvial outlets contribute to the proglacial plain, Pampa Alta Proglacial, which is widespread to the southeast.Strong and persistent fluvial erosion followed the retreat of the ice masses leading to the formation of several terrace levels in the main upper valley, La Australasia Terraces and San Fernando Terraces, and a step, Cordón Alto, that truncates the Meseta Pampa Alta. These foreland features and the relief covered by the basalts at Cerro Fraile in the cordillera, are probably a consequence of a diastrophic phase that affected both areas during this stage.Late Pliocene basaltic lavas draining into the main and tributary valleys overran this landscape. The evidence indicates that during the eruption of the basalts the glaciation was active in the cordillera and that coeval fluvial and lacustrine aggradation took place in the extra-andean valleys.During the Middle Pleistocene subsequent lava flows covered the high pampas and partially occupied the fluvial valleys again. After this last volcanic episode the glaciers reached their maximum expansion to the east.     

Evidence from the Rio Bayo valley on the extent of the North Patagonian Icefield during the Late Pleistocene-Holocene transition

This paper presents data on the extent of the North Patagonian Icefield during the Late Pleistocene-Holocene transition using cosmogenic nuclide exposure age and optically stimulated luminescence dating. We describe geomorphological and geochronological evidence for glacier extent in one of the major valleys surrounding the North Patagonian Icefield, the Rio Bayo valley. Geomorphological mapping provides evidence for the existence of two types of former ice masses in this area: (i) a large outlet glacier of the North Patagonian Icefield, which occupied the main Rio Bayo valley, and (ii) a number of small glaciers that developed in cirques on the slopes of the mountains surrounding the valley. Cosmogenic nuclide exposure-age dating of two erratic boulders on the floor of the Rio Bayo valley indicate that the outlet glacier of the icefield withdrew from the Rio Bayo valley after 10,900 ± 1000 yr (the mean of two boulders dated to 11,400 ± 900 yr and 10,500 ± 800 yr). Single-grain optically stimulated luminescence (OSL) dating of an ice-contact landform constructed against this glacier indicates that this ice mass remained in the valley until at least 9700 ± 700 yr. The agreement between the two independent dating techniques (OSL and cosmogenic nuclide exposure age dating) increases our confidence in these age estimates. A date obtained from a boulder on a cirque moraine above the main valley indicates that glaciers advanced in cirques surrounding the icefield some time around 12,500 ± 900 yr. This evidence for an expanded North Patagonian Icefield between 10,900 ± 1000 yr and 9700 ± 700 yr implies cold climatic conditions dominated at this time.     

Analysis of the institutional and social responses to the eruption and the lahars of Mount Pinatubo volcano from 1991 to 1998 (Central Luzon, Philippines)

Mount Pinatubo volcano erupted in June 1991 in the main island of Luzon belonging to the Philippines archipelago. Huge economic losses and population exodus have followed. This major crisis has been relayed with other crises due to rain-fed lahars which have been supplied with eruption deposits. These lahars have occurred every year since 1991 during the rainy season. They will probably last until 2005. After a brief presentation of the Philippine official response system to disasters, this paper draws up a critical analysis of the different kinds of institutional and social responses deployed to manage the different crisis and post-crisis phases of this event. Based on three viewpoints: from population, media and other actors, this analysis attempts to point out the strengths and weaknesses of the official management system, especially by studying the efficiency and the range of the solutions taken. So, it appears that the management of the June 1991 main crisis (eruption) was a success. On the other hand, difficulties have occurred with lahars risk management. Indeed, these lahars have obliged the authorities to protect and relocate thousands of people. In spite of persistent problems, the management system (monitoring/warning/evacuation) of lahar crises improves year after year. Failures appear especially within the rehabilitation program (protection/rehousing). Many direct (lack of means, preparedness, coordination, dialog, etc.) and indirect (politico-administrative, socio-economic, cultural contexts) factors come together to lock the wheels of the institutional response system. They defer the socio-economic start of this vital northern Philippines area. This revised version was published online in July 2006 with corrections to the Cover Date.