Ricostruzione cronostratigrafica e dinamica eruttiva dell'esplosione laterale olocenica del duomo Pucará (Ecuador)

The eruptive event occured at Cerro Pucará (Ecuador) between 7150 and 4150 y/BP is a perfect example of lateral blast associated with a single dome of andesitic composition. The eruption occurred at the peak of a constructive phase, followed by the collapse and the destruction of the south-eastern side of the ancient structure. This produced an explosive event characterized by a directed blast and a following surge events: the erupted products are dispersed over an area of approximately 35 km2 in the eastern part of the volcanic complex of Chachimbiro, up to a distance of about 7 km. The final phase of the eruption was represented by pumice fall out dispersed towards NW, up to a distance of 10 km far from the dome. Tephra deposits consist of two units, clearly visible in the proximal area. The lower unit (blast) represents most of the deposit, it is generally massive, poorly-sorted, matrix-supported and composed primarily of dense material with rare fragments of charred soil. The upper unit (surge) has a high concentration of vesicular juvenile material and a clear internal stratification variable from planar to wavy lamination and aggradational bedforms. The average thickness of the deposits tends to increase in canyons regardless of distance, with values up to 3.8 m at 5 km from the volcano: a higher thickness variation was observed for the blast unit respect to the surge deposits. The increase in thickness generally corresponds to an increase in the maximum clast size, with a trend interpreted as the result of the interaction between the downcurrent variation of the flow capacity and pre-existing topography. The deposits are made up of 90% of juvenile material: 85% is represented by dense clasts and 5-7% by vesicular pumice. The ratio between the two components varies from the base to the top of the sequence, with the surge characterized by an increase in both accidental (40%) and vesicular juvenile clasts (25%). Density and vesicularity have a strong bimodal distribution, with average ranges of 2000-1200 kg/m3 and 22-52%, respectively. these variations along the stratigraphic height were related to change in magma conditions during the event as a result of the deepening of the fragmentation level. Sorting vs median diameter plot obtained from grain size analyses shows that the Pucará deposits are comparable to other studied lateral-blast events. Deposits are generally poorly-sorted, with an excess in the coarse fraction. The features of deposits identify the Pucará explosive event as a perfect example of lateral blast compared with several cases reported in the volcanological literature, such as: Bezymianny (Kamchatka, Russia), 30 March 1956; Mount St. Helens (Washington, USA), 18 May 1980; Soufrière Hills (Montserrat, Caribbean), 26 December 1997; The Pelée (Martinique, Caribbean), 20 May 1902; Mt Lamington (Papua New Guinea), 21 January 1951