MONDACA VOLCANO LAHAR OF DECEMBER 3, 1762, MAULE REGION (35°28’S): ONE OF THE LARGEST VOLCANIC DISASTERS IN CHILEAN HISTORY

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 km, 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. IN PR ES S Andean Geology 48 (3): xxx-xxx. September, 2021 doi: 10.5027/andgeoV48n3-3361 2 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.

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.
Aunque fue una erupción fundamentalmente efusiva que no pudo ser apreciada desde Curicó, sus consecuencias colaterales afectaron una vasta zona de la Depresión Central al sur de Curicó, lo que la califica como una de los mayores desastres volcánicos en la historia de Chile. Probablemente, debido a la baja densidad poblacional en esa época, las consecuencias de esa erupción fueron menores. During a detailed study of the geology and hazards of the PPVC, Naranjo et al. (1999) concluded that, according to the places mentioned, Barros-Arana (1886) had also misinterpreted Molina's (1788) error, maintaining that the aforementioned eruption had occurred at the Peteroa volcano. In fact, the geological evidence, as the presence of remarkable fresh-looking debris flow deposits along the Lontué valley, indicates that the Mondaca volcano (unknown in 1762) lava emplacement was responsible for a dammed lake and its subsequent collapse, which originated the December 3, 1762 debris flow (Fig.1).
This conclusion is also supported by Domeyko (1903), who indicated that, approximately in the mid-nineteenth century, on the southern shore of Mondaca Lake, beaches with abundant pumice pebbles were recognized, where thermal waters still sprouted that were used for medicinal purposes. This author also pointed out that the plains arranged upstream of Mondaca Lake "formed an elliptical valley that took the place of some ancient lake".
During a 1:500,000 geological mapping, González and Vergara (1962) assigned the Mondaca volcano to the Current Volcanic Cones unit, calling it as the "Lengua de Vulcano", which dammed, towards the east, the present Mondaca Lake, to the north of the I N P R E S S Descabezado Grande volcanic group. According to these authors, it corresponds to a "vitreous flow of andesitic-dacitic lava, generated in a monogenetic eruption that dammed the current lake". They also argued that, "due to their fresh morphological features, the Lengua de Vulcano would correspond to a very recent eruption, probably occurred during historical times". deposits.
The purpose of this work is to provide the physical and chronological description of the different evolutionary stages of the Mondaca volcano and its products, as well as the characteristics of the laharic debris flow and its impact. Although the historical references of the process are scarce and confusing, based on its characteristics and distribution, this work also aims to provide the geological and geographical background that allows it to be associated with the so-called "eruption of December 3, 1762" (Molina, 1788).

METHODOLOGY
The methodology comprises field work to identify the distribution of volcanic units (mapping) and sampling, petrographic studies and chemical analyses, and compilation of historical background.
Following the antecedents presented by Naranjo et al. (1999)  Regarding the whole rock major elements chemistry, six selected Mondaca lavas and laharic boulders samples collected during at least 3 field seasons were analysed in the SERNAGEOMIN's Chemical Laboratory by AAS method.

GEOLOGY OF MONDACA VOLCANO
There have been identified different eruptive products for the Mondaca volcano. They include lava flows, a dome, laharic flows, tephra-fall and block and ash PDC deposits, which are described as follows.

The lava field
It is located approximately 1,900 m a.s.l. on the left (south) slope of the Lontué River

I N P R E S S
Under the microscope, slightly fractured and fragmented phenocrystals of plagioclase, pyroxene, biotite, with scarce amphibole and Fe-Ti oxides are observed, immersed in a vitreous ground mass, locally highly fractured, including spherulites. Also, these lavas exhibit varying amounts of vesicles (up to 1 cm in diameter), plagioclase microliths, and pyroxene and amphibole microcrystals. In addition to the aforementioned textures, they develop intersertal, cumulitic, glomeroporphic and sieve textures in plagioclase.

Proximal facies
Along ~25 km of the Lontué River, between the front of the Mondaca 2 lobe and the junction with the Colorado River ( Fig. 1, 2

Distal facies
A ~ 15 cm thick, matrix-supported gravel layer, with rounded pebbles of dark reddish and grey lava, is recognized 8 km to the north of the Mataquito River mouth, 180 km downriver from the Mondaca volcano source (Fig. 1). The deposit also shows coarse rounded pumice sand of a beige colour with few quartz crystals matrix (Fig. 4F). That gravel layer is at ~50 cm depth interbedded within dark beach sands (derived from metamorphic rocks), which are related to lagoon-like deposits. In addition, few sub-rounded to rounded pumice fragments of up to 15 cm in diameter were also observed. This deposit would correspond to the most distal facies of the lahar, whose proximal facies were previously described.

