12:00 〜 12:15
[SVC28-12] New insights into silicic caldera volcanoes and their eruptions
キーワード：Caldera volcano, Yellowstone, Taupo Volcanic Zone, Long Valley, Rhyolite, Magma chamber
Studies on Quaternary silicic eruptions and their source volcanoes in New Zealand and the USA have served to indicate that the processes in the magmatic build-up to and during such large eruptions are widely variable and not easily modelled except in hindsight. Field studies have focussed on assessing the dynamics and timescales of eruptions, and in particular finding evidence for time breaks during these events. These studies show that some events were short lived (days: Bishop Tuff), while others were extended over periods of months (Oruanui) to years (Lava Creek Tuff: LCT) to decades (Huckleberry Ridge Tuff: HRT) with periods of quiescence. Controls on eruption timings thus are not simply related to the size of the magma body: tectonic controls are inferred, but hard to quantify. These eruptions also show wide ranges in eruption style and vigour that confound simple models of eruption triggering through magmatic processes.
Arising from detailed field studies, sampling of juvenile materials then allows the petrological and geochemical characteristics of the pre-eruptive magmatic systems to be reconstructed. Again, these are highly variable from single homogeneous mixed (Oruanui, Taupo eruption Y) or systematically zoned (Bishop) systems, to ones where multiple related (Kidnappers) or wholly independent (HRT) magmatic systems were tapped. In the extreme case of the HRT, four independent magmatic systems were tapped, with up to three of them erupting within single phases of the eruption to produce extraordinarily diverse compositional arrays.
Arising in turn from detailed petrological studies, single-crystal approaches are opening up new perspectives in understanding how large silicic magmatic systems operate. Zircon age data serve to constrain the timing of crystallisation events in the magmatic systems and accumulation rates of the eruptible magma bodies, while diffusion profiling studies capture complementary information about the tempo of pre- and syn-eruptive events. Both show a wide range of results. Some bodies like that which fed the Bishop Tuff appear to have grown gradually over a period of c. 80,000 years, whereas the comparably-sized Oruanui magma body accumulated in only centuries. Diffusion studies also illustrate that such large systems can be reset and erupt within periods of only 10-20 years to produce another eruption (Kidnappers and Rocky Hill; Taupo eruptions Y and Z).
Overall, these studies illustrate that large silicic systems are much more nimble in their behaviour than conventionally thought, able to accumulate magma at rates exceeding 1 cubic kilometre per year, erupt huge volumes with very diverse eruption styles and timings and then recover to erupt again within years to decades. There is much more to be learned about such systems.
Arising from detailed field studies, sampling of juvenile materials then allows the petrological and geochemical characteristics of the pre-eruptive magmatic systems to be reconstructed. Again, these are highly variable from single homogeneous mixed (Oruanui, Taupo eruption Y) or systematically zoned (Bishop) systems, to ones where multiple related (Kidnappers) or wholly independent (HRT) magmatic systems were tapped. In the extreme case of the HRT, four independent magmatic systems were tapped, with up to three of them erupting within single phases of the eruption to produce extraordinarily diverse compositional arrays.
Arising in turn from detailed petrological studies, single-crystal approaches are opening up new perspectives in understanding how large silicic magmatic systems operate. Zircon age data serve to constrain the timing of crystallisation events in the magmatic systems and accumulation rates of the eruptible magma bodies, while diffusion profiling studies capture complementary information about the tempo of pre- and syn-eruptive events. Both show a wide range of results. Some bodies like that which fed the Bishop Tuff appear to have grown gradually over a period of c. 80,000 years, whereas the comparably-sized Oruanui magma body accumulated in only centuries. Diffusion studies also illustrate that such large systems can be reset and erupt within periods of only 10-20 years to produce another eruption (Kidnappers and Rocky Hill; Taupo eruptions Y and Z).
Overall, these studies illustrate that large silicic systems are much more nimble in their behaviour than conventionally thought, able to accumulate magma at rates exceeding 1 cubic kilometre per year, erupt huge volumes with very diverse eruption styles and timings and then recover to erupt again within years to decades. There is much more to be learned about such systems.