Abstract
Antuco (37.4°S, 71.4°W; Chile) is a dominantly basaltic stratovolcano whose original ~ 3300 m altitude main cone experienced a catastrophic sector collapse at ~ 7.1 cal ka BP, producing a volcanic debris avalanche deposit (VDAD) with hummocky surface and ~ 6.4 km3 of volume. We carried out geological studies of its debris avalanche deposit, which was distributed to the W and displays a longitudinal facies transformation from edifice’s megablocks and block to mixed facies in distal areas (up to 25 km from the scar). Our observations support the behavior of the avalanche beginning as a translational slide, and then as plug flow when confined within the Laja River valley. Clay abundance and high content of hydrothermally altered material may suggest active participation of water; flow velocities are estimated to ~100 m s−1. We primarily identify the steep-sided flanks of the cone, and hydrothermal alteration promoted the edifice instability, while basement seismogenic structures may have ultimately triggered the landslide. Subsequent landslide-led events include the transformation of the volcanic activity with explosive eruptions producing a sequence of dilute pyroclastic density currents (PDCs) ending ~3.4 ky BP, and extensive lava effusion rapidly reconstructing the collapsed edifice. Moreover, the Antuco VDAD also blocked the natural output of the Laja Lake, increasing its level by ~200 m and then triggering cataclysmic outburst floods by dam rupture, preserved as high-energy alluvial beds with ages between 2.8 and 1.7 ky BP. The Antuco constitutes an excellent example of a critical chain of events initiated by a stratovolcano lateral collapse and warns for detailed hazard investigations to better comprehend its related impacts.
Highlights
Volcanic lateral collapses result from the separation of huge rock bodies which move downslope beyond the flanks of the volcanic edifice (Siebert 1996)
We determined that the catastrophic lateral collapse which occurred ca. 7.1 cal ka BP reduced the total height of the volcano by at least 900 m
The resulting ~6.4 km3 volcanic debris avalanche deposit (VDAD) flowed downstream the Laja River at ~100 m s −1, up to 25 km distance; initially, it behaved as a translational slide and as a plug flow when it became confined in the valley
Summary
Volcanic lateral collapses ( known as “sector collapses”,“flank failures” or “flank collapses”) result from the separation of huge rock bodies (blocks and sheets) which move downslope beyond the flanks of the volcanic edifice (Siebert 1996). The landslide source is characterized by a scar-limited depression with an open side (Siebert 1984). These sudden gigantic landslides immediately result in fast and non-cohesive gravity granular flows termed volcanic debris avalanches, which are composed of a mixture of rocks and soil, controlled by particle–particle interactions as a result of a frictional or collisional regime (Nakamura 1978; Ui 1983; Schuster and Crandell 1984; Ui et al 1986; Roverato and Dufresne 2021). Lateral collapses may be immediately followed by violent explosive eruptions, including the sudden explosive release of gas contained within the hydrothermal and magmatic systems (blasts), such as observed in the collapses of Bandai-San in 1888 (Nakamura 1978; Moriya 1980), Lamington in 1951 (Belousov et al 2020), Young Shiveluch in 1964 (Belousov and Belousova 1996), or Mt. Saint Helens in 1980 (Voight et al 1981). These blasts can be followed by intense volcanic activity depositing thick successions of pyroclastic density currents (PDCs) and tephra fallout (e.g., Siebert et al 1987)
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