Block and ash deposits
A 30 m thick succession of block and ash type pyroclastic flows deposits were shed up to 3 km to the east on the eastern flank of the Mondaca volcano dome (Figs. 2; 5C). At its foot, block and ash deposits show inclinations of up to ~ 20 ° to the ENE (Fig. 5D). They consist of 5 to 50 cm layers of grayish-pale brownish, ash-rich and fine to medium lapilli pumice alternating with dark grey, clast-supported 10 to 25 cm, medium to coarse lapilli with a higher content of angular obsidian lithics ( Fig. 5E; F). Well-defined parallel stratification is observed and, in some sectors, diffuse crossed or wavy stratification is also present where surge facies occur. Some layers are subrounded pumice rich, occasionally with inverse and normal grading (Fig. 5F). Bomb-size fragments are highly abundant toward the stratigraphically higher sections, some of which being prismatic jointing and bread crust bombs of up to ~ 30 cm, and lithics that, exceptionally, reach 2 m in diameter. The matrix consists of vitreous and pumice juvenile lithic fragments, of ash and lapilli size ( Fig. 5E; F).
Both, dense and pumice juvenile fragment petrography is similar to the previously described lava.

Facies correlation and origin
Petrographic Although the Mondaca eruption was typically effusive, with low emission rates of viscous rhyolitic lava, the collateral consequences were catastrophic. The eruptive phases were low in magnitude, with pyroclasts generation restricted to the source area and the I N P R E S S Lontué valley flanks, therefore, the volcano could not be identified from the Central Depression, where Curicó city is located, between Teno and Lontué rivers. The river headwaters of the first correspond to the CVPP, whose building stands out in the mountains and is clearly visible from Curicó, 65 km to the ESE. On the other hand, Descabezado Grande, Cerro Azul, as well as Mondaca volcanoes stand out where the sources of Lontué River are located, approximately 82 km southeast of Curicó (Fig. 1). This city was founded in 1743, that is only 19 years before Mondaca erupted. Thus, given these geographical conditions, it is reasonable that both Molina (1788) and Barros-Arana (1886) had confused the origin of the debris flow on December 3, 1762 as coming from the PPVC, the closest volcano to Curicó.

Mondaca Volcano evolution: volumes and eruptive rates
After decent at least 450 m from 1,900 m a.s.l. on the southern slope of the Lontué River valley, the first effusive phase that gave rise to the Mondaca 1 lava-lobe took place, completely filling the valley and reaching 5 km long. A second ca. 3 km long lava-lobe to the east was formed as the viscosity (by cooling and/or lower emission rate) increased (Fig. 8A).
The Mondaca 1 lava, which reached a volume of 0.32 km 3 , occupied the entire valley, and caused the obstruction of the river giving rise to an early Mondaca Lake (Fig. 8B). The dam was caused by the encounter of the lava against a ledge of the north valley slope that, at an approximate maximum level between 1,520 and 1,550 m a.s.l., constituted the true right barrier of the lake. In that sector, the current Lontué River shows an abrupt descent of 100 m along only 300 m of horizontal distance.

I N P R E S S
Considering the current flow of the Lontué River, measured between the confluences of Los Patos and Colorado rivers (Fig. 8C), and that it is 25 m 3 /s on average (SNIA-DGA, 2020), it can tentatively be estimated that the early Mondaca Lake came to cover 7.33 km 2 , with a water volume of 0.44 km 3 , which accumulated in a minimum time of 7 months, before collapsing. Given the possible variations in the river water flow, especially during the eighteenth century, we must assume that the resulting values are only an approximation.
The overflow and rupture of such natural dam gave rise to the lahar of 3 December 1762 ( Fig. 8C).
According to morphostratigraphical relations, the second phase (Mondaca 2) was also developed in two lobes showing conspicuous lateral scarps on the Mondaca 1 lava, reaching a total volume of 0.31 km 3 (Fig. 8D). Following the maximum slope, the northwest lobe reached 8 km and was probably originated as the emission rate increased, overpassing previous phase lava flows. Increasing cooling and viscosity of the phase 2 lava caused the secondary lobe flowing to the northeast, upstream of the Lontué valley (Fig. 8D).
The emission rate for the Mondaca 3 phase continued to decline, and generated a coulée lava overlapping Mondaca 2 lava, with a volume of only 0.013 km 3 , probably as a result of viscosity increase. Coulée flank collapses caused successive block and ash flows emplaced to the east (Fig. 8E). These pyroclastic flows continued to build on the same flank during the emission of Mondaca 4 phase, a single lava flow that was emplaced to the northeast due to the containment of previous lava phases. The lava of the last eruptive phase reached a volume of 0.2 km 3 and covered phases 1 and 3 lava flows (Fig. 8F). This final phase originated as the result of an increase in the emission rate. In addition, block and ash flows continued to emplace to the east, eroding the fall deposits that had accumulated during the previous phases.
The materials erupted during the various phases of the Mondaca volcano were emitted effusively, through a low-magnitude eruption, with subordinated pyroclasts production, since it was not observed from the Central Depression, 60 km to the west. The rate of viscous lava emission was also very low and, considering the duration for the lake formation associated with phase 1, it must have occurred over a period greater than seven months. Although there is no evidence to determine the end of the eruption, if Domeyko´s (1903) testimony is assumed, the eruption was over by the mid-19th century.

