Jorge Clavero

Ice volumetric changes on active volcanoes in southern Chile

Abstract Most of the glaciers in southern Chile have been retreating and shrinking during recent decades in response to atmospheric warming and decrease in precipitation. However, some glacier fluctuations are directly associated with the effusive and geothermal activity of ice-covered active volcanoes widely distributed in the region. The aim of this paper is to study the ice volumetric changes by comparing several topographic datasets. A maximum mean ice thinning rate of 0.81 ± 0.45 m a−1 was observed on the ash/debris-covered ablation area of Volcan Villarrica between 1961 and 2004, whilst on Volcan Mocho the signal-to-noise ratio was too small to yield any conclusion. An area reduction of 0.036 ±0.019 km2 a−1 since 1976 was obtained on Glaciar Mocho, while on Volcan Villarrica the area change was −0.090 ± 0.034 km2 a−1 between 1976 and 2005. Glaciers on active volcanoes are therefore shrinking, mainly in response to climatic driving factors. However, volcanic activity is affecting glaciers in two opposite ways: ash/debris advection is helping to reduce surface ablation at lower reaches by insulating the ice from solar radiation, while geothermal activity is probably enhancing melting and water production at the bedrock, resulting in negative ice-elevation changes.

Eruptive parameters and dynamics of the April 2015 sub-Plinian eruptions of Calbuco volcano (southern Chile)

We conducted geological and petrological analyses of the tephra fallout and pyroclastic density current (PDC) products of the 22-23 April 2015 Calbuco eruptions. The eruptive cycle consisted of two sub-Plinian phases that generated > 15 km height columns and PDCs that travelled up to 6 km from the vent. The erupted volume is estimated at 0.38 km3 (non-DRE), with approximately 90% corresponding to tephra fall deposits and the other 10% to PDC deposits. The erupted products are basaltic-andesite, 54-55 wt.% SiO2, with minor amounts of andesite (58 wt.% SiO2). Despite the uniform composition of the products, there are at least four types of textures in juvenile clasts, with different degrees of vesicularity and types and content of crystals. We propose that the eruption triggering mechanism was either exsolution of volatiles due to crystallization, or a small intrusion into the base of the magma chamber, without significant magma mixing or with a magma compositionally similar to that of the residing magma. In either case the triggering mechanism generated convection and sufficient overpressure to promote the first eruptive phase. The start of the eruption decompressed the chamber, promoting intense vesiculation of the remaining magma and an increase in eruption rate towards the end of the eruption.

Glacier shrinkage and negative mass balance in the Chilean Lake District (40°S) / Rétrécissement glaciaire et bilan massique négatif dans la Région des Lacs du Chili (40°S)

Abstract Ice-capped volcanoes of the Chilean Lake District have shown significant glacier retreat during recent decades, probably in response to tropospheric warming and precipitation decrease. Volcán Mocho-Choshuenco (39°55′S, 72°02′W) is one of the main active volcanoes in this part of the country. A mass balance programme was initiated on its southeastern glacier in 2003, in view of its representative conditions as an ice body that is presumably not affected by current volcanic activity. The glaciers of this volcano have been retreating and shrinking in recent decades; by 2003 there had been a reduction of 40% of the original area of 28.4 km2 in 1976. A maximum decrease of area was observed in the most recently analysed period, a rate of 0.45 km2 year-1 between 1987 and 2003. The glacier average net mass balance of 2003/04 yielded −0.88 m w.e. (water equivalent) per year (±0.18), with an average net accumulation and ablation of 2.59 and −3.47 m w.e. per year, respectively. This is the first direct measurement of glacier mass balance in southern Chile, where very little is known about glacier variations and glacier–volcano interactions.

Altered mineral uptake into fresh arc magmas: Insights from u-th isotopes of samples from andean volcanoes under differential crustal stress regimes

Abstract Uranium–thorium (U–Th) isotope compositions of whole rocks, groundmasses and minerals from mafic to intermediate Andean arc magmas were determined to assess the influence of crustal stress on rates of pre-eruptive crystallization and the significance of crystal uptake. Volcanoes investigated include Lascar in the central Andes, situated in a compressional regime, and Callaqui and Lonquimay in the southern Andes, situated in a transtensional regime. In the southern Andes, Calbuco volcano, which experienced variations in the local stress field due to volcano unloading triggered by large debris avalanches, was also targeted. In U–Th equiline space, whole rock and groundmass coincide in each studied sample, and are close to secular equilibrium or in slight U-excess. No isochronal relationships are displayed by the mineral separates, although many are out of secular equilibrium. Further, (234U/238U) activity ratios of some mineral separates from the southern Andes are in disequilibrium and vary significantly between different phases of individual rock samples. The combined data indicate that many crystals are foreign to the melts they are carried in, and that some mineral phases have experienced incipient weathering before their incorporation into the melt. Using Calbuco as an example, we speculate that volcano loading may affect secondary alteration processes at depth.