A large late-glacial eruption of the Hudson Volcano, Southern Chile.

摘要

Lakes formed in the Aysen region of southern Chile after the retreat of mountain glaciers, beginning by at least ~17,900 cal yrs BP, contain numerous late-glacial and Holocene tephra layers derived from >70 eruptions of the volcanoes in the region, including Hudson, the southernmost in the Andean Southern Volcanic Zone (SVZ). Sediment cores from six of these lakes each contain an unusually thick late-glacial age tephra layer, which based on its distribution and bulk trace-element composition was derived from a large explosive eruption of the Hudson volcano between 17,300 and 17,440 cal yrs BP, and is termed Ho. In these cores, located ~100 km northeast of Hudson, the Ho tephra layers range between 35 to 88 cm in thickness. Comparison with three previously documented large explosive Holocene Hudson eruptions (H1, H2, H3 1991 AD) suggests that Ho was larger, with an estimated tephra volume of >20 km3, the largest post-glacial eruption documented for any volcano in the southern Andes. In total, Hudson has erupted ≥45 km3 of pyroclastic material in the last ~17,500 years, making it the most active volcano in the southern Andes in terms of the total volume of pyroclastic material erupted since the beginning of deglaciation in the region. Chemical stratification is not seen in the Ho deposits, but this eruption was bi-modal, with a much greater proportion of dark glassy basaltic-andesite dense fragments and pumice, which range between 55 to 59 wt % SiO2, and volumetrically less significant lighter colored dacite pumice with 66 wt % SiO2. In contrast, H1 was andesitic in composition, H2 was more felsic than H1, being composed essentially of dacite, and although H3 in 1991 AD was again bi-modal, it erupted a much smaller proportion of mafic compared to felsic material than Ho. Thus, the repetitive large explosive eruptions of Hudson volcano have evolved to progressively less mafic overall compositions from late-glacial to historic times, and their volumes have decreased. All analyzed phases of different Hudson eruptions, have similar Sr-isotopic composition (0.70444 +/- 0.00007), indicating that crystal-liquid fractionation rather than crustal assimilation was the main process responsible for these chemical variations.