Abstract
The Mondaca Volcano comprises a thick rhyolitic lava-field and a dome of similar composition, located near the Lontué River Valley headwaters in the northern part of the Southern Andes Volcanic Zone. It reaches a total volume of ~0.85 km3, and it is formed by 4 subunits, named Mondaca 1, 2, 3 and 4, which correspond to successive rhyolitic blocky lava flows, emitted from a rounded dome structure. They present well-preserved flow structures and, in the surroundings, restricted to the south and east of the dome, pyroclastic fall, as well as block and ash deposits are also exhibited. Downstream, along the Lontué River, a laharic deposit is recognized. The lahar was produced after the collapse of an ephemeral ~0.44 km3 lake generated after the river obstruction by viscous lavas, during the 1762 first eruptive phase. Proximal lahar facies are well exposed between 5 and 30 km from their source. The profuse agricultural activity has completely obliterated the lahar’s medium facies deposits along the Central Depression, but are well identified at the mouth of the Mataquito River, 180 km downstream, as a beige-coloured layer, interbedded within dark coastal beach-sands. The identification of overflows and super-elevation deposits formed during the debris flow emplacement along the Lontué River valley, allows to determine a high flow mobility, with estimated velocities that locally reached up to 114 km/h. Petrographic characteristics in addition to chemical composition of lavas from the volcano, pyroclasts and juvenile blocks of the laharic deposit, indicate that all they correspond to high K calcoalkaline rhyolites with subalkaline affinity. These backgrounds, together with the geographical continuity between the lavas and debris deposits along the Lontué and Mataquito rivers, verify facies correlation and common origin as the result of the 1762 Mondaca Volcano eruption complex evolution. Although it was a mainly effusive eruption that could not be observed from Curicó, the collateral consequences would have been catastrophic over a vast area to the south of that city, and evidences one of the largest volcanic disasters in Chilean history. Probably because of the low density polulation at that time, the consequences could have been minor.
Highlights
The Mondaca Volcano is a thick rhyolitic lavafield including a dome structure emplaced on the southern Lontué glacial valley, less than 20 km from its headwaters, in the northern part of the Southern Andes Volcanic Zone, Maule Region (35°28’ S, Fig. 1)
It was a mainly effusive eruption that could not be observed from Curicó, the collateral consequences would have been catastrophic over a vast area to the south of that city, and evidences one of the largest volcanic disasters in Chilean history
Petrographic and geochemical characteristics of the lavas and juvenile fragments, in addition to the geographical continuity between the lava facies and debris-flow deposits along the Lontué and Mataquito rivers, unequivocally indicate that they are the result of the complex eruption that gave origin to the Mondaca Volcano
Summary
The Mondaca Volcano is a thick rhyolitic lavafield including a dome structure emplaced on the southern Lontué glacial valley, less than 20 km from its headwaters, in the northern part of the Southern Andes Volcanic Zone, Maule Region (35°28’ S, Fig. 1). The oldest reference related to such an event was made by Gómez de Vidaurre (1776) and taken by Molina (1788), who mentioned an eruption that occurred on December 3, 1762, which he erroneously attributed to the Peteroa volcano, located approximately 30 km northeast of the Mondaca Volcano. Thereafter, Barros Arana (1886) highlighted that, apparently, Molina (1788) was wrong, since he did not examine for himself the effects of the eruption that occurred in the area on December 3 of 1762, attributed to the Peteroa. Having been a prominent phenomenon, no other chronicles of the effects of the eruption have been found, probably because of the low polulation density at that time
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