Mondaca Volcano lahar of December 3, 1762, Maule Region (35°28’S): one of the largest volcanic disasters in Chilean history

Abstract

The Mondaca volcano is a rhyolitic thick lava-field, located in the vicinity of the nascent Lontué River Valley in the northern part of the Southern Andes. It reached a total volume of ~ 0.85 km3, and formed 4 subunits, named Mondaca 1, 2, 3 and 4, which correspond to successive emissions of rhyolitic blocky lavas, sourced at a rounded dome structure. They present well-preserved flow structures and, in the vicinity, restricted to the south and east of the dome, pyroclastic fall, as well as block’n ash deposits are also exhibited. Downstream, along the Lontué River, a laharic deposit is recognized. The lahar was generated after the collapse of an ephemeral ~0.44 km3 lake generated after the river obstruction during the 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 medial facies deposits along the central valley, 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 superelevation deposits formed during the debris flow emplacement along the Lontué river valley, allows to determine high flow mobility, with estimated velocities that locally reached between 20 and 114 km/h.
 Petrographic characteristics in addition to whole-rock chemical analyses of lavas, pyroclasts and juvenile blocks of the laharic deposit, indicate that all they correspond to High K calcoalkaline rhyolites with subalkaline affinity. These antecedents, together with the geographical continuity between the lavas and debris deposits along Lontué and Mataquito rivers, corroborate facies correlation and common origin as the result of the complex evolution of the Mondaca volcano. Being a fundamentally 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 catastrophes in Chilean history.