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Tesi etd-03012008-140325
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Autore PISTOLESI, MARCO
URN etd-03012008-140325
Titolo Tephra and debris flow stratigraphy of the post-XII century activity at Cotopaxi volcano, Ecuador: a tool for lahar hazard assessment.
Settore scientifico disciplinare GEO/08 - GEOCHIMICA E VULCANOLOGIA
Corso di studi SCIENZE DELLA TERRA
Commissione
Nome Commissario Qualifica
Prof. Mauro Rosi Relatore
Prof. Raffaello Cioni Relatore
Parole chiave
  • stratigrafia
  • rischio
  • lahar
  • cotopaxi
  • tephra
Data inizio appello 2008-03-21
Disponibilità mixed
Data di rilascio2048-03-21
Riassunto analitico
Cotopaxi, one of the highest and most dangerous volcanoes on Earth, is a perfect, ice capped cone reaching an elevation of 5897 m. The volcano is located about 60 km south of Quito, capital of the Republic of Ecuador, and is surrounded by several villages and country’s rural infrastructures.
During the past centuries volcanic eruptions and concurrent rapid snow/ice melting have resulted in large debris flows (lahars) which have caused major devastations to the settlements around the volcano and traveled downstream for hundreds of km from the source. As a result of historical activity, lahar hazard assessment is a major focus of volcanic hazard work in Ecuador, as it represents the basis for effective mitigation actions in the field of both civil protection and land-use planning. Assessment of the lahar impact scenario along the main valleys have been developed in previous works (Mothes et al., 1998, 2004; Pareschi et al., 2004) using numerical models and assuming the last eruption occurred in 1877 as maximum expected event. The assumption of the 1877 as the most probable maximum scenario roots on the fact that the release of water during this event was maximized by the effective interaction of pyroclastic flows with the glacier, whose surface was larger than the present ice cap extension and due to approximately the 1/3 shrinkage of the glacier coverage of Cotopaxi, the volume of a lahar potentially generated under present conditions should be probably less than that of the 1877 lahar.
In this work tephra fallout architecture of Cotopaxi over the period 1150 to present is presented. 21 main tephra beds were identified in the field and described from physical and chemical points of view. Tephra deposits were furthermore characterized with volume and column height calculations, the latter also including some methodological investigations regarding maximum clast measurements.
A detailed mapping of the lahar deposits was also conducted, aimed at assessing the relative scale of different debris flow events, based on thickness, maximum block size and extension of the deposits. Precise and unequivocal identification and chronostratigraphic attribution of different lahars was made, within a radius of 25 km from the volcano, by identifying and tracing fallout beds interlayered with the lahar units. In a similar way, we also assessed the temporal relationships between pyroclastic flows and lahars.
Lahar simulations with different volumes were performed with a semi-empirical model (LAHARZ) within the western-southern area (Latacunga valley), where according to the archival records, tens of lahars have flowed during historic time causing great ruin of bridges, haciendas, and of the economy in general.
Stratigraphy of tephra was used as a tool to unravel lahar deposit complexity by using a multifaceted field approach of linking tephra, pyroclastic flow and lahar deposits.
According to the description of the historical chronicles, during the 1877 eruption (and presumably during most of previous historical eruptions as well), the spilling out from the crater of pyroclastic flows (scoria flows) was the major cause of extensive and rapid melting of the ice cap and the formation of large scale debris flows. Results of this work suggest that the totality of the recent events studied were produced by this mechanism. The pre-XV century period is characterized by the most powerful lahar events; all but one lahar deposits were related to tephra or scoria flow events.
The study of the lahar deposits indicates also that over the time interval under consideration, the volcano produced debris flows of widely variable scale. In this scale the 1877 event ranks as a moderate lahar, contrasting with other large (AD 1768, the largest for which eyewitnesses descriptions are available) or very large events (XII - XVI century).
As a result, lahar hazard assessment should take into account the possibility of an underestimation with 1877 event taken as maximum expected scenario, which could result in a life loss toll of unimaginable proportions.
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