Tephra and debris flow stratigraphy of the post-XII century activity at Cotopaxi volcano, Ecuador: a tool for lahar hazard assessment.

• Main Author: PISTOLESI, MARCO
• Other Authors: Rosi, Mauro, Cioni, Raffaello
• Format: Text
• Language: Italian
• Published: Pisa University 2008
• Subjects: GEO/08; Mothes; Quito; Ice cap
• Online Access: http://etd.adm.unipi.it/theses/available/etd-03012008-140325/

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 ...