High impact effects of December 3rd, 1762 laharic debris-flow
The overlap of the Mondaca 2 lava flow over the laharic debris flow deposit demonstrate that the former ~0.44 km 3 Mondaca lake occurred as a direct consequence of the damming effect caused by Mondaca 1 lava flow. The high mobility of the lahar flow is demonstrated when compared to the parameters that characterized the lahar flow produced by the Kelut volcano crater lake rupture during the 1919 eruption, Indonesia (Nawiyanto and Sasmita, 2018). Figure 9 shows the rate between the descending height (H) and the distance travelled (L) by the Mondaca lahar, from 3 December 1762, the lahar associated with the Kelut volcano in 1919, and the lahar produced by the glacier outburst (jökullhaup) to the south of Llaima volcano during the 1640 AD eruption, one of the largest lahars in the Chilean Southern Andes in historical times (Petit-Breuilh, 1996;Naranjo and Moreno, 2005). The H/L ratio was defined by Ui (1983) as the "apparent coefficient of friction", and ranges between 0.18 to 0.06 for volcanic dry avalanches. Consequently, we suggest that, with a coefficient of friction of 0.009 and a travelled distance of almost 180 km, the lahar generated by the emptying of the ephemeral Mondaca lake is one of the largest volcanic disasters originated during history in Chile (Fig. 9).

I N P R E S S
As far as the debris flow velocity is concern, field evidences are shown at different sectors along proximal facies. Lahar flow mobility was significantly high as the flow runout deposited sediments in elevated areas around bends where it generated well-developed lateral trimlines. The flow also climbed topographic obstacles perpendicular to the flow direction, depositing material between 5 and 42 m higher than the flow base. These debris flow superelevation, generated at some bends, allow to have velocity estimations, using the formula given by Pierson and Scott (1985, and references therein), which takes into account geometric features of such curves: where U is mean fluid velocity, g is gravity acceleration (9.8 m/s 2 ), Δh is elevation difference between mudlines on the inside and outside of the channel bend, rc is river-bend radius of curvature, and b is channel width. Velocity estimates computed from this equation are minimum values because frictional energy losses are not taken into account, but the computed values are assumed to be within 15% of actual velocities (Pierson, 1985). We have estimated an average velocity of ~50 km/h for the laharic debris flow, based on nine measurements along Lontué River between 15 and 30 km downstream from the source.
These values varied in the range of 20 and 114 km/h, which could be minimum and maximum local flow velocities, due to variations in the width and depth of the river bed, curvature radius and local river slope.
One of the main lahar effects took place on the alluvial fan from the Lontué valley apex over the Central Depression (Fig. 1). From there, it is estimated that it would have been emplaced over a ca. 300 km 2 area with great agricultural potential, thus constituting the largest and most disastrous impact of the eruption.

CONCLUSIONS
The Mondaca volcano consists in a 0.85 km 3 rhyolitic blocky lava field and a dome with similar composition, located in the vicinity of the Lontué River Valley hedwaters, formed by 4 sub-units or eruptive pulses. During the early stages, the lavas blocked the river waters and caused a dam that, upstream, formed a 7.33 km 2 and 0.44 km 3 lake, which accumulated water during a ~7 month period. The collapse of the natural dam triggered a highly mobile laharic debris flow, which, on December 3, 1762, flowed westward 180 km along the Lontué River, to the sea through the Mataquito River at an estimated average velocity of ~50 km/h.

I N P R E S S
Petrographic and geochemical characteristics of the lavas and juvenile fragments of the lahar debris flow deposit, in addition to their geographical continuity along these rivers, indicate, unequivocally, that they were the result of the complex eruption that gave rise to the Mondaca volcano. The distribution along the Lontué-Mataquito rivers and the high mobility of the lahar indicate that this eruption produced one of the largest volcanic disasters in Chilean history and affected an area estimated in 300 km 2 in the Central Depression, although its effects were only indirectly documented.