U-Th zircon dating reveals a correlation between eruptive styles and repose periods at the Nisyros-Yali volcanic area, Greece

Abstract

Water-rich silicic magmas are capable of erupting effusively and explosively, and this drastic change in eruptive styles, termed effusive-explosive transition, has important implications in managing volcanic hazards. Some volcanoes exhibit effusive-explosive transitions during the same eruptive event, while others show this behavior between different eruptions. In the latter case, magma chamber processes induce physical-chemical changes in the magma, which can favor either effusivity or explosivity. This is the case for the Nisyros-Yali volcanic center, from the South Aegean Sea. In the recent stages of activity (past 120 ky), the volcanic area generated eight rhyolitic effusive and explosive events (five on the island of Nisyros and three on the island of Yali), including two caldera-forming eruptions. Changes of water content, temperature and pre-eruptive water-saturation between effusive and explosive deposits point to a potential time-dependency between the two eruptive styles. We investigate this time-dependency by applying UTh disequilibrium dating to zircon crystals. Our eruptive age estimates of the investigated units range from 118.7 ± 10 ka to 19.9 ± 1.5 ka for Nisyros, and from 40 ± 5.2 ka to 22.7 ± 1.6 ka for Yali. Yali volcano has developed after the two caldera-forming events on Nisyros, which occurred at 63.1 ± 4.7 ka and 58.4 ± 2.7 ka. Yali marks the transition to a more geometrically complex system, where the upper-crustal silicic mush hosts at least two eruptible magma chambers (one under Yali, and one under Nisyros). The eruptive styles at both volcanoes seem to be correlated with the length of the repose periods. Effusive events occur after longer periods of volcanic quiescence, while explosive events are generated after shorter periods of repose of ~5–10 ky, which can be extended based on eruption age uncertainty to <18 ky for Nisyros and <12 ky for Yali. This observation is explained by the physical state of the volatiles in the magma chamber, with longer repose periods favoring volatile build-up. This can lead to water-supersaturation at storage pressures which was shown to favor effusivity. Based on this interpretation, both Nisyros and Yali volcanoes are presently in the effusive time window, which makes it probable for the next eruptions to be non-explosive.